Macroraptorial sperm whales were highlypredatory whales of thesperm whale superfamily (Physeteroidea) of theMioceneepoch that hunted largemarine mammals, including other whales, using their large teeth. They consist of six genera:Acrophyseter,Albicetus,Brygmophyseter,Livyatan,Scaldicetus, andZygophyseter.[1] All species are known by at least a skull, and are informally grouped without afamily designation. They were all likely theapex predator of their habitats, comparable to the modern daykiller whale (Orcinus orca), and achieved great lengths, with one species—Livyatan—measuring about 13.5–17.5 m (44–57 ft). The term "macroraptorial" combines the prefixes "macro-", derived from the Greek "makros" (μακρός) (meaning "long", "large" or "great") and "raptorial" from both the Latin "rapere" (ra.pi.o and räpio) and (ra.pi.o and räːpio) and "raptus" (rap.tʊs and räpt̪ʊs̠) and (rap.tus and räpt̪us) in both Classical Latin and in Italianate Ecclesiastical Latin (meaning "to seize", "to grasp", "to carry off", "to grab" or "to snatch"; arguably the same origin as the unrelated act ofrape).
Zygophyseter was discovered in thePietra Leccese Formation in Italy from a skull, teeth, and vertebrae;[2]Brygmophyseter was discovered in theBessho Formation in Japan from a nearly-complete skeleton;[3] andAcrophyseter andLivyatan both originate from thePisco Formation in Peru and are known by only a skull.[4]Albicetus was discovered at theSanta Barbara Lighthouse in 1909, in what is believed to most likely be part of theMonterey Formation. It was first classified within the genusOntocetus (now a genus ofwalrus), and then moved to a wastebasket taxonScaldicetus until Boersma and his colleagues assigned a new generic name.[5]
| Genus | SI | Imperial |
|---|---|---|
| Acrophyseter | ≈4–4.5 m | ≈13–15 ft |
| Albicetus | ≈5.9–6.3 m | ≈19–21 ft |
| Brygmophyseter | ≈6.5–7 m | ≈21–23 ft |
| Zygophyseter | ≈6.5–7 m | ≈21–23 ft |
| Livyatan | ≈13.5–17.5 m | ≈44–57 ft |
Macroraptorial sperm whales had large, functional, conical teeth in both jaws, as opposed to the modern sperm whale whose teeth are small and nonfunctional in the upper jaw. The teeth were deeply rooted into the gumline and could interlock, probably to aid in holding struggling prey. The teeth ofLivyatan, at a length of 31 cm (12 in), were one of the biggest teeth of any animal, excluding tusks. The macroraptorials also had well-developed muscles used in biting—thetemporalis andmasseter. They also developedbuccal exostoses in the mouth, bony growths which act as buttresses during biting, further increasing the bite force.[2][3][4][6]
Like modern sperm whales, the macroraptorials had a curved basin—the supracranial basin—on the top of the skull. This encompassed the entire breadth of the snout inLivyatan andBrygmophyseter;[3][7]Zygophyseter andAcrophyseter, instead, had reduced basins indicating beaks.[2][4] The elongatedlumbar vertebrae ofZygophyseter indicate it had largermultifidus andlongissimus back muscles and was, thus, faster than the modern sperm whale which,[2] comparable to other largeopen-ocean animals, travels horizontally at 4 km/h (2.5 mph).[8]
Using their large and deeply rooted teeth, wide-opening jaws, and great size, they likely fed on a variety of sea life, including fish,cephalopods,seals, and small whales and dolphins, occupying aniche similar to the modern daykiller whale (Orcinus orca).[4][7][9] In fact,Zygophyseter is known colloquially as the "killer sperm whale" in reference to this.[9] Likewise, they may have employed a similar hunting strategy of pursuing prey to tire it out before eventually drowning it. However, given their size, they probably did not need to hunt in groups.Livyatan probably targeted medium-sized whales ranging in size from 7–10 m (23–33 ft). Macroraptorials probablycompeted with the extinct giant sharkmegalodon for the same food sources.[7][10][11][12]
In sperm whales, the supracranial basin holds the spermaceti organ, a series ofoil andwax reservoirs which aids inecholocation. Speculatively, the organ may also serve a secondary function, such asvocalizing, acoustic stunning of prey, head-butting between males, ramming into prey, or buoyancy control by increasing or decreasing the temperature of the wax to change the density and weight.[7]
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The macroraptorial sperm whales are aparaphyletic fossil group ofhyper-predatorystemsperm whales. All share large, functional,enamel-coated teeth on both the upper and lower jaws, which were used in capturing large prey. In contrast, the modernsperm whale (Physeter macrocephalus) lacks enamel, teeth in the upper jaw, and the ability to use its teeth to catch prey.[13] However,Livyatan belongs to a different lineage than the other macroraptorials, and the development of large size and thespermaceti organ, an organ that is characteristic of sperm whales, are thought to have evolved independently from the other macroraptorials. The large teeth either evolved once in the group with abasilosaurid-like common ancestor, or independently inLivyatan. The largetemporal fossa depressions on the skull of raptorials is probably descended from a common ancestor (plesiomorphy). The presence of enamel is thought to be an ancient andbasal characteristic, as it is present on the teeth of fetal modern sperm whales. Macroraptorials appeared during anadaptive radiation event ofbaleen whales in theMiocene, an increase in whale populations and diversity, implying the macroraptorials evolved specifically to exploit baleen whales.[4][7] A 30 cm (12 in) tooth found in Australia indicates macroraptorials still existed 5 mya in thePliocene.[14]
It has also been suggested that the macroraptorials be placed into thesubfamilyHoplocetinae, a subfamily characterized by robust and enamel-coated teeth, alongside the generaDiaphorocetus,Idiorophus, andHoplocetus, which are known from the Miocene to theEarly Pleistocene. However, most of these whales are known from fragmentary remains or have been used aswastebasket taxa for indiscernible stem sperm whale remains.[9]
| Macroraptorials are in bold[4][5][7][13] |
