Oldest marine reptile fossil from the “Age of Dinosaurs” is found in the Arctic

Ichthyosaurs were a successful group of marine reptiles that thrived during the “Age of Dinosaurs”, some reaching up to around 21 meters in length – being smaller only than the largest of whales.

But its origins are a bit mysterious. Fossils dating back to around 250 million years ago unearthed in a rugged and remote location – the island of Spitsbergen, Norway – are now providing startling insight into the rise of ichthyosaurs.

Researchers say they have found remains of the ichthyosaur, which lived approximately 2 million years after Earth’s worst mass extinction that ended the Permian Period, wiping out about 90% of the planet’s species amid massive Siberian volcanism.

The 11 tail vertebrae discovered indicate the animal was around 3 meters long, making it a top predator.

Like whales, which are mammals, and the many other lineages of reptiles that inhabited Earth’s oceans, ichthyosaurs evolved from ancestors that walked on land and underwent a transition from land to sea.

The researchers had thought that any ichthyosaur that lived 250 million years ago would have been an early form, not far removed from its Earth precursors. Fossils showed that this one, which has not yet received a scientific name, was anatomically quite advanced.

“The real surprise was that after a set of geochemical, computerized microtomographic and bone microstructural analyses, the vertebrae turned out to be from a highly advanced, fast-growing, probably warm-blooded, large-bodied ichthyosaur about 3 meters long and fully oceanic,” said Benjamin Kear, curator of vertebrate paleontology at the Museum of Evolution at Uppsala University in Sweden and lead author of the research published in the journal Current Biology.

“The implications of this discovery are manifold, but most importantly it indicates that the long-awaited transient ichthyosaur ancestor must have appeared much earlier than previously suspected,” added Kear.

With this discovery, it could be that ichthyosaur origins predate the mass extinction event by as much as perhaps 20 million years, Kear said.

The Triassic Period that followed the mass extinction was the opening act of the age of dinosaurs, although the oldest known dinosaurs did not appear until about 230 million years ago.

The site where the fossils were found is a classic Arctic landscape with high snow-capped mountains along the shores of a deep fjord.

The fossils were exposed along a river channel fed by melting snow that cuts through layers of rock that were once mud on the seafloor.

While today there are polar bears and beluga whales in Spitsbergen, 250 million years ago the sea there would have been teeming with fish, sharks, squid-like ammonoids and crocodile-like marine amphibians called temnospondyls.

Mass extinction has shaken land and marine ecosystems and opened up opportunities for new species to fill ecological roles left by extinct creatures. Ichthyosaurs quickly became dominant and persisted until about 90 million years ago.

Many ichthyosaurs looked like dolphins, except with vertical tails instead of horizontal ones. Others resembled large whales. The largest of these was Shastasaurus, at around 21 meters tall. They ate fish and squid. Fossils show ichthyosaurs giving life to their young.

Until now, the oldest known member of the ichthyosaur lineage was a 40cm-long creature called Cartorhynchus, which lived 248 million years ago in China.

Researchers in recent decades have identified the earliest forms of whales, including one called Ambulocetus, nicknamed the “walking whale” because it retained limbs that still allowed it to move on land.

“The most exciting thing is that the mysterious ‘walking ichthyosaur ancestor’ is still out there, no doubt waiting to be discovered,” said Kear.

“Only now will we have to start looking in even older rocks, which is exactly what we’ll be doing on our next fossil hunting trip to Spitsbergen this summer.”