How Modern Whales Swim is Linked to the Skeleton of Extinct Dolphin

The evolution of whales dates back to species from over 50 million years ago when their ancestor species were land animals. The recent discovery of an extinct dolphin in South Carolina has been linked to the swimming adaptations in modern whales.

Recently published in Current Biology, it is the first detailed report of a 15-foot-long extinct dolphin called Ankylorhiza tiedemani. The giant dolphin roamed the earth about 25 million years ago during the Oligocene epoch in the Cenozoic Era which was previously discovered from a partial snout fossil or a rostrum.

There was prominent evidence of the dolphin's anatomy that made it a top underwater predator namely its sharp teeth, flippers, and vertebral column. The team implied that its postcranial skeleton resembles the modern toothed whales like the baleen whale in what they call a parallel evolution - two different species evolving but with similar trails and ancestral conditions.

Parallel Evolution

'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 Robert Boessenecker of the College of Charleston in South Carolina. Shared traits in the parallel evolution include the tailstock narrowing, tail vertebrae increasing in number, and the humerus of the flipper shortening through time.

Species of seals and sea lions, which have evolved in their swimming adaptations from evidence in postcranial skeletons, did not develop the same way. Boessenecker said, '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.'

The first skull fragments of the Ankylorhiza were discovered during the 1880s in the Wando River while the first skeleton was discovered in the 1970s by Alber Sanders, the curator of the Charleston Museum Natural History. 20 years later, scientists conducted a study from a nearly complete skeleton, discovered by paleontologist Mark Havenstein during the construction in a subdivision.

Experts continue to uncover more evidence from the extinct dolphin, but have ruled out that it was 'very clearly preying upon large-bodied prey like a killer whale.' Furthermore, the Ankylorhiza was the first apex predator, or alpha species, from the echolocating whale family.

Dolphins and Whales

When the giant dolphin species became extinct nearly 23 million years back, Livyatans, or killer sperm whales, and Squalodons, an extinct shark-toothed dolphin species, evolved and took over the niche within five million years. When the killer sperm whales died five million years ago, the niche was open for the evolution of killer whales.

Dolphins and whales have evolved through a long and complicated history not easily identified in modern species, explained Boessenecker. 'The fossil record has really cracked open this long, winding evolutionary path, and fossils like Ankylorhiza help illuminate how this happened."

Boessenecker concluded, '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.'

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