New fossils unveil dolphin ancestors from 28 million years ago

Ever wondered what dolphins' ancestors looked like? Meet Olympicetus thalassodon, a newly discovered species that lived in the North Pacific coast approximately 28 million years ago.
Mrigakshi Dixit
Life reconstruction of Olympicetus thalassodon
Life reconstruction of Olympicetus thalassodon

Art by Cullen Townsend. 

Scientists have discovered fossilized remains of dolphins' ancient ancestors— toothed whales (odontocetes)— according to a study published on June 23 in the journal PeerJ.

Olympicetus thalassodon, the new species, lived in the waters of the North Pacific coast approximately 28 million years ago.

The Natural History Museum of Los Angeles County (NHMLAC) experts believe that this new species discovery may help them better understand the early history and diversity of modern dolphins, porpoises, and other toothed whales. 

The discovery of the fossil remains 

The fossilized bones were discovered in the Pysht Formation, a geologic unit located along the Olympic Peninsula in Washington State. The formation dates back to between 26.5 to 30.5 million years. 

The scientists also unearthed the fossil specimens of two other closely related odontocetes from this same location. This indicates that the new species belonged to the group of early odontocete, or toothed whales. Odontocetes first evolved around 33.7 million to 23.8 million years ago, during the Oligocene Epoch.

New fossils unveil dolphin ancestors from 28 million years ago
Recovered Simocetidae fossils.

Upon examination of the remains, it was found that the Olympicetus and its relatives belonged to the Simocetidae family, which is only known from the North Pacific. It is also considered as one of the first diverging groups of toothed whales. 

Olympicetus thalassodon and its close relatives show a combination of features that truly sets them apart from any other group of toothed whales," explained Jorge Velez-Juarbe, who is an associate curator of marine mammals at NHMLAC, in an official release

"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."

Diversity of Simocetids

The team noticed several distinct biological features in the newly found fossils of the various extinct creatures from the site.

Body size, dentition, and other feeding-related characteristics differed in the recovered fossils — this indicates that simocetids most likely had distinct ways of "prey acquisition and likely prey preferences."

“The teeth of Olympicetus are truly weird, they are what we refer to as heterodont, meaning that they show differences along the toothrow. This stands out against the teeth of more advanced odontocetes whose teeth are simpler and tend to look nearly the same,” added Velez-Juarbe.  

Other aspects of the biology of these early-toothed whales, such as whether they could echolocate like their modern relatives, remain unknown. Their skulls do suggest the presence of a melon, a crucial echolocation-related feature. Up next, the team will examine the remains to better understand this feature.

According to the study, Simocetids were part of a remarkable diversity of fauna, as evidenced by the excavated fossil remains. Plotopterids, an extinct group of flightless, penguin-like birds, were among the fauna. It also included desmostylians (early relatives of seals and walruses) and toothed baleen whales.

The full study results have been published in the journal PeerJ and can be found here.

Study abstract:

Odontocetes first appeared in the fossil record by the early Oligocene, and their early evolutionary history can provide clues as to how some of their unique adaptations, such as echolocation, evolved. Here, three new specimens from the early to late Oligocene Pysht Formation are described further increasing our understanding of the richness and diversity of early odontocetes, particularly for the North Pacific. Phylogenetic analysis shows that the new specimens are part of a more inclusive, redefined Simocetidae, which now includes Simocetus rayi, Olympicetus sp. 1, Olympicetus avitus, O. thalassodon sp.nov., and a large unnamed taxon (Simocetidae gen. et sp. A), all part of a North Pacific clade that represents one of the earliest diverging groups of odontocetes. Amongst these, Olympicetus thalassodon sp. nov. represents one of the best known simocetids, offering new information on the cranial and dental morphology of early odontocetes. Furthermore, the inclusion of CCNHM 1000, here considered to represent a neonate of Olympicetus sp., as part of the Simocetidae, suggests that members of this group may not have had the capability of ultrasonic hearing, at least during their early ontogenetic stages. Based on the new specimens, the dentition of simocetids is interpreted as being plesiomorphic, with a tooth count more akin to that of basilosaurids and early toothed mysticetes, while other features of the skull and hyoid suggest various forms of prey acquisition, including raptorial or combined feeding in Olympicetus spp., and suction feeding in Simocetus. Finally, body size estimates show that small to moderately large taxa are present in Simocetidae, with the largest taxon represented by Simocetidae gen. et sp. A with an estimated body length of 3 m, which places it as the largest known simocetid, and amongst the largest Oligocene odontocetes. The new specimens described here 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.

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