Sclerocormus
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''Sclerocormus'' is an
extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of
ichthyosauriform The Ichthyosauriformes are a group of marine reptiles, belonging to the Ichthyosauromorpha, that lived during the Mesozoic. The stem clade Ichthyosauriformes was in 2014 defined by Ryosuke Motani and colleagues as the group consisting of all ic ...
from the early
Triassic period The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is the ...
. The fossil was discovered in the central Anhui Province,
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
. It is currently only known from one specimen, however the
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
is mostly complete and as such further increases the understanding of the early evolution of
ichthyosaurs Ichthyosauria is an taxonomy (biology), order of large extinction, extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides. Ichthyosaurians thrived during much of ...
.


Description

''Sclerocormus'' is much larger than its closest relative ''
Cartorhynchus ''Cartorhynchus'' (meaning "shortened snout") is an extinct genus of basal (phylogenetics), early ichthyosauriformes, ichthyosauriform marine reptile that lived during the Early Triassic epoch (geology), epoch, about 248 million years ago. The g ...
'', with a total body length of and body mass of more than . Its proportions were unusual amongst basal ichthyosauriformes, with a short, heavily built trunk, a very long tail over 92 cm long (58% of the total body length), and a small skull with a short, narrow snout and toothless jaws. Like ''Cartorhynchus'', the skull of ''Sclerocormus'' is wide, with a short,
edentulous Toothlessness or edentulism is the condition of having no teeth. In organisms that naturally have teeth, it is the result of tooth loss. Organisms that never possessed teeth can also be described as edentulous. Examples are the members of the fo ...
snout much narrower than the skull roof. The skull is unusually short at only 6.25% of its body length, compared to 12% in ''
Chaohusaurus ''Chaohusaurus'' is an extinct genus of basal ichthyosauriform, depending on definition possibly ichthyosaur, from the Early Triassic of Chaohu and Yuanan, China. Discovery and naming The type species ''Chaohusaurus geishanensis'' was name ...
'' and 15% in '' Hupehsuchus''. Unusually, the
nasals In phonetics, a nasal, also called a nasal occlusive or nasal stop in contrast with an oral stop or nasalized consonant, is an occlusive consonant produced with a lowered velum, allowing air to escape freely through the nose. The vast majorit ...
of ''Sclerocormus'' extend to the tip of the snout, a trait which it shares with ''Cartorhynchus''. The snout is very short, only 30% of the total skull length. By contrast, the orbits are very large, occupying over a third of the skull length. The upper temporal fenestrae are large, possibly indicating a strong bite force despite its lack of teeth. The trunk is short and heavily built, with broad and flattened ribs. The ribcage is deepest at the shoulders and gradually becomes shallower, forming a straight, constant slope, similar to ''Cartorhynchus'' but in contrast to the more rounded underside seen in other
ichthyosauromorphs The Ichthyosauromorpha are an extinct clade of Mesozoic marine reptiles consisting of the Ichthyosauriformes and the Hupehsuchia. The node clade Ichthyosauromorpha was first defined by Ryosuke Motani ''et al.'' in 2014 as the group consisting ...
. An extensive gastral rib basket runs along the underside in two parallel series on each side. Particularly, the inner of the two series are composed of flat, overlapping triangular pieces of bone that resemble the condition in hupehsuchians. The cervical and dorsal vertebrae bear tall, vertical neural spines that are broadened so as to leave little space between each spine. This resembles the condition in Hupehsuchia, but is unlike the well-spaced, posteriorly inclined spines of basal
ichthyopterygia Ichthyopterygia ("fish flippers") was a designation introduced by Richard Owen, Sir Richard Owen in 1840 to designate the Jurassic ichthyosaurs that were known at the time, but the term is now used more often for both true Ichthyosauria and their ...
ns. The caudal neural spines in contrast are lower than they are wide and have rounded tips. The tail itself is very long and slender, composed of at least 67 caudal vertebrae, and does not appear to have had a fluke. The haemal arches possess a unique morphology, where only the ninth pair and onwards are fused distally, however unlike other
diapsids Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The earliest traditionally identified diapsids, the araeosc ...
they are not V- or Y-shaped in cranial view, but U-shaped. Small dermal ossicles were reported from the cervical region, ranging in size between 2–5 mm, and curiously resemble the pelvic ossicles of saurosphargids. There are no ossicles present over the neural spines, unlike in hupehsuchians.


Discovery and naming

The full
binomial name In taxonomy, binomial nomenclature ("two-term naming system"), also called binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, altho ...
, ''Sclerocormus parviceps'', comes from the
Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...
words for 'stiff trunk' (σκληρός, ''skleros'' and κορμός, ''kormos'') and the
Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
words for 'small skull' (''parvus'' and ''caput'').


Classification

In the phylogenetic analysis performed by Jiang ''et al.'' 2016, ''Sclerocormus'' was found to be a basal ichthyosaurform and as a
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 ...
to ''Cartorhynchus''. Jiang ''et al.'' erected the new clade Nasorostra ('nose beak') for these two taxa, itself a sister taxon to Ichthyopterygia. As with other recent analyses, Ichthyosauriformes and Hupehsuchia are recognised as sister clades in Ichthyosauromorpha. As the holotype of ''Cartorhynchus'' was potentially not fully mature, it was possible ''Sclerocormus'' simply represented a large or mature ''Cartorhynchus''. However, it was established this was not the case, as there were significant anatomical differences between the two genera, including different presacral vertebral counts and the form of the gastralia, that could distinguish between the two taxa. This cladogram represents a simplified form of the phylogeny from Jiang ''et al''. 2016:


Palaeobiology

Due to its heavy build and body shape, ''Sclerocormus'' likely inhabited shallow waters, as with ''Cartorhynchus'', and was probably a slow swimmer. The narrow, toothless snout indicates ''Sclerocormus'' was likely a suction feeder, using its snout to generate syringe-like pressure concentration to suck in soft-bodied prey. Due to the small size of its jaws and head relative to its body, ''Sclerocormus'' would have been limited to feeding on prey much smaller than itself.


References

{{Taxonbar, from=Q24228715 Ichthyosauriformes Early Triassic reptiles of Asia Paleontology in Anhui Fossil taxa described in 2016