fossil record

According to the fossil record, cetaceans -- whales, dolphins and their relatives -- evolved from four-legged land mammals that returned to the oceans beginning some 50 million years ago.

But until now, no one had found those earlier forms in the fossil record.

"Like many modern-day animal groups, ecdysozoans were prevalent in the Cambrian fossil record and we can see evidence of all three subgroups right at the beginning of this period, about 540 million years ago," said Ian Hughes, a graduate student in marine biology at Harvard University and the paper's first author.

But a UC Riverside-led team has now identified the oldest known ecdysozoan in the fossil record and the only one from the Precambrian period.

The earliest caviomorphs, which arrived during the Mid-Eocene Climatic Optimum, a period of elevated temperatures, were tiny, but Martinelli remarks there’s evidence of other extinct giant rodents in the fossil record.