The origin of marine reptiles occurred much earlier than was previously thought, the “exciting” discovery of the oldest known ichthyosaur has revealed. Ichthyosaurs — which look like modern dolphins but actually are more closely related to crocodiles — were among the first land animals to adapt to life in the sea. They ruled the marine food chain for more than 160 million years during the so-called Age of the Dinosaurs. New evidence, however, suggests they also predated their counterparts on land.
According to the scientific textbooks, reptiles first ventured into the open sea in the wake of the end-Permian mass extinction, which occurred 251.9 million years ago.
(This cataclysmic event — dubbed “the Great Dying” — is thought to have been triggered by rising global temperatures induced by volcanically-released carbon dioxide, and resulted in the loss of a whopping 81 per cent of marine species.
At the same time, however, the mass extinction paved the way for the ascent of the dinosaurs, who would dominate until the Chixculub asteroid impact 66 million years ago.)
It had been thought that land-based reptiles with walking legs moved to occupy shallow coastal environments to take advantage of predator niches left vacant by the Great Dying.
Over time, experts thought, these early amphibious reptiles would have become more efficient at swimming — evolving their limbs into flippers; developing a more streamlined, “hydrodynamic” body shape; and severing their last tie to the land by giving birth to live young, rather than need to lay eggs on solid ground.
The new fossils unearthed from Spitsbergen, however, are throwing doubt on this account of how marine reptiles first evolved.
In their study, the researchers analysed boulder-sized “concretions” of limestone in Flower Valley on the southern shore of Isfjorden, on the western side of the Norwegian island of Spitsbergen.
These some 250-million-year-old rocks — exposed from the surrounding mudstone by the action of a fast-flowing river — formed from lime-based sediments that settled around decomposing animal remains on an ancient seabed, preserving them as fossils in beautiful detail.
In 2014, an expedition saw a large number of concretions collected from to Flower Valley and shipped back to the the Natural History Museum at the University of Oslo for analysis in tandem with researchers from the the Museum of Evolution at Uppsala University, in Sweden.
From within the concretions, the palaeontologists extracted bony fish and bizarre crocodile-like amphibian bones — along with 11 articulated vertebrae from the tail of an ichthyosaur.
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What surprised the researchers, however, is that the vertebrae were found within rocks that were supposedly too old to contain ichthyosaurs.
Geochemical analysis of the surrounding concretions confirmed that the fossils were deposited around two million years after the end of the Permian–Triassic mass extinction.
Furthermore, rather than representing a “textbook” example of how palaeontologists think an amphibious ichthyosaur would look, the vertebrae are identical to their geologically-younger, larger bodied descendants.
The bones also preserve internal microstructures which are indicative of adaptations towards fast growth, elevated metabolism and — most importantly — a fully marine lifestyle.
In a press release from Uppsala University, the researchers explained: “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.”
This, they added, is “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 full findings of the study were published in the journal Current Biology.
