This is primarily due to the taxonomic confusion and nomenclatural changes surrounding the genera Platyosphys and Pachycetus, both of which trace their origins to fossils discovered during the late 19th century.

[3] Things largely went quiet around both the German and the Ukrainian remains in the following 70 years, with Pachycetus slipping into obscurity and Platyosphys likewise receiving little attention.

With the holotype presumably lost sometime during WW2, Pavel Gol'din and Evgenij Zvonok argued that Platyosphys should be considered a nomen dubium given the lack of material to compare more recent finds with.

[8][9][2] While this would have provided a more accessible basis for future referrals, the proposition was not unanimously accepted by other researchers, with some later publications highlighting the fact that a missing holotype alone is not enough reason to disregard the use of Platyosphys.

Supporters of this line of thinking argue that the illustrations provided by Brandt and later authors are sufficient in diagnosing and comparing the material to more recent finds, therefore maintaining the validity of Platyosphys.

Two of the researchers remarking on the matter, Phillip D. Gingerich and Samir Zhouri, subsequently question the validity of P. uheni (alongside that of the poorly preserved P. einori) while naming a species of their own: Platyosphys aithai.

[10][2] The name Pachycetus eventually returned to prominence in 2020, when Henk Jan van Vliet and his colleagues noticed the similarities between the German remains and those of Platyosphys, lumping the two genera into a single taxon.

[2] Things are slightly different for "Zeuglodon" wanklyni, which has never been formally included under Pachycetus but may represent an additional species according to Gingerich, Amane and Zhouri.

[2] Smaller specimens of Pachycetus are also known from isolated remains collected in Spain[12] and the North Sea off the coast of Belgium and the Netherlands,[13] though their fragmentary nature means that it is uncertain whether or not they represent distinct species.

The fact that the cancellous bone took on a cone shape was originally noted to be distinct to pachycetines, but later studies argued that this condition is much more widespread across basilosaurids and not diagnostic.

[10][2] The surface of the bones is littered with numerous vascular canals, which give them a distinct pattern described as "pitted" or "pockmarked" by researchers.

[10][2][11][8][13][5][4] The only innominate bone known from a pachycetine is that of Pachycetus wardii, which appears to show a much more basal condition than that of other basilosaurids and features a much larger obturator foramen.

[1] Though this broader relationship is maintained across studies, few make any more definitive statements of how pachycetines may relate to other contemporary archaeocetes within this family, which is commonly found to be paraphyletic.

Within this grouping, the two species of Basilotritus they erected formed a clade of their own alongside specimen MUSM 1443, which was found to be more derived than Supayacetus yet more basal than Zygorhiza.

[14] Eocetus Chrysocetus Ocucajea Tutcetus rayanensis Zygorhiza Saghacetus Ancalecetus Basilosaurus Dorudon Pontogeneus peruvianus Supayacetus Pachycetus paulsonii Pachycetus wardii Antaecetus aithai Neoceti Most pachycetines with well-dated fossil remains suggest that the family was most prominent during the Bartonian, however, some localities that have yielded pachycetines might extend their range.

Regardless of the first confirmed appearance, it is likely that the origin of Pachycetinae is tied to the global warming that occurred during the Lutetian thermal maximum and the middle Eocene climatic optimum.

The brief period of cooler temperatures between these may be another point of origin for pachycetines, as it was during this time that the sea levels began to rise, leading to the conditions seen during the Bartonian.

This interpretation is supported by the divergence time calculated by Antar and colleagues, who suggest that pachycetines split from other archaeocetes some 43 to 42 million years ago.

For instance, the elongation of the lumbar vertebrae highly resembles what is seen in basilosaurines and suggests that these whales swam by undulating their entire bodies.

Because of this, the mobility of these whales would be much lower than that of other basilosaurids and likely restricted to undulating their spine dorsoventrally (up-and-down), leading to researchers comparing pachycetines to manatees.

In combination, these could indicate that pachycetines spent a lot of time feeding on the seafloor in relatively shallow water, that it was a slow swimmer and/or that it required large oxygen stores.

Some Ukrainian localities for instance yield up to 35 species of shark including Scyliorhinus sp., Hemiscyllium bruxelliensis, Carcharias acutissimu and Clerolamna umovae.

[11] The same applies to the Piney Point Formation of Virginia, which represents shallow marine sediments deposited at a depth of approximately 60–120 ft (18–37 m).

Molluscs from this formation further suggest mild to warm water temperatures whereas the terrestrial climate is interpreted as having been tropical with dry winters.