Reconstruction of an early ichthyosaur and 250-million-year-old ecosystem found in Spitsbergen. Credit: Illustration by Esther Van Hulsen
For nearly 190 years, scientists have searched for the origins of ancient marine reptiles since the time of the dinosaurs. Now a team of Swedish and Norwegian paleontologists have discovered the remains of the earliest known ichthyosaur, or ‘fish-lizard’, on the remote Arctic island of Spitsbergen.
Ichthyosaurs are an extinct group of marine reptiles whose fossils have been recovered around the world. They were among the first land-dwelling animals to live in the open ocean, and developed a ‘fish-like’ body shape similar to modern whales. Ichthyosaurs were at the top of the food chain in the oceans, while dinosaurs roamed the land and dominated marine habitats for over 160 million years.
Computed tomography image and cross-section showing the internal skeletal structure of a vertebrate from an early ichthyosaur. Credit: Øyvind Hammer and Jørn Hurum
According to textbooks, reptiles first entered the open ocean after the Permian mass extinction, which destroyed marine ecosystems and paved the way for the dawn of the age of dinosaurs nearly 252 million years ago. As the story goes, terrestrial reptiles with walking legs invaded shallow coastal environments to take advantage of the marine hunting grounds left vacant by this cataclysmic event. Over time, these early amphibious reptiles became more adept at swimming and eventually evolved their limbs into flippers, developing a ‘fish-like’ body shape and living young; Thus severing their final link to land without the need to come ashore to spawn.
New fossils discovered in Spitsbergen are now revising this long-accepted theory.
Near the hunting lodges on the south coast of the Ice Fjord in western Spitsbergen, the Valley of Flowers cuts through snow-capped mountains to reveal layers of rock that were once mud at the bottom of the ocean 250 million years ago. A fast-flowing river with melting snow erodes the mudstone, revealing rounded limestone boulders. These are formed from calcareous sediments that settled around the remains of decomposed animals on ancient seabeds, then preserved them in fascinating three-dimensional detail. Paleontologists today hunt for these concretions to study the fossilized remains of long-dead sea creatures.
The fossiliferous rocks in Spitsbergen yield the earliest ichthyosaur remains. Credit: Benjamin Gear
During a trip in 2014, a large number of concretions were collected from the Valley of Flowers and sent to the Natural History Museum.[{” attribute=””>University of Oslo for future study. Research conducted with The Museum of Evolution at Uppsala University has now identified bony fish and bizarre ‘crocodile-like’ amphibian bones, together with 11 articulated tail vertebrae from an ichthyosaur. Unexpectedly, these vertebrae occurred within rocks that were supposedly too old for ichthyosaurs. Also, rather than representing the textbook example of an amphibious ichthyosaur ancestor, the vertebrae are identical to those of geologically much younger larger-bodied ichthyosaurs, and even preserve internal bone microstructure showing adaptive hallmarks of fast growth, elevated metabolism and a fully oceanic lifestyle.
Geochemical testing of the surrounding rock confirmed the age of the fossils at approximately two million years after the end-Permian mass extinction. Given the estimated timescale of oceanic reptile evolution, this pushes back the origin and early diversification of ichthyosaurs to before the beginning of the Age of Dinosaurs; thereby forcing a revision of the textbook interpretation and revealing that ichthyosaurs probably first radiated into marine environments prior to the extinction event.
Excitingly, the discovery of the oldest ichthyosaur rewrites the popular vision of Age of Dinosaurs as the emergence timeframe of major reptile lineages. It now seems that at least some groups predated this landmark interval, with fossils of their most ancient ancestors still awaiting discovery in even older rocks on Spitsbergen and elsewhere in the world.
The paper is published in the prestigious international life sciences journal Current Biology.
Reference: “Earliest Triassic ichthyosaur fossils push back oceanic reptile origins” by Benjamin P. Kear, Victoria S. Engelschiøn, Øyvind Hammer, Aubrey J. Roberts and Jørn H. Hurum, 13 March 2023, Current Biology.
DOI: 10.1016/j.cub.2022.12.053
