Researchers report that they've found the missing link between an ancient aquatic predator and its ancestors on land. Ichthyosaurs, the dolphin-like reptiles that lived in the sea during the time of the dinosaurs, evolved from terrestrial creatures that made their way back into the water over time.
But the fossil record for the lineage has been spotty, without a clear link between land-based reptiles and the aquatic ichthyosaurs scientists know came after. Now, researchers report in Nature that they've found that link — an amphibious ancestor of the swimming ichthyosaurs named Cartorhynchus lenticarpus.
"Many creationists have tried to portray ichthyosaurs as being contrary to evolution," said lead author Ryosuke Motani, a professor of earth and planetary sciences at the University of California Davis. "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?"
Now the gap has been filled, he said.
Motani and his colleagues, who include researchers from Peking University, Anhui Geological Museum, the Chinese Academy of Science, University of Milan and the Field Museum in Chicago, found the fossil in China's Anhui Province in 2011. The creature is about a foot and a half long and lived 248 million years ago
"Initially I was really puzzled by this fossil," Motani said. "I could tell it was related [to ichthyosaurs], but I didn't know how to place it. It took me about a year before I was sure I had no doubts."
Intense analysis put it smack dab in the middle of the ichthyosaur family. But unlike previously discovered fossils in the lineage, this one wasn't perfectly suited to life in the ocean. The key differences between this specimen and previously known ichthyosaurs set it up as the perfect amphibious intermediary.
Ocean-bound ichthyosaurs had very long snouts (leading to their frequent comparison to modern dolphins) that were made for capturing fish and squid. This new animal had a shorter snout — more like a land-based reptile. It also had large flippers and flexible wrists, which would have allowed it to flop around on land like a seal.
One of the most important differences between this new ichthyosaur and its supposed descendants comes down to being big boned: When other vertebrates have evolved from land to sea living, they've gone through stages where they're amphibious and heavy. Their thick bones probably allowed them to fight the power of strong coastal waves and stay grounded in shallow waters. Sure enough, this new fossil has much thicker bones than previously examined ichthyosaurs.
The animal lived about 4 million years after the worst mass extinction in our planet's history — so Motani and his colleagues believe the creature gives insight into how long it took the ecosystem to recover.
"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 said. "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."
This single fossil hasn't revealed all of the ichthyosaurs' secrets. Motani hopes to find the preceding evolutionary ancestor next — one that was also amphibious, but spent slightly more of its time on land. "We're looking for that one now," Motani said.