Some early ichthyosaurs used rounded, pebble-like teeth to crush the shells of snails and clam-like bivalves, according to new research.

Roughly 252 million years ago, the end-Permian Mass extinction wiped out 90 percent of the planet's marine species and 70 percent of terrestrial species. Ichthyosaurs were one of the few benefactors. The marine tetrapods rapidly diversified, taking advantage of a variety of vacated environs.

The fossil record suggests ichthyosaurs quickly established themselves as the sea's top predators, but the details of their rapid radiation aren't well understood.

The study, published Friday in the journal Scientific Reports, offers insights into the strategies ichthyosaurs used to adapt to different ecological niches.

CT scans of the remains of one of the earliest ichthyosaurs revealed the presence of small, rounded, pebble-like teeth inside the reptile's short snout. The discovery suggests the first ichthyosaurs preyed on shellfish.

"We don't know exactly the ancestry of ichthyosaurs. They're reptiles, and they're probably archosaurs -- that is to say, they're more closely related to crocodiles and dinosaurs and birds than they are to lizards and snakes -- but even that isn't 100 percent," study co-author Olivier Rieppel, a paleontologist at Chicago's Field Museum, said in a news release. "By studying this early ichthyosaur's unusual rounded teeth, we get a better understanding of how these animals evolved and what their lifestyles were like."

Later ichthyosaurs evolved dolphin-like shapes and long snouts full of sharp teeth. They were sleek and formidable hunters. The earliest known ichthyosaur, Cartorhynchus lenticarpus, was much less fearsome. The species measured no more than a foot in length, and its wrist bones suggest the species was able to scoot onto shore and scuttle about like a seal.

"Ichthyosaurs became creatures of the open ocean, but the smaller species like Cartorhynchus probably lived closer to the shore and picked up invertebrates to eat from the sea floor," Rieppel said.

The earliest investigations of Cartorhynchus lenticarpus suggested the first ichthyosaurs were toothless suction feeders. But later, researchers found teeth hiding in the back of the reptile's mouth.

"In this study, we took CT scans of the fossil to see the teeth that were hidden in its skull, and we found that they had an unusual pebble-like shape," said Rieppel.

Analysis of the early ichthyosaur's pebble-like teeth revealed significant wear and tear, suggesting the only known Cartorhynchus lenticarpus specimen was full-grown.

When scientists looked at the teeth of other early ichthyosaurs, they found similarly rounded teeth. The discovery suggests the trait was common among early ichthyosaurs and likely evolved independently across several different ichthyosaur lineages.

Evidence of sharp, pointed teeth can also be found among early ichthyosaur species, evidence that the marine reptiles quickly adapted to a diversity of ecological niches.

"There were no marine reptiles prior to the Triassic," said Rieppel. "That's what makes these early ichthyosaurs so interesting -- they tell us about the recovery from the mass extinction, because they entered the sea only after it."

The earliest ichthyosaurs expanded across the a marine environment that had been emptied of other marine species, leaving plenty of room for rapid expansion and radiation. Ancient teeth -- small and rounded or long and jagged -- can provide clues as to what that radiation looked like.

"Fossils are clues about what the world was like long ago," said Rieppel. "By gaining a better understanding of how these ichthyosaurs evolved, we get a better sense of how life rebounds after extinctions, and that lesson is still relevant today."

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