Ichthyosaur fossils push back the origin of marine reptiles

March 13 () –

Swedish and Norwegian paleontologists have discovered remains of the oldest known ichthyosaur on the remote Arctic island of Spitsbergen, that evolved before the Permian mass extinction.

Ichthyosaurs were an extinct group of marine reptiles whose fossils have been recovered from around the world. They were among the first land animals to adapt to life in the open sea and developed a body shape similar to that of today’s whales.. Ichthyosaurs occupied the top of the food chain in the oceans while dinosaurs roamed the land and dominated marine habitats for more than 160 million years.

According to textbooks, reptiles first ventured into the open sea after the late Permian mass extinction, which devastated marine ecosystems and paved the way for the dawn of the Age of Dinosaurs. almost 252 million years ago. As the story goes, walking legged terrestrial reptiles invaded shallow coastal environments to take advantage of niches for marine predators left vacant by this cataclysm. Over time, these early amphibious reptiles became more efficient swimmers and eventually modified their limbs into flippers, developed a fish-like body shape, and began giving birth to live young, thus breaking its last link with the land by not needing to disembark to lay eggs.

The new fossils discovered in Spitsbergen are revising this long-accepted theory.

Near the hunting lodges, on the southern shore of Icefjord, west of Spitsbergen, the Flower Valley cuts through snow-capped mountains, exposing layers of rock that were once mud on the sea floor some 250 million years ago. of years. The swift current of a snow-fed river has eroded the mud, exposing rounded limestone rocks called concretions. These formed from calcareous sediments that settled around decaying animal remains on the ancient seabed, later preserving them in spectacular three-dimensional detail. Today, paleontologists search for these concretions to examine the fossil footprints of long-dead sea creatures.

During an expedition carried out in 2014a large number of concretions were collected in the Flower Valley and sent to the Museum of Natural History of the University of Oslo for further study.

Research conducted in collaboration with the Uppsala University Museum of Evolution has identified bony fish and strange crocodile-like amphibian bones, as well as eleven jointed vertebrae from the tail of an ichthyosaur.

Unexpectedly, these vertebrae were found in rocks supposedly too old for ichthyosaurs. Also, instead of depicting the textbook example of an ichthyosaur amphibian ancestor, the vertebrae are identical to those of geologically much younger and larger-bodied ichthyosaursand even retain the internal bone microstructure that displays the adaptive characteristics of rapid growth, high metabolism, and a totally oceanic lifestyle.

Geochemical tests of the surrounding rock confirmed the age of the fossils at approximately two million years after the late Permian mass extinction. Taking into account the estimated time scale of the evolution of oceanic reptiles, this delays the origin and early diversification of ichthyosaurs until before the start of the Age of Dinosaursforcing a revision of textbook interpretation and revealing that ichthyosaurs likely first radiated in marine environments before extinction.

According to the authors, who publish the finding in Current Biology, it is exciting that the discovery of the oldest ichthyosaur rewrites the popular view of the Age of Dinosaurs as the time frame of the appearance of the main lineages of reptiles. It now appears that at least some groups predated this historical interval, and fossils of their oldest ancestors are still waiting to be discovered in even older rocks on Spitsbergen and elsewhere around the world.