Amazing stuff!
"... But for some species that evolution also involved a return trip. Dozens of major mammal and reptile groups ultimately made their way back to the beach and into the water. ...
“These secondarily aquatic groups adapted in strikingly similar ways to their new aquatic home — evolving flippers and a suite of other features that made them better swimmers,” ...
For the study, the researchers analyzed hundreds of specimens in the collection of the Yale Peabody Museum and dozens of other institutions around the world, taking new measurements (more than 11,000), photographs, and CT scans. The team also made use of classic paleontology methods, phylogenetic machine-learning algorithms, and even World War II-era naval statistics — all to reconstruct one of the most significant evolutionary transitions in natural history. ...
The lack of a robust, comprehensive explanation for when various animal groups fully returned to the water has divided the scientific community for years. Researchers had to rely upon their own interpretations of fragmentary fossil samples that had uncertain relationships to one another — with some features suggesting the animals lived on land and other features suggesting they lived in the water. ...
For example, the researchers were able to weigh in on the controversy surrounding Spinosaurus, the ancient and ambiguously aquatic dinosaur that has been the subject of intense debate among paleontologists. Spinosaurus lived from roughly 113 to 94 million years ago in what is now northern Africa.
One body of evidence suggests that Spinosaurus frequently dived and hunted for prey underwater, like a seal or penguin; another body of evidence suggests that Spinosaurus walked and foraged for food near the water’s edge, like a modern-day heron.
“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.” ..."
From the highlights and abstract:
"Highlights
• Amniote limb proportions predict flippers and aquatic habits with >90% accuracy
• Mesosaurs and other Paleozoic reptiles did not evolve highly/fully aquatic habits
• Mesozoic marine reptiles show lineage-specific patterns of aquatic adaptation
• Phylogenetic ROC analysis reconstructs cryptic phenotypes in extinct species
Summary
Among mammals and reptiles, the recurring evolution of fully aquatic forms from land-dwelling ancestors highlights the remarkable powers and implications of natural selection.
The most aquatically specialized of these groups have limb morphologies that betray a fully marine lifestyle, but the transitional forms near the base of each lineage have more ambiguous features, making it difficult to determine which fossil species were aquatic.
Here, we use a scalable phylogenetic machine-learning pipeline to test previously proposed osteological correlates of interdigital webbing, soft-tissue flippers, and aquatic habits in amniotes.
We collect >11,000 original measurements from amniote limbs and use these measurements to train and test phylogenetic logistic regression models that can predict aquatic affinities in extinct species. We then interpret and select among competing predictor models with receiver-operating characteristic analysis. Ultimately, relative hand length makes the best predictions, reconstructing soft-tissue flippers and aquatic habits with >90% accuracy across amniotes and clarifying the aquatic habits of fossil species with historically ambiguous ecologies.
Placing these predictions on the phylogenetic tree of amniotes reveals semi-terrestrial habits in mesosaurs and all other sampled stem reptiles, highly/fully aquatic habits in all known ichthyosauromorphs, and multiple independent origins of highly/fully aquatic habits among sauropterygians, mosasaurs, and theropod dinosaurs.
Taken more broadly, these results enable a broader comparative assessment of amniote limb proportions that reveals distinct evolutionary landscapes in limb morphometry for highly/fully aquatic vs. semi-terrestrial amniotes, as well as between total-group mammals and reptiles."
Back to the beach: Why did evolution return some animals to the water? | Yale News "A new study offers insights into which animal groups’ evolutionary path brought them from the land and back to water."
Graphical abstact
The new method uses machine-learning models, trained on modern species, to predict the aquatic habits and associated soft-tissue adaptations of ancient extinct species.