Acrophyseter

''Acrophyseter'' is a genus of extinct sperm whale that lived in the Late Miocene off the coast of what is now Peru. The genus comprises two species: ''A. deinodon'' and ''A. robustus''. It is part of a group of macroraptorial sperm whales that all share several features for hunting large prey, such as deeply rooted and thick teeth. ''Acrophyseter'' measured in length, making it the smallest macroraptorial sperm whale currently known. Because of its short pointed snout and strongly curved front teeth, it probably fed on the marine vertebrates of its time, such as seals and other whales.

History of discovery

All known fossils of ''Acrophyseter'', including those of the two named species, were discovered in the Pisco Formation, located in southern Peru. The type species, ''A. deinodon'', was described in 2008 by Olivier Lambert, Giovanni Bianucci and Christian De Muizon from a skull, catalogued MNHN SAS 1626, discovered in the Sud-Sacaco locality. This site is dated between the Tortonian and Messinian stages of the Miocene, around 8.5–6.7 million years ago. The specimen represents a mature individual and consists of a skull and jaw with most of the teeth intact.

In 2017, another species, ''A. robustus'', was described by the same authors from a skull, catalogued as MUSM 1399, discovered in the Cerro la Bruja locality. This locality is older than Sud-Sacaco, dating between the Serravallian and Tortonian stages of the Miocene, at the least older than 9.2 million years. Later, a second ''A. deinodon'' specimen consisting of a right parietal bone was recovered from the Aguada de Lomas locality and recorded as MNHM F-PPI 272. The rocks at Aguada de Lomas are younger than both previously mentioned localities, and the specimen was dated to the Messinian stage of the Miocene, 6.9–6.7 mya. There have been doubts about its referral to ''A. deinodon'', with some suggesting that it actually represents ''A. robustus'' instead.

A third ''Acrophyseter'' skull, catalogued MUSM 2182, was discovered in the Cerro los Quesos locality, dating from the same period as Aguada de Lomas. Its specific attribution has not been formally established, although it shares some similarities with the holotype skull of ''A. robustus''.

The genus name ''Acrophyseter'' is derived from the Greek —meaning acute, which describes the short, pointed, upturned snout—and —which is the genus name for the modern sperm whale ''Physeter macrocephalus''. The species name ''deinodon'' is from the Greek —meaning terrible—and —tooth. The species name ''robustus'' comes from Latin and references the thick bone constituting the edges of the supracranial basin and the base of the rostrum.

Description

Body length estimates for ''Acrophyseter'' range between . ''A. deinodon'' was estimated to be using the distance between the cheekbones in comparison to the dimensions of the related ''Zygophyseter.'' This makes it the smallest of the macroraptorial sperm whales.

Unlike modern sperm whales, ''A. deinodon'' had teeth in both its upper and lower jaws. The teeth were robust and deeply set into the roots, particularly the front teeth, the tooth roots were comparatively thick when compared to the thin tooth crown. The front teeth were more conical than those further back in the mouth. The lower back teeth were tightly packed, and the space between the teeth increased from front to back, suggesting they were used in a shearing motion. This suggests a different feeding strategy from modern sperm whales, which all use suction-feeding due to a lack of teeth in the upper jaw. The front teeth were more worn on the sides, whereas the lower teeth were more worn along the middle.

''A. deinodon'' had 12 teeth in the upper jaw and 13 teeth in the lower jaw and possessed tooth enamel like other macroraptorial sperm whales. The premaxillae bore three teeth, and the maxillae had nine teeth. Unlike in other sperm whales, the top of the premaxillae near the vomer lacked a deep groove. The last lower teeth may have contacted the roof of the mouth, and cementum was continually added to the teeth as they were growing, as in killer whales (''Orcinus orca''). The tooth count of ''A. robustus'' is unknown, though it is thought to be similar to or the same as that of ''A. deinodon''. Discovered along the tooth sockets were buccal exostoses: bony growths which may have developed during biting to strengthen the teeth, acting as buttresses. The back teeth had larger buccal exostoses as they experienced more pressure during biting.

Like other sperm whales, ''Acrophyseter'' had a deep basin on the top of its skull, the supracranial basin. This is overhung by the nuchal crest on the back of the skull. The supracranial basin, in turn, overhangs the orbit around the eye but does not extend onto the snout, unlike in other macroraptorial sperm whales. The temporal fossae on the sides of the skull were as high as they were long, unlike in ''Zygophyseter'' and ''Brygmophyseter.'' This displaces the brow ridge, which slopes down at an angle of around 55 degrees. ''Acrophyseter'''s cheekbones were thin plates which limited the ear canals. The snout was short and, unlike in other sperm whales, had a distinct upward curve. The masseter muscles, used in chewing, were located between the condyloid process, which connects the jaw with the skull, and the teeth. Unlike later species of sperm whales, ''Acrophyseter'' had two nostrils. The left nostril was five times bigger than the right nostril, measuring across, respectively.

Taxonomy

''Acrophyseter'' belongs to a group of macroraptorial sperm whales together with ''Brygmophyseter'', ''Livyatan'' and ''Zygophyseter.'' They all have large, deeply rooted teeth coated in enamel in both the upper and lower jaws and were adapted for hunting large prey items. Macroraptorial sperm whales are thought to have either evolved these adaptations from a basilosaurid-like ancestor or independently once or twice within the group. The extinct subfamily Hoplocetinae has been proposed to house this group, alongside the genera ''Scaldicetus'', ''Diaphorocetus'' and ''Hoplocetus''. However, this grouping is paraphyletic, meaning it does not consist of a common ancestor and all of its descendants. Relationships between ''Acrophyseter'' and other sperm whales are shown below, with the macroraptorial sperm whales in bold and the clades Kogiidae and Physeteridae collapsed.

Paleobiology

Features like the short, pointed snout, and robust, curved front teeth suggest that ''Acrophyseter'' targeted large prey. Their back teeth were potentially utilised in a shearing motion. It is thought that ''Acrophyseter'' would have preyed upon the numerous marine vertebrates that lived alongside it.

Paleoecology

The localities of the Pisco Formation where remains of the animal have been found have yielded the remains of numerous marine vertebrates: the whales '' Piscolithax'' and '' Piscobalaena'', the dolphins '' Brachydelphis'', '' Atocetus iquensis,'' and '' Belonodelphis'', the seal '' Acrophoca'', the penguins ''Spheniscus urbinai'' and ''Spheniscus muizoni'', the marine sloth '' Thalassocnus natans'', the crocodile ''Piscogavialis,'' and the megalodon and broad-toothed mako sharks (''Cosmopolitodus hastalis''). ''Acrophyseter'' was restricted primarily to preying on seals, dolphins, marine sloths, seabirds, and actinopterygians.

References

{{Taxonbar, from=Q779770

Physeteroidea
Prehistoric toothed whales
Prehistoric cetacean genera
Miocene cetaceans
Miocene mammals of South America
Neogene Peru
Fossils of Peru
Pisco Formation
Fossil taxa described in 2008