Ancient "fish-lizards" were swimming around in Earth's oceans 250 million years ago, long before scientists thought they first emerged, a new study finds.

Researchers discovered the fossilized remains of an ichthyosaur on Spitsbergen, a remote Arctic island in the Svalbard archipelago in Norway in 2014. Ichthyosaurs are an extinct, fish-like lizard whose body shape resembled that of modern-day dolphins and toothed whales. The remains, which consist of 11 tail vertebrae, were trapped within a limestone boulder that dated to the early Triassic period, which makes the fossils the oldest ichthyosaur remains ever uncovered and the oldest evidence of marine reptiles.

Scientists previously assumed that ichthyosaurs had emerged along with all other marine reptiles after the Permian mass extinction event, also known as the "Great Dying," which occurred around 251.9 million years ago and wiped out around 90% percent of all life on Earth at the time. Until now, the oldest-known marine reptile fossils belonged to smaller and less aquatically advanced groups and dated to 249 million years ago, which suggested that marine reptiles had emerged shortly after the destructive event.

But in a new study, published March 13 in the journal Current Biology, researchers argue that the size and composition of the ichthyosaur bones are evidence that the gigantic ocean predators may have emerged before the Permian extinction took place.

Related: 55-foot-long Triassic sea monster discovered in Nevada

Icthyosaurs and other marine reptiles are believed to have descended from land-dwelling reptiles that slowly transitioned into the water to fill an ecological niche that had been left open after the disappearance of oceanic predators. As a result, the first marine reptile species were not perfectly suited to an aquatic lifestyle and likely had dense bones, less streamlined bodies and did not grow to large sizes.

In April 2022, researchers announced the discovery of a tooth from one of the largest ichthyosaurs to ever swim in Earth's oceans that was likely larger than the current record holder Shastasaurus sikanniensis, which measured 69 feet (21 meters) long.

RELATED STORIES

—This 130 million-year-old ichthyosaur was a 'hypercarnivore' with knife-like teeth

—Meet 'Fiona' the pregnant ichthyosaur, Chile's oldest marine reptile mom

—Enormous sea dragon fossil from 180 million years ago discovered in England

The newly uncovered ichthyosaur's vertebrae are the same size as those found in later ichthyosaurs, which grew to around 9.8 feet (3 m) long. The bones also have a spongy structure that appears to be well adapted to aquatic life. The team therefore suspects that the ichthyosaur lineage likely emerged before the end-Permian mass extinction because they were unlikely to have evolved these advanced traits in the less than 2 million years after the catastrophic event occurred.

The results could force palaeontologists to reconsider what they thought they knew about the Permian mass extinction event.

"It now seems that at least some groups predated this landmark interval," the researchers wrote in a statement. Fossils from other ancient ichthyosaur ancestors and other dinosaur-era reptiles may also be waiting to be found elsewhere in the world, they added.