Los dinosaurios son variados desde los puntos de vista taxonómico , morfológico y ecológico . Las aves, con más de 11.000 especies vivas , se encuentran entre los grupos más diversos de vertebrados. Utilizando evidencia fósil, los paleontólogos han identificado más de 900 géneros distintos y más de 1.000 especies diferentes de dinosaurios no aviares. Los dinosaurios están representados en todos los continentes tanto por especies existentes (aves) como por restos fósiles. Durante la primera mitad del siglo XX, antes de que las aves fueran reconocidas como dinosaurios, la mayoría de la comunidad científica creía que los dinosaurios habían sido lentos y de sangre fría . Sin embargo, la mayoría de las investigaciones realizadas desde la década de 1970 han indicado que los dinosaurios eran animales activos con metabolismos elevados y numerosas adaptaciones para la interacción social. Algunos eran herbívoros , otros carnívoros . La evidencia sugiere que todos los dinosaurios ponían huevos y que la construcción de nidos era un rasgo compartido por muchos dinosaurios, tanto aviares como no aviares.
Aunque los dinosaurios eran ancestralmente bípedos , muchos grupos extintos incluían especies cuadrúpedas , y algunas podían cambiar de postura. Las elaboradas estructuras de exhibición, como cuernos o crestas, son comunes a todos los grupos de dinosaurios, y algunos grupos extintos desarrollaron modificaciones esqueléticas , como armadura ósea y espinas . Si bien el linaje aviar superviviente moderno de los dinosaurios (aves) es generalmente pequeño debido a las limitaciones del vuelo, muchos dinosaurios prehistóricos (no aviares y aviares) tenían un cuerpo grande: se estima que los dinosaurios saurópodos más grandes alcanzaron longitudes de 39,7 metros (130 pies) y alturas de 18 m (59 pies) y fueron los animales terrestres más grandes de todos los tiempos. La idea errónea de que los dinosaurios no aviares eran uniformemente gigantescos se basa en parte en el sesgo de preservación , ya que es más probable que los huesos grandes y resistentes duren hasta que se fosilicen. Muchos dinosaurios eran bastante pequeños, algunos medían unos 50 centímetros (20 pulgadas) de largo.
Los primeros fósiles de dinosaurios fueron reconocidos a principios del siglo XIX, con el nombre "dinosaurio" (que significa "lagarto terrible") acuñado por Sir Richard Owen en 1842 para referirse a estos "grandes lagartos fósiles". [7] [8] [9] Desde entonces, los esqueletos de dinosaurios fósiles montados han sido atracciones importantes en museos de todo el mundo, y los dinosaurios se han convertido en una parte duradera de la cultura popular . Los grandes tamaños de algunos dinosaurios, así como su naturaleza aparentemente monstruosa y fantástica, han asegurado su aparición regular en libros y películas superventas, como la franquicia Jurassic Park . El persistente entusiasmo público por los animales ha resultado en una financiación significativa para la ciencia de los dinosaurios, y los nuevos descubrimientos son cubiertos regularmente por los medios de comunicación.
Las aves son los únicos dinosaurios supervivientes. En la taxonomía tradicional , las aves se consideraban una clase separada que había evolucionado a partir de los dinosaurios, un superorden distinto . Sin embargo, la mayoría de los paleontólogos contemporáneos rechazan el estilo tradicional de clasificación basado en la similitud anatómica, a favor de la taxonomía filogenética basada en la ascendencia deducida, en la que cada grupo se define como todos los descendientes de un género fundador determinado. [13] Las aves pertenecen al subgrupo de dinosaurios Maniraptora , que son celurosaurios , que son terópodos, que son saurisquios. [14]
La investigación de Matthew G. Baron, David B. Norman y Paul M. Barrett en 2017 sugirió una revisión radical de la sistemática de los dinosaurios. El análisis filogenético de Baron et al. recuperó a los Ornithischia como más cercanos a los Theropoda que a los Sauropodomorpha, en oposición a la unión tradicional de los terópodos con los sauropodomorfos. Esto haría que los saurópodos y sus parientes quedaran fuera de los dinosaurios tradicionales, por lo que redefinieron a Dinosauria como el último ancestro común de Triceratops horridus , Passer domesticus y Diplodocus carnegii , y todos sus descendientes, para garantizar que los saurópodos y sus parientes permanezcan incluidos como dinosaurios. También resucitaron el clado Ornithoscelida para referirse al grupo que contiene a Ornithischia y Theropoda. [15] [16]
Usando una de las definiciones anteriores, los dinosaurios pueden ser descritos generalmente como arcosaurios con extremidades traseras erguidas debajo del cuerpo . [17] Otros animales prehistóricos, incluyendo pterosaurios , mosasaurios , ictiosaurios , plesiosaurios y Dimetrodon , aunque a menudo popularmente concebidos como dinosaurios, no están clasificados taxonómicamente como dinosaurios. Los pterosaurios están distantemente relacionados con los dinosaurios, siendo miembros del clado Ornithodira . Los otros grupos mencionados son, como los dinosaurios y los pterosaurios, miembros de Sauropsida (el clado de reptiles y aves), excepto Dimetrodon (que es un sinápsido ). Ninguno de ellos tenía la postura erecta de las extremidades traseras característica de los verdaderos dinosaurios. [18]
Los dinosaurios fueron los vertebrados terrestres dominantes de la Era Mesozoica , especialmente los períodos Jurásico y Cretácico. Otros grupos de animales estaban restringidos en tamaño y nichos; los mamíferos , por ejemplo, rara vez superaban el tamaño de un gato doméstico y generalmente eran carnívoros del tamaño de un roedor de presas pequeñas. [19] Los dinosaurios siempre han sido reconocidos como un grupo extremadamente variado: más de 900 géneros de dinosaurios no aviares han sido identificados con confianza (2018) con 1124 especies (2016). Las estimaciones sitúan el número total de géneros de dinosaurios preservados en el registro fósil en 1850, casi el 75% aún sin descubrir, [20] [21] [22] y el número que alguna vez existió (dentro o fuera del registro fósil) en 3400. [23] Una estimación de 2016 situó el número de especies de dinosaurios que vivieron en el Mesozoico en 1.543-2.468, [24] [25] en comparación con el número de aves actuales (dinosaurios aviares) en 10.806 especies. [26]
Los dinosaurios extintos, así como las aves modernas, incluyen géneros que son herbívoros y otros carnívoros, incluidos los que se alimentan de semillas, los que se alimentan de peces, los insectívoros y los omnívoros. Si bien los dinosaurios eran ancestralmente bípedos (como lo son todas las aves modernas), algunos evolucionaron a cuadrúpedos y otros, como Anchisaurus e Iguanodon , podían caminar con la misma facilidad sobre dos o cuatro patas. Las modificaciones craneales como los cuernos y las crestas son rasgos comunes de los dinosaurios, y algunas especies extintas tenían armadura ósea. Aunque los géneros más conocidos son notables por su gran tamaño, muchos dinosaurios mesozoicos eran del tamaño de un humano o más pequeños, y las aves modernas son generalmente de tamaño pequeño. Los dinosaurios habitan hoy en día todos los continentes, y los fósiles muestran que habían alcanzado una distribución global a más tardar en la época del Jurásico Temprano . [27] Las aves modernas habitan la mayoría de los hábitats disponibles, desde los terrestres hasta los marinos, y hay evidencia de que algunos dinosaurios no aviares (como el Microraptor ) podían volar o al menos planear, y otros, como los espinosáuridos , tenían hábitos semiacuáticos . [28]
Distinguir características anatómicas
Aunque los descubrimientos recientes han dificultado la presentación de una lista universalmente aceptada de sus características distintivas, casi todos los dinosaurios descubiertos hasta ahora comparten ciertas modificaciones con el esqueleto ancestral de los arcosaurios, o son claramente descendientes de dinosaurios más antiguos que muestran estas modificaciones. Aunque algunos grupos posteriores de dinosaurios presentaron versiones modificadas de estos rasgos, se consideran típicos de Dinosauria; los primeros dinosaurios los tenían y los transmitieron a sus descendientes. Tales modificaciones, que se originan en el ancestro común más reciente de un determinado grupo taxonómico, se denominan sinapomorfías de dicho grupo. [29]
Diagrama etiquetado de un cráneo típico de arcosaurio, el cráneo de Dromaeosaurus
Una evaluación detallada de las interrelaciones de los arcosaurios realizada por Sterling Nesbitt [30] confirmó o encontró las siguientes doce sinapomorfías inequívocas, algunas de ellas ya conocidas:
En el cráneo , hay una fosa supratemporal (excavación) delante de la fenestra supratemporal , la abertura principal en el techo posterior del cráneo.
Epífisis , procesos que apuntan oblicuamente hacia atrás en las esquinas superiores traseras de las vértebras cervicales anteriores (frontales), detrás del atlas y el axis , las dos primeras vértebras cervicales.
Vértice de una cresta deltopectoral (una proyección en la que se adhieren los músculos deltopectorales ) ubicada a un 30 % o más de la longitud del húmero (hueso de la parte superior del brazo)
Radio , un hueso del brazo inferior, más corto que el 80% de la longitud del húmero.
El cuarto trocánter (proyección donde el músculo caudofemoral se une al eje posterior interno) del fémur (hueso del muslo) es una brida afilada.
Cuarto trocánter asimétrico, con el margen distal inferior formando un ángulo más pronunciado con el eje.
En el astrágalo y el calcáneo , huesos superiores del tobillo, la faceta articular proximal, la superficie de conexión superior, para el peroné ocupa menos del 30% del ancho transversal del elemento.
Los exoccipitales (huesos en la parte posterior del cráneo) no se encuentran a lo largo de la línea media en el piso de la cavidad endocraneal, el espacio interno de la caja craneal.
En la pelvis, las superficies articulares proximales del isquion con el íleon y el pubis están separadas por una gran superficie cóncava (en el lado superior del isquion se encuentra una parte de la articulación abierta de la cadera entre los contactos con el hueso púbico y el íleon).
La cresta cnemial de la tibia (parte saliente de la superficie superior de la tibia) se arquea anterolateralmente (se curva hacia el frente y el lado externo).
Cresta distintiva orientada proximalmente (vertical) presente en la cara posterior del extremo distal de la tibia (la superficie trasera del extremo inferior de la tibia)
Superficie articular cóncava del peroné del calcáneo (la superficie superior del calcáneo, donde toca el peroné, tiene un perfil hueco)
Nesbitt encontró una serie de posibles sinapomorfías adicionales y descartó una serie de sinapomorfías sugeridas previamente. Algunas de ellas también están presentes en los silesáuridos , que Nesbitt recuperó como un grupo hermano de Dinosauria, incluyendo un gran trocánter anterior, metatarsianos II y IV de longitud subigual, contacto reducido entre el isquion y el pubis, la presencia de una cresta cnemial en la tibia y de un proceso ascendente en el astrágalo, y muchas otras. [10]
Articulaciones de la cadera y posturas de las extremidades posteriores de: (de izquierda a derecha) reptiles típicos (despatarrados), dinosaurios y mamíferos (erectos) y rauisuquios (erectos en forma de pilar)
Los dinosaurios comparten una variedad de otras características esqueléticas. Sin embargo, debido a que son comunes a otros grupos de arcosaurios o no estaban presentes en todos los dinosaurios primitivos, estas características no se consideran sinapomorfias. Por ejemplo, como diápsidos , los dinosaurios tenían ancestralmente dos pares de fenestras infratemporales (aberturas en el cráneo detrás de los ojos), y como miembros del grupo de diápsidos Archosauria, tenían aberturas adicionales en el hocico y la mandíbula inferior. [31] Además, ahora se sabe que varias características que alguna vez se pensó que eran sinapomorfias aparecieron antes de los dinosaurios, o estaban ausentes en los primeros dinosaurios y evolucionaron independientemente por diferentes grupos de dinosaurios. Estas incluyen una escápula alargada , u omóplato; un sacro compuesto de tres o más vértebras fusionadas (se encuentran tres en algunos otros arcosaurios, pero solo dos en Herrerasaurus ); [10] y un acetábulo perforado , o cavidad de la cadera, con un agujero en el centro de su superficie interior (cerrado en Saturnalia tupiniquim , por ejemplo). [32] [33] Otra dificultad para determinar características distintivas de los dinosaurios es que los primeros dinosaurios y otros arcosaurios de la época del Triásico Tardío a menudo son poco conocidos y eran similares en muchos aspectos; estos animales a veces han sido identificados erróneamente en la literatura. [34]
Los dinosaurios se paran con sus extremidades traseras erguidas de una manera similar a la mayoría de los mamíferos modernos , pero distinta de la mayoría de los otros reptiles, cuyas extremidades se extienden hacia ambos lados. [35] Esta postura se debe al desarrollo de un receso lateral en la pelvis (generalmente una cavidad abierta) y una cabeza distinta correspondiente orientada hacia adentro en el fémur. [36] Su postura erguida permitió a los primeros dinosaurios respirar fácilmente mientras se movían, lo que probablemente permitió niveles de resistencia y actividad que superaron los de los reptiles "extendido" . [37] Las extremidades erectas probablemente también ayudaron a soportar la evolución del gran tamaño al reducir las tensiones de flexión en las extremidades. [38] Algunos arcosaurios no dinosaurios, incluidos los rauisuquios , también tenían extremidades erectas, pero lo lograron mediante una configuración "erecta de pilar" de la articulación de la cadera, donde en lugar de tener una proyección desde la inserción del fémur en una cavidad en la cadera, el hueso pélvico superior se rotó para formar una plataforma sobresaliente. [38]
Historia del estudio
Historia precientífica
Los fósiles de dinosaurios se conocen desde hace milenios, aunque no se reconoció su verdadera naturaleza. Los chinos los consideraban huesos de dragón y los documentaron como tales. Por ejemplo, Huayang Guo Zhi (華陽國志), un diccionario geográfico compilado por Chang Qu (常璩) durante la dinastía Jin occidental (265-316), informó del descubrimiento de huesos de dragón en Wucheng en la provincia de Sichuan . [39] Los pobladores del centro de China han desenterrado durante mucho tiempo "huesos de dragón" fosilizados para su uso en medicinas tradicionales . [40] En Europa , generalmente se creía que los fósiles de dinosaurios eran los restos de gigantes y otras criaturas bíblicas . [41]
Las descripciones académicas de lo que ahora se reconocerían como huesos de dinosaurio aparecieron por primera vez a fines del siglo XVII en Inglaterra. Parte de un hueso, ahora conocido como el fémur de un Megalosaurus , [42] fue recuperado de una cantera de piedra caliza en Cornwell cerca de Chipping Norton , Oxfordshire, en 1676. El fragmento fue enviado a Robert Plot , profesor de química en la Universidad de Oxford y primer curador del Museo Ashmolean , quien publicó una descripción en su The Natural History of Oxford-shire (1677). [43] Identificó correctamente el hueso como la extremidad inferior del fémur de un animal grande y reconoció que era demasiado grande para pertenecer a alguna especie conocida. Por lo tanto, concluyó que era el fémur de un humano enorme, tal vez un titán u otro tipo de gigante que aparece en las leyendas. [44] [45] Edward Lhuyd , un amigo de Sir Isaac Newton , publicó Lithophylacii Britannici ichnographia (1699), el primer tratamiento científico de lo que ahora sería reconocido como un dinosaurio cuando describió y nombró un diente de saurópodo , " Rutellum impicatum ", [46] [47] que había sido encontrado en Caswell, cerca de Witney , Oxfordshire. [48]
Entre 1815 y 1824, el reverendo William Buckland , el primer lector de geología en la Universidad de Oxford, recolectó más huesos fosilizados de Megalosaurus y se convirtió en la primera persona en describir un dinosaurio no aviar en una revista científica . [42] [49] El segundo género de dinosaurio no aviar en ser identificado, Iguanodon , fue descubierto según la leyenda en 1822 por Mary Ann Mantell , la esposa del geólogo inglés Gideon Mantell, quien de hecho había requerido restos años antes. Gideon Mantell reconoció similitudes entre sus fósiles y los huesos de las iguanas modernas . Publicó sus hallazgos en 1825. [50] [51]
El estudio de estos "grandes lagartos fósiles" pronto se convirtió en un tema de gran interés para los científicos europeos y estadounidenses, y en 1842 el paleontólogo inglés Sir Richard Owen acuñó el término "dinosaurio", utilizándolo para referirse a la "tribu o suborden distintiva de reptiles saurios" que entonces se reconocían en Inglaterra y en todo el mundo. [7] [8] [9] [52] [53] El término se deriva del griego antiguo δεινός (deinos) 'terrible, potente o terriblemente grande' y σαῦρος (sauros) 'lagarto o reptil'. [52] [54] Aunque el nombre taxonómico a menudo se ha interpretado como una referencia a los dientes, garras y otras características temibles de los dinosaurios, Owen también pretendía que evocara su tamaño y majestuosidad. [55] Owen reconoció que los restos que se habían encontrado hasta ahora, Iguanodon , Megalosaurus y Hylaeosaurus , compartían características distintivas, y por eso decidió presentarlos como un grupo taxonómico distinto. Como aclaró el geólogo e historiador británico Hugh Torrens, Owen había dado una presentación sobre reptiles fósiles a la Asociación Británica para el Avance de la Ciencia en 1841, pero los informes de la época muestran que Owen no mencionó la palabra "dinosaurio", ni reconoció a los dinosaurios como un grupo distinto de reptiles en su discurso. Presentó a Dinosauria solo en la versión de texto revisada de su charla publicada en abril de 1842. [7] [8] Con el respaldo del Príncipe Alberto , esposo de la Reina Victoria , Owen estableció el Museo de Historia Natural de Londres , para exhibir la colección nacional de fósiles de dinosaurios y otras exhibiciones biológicas y geológicas. [56]
Descubrimientos en América del Norte
En 1858, William Parker Foulke descubrió el primer dinosaurio americano conocido, en fosas de marga en la pequeña ciudad de Haddonfield, Nueva Jersey . (Aunque se habían encontrado fósiles antes, su naturaleza no había sido correctamente discernida.) La criatura fue nombrada Hadrosaurus foulkii . Fue un hallazgo extremadamente importante: Hadrosaurus fue uno de los primeros esqueletos de dinosaurio casi completos encontrados ( el primero fue en 1834, en Maidstone, Inglaterra ), y era claramente una criatura bípeda. Este fue un descubrimiento revolucionario ya que, hasta ese momento, la mayoría de los científicos habían creído que los dinosaurios caminaban sobre cuatro patas, como otros lagartos. Los descubrimientos de Foulke provocaron una ola de interés en los dinosaurios en los Estados Unidos, conocida como dinosauriomanía. [57]
La manía de los dinosaurios se ejemplificó con la feroz rivalidad entre Edward Drinker Cope y Othniel Charles Marsh , quienes compitieron para ser los primeros en encontrar nuevos dinosaurios en lo que llegó a conocerse como las Guerras de los Huesos . Esta lucha entre los dos científicos duró más de 30 años, y terminó en 1897 cuando Cope murió después de gastar toda su fortuna en la caza de dinosaurios. Muchos especímenes valiosos de dinosaurios fueron dañados o destruidos debido a los métodos toscos de la pareja: por ejemplo, sus excavadores a menudo usaban dinamita para desenterrar huesos. Los paleontólogos modernos encontrarían tales métodos rudimentarios e inaceptables, ya que las explosiones destruyen fácilmente la evidencia fósil y estratigráfica. A pesar de sus métodos poco refinados, las contribuciones de Cope y Marsh a la paleontología fueron enormes: Marsh desenterró 86 nuevas especies de dinosaurios y Cope descubrió 56, un total de 142 nuevas especies. La colección de Cope se encuentra ahora en el Museo Americano de Historia Natural de la ciudad de Nueva York, mientras que la de Marsh está en el Museo Peabody de Historia Natural de la Universidad de Yale . [58]
La Segunda Guerra Mundial provocó una pausa en la investigación paleontológica; después de la guerra, la atención de la investigación también se desvió cada vez más hacia los mamíferos fósiles en lugar de los dinosaurios, que eran vistos como lentos y de sangre fría. [59] [60] Sin embargo, a fines de la década de 1960, el campo de la investigación sobre dinosaurios experimentó un aumento de actividad que aún continúa. [61] Varios estudios seminales condujeron a esta actividad. Primero, John Ostrom descubrió el terópodo dromeosáurido Deinonychus , parecido a un pájaro , y lo describió en 1969. Su anatomía indicaba que era un depredador activo que probablemente era de sangre caliente, en marcado contraste con la imagen predominante de los dinosaurios en ese momento. [59] Al mismo tiempo, Robert T. Bakker publicó una serie de estudios que también defendían estilos de vida activos en los dinosaurios basados en evidencia anatómica y ecológica (ver § Fisiología), [62] [63] que posteriormente se resumieron en su libro de 1986 The Dinosaur Heresies . [64]
Las nuevas revelaciones fueron respaldadas por un aumento en los descubrimientos de dinosaurios. Los paleontólogos que trabajaron en regiones previamente inexploradas, como India, Sudamérica, Madagascar, la Antártida y, más significativamente, China, hicieron importantes descubrimientos de dinosaurios. En el caso de los terópodos, los sauropodomorfos y los ornitisquios, el número de géneros nombrados comenzó a aumentar exponencialmente en la década de 1990. [20] A partir de 2008, [update]se nombraron más de 30 nuevas especies de dinosaurios cada año. [65] Al menos los sauropodomorfos experimentaron un aumento adicional en el número de especies nombradas en la década de 2010, con un promedio de 9,3 nuevas especies nombradas cada año entre 2009 y 2020. Como consecuencia, se nombraron más sauropodomorfos entre 1990 y 2020 que en todos los años anteriores combinados. [66] Estas nuevas localidades también llevaron a mejoras en la calidad general de los especímenes, ya que las nuevas especies se nombraban cada vez más a partir de esqueletos más completos, a veces de varios individuos, y no a partir de fósiles fragmentados. La mejora de los especímenes también llevó a que las nuevas especies se invalidaran con menos frecuencia. [65] Las localidades asiáticas han producido los especímenes de terópodos más completos, [67] mientras que las localidades norteamericanas han producido los especímenes de sauropodomorfos más completos. [66]
Antes del renacimiento de los dinosaurios, estos se clasificaban principalmente utilizando el sistema tradicional basado en rangos de la taxonomía de Linneo . El renacimiento también estuvo acompañado por la aplicación cada vez más extendida de la cladística , un método de clasificación más objetivo basado en la ascendencia y los rasgos compartidos, que ha demostrado ser tremendamente útil en el estudio de la sistemática y la evolución de los dinosaurios. El análisis cladístico, entre otras técnicas, ayuda a compensar un registro fósil a menudo incompleto y fragmentario. [68] [69] Los libros de referencia que resumen el estado de la investigación sobre los dinosaurios, como The Dinosauria de David B. Weishampel y colegas , hicieron que el conocimiento fuera más accesible [70] y estimularon un mayor interés en la investigación sobre los dinosaurios. La publicación de la primera y segunda ediciones de The Dinosauria en 1990 y 2004, y de un artículo de revisión de Paul Sereno en 1998, estuvieron acompañados por aumentos en el número de árboles filogenéticos publicados para los dinosaurios. [71]
Preservación molecular y de tejidos blandos
Impresiones de la piel de un espécimen de Edmontosaurus encontradas en 1999
Los fósiles de dinosaurios no se limitan a los huesos, sino que también incluyen huellas o restos mineralizados de cubiertas de piel, órganos y otros tejidos. De estos, las cubiertas de piel basadas en proteínas de queratina se conservan más fácilmente debido a su estructura molecular reticulada e hidrófoba . [72] Se conocen fósiles de cubiertas de piel basadas en queratina o cubiertas de piel ósea de la mayoría de los grupos principales de dinosaurios. Se han encontrado fósiles de dinosaurios con impresiones de piel escamosa desde el siglo XIX. Samuel Beckles descubrió una extremidad anterior de saurópodo con piel preservada en 1852 que se atribuyó incorrectamente a un cocodrilo; fue atribuida correctamente por Marsh en 1888 y sujeta a un estudio adicional por Reginald Hooley en 1917. [73] Entre los ornitisquios, en 1884 Jacob Wortman encontró impresiones de piel en el primer espécimen conocido de Edmontosaurus annectens , que fueron destruidas en gran parte durante la excavación del espécimen. [74] Owen y Hooley posteriormente describieron impresiones de piel de Hypsilophodon e Iguanodon en 1885 y 1917. [73] Desde entonces, las impresiones de escamas se han encontrado con mayor frecuencia entre los hadrosáuridos, donde las impresiones se conocen de casi todo el cuerpo en múltiples especímenes. [75]
A partir de la década de 1990, los descubrimientos importantes de fósiles excepcionalmente conservados en depósitos conocidos como Lagerstätten de conservación contribuyeron a la investigación sobre los tejidos blandos de los dinosaurios. [76] [77] Principalmente entre estos se encontraban las rocas que produjeron las biotas de Jehol (Cretácico Inferior) y Yanliao (Jurásico Medio a Superior) del noreste de China, de las cuales Xing Xu y colegas han descrito cientos de especímenes de dinosaurios que llevan impresiones de estructuras similares a plumas (tanto estrechamente relacionadas con las aves como de otro tipo, ver § Origen de las aves) . [78] [79] En los reptiles y mamíferos actuales, las estructuras celulares que almacenan pigmento conocidas como melanosomas son parcialmente responsables de producir la coloración. [80] [81] Se han reportado tanto rastros químicos de melanina como melanosomas de forma característica en plumas y escamas de dinosaurios Jehol y Yanliao, incluidos terópodos y ornitisquios. [82] Esto ha permitido múltiples reconstrucciones de cuerpo completo de la coloración de los dinosaurios , como para Sinosauropteryx [83] y Psittacosaurus [84] por Jakob Vinther y colegas, y técnicas similares también se han extendido a fósiles de dinosaurios de otras localidades. [80] (Sin embargo, algunos investigadores también han sugerido que los melanosomas fosilizados representan restos bacterianos. [85] [86] ) El contenido del estómago en algunos dinosaurios Jehol y Yanliao estrechamente relacionados con las aves también ha proporcionado indicaciones indirectas de la dieta y la anatomía del sistema digestivo (por ejemplo, cultivos ). [87] [88] Se ha informado de evidencia más concreta de anatomía interna en Scipionyx del Pietraroja Plattenkalk de Italia. Conserva porciones de los intestinos, colon, hígado, músculos y tráquea. [89]
Al mismo tiempo, una línea de trabajo dirigida por Mary Higby Schweitzer , Jack Horner y colegas informaron sobre varias apariciones de tejidos blandos y proteínas preservadas dentro de fósiles de huesos de dinosaurios. Schweitzer y otros habían encontrado varias estructuras mineralizadas que probablemente representaban glóbulos rojos y fibras de colágeno en huesos de tiranosáuridos ya en 1991. [90] [91] [92] Sin embargo, en 2005, Schweitzer y colegas informaron que un fémur de Tyrannosaurus conservaba tejido blando y flexible en su interior, incluidos vasos sanguíneos , matriz ósea y tejido conectivo (fibras óseas) que habían conservado su estructura microscópica. [93] Este descubrimiento sugirió que los tejidos blandos originales podrían conservarse a lo largo del tiempo geológico, [72] habiéndose propuesto múltiples mecanismos. [94] Más tarde, en 2009, Schweitzer y colegas informaron que un fémur de Brachylophosaurus conservaba microestructuras similares, y las técnicas inmunohistoquímicas (basadas en la unión de anticuerpos ) demostraron la presencia de proteínas como colágeno, elastina y laminina . [95] Ambos especímenes produjeron secuencias de proteínas de colágeno que eran viables para análisis filogenéticos moleculares , que los agruparon con aves como se esperaría. [95] [96] También se ha informado de la extracción de ADN fragmentario para ambos fósiles, [97] junto con un espécimen de Hypacrosaurus . [98] En 2015, Sergio Bertazzo y colegas informaron de la preservación de fibras de colágeno y glóbulos rojos en ocho especímenes de dinosaurios del Cretácico que no mostraban ningún signo de preservación excepcional, lo que indica que el tejido blando puede conservarse con más frecuencia de lo que se pensaba anteriormente. [99] Se han rechazado las sugerencias de que estas estructuras representan biopelículas bacterianas [100] , [101] pero la contaminación cruzada sigue siendo una posibilidad difícil de detectar. [102]
Los dinosaurios divergieron de sus ancestros arcosaurios durante las épocas del Triásico medio y tardío, aproximadamente 20 millones de años después del devastador evento de extinción del Pérmico-Triásico que acabó con un estimado del 96% de todas las especies marinas y el 70% de las especies de vertebrados terrestres hace aproximadamente 252 millones de años. [103] [104] Los fósiles de dinosaurios más antiguos conocidos a partir de restos sustanciales datan de la época Carniana del período Triásico y se han encontrado principalmente en las Formaciones Ischigualasto y Santa María de Argentina y Brasil, y la Formación Pebbly Arkose de Zimbabue. [105]
La Formación Ischigualasto ( datada radiométricamente en 231-230 millones de años [106] ) ha producido el saurisquio temprano Eoraptor , originalmente considerado un miembro de Herrerasauridae [107] pero ahora considerado un sauropodomorfo temprano, junto con los herrerasáuridos Herrerasaurus y Sanjuansaurus , y los sauropodomorfos Chromogisaurus , Eodromaeus y Panphagia . [108] El probable parecido de Eoraptor con el ancestro común de todos los dinosaurios sugiere que los primeros dinosaurios habrían sido pequeños depredadores bípedos . [109] [110] [111] La Formación Santa María (datada radiométricamente como más antigua, con 233,23 millones de años [112] ) ha producido los herrerasáuridos Gnathovorax y Staurikosaurus , junto con los sauropodomorfos Bagualosaurus , Buriolestes , Guaibasaurus , Macrocollum , Nhandumirim , Pampadromaeus , Saturnalia y Unaysaurus . [108] La Formación Pebbly Arkose, que es de edad incierta pero probablemente comparable a las otras dos, ha producido el sauropodomorfo Mbiresaurus , junto con un herrerasáurido sin nombre. [105]
Restos menos bien conservados de los sauropodomorfos Jaklapallisaurus y Nambalia , junto con el saurisquio temprano Alwalkeria , se conocen de las Formaciones Maleri Superior e Inferior de la India. [113] La Formación Chañares de Argentina, de la edad Carniense , preserva ornitodiranos primitivos, parecidos a los dinosaurios, como Lagosuchus y Lagerpeton en Argentina , lo que la convierte en otro sitio importante para comprender la evolución de los dinosaurios. Estos ornitodiranos respaldan el modelo de los primeros dinosaurios como depredadores pequeños y bípedos. [108] [114] Los dinosaurios pueden haber aparecido tan temprano como la época Anisiana del Triásico, hace aproximadamente 243 millones de años, que es la edad de Nyasasaurus de la Formación Manda de Tanzania. Sin embargo, sus fósiles conocidos son demasiado fragmentarios para identificarlo como un dinosaurio o solo un pariente cercano. [115] La referencia de la Formación Manda al Anisiano también es incierta. De todos modos, los dinosaurios coexistieron con ornitodiranos no dinosaurios durante un período de tiempo, con estimaciones que van desde 5 a 10 millones de años [116] hasta 21 millones de años. [112]
Cuando aparecieron los dinosaurios, no eran los animales terrestres dominantes. Los hábitats terrestres estaban ocupados por varios tipos de arcosauromorfos y terápsidos , como los cinodontes y los rincosaurios . Sus principales competidores eran los pseudosuquios , como los aetosaurios , los ornitosúquidos y los rauisuquios, que tuvieron más éxito que los dinosaurios. [117] La mayoría de estos otros animales se extinguieron en el Triásico, en uno de dos eventos. Primero, hace unos 215 millones de años, se extinguieron una variedad de arcosaurios basales , incluidos los protorosaurios . Esto fue seguido por el evento de extinción Triásico-Jurásico (hace unos 201 millones de años), que vio el final de la mayoría de los otros grupos de arcosaurios tempranos, como los aetosaurios, los ornitosúquidos, los fitosaurios y los rauisuquios. Los rincosaurios y los dicinodontes sobrevivieron (al menos en algunas áreas) al menos hasta principios y mediados del Noriense y finales del Noriense o principios del Rético , respectivamente, [118] [119] y la fecha exacta de su extinción es incierta. Estas pérdidas dejaron atrás una fauna terrestre de crocodilomorfos , dinosaurios, mamíferos, pterosaurios y tortugas . [10] Las primeras líneas de dinosaurios primitivos se diversificaron a través de las etapas Carniense y Noriana del Triásico, posiblemente ocupando los nichos de los grupos que se extinguieron. [12] También es notable que haya una mayor tasa de extinción durante el evento pluvial Carniense . [120]
Evolución y paleobiogeografía
El supercontinente Pangea a principios del Mesozoico (hace unos 200 millones de años)
La evolución de los dinosaurios después del Triásico siguió los cambios en la vegetación y la ubicación de los continentes. En el Triásico Tardío y el Jurásico Temprano, los continentes estaban conectados como la única masa terrestre Pangea , y había una fauna de dinosaurios mundial compuesta principalmente por carnívoros celofisoides y herbívoros sauropodomorfos tempranos. [121] Las plantas gimnospermas (particularmente las coníferas ), una fuente potencial de alimento, irradiaron en el Triásico Tardío. Los primeros sauropodomorfos no tenían mecanismos sofisticados para procesar los alimentos en la boca, por lo que deben haber empleado otros medios para descomponer los alimentos más adelante en el tracto digestivo. [122] La homogeneidad general de las faunas de dinosaurios continuó en el Jurásico Medio y Tardío, donde la mayoría de las localidades tenían depredadores que consistían en ceratosaurios , megalosauroides y alosauroides , y herbívoros que consistían en ornitisquios estegosaurios y grandes saurópodos. Ejemplos de esto incluyen la Formación Morrison de América del Norte y los lechos Tendaguru de Tanzania. Los dinosaurios en China muestran algunas diferencias, con terópodos metriacantosáuridos especializados y saurópodos inusuales de cuello largo como Mamenchisaurus . [121] Los anquilosaurios y los ornitópodos también se estaban volviendo más comunes, pero los sauropodomorfos primitivos se habían extinguido. Las coníferas y los pteridofitos eran las plantas más comunes. Los saurópodos, como los sauropodomorfos anteriores, no eran procesadores orales, pero los ornitisquios estaban desarrollando varios medios para tratar la comida en la boca, incluidos posibles órganos similares a las mejillas para mantener la comida en la boca y movimientos de la mandíbula para moler la comida. [122] Otro evento evolutivo notable del Jurásico fue la aparición de las aves verdaderas, descendientes de los celurosaurios maniraptoros. [14]
En el Cretácico Inferior y la desintegración de Pangea, los dinosaurios se diferenciaban claramente por su masa continental. En la primera parte de este período se produjo la expansión de los anquilosaurios, los iguanodontes y los braquiosáuridos por Europa, América del Norte y el norte de África . Más tarde, estos se complementaron o reemplazaron en África con grandes terópodos espinosáuridos y carcarodontosáuridos , y saurópodos rebaquisáuridos y titanosaurios , que también se encuentran en América del Sur . En Asia , los celurosaurios maniraptoros como los dromeosáuridos, los troodóntidos y los oviraptorosaurios se convirtieron en los terópodos comunes, y los anquilosáuridos y los primeros ceratopsianos como Psittacosaurus se convirtieron en importantes herbívoros. Mientras tanto, Australia albergaba una fauna de anquilosaurios basales, hipsilofodontes e iguanodontes. [121] Los estegosaurios parecen haberse extinguido en algún momento a finales del Cretácico Inferior o principios del Cretácico Superior . Un cambio importante en el Cretácico Inferior, que se amplificaría en el Cretácico Superior, fue la evolución de las plantas con flores . Al mismo tiempo, varios grupos de herbívoros dinosaurios desarrollaron formas más sofisticadas de procesar los alimentos por vía oral. Los ceratopsianos desarrollaron un método de corte con dientes apilados uno sobre otro en baterías, y los iguanodontes refinaron un método de trituración con baterías dentales , llevado al extremo en los hadrosáuridos. [122] Algunos saurópodos también desarrollaron baterías dentales, mejor ejemplificadas por el rebbachisáurido Nigersaurus . [123]
En el Cretácico Superior existían tres faunas generales de dinosaurios. En los continentes del norte de América del Norte y Asia, los principales terópodos eran los tiranosáuridos y varios tipos de terópodos maniraptores más pequeños, con un conjunto de herbívoros predominantemente ornitisquios de hadrosáuridos, ceratopsios, anquilosáuridos y paquicefalosaurios. En los continentes del sur que habían formado el supercontinente Gondwana , ahora en división, los abelisáuridos eran los terópodos comunes y los saurópodos titanosaurios los herbívoros comunes. Finalmente, en Europa, prevalecían los dromeosáuridos, los iguanodontes rabdodóntidos , los anquilosáuridos nodosáuridos y los saurópodos titanosaurios. [121] Las plantas con flores se extendían en gran medida, [122] y las primeras gramíneas aparecieron a finales del Cretácico. [124] Los hadrosáuridos trituradores y los ceratopsianos esquiladores se volvieron muy diversos en América del Norte y Asia. Los terópodos también se expandieron como herbívoros u omnívoros , y los terizinosaurios y los ornitomimosaurios se volvieron comunes. [122]
La extinción masiva del Cretácico-Paleógeno, que ocurrió hace aproximadamente 66 millones de años al final del Cretácico, provocó la extinción de todos los grupos de dinosaurios, excepto las aves neornitinas. Algunos otros grupos de diápsidos, incluidos los crocodilianos , los dirosaurios , los sebecosuquios , las tortugas, los lagartos , las serpientes , los esfenodontes y los coristoderos , también sobrevivieron al evento. [125]
Los linajes supervivientes de aves neornitinas, incluidos los ancestros de las modernas ratites , patos y pollos , y una variedad de aves acuáticas , se diversificaron rápidamente al comienzo del período Paleógeno , entrando en nichos ecológicos que quedaron vacantes por la extinción de los grupos de dinosaurios mesozoicos, como los enantiornitinos arbóreos, los hesperornitinos acuáticos e incluso los terópodos terrestres más grandes (en forma de Gastornis , eogruiidos , batornítidos , ratites, geranoididos , mihirungs y " pájaros del terror "). A menudo se afirma que los mamíferos superaron a los neornitinos en la competencia por el dominio de la mayoría de los nichos terrestres, pero muchos de estos grupos coexistieron con ricas faunas de mamíferos durante la mayor parte de la Era Cenozoica . [126] Las aves del terror y los batornítidos ocuparon gremios carnívoros junto con los mamíferos depredadores, [127] [128] y las ratites todavía tienen bastante éxito como herbívoros de tamaño mediano; los eogruidos de manera similar duraron desde el Eoceno hasta el Plioceno , extinguiéndose solo muy recientemente después de más de 20 millones de años de coexistencia con muchos grupos de mamíferos. [129]
Clasificación
Los dinosaurios pertenecen a un grupo conocido como arcosaurios, que también incluye a los cocodrilos modernos. Dentro del grupo de los arcosaurios, los dinosaurios se diferencian principalmente por su forma de andar. Las patas de los dinosaurios se extienden directamente debajo del cuerpo, mientras que las patas de los lagartos y los cocodrilos se extienden hacia ambos lados. [29]
En conjunto, los dinosaurios como clado se dividen en dos ramas principales, Saurischia y Ornithischia. Saurischia incluye aquellos taxones que comparten un ancestro común más reciente con las aves que con Ornithischia, mientras que Ornithischia incluye todos los taxones que comparten un ancestro común más reciente con Triceratops que con Saurischia. Anatómicamente, estos dos grupos se pueden distinguir más notablemente por su estructura pélvica . Los primeros saurisquios —«cadera de lagarto», del griego sauros ( σαῦρος ) que significa «lagarto» e isquion ( ἰσχίον ) que significa «articulación de la cadera»— conservaron la estructura de la cadera de sus ancestros, con un hueso púbico dirigido cranealmente , o hacia adelante. [36] Esta forma básica fue modificada al rotar el pubis hacia atrás en diversos grados en varios grupos ( Herrerasaurus , [130] terizinosáuridos, [131] dromeosáuridos, [132] y aves [14] ). Saurischia incluye a los terópodos (exclusivamente bípedos y con una amplia variedad de dietas) y a los sauropodomorfos (herbívoros de cuello largo que incluyen grupos avanzados, cuadrúpedos). [28] [133]
Por el contrario, los ornitisquios (del griego ornitheios (ὀρνίθειος) que significa "de un pájaro" e ischion (ἰσχίον) que significa "articulación de la cadera") tenían una pelvis que superficialmente se parecía a la de un pájaro: el hueso púbico estaba orientado caudalmente (apuntando hacia atrás). A diferencia de las aves, el pubis de los ornitisquios también solía tener un proceso adicional que apuntaba hacia adelante. Los ornitisquios incluyen una variedad de especies que eran principalmente herbívoras.
A pesar de los términos "cadera de ave" (Ornithischia) y "cadera de lagarto" (Saurischia), las aves no forman parte de Ornithischia. Las aves pertenecen a Saurischia, los dinosaurios "con cadera de lagarto" (las aves evolucionaron a partir de dinosaurios anteriores con "caderas de lagarto"). [29]
Taxonomía
La siguiente es una clasificación simplificada de los grupos de dinosaurios basada en sus relaciones evolutivas y las de los principales grupos de dinosaurios Theropoda, Sauropodomorpha y Ornithischia, compilada por Justin Tweet. [134] Se pueden encontrar más detalles y otras hipótesis de clasificación en artículos individuales.
† Ceratopsidae (ceratopsianos grandes y elaboradamente ornamentados)
† Chasmosaurinae (ceratópsidos con cuernos frontales agrandados)
† Centrosaurinae (ceratópsidos caracterizados principalmente por volante y ornamentación nasal)
† Nasutoceratopsini (centrosaurios con cavidades nasales agrandadas)
† Centrosaurini (centrosaurinos con cuernos nasales agrandados)
† Pachyrhinosaurini (en su mayoría tenían protuberancias nasales en lugar de cuernos)
† Ornitópodos (varios tamaños; bípedos y cuadrúpedos; desarrollaron un método de masticación utilizando la flexibilidad del cráneo y numerosos dientes)
† Ceratosauria (carnívoros generalmente con cuernos elaborados que existieron desde el período Jurásico al Cretácico, originalmente incluían Coelophysoidea)
Cronología de los principales grupos de dinosaurios según Holtz (2007).
Paleobiología
Knowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin, internal organs and other soft tissues.[89][93] Many fields of study contribute to our understanding of dinosaurs, including physics (especially biomechanics), chemistry, biology, and the Earth sciences (of which paleontology is a sub-discipline).[135][136] Two topics of particular interest and study have been dinosaur size and behavior.[137]
Size
Scale diagram comparing the average human to the longest known dinosaurs in five major clades:
Current evidence suggests that dinosaur average size varied through the Triassic, Early Jurassic, Late Jurassic and Cretaceous.[110] Predatory theropod dinosaurs, which occupied most terrestrial carnivore niches during the Mesozoic, most often fall into the 100-to-1,000 kg (220-to-2,200 lb) category when sorted by estimated weight into categories based on order of magnitude, whereas recent predatory carnivoran mammals peak in the 10-to-100 kg (22-to-220 lb) category.[138] The mode of Mesozoic dinosaur body masses is between 1 and 10 metric tons (1.1 and 11.0 short tons).[139] This contrasts sharply with the average size of Cenozoic mammals, estimated by the National Museum of Natural History as about 2 to 5 kg (4.4 to 11.0 lb).[140]
The sauropods were the largest and heaviest dinosaurs. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat, and the largest was an order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric mammals such as Paraceratherium (the largest land mammal ever) were dwarfed by the giant sauropods, and only modern whales approach or surpass them in size.[141] There are several proposed advantages for the large size of sauropods, including protection from predation, reduction of energy use, and longevity, but it may be that the most important advantage was dietary. Large animals are more efficient at digestion than small animals, because food spends more time in their digestive systems. This also permits them to subsist on food with lower nutritive value than smaller animals. Sauropod remains are mostly found in rock formations interpreted as dry or seasonally dry, and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments.[142]
Largest and smallest
Scientists will probably never be certain of the largest and smallest dinosaurs to have ever existed. This is because only a tiny percentage of animals were ever fossilized and most of these remain buried in the earth. Few non-avian dinosaur specimens that are recovered are complete skeletons, and impressions of skin and other soft tissues are rare. Rebuilding a complete skeleton by comparing the size and morphology of bones to those of similar, better-known species is an inexact art, and reconstructing the muscles and other organs of the living animal is, at best, a process of educated guesswork.[143]
The tallest and heaviest dinosaur known from good skeletons is Giraffatitan brancai (previously classified as a species of Brachiosaurus). Its remains were discovered in Tanzania between 1907 and 1912. Bones from several similar-sized individuals were incorporated into the skeleton now mounted and on display at the Museum für Naturkunde in Berlin;[144] this mount is 12 meters (39 ft) tall and 21.8 to 22.5 meters (72 to 74 ft) long,[145][146] and would have belonged to an animal that weighed between 30000 and 60000 kilograms (70000 and 130000 lb). The longest complete dinosaur is the 27 meters (89 ft) long Diplodocus, which was discovered in Wyoming in the United States and displayed in Pittsburgh's Carnegie Museum of Natural History in 1907.[147] The longest dinosaur known from good fossil material is Patagotitan: the skeleton mount in the American Museum of Natural History in New York is 37 meters (121 ft) long. The Museo Municipal Carmen Funes in Plaza Huincul, Argentina, has an Argentinosaurus reconstructed skeleton mount that is 39.7 meters (130 ft) long.[148]
Maraapunisaurus, one of the largest animals to walk the earth.Bruhathkayosaurus, potentially the largest terrestrial animal to ever exist.
There were larger dinosaurs, but knowledge of them is based entirely on a small number of fragmentary fossils. Most of the largest herbivorous specimens on record were discovered in the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80000 to 100000 kilograms (88 to 110 short tons) and reached lengths of 30 to 40 meters (98 to 131 ft); some of the longest were the 33.5-meter (110 ft) long Diplodocus hallorum[142] (formerly Seismosaurus), the 33-to-34-meter (108 to 112 ft) long Supersaurus,[149] and 37-meter (121 ft) long Patagotitan; and the tallest, the 18-meter (59 ft) tall Sauroposeidon, which could have reached a sixth-floor window. There were a few dinosaurs that was considered either the heaviest and longest. The most famous one include Amphicoelias fragillimus, known only from a now lost partial vertebral neural arch described in 1878. Extrapolating from the illustration of this bone, the animal may have been 58 meters (190 ft) long and weighed 122400 kg (269800 lb).[142] However, recent research have placed Amphicoelias from the long, gracile diplodocid to the shorter but much stockier rebbachisaurid. Now renamed as Maraapunisaurus, this sauropod now stands as much as 40 meters (130 ft) long and weigh as much as 120000 kg (260000 lb).[150][151] Another contender of this title includes Bruhathkayosaurus, a controversial taxon that was recently confirmed to exist after archived photos were uncovered.[152]Bruhathkayosaurus was a titanosaur and would have most likely weighed more than even Marrapunisaurus. Recent size estimates in 2023 have placed this sauropod reaching lengths of up to 44 m (144 ft) long and a colossal weight range of around 110000–170000 kg (240000–370000 lb), if these upper estimates up true, Bruhathkayosaurus would have rivaled the blue whale and Perucetus colossus as one of the largest animals to have ever existed.[153]
The largest carnivorous dinosaur was Spinosaurus, reaching a length of 12.6 to 18 meters (41 to 59 ft) and weighing 7 to 20.9 metric tons (7.7 to 23.0 short tons).[154][155] Other large carnivorous theropods included Giganotosaurus, Carcharodontosaurus, and Tyrannosaurus.[155]Therizinosaurus and Deinocheirus were among the tallest of the theropods. The largest ornithischian dinosaur was probably the hadrosaurid Shantungosaurus giganteus which measured 16.6 meters (54 ft).[156] The largest individuals may have weighed as much as 16 metric tons (18 short tons).[157]
The smallest dinosaur known is the bee hummingbird,[158] with a length of only 5 centimeters (2.0 in) and mass of around 1.8 g (0.063 oz).[159] The smallest known non-avialan dinosaurs were about the size of pigeons and were those theropods most closely related to birds.[160] For example, Anchiornis huxleyi is currently the smallest non-avialan dinosaur described from an adult specimen, with an estimated weight of 110 g (3.9 oz)[161] and a total skeletal length of 34 centimeters (1.12 ft).[160][161] The smallest herbivorous non-avialan dinosaurs included Microceratus and Wannanosaurus, at about 60 centimeters (2.0 ft) long each.[162][163]
Many modern birds are highly social, often found living in flocks. There is general agreement that some behaviors that are common in birds, as well as in crocodilians (closest living relatives of birds), were also common among extinct dinosaur groups. Interpretations of behavior in fossil species are generally based on the pose of skeletons and their habitat, computer simulations of their biomechanics, and comparisons with modern animals in similar ecological niches.[135]
The first potential evidence for herding or flocking as a widespread behavior common to many dinosaur groups in addition to birds was the 1878 discovery of 31 Iguanodon, ornithischians that were then thought to have perished together in Bernissart, Belgium, after they fell into a deep, flooded sinkhole and drowned.[164] Other mass-death sites have been discovered subsequently. Those, along with multiple trackways, suggest that gregarious behavior was common in many early dinosaur species. Trackways of hundreds or even thousands of herbivores indicate that duck-billed (hadrosaurids) may have moved in great herds, like the American bison or the African springbok. Sauropod tracks document that these animals traveled in groups composed of several different species, at least in Oxfordshire, England,[165] although there is no evidence for specific herd structures.[166] Congregating into herds may have evolved for defense, for migratory purposes, or to provide protection for young. There is evidence that many types of slow-growing dinosaurs, including various theropods, sauropods, ankylosaurians, ornithopods, and ceratopsians, formed aggregations of immature individuals. One example is a site in Inner Mongolia that has yielded remains of over 20 Sinornithomimus, from one to seven years old. This assemblage is interpreted as a social group that was trapped in mud.[167] The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as pack hunters working together to bring down large prey.[168][169] However, this lifestyle is uncommon among modern birds, crocodiles, and other reptiles, and the taphonomic evidence suggesting mammal-like pack hunting in such theropods as Deinonychus and Allosaurus can also be interpreted as the results of fatal disputes between feeding animals, as is seen in many modern diapsid predators.[170]
The crests and frills of some dinosaurs, like the marginocephalians, theropods and lambeosaurines, may have been too fragile to be used for active defense, and so they were likely used for sexual or aggressive displays, though little is known about dinosaur mating and territorialism. Head wounds from bites suggest that theropods, at least, engaged in active aggressive confrontations.[171]
From a behavioral standpoint, one of the most valuable dinosaur fossils was discovered in the Gobi Desert in 1971. It included a Velociraptor attacking a Protoceratops,[172] providing evidence that dinosaurs did indeed attack each other.[173] Additional evidence for attacking live prey is the partially healed tail of an Edmontosaurus, a hadrosaurid dinosaur; the tail is damaged in such a way that shows the animal was bitten by a tyrannosaur but survived.[173]Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod Majungasaurus.[174]
Comparisons between the scleral rings of dinosaurs and modern birds and reptiles have been used to infer daily activity patterns of dinosaurs. Although it has been suggested that most dinosaurs were active during the day, these comparisons have shown that small predatory dinosaurs such as dromaeosaurids, Juravenator, and Megapnosaurus were likely nocturnal. Large and medium-sized herbivorous and omnivorous dinosaurs such as ceratopsians, sauropodomorphs, hadrosaurids, ornithomimosaurs may have been cathemeral, active during short intervals throughout the day, although the small ornithischian Agilisaurus was inferred to be diurnal.[175]
Based on fossil evidence from dinosaurs such as Oryctodromeus, some ornithischian species seem to have led a partially fossorial (burrowing) lifestyle.[176] Many modern birds are arboreal (tree climbing), and this was also true of many Mesozoic birds, especially the enantiornithines.[177] While some early bird-like species may have already been arboreal as well (including dromaeosaurids) such as Microraptor[178]) most non-avialan dinosaurs seem to have relied on land-based locomotion. A good understanding of how dinosaurs moved on the ground is key to models of dinosaur behavior; the science of biomechanics, pioneered by Robert McNeill Alexander, has provided significant insight in this area. For example, studies of the forces exerted by muscles and gravity on dinosaurs' skeletal structure have investigated how fast dinosaurs could run,[135] whether diplodocids could create sonic booms via whip-like tail snapping,[179] and whether sauropods could float.[180]
Communication
Modern birds communicate by visual and auditory signals, and the wide diversity of visual display structures among fossil dinosaur groups, such as horns, frills, crests, sails, and feathers, suggests that visual communication has always been important in dinosaur biology.[181] Reconstruction of the plumage color of Anchiornis suggest the importance of color in visual communication in non-avian dinosaurs.[182] Vocalization in non-avian dinosaurs is less certain. In birds, the larynx plays no role in sound production. Instead, birds vocalize with a novel organ, the syrinx, farther down the trachea.[183] The earliest remains of a syrinx were found in a specimen of the duck-like Vegavis iaai dated 69 –66 million years ago, and this organ is unlikely to have existed in non-avian dinosaurs.[184]
Restoration of a striking and unusual visual display in a Lambeosaurus magnicristatus. The crest may also have acted as a resonating chamber for sounds.
On the basis that non-avian dinosaurs did not have syrinxes and that their next close living relatives, crocodilians, use the larynx, Phil Senter, a paleontologist, has suggested that the non-avians could not vocalize, because the common ancestor would have been mute. He states that they mostly on visual displays and possibly non-vocal sounds, such as hissing, jaw-grinding or -clapping, splashing, and wing-beating (possible in winged maniraptoran dinosaurs).[181] Other researchers have countered that vocalizations also exist in turtles, the closest relatives of archosaurs, suggesting that the trait is ancestral to their lineage. In addition, vocal communication in dinosaurs is indicated by the development of advanced hearing in nearly all major groups. Hence the syrinx may have supplemented and then replaced the larynx as a vocal organ, without a "silent period" in bird evolution.[185]
In 2023, a fossilized larynx was described, from a specimen of the ankylosaurid Pinacosaurus. The structure was composed of cricoid and arytenoid cartilages, similar to those of non-avian reptiles; but the mobile cricoid–arytenoid joint and long arytenoid cartilages would have allowed air-flow control similar to that of birds, and thus could have made bird-like vocalizations. In addition, the cartilages were ossified, implying that laryngeal ossification is a feature of some non-avian dinosaurs.[186] A 2016 study concludes that some dinosaurs may have produced closed-mouth vocalizations, such as cooing, hooting, and booming. These occur in both reptiles and birds and involve inflating the esophagus or tracheal pouches. Such vocalizations evolved independently in extant archosaurs numerous times, following increases in body size.[187] The crests of some hadrosaurids and the nasal chambers of ankylosaurids may have been resonators.[188][189]
All dinosaurs laid amniotic eggs. Dinosaur eggs were usually laid in a nest. Most species create somewhat elaborate nests which can be cups, domes, plates, beds scrapes, mounds, or burrows.[190] Some species of modern bird have no nests; the cliff-nesting common guillemot lays its eggs on bare rock, and male emperor penguins keep eggs between their body and feet. Primitive birds and many non-avialan dinosaurs often lay eggs in communal nests, with males primarily incubating the eggs. While modern birds have only one functional oviduct and lay one egg at a time, more primitive birds and dinosaurs had two oviducts, like crocodiles. Some non-avialan dinosaurs, such as Troodon, exhibited iterative laying, where the adult might lay a pair of eggs every one or two days, and then ensured simultaneous hatching by delaying brooding until all eggs were laid.[191]
When laying eggs, females grow a special type of bone between the hard outer bone and the marrow of their limbs. This medullary bone, which is rich in calcium, is used to make eggshells. A discovery of features in a Tyrannosaurus skeleton provided evidence of medullary bone in extinct dinosaurs and, for the first time, allowed paleontologists to establish the sex of a fossil dinosaur specimen. Further research has found medullary bone in the carnosaur Allosaurus and the ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that the production of medullary tissue is a general characteristic of all dinosaurs.[192]
Another widespread trait among modern birds (but see below in regards to fossil groups and extant megapodes) is parental care for young after hatching. Jack Horner's 1978 discovery of a Maiasaura ("good mother lizard") nesting ground in Montana demonstrated that parental care continued long after birth among ornithopods.[193] A specimen of the oviraptoridCitipati osmolskae was discovered in a chicken-like brooding position in 1993,[194] which may indicate that they had begun using an insulating layer of feathers to keep the eggs warm.[195] An embryo of the basal sauropodomorph Massospondylus was found without teeth, indicating that some parental care was required to feed the young dinosaurs.[196] Trackways have also confirmed parental behavior among ornithopods from the Isle of Skye in northwestern Scotland.[197]
However, there is ample evidence of precociality or superprecociality among many dinosaur species, particularly theropods. For instance, non-ornithuromorph birds have been abundantly demonstrated to have had slow growth rates, megapode-like egg burying behavior and the ability to fly soon after birth.[198][199][200][201] Both Tyrannosaurus and Troodon had juveniles with clear superprecociality and likely occupying different ecological niches than the adults.[191] Superprecociality has been inferred for sauropods.[202]
Genital structures are unlikely to fossilize as they lack scales that may allow preservation via pigmentation or residual calcium phosphate salts. In 2021, the best preserved specimen of a dinosaur's cloacal vent exterior was described for Psittacosaurus, demonstrating lateral swellings similar to crocodylian musk glands used in social displays by both sexes and pigmented regions which could also reflect a signalling function. However, this specimen on its own does not offer enough information to determine whether this dinosaur had sexual signalling functions; it only supports the possibility. Cloacal visual signalling can occur in either males or females in living birds, making it unlikely to be useful to determine sex for extinct dinosaurs.[203]
Physiology
Because both modern crocodilians and birds have four-chambered hearts (albeit modified in crocodilians), it is likely that this is a trait shared by all archosaurs, including all dinosaurs.[204] While all modern birds have high metabolisms and are endothermic ("warm-blooded"), a vigorous debate has been ongoing since the 1960s regarding how far back in the dinosaur lineage this trait extended. Various researchers have supported dinosaurs as being endothermic, ectothermic ("cold-blooded"), or somewhere in between.[205] An emerging consensus among researchers is that, while different lineages of dinosaurs would have had different metabolisms, most of them had higher metabolic rates than other reptiles but lower than living birds and mammals,[206] which is termed mesothermy by some.[207] Evidence from crocodiles and their extinct relatives suggests that such elevated metabolisms could have developed in the earliest archosaurs, which were the common ancestors of dinosaurs and crocodiles.[208][209]
This 1897 restoration of Brontosaurus as an aquatic, tail-dragging animal, by Charles R. Knight, typified early views on dinosaur lifestyles.
After non-avian dinosaurs were discovered, paleontologists first posited that they were ectothermic. This was used to imply that the ancient dinosaurs were relatively slow, sluggish organisms, even though many modern reptiles are fast and light-footed despite relying on external sources of heat to regulate their body temperature. The idea of dinosaurs as ectothermic remained a prevalent view until Robert T. Bakker, an early proponent of dinosaur endothermy, published an influential paper on the topic in 1968. Bakker specifically used anatomical and ecological evidence to argue that sauropods, which had hitherto been depicted as sprawling aquatic animals with their tails dragging on the ground, were endotherms that lived vigorous, terrestrial lives. In 1972, Bakker expanded on his arguments based on energy requirements and predator-prey ratios. This was one of the seminal results that led to the dinosaur renaissance.[62][63][59][210]
One of the greatest contributions to the modern understanding of dinosaur physiology has been paleohistology, the study of microscopic tissue structure in dinosaurs.[211][212] From the 1960s forward, Armand de Ricqlès suggested that the presence of fibrolamellar bone—bony tissue with an irregular, fibrous texture and filled with blood vessels—was indicative of consistently fast growth and therefore endothermy. Fibrolamellar bone was common in both dinosaurs and pterosaurs,[213][214] though not universally present.[215][216] This has led to a significant body of work in reconstructing growth curves and modeling the evolution of growth rates across various dinosaur lineages,[217] which has suggested overall that dinosaurs grew faster than living reptiles.[212] Other lines of evidence suggesting endothermy include the presence of feathers and other types of body coverings in many lineages (see § Feathers); more consistent ratios of the isotope oxygen-18 in bony tissue compared to ectotherms, particularly as latitude and thus air temperature varied, which suggests stable internal temperatures[218][219] (although these ratios can be altered during fossilization[220]); and the discovery of polar dinosaurs, which lived in Australia, Antarctica, and Alaska when these places would have had cool, temperate climates.[221][222][223][224]
In saurischian dinosaurs, higher metabolisms were supported by the evolution of the avian respiratory system, characterized by an extensive system of air sacs that extended the lungs and invaded many of the bones in the skeleton, making them hollow.[225] Such respiratory systems, which may have appeared in the earliest saurischians,[226] would have provided them with more oxygen compared to a mammal of similar size, while also having a larger resting tidal volume and requiring a lower breathing frequency, which would have allowed them to sustain higher activity levels.[141] The rapid airflow would also have been an effective cooling mechanism, which in conjunction with a lower metabolic rate[227] would have prevented large sauropods from overheating. These traits may have enabled sauropods to grow quickly to gigantic sizes.[228][229] Sauropods may also have benefitted from their size—their small surface area to volume ratio meant that they would have been able to thermoregulate more easily, a phenomenon termed gigantothermy.[141][230]
Like other reptiles, dinosaurs are primarily uricotelic, that is, their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acid instead of urea or ammonia via the ureters into the intestine. This would have helped them to conserve water.[206] In most living species, uric acid is excreted along with feces as a semisolid waste.[231][232] However, at least some modern birds (such as hummingbirds) can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.[233] This material, as well as the output of the intestines, emerges from the cloaca.[234][235] In addition, many species regurgitate pellets,[236] and fossil pellets are known as early as the Jurassic from Anchiornis.[237]
The size and shape of the brain can be partly reconstructed based on the surrounding bones. In 1896, Marsh calculated ratios between brain weight and body weight of seven species of dinosaurs, showing that the brain of dinosaurs was proportionally smaller than in today's crocodiles, and that the brain of Stegosaurus was smaller than in any living land vertebrate. This contributed to the widespread public notion of dinosaurs as being sluggish and extraordinarily stupid. Harry Jerison, in 1973, showed that proportionally smaller brains are expected at larger body sizes, and that brain size in dinosaurs was not smaller than expected when compared to living reptiles.[238] Later research showed that relative brain size progressively increased during the evolution of theropods, with the highest intelligence – comparable to that of modern birds – calculated for the troodontid Troodon.[239]
Origin of birds
The possibility that dinosaurs were the ancestors of birds was first suggested in 1868 by Thomas Henry Huxley.[240] After the work of Gerhard Heilmann in the early 20th century, the theory of birds as dinosaur descendants was abandoned in favor of the idea of them being descendants of generalized thecodonts, with the key piece of evidence being the supposed lack of clavicles in dinosaurs.[241] However, as later discoveries showed, clavicles (or a single fused wishbone, which derived from separate clavicles) were not actually absent;[14] they had been found as early as 1924 in Oviraptor, but misidentified as an interclavicle.[242] In the 1970s, Ostrom revived the dinosaur–bird theory,[243] which gained momentum in the coming decades with the advent of cladistic analysis,[244] and a great increase in the discovery of small theropods and early birds.[31] Of particular note have been the fossils of the Jehol Biota, where a variety of theropods and early birds have been found, often with feathers of some type.[69][14] Birds share over a hundred distinct anatomical features with theropod dinosaurs, which are now generally accepted to have been their closest ancient relatives.[245] They are most closely allied with maniraptoran coelurosaurs.[14] A minority of scientists, most notably Alan Feduccia and Larry Martin, have proposed other evolutionary paths, including revised versions of Heilmann's basal archosaur proposal,[246] or that maniraptoran theropods are the ancestors of birds but themselves are not dinosaurs, only convergent with dinosaurs.[247]
Feathers are one of the most recognizable characteristics of modern birds, and a trait that was also shared by several non-avian dinosaurs. Based on the current distribution of fossil evidence, it appears that feathers were an ancestral dinosaurian trait, though one that may have been selectively lost in some species.[248] Direct fossil evidence of feathers or feather-like structures has been discovered in a diverse array of species in many non-avian dinosaur groups,[69] both among saurischians and ornithischians. Simple, branched, feather-like structures are known from heterodontosaurids, primitive neornithischians,[249] and theropods,[250] and primitive ceratopsians. Evidence for true, vaned feathers similar to the flight feathers of modern birds has been found only in the theropod subgroup Maniraptora, which includes oviraptorosaurs, troodontids, dromaeosaurids, and birds.[14][251] Feather-like structures known as pycnofibres have also been found in pterosaurs.[252]
However, researchers do not agree regarding whether these structures share a common origin between lineages (i.e., they are homologous),[253][254] or if they were the result of widespread experimentation with skin coverings among ornithodirans.[255] If the former is the case, filaments may have been common in the ornithodiran lineage and evolved before the appearance of dinosaurs themselves.[248] Research into the genetics of American alligators has revealed that crocodylian scutes do possess feather-keratins during embryonic development, but these keratins are not expressed by the animals before hatching.[256] The description of feathered dinosaurs has not been without controversy in general; perhaps the most vocal critics have been Alan Feduccia and Theagarten Lingham-Soliar, who have proposed that some purported feather-like fossils are the result of the decomposition of collagenous fiber that underlaid the dinosaurs' skin,[257][258][259] and that maniraptoran dinosaurs with vaned feathers were not actually dinosaurs, but convergent with dinosaurs.[247][258] However, their views have for the most part not been accepted by other researchers, to the point that the scientific nature of Feduccia's proposals has been questioned.[260]
Archaeopteryx was the first fossil found that revealed a potential connection between dinosaurs and birds. It is considered a transitional fossil, in that it displays features of both groups. Brought to light just two years after Charles Darwin's seminal On the Origin of Species (1859), its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for the small theropod Compsognathus.[261] Since the 1990s, a number of additional feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. Many of these specimens were unearthed in the lagerstätten of the Jehol Biota.[254] If feather-like structures were indeed widely present among non-avian dinosaurs, the lack of abundant fossil evidence for them may be due to the fact that delicate features like skin and feathers are seldom preserved by fossilization and thus often absent from the fossil record.[262]
Skeleton
Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent another important line of evidence for paleontologists. Areas of the skeleton with important similarities include the neck, pubis, wrist (semi-lunate carpal), arm and pectoral girdle, furcula (wishbone), and breast bone. Comparison of bird and dinosaur skeletons through cladistic analysis strengthens the case for the link.[263]
Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to a 2005 investigation led by Patrick M. O'Connor. The lungs of theropod dinosaurs (carnivores that walked on two legs and had bird-like feet) likely pumped air into hollow sacs in their skeletons, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said.[264][265] In 2008, scientists described Aerosteon riocoloradensis, the skeleton of which supplies the strongest evidence to date of a dinosaur with a bird-like breathing system. CT scanning of Aerosteon's fossil bones revealed evidence for the existence of air sacs within the animal's body cavity.[225][266]
Behavioral evidence
Fossils of the troodonts Mei and Sinornithoides demonstrate that some dinosaurs slept with their heads tucked under their arms.[267] This behavior, which may have helped to keep the head warm, is also characteristic of modern birds. Several deinonychosaur and oviraptorosaur specimens have also been found preserved on top of their nests, likely brooding in a bird-like manner.[268] The ratio between egg volume and body mass of adults among these dinosaurs suggest that the eggs were primarily brooded by the male and that the young were highly precocial, similar to many modern ground-dwelling birds.[269]
Some dinosaurs are known to have used gizzard stones like modern birds. These stones are swallowed by animals to aid digestion and break down food and hard fibers once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths.[270]
Extinction of major groups
All non-avian dinosaurs and most lineages of birds[271] became extinct in a mass extinction event, called the Cretaceous–Paleogene (K-Pg) extinction event, at the end of the Cretaceous period. Above the Cretaceous–Paleogene boundary, which has been dated to 66.038 ± 0.025 million years ago,[272] fossils of non-avian dinosaurs disappear abruptly; the absence of dinosaur fossils was historically used to assign rocks to the ensuing Cenozoic. The nature of the event that caused this mass extinction has been extensively studied since the 1970s, leading to the development of two mechanisms that are thought to have played major roles: an extraterrestrial impact event in the Yucatán Peninsula, along with flood basalt volcanism in India. However, the specific mechanisms of the extinction event and the extent of its effects on dinosaurs are still areas of ongoing research.[273] Alongside dinosaurs, many other groups of animals became extinct: pterosaurs, marine reptiles such as mosasaurs and plesiosaurs, several groups of mammals, ammonites (nautilus-like mollusks), rudists (reef-building bivalves), and various groups of marine plankton.[274][275] In all, approximately 47% of genera and 76% of species on Earth became extinct during the K-Pg extinction event.[276] The relatively large size of most dinosaurs and the low diversity of small-bodied dinosaur species at the end of the Cretaceous may have contributed to their extinction;[277] the extinction of the bird lineages that did not survive may also have been caused by a dependence on forest habitats or a lack of adaptations to eating seeds for survival.[278][279]
Pre-extinction diversity
Just before the K-Pg extinction event, the number of non-avian dinosaur species that existed globally has been estimated at between 628 and 1078.[280] It remains uncertain whether the diversity of dinosaurs was in gradual decline before the K-Pg extinction event, or whether dinosaurs were actually thriving prior to the extinction. Rock formations from the Maastrichtian epoch, which directly preceded the extinction, have been found to have lower diversity than the preceding Campanian epoch, which led to the prevailing view of a long-term decline in diversity.[274][275][281] However, these comparisons did not account either for varying preservation potential between rock units or for different extents of exploration and excavation.[273] In 1984, Dale Russell carried out an analysis to account for these biases, and found no evidence of a decline;[282] another analysis by David Fastovsky and colleagues in 2004 even showed that dinosaur diversity continually increased until the extinction,[283] but this analysis has been rebutted.[284] Since then, different approaches based on statistics and mathematical models have variously supported either a sudden extinction[273][280][285] or a gradual decline.[286][287] End-Cretaceous trends in diversity may have varied between dinosaur lineages: it has been suggested that sauropods were not in decline, while ornithischians and theropods were in decline.[288][289]
The bolide impact hypothesis, first brought to wide attention in 1980 by Walter Alvarez, Luis Alvarez, and colleagues, attributes the K-Pg extinction event to a bolide (extraterrestrial projectile) impact.[290] Alvarez and colleagues proposed that a sudden increase in iridium levels, recorded around the world in rock deposits at the Cretaceous–Paleogene boundary, was direct evidence of the impact.[291]Shocked quartz, indicative of a strong shockwave emanating from an impact, was also found worldwide.[292] The actual impact site remained elusive until a crater measuring 180 km (110 mi) wide was discovered in the Yucatán Peninsula of southeastern Mexico, and was publicized in a 1991 paper by Alan Hildebrand and colleagues.[293] Now, the bulk of the evidence suggests that a bolide 5 to 15 kilometers (3 to 9+1⁄2 miles) wide impacted the Yucatán Peninsula 66 million years ago, forming this crater[294] and creating a "kill mechanism" that triggered the extinction event.[295][296][297]
Within hours, the Chicxulub impact would have created immediate effects such as earthquakes,[298] tsunamis,[299] and a global firestorm that likely killed unsheltered animals and started wildfires.[300][301] However, it would also have had longer-term consequences for the environment. Within days, sulfate aerosols released from rocks at the impact site would have contributed to acid rain and ocean acidification.[302][303] Soot aerosols are thought to have spread around the world over the ensuing months and years; they would have cooled the surface of the Earth by reflecting thermal radiation, and greatly slowed photosynthesis by blocking out sunlight, thus creating an impact winter.[273][304][305] (This role was ascribed to sulfate aerosols until experiments demonstrated otherwise.[303]) The cessation of photosynthesis would have led to the collapse of food webs depending on leafy plants, which included all dinosaurs save for grain-eating birds.[279]
Deccan Traps
At the time of the K-Pg extinction, the Deccan Traps flood basalts of India were actively erupting. The eruptions can be separated into three phases around the K-Pg boundary, two prior to the boundary and one after. The second phase, which occurred very close to the boundary, would have extruded 70 to 80% of the volume of these eruptions in intermittent pulses that occurred around 100,000 years apart.[306][307]Greenhouse gases such as carbon dioxide and sulfur dioxide would have been released by this volcanic activity,[308][309] resulting in climate change through temperature perturbations of roughly 3 °C (5.4 °F) but possibly as high as 7 °C (13 °F).[310] Like the Chicxulub impact, the eruptions may also have released sulfate aerosols, which would have caused acid rain and global cooling.[311] However, due to large error margins in the dating of the eruptions, the role of the Deccan Traps in the K-Pg extinction remains unclear.[272][273][312]
Before 2000, arguments that the Deccan Traps eruptions—as opposed to the Chicxulub impact—caused the extinction were usually linked to the view that the extinction was gradual. Prior to the discovery of the Chicxulub crater, the Deccan Traps were used to explain the global iridium layer;[308][313] even after the crater's discovery, the impact was still thought to only have had a regional, not global, effect on the extinction event.[314] In response, Luis Alvarez rejected volcanic activity as an explanation for the iridium layer and the extinction as a whole.[315] Since then, however, most researchers have adopted a more moderate position, which identifies the Chicxulub impact as the primary progenitor of the extinction while also recognizing that the Deccan Traps may also have played a role. Walter Alvarez himself has acknowledged that the Deccan Traps and other ecological factors may have contributed to the extinctions in addition to the Chicxulub impact.[316] Some estimates have placed the start of the second phase in the Deccan Traps eruptions within 50,000 years after the Chicxulub impact.[317] Combined with mathematical modelling of the seismic waves that would have been generated by the impact, this has led to the suggestion that the Chicxulub impact may have triggered these eruptions by increasing the permeability of the mantle plume underlying the Deccan Traps.[318][319]
Whether the Deccan Traps were a major cause of the extinction, on par with the Chicxulub impact, remains uncertain. Proponents consider the climatic impact of the sulfur dioxide released to have been on par with the Chicxulub impact, and also note the role of flood basalt volcanism in other mass extinctions like the Permian-Triassic extinction event.[320][321] They consider the Chicxulub impact to have worsened the ongoing climate change caused by the eruptions.[322] Meanwhile, detractors point out the sudden nature of the extinction and that other pulses in Deccan Traps activity of comparable magnitude did not appear to have caused extinctions. They also contend that the causes of different mass extinctions should be assessed separately.[323] In 2020, Alfio Chiarenza and colleagues suggested that the Deccan Traps may even have had the opposite effect: they suggested that the long-term warming caused by its carbon dioxide emissions may have dampened the impact winter from the Chicxulub impact.[297]
Possible Paleocene survivors
Non-avian dinosaur remains have occasionally been found above the K-Pg boundary. In 2000, Spencer Lucas and colleagues reported the discovery of a single hadrosaur right femur in the San Juan Basin of New Mexico, and described it as evidence of Paleocene dinosaurs. The rock unit in which the bone was discovered has been dated to the early Paleocene epoch, approximately 64.8 million years ago.[324] If the bone was not re-deposited by weathering action, it would provide evidence that some dinosaur populations survived at least half a million years into the Cenozoic.[325] Other evidence includes the presence of dinosaur remains in the Hell Creek Formation up to 1.3 m (4.3 ft) above the Cretaceous–Paleogene boundary, representing 40,000 years of elapsed time. This has been used to support the view that the K-Pg extinction was gradual.[326] However, these supposed Paleocene dinosaurs are considered by many other researchers to be reworked, that is, washed out of their original locations and then reburied in younger sediments.[327][328][329] The age estimates have also been considered unreliable.[330]
By human standards, dinosaurs were creatures of fantastic appearance and often enormous size. As such, they have captured the popular imagination and become an enduring part of human culture. The entry of the word "dinosaur" into the common vernacular reflects the animals' cultural importance: in English, "dinosaur" is commonly used to describe anything that is impractically large, obsolete, or bound for extinction.[331]
Public enthusiasm for dinosaurs first developed in Victorian England, where in 1854, three decades after the first scientific descriptions of dinosaur remains, a menagerie of lifelike dinosaur sculptures was unveiled in London's Crystal Palace Park. The Crystal Palace dinosaurs proved so popular that a strong market in smaller replicas soon developed. In subsequent decades, dinosaur exhibits opened at parks and museums around the world, ensuring that successive generations would be introduced to the animals in an immersive and exciting way.[332] The enduring popularity of dinosaurs, in its turn, has resulted in significant public funding for dinosaur science, and has frequently spurred new discoveries. In the United States, for example, the competition between museums for public attention led directly to the Bone Wars of the 1880s and 1890s, during which a pair of feuding paleontologists made enormous scientific contributions.[333]
The popular preoccupation with dinosaurs has ensured their appearance in literature, film, and other media. Beginning in 1852 with a passing mention in Charles Dickens'Bleak House,[334] dinosaurs have been featured in large numbers of fictional works. Jules Verne's 1864 novel Journey to the Center of the Earth, Sir Arthur Conan Doyle's 1912 book The Lost World, the 1914 animated film Gertie the Dinosaur (featuring the first animated dinosaur), the iconic 1933 film King Kong, the 1954 Godzilla and its many sequels, the best-selling 1990 novel Jurassic Park by Michael Crichton and its 1993 film adaptation are just a few notable examples of dinosaur appearances in fiction. Authors of general-interest non-fiction works about dinosaurs, including some prominent paleontologists, have often sought to use the animals as a way to educate readers about science in general. Dinosaurs are ubiquitous in advertising; numerous companies have referenced dinosaurs in printed or televised advertisements, either in order to sell their own products or in order to characterize their rivals as slow-moving, dim-witted, or obsolete.[335][336]
^Dinosaurs (including birds) are members of the natural groupReptilia. Their biology does not precisely correspond to the antiquated class Reptilia of Linnaean taxonomy, consisting of cold-bloodedamniotes without fur or feathers. As Linnean taxonomy was formulated for modern animals prior to the study of evolution and paleontology, it fails to account for extinct animals with intermediate traits between traditional classes.
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