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Corosaurus

The initial specimen of Corosaurus was found in the summer 1935 by University of Wyoming geology student Don Allsen. The specimen was preserved in an outcrop of the Alcova Limestone, and was the first vertebrate fossil to be recorded from that unit. It was found near Casper, Wyoming, in Jackson Canyon of Natrona County. After the initial excavation, more of the specimen was collected by Allsen and Horace Thomas. Extracting the specimen was complicated, as it was found in a quarry dump and fragmented. To expose the specimen from the hard calcareous concretions that it was embedded in, the matrix was ground down using a silicon carbide bit.

Knight asked paleontologist E.C. Case to determine what the specimen was, and sent it to him so he could study it. In a 1936 publication, Case determined that it represented a new genus and species of nothosaur, and named it Corosaurus alcovensis, with Corus meaning 'northwest quarter'.

"Callawayia" wolonggangense

History of study

There are two known specimens of "Callawyia" wolonggangense, the holotype SPCV 10306 and the additional assigned specimen SPCV 10305. The holotype is the more complete of the two, being a partial articulated skeleton consisting of a three-dimensionally preserved skull as well as trunk, shoulder, and forelimb bones. SPCV 10305 is a crushed skull. Both specimens are preserved in micritic limestone from the lower portion of the Xiaowa Formation, dating to the Carnian age of the Late Triassic. These fossils were discovered near the village of Xinpu in Guanling County, Guizhou Province, China. The species was named in 2007 by Chen Xiao-hong, Cheng Long, and Martin Sander. They found it to be quite similar to the Canadian ichthyosaur Callawayia neoscapularis, and therefore assigned it to that genus, thereby extending both the geographic and temporal range of the genus. However, there were differences between C. neoscapularis and the new Chinese material, so the authors named a new species, C. wolonggangense, to contain these new fossils. The name of the species refers to a hill, Wolonggang, rich in well-preserved fossils.

The assignment of this species to the genus Callawayia was contested by Michael Masich in 2010. Furthermore, he noted that the species was differentiated by the configuration of bones in the skull, which can be influenced by the quality of a specimen's preservation and how well it was extracted from the surrounding rock. He suspected that "C." wolonggangense represented the same animal as Guizhouichthyosaurus tangae, another ichthyosaur from the same place and time, but refrained from synonymizing them, provisionally reassigning "C." wolonggangense to Guizhouichthyosaurus wolonggangense until it could be studied in more detail. In a 2012 study, Shang Qing-Hua and coauthors restudied the anatomy of the skull of Guizhouichthyosaurus tangae, which they assigned to the geuns Shastasaurus as S. tangae, an assignment not accepted by other authors. They considered "Callawayia" wolonggangense to be quite similar to G. tangae and also proposed that the two species were synonymous. In a subsequent paper, Shang and Li Chun reaffirmed this synonymy, finding "C." wolonggangense to fall within the range of variation of G. tangae.

However, "C." wolonggangense has also been considered a distinct species. Cheng Ji and coauthors conducted a phylogenetic study of ichthyosaurs in 2015, including "C." wolonggangense in their analysis. Their work further rejected the assignment of this species to Callawayia, failing to find unique features linking it to C. neoscapularis. However, they also found it to not be assignable to Guizhouichthyosaurus either.

Description

"Callawayia" wolonggangense is a medium-sized ichthyosaur, based on the size of its lower jaw, measuring about 70 centimetres (2.3 ft) long. More than two thirds of length of the skull in "C." wolonggangense is formed by a long snout. The openings for the nostrils are narrow, with a furrow extending forwards from each. The orbits, which housed the eyes in life, are large and fairly round openings, and the portion of the skull behind them is narrow. The smaller paired openings at the top of the skull, the supratemporal fenestrae, are preceeded by depressions extending from their front edges. The jaws are lined with conical teeth of varying sizes, with striated crowns and grooved roots implanted into sockets.

The configuration of the skull bones is preserved in the specimens, but it is uncertain how accurately. The toothbearing bones known as premaxillae form most of the snout. The other pair of toothbearing bones in the upper jaw, the maxillae, each bear a projection that rises to contact the corresponding prefrontal, bones which form part of the orbital rim. The prefrontals are blocked from touching a pair of skull roof bones known as frontals by two other pairs of bones, the nasals and postfrontals. The nasals are located on top of the snout and are quite extensive, reaching far back on the skull roof. The frontals form both the front edges of the supratemporal fenestrae and the terraces in front of them. The remainder of the rims of the supratemporal fenestrae are formed by bones known as parietals, supratemporals, and postfrontals; with the contact of the latter two preventing the postorbitals and squamosals from reaching the rim. Each parietal was described by Chen and coauthors as bearing a prominent ridge, behind which there is a shelf-like surface. However, Ji and coauthors stated in 2015 that this feature was not present in "C." wolonggangense.

The trunk of the holotype of "C." wolonggangense is about 1.5 metres (4.9 ft) long. Exactly how many vertebrae the trunk contained is not certain, but Chen and coauthors estimated that there were between 46 and 65. The vertebral bodies are shaped like disks. Chen and coauthors described the scapulae as long, narrow bones; however, Ji and coauthors noted that their shape was in part due to them being broken. The coracoids, another pair of shoulder bones involved in the shoulder joint, have concave front and back edges, resembling fans in shape. The humeri are short, wide bones with a strong ridge on their upper sides. Each foreflipper contains four digits. The front edges of the bones in the frontmost digit bear notches.

Classification

While recognized as a shastasaurid-like ichthyosaur, the exact placement of "C." wolonggangense within Ichthyosauria is uncertain. Chen and coauthors assigned "C." wolonggangense to Callawayia in their initial description. They noted that Shastasaurus and Callawayia had some significant differences from each other, and cast doubt on the notion that Callawayia could be classified in the group Shastasauridae. They tentatively assigned the genus to Shastasauria on account of the shape of its humerus, but argued that further study would be necessary to determine how Callawayia was related to other ichthyosaurs, and refrained from assigning it to a family. Maisch, who in 2010 assigned "C." wolonggangense to Guizhouichthyosaurus, classified it in Shonisauridae, alonside Shonisaurus and Himalayasaurus.

In their 2015 phylogenetic analysis, Ji and coauthors found "C." wolonggangense to belong to Shastasauridae, in turn part of a larger group called Merriamosauria. Guizhouichthyosaurus was also found to belong to this group, though Callawayia was not. A modified version of this analysis run by Gabriele Bindellini and coauthors in 2021, who found different configurations for the classification of the shastasaurids. Some of their analyses found shastasaurids to form a natural group like that of Ji an colleagues; however, the relationships within this group were uncertain. "C. wolonggangense was sometimes found to form part of a small group in Shastasauridae along with Guizhouichthyosaurus and Besanosaurus, though this was not always the case. Furthermore, other analyses run by these authors found shastasaurids to not be a natural grouping, instead being a series of branches each more closely related to other merriamosaurs.