Keichosaurus hui

Keichosaurus hui is a small diapsid marine reptile that lived during the Middle Triassic , approximately 245–240 million years ago, in what is now southern China. It belongs to the group of primitive sauropterygians within the Pachypleurosauridae family and is one of the best-known marine reptiles thanks to the abundance and exceptional preservation of its fossils, primarily from the Guanling Formation (Guizhou Province). Recent studies consider it a key taxon for understanding the early stages of marine reptile radiation following the Permian–Triassic mass extinction.

The skeleton of Keichosaurus hui reveals a small, slender animal, with a total length ranging from 20 to 35 cm , depending on sex and developmental stage. One of the most striking features, confirmed by modern statistical analyses and reviews of large fossil samples, is the clear sexual dimorphism . Males have proportionally longer and more robust forelimbs, with a more developed humerus, while females have somewhat larger bodies and shorter limbs, which is interpreted as an adaptation related to reproduction and swimming.

The skull of Keichosaurus was small, elongated, and lightweight, with a narrow snout and jaws equipped with fine, conical, and slightly curved teeth , ideal for capturing small fish and crustaceans. Recent studies of dental microwear and cranial morphology support a primarily piscivorous diet, with a fast and precise feeding style. The relatively large eye sockets suggest good eyesight, important for hunting in shallow water.

The vertebral column was composed of numerous dorsal and caudal vertebrae, with a long, laterally compressed tail that served as the primary means of propulsion. Current research indicates that Keichosaurus swam primarily by lateral undulation of its body and tail , similar to the pattern observed in primitive aquatic reptiles, while its limbs played a secondary role in maneuverability.

The limbs were transformed into short but functional flippers. The hand and foot bones show moderate elongation of the fingers (incipient hyperphalangy), an early adaptation to aquatic life that anticipates more extreme traits observed in later plesiosaurs. The appendicular skeleton, however, retains a relatively primitive structure, indicating that Keichosaurus could still move clumsily on land or in shallow water.

Recent histological studies of bone have revealed relatively rapid growth , consistent with an active metabolism, although not comparable to that of more derived marine reptiles. Furthermore, the excellent preservation of multiple individuals at the same stratigraphic level has led to the interpretation that Keichosaurus hui lived in calm coastal marine environments , possibly lagoons or inland seas, and that it may have exhibited some degree of gregarious behavior.

From an evolutionary perspective, Keichosaurus hui occupies a fundamental position in current phylogenetic analyses, as it exhibits a combination of primitive terrestrial traits and early aquatic adaptations. Recent work confirms it as one of the best examples of how diapsid reptiles rapidly adapted to the marine environment after the Permian crisis, making it a key taxon for understanding the origin and early diversification of Mesozoic marine reptiles.

Approximate measurements of the Keichosaurus:

• Full 1:1 Scale
• Length 167 mm
• Height 190 mm
• Width 96 mm
• Snout-tail length 240 mm