''Coeruleodraco'' 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 n ...

choristodera Choristodera (from the Greek χωριστός ''chōristos'' + δέρη ''dérē'', 'separated neck') is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Triassic, to the late Miocene (168 to ...

n known from the Late

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

( Oxfordian)

Tiaojishan Formation The Tiaojishan Formation is a geological formation in Hebei and Liaoning, People's Republic of China, dating to the middle-late Jurassic period (Bathonian- Oxfordian stages). It is known for its exceptionally preserved fossils, including those of ...

in China. ''Coeruleodraco'' is significant as the most complete Jurassic choristodere taxon, as the only other named Jurassic choristodere '' Cteniogenys'' is based on fragmentary remains. Although similar to '' Philydrosaurus'' in its proportions and postcranial characters, it is distinct in retaining several apparently

plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, an ...

characters, including a short snout, paired external nares and an open lower temporal fenestra.

Description

The skeleton was small, about 20.3 cm (8.0 inches) in length, not including the tail. The tail is incomplete, but including its estimated length, the total length of the skeleton would have been closer to 40.0 cm (15.7 inches). ''Coeruleodraco'' had a generalized, lizard-like body type. This is similar to some early choristoderes (like '' Cteniogenys'', '' Philydrosaurus'', and '' Monjurosuchus''), but contrasts with other choristodere body types including long-necked forms such as ''

Hyphalosaurus ''Hyphalosaurus'' (meaning "submerged lizard") is a genus of freshwater aquatic reptiles, belonging to the extinct order Choristodera. They lived during the early Cretaceous period (Aptian age), about 122 million years ago. The genus contains ...

'', and

gharial The gharial (''Gavialis gangeticus''), also known as gavial or fish-eating crocodile, is a crocodilian in the family Gavialidae and among the longest of all living crocodilians. Mature females are long, and males . Adult males have a distinct ...

-like forms such as ''

Champsosaurus ''Champsosaurus'' is an extinct genus of crocodile-like choristodere reptile, known from the Late Cretaceous and early Paleogene periods of North America and Europe (Campanian-Paleocene). The name ''Champsosaurus'' is thought to come from , () ...

'' and other neochoristoderes. Coeruleodraco NT

The skull had a short, pointed snout with about 38 teeth on each side (a low number by choristodere standards). It also had two nares (nostril holes). While paired nares are standard in most reptile groups, almost all other choristoderes (except ''Lazarussuchus'') typically have one single hole in the skull for the nostrils, making ''Coeruleodraco'''s retention of paired nares a

("primitive") feature relative to most other choristoderes. In common with other choristoderes, ''Coeruleodraco'' has elongated prefrontal bones which meet each other at the top of the snout, separating a pair of small, triangular nasal bones in the process. The nasals are so reduced in size (especially compared to ''Philydrosaurus'') that they are completely surrounded by the prefrontals and premaxillary bones, and fail to contact the toothed maxillary bones. The

postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some v ...

and

postfrontal The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

bones (which lie behind the eye) are separate from each other; in many other choristoderes these bones fuse into a single postorbitofrontal bone. The rear of the skull is not particularly unusual, with each side of the skull having two holes known as temporal fenestrae (as with other diapsids). The upper temporal fenestra is not elongated (unlike neochoristoderes), but the lower temporal fenestra is low and long. The rear edge of each upper temporal fenestra was formed by the

squamosal bone The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...

, which was ornamented with small spines. The postcranial skeleton was very similar to that of ''Philydrosaurus''. The neck was short, and its vertebrae had low, swollen neural arches (upper portions). There were six or seven vertebrae in the neck, 16 in the torso, three in the hip, and more than 24 in the tail, which is incomplete. Vertebrae were longer than high, and the neural arches were completely fused to the centra (lower, spool-shaped portions), in contrast to the condition in neochoristoderes. Vertebrae at the base of the tail were fused to fan-shaped ribs which projected straight out. Further down the tail, these ribs became shorter and rectangular. The humerus was robust, with a roughly-textured knob above the large

entepicondyle The medial epicondyle of the humerus is an epicondyle of the humerus bone of the upper arm in humans. It is larger and more prominent than the lateral epicondyle and is directed slightly more posteriorly in the anatomical position. In birds, whe ...

. The plate-like

ischium The ischium () form ...

bone (which formed the rear lower part of the hip) had a distinct triangular prong along its rear edge.

Classification

A group of phylogenetic analyses recovered ''Coeruleodraco'' as one of the earliest diverging members of a group of small-bodied

Eurasian Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelago a ...

choristoderes. This group was likely a clade according to the parsimony analysis, in which case it was the

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

of the gharial-like neochoristoderes, although its

monophyly In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gr ...

is more uncertain under the bootstrap analysis. The only known genus of choristodere which diverged earlier than ''Coeruleodraco'' was '' Cteniogenys'', which acquired a few specialized features independently of other choristoders. In the analyses of Dong and colleagues (2020) it was recovered in a more basal position, outside the clade containing Neochoristodera + the newly named "Allochoristoderes", and in a

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...

with the newly named '' Heishanosaurus'':

References

Choristodera Prehistoric animals of China Fossil taxa described in 2019 Prehistoric reptile genera {{jurassic-reptile-stub