Ichthyosaurs, dolphin-like marine reptiles of the Mesozoic Era, are familiar creatures to most paleontologists. But not much had been uncovered about the intermediate stages in their evolutionary transition from terrestrial reptiles until the recent discovery of a fossilized skeleton in a limestone mine near Shanghai, China. A primitive member of the clade Ichthyosauriformes (which also contains all ichthyosaurs), it is the first known fossil of an ichthyosauriform which may have been amphibious – i.e. able to move on land and in water. An analysis of the specimen led by Professor Ryosuke Motani of the University of California, Davis, was published online in Nature on November 5, 2014.
Cartorynchus lenticarpus is the smallest known ichthyosauriform at about 8.4 inches long, not including a missing tail. Large, flexible front flippers may have allowed it to pull its proportionately short, lightweight body over the ground. Thickened ribs would help to weigh it down for easier movement through turbulent near-shore waters – a feature more pronounced in many primitive marine reptiles as well as modern sirenians (manatees and dugongs). Its snout is relatively short, with a mouth shaped for sucking small animals from seafloors, instead of catching fish like the long, toothy snouts of its relatives. Scientists were surprised to find that some features of its eye sockets suggest the poorest underwater vision of any ichthyosauriform.
Motani’s C. lenticarpus lived about 248 million years ago, during the Lower Triassic Period, when southern China was a humid tropical archipelago. All of the oldest known ichthyosauriforms have been found only in this region, suggesting that the clade originated there in the Lower Triassic. We have yet to determine what forces of competition, predation, or climate may have driven some of its terrestrial reptiles to invade the sea.
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