An illustration of a pod of giant tusked dolphins hunting
Robert W Boessenecker
A 24-million-year-old fossil of a giant tusked dolphin lacks several features common to modern dolphins and baleen whales. The discovery shows that the common ancestor of dolphins and whales lacked these features, meaning the same adaptations for swimming must have evolved independently in both lineages.
“We were surprised to find so many archaic features in an extinct dolphin,” says Robert Boessenecker at the College of Charleston in South Carolina.
Ankylorhiza tiedemani was one of the top predators in the sea from around 30 to 23 million years ago. The 5-metre-long dolphin would have looked much like a bottlenose dolphin, says Boessenecker, apart from its front teeth. These stick straight out and may have been used for ramming prey.
Boessenecker’s team has been studying an A. tiedemani fossil found in the 1990s. “No one had found such a complete skeleton before,” he says. It revealed an unexpected lack of modern features.
Compared to ancient ancestors all modern whales and dolphins have extra vertebrae in their tails, giving them more flexibility and swimming power, and a very narrow base to the tail, just before the tail flukes. The “upper arm” bone in their pectoral flippers is very short relative to the other bones, and they have two or three extra finger bones. This helps make the flippers larger and stiffer, improving manoeuvreability.
It has been assumed that these features all evolved before the ancestors of baleen whales split from the ancestors of echolocating dolphins around 35 million years ago, says Boessenecker, but the fossil shows these features are instead a result of convergent evolution.
“All these features evolved at least twice,” he says.
We don’t know what Ankylorhiza preyed on. But if it lived in pods it would have been able to eat just about anything it wanted, says Boessenecker – including the 13-metre-long predecessors of megalodon sharks that lived at this time.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2020.06.012
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