The animal sheds light on the evolution of toothed cetaceans such as certain whales, belugas, or orcas.
Ankylorhiza tieemani lived about 25 million years ago. It was known before only by a partial rostrum fossil: the paleontologists were therefore far from defining the profile of this dolphin. But the discovery in South Carolina of an almost complete skeleton reveals how much he was a hunter specializing in large prey.
The dolphin fossil was discovered in the 1990s during the construction of a housing estate and was donated to the Mace Brown Museum of Natural History in Charleston. It was finally studied by an American team that publishes its description in the journal Current Biology. The animal belongs to the micro-order of the odontocetes, the toothed cetaceans. It was an animal about 4.8 meters long, making it the largest of its time.
He was capable of fast swimming and his anatomy indicates that he was a formidable predator who could attack animals the size of a modern dolphin with a jaw whose power had nothing to envy to the current orcas. These teeth, on the other hand, were surprisingly simple with no cusps or cingulas, and some were badly worn, no doubt by grinding the bones of large prey.
Ankylorhiza is also the first odontocetes equipped with an echolocation organ to become an advanced predator. When it disappeared 23 million years ago, killer whales such as the Melville leviathan and shark-toothed dolphins (Squalodontes) evolved and reoccupied the niche in 5 million years. After the last sperm whales disappeared around 5 million years ago, there was an absence of similar large predators until the advent of orcas 1 to 2 million years ago.
Researchers have noted that many anatomical features of this dolphin are similar to those of modern odontocetes as well as to the mysticetes, baleen whales. It seems that all of these groups have developed identical adaptations to the same constraints, another case of convergent evolution.
Among the common evolutions, they note the narrowing and stiffening of the tail, the increase in the number of caudal vertebrae, and the shortening of the humerus in the fin. To find out a little more, we will have to wait a little longer: several other Ankylorhiza fossils are under investigation, including a second unknown species as well as juvenile specimens.