The oldest ichthyosaur reconstruction and 250 million year old ecosystem found at Spitsbergen. Credit: Illustration by Esther Van Heulsen
For nearly 190 years, scientists have searched for the origin of ancient marine reptiles from the time of the dinosaurs. A team of Swedish and Norwegian paleontologists have discovered the oldest known remains of an ichthyosaur, or “fish lizard”, on the remote Arctic island of Spitsbergen.
Ichthyosaurs are an extinct group of marine reptiles whose fossils have been found around the world. They were among the first land animals to adapt to life on the high seas, and evolved a “fish-like” body shape similar to that of modern whales. Ichthyosaurs were at the top of the food chain in the oceans while dinosaurs roamed the land, dominating marine habitats for over 160 million years.
Cross-sectional and cross-sectional images showing the internal bone structure of the vertebrae of an older ichthyosaur. Credits: Øyvind Hammer and Jørn Hurum
According to textbooks, reptiles first ventured into the open seas after the mass extinction at the end of the Permian period, which destroyed marine ecosystems and paved the way for the beginning of the age of dinosaurs around 252 million years ago. As the story goes, land reptiles with walking legs raided shallow coastal environments to take advantage of the marine predator niches left empty by this catastrophic event. Over time, these early amphibious reptiles became more efficient swimmers, eventually turning their limbs into flippers, developing a fish-like body shape, and beginning to give birth to young; Thus, they break their final bond with the land by not having to come ashore to lay their eggs.
New fossils unearthed at Spitsbergen are now revising this long-accepted theory.
Near the fishing huts 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 silt on the ocean floor some 250 million years ago. A fast-flowing river fed by melting snow erodes the mudstone to reveal rounded limestone known as concrete. It was formed from calcareous deposits that settled around the decaying remains of animals on the ancient seafloor, preserving them in stunning three-dimensional detail. Today, paleontologists are searching for these concrete blocks to examine fossilized traces of dead sea creatures from the past.
Fossil-bearing rocks at Spitsbergen that produced the remains of the first ichthyosaurs. Credit: Benjamin Kerr
Selama ekspedisi tahun 2014, sejumlah besar beton dikumpulkan dari Lembah Bunga dan dikirim kembali ke Museum Sejarah Alam di[{”attribute=””>UniversityofOsloforfuturestudyResearchconductedwithTheMuseumofEvolutionatUppsalaUniversityhasnowidentifiedbonyfishandbizarre‘crocodile-like’amphibianbonestogetherwith11articulatedtailvertebraefromanichthyosaurUnexpectedlythesevertebraeoccurredwithinrocksthatweresupposedlytoooldforichthyosaursAlsoratherthanrepresentingthetextbookexampleofanamphibiousichthyosaurancestorthevertebraeareidenticaltothoseofgeologicallymuchyoungerlarger-bodiedichthyosaursandevenpreserveinternalbonemicrostructureshowingadaptivehallmarksoffastgrowthelevatedmetabolismandafullyoceaniclifestyle[{”attribute=””>UniversityofOsloforfuturestudyResearchconductedwithTheMuseumofEvolutionatUppsalaUniversityhasnowidentifiedbonyfishandbizarre‘crocodile-like’amphibianbonestogetherwith11articulatedtailvertebraefromanichthyosaurUnexpectedlythesevertebraeoccurredwithinrocksthatweresupposedlytoooldforichthyosaursAlsoratherthanrepresentingthetextbookexampleofanamphibiousichthyosaurancestorthevertebraeareidenticaltothoseofgeologicallymuchyoungerlarger-bodiedichthyosaursandevenpreserveinternalbonemicrostructureshowingadaptivehallmarksoffastgrowthelevatedmetabolismandafullyoceaniclifestyle
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 DOI: 10.1016/j.cub.2022.12.053
