Ichthyosaurs used a barren region of the ocean as an ancient nursery

A rich deposit of fossilised marine reptiles called ichthyosaurs found in Nevada may be remains from a breeding ground dating back more than 200 million years. The huge creatures seem to have gathered in a quiet patch of ocean where few predators would threaten their young.

The discovery indicates that breeding behaviours seen in modern marine animals like whales were being performed by their reptilian equivalents during the dinosaur era.

Ichthyosaurs, which superficially resembled today’s whales and dolphins, lived in the seas from about 250 million years ago in the Triassic Period until about 90 million years ago in the Cretaceous, while dinosaurs roamed the land.

The Nevada ichthyosaurs were excavated between the 1950s and 1970s from the Berlin-Ichthyosaur State Park site by palaeontologists led by Charles Lewis Camp. His description of the animals was published posthumously in 1980, five years after his sudden death from pancreatic cancer, after which the specimens were largely ignored for several decades.

These ichthyosaurs belong to a species called Shonisaurus popularis. They were between about 11 and 15 metres long and lived some 215 million years ago, near the end of the Triassic.

Camp suggested in his description that there had been a mass stranding event, as sometimes happens to whales and dolphins today. “He imagined that they were hunting around in the shallows and the tide had gone out, and they had got inadvertently left behind,” says Neil Kelley at Vanderbilt University in Nashville, Tennessee.

This explained the absence of smaller animals like fish among the fossils, because they could have escaped even in shallow water. However, it didn’t fit the geological evidence, which showed that the fossils were laid down in deep water.

Kelley and his colleagues have now re-examined the evidence. They looked for signs of environmental disruptions like volcanic eruptions or low oxygen levels, which might have killed the ichthyosaurs. “We could not find direct evidence of any of that,” he says.

What they did find were baby ichthyosaurs. Team member Paige dePolo at the University of Edinburgh, UK, has spent years sorting the fossils, which were left in some disarray. Camp’s 1980 work mentioned that there were embryos, but didn’t describe or draw them, so dePolo had to go through every lump of rock that had been excavated. She found the tiny ichthyosaurs in “the literal last block”, she says.

The reanalysis revealed that there were a lot of large adult ichthyosaurs, at least three juveniles or embryos and virtually no other animals. The team suggests the ichthyosaurs sought out an unproductive region of the ocean – where there is little food – because there would be few predators to threaten their young. “The palaeontological evidence we have supports that there’s not other things with backbones that might eat the babies in this area,” says dePolo.

We can’t be 100 per cent sure because the fossil record is always open to interpretation, says Benjamin Kear at Uppsala University in Sweden. “Having said that, I think it’s entirely plausible.”

He says this fits with other lines of evidence. For example, ichthyosaurs only gave birth to a handful of young at a time, which hints at parental care. They may have lived in pods like some whales and dolphins.

There are signs that other marine reptiles used breeding grounds. In 2006, Kear and his colleagues described fossils of long-necked plesiosaurs that lived in an inland seaway near the South Pole. The remains include lots of small, young animals, all living in “a huge embayment” that would have served as “a sheltered calving ground”.

Sign up to Wild Wild Life, a free monthly newsletter celebrating the diversity and science of animals, plants and Earth’s other weird and wonderful inhabitants