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Zygophyseter is an extinct species of macroraptorial sperm whales that was by far one of the most unique to ever exist. It is represented by an almost complete skeleton and lived in what is now Italy during the Miocene.

Zygophyseter-size

Paleoecology[]

Zygophyseter is thought to have been the ancestor to modern-day dolphins, due to the fact that it remained quite small for a whale, and even has a "bill", while another closely related whale that lived at the same time, Livyatan, grew absurdly large, but lacked the "bill" of Zygophyseter. It is thought that Zygophyseter and Livyatan had a common ancestor and that there was a divergence that resulted in these two genera. Zygophyseter eventually evolved into dolphins and orcas while Livyatan would become closer to the sperm whales of today, and possibly also baleen whales.

Due to its transitioning form, Zygophyseteris thought to have fed like dolphins as well, albeit slower. It would have most likely faced predatory threats from its own relative Livyatan as well as the gigantic shark C. Megalodon. Zygophyseter likely went exinct due to its own descendants, now closer to modern dolphins, performing the niche that this prehistoric whale did, but better. Zygophyseter was not able to keep up, and went extinct.

Paleobiology[]

Since the teeth of Zygophyseter are large, exhibit wearing not unlike the teeth of modern-day killer whales, and had functionality in both the upper and lower jaws, it was likely a macropredator. The position of the condyloid processes between the jaw and the skull, like in the modern sperm whale, allowed it to open its jaw wider in order to grab large prey. Its apparent similarity to the feeding habits of the killer whale gave it its nickname "killer sperm whale".

A 2021 multi-author study led by Emanuele Peri reconstructed the bite force of Zygophyseter using finite element analysis of the skull. The model calculated an anterior bite force (the bite force at the front end of the jaws) of 4,812 newtons (1,082 lbf) and posterior bite force (at the back end of the jaws) of 10,823 newtons (2,433 lbf) from a bite simulated at a 35° jaw gape. This is roughly the same bite force that could be exerted by an adult great white shark that is 5.01–5.36 meters (16.4–17.6 ft) long and is stronger than that in other strong-biting animals like lions, though not as strong as in saltwater crocodiles and Basilosaurus isis. Nevertheless, the posterior bite force of Zygophyseter was strong enough to crush bone.

The significant disparity between the anterior and posterior bite forces and the pattern of stress distribution in the finite element analysis model suggests that Zygophyseter employed a "grip-and-shear" feeding strategy, in which the animal would grasp prey with its front teeth and cut them using its back teeth. This strategy somewhat unique, being absent in modern marine macropredators such as sharks and orcas, which instead use a "grip-and-tear" method that dismembers prey by holding and shaking them, and was only previously present in some basilosaurids. However, it is likely that the feeding strategy evolved independently in Zygophyseter and related macroraptorial sperm whales, as it was absent in more ancestral genera like Eudelphis. Given the similar bite force between Zygophyseter and a fully-grown great white shark, it was hypothesized that the cetacean occupied a similar ecological niche that primarily fed on local large fish such as marlin and wahoos and small to medium-sized marine mammals such as seals, dugongs, and small cetaceans. However, neither stomach contents nor cut marks on the bones of prey species have been discovered, and thus its diet is speculative.

Description[]

A characteristic of related raptorials, Zygophyseter had buccal exostoses, bony outgrowths in the alveolar ridge in the mouth, which are thought to have increased their bite force. Like other raptorials, it had large temporal fossae, probably for supporting strong temporal and masseter muscles, the strongest muscles between the skull and the jaw, meaning this adaptation allowed it to shut its jaws harder. The zygomatic bone (cheekbone) projects outward (anteriorly), indicating it had a beak, which featured an abrupt narrowing; this may have allowed it to clamp down on prey more effectively.

The head probably took up 21–23% of the total body size, compared to that of the modern sperm whale which takes up around one fourth to one third of the total body size. Like in other sperm whales, the blowhole was slanted towards the left side of the animal, and it may have lacked a right nasal passage. The falciform process on the squamosal bone was large and ventrally facing; as opposed to the ones in the Kogiidae (Kogia and Praekogia) which are either reduced or absent. These may have been reduced in kogiids due to adaptations to deep-sea diving.

Like in modern sperm whales, Zygophyseter had a very large basin above the braincase, known as the supracranial basin, which probably housed the spermaceti organ and the melon. These are used in the generation and focusing of sound for biosonar in the modern sperm whale, indicating Zygophyseter had some mechanisms for biosonar; that is to say this animal could have used echolocation. The zygomatic processes of the temporal bone on the cheeks were elongated probably because they supported the spermaceti organ. The skull features a pronounced slope into the supracranial basin. It probably had an echolocation system similar to that of the modern sperm whale, and Zygophyseter may have, in comparison to the echolocative abilities of other modern toothed whales, produced smaller bandwidths and lower center frequencies. This would have made it inept at detecting anything that did not have a diameter of at least 1 meter (3 ft 3 in).

Taxonomy[]

The type and only specimen, labelled MAUL 229/1, is of an almost complete skeleton discovered in southern Italy by geologist Angelo Varola in the marine lime mudstone of the Pietra Leccese Formation near the city of Lecce. It was described in 2006 by geologists Giovanni Bianucci and Walter Landini from the University of Pisa. The genus name Zygophyseter comes from the Latin word zygomaticus, which emphasizes the elongation of the zygomatic process of the only known species Z. varolai, and the term physeter refers to the modern-day sperm whale (Physeter macrocephalus) of the family Physeteridae. The species name honors the discoverer.

Zygophyseter is part of a fossil stem group of hyper-predatory macroraptorial sperm whales (often shortened to "raptorial") which also includes Brygmophyseter, Acrophyseter, and Livyatan. This group is characterized by having large, functional teeth on both the upper and lower jaw with an enamel coating; whereas the modern sperm whale lacks enamel, teeth in the upper jaws, and functionality in the teeth for catching prey. Zygophyseter is more closely related to Brygmophyseter and Acrophyseter than to Livyatan, and the enlarged teeth of this group are thought to have evolved either from a common basilosaurid-like ancestor, or independently once or twice within the group.

Some fossil remains, mostly teeth, of the genus Scaldicetus were reassigned to these raptorials, including Z. varolai. Scaldicetus is now considered to be a grade taxon with reported specimens probably united only by similar physical characteristics rather than a shared ancestry as a clade. It has been proposed that these raptorials be placed into the extinct, possibly paraphyletic (which would make it invalid) subfamily Hoplocetinae, alongside Scaldicetus, Diaphorocetus, Idiorophus, and Hoplocetus.

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