The newly named genus Ankylorhiza is derived from the Greek word "ankylo" meaning bound, stiff, or fused, and "rhiza", meaning root — meaning fused roots, and referring to the mostly single-rooted condition of the teeth — a surprisingly toothy grin for an early dolphin.
We think of dolphins as the gentle, squeaky darlings of the ocean but back in the Oligocene, they were formidable predators. Picture a mug full of sharp teeth and a body designed for speed. Ankylorhiza tiedemani was the largest member of the Odontoceti, a parvorder or suborder of cetaceans that includes dolphins, porpoise and our toothed whale friends and includes all the whales which eat prey larger than plankton. This toothy group includes sperm whales, beaked whales, river and oceanic dolphins, pilot whales and their cetacean brethren with teeth rather than the baleens we find in Mysticeti whales.
More bits and pieces of this brute were unearthed in the 1970s and 1990s. We usually find just the skulls of our aquatic friends but the nearly complete skeleton that found its way to the Mace Brown Museum of Natural History at the College of Charleston included a well-preserved skull, the ribcage, most of the vertebral column and a lone flipper. These additional bits of the skeleton provided the information necessary to truly tease out this ancient tale. Together, the bones tell the story of a 4.8 m predator who would have diverged from baleen whales — but continued to evolve convergent similarities — about 35-36 million years ago.
This beast of a dolphin hunted our ancient seas some 24 million years ago. He was a fast swimmer with a narrow tailstock, some added tail vertebra and a shorter humorous — upper arm bone — in his flippers. Some dolphins can exceed speeds of 50 km/h, a feat accomplished by thrusting the flukes while adjusting attack angle with their flippers. These movements are driven by robust axial musculature anchored to a relatively rigid torso consisting of numerous short vertebrae and controlled by hydrofoil-like flippers.
Eocene skeletons of whales illustrate the transition from semiaquatic to aquatic locomotion, including the development of a fusiform body and reduction of hindlimbs, but the rarity of Oligocene whale skeletons has hampered efforts to understand the evolution of fluke-powered, but forelimb-controlled, locomotion. Modern whales and dolphins are superbly adapted for marine life, with tail flukes being a key innovation shared by all extant species. Did ancient dolphins have these modifications for speed? Most thought not. We have the benefit of modern species to make tentative comparisons but need ancient specimens to confirm the hypothesis.
Kudos to Robert Boessnecker and team for their paper in the journal Current Biology. In it, they report a nearly complete skeleton of the extinct large dolphin Ankylorhiza tiedemani comb. n. from the Oligocene of South Carolina, previously known only from a partial rostrum. Its forelimb is intermediate in morphology between stem cetaceans and extant taxa, whereas its axial skeleton displays incipient rigidity at the base of the tail with a flexible lumbar region.
The position of Ankylorhiza near the base of the odontocete radiation implies that several postcranial specializations of extant cetaceans, including a shortened humerus, narrow peduncle, and loss of radial tuberosity, evolved convergently in odontocetes and mysticetes. Craniodental morphology, tooth wear, torso vertebral morphology, and body size all suggest that Ankylorhiza was a macrophagous predator that could swim relatively fast, indicating that it was one of the few extinct cetaceans to occupy a niche similar to that of killer whales.
If you fancy a read, here's the reference:
Robert W. Boessenecker et al. Convergent Evolution of Swimming Adaptations in Modern Whales Revealed by a Large Macrophagous Dolphin from the Oligocene of South Carolina. Current Biology, published online July 9, 2020; doi: 10.1016/j.cub.2020.06.012