Llanocetus

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Llanocetus (“Llano's whale”) is a genus of extinct toothed baleen whales from the Late Eocene of Antarctica. The whale reached gigantic proportions, with the juvenile specimen reaching an estimated 8 m (26 ft) in length. Like contemporary baleen whales, Llanocetus completely lacked baleen in its jaws. It was probably a suction feeder like the modern beaked and pygmy right whales.

History

Llanocetus was described in 1989 by paleontologist Edward Mitchell based on the braincase and a partial mandible with two teeth, specimen USNM 183022, from the La Meseta Formation of Seymour Island, Antarctica.[1] Dating back to the Late Eocene, it is the second-oldest known baleen whale, behind Mystacodon.[2] However, Mitchell classified it into the new infraorder Crenaticeti as an intermediary between Archaeoceti–the forerunners of modern whale types–and more modern baleen whales,[1] but Crenaticeti was later invalidated in 2016.[3]

The genus name is in honor of the American biologist George A. Llano who helped Mitchell gain access to the specimen. The species name derives from Latin denti, teeth, and crena, notch, in reference to the tooth shape which is similar to the crabeater seal.[1]

The complete skull was described in 2018 by paleontologists Robert Ewan Fordyce and Felix Marx. It is grouped into the Eocene family Llanocetidae along with Mystacodon and an as yet undescribed specimen OU GS10897 from New Zealand.[2] Though Mitchell, based on the size and density of the bone, thought the specimen was an adult,[1] Fordyce and Marx concluded it was a juvenile due to the loss of both epiphyses–which are the surfaces that the vertebra connects to the other vertebrae to–on the single preserved neck vertebra.

Description

Llanocetus had several ancient characteristics reminiscent of archaeocetes. The dental formula, Template:DentalFormula, indicating number of, in order, incisors, canines, premolars, and molars in one half of a jaw, is similar to basilosaurid archaeocetes. However, the broad snout is unlike archaeocetes. Wearing patterns on the cheek teeth, the molars and premolars, indicate they sheared passed each other while biting, which would have given Llanocetus the ability to slice through flesh, and serration wearing indicates a gripping function. It had wide gaps (diastema) between the teeth.[2] Its teeth had one of the thickest enamel layers of any known baleen whale, 830–890 μm at the top and 350–380 μm at the base, which is also consistent with a shearing action.[4] It had a crest on the mandible which may have supported proper musculature to pucker its lips.[2]

All baleen whales have in their mouth palatal sulci, which carry blood, between tooth sockets, which has generally thought to be indicative of baleen. However, in Llanocetus, these sulci are present within the tooth sockets, meaning sulci are not always indicative of baleen, and the whale probably did not have baleen. Palatal sulci are generally associated with large gums, and this enhanced blood supply in these early whales may have eventually led to the secondary evolution of baleen in later whales.[2]

The estimated minimum length for this juvenile specimen is 8 m (26 ft), comparable to a modern day adult minke whale, and exceeding the size of most whales until the Late Miocene. For most of the Oligocene and Miocene, whales generally stayed well below 6 m (20 ft) in length. Large size in baleen whales was thought to be linked to the evolution of baleen and bulk feeding behavior, however Llanocetus is evidence that gigantism evolved multiple times in baleen whales. The size of Llanocetus is probably due to its polar environment or potential long-distance traveling.