Locomotion and lifestyle One of the most notable traits of
Pachycetus and pachycetines in general is the great increase in bone density achieved via pachyostosis and osteosclerosis. This would have greatly increased the animal's weight and served as
ballast. This however came at the cost of making the animal much slower to accelerate and less maneuverable while swimming. The animal's maneuverability was also negatively affected by the anatomy of the transverse processes of the vertebrae. Since the transverse processes were much more elongated than those of
Basilosaurus, there is less space between each process and those preceding and following it. This in turn means less space for muscle contraction, which leads to a decrease in lateral flexibility. While
Pachycetus would still share a swimming style broadly similar to that of
Basilosaurus, undulating the entire body to move, it was much more restricted in its movement. Gingerich and colleagues argue that
Pachycetus would have had to rely on dorsoventral undulation, meaning it was limited to moving its body up and down. This limited mobility, combined with the overall increase in density, is at times compared to modern
sirenians. While acceleration and mobility were limited, there may have also been a variety of advantages in the denser bones, especially if coupled with an increase in lung volume. Slow swimmers, animals requiring large oxygen stores and shallow water inhabitants feeding close to the ocean floor are all listed as profiting from such anatomy, with Gingerich and colleagues interpreting pachycetines as slow inhabitants of the
neritic zone. Gol'din and Zvonok come to a similar conclusion while further highlighting how the large vertebral processes would indicate that
Pachycetus, though slow-moving, was a powerful swimmer regardless. Given that these adaptations are poorly suited for pursuing prey items, it has been hypothesized that pachycetines were ambush-hunter.
Diet Some inferences have been made on the diet of
Pachycetus based on the patterns of tooth wear. Teeth from Ukraine show a mixed pattern of latitudinal and cross-oriented scratches as well as deeper gouges that cover the surface of the teeth. Prior studies have linked coarse patterns like gouges to a carnivorous diet like in
Basilosaurus and
orcas while the scratches mirror those seen in modern piscivorous
toothed whales. Gol'din and colleagues especially highlight the strongly abraded tips of the teeth of the Nagornoye teeth they assigned to
Basilotritus. This abrasion is seen across all types of teeth, but is least pronounced on the canines. The wear is described as uniform, leaving smooth margins and occurring perpendicular to the long axis of the tooth. In orcas, similar rapid abrasion is associated with the animals preying on sharks, causing the teeth to be worn down by their
placoid scales. It is noted by the team that the same sediments yielding these teeth also preserve 35 species of sharks including
Scyliorhinus sp.,
Hemiscyllium bruxelliensis,
Carcharias acutissimu and
Clerolamna umovae. A similarly diverse
chondrichthyian fauna was present in the Americas, with the Piney Point Formation yielding the remains of 19 shark species and 9 species or rays. They therefore hypothesize that the animal was specialised in feeding on sharks.
Paleoenvironment Helmstedt, where much of the German material of
Pachycetus originated, was once located at the southern edge of the Paleogene North Sea where rivers from the continent filled an
estuary. During the Paleogene the ocean retreated and expanded repeatedly, creating a mix of terrestrial and marine conditions. Multiple
geological formations within the region around Helmstedt trace their origins to this period, including the Lutetian to Bartonian
Annenberg Formation and the Bartonian to Priabonian
Gehlberg Formation that is most likely to have yielded the bones of
Pachycetus. During the time periods these formations were formed the sea was rather shallow, reaching a depth of only around at the time the Anneberg Formation was deposited and a depth between during the deposition of the Gehlberg Formation.
Pachycetus would have shared this environment with a second, much smaller basilosaurid originally named
Pachycetus humilis, though it is now believed to have been a dorudontine. Similar to the German remains, the American fossils of
Pachycetus wardii are also known from shallow marine sediments. The Piney Point Formation represents a shallow marine shelf that was deposited at a depth of around . Molluscs from this formation suggest mild to warm temperatures and the terrestrial
flora supported members of
Fagaceae,
Juglandaceae,
ferns,
tupelos and
palms, which indicate that the climate was tropical with dry winters. Ukrainian material is known from sediments containing
glauconite, formed at depths between and most common between , which has been interpreted to mean that
Pachycetus at least occasionally left the shallows to venture into the open ocean. One hypothesis suggests that
Pachycetus may have entered deeper waters while migrating, which would have been made possible by their powerful swimming musculature and its good ballast. == References ==