Pachycetus

Pachycetus (from Ancient Greek παχύς (pakhús), meaning "thick", and Latin cetus, meaning "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.