Put a dolphin’s front flipper in an X-ray machine, and you will see a surprise: an arc of humanlike finger bones. The same goes for a sea turtle, a seal, a manatee and a whale. All of these animals had four-legged ancestors that lived on land. As their various lineages adapted to life in the water, what had been multidigit limbs slowly transformed into flippers.
For a paper published in Biology Letters, researchers compared the flipper bone structures of 19 marine species with terrestrial ancestors, from species around today, like dolphins and sea turtles, to now-extinct creatures, like mosasaurs and ichthyosaurs that swam the oceans in the dinosaur era.
A majority, including most of the still-living animals, stuck close to the original blueprint, the researchers found. But some now-extinct creatures tried more creative strategies to adapt to aquatic life that have since been lost to time.
For this study, the researchers wanted to know how one function — swimming — had inspired the development of a number of unique limb structures. To do this, they needed “to compare things that are not directly comparable,” like a sea turtle flipper and the five-fingered limb it came from, said Evangelos Vlachos, a researcher at the Museum of Paleontology Egidio Feruglio in Chubut, Argentina, and one of the paper’s authors.
Network analysis allowed the researchers to convert each animal’s skeletal fin structure into an abstract web of nodes and connections. By comparing this network with the similarly broken-down limb structure of a landlubbing ancestor, they could see which of the creature’s bones had been gained, lost, fused, connected or otherwise rejiggered since its terrestrial days.
Almost all of the animals kept their fingers, the researchers saw. The digits are connected to one another by surrounding skin and tissue, and can’t move independently. It is as though they are inside “a baby mitten,” Vlachos said.
Other than penguins (whose ancestors evolved wings before they returned to the water), all of the living aquatic animals in the study pursued this strategy, Vlachos said. Some now-extinct creatures, such as plesiosaurs and ancient crocodiles, had fingers in their flippers as well.
The exception was ichthyosaurs — thick-bodied reptiles that ruled the seas through the early Jurassic. Their ancestors also had fingers. But over time, they connected to each other until they were less like a multibranched tree and more like a dense bush. “They ‘lost’ their digits by reintegrating them,” Vlachos said.