The experts analyzed the first nearly complete skeleton of the extinct dolphin Ankylorhiza – a 15-foot-long specimen that was discovered in South Carolina. The species was previously known from only a partial snout fossil.
The analysis produced ample evidence to show that Ankylorhiza was an apex predator, from the anatomy of the skull to the teeth and flippers. According to the researchers, the dolphin was “very clearly preying upon large-bodied prey like a killer whale.”
Ankylorhiza belongs to the group Odontoceti, or toothed whales, which are distinct from baleen whales. The experts noted that many features of Ankylorhiza’s postcranial skeleton indicate that modern baleen whales and modern toothed whales must have evolved similar features independently, driven by parallel evolution in similar habitats.
Study co-author Robert Boessenecker is a paleontologist at the College of Charleston in South Carolina.
“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 Boessenecker.
“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.”
“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.”
According to the researchers, another intriguing aspect of the study is that Ankylorhiza is the first echolocating whale to become an apex predator. They explained that when Ankylorhiza became extinct about 23 million years ago, killer sperm whales and the shark-toothed dolphin Squalodon evolved to take over as the top predators within 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,” said Boessenecker. “The fossil record has really cracked open this long, winding evolutionary path, and fossils like Ankylorhiza help illuminate how this happened.”
Additional fossils of Ankylorhiza are awaiting study, including specimens of juveniles that will provide insight into the dolphin’s growth. Boessenecker said that there is still much to learn from fossilized dolphins and baleen whales from South Carolina.
“There are many other unique and strange early dolphins and baleen whales from Oligocene aged rocks in Charleston, South Carolina.”
“Because the Oligocene epoch is the time when filter feeding and echolocation first evolved, and since marine mammal localities of that time are scarce worldwide, the fossils from Charleston offer the most complete window into the early evolution of these groups, offering unparalleled evolutionary insight.”
The study is published in the journal Current Biology.