Citipes
Citipes Temporal range: Late Cretaceous
~76.9 to 75.8 Ma - | |
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A diagram of all known material from Citipes with the material described in 2020 shown in pink | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | Saurischia |
Clade: | Theropoda |
Family: | †Caenagnathidae |
Subfamily: | † Elmisaurinae (?)
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Genus: | †Citipes Funston, 2020 |
Type species | |
†Ornithomimus elegans Parks , 1933 | |
Species | |
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Synonyms | |
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Citipes (
Discovery and naming
Initial discovery
The material which would eventually be named Citipes was discovered in 1926 during one of the
It was named as a new species of the common genus
Taxonomic history
ROM 781 was reassigned to the dubious genus Macrophalangia by Dale Russell in 1972.
A more thorough re-examination of ROM 781 was conducted in 1988 by
Elmisaurus elegans was distinguished from the
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
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.[6]
Current understanding
In 2020, Gregory Funston conducted a review of all
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.[1]
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.[1]
Description
Citipes is a small-bodied
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 poterior 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.[1]
- 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 160 millimetres (6.3 in).[2]
- 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
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
- 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.[1] Funston also argues that the designation of a paratype based solely on the size and locality of the specimen (in comparison to ROM 781)[7] 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.[1]
The first of the new specimens, TMP 1981.023.0039, is an anterior
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.[1]
Several other caenagnathid specimens from the collections of the
Classification
Phylogeny
The first systematic phylogenetic analysis to include ROM 781 was the one conducted by Nicholas Longrich, Ken Barnes, Scott Clark, and Larry Millar in their paper describing the genus Leptorhynchos in 2013. In their analysis, they assign ROM 781 and several other specimens to a second species in the genus Leptorhynchos — L. elegans. 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.[6]
The
Longrich and colleagues broadly determined that most
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.[6]
Oviraptorosauria |
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In his review of
This analysis recovered similar results to most prior analyses. Namely, that
Oviraptorosauria |
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Possible synonymity
In a study of the pelvic morphology of all caenagnathid material from the Dinosaur Park Formation in 2019, Matthew Rhodes and colleagues categorize every specimen into one of three categories. The morph which they believe belongs to a new taxon,[14] later named Citipes,[1] is described as the "small, robust morphotype" and is distinct from all other caenagnathid pelves in the formation. The most immediately visible distinction is that the pelves of the small, robust morph are curved into a slight letter-s shape when viewed from above or below. Other differences are present but less obvious such as the very small pubic peduncle and the expanded dorsal ridge on the posterior part of the ilium.[14]
Rhodes and colleagues suggested that these morphological distinctions in the hip bones of caenagnathids were unlikely to be the result of ontogeny because the level of coossification between the ilia and sacra is similar in very differently-sized specimens.[14] Funston elaborated on this finding in 2020 in his description of Citipes. The similar ontogenetic age of differently-sized postcranial remains indicated the probable presence of three caenagnathid taxa in Dinosaur Park. Two of these taxa had already been named, and the remaining group of specimens — comprising the small, robust morph — were morphologically distinct from the Texan species of Leptorhynchos which they had been named as in 2013.[1] Phylogenetic analyses have never found the two "species" of Leptorhynchos as sister taxa,[6] but there have not been any comprehensive reviews of Funston's phylogenetic analysis to date, nor have any new analyses been conducted as of yet.
Paleobiology
Growth and histology
The ontogeny of caenagnathids is difficult to study due to the incompleteness of known remains. This is further complicated by the relative lack of overlapping skeletal material from the various named taxa of the Dinosaur Park Formation. Gregory Funston and colleagues conducted a histological study of caenagnathid dentaries (the most common remains found in Dinosaur Park) and noticed that the mandibular symphysis becomes fully ossified relatively early in caeagnathid growth, and it is thus a poor indicator of the precise age of a specimen. Another impediment to precise age identification are the possible irregularities in the deposition of bone tissues as the animals grow. Funston and colleagues notice that the lines of arrested growth (or LAGs), which usually correspond to slower growth during the winter months, may not have been formed in caenagnathids under 1-2 years of age, which may lead to the underestimation of the ages of some specimens.[15]
Pathology
One of the specimens referred to Citipes (at the time, Leptorhynchos elegans) is TMP 1992.036.0476, which is a mostly complete
Paleoecology
Diet and niche partitioning
The Dinosaur Park Formation can be divided into several discrete faunal assemblages which were not all contemporaneous.[17] However, it is presently unclear if any of the known caenagnathid genera are restricted to one or more of these faunal zones. Caenagnathid remains have not been recovered as often as those of other coelurosaurs, but this is partially accounted for by their relatively small size and lack of teeth, which may present a sampling bias.[1]
While caenagnathid remains are relatively uncommon, they are widely dispersed across Dinosaur Provincial Park, with no clear pattern of which genera are found at specific localities. This suggests that they were present throughout the faunal zones of the Dinosaur Park Formation, which implies there was some sort of niche differentiation between the three taxa. The most immediately obvious morphological difference between
Caenagnathus is substantially larger than either Citipes or Chirostenotes,[1] and it is believed to be primarily herbivorous because the better-understood and closely-related taxon Anzu is believed to have been a herbivore.[18] Chirostenotes, on the other hand, has a sharply upturned tip of the lower jaw, which is possibly an adaptation for a more carnivorous mode of life,[19] although this remains speculative and untestable due to the incompleteness of these remains.[1]
Predation
In 2023, François Therrien and colleagues described TMP 2009.12.14 — an exceptionally preserved juvenile specimen of Gorgosaurus, which contained remains from at least two Citipes individuals. The legs of the Citipes individuals were flexed and contorted into a very compact shape, which suggests that they were inside the stomach of the Gorgosaurus, rather than being preserved alongside it or washed together post-mortem. This demonstrated that Gorgosaurus occasionally fed upon Citipes.[20]
The femur of the Gorgosaurus was sectioned and subjected to histological analysis, which revealed that it was between 5-7 years-old when it died and was only about one-eighth the mass of an adult Gorgosaurus. Histology of the Citipes bones showed a complete lack of growth marks, leading Therrien and colleagues to suggest that these individuals were within their first year of life when they were eaten by the Gorgosaurus.[20]
Notably, the Citipes individuals do not appear to have been eaten in their entirety. Only the hind limbs and elements of the
In their description of TMP 2009.12.14, Therrien and colleagues conducted a regression analysis of the relationship between the masses of extant reptilian and mammalian predators and the masses of potential sympatric prey items. They used this, along with the estimated masses of juvenile and adult Gorgosaurus to predict the possible range of prey sizes for young and adult individuals of that genus. Their findings indicated that Gorgosaurus individuals of both major ontogenetic ranges, juvenile and adult, were within the size range that both juvenile and adult Citipes could be considered prey items.[20]
Paleoenvironment
Citipes was discovered at the Little Sand Hill Creek locality, which is located within Dinosaur Provincial Park in Alberta, Canada. The age of this locality relative to the rest of the Dinosaur Park Formation is not known confidently known.[1] However, the Dinosaur Park Formation as a whole is relatively confidently known to have been deposited between 77.03 and 75.46 million years ago.[21]
The Dinosaur Park Formation is composed of sediments that were derived from the erosion of the mountains to the west. It was deposited on an alluvial to coastal plain by river systems that flowed eastward and southeastward to the Bearpaw Sea, a large inland sea that was part of the Western Interior Seaway. That sea gradually inundated the adjacent coastal plain, depositing the marine shales of the Bearpaw Formation on top of the Dinosaur Park Formation.[22]
The Dinosaur Park Formation is roughly 75 metres (250 ft) thick. The lower portion of the formation was laid down in fluvial
These geological features are interpreted as a low-relief setting of
Contemporary fauna
The Dinosaur Park Formation preserves one of the most diverse non-avian dinosaur assemblages known in the fossil record. At least six species of hadrosaurids including the well-known Parasaurolophus and Corythosaurus, are known from various layers in addition to eight or more species of ceratopsids in such well-known genera as Styracosaurus, Chasmosaurus, and Centrosaurus. Ankylosaurs are also abundant, being represented by members of both nodosauridae, such as Edmontonia, as well as ankylosauridae, such as Euoplocephalus.[22]
Preying on this wide variety of large
The waterways of the Dinosaur Park Formation also played host to several
The Dinosaur Park Formation also preserves a wide array of smaller animals which would have been abundant in the waterways of the depositional environment. These included a variety turtles such as the large
See also
- 2020 in archosaur paleontology
- 2023 in archosaur paleontology
- Judith River Formation and Two Medicine Formation - roughly coeval geological formations in Alberta and Montana
- List of North American dinosaurs
- List of stratigraphic units with dinosaur body fossils
- Cretaceous land vertebrate ages in North America (Judithian)
- Timeline of oviraptorosaur research
References
- ^ ISSN 2292-1389.
- ^ a b c Parks, William A. (1933). "New Species of Dinosaurs and Turtles from the Upper Cretaceous Formations of Alberta". University of Toronto Studies, Geological Series. 43: 4–44.
- ^ doi:10.1139/e72-031.
- ^ doi:10.1139/e89-111.
- ^ .
- ^ S2CID 128444961.
- ^ S2CID 128898931.
- OCLC 40272928.
- ^ Sternberg, C.M. (1932). "Two new theropod dinosaurs from the Belly River Formation of Alberta". Canadian Field-Naturalist. 46 (5): 99–105.
- theropodsOviraptoridae n. fam. in Mongolia]. Doklady Akademii Nauk SSSR. 226 (3): 685−688.
- doi:10.1139/e88-097.
- ISBN 978-0691180311.
- PMID 33204472.
- ^ S2CID 219745025.
- PMID 31270950.
- ^ Funston, Gregory F. (2019). "Chapter 3 — Caenagnathidae". Anatomy, systematics, and evolution of Oviraptorosauria (Dinosauria, Theropoda) (PhD in Systematics and Evolution thesis). Department of Biological Sciences, University of Alberta.
- .
- doi:10.1139/e93-196.
- .
- ^ PMID 38064561.
- PMID 29166406.
- ^ ISBN 0-253-34595-2.
- ISBN 978-0-253-34595-0.
- ISBN 978-0-253-34595-0.
- PMID 33614274.)
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: CS1 maint: multiple names: authors list (link - S2CID 203406859.