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 ...
Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
Guizhou Province
Guizhou (; formerly Kweichow) is a landlocked province in the southwest region of the People's Republic of China. Its capital and largest city is Guiyang, in the center of the province. Guizhou borders the autonomous region of Guangxi to t ...
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 preserved ...
specimen was discovered in 1957. They are among the most common sauropterygian fossils recovered and are often found as nearly complete, articulated skeletons, making them popular among collectors. ''Keichousaurus'', and the pachypleurosaur family broadly, are sometimes classified within Nothosauroidea, but are otherwise listed as a separate, more primitive lineage within Sauropterygia.
Description
''Keichousaurus'', like all sauropterygians, was highly adapted to the aquatic environment. Most specimens had small body, males
sexually mature
Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural defini ...
with snout-vent length (SVL), and in females by SVL. Mean SVL for mature males is approximately SVL, and for mature females, at most SVL. It had both long necks and long tails, with elongated, five-toed feet. The pointed head and sharp teeth in this genus also indicate that they were
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% ...
-eaters. Some recovered specimens feature an especially developed
ulna
The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
suggesting they may have spent some time on land or in marshes.
In addition fossil evidence suggest also a pair of fossilized pregnant marine reptiles called ''Keichousaurus hui'', show they had a mobile pelvis to give birth to live young rather than laying eggs.
Morphology
''Keichousaurus hui'' was found in 1958 in
Guizhou
Guizhou (; formerly Kweichow) is a landlocked province in the southwest region of the People's Republic of China. Its capital and largest city is Guiyang, in the center of the province. Guizhou borders the autonomous region of Guangxi to t ...
, China by palaeontologist
Hu Chengzhi
Hu Chengzhi (; 23 August 1917 – 12 April 2018) was a Chinese paleontologist and paleoanthropologist. He made the plaster casts of the Peking Man skull in the 1930s, and identified the Yuanmou Man (''Homo erectus yuanmouensis'') based on fossil ...
. This fossil is distinguished by its broad ulna which makes it unlike other European genera. The broad ulna increased the surface area of the forelimbs, making it more effective in locomotion. ''Keichousaurus'' shows many characteristics of its family Pachypleurosauridae such as its short snout and elongated temporal openings. ''Keichousaurus'' also had a long serpentine neck with a relatively small head and long tail. The anterior caudal vertebrae possess lateral transverse processes. The morphology of ''Keichousaurus'' is most like that of ''
Dactylosaurus
''Dactylosaurus'' is a genus of nothosaur in the family Pachypleurosauridae. Along with '' Anarosaurus'', ''Dactylosaurus'' was one of the earliest known pachypleurosaurs to come from Europe.allometric growth.
Locomotion
The locomotion of ''Keichousaurus'' probably resembled (in part) the "underwater flight" that plesiosaurs employed. The flattened forelimbs would likely have acted as hydrofoils. The hindlimbs show less specialization, and may therefore have acted as stabilizers and control surfaces, such as is seen in extant sea turtles. The intermediate nature of the limb morphology implies that there was also, to some extent, the kind of 'crawling through the water' seen in small freshwater turtles. The powerfully built pectoral girdle allowed for the attachment of strong muscles, but their location beneath the shoulder favours the underwater flight model. Despite the specialization of the limbs, the tail also shows adaptations to an aquatic existence. Lateral transverse processes of the anterior caudal vertebrae show that powerful muscles enabled the tail to beat or at least undulate from side to side. This would imply some distal lateral compression, but this is not recorded in soft-tissue preservation. The long neck was primarily concerned with prey-capture. The combination of powerful limbs and tail would have made ''K. hui'' an adept and maneuverable predator.
Reproduction
Although there is no direct evidence, ''Keichosaurus'' was potentially ovoviviparous (eggs form and hatch within uterus). Fossil ''Keichousaurus'' display a simplified elbow joint and a lack of ossification in the olecranon process of the ulna. This would make crawling up the beach to lay eggs awkward. Specimens at different developmental stages, found in the same type of sediment at the same locality, also support an ovoviviparous reproduction model. However, fossils have been found of female ''Keichousaurus'' with fetuses within the lower portion of the thoracic cavity. Their position implies that they are not victims of
cannibalism
Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is well documented, b ...