| Pachycetus Temporal range: Bartonian Though the best-dated remains are Bartonian in age, some could suggest its presence during the Lutetian and Priabonian.
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Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Infraorder: | Cetacea |
| Family: | †Basilosauridae |
| Subfamily: | †Pachycetinae |
| Genus: | †Pachycetus van Beneden, 1883 |
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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 best known remains generally suggest that Pachycetus lived during the Bartonian, however, fossil finds have also been recovered from sediments of less certain age that could suggest that it may have also lived during the Late Lutetian and Early Priabonian. Pachycetus is primarily known from vertebrae and ribs and is characterized by its highly osteosclerotic and pachyostotic skeleton. This means the bones not only featured thickened rings of cortical bone surrounding the internal cancellous bone, but the cortical bone was furthermore much denser than in other basilosaurids. Two species of Pachycetus are recognized: Pachycetus paulsonii from Europe and Pachycetus wardii from the United States. A third species might be represented by "Zeuglodon" wanklyni.
Pachycetus has a long and complex history, with its earliest recorded remains having been found in what is now Ukraine in the late 19th century. The fossils were initially dubbed Zeuglodon rossicus, only to be immediately renamed to Zeuglodon paulsonii. Simultaneously, fossils from the north of Germany were described as Pachycetus, a name that would eventually fade into obscurity as time passed on. 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. By the 2020s it was recognized that the German Pachycetus and the Ukrainian Platyosphys most likely represented a single taxon, which came to combine the generic name of the former and the species name of the latter. A species from the USA previously known as "Eocetus" wardii was also introduced to the genus and together with Antaecetus these animals would become the basis of the family Pachycetinae.
Given the limited material known of this genus, its biology is only poorly understood. The dense skeletal structure is commonly compared to that seen in modern-day sirenians (manatees and dugongs) and it has been suggested that it was a powerful if comparably inflexible swimmer that swam by moving its entire body up and down similar to Basilosaurus. It has further been suggested that Pachycetus preferred shallow waters and fed close to the seafloor, a suggestion largely congruent with the German and American sediments it has been recovered from. 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. Some supporting evidence for this hypothesis may be found in the high number of sharks and rays found within the same sediments as Pachycetus.