Abstract
A new basal basilosaurid cetacean, Basilotritus uheni n. gen. n. sp., comes from the late middle Eocene (Bartonian) of Ukraine. It is the earliest dated record of a cetacean from Eastern Europe. The tympanic bulla of Basilotritus uheni shares basilosaurid synapomorphies but possesses unusual traits inherited from protocetids. Cetaceans related to Basilotritus uheni and referred to as Eocetus or “Eocetus” have been recorded from Africa, Europe, North America and South America. “Eocetus” wardii from North America is recombined as Basilotritus wardii. Platyosphys paulsonii and Platyosphys einori from Ukraine are considered as nomina dubia; specimens prior referred to as Platyosphys sp. are similar or related to Basilotritus. Other records of the Eocene cetaceans from Ukraine and south Russia are identified as Basilotritus or related genera. Early basilosaurids are demonstrated to be a paraphyletic, morphologically and geographically diverse group of the genera that colonized the world ocean as late as in Bartonian age and were probably the ancestors of Neoceti, as well as of more derived basilosaurids.