Researchers have discovered the fossils of an ancient amphibious reptile that might just be the missing link between ichthyosaurs and their landlubber ancestors. Various features of this new proto-ichthyosaur—including a short snout, big seal-like flippers, and a heavy build—suggest that they were comfortable on land and in the sea. The findings were published in Nature this week.
Ichthyosaurs (or “fish lizards”) were exclusively aquatic reptiles that were shaped like dolphins, complete with flippers for limbs and elongated snouts. Reptiles evolved to live on land, but some groups returned to the oceans later on, Science explains. Such as the ichthyosaurs, who appeared in the fossil record inexplicably—without any intermediate forms to explain their transition back to the sea. "Many creationists have tried to portray ichthyosaurs as being contrary to evolution," Ryosuke Motani from the University of California, Davis, tells The Washington Post. "We knew based on their bone structure that they were reptiles, and that their ancestors lived on land at some time, but they were fully adapted to life in the water. So creationists would say, well, they couldn't have evolved from those reptiles, because where's the link?"
Well, now that gap’s being filled by a "primitive ichthyopterygian" named Cartorhynchus lenticarpus. In 2011, Motani and an international team excavated the 248-million-year-old fossil from a Lower Triassic formation at Chaohu, Anhui Province, China. The animal was estimated to be 40 centimeters long and weighed only two kilograms, making it the smallest ichthyosaur-like creature known. The small bones in its forelimbs had ample space in between, suggesting that they were actually large, cartilaginous flippers—not legs—which enabled limited locomotion on land. Its bendy flippers and flexible wrists mean Cartorhynchus likely moved on land like a sea lion.
The genus, Cartorhynchus, means “shortened snout” in Greek, and the species name, lenticarpus, means “flexible wrist” in Latin. The fossil was nearly complete, with only its tail missing:
It certainly seems that Cartorhynchus was on its way to becoming an aquatic reptile, though its head, body trunk, and snort snout (probably for suction feeding) aren’t fully streamlined and specialized for preying on fish. Also, its skeleton was heavily built. While these hefty bones helped it stand up to rough waves in coastal waters, later ichthyosaurs were much lighter. Taken together, Cartorhynchus is the earliest amphibious reptile known.
This fossils date back to a few million years after the Great Dying, otherwise known as the Permian-Triassic mass extinction event, which wiped out at least half of all marine families. With all these newly-vacated niches to be filled, terrestrial reptiles were probably experimenting with life at sea again. And Cartorhynchus was just one species in that gradual evolution. "This animal probably had a happy life. It was in the tropics, and it was probably a bottom feeder that fed on soft-bodied things like squid and animals like shrimp," Motani tells The Washington Post. "And for a predator like that to exist, there has to be plenty of prey. This was probably one of the first predators to appear after that extinction."
Images: Stefano Broccoli (Milano) created this image (top), Dr. Ryosuke Motani took the photo and created this image, the specimen is deposited in the Anhui Geological Museum (middle)