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  6. New fossil species of toothless dolphin expands knowledge of early whale evolution

New fossil species of toothless dolphin expands knowledge of early whale evolution

Robert Boessenecker © Robert Boessenecker

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Illustration of Inermorostrum xenops in the Oligocene-epoch ocean.  

OTTAWA, August 23, 2017 –The discovery of a fossil skull by a diver in Charleston, South Carolina has led to the identification of a new species of extinct dolphin. The research team included Dr. Danielle Fraser, a paleontologist at the Canadian Museum of Nature. The toothless dolphin, which lived about 28-30 million years ago, provides new evidence of the evolution of feeding behaviour in whales (which includes dolphins). 

According to College of Charleston adjunct geology professor Robert W. Boessenecker, this newly discovered dolphin is a dwarf dolphin with a short snout and entirely lacks teeth. The genus name Inermorostrum xenops means “defenseless snout”, referring to its toothless condition.

Boessenecker is the lead author of the study, which was published in the journal, Proceedings of the Royal Society B. Fraser helped determine how the new specimen improves knowledge of early whale evolution. Researchers from the New York Institute of Technology (Jonathan Geisler and Morgan Churchill) established the evolutionary relationship of Inermorostrum with other known fossil whale specimens.  

Inermorostrum is the earliest known suction feeder, which describes one of two ways that toothed whales ingest food. Living toothed whales include dolphins and porpoises, beaked whales, and sperm whales.

Boessenecker says this newly discovered species belongs to an early group of echolocating dolphins, the Xenorophidae, which represent the earliest diversification of the toothed whales. Toothed whales evolved only four million years before Inermorostrum appeared; the ancestors were the basilosaurid archaeocetes, which had a precisely occluding set of teeth. Inermorostrum indicates that it took only four million years to evolve a toothless, suction-feeding specialist from ancestral whales.

“We studied the evolution of snout length in whales, and found that during the Oligocene (25-35 million years ago) and early-Miocene epochs (20-25 million years ago), the echolocating whales rapidly evolved extremely short snouts and extremely long snouts. This represents an adaptive radiation in feeding behavior and specializations,” says Boessenecker. “We also found that short snouts and long snouts have both evolved numerous times on different parts of the evolutionary tree—and that modern dolphins such as the bottlenose dolphin, which have a snout twice as long as it is wide, represent the optimum length as it permits both fish catching and suction feeding."

Boessenecker proposes that this new species of dolphin was primarily a suction feeder, perhaps feeding on fish, squid, and other soft-bodied invertebrates from around the seafloor. The feeding behaviour would perhaps be similar to that of a walrus. Furthermore, a series of deep channels and holes for arteries on the snout indicate the presence of extensive soft tissues, likely enlarged lips and also perhaps whiskers.

The researchers estimate that the dolphin grew to only about four feet long (1.2 metres), smaller than its closest relatives. Modern bottlenose dolphins are commonly seven to twelve feet (2.1 – 3.5 metres) in length.

“The discovery of a suction-feeding whale this early in their evolution is forcing us to revise what we know about how quickly new forms appeared, and what may have been driving early whale evolution” explains Fraser. “Increased ocean productivity may have been one important factor,” she says.

Many species of Oligocene whales have been described from South Carolina, with several discovered in and around Charleston. The area is among a few in the world, including others in New Zealand, Japan, and the Pacific Northwest, to offer a window into early toothed whale evolution.

The skull of Inermorostrum was discovered by a diver in the Wando River in Charleston and is now on display in the Mace Brown Museum of Natural History at College of Charleston. "This discovery once again highlights deposits around Charleston as recording a hotspot of ancient dolphin and whale diversity" says Boessenecker.

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Information for media:

Dan Smythe
Head, Media Relations
Canadian Museum of Nature
613.566.4781; 613.698.9253 (cell)
dsmythe@mus-nature.ca

Mike Robertson
Senior Director of Media Relations
College of Charleston
843.953.5667; 843.870-1277 (cell)
Robertsonm@cofc.edu