Discovery of oldest ichthyosaur pushes back origin to pre-dinosaur era

The new fossil substantiates the theory that ichthyosaurs, who were land-based reptiles with walking legs, invaded coastal environments to “take advantage of marine predator niches” that was left vacant by the end-Permian mass extinction. These reptiles modified their limbs into flippers, developed a ‘fish-like’ body shape, and started giving birth to live young. 

The paper is published in the life sciences journal Current Biology.

Research identified bony fish and bizarre ‘crocodile-like’ amphibian bones

How were the fossil remains preserved?

Close on the shore of Ice Fjord in western Spitzbergen, Flower’s valley cuts through snow-capped mountains. It exposes rock layers that were once just mud on the seabed, around 250 million years ago. A river eroded the mudstone to reveal limestone boulders called concretions. These in turn formed sediments that settled around decomposing animal remains on the seabed, thereby preserving them wonderfully in three-dimensional detail, the release describes.

In 2014, a large number of concretions were collected from Flower’s valley and collated at the Natural History Museum at the University of Oslo for future study. It was research conducted with The Museum of Evolution at Uppsala University that identified bony fish and bizarre ‘crocodile-like’ amphibian bones, together with 11 articulated tail vertebrae from an ichthyosaur. 

Thanks to geochemical testing, the age of the fossils was confirmed to be approximately two million years after the end-Permian mass extinction. The finding pushed back the origin and early diversification of ichthyosaurs to before the beginning of the Age of Dinosaurs.

In exciting news, it means that fossils of major ancient reptile lineages are still awaiting discovery in even older rocks on Spitsbergen and elsewhere in the world.

Study Abstract:

Reptiles first radiated into oceanic environments after the cataclysmic end-Permian mass extinction (EPME) 251.9 million years (Ma) ago. The geologically oldest fossils evincing this adaptive transition have been recovered from upper-Lower Triassic (lower Spathian) strata, ∼248.8 Ma2, and postdate a landmark turnover of amphibian-dominated to reptile-dominated marine ecosystems spanning the late Smithian crisis (LSC), ∼249.6 Ma4 —less than ∼2.3 Ma after the EPME. Here, we report ichthyopterygian (the group including ‘fish-shaped’ ichthyosaurians remains from the Arctic island of Spitsbergen that predate the LSC in later-middle to early-late Smithiandeposits up to ∼250 Ma. Unexpectedly, however, their large size and spongy internal bone structure indicate a fully pelagic ichthyopterygian. Given this unambiguous occurrence ∼2 Ma after the EPME, these pioneering seagoing tetrapods can now be feasibly recast as mass extinction survivors instead of ecological successors within the earliest Mesozoic marine predator communities.