The nearly 16-foot long dolphin (Ankylorhiza tiedemani comb. n.) lived around 25 million years ago in what is now South Carolina, according to the study published last week in the journal Current Biology. Fossil whales from the Oligocene period (23 million to 33.9 million years ago) are rare and offer paleontologists a unique opportunity to clarify the early evolution of whales.
Besides its large size, this Oligocene dolphin’s teeth suggested to researchers it was a formidable foe to prehistoric prey.
“Ankylorhiza has proportionally large teeth with thickened roots, an adaptation for higher bite force, [and] the teeth have longitudinal ridges which cut through flesh more efficiently,” Robert Boessenecker, a paleontologist at the College of Charleston and the lead author of the new research, tells Jonathan Chadwick of the Daily Mail.
Speaking with CNN, Boessenecker says these strong, sharp teeth would have allowed Ankylorhiza to rip apart large-bodied prey by violently shaking its head, “which is precisely what killer whales do with seals.” The extinct dolphin also had incisors that protruded forward like tusks, which might have been used for ramming its victims.
Other aspects of the fossil that researchers say show it was adapted to hunting bigger quarry include its shorter, more powerfully built snout, and the alignment of vertebrae in its neck, which, according to the paper, would have granted the beast similar neck mobility to modern killer whales. The researchers write that Ankylorhiza would have been a fast swimmer, meaning it could have chased down and killed other whales and dolphins of the era as well as ancient manatees or sea cows, sea turtles, sea birds, sharks and fish.
The first fossil of this species—a fragmentary skull—was discovered in the 1880s during dredging of South Carolina’s Wando River, according to a release, but the first skeleton wasn’t found until the 1970s. The nearly complete fossil skeleton that is the subject of the current study was unearthed in the 1990s during excavations related to the construction of a new housing subdivision.
Ankylorhiza is a member of the odontocetes or toothed whales, a group that includes modern dolphins, porpoises, killer whales and other species that have teeth, including pilot whales, belugas and sperm whales. The evolutionary paths of odontocetes and the mysticetes, or the baleen whales, which include filter-feeding humpbacks and blue whales, diverged at least 36 million years ago.
Despite many millions of years of evolving independently, the surviving species of toothed and baleen whales sport many similar adaptations to swimming underwater. Researchers had previously thought these similarities were due to the traits having been present in the two groups’ last common ancestor. But the new study of the Ankylorhiza fossil suggests each group separately evolved the shared features in parallel.
"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," says Boessenecker in a statement. Examples include narrowing the base of the tail, increasing the number of tail vertebrae and shortening the upper arm bone (humerus) in the flipper, per the statement.
"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," Boessenecker adds. "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."
A second species from the Ankylorhiza genus is set to be described in a follow-up paper as well as juvenile specimens of Ankylorhiza tiedemani comb. n., according to the statement. Boessenecker says other “unique and strange early dolphins and baleen whales” may yet emerge from the Oligocene-aged rocks of Charleston, South Carolina, “offering unparalleled evolutionary insight” into a poorly understood era of whale evolution.