Paleontologists from Uppsala University and the University of Oslo have found the 250-million-year-old fossilized ichthyosaur remains in the Vikinghøgda Formation on the Arctic island of Spitsbergen.
Reconstruction of the earliest ichthyosaur and the 250-million-year-old ecosystem found on Spitsbergen. Image credit: Esther van Hulsen.
Ichthyosaurs are a group of dolphin-shaped marine reptiles that played an important role as apex predators in Mesozoic oceanic ecosystems.
These creatures originated and diversified from around 249 million years ago, around 3 million years after the end-Permian mass extinction event, and their fossil record spans about 160 million years.
They had an elongated body, a relatively small head, a long snout, flipper shaped limbs, and dolphin-like tail flukes.
Most hunted fish or other small prey, but some fed on other marine reptiles, much like killer whales that hunt other marine mammal species today.
Their fossils have been found on every continent but they are particularly well known from Europe, east Asia and North America.
“According to the textbooks, reptiles first ventured into the open sea after the end-Permian mass extinction, which devastated marine ecosystems and paved the way for the dawn of the Age of Dinosaurs nearly 252 million years ago,” said Uppsala University paleontologist Benjamin Kear and his colleagues.
“As the story goes, land-based reptiles with walking legs invaded shallow coastal environments to take advantage marine predator niches that were left vacant by this cataclysmic event.”
“Over time, these early amphibious reptiles became more efficient at swimming and eventually modified their limbs into flippers, developed a ‘fish-like’ body shape, and started giving birth to live young; thus, severing their final tie with the land by not needing to come ashore to lay eggs.”
In 2014, the researchers unearthed a series of 11 articulated tail vertebrae from an ichthyosaur in the Lusitaniadalen Member of the Vikinghøgda Formation, on the southern shore of Ice Fjord in western Spitsbergen.
“Unexpectedly, these vertebrae occurred within rocks that were supposedly too old for ichthyosaurs,” they noted.
“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,” they said.
“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,” the scientists concluded.
“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 study appears today in the journal Current Biology.
Benjamin P. Kear et al. 2023. Earliest Triassic ichthyosaur fossils push back oceanic reptile origins. Current Biology 33 (5): 178-179; doi: 10.1016/j.cub.2022.12.053
Source : Breaking Science News