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Earliest ichthyosaur fossil discovered on remote Arctic Island
Ichthyosaur remains discovered in Svalbard have shone new light on the evolution of the ancient marine reptiles.
New research suggests that ichthyosaurs may have pre-dated the age of the dinosaurs after surviving Earth's largest mass extinction event.
Ichthyosaurs are an extinct group of marine reptiles that once swam in every major ocean. They were thought to have evolved during the Early Triassic, before dominating the oceans throughout the dinosaur age.
However, the discovery of the earliest known ichthyosaur fossil could rewrite the evolutionary history of these marine reptiles.
A new study published in Current Biology reveals that ichthyosaurs evolved much earlier than previously thought and may have even been around before the huge Permian-Triassic mass extinction event that killed off some 90% of all species.
Dr Aubrey Roberts, an associate scientist at the Museum and co-author of the study, says, 'According to the textbooks, ichthyosaurs evolved a few million years after the largest mass extinction ever seen on our planet 252 million years ago.'
'However, our latest findings indicate that this might not be the case.'
'Our research has dated the fossils to be 250 million years old and are the remains of an already fully developed marine ichthyosaur, only a couple of million years after the mass extinction event.'
'For a palaeontologist, this is quite a short period of time and suggests that ichthyosaurs likely evolved before the mass extinction event.'
Cross-section showing the internal bone structure of vertebrae from the earliest ichthyosaur. Image © Øyvind Hammer and Jørn Hurum via Kear et al.
How did the ichthyosaurs evolve?
Ichthyosaurs were a group of marine reptiles that roamed the oceans for at least 160 million years. Their fossils date from the Early Triassic until the Late Cretaceous, around 90 million years ago.
The marine reptiles were not dinosaurs, but part of a separate group of marine vertebrates similar in appearance to modern-day dolphins and whales, with streamlined bodies and powerful tails for swimming.
They adapted to become one of the ocean's top predators, possessing many sharp teeth perfect for catching fish and squid. While some species were less than a metre in length, others grew to reach a whopping 25 metres, rivalling some modern whales.
Ichthyosaur fossils were among the first to be found in the UK, with Mary Anning and her brother Joseph discovering the first specimen in the early 1800s. In 2021, Britain's largest-ever ichthyosaur fossil was discovered in Rutland Water during routine maintenance work.
Studies suggest that the reptiles evolved from a land-living ancestor to a fully marine way of life, similar to how dolphins and other marine mammals evolved from land-dwelling mammals, but nobody is quite sure what this ancestor would have looked like.
Despite the discovery of many well-preserved ichthyosaur fossils in Western Europe and North America, gaps in the fossil record make the evolutionary history of this group of animals uncertain.
Teamleader Prof. Jørn Hurum (left) and team member Øyvind Enger (right) searching for fossils in Flower's valley. Image © Victoria Engelschiøn.
How was this early ichthyosaur discovered?
In 2014, palaeontologists embarked on an expedition to the Arctic to collect fossils from Flower's Valley in western Spitsbergen.
Fast-flowing rivers created by melting snow from the surrounding mountains exposed rock that would have once been mud at the bottom of a sea 250 million years ago. Sediments would have settled around decomposing animal remains on the ancient seabed, preserving them in spectacular three-dimensional detail.
Rock samples from Spitsbergen were then taken to the University of Oslo's Natural History Museum for examination. Among the fossils of bony fish and crocodile-like amphibians were the remains of 11 articulated tail vertebrae from an ichthyosaur.
Geochemical testing of the surrounding rock confirmed the age of the fossils at approximately two million years after the end-Permian mass extinction.
Researchers initially thought the rocks were therefore too old for ichthyosaurs and instead thought that the bones may have belonged to an early ancestor. However, analysis showed that the vertebrae were identical to much younger larger-bodied ichthyosaurs.
The preserved internal bone microstructure also showed that the ichthyosaur had an elevated metabolism and an entirely oceanic lifestyle.
While any earlier ichthyosaur fossils are still yet to be found, given the estimated timescale of oceanic reptile evolution researchers believe that this discovery pushes back the origin and early diversification of ichthyosaurs to before the Permian-Triassic mass extinction.
If ichthyosaurs had radiated into marine environments before the extinction event, this would rewrite the popular vision of the age of dinosaurs as the time when major reptile lineages appeared.
'The next step is for palaeontologists to search for the transitional fossils,' explains Aubrey.
'In other words, the semi-aquatic ichthyosaur ancestors in rocks from before the Permian-Triassic mass extinction. However, the challenge is finding fossiliferous rocks from the right environment that might preserve them.'
