Olympicetus thalassodon is one of several that are helping us understand the early history and diversification of modern dolphins, porpoises and other toothed whales.
Life reconstruction of Olympicetus thalassodon pursuing a school of fishes alongside plotopterid birds (background) somewhere in the eastern North Pacific Ocean. Image credit: Cullen Townsend.
Olympicetus thalassodon swam along the North Pacific coastline around 28 million years ago (Oligocene epoch).
“Olympicetus thalassodon and its close relatives show a combination of features that truly sets them apart from any other group of toothed whales,” said Dr. Jorge Velez-Juarbe, a paleontologist in the Department of Mammalogy at the Natural History Museum of Los Angeles County.
“Some of these characteristics, like the multi-cusped teeth, symmetric skulls, and forward position of the nostrils makes them look more like an intermediate between archaic whales and the dolphins we are more familiar with.”
In addition to Olympicetus thalassodon, Dr. Velez-Juarbe examined the fossilized remains of two other closely related toothed whales.
The specimens were found in the Pysht Formation, exposed along the coast of the Olympic Peninsula in Washington State, the United States.
Dr. Velez-Juarbe found that Olympicetus thalassodon and its close kin belonged to Simocetidae, a family so far known only from the North Pacific and one of the earliest diverging groups of toothed whales.
Simocetids formed part of an unusual fauna represented by fossils found in the Pysht Formation and which included plotopterids (an extinct group of flightless, penguin-like birds), the bizarre desmostylians, early relatives of seals and walruses, and toothed baleen whales.
Differences in body size, teeth and other feeding-related structures suggest that simocetids showed different forms of prey acquisition and likely prey preferences.
“The teeth of Olympicetus thalassodon are truly weird, they are what we refer to as heterodont, meaning that they show differences along the toothrow,” Dr. Velez-Juarbe said.
“This stands out against the teeth of more advanced odontocetes whose teeth are simpler and tend to look nearly the same.”
However, other aspects of the biology of these early toothed whales remain to be elucidated, such as whether they could echolocate like their living relatives, or not.
Some aspects of their skull can be related to the presence of echolocating-related structures, such as a melon.
An earlier study had suggested that neonatal individuals could not hear ultrasonic sounds, so the next step would be to investigate the earbones of subadult and adult individuals to test whether this changed as they grew older.
“The new specimens add to a growing list of Oligocene marine tetrapods from the North Pacific, further promoting faunistic comparisons across other contemporaneous and younger assemblages, that will allow for an improved understanding of the evolution of marine faunas in the region,” Dr. Velez-Juarbe said.
His paper was published in the journal PeerJ.
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J. Velez-Juarbe. 2023. New heterodont odontocetes from the Oligocene Pysht Formation in Washington State, U.S.A., and a reevaluation of Simocetidae (Cetacea, Odontoceti) PeerJ 11: e15576; doi: 10.7717/peerj.15576
