Dolphins may seem cute and friendly, but the largest member of the dolphin family is actually none other than the ultimate hunter of the sea — the killer whale, also known as an orca.
Now researchers have confirmed that an ancient dolphin that lived during the Oligocene Epoch — 33.9 million to 23 million years ago — was the first cetacean (a type of mammal) using echolocation to navigate underwater and fill the role of apex predator, much like the current-day killer whale.
Echolocation allows dolphins to "see" through sound underwater. They do so by emitting calls to locate distant objects in the water, then interpret the echoes of sound waves that bounce off of those objects.
The skeleton helps to fill the gaps in the evolutionary narrative of these marine mammals who returned to the sea.
Cetaceans are an order of mammal including dolphins, whales and porpoises. Odontocetes, or toothed whales, are an order of cetaceans that includes dolphins, porpoises and all other whales that have teeth, such as sperm whales.
The specimen, named Ankylorhiza tiedemani, was discovered partly in rock formations in South Carolina, said the study published Thursday in the journal Current Biology.
Its 15-foot-long body size, a shorter and stronger snout, tooth wear and vertebral formation indicated that Ankylorhiza was the first Odontocete predator that could eat both small- and large-bodied prey and swim faster than other whales. This indicates for the first time that it was one of the few extinct cetaceans to fulfill an ecological position similar to that of killer whales.
"We see that same pattern in the fossil record of terrestrial carnivores," said Anthony Friscia, an adjunct associate professor of integrative biology and psychology at the University of California, Los Angeles, who wasn't part of the study.
"For instance, you see a 'cat-like' predator arise many different times before you get the modern radiation of cats. This kind of repeated evolution of similar ecologies is the basis of so many studies of how evolution works in the long term."
How a rare skeleton was discovered
The rarity of Oligocene Epoch whale skeletons has hindered research efforts to understand the evolution of modern whales' locomotion that is powered by their flukes (tails) but controlled by their forelimbs, the study said.
"We have been waiting for such fossils for decades," said Olivier Lambert, director of operations of Earth and History of Life and Evolution of the Paleobiosphere at the Royal Belgian Institute of Natural Sciences. Lambert wasn't involved in the research.
The skeleton suggested that the features involving their flippers and locomotion might have evolved more recently than 35 million years ago, which was the previous assumption, said study coauthor Robert Boessenecker, a research associate and adjunct instructor in the department of biology and environmental geosciences at the College of Charleston in South Carolina.
"If you're a mammal or reptile invading the water, there's only a certain number of things you can do in order to evolve efficient swimming. And those same features have convergently evolved again and again in different groups," Boessenecker explained. "In this case, they even continued evolving into parallel lineages with common ancestry."
In the 1880s, the partial snout of the dolphin — a toothed whale in the group Odonoceti — was recovered during dredging of the Wando River in South Carolina.
The first skeleton of the dolphin was discovered in the 1970s by then and late Charleston Museum Bunting Natural History curator Albert Sanders. Another nearly complete skeleton, described in the current study, was unearthed during the 1990s, when paleontologist Mark Havenstein found it during construction of a housing subdivision in South Carolina.
It was then donated to the Mace Brown Museum of Natural History for further study, but categorized as belonging to Squalodon, an extinct genus of whales — which researchers of the study said was an incorrect classification.
After Boessenecker was hired by the museum to study these fossils, he took a closer look at the skeleton in 2015. That the skeleton didn't belong in the Squalodon genus was widely known in the research community by then, he said, but no one had done the definitive research to explain why.
Researchers also wanted to figure out why and how baleen whales evolved from toothed whales. They found that features of the dolphin's skeleton beyond its neck implied that modern baleen and toothed whales, though separate, might have evolved similar characteristics due to the parallel evolution in the similar aquatic environments they inhabited.
"The resulting pattern is unexpected given just what we know about the [living animals]," said John Gatesy, a senior research scientist at the American Museum of Natural History who wasn't involved in the study.
Characteristics that were interpreted as traits shared by living cetaceans instead evolved in separate lines of descent, Gatesy added — so modern whales reached where they are today by multiple, similar pathways that trace back to their ancestors.
Evolution of echolocation
Ankylorhiza was the first echolocating whale to become an apex predator because of a cranial joint that allowed a range of motion identical to a modern orca, Boessenecker said.
Ankylorhiza had large teeth with thick roots, which might have strengthened the teeth against fractures while shaking prey to smaller pieces since it didn't have molars — "which is precisely what killer whales do with seals," Boessenecker said.
The dolphin's incisor tusks likely meant it could ram other animals with its teeth. "That's hard to test, but modern dolphins do ram tusks and kill them," Boessenecker said.
After this ancient dolphin went extinct about 23 million years ago, shark-toothed dolphins and giant killer sperm whales evolved to occupy Ankylorhiza's position within 5 million years. Giant killer sperm whales had massive teeth and likely preyed upon smaller whale species, while today's sperm whales eat mostly giant squid.
After killer sperm whales faded away about 5 million years ago, the ecological spot was open until the evolution of killer whales during the ice ages, roughly 2 million years ago.
"There are many other unique and strange early dolphins and baleen whales from Oligocene aged rocks in Charleston, South Carolina," Boessenecker said in a press release.
"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."