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Corosaurus

History of study

The first specimen of Corosaurus was found in the summer of 1935, in Jackson Canyon in Natrona County near Goose Egg Ranch and the city of Casper, Wyoming. Geology student Don Allsen from the University of Wyoming discovered various pieces of the specimen spread among the rubble in a quarry dump in the Alcova Limestone. After the initial discovery, Allsen and Horace Thomas returned to the site and collected more material. Recovery of the specimen was hampered by it having been jumbled and broken up in the dump. This skeleton was the first vertebrate fossil to be found in the Alcova Limestone. The head of the State Geologic Survey of Wyoming, Samuel Knight, sent the specimen in November of the same year to paleontologist E.C. Case to study.

The bones of the specimen were preserved in limestone concretions within a sandy shale. This limestone was very hard, more so than the bones, and very dense as well. Because of this, it was difficult to remove the rock surrounding the bones. Traditional tools such as needles were found to be insufficient, so wheels made of silicon carbide were used to grind down the matrix instead. This specimen, cataloged as No. 51000 at the time but later renumbered UW 5485, is still the most complete specimen of Corosaurus. The skeleton is curled into a loop and twisted about 180°. In 1936, Case reported that it consisted of a skull, 58 vertebrae, ribs, gastralia, and portions of the forelimbs and shoulder girdle. However, in 1963, Rainer Zangerl reported additional large shoulder girdle elements Case did not mention, indicating that the specimen had likely been prepared further since Case's description. Zangerl also suspected that multiple individuals were represented in the material assigned to UW 5485, noting in particular the gastralia; in 1991 Glenn Storrs noted that some isolated elements cataloged with this specimen were duplicates of those in the main skeleton. Furthermore, due to the limestone blocks not all fitting together, Zangerl suspected that parts of the specimen had been lost, but Glenn Storrs demonstrated in 1991 that this was not the case.

In 1936, a paper describing the specimen was published by Case, who determined that it represented a new genus and species of nothosaur. This was the first time a North American nothosaur had been positively identified; while some vertebrae and ribs from the Hosselkus Limestone in California had previously been assigned to Nothosaurus, this specimen was reidentified as pertaining to the ichthyosaur Shastasaurus. Case named the new nothosaur Corosaurus alcovensis, which translates to "northwest-quarter reptile of Alcova", with UW 5485 as the holotype. Case had never worked with nothosaurs before, but subsequent researchers nevertheless considered Case's descriptions to be quite good.

Zangerl was interested in Corosaurus, and went looking for more specimens in 1948. This expedition found multiple specimens in Natrona County, and sent them to the Chicago Natural History Museum (now the Field Museum of Natural History). As with Case's specimen, preparing the fossils was difficult and very time-consuming. In 1963, he had a study on Corosaurus published, based on the holotype and the new specimen FMNH PR480, found near Freeland, Wyoming. This new specimen includes hip and hindlimb bones, not preserved in the holotype, as well as vertebrae from the middle of the torso to the tip of the tail. At least twelve more individuals, consisting of four partial skeletons and additional isolated remains, were also recovered, though they had not yet been prepared. FMNH PR480 had not been fully prepared either, though Zangerl still considered the available information important enough and the preparation difficult enough to publish preliminary research.

The skeleton of Corosaurus was still not completely known, so in the summer 1983, more specimens were sought around Casper. Scree from cliffs along Corral Creek on Milne Ranch near Muddy Mountain yielded most of the Corosaurus fossils found by this expedition, though except for two partial skeletons, all other specimens were fragmentary. After excavation, the specimens were brought to the Yale Peabody Museum. As with the two previous studies, preparation was still difficult. Since the bones were permineralized with calcite, they would dissolve in acid. Furthermore, the bones were too dense to be permeated with chemical protectants from the acid. The density of the rock was similar enough to that of the bones that hidden elements could not be viewed using X-rays. Therefore, despite being so painstaking, mechanical preparation had to be used to reveal the specimens, some of which had been damaged by previous attempts. Latex and epoxy were used to make casts when bones were only preserved as natural molds. Based on these new specimens and restudy of older ones, Corosaurus was reviewed by Storrs in a monograph published in 1991. At this point, the only unknown parts of Corosaurus were the palate, toes, and interclavicle (one of the shoulder bones).

An expedition during the summer of 1996 recovered another partial skeleton of Corosaurus from the north side of Muddy Mountain, in addition to fragments found throughout the general region of Casper. The partial skeleton was cataloged at the Field Museum as FMNH 2018. This time, chemical methods were used to make preparation easier. A solution of sodium salts (known as Waller solution) was used to remove iron from the rock, making it more easily dissolved. Formic acid was then used to remove the weakened matrix, with the specimens being subjected to repeated cycles of treatment with Waller solution and acid. To avoid damaging the bones, they were coated with acrylic resin. Additional preparation of the holotype and some of the other Field Museum specimens was also performed.

Description

Skull

The holotype is the only specimen that preserves the cranium. It is wide and flat, measuring about 13 centimeters (5.1 in) long and 7.5 centimeters (3.0 in) broad at its widest. The snout is short and is not constricted, with the external nares (openings for the nostrils) located located about halfway along its length. The external nares and the orbits (openings for the eyes) are partially angled upwards. Behind the orbits are larger paired openings known as supratemporal fenestrae. While higher, Case reconstructed the back part of the skull as too tall.

Each side of the mandible is long, low, and narrow, forming the mandibular symphysis where they meet in front at an acute angle. The symphysis is the sturdiest part of the mandible, and is rather short and scoop-like in shape. There is a ridge on the inner surface of each side of the mandible. In front of the jaw joint, there is a prominent coronoid process; behind the jaw joint, there is a large retroarticular process.

The teeth of Corosaurus are all the same shape, though they vary in size. In the upper jaw, the teeth at the front are splayed outwards, while those further back point downwards. There are large, fang-like teeth at the front of the maxillae. There are especially large teeth at the front of the lower jaw, splayed outwards and to the front, while those below the maxillae are smaller and upright. The teeth are pointed and recurved, with striations running from their tips to their bases. There are no cutting edges on the teeth. The tooth roots are large and set in sockets.

Postcranium

The vertebral column of Corosaurus consists of 18 cervical (neck), 23 dorsal (trunk), 3 sacral (hip), and likely about 40 caudal (tail) vertebrae, for a total of around at least 85. The largest neck vertebrae are those closest to the trunk, with size decreasing towards the skull. The vertebral bodies in the neck are about as tall as they are long. The second neck vertebra has a small triangular neural spine, while those of the other neck vertebrae are quadrangular. The cervical ribs each articulate with the vertebral bodies with two articular surfaces. The tips of the cervical ribs bear two prongs, one pointing forwards and the other pointing backwards. While fairly similar to those in the neck, the trunk vertebrae are larger than those in the other regions. The trunk vertebrae have robust sideways extensions known as transverse processes, about as long as the quadrangular neural spines are tall. The trunk vertebrae also posses an extra set of articulations, formed by protuberances and sockets known as zygosphenes and zygantra. The long, curved, unthickened ribs in the trunk have only one surface for articulation with the vertebrae.

The hip vertebrae resemble the trunk vertebrae and fit together firmly. In some individuals they are unfused, in others, there may be a limited amount of fusion. The sacral ribs, which articulate with the pelvis, are long and extend outwards and downwards. The tail vertebrae have constricted, cylindrical bodies. Those close to the hips are not very elongated, though they become increasingly elongated towards the tip of the tail. From the base to the tip of the tail, the shape of the neural spines changes from quadrangular to round and reclined to absent. The first 14 tail vertebrae bear caudal ribs, which resemble longer, unexpanded sacral ribs. The caudal ribs are either fused or very firmly attached to the vertebrae. V-shaped bones known as chevrons articulate with the undersides of the tail vertebrae, and first occur beneath the fifth or seventh caudal vertebrae, depending on the specimen.

A series of dense, interlocking bones known as gastralia are positioned along the underside of the trunk. They consist of V-shaped elements along the midline with rod-like elements to the sides. Two sets of gastralia seem to be present per the length of a vertebra. Pronged ends may be present on the midline elements, which Storrs interpreted as a congenital deformity.

The clavicles (collarbones) of Corosaurus each consist of two processes oriented roughly orthogonally to each other, forming an L-shape. The outer process is thin and flat while the inner process is short and robust, bearing a groove for the articulation of the interclavicle. The interclavicle supported the clavicles, which lacked firm contact with each other and the scapulae. As preserved, the interclavicle appears to be small and triangular, concave in front and pointed at the back. The shape of the clavicles suggest that there may have been long projections from the sides of the interclavicle. The remainder of the shoulder girdle consists of the sturdy scapulae and coracoids. The front edge of the scapula has an undualting profile, while the scapular blade is slender and backswept. There is a notch between the blade and the thick, rough portion of the scapula involved in the shoudler joint. The coracoids are a pair large, subrectangular, plate-like bones, with weakly concave front and back edges. There are no notches on the coracoids, which are thickest where they meet along the midline. The shoulder joint is somewhat inclined forwards. The coracoids have dense bone structure and in life would have had cartilaginous extensions.

"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.