Leptorhynchos (/ˌlɛptəuˈrɪŋkəus/) is an extinct genus of caenagnathid theropod from the Late Cretaceous of what is now the US state of Texas, although it has been suggested to also exist in Alberta and South Dakota. The type species is L. gaddisi, and it is currently the only widely accepted valid species. The generic name of Leptorhynchos comes from the Greek "leptos" meaning "small" and "rhynchos" meaning "beak". The specific epithet is in honor of the Gaddis family, who owned the land on which the holotype was discovered.[2]
Leptorhynchos was named in 2013 by Nick Longrich, Ken Barnes, Scott Clark, and Larry Millar. In their description, they conducted a specimen-level review of North American caenagnathids in an attempt to resolve the perennial taxonomic confusion surrounding the group. This included the type specimens of Caenagnathus, Chirostenotes, and Hagryphus as well as specimens of more ambiguous affinities such as ROM 781, which was previously named as a species of Ornithomimus. Their analysis also included the specimen TMM 45920-1, which would later be designated as the holotype of Leptorhynchos gaddisi.[2][3]
The results of Longrich and colleagues' analysis was the naming of the new genus, Leptorhynchos, and the assignment of "Ornithomimus" elegans (specimen ROM 781) as a second species in that genus. Several other specimens, such as MOR 1107, were referred to the genus but not assigned to a particular species. Their description contained an initial and then a revised diagnosis of the genus and both species.[2][3] Remains of other small caenagnathids from Laramidia discovered in the Hell Creek and Scollard formations have variously been attributed to the genus, but without being assigned to either of the named species.
L. gaddisi was named as the type species of the new genus by Longrich and colleagues. They based the diagnosis of this species on the anteriorly-projected beak tip, rounded ventral margin of the dentary symphysis, and a spoon-shape to the beak tip in dorsal view.[3] Longrich and colleagues did not provide a mass estimate in their description but Rubén Molina-Pérez and Asier Larramendi suggested a length of 1.75 metres (5.7 ft) and a mass of roughly 19 kilograms (42 lb).[4] L. gaddisi is generally regarded as the only valid species of Leptorhynchos.[5]
The fossil specimen ROM 781 was described in 1933 as a new species of the genus Ornithomimus. I was later reassigned to the genus Magrophalangia (now a junior synonym of Chirostenotes), then to Elmisaurus, and then to Chirostenotes before being named by Longrich and colleagues as the type specimen of the new species L. elegans.[5] They made this diagnosis based on the strongly upturned beak and straight anterior margin of the dentary symphysis and also referred the specimen TMP 1992.36.390 to the species. However, in their phylogenetic analysis which accompanied the description, the authors remarked that the two species form a polytomy with one another within Caenagnathidae and they may not represent the same genus.[2] In 2020, Gregory Funston reassigned L. elegans to the new genus, Citipes.[5] Leptorhynchos has not been included in any phylogenetic analyses with Citipes since the most recent referral, so the veracity of this reassignment has yet to be tested by new authors in order to affirm or contest it.[6]
In their description of the genus, Longrich and colleagues referred the specimen MOR 1107 (part of a mandible) to Leptorhynchos, but they did not assign it to either of the species they named. Although the bones are convex in profile, similar to Chirostenotes, its smaller size led them to refer it to Leptorhynchos.[2] Other small caenagnathid remains have been variously referred to the genus since it was named, although none have been named as a their own species. Among these are caenagnathid remains from the late Maastrichtian Scollard Formation,[7] the Frenchman Formation,[8] and the Hell Creek Formation.[9] Subsequent authors have disputed these referrals.[6] More caenagnathid remains have also recently been described from the Aguja Formation, which have been speculated to belong to Leptorhynchos.[10]
The holotype of Leptorhynchos gaddisi, given the designation TMM 45920-1, is a single fused mandible. Additional specimens were also referred to the species in the paper describing it. These include a caudal vertebra, the partial third and fourth metatarsals, and a single toe claw. These specimens were not associated with the holotype, but were referred to L. gaddisi based on their size and discovery in the same geologic layer.[2] Later authors have questioned this referral[5] and Leptorhynchos has not been included in some recent analyses.[5][6]
The dentaries of the holotype were fully fused with no visible sutures, which suggest that the animal was fully mature when it died and the small size of the specimen is not the result of ontogeny. This was one of the primary reasons that Leptorhynchos was named as a new species instead of an immature Chirostenotes, which it closely resembled in shape but which was much larger. The interior surface of the jaw had several furrows and a lingual shelf with a ridge along its margin. The mandible itself was much more elongated than members of oviraptoridae, but was proportionally relatively short for a caenagnathid.[2]
The referred specimens from the Aguja Formation are tentatively assigned to this taxon. They include a single partial vertebra, two partial metatarsals, and a pedal ungual. The vertebra is unremarkable with twin pleurocoels on each side of the vertebral centrum. The metatarsals strongly resemble those of Citipes, which was part of the basis for the initial referral of that taxon to the genus Leptorhynchos. The metatarsals are also fused to the tarsal bones, which is the condition seen in both Citipes and Elmisaurus.[2]
In their description of the holotype, Longrich and colleagues conducted a phylogenetic analysis. Their analysis used the dataset of Longrich's earlier publication in 2010 with several new taxa added for a total of 28 taxa coded for 205 characters. Their data set also included recent information about Nemegtomaia and Nomingia which had been published since the analysis in 2010. The monophyly of caenagnathidae was supported in their analysis based on the following diagnostic characters: fused dentaries, a ventrally bowed process on the underside of the dentary, the lack of contact between the dentary and the mandibular fenestra, and a shallow surangular bone. The analysis did resolve several taxa at the base of caenagnathidae including Gigantoraptor and Microvenator, but there was less resolution in the more derived area of the tree. In particular, their strict consensus tree did not support the unambiguous monophyly of the genus Leptorhynchos; the two species they assigned to the genus were in a polytomy with Hagryphus and a clade containing Caenagnathus, Chirostenotes, and the specimen BMNH 2033 (which is from the Hell Creek Formation and was coded as a distinct taxon). The authors remark that they referred ROM 781 to Leptorhynchos because of the similar size of the two species, but noted that they may belong to separate genera, pending further research.[2]
Longrich and colleagues broadly determined that most caenagnathid genera are diagnosed by characteristics of the mandible and manus, which allows researchers to diagnose genera and species based on very incomplete remains, but which also means that more complete remains are undiagnostic if they do not preserve the mandible or manus. However, the authors note that the shape of the beak in modern birds is an important diagnostic trait, as well as in some dinosaurs like Triceratops and Edmontosaurus, so they argue that it is not unreasonable to assume that this may be sufficient in naming new genera or species. Furthermore, the degree of fusion exhibited by the jaw bones, as well as their surface texture, can be indicative of ontogenetic age in many oviraptorosaurs, which can be used to determine if an individual specimen is a new taxon or simply a different life stage of an existing taxon.[2]
For specimens which do not contain the relevant diagnostic characters or any indication of their ontogenetic age, they have been assigned to existing taxa according to size and locality. Longrich and colleagues note that this approach has shortcomings, but they argue that the ecological abundance of adult animals in any ecosystem means that most fossilized animals will be adults anyways, which they suggest is sufficient for the purposes of their analysis. A consensus of the 116 most parsimonious trees in their analysis is shown below.[2]