Pachycetus
Pachycetus is an extinct genus of pachycetine basilosaurid from Middle Eocene of the eastern United States and Europe. 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 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.
History and naming
Early discoveries, ''Pachycetus'' and ''Platyosphys''
Pachycetus has a long and complex history, in part due to the often fragmentary and isolated nature of much of the material referred to this genus. 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. The material consisted of three complete and one partial vertebral centra, discovered near the Tiasmyn River south of Chyhyryn, Ukraine. 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. Paulson initially dubbed the material Zeuglodon rossicus, but Brandt changed the name to Zeuglodon paulsonii, reasoning that the name was better suited in case material outside of the then Russian Empire would be found. Paulson seemingly didn't object to the change, though later authors would occasionally refer to the taxon as Zeuglodon rossicum regardless.Around the same time another archaeocete was discovered within the Barton Clay of the UK. The nearly complete skull discovered in 1872, which was damaged during collection, was described in 1876 by Harry Govier Seeley and named Zeuglodon wanklyni. Though the holotype has been lost since then, various other elements referred to Z. wanklyni have been described in subsequent years. In Germany meanwhile Hanns Bruno Geinitz published on a vertebral centrum recovered from the phosphate beds 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. He furthermore described a second species, Pachycetus humilis based on some of the smaller fossils sent to him by Geinitz. Kuhn published a redescription of the two taxa in 1935, though he was less convinced of their distinctiveness. He rejected the name Pachycetus and instead suggested that the fossils may have been those of Zeuglodon isis and Zeuglodon osiris. Kuhn furthermore described several additional fossils and correctly hypothesized that they were Eocene in age, while Van Beneden had assumed them to be Oligocene.
Additional archaeocete material from eastern Europe was described in 1894 by Leonid Ivanovich Lutugin, who discusses a vertebra found within the Donets River basin that he referred to the genus Zeuglodon, and even more complete material was discovered in 1909 near Koropove. In addition to the initial three vertebra discovered by Alexandre S. Fedorowskij, another seven vertebrae were recovered by farmworkers from the region. The ten vertebrae, regarded as lumbar, saccral and caudal vertebrae by Fedorowskij, were described in 1912 and assigned to the species Zeuglodon rossicus before being introduced into the collection of Kharkiv University, though their whereabouts following WW2 are not known and the remains are currently considered lost.
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. He furthermore referred Zeuglodon wanklyni to the genus Zygorhiza, but failed to mention the material from Germany. 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. Things went quiet for the following 70 years until 2001, when Volodymyr P. Gritsenko described Platyosphys einori from material discovered in Pyrohiv south of Kyiv. 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.
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. The type specimen of E. wardii consisted of a variety of bones including fragments of the skull, various vertebrae, ribs and parts of the innominate bone of the pelvic region. Additional specimens were referred to this species in 2001 by Uhen himself and in 2011 by Weems and colleagues, who expanded the range of E. wardii into Virginia. By that point however the assignment of the fossils to Eocetus had fallen out of favour, with Geisler, Sanders and Luo suggesting it be named "Eocetus" wardii until further study could determine its generic identification.