Macropredatory sperm whales

Miocene-aged rocks of Peru's Pisco Formation preserve a large number of fossil physeteroids from at least three species in two genera showing that ancient physeteroids, unlike modern sperm whales, were so-called macroraptorial predators, focusing on large prey animals including marine mammals. The Pisco Formation is exposed near Peru's Pacific coast south of Lima. The available prey items preserved in rocks of the Pisco Formation include baleen whales, seals, penguins, crocodiles, and sharks, as well as abundant fish and squid.

In 2016, paleontologists Olivier Lambert, Giovanni Bianucci, and Christian de Muizon published a study reviewing several specimens of macroraptorial physeteroids from the Pisco Formation. The smaller-bodied genus is called Acrophyseter and is known from numerous locations in the Pisco Formation in rocks from roughly seven to nine million years old. There are at least two species of Acrophyseter known, A. deinodon from 7.3 million-year-old rocks of the Sud Sacaco site in the Arequipe Region, and a newly described species, A. robustus from 9.2 million-year-old rocks of the Cerro la Bruja site in the Ica Region. The authors also describe an Acrophyseter skull that is from 6.8 million-year-old rocks from the Cerro los Quesos site in the Ica Region that can't be assigned to any species.

All species of Acrophyseter have long, robust skulls with upturned snouts. The upper and lower jaws were lined with thick, recurved teeth, resembling half bananas and similar to what's found in living orcas. In life, Acrophyseter species were between four and five meters (13 to 16 feet) in length, roughly the same size as modern belugas, but with larger more orca-like heads. The two species of Acrophyseter differ in details of their skull anatomy with A. robustus having a skull that is overall more robust, longer, and narrower than that of A. deinodon. In life their feeding habits were probably similar to orcas focusing on marine mammals in addition to fish and squid.

The second genus, the enormous Livyatan, is known from a single species, L. melvillei found in nine million-year-old rocks. Livyatan was discovered at the Cerro Colorado locality in the Ica Region in rocks near the base of the Pisco Formation. It is named after the Leviathan of the Bible, thought to represent a whale, and Herman Melville, author of Moby Dick.

Livyatan was truly a giant, with a skull estimated to have been three meters (10 feet) long and a total body length of 13 to 17 meters (42 to 57 feet). This is nearly the same size as living sperm whales. Like Acrophyseter and living orcas but unlike living sperm whales, both the upper and lower jaws have a number of quite large recurved teeth indicating that it too focused on large prey.

Although known only from Peru, it's possible that like living whales of similar size, both Acrophyseter and Livyatan were present in a much wider area. Among living whales, the largest species such as sperm whales and orcas have a nearly global distribution, while smaller species have ranges that are restricted but still quite large.

Lambert and colleagues conducted a phylogenetic analysis to identify where Acrophyseter and Livyatan belong on the physeteroid family tree. They found that Acrophyseter and Livyatan fall outside the living sperm whale clade, but are not particularly closely related to each other. Acrophyseter was found to be closely related to Brygmophyseter from the Middle Miocene of Japan and Zygophyseter from the Late Miocene of Italy, both mid-sized top predators like Acrophyseter. Livyatitan was found to be more closely related to the living sperm whales than to the mid-sized Acrophyseter clade.

The Miocene macroraptorial physteroids found in Peru were among the ocean's top predators at the time, and filled the roles once filled by mosasaurs and pliosaurs in the Mesozoic, however five million years ago, all the macroraptorial physeteroids were extinct, and the only physeteroids left living were close relatives of the giant Physeter whales and smaller Kogia whales. The top predator role they once filled has now been filled by orcas among whales.

Read the original research in the Zoological Journal of the Linnean Society

//onlinelibrary.wiley.com/doi/10.1111/zoj.12456/abstract

Pete
Buchholz

Senior Writer

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