Cartorhynchus (meaning "shortened snout") is an extinct genus of early ichthyosauriform marine reptile that lived during the Early Triassic epoch, about 248 million years ago. The genus contains a single species, Cartorhynchus lenticarpus, named in 2014 by Ryosuke Motani and colleagues from a single nearly-complete skeleton found near Chaohu, Anhui Province, China. Along with its close relative Sclerocormus, Cartorhynchus was part of a diversification of marine reptiles that occurred suddenly (over about one million years) during the Spathian substage, soon after the devastating Permian-Triassic extinction event, but they were subsequently driven to extinction by volcanism and sea level changes by the Middle Triassic.
Measuring about long, Cartorhynchus was a small animal with a lizard-like body and a short torso; it probably swam in an eel-like manner at slow speeds. Its limbs bore extensive cartilage and could bend like flippers, which may have allowed it to walk on land. The most distinctive features of Cartorhynchus were its short, constricted snout, and its multiple rows of molar-like teeth which grew on the inside surface of its jaw bones. These teeth were not discovered until the specimen was subjected to CT scanning. Cartorhynchus likely preyed on hard-shelled invertebrates using suction feeding, although how it exactly used its inward-directed teeth is not yet known. It was one of up to five independent acquisitions of molar-like teeth among ichthyosauriforms.
Discovery and naming
In 2011, the only known specimen of Cartorhynchus was discovered in Bed 633 from the second level of the Majiashan Quarry near downtown Chaohu, Anhui Province, China; the rock strata in this quarry belong to the Upper Member of the Nanlinghu Formation. The specimen consists of a nearly-complete skeleton missing only part of the tail and some of the bones from the left part of the rear skull. The specimen's preservation likely resulted from it having been deposited in sediment right side down, thus leaving the left side exposed to the elements. It received a field number of MT-II, and later a specimen number of AGB 6257 at the Anhui Geological Museum.
In 2014, the specimen was described by Ryosuke Motani and colleagues in Nature as representing a new genus and species, Cartorhynchus lenticarpus. They derived the generic name Cartorhynchus from the Greek words (καρτός, "shortened") and (ῥύγχος, "snout"), and the specific name lenticarpus from the Latin words ("flexible") and ("wrist"). Both names refer to anatomical characteristics that it would have had in life. The specimen was thought to be toothless until an isolated tooth was discovered during further attempts to remove rock from between the closed jaws. Since the specimen was too fragile to expose the interior of the jaws, Jian-Dong Huang, Motani, and other colleagues subsequently scanned and rendered the specimen in 3D using micro-computed tomography (micro-CT), performed at the Yinghua Testing Company in Shanghai, China. In 2020, results from their follow-up work were published in Scientific Reports.
Description
At the time of its discovery, Cartorhynchus was the smallest-known member of the Ichthyosauriformes. The preserved specimen had a length of ; assuming that it had tail proportions comparable to close relatives, Motani and colleagues estimated a full body length of and a weight of . In 2021, Sander and colleagues produced a much lower weight estimate of .
Skull
Cartorhynchus had an unusually short and constricted snout, which only occupied half of the skull's length, and a deep jaw. The tip of the snout was only wide. Unlike most reptiles, its nasal bone reached the front of the snout. Due to its likewise elongated premaxilla, its bony nostrils were located relatively far back on the skull, and its frontal bone also lacked an expansion at its rear outer corner. All of these characteristics were shared with its close relative Sclerocormus. However, unlike the latter, the frontal bone did not contribute to the eye socket in Cartorhynchus; the prefrontal and postfrontal bones did not meet above the eye socket; and the location of the large hole for the pineal gland on the skull roof differed: it was at the contact between the frontal and parietal bones in Sclerocormus, but solely on the parietals in Cartorhynchus. Cartorhynchus also had a characteristically large hyoid bone.