Pachycetus (meaning "thick whale") is an extinct genus of pachycetine basilosaurid from Middle Eocene of the eastern United States (North Carolina & Virginia) and Europe (chiefly Germany and Ukraine).

The Ukrainian material would eventually come to be named Platyosphys before being changed to Basilotritus, though the reasoning for the latter was regarded as unjustified and was not accepted in much of the subsequent literature.

Some fossils from Ukraine do however suggest that Pachycetus at least occasionally entered deeper waters, which has been interpreted as potential evidence for migratory behavior.

Tooth wear has been used as evidence for the fact that Pachycetus might have fed primarily on chondrichthyans like sharks and rays, arguing that the high levels of abbrasion seen on some of the teeth was caused by contact with their preys placoid scales.

The earliest recorded mention of material now assigned to Pachycetus dates back to 1871, when fossils of this whale where reported by botanist Afanasii Semenovich Rogovich at the Third Russian Congress of Naturalists in Kyiv.

These remains were eventually described by Otto Mikhaĭlovich Paulson at the behest of Johann Friedrich Brandt, who published the description in his 1873 monograph on European fossil whales.

In Germany meanwhile Hanns Bruno Geinitz published on a vertebral centrum recovered from the phosphate beds (likely the Gehlberg Formation) of Helmstedt, Lower Saxony, which represented the first example of a fossil whale being found within the country.

Later publications also make mention of another centra and a rib, which alongside the first centrum and some other whales bones were sent to Pierre-Joseph Van Beneden in Leuven, Belgium.

Taking note of the robust nature of the bones and their similarity to those of sirenians, Van Beneden described them as a new genus of what he thought to be a mysticete whale, which he named Pachycetus robustus.

[2] In his 1936 monograph on archaeocetes, Remington Kellogg recognized the distinct nature of the taxon and coined the new genus Platyosphys for Zeuglodon paulsonii, primarily basing his work on the material previously described by Fedorowskij.

[4] It is thought by van Vliet that this was likely due to the fact that Kuhn had only identified the Helmstedt material as archaeocetes a year earlier, with Kellogg simply having been unaware of this research.

[5] Although Gritsenko took note of the advanced pachyostosis of the material, likening it to what is seen in modern sirenians, he failed to provide an adequate diagnosis for this new species.

The validity of P. einori eventually came into question, with other authors highlighting that the material is very poorly preserved, casting doubt over the Gritsenko's claims of it being distinct, and overall not diagnostic enough to be referred to any species or genus.

[6][2][7] In 1999 Mark D. Uhen described Eocetus wardii based on material from the collection of the United States National Museum of Natural History, which had been discovered in 1977 in North Carolina.

Instead the descriptions provided by Brandt, Fedorowskij and Kellogg were deemed sufficient for Platyosphys paulsonii to remain valid while the status of Basilotritus uheni was questioned.

In a paper published in 2020, van Vliet noted that the vertebrae of Platyosphys and Basilotritus were identical to those from Germany originally described as Pachycetus.

[1] Several other names have been proposed in the past, including Basilotritus ("third king" in allusion to Basilosaurus and Basiloterus)[6] and Platyosphys ("broad loin" in reference to the wide transverse processes of the lumbar vertebrae).

A prominent keel emerges from the medial margin of the bulla and the involucrum is much steeper towards the front, both features setting the taxon aparat from most other basilosaurids.

Overall its size and shape resemble the ear bones of Zygorhiza and Gol'din and Zvonok describe it as displaying a mix of features seen in basilosaurids and protocetids.

[2] However, Van Vliet and colleagues argue that the presence of cones is more widespread than presumed by Gingerich and Zouhri and in fact common across basilosaurids.

Like in other members of its family and derived ceteaceans, the innominate bones of Pachycetus did not articulate at the sacrum, clearly setting it apart from the more basal protocetids.

[8][2] Additional material has been described for the European species and Gol'din and Zvonok include in their diagnosis for Basilotritus uheni that the section formed by the ilium extends forward whereas the ischiac part is highly reduced.

[11] Pakicetus Maiacetus Rodhocetus Artiocetus Qaisracetus Aegyptocetus Protocetus Carolinacetus Gaviacetus Eocetus Georgiacetus Babiacetus Supayacetus

Basilotritus uheni Basilotritus wardii Zygorhiza Chrysocetus Saghacetus Ancalecetus Cynthiacetus Basilosaurus Dorudon Ocucajea Kekenodontidae Neoceti Although no full analysis was performed, Van Vliet hypothesized that Pachycetus may have been a basal member of basilosauridae owning in part to its well developed innominate bones,[1] which somewhat mirrors the position the taxon occupied in the work of Gol'din and Zvonok.

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.

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.

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.

[11] 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.

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.

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.