| Summary: | A review of the sperm whales (Cetacea, Odontoceti, Physeteroidea) from the Miocene of the southern margin of the North Sea Basin is undertaken, mostly based on the cranial material of the area of Antwerp (north of Belgium) described for the first time by P.-J. Van Beneden, B. A. L. du Bus, and O. Abel more than a century ago. This work leads to the detailed redescription of the species Eudelphis mortezelensis, Physeterula dubusi, Placoziphius duboisi, and Thalassocetus antwerpiensis , the identification in the North Sea Basin of the eastern North American species Orycterocetus crocodilinus , and the description of a new undetermined physeterid. The stratigraphic information associated to some of these Miocene taxa is refined ( E. mortezelensis, O. crocodilinus , and P. duboisi ), whereas a more important incertitude persists for others. These results further emphasize the physeteroid diversity during the Miocene. The performed phylogenetic analysis places Eudelphis as the most basal stem-physeteroid, displaying the most salient features of the superfamily (supracranial basin, strong asymmetry of the bony nares and premaxillae) but retaining enamel on teeth and a rather conservative skull morphology (deep maxillary alveoli, large left pre-maxillary foramen, distinct falciform process of the squamosal.). Together with Orycterocetus, Placoziphius is provisionally kept outside the family Physeteridae, defined as the clade grouping all the physeteroids more closely related to Physeter than to Kogia . The large species Physeterula dubusi is the most stemward physeterid, retaining functional upper teeth lacking enamel. Among the physeterids the new undetermined taxon is sister-group to the clade Aulophyseter + Physeter , sharing with these two genera the preorbital process distinctly lower than the lateral margin of the rostrum base. With a sagittal crest in its supracranial basin the small Thalassocetus antwerpiensis is confirmed as an archaic kogiid. The evolutionary history of the supracranial basin and the oral apparatus are discussed. The parsimony analysis suggests that the spermaceti organ remained small in the supracranial basin of most physeteroids, the basin probably functioning as a parabolic structure for reflecting and focusing the echolocative sounds. It is proposed that the spermaceti organ only considerable increased in size in the lineage of the Recent species Physeter macrocephalus , possibly as a sexually dimorphic sound transmitting organ. Preceded by the loss of enamel, the reduction of the upper dentition associated to a decrease of the size of the temporal fossa occurred in parallel in the physeterids and the kogiids, much likely related to a major change in diet and food processing.
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