Cartorhynchus lenticarpus

Cartorhynchus lenticarpus is an extinct species of primitive marine reptile belonging to the Ichthyosauriformes clade, recognized as one of the most basal representatives of this group that would later give rise to the classic ichthyosaurs of the Mesozoic. Its only known specimen was formally described by Ryosuke Motani and colleagues in 2014 from a nearly complete skeleton found in the Nanlinghu Formation , near Chaohu, Anhui Province (China) , and dates to the Early Triassic, approximately 248 million years ago (Spathian) , shortly after the Permian-Triassic extinction event.

The genus name Cartorhynchus comes from Greek and means “shortened snout”, while the specific name lenticarpus derives from Latin for “flexible wrist”, referring to the morphology of its limbs.

General morphology and size. Cartorhynchus is notable for its small size: the preserved specimen measures 21.4 centimeters in trunk length , but assuming comparable tail proportions to those of other basal ichthyosauriforms, the estimated total length of the animal would be approximately 40 centimeters . This makes it one of the smallest known ichthyosauriforms.

The body was relatively short with a robust skull and an unusually short, narrow snout that occupied only about half the length of the skull. The eye sockets were large, indicating significant visual sensory importance, and the limbs already showed transformation into flipper-like or fin-like forms, although with flexible joints suggesting a dual function.

Dentition and feeding. More recent studies using computed tomography scans have revealed that Cartorhynchus possessed complex internal dentition with multiple rows of molariform teeth , which was hidden from view in the original fossil and was not evident in the initial preparation of the material. This robust, rounded dentition is indicative of adaptation to consuming hard-shelled prey (durofauna), although the exact function of the tooth rows and how they were used in feeding is still being investigated.

Locomotor adaptations and lifestyle. The combination of anatomical features suggests that Cartorhynchus had a lifestyle intermediate between terrestrial and aquatic . Its limbs, with extensive cartilage and flexible wrists, likely allowed it to move both in water and, to a limited extent, on firm surfaces , analogous to the locomotion of modern seals, leading to its description as a possible "amphibious ichthyosaur." Its less specialized body morphology for swimming compared to later ichthyosaurs indicates that it swam slowly, probably using undulating movements of its body and tail, rather than by active propulsion with highly efficient flippers.

Habitat and paleoenvironmental context. The only known fossil of Cartorhynchus comes from shallow marine sediments deposited in a tropical coastal environment within what was a warm, humid archipelago in southern China during the Early Triassic. This environment would have included transitions between intertidal zones, beaches, and shallow marine platforms, providing an ecological gradient where partially aquatic life forms could exploit both marine and marginal resources. The region appears to have been an early center of diversification for ichthyosauromorphs, with other basal forms such as Sclerocormus parviceps found in the same deposits.

Evolutionary significance. Cartorhynchus lenticarpus occupies a key position in the fossil record for understanding the initial steps in the transition from terrestrial reptiles to fully adapted marine forms. Its intermediate characteristics provide morphological evidence of how ichthyosaurs may have initiated the invasion of the aquatic environment after the devastating mass extinction at the end of the Permian, thus marking one of the first diversified marine tetrapod lineages after that event.

Approximate measurements of Cartorhynchus:

• Full 1:2 Scale
• Length 125 mm
• Height 28 mm
• Width 80 mm
• Snout-tail length 200 mm

• Full 1:1 Scale
• Length 250 mm
• Height 56 ​​mm
• Width 160 mm
• Snout-tail length 400 mm