Extinct genus of mammals
Odobenocetops () is an extinct genus of small toothed whale known from Chile and Peru. Its fossils are found in Miocene-aged marine strata of the Bahía Inglesa Formation and Pisco Formation. Two species of "Odobenocetops" are currently recognized, O. peruvianus and the slightly younger O. leptodon.
"Odobenocetops" is mostly known for its large asymmetric tusks, which emerge from pronounced processes formed by the premaxillae, known as the alveolar sheaths. These tusks are thought to be sexually dimorphic and are only strongly pronounced in male individuals, while females appear to possess two similarly sized tusks. In the holotype of "O. peruvianus" the elongated right tusk is broken, leaving its precise length ambiguous. "O. leptodon" on the other hand preserves complete tusks, showing that at least in this species the longer tusk reached a total length of long, of which is located outside of the alveolar sheath. While these tusks are reminiscent of the tusk seen in the closely related narwhals, they evolved independently. Their purpose remains unknown, but the most common interpretation is that they served a non-violent social role, as they are too fragile for combat. The alveolar sheaths on the other hand may have been used as orientation guides during foraging. Besides the two tusks in the upper jaw, "Odobenocetops" is thought to have been toothless.
Another difference between this genus and other whales is that the melon, an organ important for echolocation, is reduced in "O. leptodon" and vestigial or entirely absent in "O. peruvianus". At least the older species compensated for this by having large, dorsally located eyes giving it binocular vision. The fact that only the older species lost its melon has been taken as evidence that they were sister taxa, rather than one species evolving directly from the other. In addition to their vision or echolocation, Christian de Muizon argues that they may have possessed tactile hair, which are also found in walrus and to a lesser extent in Amazon river dolphins.
"Odobenocetops" is among the cetaceans with the greatest range of head motion, exceeding even the values of the beluga whale. This may have helped while foraging, extending the neck in a way that keeps their tusks roughly parallel to the rest of the body. Due to the anatomy of the palate and other similarities to the walrus, it is thought that this whale was a suctionfeeding molluscivore, searching for bivalves on the ocean floor, uncovering them with precise jets of water, grasping the uncovered molluscs with a powerful upper lip and using its tongue like a piston to suck out the soft parts of their prey, leaving the shell intact.
History and naming.
The first fossil material, a single skull missing much of its left side, was recovered in 1990 from the Sud Sadaco horizon of the Pisco Formation in Peru. Although initially thought to correlate with the earliest Pliocene, later studies have found that these sediments were deposited during the Miocene. This skull, designated USNM 460306 initially and later USNM 488252, was described by Christian de Muizon in 1993, establishing the genus "Odobenocetops" with "O. peruvianus" serving as the type species. Due to this genus' strange anatomy Muizon also coined the family Odobenocetopsidae. Several additional fragmentary fossils, namely periotic and tympanic bones, were later referred to the genus as well.
More substantial material was found in the form of three additional specimens, one of which is thought to represent a female "O. peruvianus" while the other two were described as a second, younger species named "O. leptodon". The holotype specimen of "O. leptodon" is a nearly complete skull with the associated atlas, the topmost of the neck vertebrae. The other specimen of "O. leptodon" is a much less complete skull, badly weathered and missing the right tusk, but preserving an assortment of postcranial elements such as ribs, vertebrae and a partial forelimb. While the referred "O. peruvianus" skull was found in the SAS horizon like the type specimen, the new species stems from the SAO horizon, which is slightly younger.
The generic name "Odobenocetops" comes from the Greek "odon" for "tooth", "baino" which means "walk", the Latin word "cetus" for "whale" and "ops", "like". In combination the name means "cetacean that seems to walk on its teeth", a name chosen both to reflect the animal's potential feeding position as well as referring to the similarity with the extant walrus ("Odobenus"). The species name of "O. peruvianus" refers to Peru, the country it was found in.
Description.
Size.
The body length has been estimated to range from . It is possible that "Odobenocetops" reached a mass similar to that of modern narwhals, between .
Skull.
The skull of "O. peruvianus" is large, measuring throughout its preserved length. The skull has a characteristic profile, appearing strongly concave between the elevated snout and skullroof. When viewed from above, it is also clearly separated into two large portions. The anterior most portion, which includes the premaxillae, tusks and nares is separated from the back of the head by a strong constriction, giving the skull somewhat of an hourglass-shape. The skulls of modern whales show a great variety of adaptations towards aquatic life, clearly setting them apart from all other mammals. Among these adaptations is what is commonly referred to as "telescoping", a term that generally describes the fact that bones typically far apart are very closely spaced in cetaceans and largely overlap.
However, "Odobenocetops" is unique due to how its skull appears to reverse the telescoped condition of the cetacean skull. This is achieved through the maxilla and frontal bones regressing towards the tip of the snout and the bony nares being moved forward. Subsequently, this gives the rostrum its characteristic short and round appearance, in contrast to the elongated skulls found in other cetaceans. Related to this the type species "O. peruvianus" is thought to have lacked a melon (an important sensory organ), or at the least only had a vestigial melon. The bony nares are now located near the tip of the skull, in contrast to the blowholes of whales and dolphins located on the top of the skull. In other odontocetes, parts of the frontal and maxillae cover the temporal fossae. In "Odobenocetops", these bones are reduced and narrowed so that the temporal fossae are open dorsally. Additionally the parietal bones are well exposed dorsally, which corresponds with a well developed temporalis muscle. The periotic and tympanic bones are similar to those in other dolphins. The eye-sockets are oriented upwards and sideways, and not fully laterally like in other dolphins. The palate is arched, large and deep like in walruses and besides the two tusks in the premaxilla, "Odobenocetops" was toothless. The tip of the snout, specifically the premaxilla, is covered in important insertion points for facial musculature while also housing a great number of neurovascular foramina. This has been interpreted as supporting a strong upper lip and potentially even vibrissae similar to those in a walrus.
The skull of "O. leptodon" differs from that of "O. peruvianus" in several ways. The palate is much deeper, longer and wider and the anterior border is curved more gently, giving it a U-shape rather than a V-shape as in "O. peruvianus". The palate itself is also asymmetrical and was likely positioned parallel to the seafloor, but not at a right angle with the sagittal plane. The apex of the snout in general is more massive than in the type species and at the tip of the rostrum, between the premaxillae, there is a unique pair of supplementary bones not present in the older species at all. These bones, which may have been the rostral or prenasal bones, are similar to what is seen in mammals such as elephant shrews, moles, tapirs, saigas and pigs. A unifying factor of these groups is the presence of a strong upper lip, further supporting what was already inferred for "O. peruvianus". The premaxillary foramina are missing in "O. leptodon" and a dorsal fossa (shallow depression) is present on the premaxilla. This fossa suggests the presence of a melon in "O. leptodon", an organ either absent or strongly reduced in "O. peruvianus". On the other hand, the orbit in "O. leptodon" has an anterior edge that is only slightly concave, whereas it is deeply notched in "O. peruvianus" which suggests that the binocular vision of "O. leptodon" wasn't as well-developed as in the older form. Although no mandibles are known for either species, it has been inferred that it was short and toothless based on the well developed pterygoid and temporalis muscles.
Like the rest of the skull, the periotic bone surrounding the inner ear is highly derived and does not closely resemble the typical anatomy expected from a cetacean. Still, the cochlear canal follows the general anatomy also present in its closest relatives the beluga and narwhal. The inner ear also has large semicircular canals, vestibular aqueduct and a large count of facial and vestibular nerve fibres. Other toothed whales have semicircular canals that are notably smaller than the cochlear canal. Again "Odobenocetops" most closely resembles the related beluga. It is possible that this correlates with increased mobility of the neck and head.
Tusks.
In addition to the unique shape of the skull, "Odobenocetops" is most easily distinguished from other cetacean by the presence of a pair of asymmetric tusks composed entirely of dentine. These tusks are housed by large processes, known as the alveolar sheaths, formed by the premaxilla, which are directed back- and downwards at a 60° angle from the horizontal plane of the skull. Such a tusk is only known in a single other cetacean, the extant narwhal, in which they are also asymmetric. However, in narwhals the tusk is implanted in the left maxilla, whereas the tusk in "Odobenocetops" originates in the right premaxilla. The tusks in these two genera are therefore not homologous, and the occurrence of tusks in "Odobenocetops" is a convergence with narwhals. In the holotype of "Odobenocetops peruvianus" both tusks are incomplete. Based on the preserved elements it is estimated that the longer, right tusk measured between in length. The left tusk was notably smaller, being estimated at no more than long and possibly entirely contained in the premaxillary process. Following the discovery of "O. leptodon", Muizon speculated that both tusks could have obtained larger sizes in other individuals. This however would require additional specimens to corroborate the hypothesis. The second skull, described by Muizon in 1999, differs significantly from the 1993 skull despite being found in the same horizon. Unlike the drastic size difference between the left and right tusk of the type specimen, the tusks of the second specimen were both mostly symmetrical and short. Rather than taking this as evidence for an additional species, Muizon suggests that this skull may have belonged to a female individual displaying sexual dimorphism. This is supported by the dimorphism seen in modern narwhals, in which only males possess the iconic tusk while most females lack them. The pulp cavity of the elongated tusk was long, indicating that it grew continuously. The short tusk also appeared to still be growing, even if at a much slower speed.
The holotype of the younger species "O. leptodon" was found with both tusks "in situ", the right one was needle-like and long, of which being located outside of the premaxillary process, the left tusk was only long, similar in length to the small tusk of "O. perivianus". Despite the small tusk only being slightly longer than that of the type species, the wear facet of it indicates that it was erupted and not confined within the bony process of the premaxilla. The larger tusk also preserves a clear wear facet at its tip, giving the apex of the tusk a strongly oblique shape due to how the tusk was used.
Given the tusks length and slenderness, it was likely very fragile, which is considered to be an argument against it being held at a 45° angle from the body. This is corroborated by the anatomy of the neck. The way the atlas and the occipital condyle articulate suggests that in neutral position, the neck would have been angled slightly downward, which inclines the tusk slightly upward into a position parallel to the rest of the body. Due to the fact that the tusk additionally projects slightly towards the side, "Odobenocetops" could lower its head even further than that without having