Cartorhynchus

Cartorhynchus (meaning "shortened snout") was an extinct group of early marine reptiles that lived during the Early Triassic epoch, about 248 million years ago. It was discovered from a nearly-complete skeleton found near Chaohu, Anhui Province, China. This small creature measured about 40 centimetres (16 inches) long and had a lizard-like body with a short torso.

Cartorhynchus probably swam slowly in an eel-like manner and could walk on land using its flipper-like limbs. Its most special features were its short, narrow snout and many rows of molar-like teeth inside its jaws. Scientists found these teeth only after using special scanning tools. These teeth suggest that Cartorhynchus may have eaten hard-shelled sea creatures using suction, but exactly how it used its teeth is still a mystery. It was one of several groups of ancient marine reptiles to develop such teeth independently.

Discovery and naming

In 2011, scientists found the only known example of Cartorhynchus near Chaohu in Anhui Province, China. This area had layers of rock from a long time ago. The fossil they found was almost complete, missing just a small part of the tail and some bones from the back of the skull.

In 2014, researchers named this new species Cartorhynchus lenticarpus. They chose the name to describe special features it had. Later, they used special scanning tools to look inside the fossil without damaging it.

Description

When it was found, Cartorhynchus was the smallest known member of the Ichthyosauriformes. The fossil that scientists studied was 21.4 centimetres (8 inches) long. By looking at its close relatives, scientists think the full length of Cartorhynchus might have been about 40 centimetres (1 foot 4 inches).

Cartorhynchus had a very short snout, making up only half of its skull. Its nose bone reached the front of its snout, unlike most reptiles. It also had a deep jaw and a large bone under its tongue called a hyoid bone.

At first, scientists thought Cartorhynchus had no teeth. But later, they found small, rounded teeth hidden inside its jaws. These teeth were arranged in three rows, with the outer row having the most and largest teeth.

Cartorhynchus had 5 neck bones and 26 back bones, totaling 31 bones before its hip. Its ribcage was strong and deepest near its shoulder. Its limbs were not fully hardened with bone, suggesting they had a lot of flexible cartilage, making them flipper-like. The front flippers curved backward, and the back flippers curved forward.

Classification

Because we don't have many complete fossils, it was hard to understand where ichthyosaurs came from. For a long time, people thought ichthyosaurs just appeared suddenly in the Middle Triassic with strong swimming skills. But the discovery of Cartorhynchus and its relative Sclerocormus helped fill in some of these gaps.

Scientists found that Cartorhynchus and Sclerocormus were very close to each other and formed a group called Nasorostra. They were also closely related to ichthyosaurs. This helped scientists understand that these ancient sea reptiles were part of a bigger group called Ichthyosauriformes, which also included another group called Hupehsuchia. Even when scientists left out features related to swimming, these relationships still held true, showing they were strong links.

Evolutionary history

The appearance of these ancient sea reptiles happened as marine life was recovering after a big event that wiped out many species, called the Permian-Triassic extinction. It used to be thought that marine life took a long time to recover, but discoveries like Cartorhynchus show that marine reptiles actually appeared quickly in the Early Triassic.

These ancient sea reptiles appeared during a time called the Spathian substage and quickly developed many different ways to live in the ocean. They had different types of teeth, which suggests they ate hard-shelled animals. Over time, their teeth changed, which may have helped them adapt to new foods as the ocean changed.

However, these early sea reptiles went extinct at the end of the Early Triassic. Changes in sea levels and volcanic activity made the oceans harder to live in. Other groups of sea reptiles then began to diversify and thrive.

Palaeobiology

Diet

Scientists in 2014 thought that Cartorhynchus may have caught food by using suction with its narrow snout. They based this idea on the strength of its hyoid bone, which would have supported its tongue. Later, in 2020, other scientists found that Cartorhynchus had teeth suited for eating hard-shelled animals. They believed it could still use suction to catch small prey. The teeth showed wear patterns that puzzled scientists, and they thought the jaw might have changed shape after the animal died.

Limbs and locomotion

Cartorhynchus had limbs that were not fully hardened, even though its skull and spine were well-formed. Scientists suggested it might have been an adult because many early marine reptiles kept soft limbs into adulthood. They thought its large, flipper-like front limbs could have helped it move on land, similar to young sea turtles. Its body was light, and its short trunk and strong ribs would have helped it stay stable in shallow water.

A study in 2019 used computer models to guess how much energy Cartorhynchus used to swim. Because of its body shape, it likely swam in an eel-like way, which uses more energy than the swimming style of later, larger ichthyosaurs. However, Cartorhynchus probably swam slowly, so it did not need as much energy. Larger ichthyosaurs had bodies that let them swim more efficiently, but their size also added to their energy needs.

Palaeoecology

The layer where Cartorhynchus was found is made of grey clay-bearing limestone, located in the Majiashan Quarry. This layer is part of the Nanlinghu Formation and dates back to about 248.41 million years ago.

Along with Cartorhynchus, other ancient sea reptiles like Sclerocormus and Chaohusaurus were also found in this area. Fish such as Chaohuperleidus and Saurichthys lived there too. Small ammonites, bivalves, and an arthropod called Ankitokazocaris may have been possible food for Cartorhynchus.