Put a dolphin’s front flipper in an X-ray machine, and you’ll 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 Wednesday 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.
To compare and contrast the flippers of this diverse set of animals, the researchers used a technique called network analysis.
The tool has long been popular in the social sciences for tracing the spread of ideas, gossip and even viruses. Anatomists are using it to investigate “one of the oldest and most confounding problems in biology: the evolutionary relationship between structure and function,” said Julia Molnar, a researcher at the New York Institute of Technology, who has done similar work but was not involved with the new paper.
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’s as though they are inside “a baby mitten,” Dr. 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, Dr. 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,” Dr. Vlachos said.
The researchers next wanted to quantify how far each creature’s flippers had come from the original design. So they broke the networks down further, into metrics like complexity and modularity, and plotted them.
Even within the “baby mitten” group, different animals had experienced different levels of transformation. Sea turtle flippers, for example, have remained almost the same for millions of years, while manatees fused some of their bones together and most baleen whales lost a finger.
Overall, though, the animals that still share our seas did not change their flippers much. “They experiment,” Dr. Vlachos said. But “they never left this comfort zone.”
In contrast — and as expected — the ichthyosaurs were outliers, with more homogeneous and well-integrated flipper bones than the other study species. But many of those now-extinct animals that kept their fingers still changed their flippers substantially.
One prehistoric crocodile-like reptile, for example, had seven fingers. And ancient whales called basilosaurids experienced enough bone integration that they were inching toward penguin territory.
Did any of these creatures’ adaptations ultimately set them on a course for extinction? The researchers plan to investigate further. While pinning down a reason is complicated, Dr. Vlachos said, comparing the extremity of different adaptations “might give you some clues.”