Skull morphology and phylogenetic relationships of a new diminutive balaenid from the Lower Pliocene of Belgium

A new small balaenid is described and compared to all fossil and living balaenid taxa. The specimen represents a new genus and species and is named Balaenella brachyrhynus. It was discovered in the Lower Pliocene of Kallo (northwest Antwerp, Belgium) and adds new information on the diversity and evolution of Balaenidae. Based on both comparative morphology and phylogenetic analysis, Balaenella brachyrhynus is morphologically closer to the genus Balaena, including the living Greenland Bowhead whale (B. mysticetus), and two Pliocene species (B. montalionis and B. ricei) from central Italy and the eastern USA. Balaenella brachyrhynus has very short nasals, a short rostrum relative to the total skull length and a horizontal supraoccipital. A cladistic treatment of 81 morphological character states scored for 10 balaenids and nine non-balaenid cetaceans revealed that the other small balaenids generally included in the genus Balaenula (including Balaenula astensis, B. balaenopsis and a Pliocene Balaenula sp. from Japan) are closer to the living genus Eubalaena (the Right whale). As the new skull is so different from the nominal Balaenula species, and as it is more closely related to Balaena than to Eubalaena, it is concluded that a small body size was a common condition in different Balaenidae clades.