| Acrophyseter Temporal range: Late Miocene
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| Acrophyseter deinodon skull | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Infraorder: | Cetacea |
| Superfamily: | Physeteroidea |
| Family: | incertae sedis |
| Genus: | †Acrophyseter Lambert, Bianucci, and Muizon, 2008 |
| Type species | |
| A. deinodon | |
| Species | |
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Acrophyseter is a genus of stem-sperm whales that lived around 12-7 million years ago in waters off the coast of Peru. It consists two species: A. deinodon and A. robustus.[1]
Taxonomy
Acrophyseter, together with Brygmophyseter, Livyatan, and Zygophyseter, belong to a group of raptorial stem sperm whales, which have adaptations to hunting large prey. They all have large, deeply rooted teeth coated in enamel in both the upper and lower jaws, unlike the modern sperm whale (Physeter macrocephalus) which lacks enamel and teeth in the upper jaw.[2] Raptorials are thought to have either evolved these adaptations from a basilosaurid-like ancestor or independently once or twice within the group.[3] The extinct subfamily Hoplocetinae has been proposed to house this group, alongside the genera Scaldicetus, Diaphorocetus, Idiorophus, and Hoplocetus. This subfamily is paraphyletic, in that it does not consist of a common ancestor and all of its descendants.[4]
The genus Acrophyseter houses two species. The type species, A. deinodon, was discovered in the Pisco Formation in Peru, dating back to the Tortonian–Messinian stages of the Miocene around 6 million years ago (mya); the holotype specimen, MNHN SAS 1626, represents a mature individual and consists of a skull and jaw with most of the teeth intact.[1] The second species, A. robustus, is known from a skull also from the Pisco Formation, named MUSM 2182, dating back to the Serravallian–Tortonian stages of the Miocene.[3] A second A. deinodon specimen was named MNHM F-PPI 272, though it is possible it actually represents A. robustus. A. robustus had a more pointed snout, straighter teeth, a decreasing width of the mandible from front to back, a groove on the side of its snout, and a more well-define supracranial basin which housed the melon organ than A. deinodon.[3]
The genus name Acrophyseter is derived from the Greek akros, meaning acute, which describes the short, pointed, upturned snout; and Physeter, which is the genus name for the modern sperm whale (Physeter macrocephalus). The species epithet deinodon is from the Greek deinos, meaning terrible, and odon, tooth.[1]
| Relationships between Brygmophyseter and other sperm whales, raptorials in bold[5][3] |
Description
Acrophyseter was estimated to be 3.9–4.3 metres (13–14 ft) using the distance between the cheek bones in comparison to the dimensions of Zygophyseter, which is relatively small, being around half the size of Zygophyseter.[3]
Unlike the modern sperm whales, A. deinodon had teeth on 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 with the thin tooth crown. The front teeth were more conical than the back teeth. The lower back teeth were close together, and the space between the teeth increased from front to back, suggesting they were used for shearing, unlike the suction-feeding modern-day sperm whales which lack teeth in their upper jaws. The front teeth were more worn on the sides, whereas the bottom teeth were more worn along the middle. It had 12 teeth in the upper jaw and 13 teeth in the bottom jaw, and like other raptorials it had tooth enamel. The premaxillae bore three teeth, and the maxillae had nine teeth. The last bottom teeth may have contacted the roof of the mouth.[1] The tooth count of A. robustus is unknown though thought to be similar or the same. Cementum was continually added to the teeth as they were growing, as in killer whales (Orcinus orca).[3]
Like other sperm whales, Acrophyseter had a deep basin on the top of its skull, the supracranial basin. This basin in Acrophyseter overhung the orbit around the eye, but did not extend onto the snout, unlike in other raptorials. Unlike later species of sperm whales, Acrophyseter had two nostrils. The temporal fossae on the sides of the skull were as high as they were long, unlike in Zygophyseter and Brygmophyseter, which displaced the brow ridge. The area between the condyloid process, which connects the jaw with the skull, and the teeth was probably where the masseter muscles were. The brow ridge slopes down at an angle of around 55°. The nuchal crest on the back side of the skull had overhung the supracranial basin. The cheekbones were thin plates which limited the ear canals. The snout was short and, unlike in other sperm whales, had a distinct upward curve. Unlike other sperm whales, the top of the premaxillae near the vomer lacked a deep groove.[1] The left nostril was five times bigger than the right nostril, measuring 30 and 7.2 millimetres (1.18 and 0.28 in) across respectively.[3]
Paleoecology
The short and pointed snout, coupled with the robust, curved front teeth suggests Acrophyseter targeted large prey and perhaps used their back teeth for shearing. It may have preyed upon smaller whales such as Piscolithax, seals such as Acrophoca, and seabirds such as the penguin Spheniscus urbinai.[1] Other marine vertebrates that were discovered in the Pisco Formation are the whale Piscobalaena, the marine sloth Thalassocnus natans, the crocodile Piscogavialis, megalodon, and the broad-toothed mako.[3]
References
- ^ a b c d e f Lambert, Olivier; Bianucci, Giovanni; Demuizon, Christian (2008). "A New Stem-Sperm Whale (Cetacea, Odontoceti, Physeteroidea) from the Latest Miocene of Peru". Comptes Rendus Palevol. 7: 361–369. doi:10.1016/j.crpv.2008.06.002.
- ^ Bianucci, G.; Landini, W. (2006). "Killer Sperm Whale: a New Basal Physeteroid (Mammalia, Cetacea) from the Late Miocene of Italy". Zoological Journal of the Linnean Society. 148: 103–131. doi:10.1111/j.1096-3642.2006.00228.x.
- ^ a b c d e f g h Lambert, O.; Bianucci, G.; de Muizon, C. (2017). "Macroraptorial Sperm Whales (Cetacea, Odontoceti, Physeteroidea) from the Miocene of Peru". Zoological Journal of the Linnean Society. 179: 404–474. doi:10.1111/zoj.12456.
- ^ Toscano, A.; Abad, M.; Ruiz, F.; Muñiz, F.; Álvarez, G.; García, E.; Caro, J. A. (2013). "Nuevos Restos de Scaldicetus (Cetacea, Odontoceti, Physeteridae) del Mioceno Superior, Sector Occidental de la Cuenca del Guadalquivir (Sur de España)" [New Remains of Scaldicetus (Cetacea, Odontoceti, Physeteridae) from the Upper Miocene, Western Sector of the Guadalquivir Basin]. Revista Mexicana de Ciencias Geológicas (in Spanish). 30 (2).
- ^ Berta, A. (2017). The Rise of Marine Mammals: 50 Million Years of Evolution. Baltimore, Maryland: Johns Hopkins University Press. pp. 112–113. ISBN 978-1-4214-2326-5.
