''Chinatichampsus'' is an
extinct genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of
crocodilian from the
Devil's Graveyard Formation of Texas, specifically the Dalquest Desert Research Site. It is a
monotypic
In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...
genus, containing only the type species ''Chintanichampsus wilsonorum''. A single specimen, TMM 45911–1, was first discovered in 2010. ''Chinatichampsus'' is the most basal
Eocene
The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...
caimanine
A caiman (also cayman as a variant spelling) is an alligatorid belonging to the subfamily Caimaninae, one of two primary lineages within the Alligatoridae family, the other being alligators. Caimans inhabit Mexico, Central and South America fro ...
, dating to between 42.8 and 41.5 million years ago, and is considered to be more
basal than ''
Protocaiman''.
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History of discovery
The
holotype
A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...
of ''Chinatichampsus'', TMM 45911–1, was discovered in 2010, in the
Midwestern State University
Midwestern State University (MSU Texas) is a public liberal arts university in Wichita Falls, Texas. In 2020 it had 5,141 undergraduate students. It is the state's only public institution focused on the liberal arts.
History
Founded in 1922 as ...
's Dalquest Desert Research Site. Strata from this locality are assigned to the
Devil's Graveyard Formation, and the
conglomeratic sandstones
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) b ...
in which TM 45911-1 was discovered have been dated to between 42.8 and 41.5 million years ago. This corresponds to the late
Uintan–early
Duchesnean
The Duchesnean North American Stage on the geologic timescale is a North American Land Mammal Age (NALMA), with an age from 42 to 38 million years BP, representing . It falls within the Eocene epoch. The Duchesnean is preceded by the Uintan and ...
faunal stages. The fossil locality in which it was discovered has been nicknamed "Crocolicious".
The
generic name of ''Chinatichampsus'' is derived from the
Chinati Mountains and the Greek ''champsus'' (crocodile). The
specific epithet honours Cornelia and Samuel Wilson, the discoverers of the holotype specimen.
Description
![Chinatichampsus side view](https://upload.wikimedia.org/wikipedia/commons/5/55/Chinatichampsus_side_view.jpg)
The holotype specimen of ''Chinatichampsus'' consists of a cranium, lacking most of the left
dorsal
Dorsal (from Latin ''dorsum'' ‘back’) may refer to:
* Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism
* Dorsal, positioned on top of an aircraft's fuselage
* Dorsal c ...
surface. The left
premaxilla
The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...
is missing, and most of the left side of the cranium, as well as the right premaxilla, right nasal and right prefrontal, are also entirely absent. Though no parts of the postcranial skeleton were recovered, its describers hypothesize that it is a morphologically mature individual, specifically one older than two years of age, due to the size of the cranium and the proportions of the cranial
fenestra
A fenestra (fenestration; plural fenestrae or fenestrations) is any small opening or pore, commonly used as a term in the biological sciences. It is the Latin word for "window", and is used in various fields to describe a pore in an anatomical st ...
e.
None of the external bone of the left maxilla remains, and only the medial surface of the large third or fourth left maxillary tooth and traces of the posterior maxillary
alveoli are present. Remnants of
matrix infillings of the nasal passage and
maxillary sinus
The pyramid-shaped maxillary sinus (or antrum of Highmore) is the largest of the paranasal sinuses, and drains into the middle meatus of the nose through the osteomeatal complex.Human Anatomy, Jacobs, Elsevier, 2008, page 209-210
Structure
It i ...
are visible, though.
The right maxilla does preserve most of its external surface, though the outermost bony surface is either missing or crushed. At the posteromedial intersection of the premaxillary-maxillary suture with the nasal is a prominent crest, differing from the preorbital and rostral ridges observed in other
caimanines in that it is oriented anteroposteriorly, rather than anterolaterally or mediolaterally.
Most of the dorsal surfaces of the relatively short and wide nasals are entirely missing. Along the cranium's midline are paired molds of the grooved ventral surfaces of the nasals.
The anteriorly elongated nasals appear to have tapered slightly towards their anterior articulations with the maxillae and premaxillae. Though the anteriormost portion of the cranium is not very well-preserved, it seems there was at least a slight projection of the nasals beyond the posterior extent of the naris. Whether an internarial bar would have entirely bisected the nares is unclear.
Between 13 and 15 maxillary teeth are preserved. The number of maxillary teeth and their positions is ambiguous due to the position of the maxillary-premaxillary suture and the nature of a small hole at the posterior end of the maxillary tooth row. Assuming the largest preserved alveolus is the fourth, as is typical for an alligatorid, and that it is indeed an alveolus, there would be 15 maxillary teeth. The posterior maxillary teeth are rounded and slightly bulbous with the posteriormost preserved tooth being almost fungiform.
The robust pterygoids are roughly trapezoidal in ventral view and wider posteriorly than anteriorly, and form the entire posterior margin of the suborbital fenestrae. The pterygoids completely surround the choanae in ''Chinatichampsus''. These choanae project anteroventrally and have posterior edges that have raised because of a surrounding concavity. A deep notch bisects the posterior edge. The choanal septum is not visible in lateral view.
Both ''Chitanichampsus'' and the slightly more derived ''
Eocaiman
''Eocaiman'' is an extinct genus of caiman containing species living from the Early Paleocene to Miocene in what is now Argentina (Salamanca Formation, Salamanca and Sarmiento Formations), Itaboraí Formation of Brazil and Colombia (Honda Group, ...
'' share several characters
plesiomorphic for crown-caimanines and ''
Necrosuchus
''Necrosuchus'' is an extinct genus of caiman from modern day Argentina that lived during the Paleocene epoch (Selandian age, about 60 million years ago). It inhabited the fluvio-lacustrine environment of the Patagonian Salamanca Formation.
Hist ...
'' including the presence of posterior maxillary processes between the lacrimal and prefrontal, an extension of the
quadratojugal The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians.
Anatomy and function
In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...
with the dorsal angle of the
infratemporal fenestra, and a small supraoccipital exposure on the skull table. It can be differentiated from ''Eocaiman'' because of the rounded shape of the expanded anterior margins of the palatines, rather than the quadrangular anterior margins of ''Eocaiman.''
Taxonomy
''Chinatichampsus'' was originally believed to have affinities with the
Alligatorinae
Alligatorinae is a subfamily within the family Alligatoridae that contains the alligators and their closest extinct relatives, and is the sister taxon to Caimaninae (the caimans). Many genera in Alligatorinae are described, but only the genus ...
, partly based on comparisons with a currently undescribed alligatorid.
[Stocker, Brochu & Kirk (2012) Stocker MR, Brochu CA, Kirk EC. Spatial and temporal shifts in Paleogene crocodyliform diversity and a new globidont alligatoroid from the Middle Eocene of West Texas. Journal of Vertebrate Paleontology, Program and Abstracts. 2012;2012:180] In contrast, the phylogeny of Stocker, Brochu and Kirk (2021) found ''Chinatichampsus'' to be one of the most
basal caimanines, branching off before ''
Protocaiman'', but after ''
Brachychampsa'', ''
Stangerochampsa'' and ''
Albertochampsa''.
Below is a
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
tree by Stocker, Brochu & Kirk (2021):
References
{{Taxonbar, from=Q106610344, from2=Q108073337
Alligatoridae
Fossil taxa described in 2021
Prehistoric pseudosuchian genera