Scientists have been looking for the origins of ancient sea-going reptiles from the Age of Dinosaurs for nearly 190 years.
On the remote Arctic island of Spitsbergen, a team of Swedish and Norwegian paleontologists has discovered the remains of the earliest known ichthyosaur ("fish-lizard").
The team's findings are described in a paper published in Current Biology.
Oldest Ichthyosaurs
Ichthyosaurs are a type of extinct marine reptile whose fossils have been discovered all over the world, as per Phys.org.
They were among the first land animals to adapt to life in the open sea, developing a fish-like body shape similar to that of modern whales.
While dinosaurs roamed the land, ichthyosaurs were at the top of the food chain in the oceans and dominated marine habitats for over 160 million years.
This long-held theory is being challenged by new fossils discovered on Spitsbergen.
Flower's valley, near the hunting cabins on the southern shore of Ice Fjord in western Spitsbergen, cuts through snow-capped mountains, exposing rock layers that were once mud at the sea's bottom around 250 million years ago.
The mudstone has been eroded away by a fast-flowing river fed by snow melt, revealing rounded limestone boulders known as concretions.
These were formed by limey sediments that settled around decomposing animal remains on the ancient seafloor, preserving them in stunning three-dimensional detail. Paleontologists today look for concretions to study the fossilized remains of long-dead sea creatures.
A large number of concretions were collected from Flower's valley during an expedition in 2014 and shipped back to the Natural History Museum at the University of Oslo for future study.
The Museum of Evolution at Uppsala University collaborated on the discovery of bony fish and strange crocodile-like amphibian bones, as well as 11 articulated tail vertebrae from an ichthyosaur.
These vertebrae were discovered in rocks that were thought to be too old for ichthyosaurs.
Furthermore, rather than being a 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 demonstrating adaptive hallmarks of rapid growth, elevated metabolism, and a fully oceanic lifestyle.
Geochemical testing of the surrounding rock confirmed the fossils' age at around two million years after the end-Permian mass extinction.
Given the estimated timescale of oceanic reptile evolution, this places the origin and early diversification of ichthyosaurs before the beginning of the Age of Dinosaurs, requiring a revision of textbook interpretation and revealing that ichthyosaurs most likely first radiated into marine environments prior to the extinction events.
The discovery of the oldest ichthyosaur rewrites the popular view of the Age of Dinosaurs as the timeframe for the emergence of major reptile lineages.
It now appears that at least some groups predated this epoch, with fossils of their most ancient ancestors still waiting to be discovered in even older rocks on Spitsbergen and elsewhere around the world.
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Ichthyosaurs icons of evolution
Ichthyosaurs are among the most well-known fossils. Their remains have been found in rocks dating from the late Early Triassic period (250 Ma) to the early Late Cretaceous period (90 Ma), as per Capela.
Although they were often referred to as "swimming dinosaurs," ichthyosaurs were actually a group of fully aquatic marine reptiles, not dinosaurs.
Almost every palaeontological collection in the world has an ichthyosaur fossil - at the very least, an isolated vertebra, but more often, an isolated fin, skull, or even an entire skeleton.
In the absence of skeletons, casts of skeletons can be found in many collections. In fact, I'm typing this with ichthyosaur bones, teeth, and casts on my desk.
There was a lot of interest in ichthyosaurs at the beginning of the nineteenth century, especially. Everyone wanted one to add to their collection.
This was largely due to the efforts of Mary Anning (1799-1847), a palaeontologist who lived in Lyme Regis, a coastal town in Dorset on England's south coast.
The discovery of complete fossilized skeletons near Lyme Regis was a watershed moment for the fledgling science of palaeontology.
Ichthyosaurs arose from the depths of time and gave us a glimpse of how strange our world once was. They became a worldwide sensation.
Whales and dolphins, whose ancestors were also running around on land, are well-documented examples of land-dwelling animals adapting to a marine environment.
In fact, ichthyosaurs, like whales and dolphins today, gave birth to live young in the sea.
Interestingly, ichthyosaur viviparity does not appear to have evolved during their adaptation to an entirely marine life style, but rather appears to have been present in their ancestors before they took to the sea.
Hupehsuchians are long-jawed marine reptiles from China's Early Triassic period.
If you looked at one of their fossils, researchers wouldn't blame you for thinking it was an ichthyosaur or ichthyosaur ancestor, because they share some characteristics with Early Triassic ichthyosaurs.
However, this group, as well as two rather interesting and recently described fossils from the early Triassic of China called Cartorhynchus and Sclerocormus, lack key ichthyosaur features and are instead early ichthyosaur relatives.
The interesting thing is that, unlike ichthyosaurs, these were most likely semi-aquatic. This demonstrates that following the world's greatest mass extinction event, the Permo-Triassic extinction event at 252 Ma, a slew of new, unusual, and diverse marine reptiles appeared and evolved quickly.
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