New research from Yale University combines fossil evidence, modern animal data, machine learning and statistical techniques developed during World War II to reconstruct how vertebrates returned from land to aquatic environments. The analysis suggests that Spinosaurus spent most of its time in water, while mesosaurs and other early marine reptiles remained dependent on land and periodically returned to it.
The findings, published in Current Biology, examine when mammals and reptiles became adapted to aquatic life and which groups evolved fully aquatic lifestyles. Dozens of major mammal and reptile groups independently made the transition back to water, developing flippers and other swimming adaptations through convergent evolution.
“These secondary aquatic groups adapted to their new aquatic home in strikingly similar ways: evolving flippers and a series of other features that made them better swimmers,” said lead author Caleb Gordon. Gordon received his PhD from the Yale School of Arts and Sciences earlier this year and is now a postdoctoral fellow at the Florida Museum of Natural History.
“As a result, they provide a textbook example of convergent evolution and can tell us a lot about the processes that drive and suppress adaptive change in response to similar environmental cues,” Gordon said.
Reconstructing evolution with fossils and artificial intelligence
To investigate these evolutionary transitions, the researchers examined hundreds of fossils and modern specimens held at Yale University’s Peabody Museum and dozens of institutions worldwide. They collected more than 11,000 measurements, along with photographs and CT scans.
The team combined traditional paleontological methods with machine-learning models that identify evolutionary relationships. It also used statistical techniques originally developed by the Navy during World War II. Together, these methods provided a new way to study one of the most significant transitions in vertebrate evolution.
“Reconstructing the ecology of extinct life forms in a scientifically rigorous way, rather than just telling a story, is a precise and delicate task,” said the study’s senior author, Bad Anjan Brar, identified in the source as an associate professor of Earth and Planetary Sciences in Yale’s School of Arts and Sciences and an associate curator of vertebrate paleontology and vertebrate zoology at the Peabody Museum.
“This requires careful interweaving of data from modern organisms, where our understanding is necessarily much deeper, and knowledge of the genealogies of these organisms compared to their fossil forms,” he said. “Caleb’s work accomplishes all of this and more on a massive evolutionary scale.”
For decades, scientists have disagreed about when some prehistoric animals became fully adapted to aquatic life. Fossils often provide incomplete or contradictory evidence, making it difficult to determine whether an extinct species lived mainly on land, in water or in both environments.
Some skeletal features can indicate aquatic adaptation, while others suggest that an animal retained the ability to move and feed on land. These conflicting signals have led researchers to different conclusions about the behavior of ancient species.
“Paleontologists often get stuck in cases like this because the different lines of evidence don’t agree on what the ancient animals looked like,” Gordon said.
To address the uncertainty, Gordon developed a method using relatively simple machine-learning models trained on living animals. By identifying relationships between skeletal proportions and known lifestyles, the models can estimate how extinct species used aquatic environments. They can also predict soft-tissue adaptations, such as swimming structures, that are rarely preserved in fossils.
Spinosaurus may have spent most of its life underwater
One of the study’s most notable findings concerns Spinosaurus, a giant prehistoric dinosaur whose relationship with water has been debated extensively.
Spinosaurus lived in what is now North Africa between about 113 million and 94 million years ago. Scientists generally agree that it was associated with aquatic environments, but they continue to debate how it moved, hunted and divided its time between land and water.
Some evidence suggests that the dinosaur frequently dived and pursued prey underwater, in a manner comparable to seals and penguins. Other findings have supported a lifestyle more like that of modern herons, which forage along coastlines and in shallow water.
The Yale team’s analysis provides new support for the more aquatic interpretation.
“Our findings shed new light on how long Spinosaurus was submerged and may support the theory of underwater hunting,” Gordon said. “We confidently reproduced the highly aquatic habits of Spinosaurus, indicating that Spinosaurus spent the majority of its time in water.”
The finding does not resolve every question about how the dinosaur hunted, but it supports the view that the giant predator spent more of its life underwater than on land.
Early marine reptiles still returned to land
The researchers also examined mesosaurs, small marine reptiles that lived between 290 million and 274 million years ago in what is now South Africa and parts of eastern South America.
Mesosaurs have long interested paleontologists because they gave birth to live young. Some researchers have interpreted this feature as evidence that the animals lived entirely in water. The new analysis, however, suggests that their limbs were not fully adapted to a permanently aquatic lifestyle.
Instead, their skeletal proportions indicate that mesosaurs regularly moved between aquatic and terrestrial environments.
“Like modern crocodiles and platypuses, they spent a lot of time on land,” Gordon said. “Mesosaurus didn’t leave Earth completely.”
The findings also point to a broader pattern among early marine reptiles. According to the researchers, all Paleozoic marine reptiles—those that lived before the appearance of dinosaurs—continued to return to land periodically. No species appears to have evolved a fully aquatic lifestyle; even the most water-adapted species remained amphibious.
Forelimb proportions were particularly useful for distinguishing between these lifestyles. Using this information, the model predicted with more than 90% accuracy whether an animal had soft-tissue flippers and whether it had a highly aquatic or fully aquatic lifestyle.
World War II statistics applied to evolutionary questions
One unusual aspect of the study was its use of statistical methods developed in the 1940s.
“Caleb revived the statistical methods of a bygone era,” said co-author Jacques Gauthier, a professor of Earth and Planetary Sciences at Yale University and curator of vertebrate paleontology at the Peabody Museum. “It was developed during World War II to estimate the probability that a blip approaching a radar screen was an enemy aircraft. Caleb cleverly used it to find out whether the limbs of extinct animals had been modified to help them swim.”
By adapting this approach, the researchers could estimate whether extinct species had specialized swimming adaptations even when direct evidence of those traits was unavailable.
The method could eventually be applied to other major evolutionary transitions, including when human ancestors developed the ability to walk on two legs and how dinosaurs, the ancestors of birds, evolved flight.
A systematic way to study extinct animals
Most prehistoric vertebrates are known primarily from fossilized skeletons, making the reconstruction of their behavior one of paleontology’s greatest challenges. Bones contain valuable information, but they rarely provide a complete picture of how an animal lived.
Scientists have traditionally relied on individual skeletal features when interpreting the lifestyles of extinct species. Those interpretations can differ substantially, particularly when fossils are incomplete or when different anatomical features point to different conclusions.
The new approach offers a more systematic way to evaluate the evidence. It uses patterns observed in modern animals to make testable predictions about species that disappeared millions of years ago.
“All we have left of most of the vertebrates that lived on Earth are bones,” Buller said. “We want to know so much about these creatures and the series of lost worlds they lived on, but there is little to base our conclusions on. People have tried to discern lifestyle habits from skeletal remains, but they have generally based their arguments on one favorable set of characteristics. Caleb’s method removes subjectivity from this process and allows the data to speak for itself.”
Additional co-authors were Lisa Friesem, a doctoral student at Yale’s School of Arts and Sciences, and Christopher Griffin, an assistant professor of geosciences at Princeton University who previously worked as a postdoctoral fellow at Yale University.
The research was supported by the Yale Biosphere Institute, the International Union for Conservation of Nature’s Crocodile Expert Group and the National Science Foundation.
Source: www.sciencedaily.com