Toothless Dwarf Dolphin Provides Insights into Cetacean Evolution

Tuesday, October 10, 2017

 Inermorostrum xenops

A new species of extinct dwarf dolphin that lived about 30 million years ago (Oligocene epoch) and possessed adaptations for suction feeding has been identified from a fossil found in the Wando River in Charleston, South Carolina.

Named Inermorostrum xenops, the newfound dolphin provides new evidence of the evolution of feeding behavior in cetaceans (whales, dolphins and porpoise).

It had a short snout, entirely lacked teeth, and was only 4 feet (1.2 m) long, smaller than its closest relatives and significantly smaller than today’s bottlenose dolphins (7-12 feet, or 2.1-3.7 m, in length).

Inermorostrum xenops fed primarily on fish, squid, and other soft-bodied invertebrates from the seafloor, similar to the feeding behavior of a walrus,” said Professor Robert Boessenecker, from the University of California Museum of Paleontology and College of Charleston.

“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 even whiskers.”

A skull of Inermorostrum xenops. Abbreviations: Fr – frontal, gpf – greater palatine foramen, ioc – infraorbital canal, iof – infraorbital foramen, La – lacrimal, Mx – maxilla, Na – nasal, np – nasal process of premaxilla, og – optic groove, Pl – palatine, pf – premaxillary foramen, ps – palatine sulcus, psf – premaxillary sac fossa, Px – premaxilla, rb – rostral basin, rf – reentrant foramina, Vo – vomer, xn – external nares. Image credit: Boessenecker et al, doi: 10.1098/rspb.2017.0531.

Professor Boessenecker and his colleagues from the University of Wisconsin Oshkosh, the Canadian Museum of Nature and the New York Institute of Technology also studied the evolution of snout length in cetaceans.

They 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, representing an adaptive radiation in feeding behavior and specializations.

They also found that short snouts and long snouts have both evolved numerous times on different parts of the evolutionary tree — and that modern dolphins like 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.

“The identification of Inermorostrum xenops opens up new questions about the evolution of early whales,” said Dr. Danielle Fraser, a paleontologist at the Canadian Museum of Nature.

“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.”

“Increased ocean productivity may have been one important factor,” she said.

The research is published in the Proceedings of the Royal Society B.


Robert W. Boessenecker et al. 2017. A toothless dwarf dolphin (Odontoceti: Xenorophidae) points to explosive feeding diversification of modern whales (Neoceti). Proceedings of the Royal Society B 284 (1861); doi: 10.1098/rspb.2017.0531