archaeocetes. It has been proposed that Antaecetus was a slow moving inhabitant of shallow coastal waters, where it would ambush
fish and marine molluscs. Only a single species of Antaecetus is known, Antaecetus aithai, which was originally described as a species of Pachycetus.
History and naming
Initially thought to represent a species of Platyosphys (now
tympanic bulla and various vertebrae was recovered in 2014 along with remains of various other basal whales, leading to the recognition of the Gueran depression as an important assemblage for research on early cetaceans.[2] Several years after this initial paper was published a second publication was released, dealing in part with the taxonomy of Platyosphys in an attempt to clarify its status. At the same time, following the discovery of an additional, much better preserved specimen from El Brije, P. aithai was reexamined and found to be sufficiently distinct to warrant it being placed in its own genus, which the authors dubbed Antaecetus. The principal specimen of said study consists of a cranium, the second to eleventh thoracic vertebrae and an additional ten lumbar vertebrae. The same team further established the clade Pachycetinae based on shared characteristics of the skeleton that distinguish Antaecetus and Pachycetus from other basilosaurids. Material of Antaecetus has also been recovered from Fayum, Egypt, but not yet described or catalogued.[1]
Antaecetus was named after
fossil collector who was responsible for guiding researchers to the Gueran locality.[2]
Description
Antaecetus (top) compared to Saghacetus (bottom)
The skull of Antaecetus is noticeably smaller than that of Pachycetus relative to its body, measuring a total of 69.5 cm (27.4 in) from the tip of the
squamosals. In its overall form the skull of Antaecetus resembles the typical basilosaurid condition, featuring a narrow rostrum connected to a much wider basicranium.[1]
The teeth are much more gracile than those of Pachycetus and lack the crenulated
protocones of the molars have been lost, the teeth of Antaecetus still retain an expansion in this area hinting at where the structure was once located. Both molars have a weak lateral cingulum, but differ in that the first has a stronger medial cingulum while there is none in the second molar.[1]
The full skeleton of the El Brije Antaecetus specimen
The anterior vertebrae of the
neural spines are described as slender and relatively short. The lumbar vertebrae strongly resemble those of Pachycetus, with both the centra themselves and the robust transverse processes being elongated, the latter stretching almost the entire length of their respective centra.[2][1]
Another significant trait of the vertebrae is their density relative to those of other basilosaurids. The vertebrae are
nutrient foramina that penetrate the cortical bone.[2] The ribs of Antaecetus are robust, but only the anterior ones are pachyostotic.[1]
Antaecetus is described as having been a medium-sized basilosaurid, significantly smaller than the related species Pachycetus paulsonii, slightly smaller than Pachycetus wardii and in a similar size range as Pappocetus, a contemporary protocetid. The lumbar vertebrae of large males are in the same size range as those of female Eocetus and the skull is similar in size to that of Saghacetus.[2][1]
Phylogeny
A
Neoceti, rendering Basilosauridae paraphyletic as well.[4]
The elongation of the vertebrae in Antaecetus resembles what is known from
ocean floor. The articulation of the ribs would also fall in line with this interpretation, as ligamentous or cartilaginous rib articulation allows for the thorax to expand during air intake while also allowing the animal to collapse the thorax at the ocean floor to minimize buoyancy. At the same time, the increased density took a further toll on the mobility of Antaecetus, likely making it much slower to accelerate in addition to its poor maneuverability.[1]
From this Gingerich, Ayoub Amane and Zhouri hypothesize that Antaecetus, like Pachycetus, was a slow-moving animal inhabiting shallow coastal waters. However, its precise ecology proved to be more elusive, with its small, gracile teeth unfit to deal with either vegetation or hard-shelled prey (ruling out lifestyles akin to those of
sea otters). Feeding directly from the ocean floor is also dismissed, as contact with sediment would lead the teeth to undergo rapid abrasion. The poor maneuverability and slow speed of Antaecetus indicate that it was not built for pursuing prey either, leaving the possibility that Antaecetus was an ambush hunter preying on fish and invertebrates.[1]
The remains of Antaecetus are known from the Aridal Formation of Morocco, where the animal coexisted with at least 5 additional early whales of different sizes. Antaecetus was one of the larger whales of this assemblage, together with the large protocetid Pappocetus and the basilosaurid Eocetus schweinfurthii, which was the largest cetacean of the locality. The remaining three taxa were all smaller than Antaecetus, including two indeterminate protocetids and the small basilosaurid Chrysocetus fouadassaii, which in terms of size falls in between the ranges of the afforementioned protocetids.[2]
