When Did Some Ancient Extinct Species Return To The Sea? Machine Learning Helps Find The Answer

When the ancestors of terrestrial life on Earth crawled or slithered out of the sea, they formed the starting point for many of the world’s species. Some of these species even made the return journey back into the water from a life on land. Now, a new study has taken a closer look at explaining when and how this happened, and which species went back to a fully aquatic lifestyle.

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To do this, the team behind the study looked in finer detail at hundreds of museum specimens at the Yale Peabody Museum and other institutions, taking more than 11,000 new measurements, photographs, and CT scans. They even used a statistical test developed during World War II to work out the probability of features seen on the fossils indicating whether or not the creatures they belonged to had an aquatic or terrestrial lifestyle.

“Reconstructing the lives of extinct life forms in a scientifically rigorous way, as opposed to just telling stories, is a precise and delicate undertaking,” said Bhart-Anjan Bhullar, associate professor of Earth and planetary sciences in Yale’s Faculty of Arts and Science, associate curator of vertebrate paleontology and vertebrate zoology at the Peabody Museum, and senior author of the new study, in a statement. 

“It requires a careful interweaving of data from modern organisms, of which our understanding is necessarily far deeper, and knowledge of these living organisms’ genealogy relative to fossil forms."

To make matters even more confusing, some of the species they looked at are easily seen as aquatic because of their limbs. However, the earlier ancestors of these species have more ambiguous limbs, which could either indicate a terrestrial or aquatic lifestyle or a species that features both, like a modern-day platypus or otter.

“In these cases, paleontologists are often stuck, as different lines of evidence disagree about what the ancient animal was like,” said Caleb Gordon, lead author and a postdoctoral researcher at the Florida Museum of Natural History.

The team used all the measurements that they had collected to train a model that could predict the aquatic affinity of each species. They found that hand length measurements made the best prediction, as these could be reconstructed into flippers with around 90 percent accuracy. 

The machine learning model also revealed only semi-terrestrial habitats for mesosaurs, a group of marine reptiles from 290 million years ago, but fully aquatic habitats for species of ichthyosaurs and mosasaurs. The team even used their data and results to weigh in on the ongoing controversy around Spinosaurus.

“Our results shed new light on how much time Spinosaurus spent submerged, which could support the underwater hunting view,” Gordon said. “We confidently recovered highly aquatic habits for Spinosaurus, indicating that it spent the vast majority of its time submerged in the water.”

The team even thinks that the framework that they have created could be applied to different research questions in the future, helping to expand the knowledge of evolutionary history across different areas.

The paper is published in Current Biology.