According to a new research published in the journal Nature Communications, ichthyosaurs were driven to extinction by climate change and their own failure to evolve quickly.
Life restoration of Ichthyosaurus anningae. Image credit: James McKay.
Ichthyosaurs were predatory marine reptiles that ranged in size from 1 to 69 feet (0.3 – 21 m) long. They swam the world’s oceans while dinosaurs walked the land.
Despite their profound adaptations to the aquatic realm and their apparent success throughout Triassic and Jurassic periods, these marine reptiles disappeared roughly 30 million years before the end-Cretaceous mass extinction (65 million years ago) that marked the end for dinosaurs and the beginning of the age of mammals.
Current hypotheses for this early demise involve relatively minor biotic events (increased competition with other marine reptiles, or a diversity drop in their assumed principal food resource), but are at odds with recent understanding of the ichthyosaur fossil record.
“Recent data challenge this view of ichthyosaur history, indicating that Early Cretaceous ichthyosaurs were taxonomically, phylogenetically and – possibly – ecologically diverse, even a few million years before their extinction,” said study lead author Dr. Valentin Fischer, of the University of Liège, Belgium, and the University of Oxford, and his colleagues.
The researchers analyzed the extinction of this marine group thoroughly for the first time.
Using a battery of cutting-edge techniques to quantify ancient biodiversity and its fluctuations, they were able to reconstruct the evolution of the ichthyosaurs during the last 120 million years of their lifetime and assess the causes of their extinction.
“We compared the diversity of ichthyosaurs with the geological record of global change, emphasizing the dynamics of these datasets,” Dr. Fischer said.
According to the team, a two-phase event suppressed their ecological diversity and wiped out the group at the beginning of the Late Cretaceous, about 100 million years ago.
At that time, the Earth’s poles were essentially ice-free, and sea levels were much higher than today.
Analyses revealed that this two-phase extinction can be associated both with reduced evolutionary rates (a failure to evolve novel body plans for a prolonged period) and intense climate change (strong variations in sea surface temperatures and sea levels).
“Although the rising temperatures and sea levels evidenced in rock records throughout the world may not directly have affected ichthyosaurs, related factors such as changes in food availability, migratory routes, competitors and birthing places are all potential drivers, probably occurring in conjunction to drive ichthyosaurs to extinction,” Dr. Fischer said.
This study supports a growing body of evidence suggesting that a major, global, change-driven turnover profoundly reorganized marine ecosystems at the beginning of the Late Cretaceous, giving rise to the highly peculiar and geologically brief Late Cretaceous marine world.
“Ichthyosaurs disappeared in the course of this turnover, while numerous lineages of bony fishes and sharks evolved,” the scientists said.
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Valentin Fischer et al. 2016. Extinction of fish-shaped marine reptiles associated with reduced evolutionary rates and global environmental volatility. Nature Communications 7, article number: 10825; doi: 10.1038/ncomms10825
