Citipes () is an extinct genus of caenagnathid theropod from the Late Cretaceous Dinosaur Park Formation in Alberta, Canada. The genus contains only one species, the type species, C. elegans. The generic name of Citipes is Latin for "fleet-footed", and the specific epithet "elegans" is Latin for "elegant". The type specimen of Citipes has a convoluted taxonomic history, and has been previously assigned to the genera Ornithomimus, Macrophalangia, Elmisaurus, Chirostenotes, and Leptorhynchos before being given its own genus in 2020.
Discovery and naming Initial discovery
The material which would eventually be named Citipes was discovered in 1926 during one of the University of Toronto's expeditions to the Sand Creek area of the Red Deer River. This locality was a part of the Belly River Group, which is now recognized as the Dinosaur Park Formation. However, it would be another seven years before a full description of this material was published in the University of Toronto's geology journal in 1933 by Dr. William A. Parks. The material, consisting of three metatarsals, was reposited at the Royal Ontario Museum and was given the designation ROM 781.
It was named as a new species of the common genus Ornithomimus — Ornithomimus elegans. This name was assigned based on three metatarsals, which were believed to belong to an ornithomimosaur because of the lack of a fifth metatarsal bone. Oviraptorosauria was not yet recognized as a unique clade, and several oviraptorans, including Oviraptor and Chirostenotes, were believed to be ornithomimids. It would not be until 1976 that oviraptorosaurs became recognized as their own unique clade.
Taxonomic history ROM 781 was reassigned to the dubious genus Macrophalangia by Dale Russell in 1972. However, this genus was made a junior synonym of Chirostenotes a few years later.
A more thorough re-examination of ROM 781 was conducted in 1988 by Philip J. Currie, who published a re-description of the material in the Canadian Journal of Earth Sciences. Currie referred the specimen to a new species in the recently described genus Elmisaurus — Elmisaurus elegans. He also referred two new specimens to the species. The first of these new specimens, ROM 37163, consists of a partial metatarsus and was collected sometime between the years of 1920 and 1954 by an expedition of the Royal Ontario Museum. The second, TMP 82.39.4, consists of another partial tarsometatarsus, and it was collected in 1982 by Linda Strong-Watson. Both of these specimens were discovered in the Dinosaur Park Formation and resembled the holotype enough to warrant referral.
Elmisaurus elegans was distinguished from the type species, E. rarus, by the presence of a weaker ridge on the fourth metatarsal bone, and a process on the distal ends of metatarsals II and IV. At the time of this publication, there was an ongoing debate about the validity of the caenagnathid genera Chirostenotes, Caenagnathus, and Elmisaurus, and Currie used his re-description of ROM 781 to argue that Elmisaurus was a distinct and valid genus.
The classification of ROM 781 was amended again in 1997 by Hans-Dieter Sues in 1997 when he published an extensive monograph describing a newly discovered specimen of the genus Chirostenotes. In his monograph, Sues referred Elmisaurus elegans to the genus Chirostenotes as the new species C. elegans, and he also regarded the genera Caenagnathus and Elmisaurus as subjective junior synonyms of Chirostenotes. Some specimens were referred to C. pergracilis and others to the newly erected C. elegans. Sues distinguished C. elegans from C. pergracilis by the presence of co-ossification of the second and fourth metatarsals, and it was included in the genus because Sues regarded Chirostenotes as the only valid caenagnathid genus in Campanian Laramidia.
In 2013, accompanying their description of the genus Leptorhynchos, Nicholas Longrich, Ken Barnes, Scott Clark, and Larry Millar, referred ROM 781 to their newly described genus. Their reasoning for this reassignment was the referral of additional specimens — TMP 1992.36.390, TMP 1979.8.622, TMP 1991.144.1 (all lower jaw fragments), and TMP 1982.39.4 (a partial tarsometatarsus) — to the species based on their small size in comparison with all other described caenagnathids. Hagryphus, Caenagnathus, and Chirostenotes were all much larger than the newly named Leptorhynchos, and so they moved Chirostenotes pergracilis to the new genus as the species Leptorhynchos elegans. Longrich and colleagues distinguished L. elegans from the type species, L. gaddisi, by the presence of a strongly upturned beak tip and a chin that is square-shaped in lateral view.
Current understanding
In 2020, Gregory Funston conducted a review of all caenagnathid material known from the Dinosaur Park Formation in order to determine their actual diversity in that depositional environment. Funston personally examined and scored material in the collections of the Canadian Museum of Nature, the Royal Ontario Museum, the Royal Tyrell Museum, the University of Alberta, and the Mongolian Academy of Sciences. He also collected histological samples from several bones in order to examine them for "lines of arrested growth", which can be used to estimate the ontogenetic age of the specimens.
There has historically been significant uncertainty in the scientific literature as to which ontogenetic stage each of the specimens were in and whether or not the smaller caenagnathid remains belonged to juveniles or adults of smaller species. Funston's analysis resulted in revised diagnoses for Caenagnathus and Chirostenotes, which Funston argues are distinct and valid genera. This analysis also resulted in the specimens assigned to Leptorhynchos elegans, as well as some recently discovered specimens, being referred to the newly erected genus, Citipes.
Most of the caenagnathid material from the Dinosaur Park Formation was assigned to one of these three genera based on size and diagnoses were amended from the apomorphies of those specimens. Some specimens were not able to be referred to any of these genera with certainty due to the incompleteness of the remains.
Description Citipes is a small-bodied caenagnathid. In his review of Dinosaur Park caenagnathids, Gregory Funston concluded that it had a substantially smaller adult size than the contemporaneous genera Caenagnathus and Chirostenotes. Histological analysis of two metatarsals assigned to the genus indicated that the individuals that the bones came from were close to adulthood, but not yet fully matured. Funston is confident in his assessment that Citipes was relatively small in life, with adults measuring long. Rubén Molina-Pérez and Asier Larramendi suggested a more specific estimate of long and tall at the hip with an estimated mass of .
In his comprehensive re-description of the caenagnathids from the Dinosaur Park Formation, Gregory Funston provides amended diagnoses for all three named genera in his analysis. For Citipes, he identifies several autapomorphies including: coossification of the distal ends of tarsals III and IV, a lack of posterior protrusions on the proximal ends of metatarsals II and IV, and a prominent ridge on the posterior surface of metatarsal III. These autapomorphies not only distinguish Citipes from its contemporaries, but also from the closely related genera Elmisaurus and Leptorhynchos.
Holotype The holotype of Citipes was described in 1933 by William Parks as a species of Ornithomimus. It consists of three metatarsals which are bowed slightly inwards such that metatarsals II and IV meet at their proximal ends. The length of the metatarsals is stated by Parks as roughly . Referred material (1989-2013) In 1989, the specimens ROM 37163 (distal metatarsal II) and TMP 82.39.4 (a partial tarsometatarsus) were assigned to the same taxon as ROM 781 and the diagnosis of the holotype was amended. A re-examination of the holotype led to the discovery that the metatarsals II and IV were coossified and that their separation in the holotype was taphonomic. The overall morphological similarities to Elmisaurus rarus included the shape of the bones' cross-sections and the presence of an arctometatarsus, which is what led Philip Currie to refer the species to Elmisaurus.
In 2013, Nicholas Longrich and colleagues referred additional material to the newly renamed Leptorhynchos elegans. These specimens included several bones of the lower jaws as well as newly discovered tarsometatarsal elements. They also erected TMP 1992.36.290 (the anterior half of two dentaries) as the paratype of the species. In their re-assignment, Longrich and colleagues distinguished L. elegans from the genera to which it was assigned previously (Elmisaurus and Chirostenotes) due to its small size, which they believed was not reflective of its ontogenetic age because of the fusion of the tarsal elements. They also remark that the dentaries of L. elegans — exemplified by the specimens TMP 1992.36.390, TMP 1979.8.622, and TMP 1991.144.1 — differ substantially from other caenagnathids in their relatively short length and the upturned nature of the dentary symphysis.
New material assigned in 2020 During his comprehensive analysis in 2020, Gregory Funston referred two partial hip specimens to Citipes, however this assignment is somewhat uncertain because of the lack of overlapping material with the type series. Funston also argues that the designation of a paratype based solely on the size and locality of the specimen (in comparison to ROM 781) was unfounded, and he states that there is insufficient evidence to suggest that TMP 1992.36.290 belongs to the same taxon as ROM 781.
The first of the new specimens, TMP 1981.023.0039, is an anterior sacral vertebra which was found in association with two ilia (TMP 1981.023.0034-5). The shape of the centrum is characteristically low and wide, which is the condition seen in many other oviraptorosaurs. However, the anterior articular surface of the centrum is unbroken, which Funston suggests means that the sacrum was not fully fused when the animal died. Evidence of the sacral pleurocoels is also preserved on the lateral surface of the centrum. Despite the fact that these bones were not subject to a histological analysis, Funston believes they belong to an adult due to the large muscle attachment sites visible on the bone and the robustness of the iliac crest. The small size in an adult animal precludes the assignment of TMP 1981.023.0039 to either Caenagnathus or Chirostenotes, and therefore probably belongs to Citipes.
The remaining specimens, UALVP 59606, UALVP 55585, and UALVP 59606, consist of various partial tarsometatarsal bones of varying completeness and states of coossification. The latter of these was sectioned for histological analysis. Similarities with the known metatarsal elements of Elmisaurus were noted among these bones, which was reflected in the phylogenetic analysis that was conducted in the same paper.
Several other caenagnathid specimens from the collections of the Royal Tyrell Museum were described by Funston, but they were not referred to any particular genus within caenagnathidae. These remains, which included jaw bones, were small in size, but similar in morphology to Chirostenotes. Funston argues that they cannot be referred to a particular genus without being sectioned and having their ontogenetic age determined.
Classification Phylogeny