Giant Prehistoric Dolphin Hunted Killer Whale-Sized Prey

A life restoration of a pod of Ankylorhiza tiedemani hunting. Robert W Boessenecker

Rachael Funnell 09 Jul 2020, 16:13

A near-complete skeleton of an ancient, giant, extremely toothy dolphin has been found in South Carolina. The findings, published in the journal Current Biology, describe an enormous dolphin that was 4.6 meters (15 foot) long and lived in the Oligocene around 25 million years ago.

Previously only known to science from a partial rostrum (the dolphin’s snoot), Ankylorhiza tiedemani’s tooth and flipper morphology revealed that this large-toothed whale (from the genus Odontoceti) was an apex predator in the Oligocene ocean. The remarkable discovery was that the animal’s skeleton shares significant similarities with modern toothed and baleen whales, indicating that these animals shared a convergent evolution because they inhabited similar aquatic environments.

"The degree to which baleen whales and dolphins independently arrive at the same overall swimming adaptations, rather than these traits evolving once in the common ancestor of both groups, surprised us," said Robert Boessenecker of the College of Charleston in Charleston, South Carolina, in a statement. "Some examples include the narrowing of the tail stock, increase in the number of tail vertebrae, and shortening of the humerus (upper arm bone) in the flipper.

"This is not apparent in different lineages of seals and sea lions, for example, which evolved into different modes of swimming and have very different looking postcranial skeletons," he added. "It's as if the addition of extra finger bones in the flipper and the locking of the elbow joint has forced both major groups of cetaceans down a similar evolutionary pathway in terms of locomotion."

The first partial skeleton of Ankylorhiza was discovered in the 1970s by Albert Sanders, once a Charleston Museum Natural History curator. It wasn’t until the late 1990s that scientists finally got their hands on a full skeleton, which was sent to the Mace Brown Museum of Natural History.

Amazingly, the researchers state that the fossil evidence indicates that these mega dolphins were the top predators of the time, feasting on large prey, including killer whales. They also say that the remains imply that Ankylorhiza was the first echolocating cetacean to become an apex predator. It wasn’t until after this powerful predator was wiped out that sperm whales and shark-toothed dolphins began to thrive, though there was a gap of about 5 million years.

"Whales and dolphins have a complicated and long evolutionary history, and at a glance, you may not get that impression from modern species," Boessenecker said. "The fossil record has really cracked open this long, winding evolutionary path, and fossils like Ankylorhiza help illuminate how this happened."

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