A New Bowhead Whale-like Genus from the Early Pliocene of Antwerpen Illuminates Phylogenetic Relationships and Body Size Evolution in Balaenidae (Cetacea, Mysticeti)

Right and bowhead whales represent key species in modern-day marine ecosystems because of their capacity to catch enormous amounts of prey items (especially calanoid copepod crustaceans). However, their evolutionary history is still not completely understood. Morphology- and molecule-based phylogenetic analyses failed to find a consensus view about the relationships of balaenids and other mysticete species and in determining the intra-family relationships of extant and fossil taxa. The fossil record may help in increasing the precision of morphology-based phylogenetic results when new balaenid species are discovered because the new fossils may add new character combinations able to change previously published hypotheses of relationships. Here, we describe a new balaenid genus and species, Quasibalaena portisantuerpiae, based on a partial skull from the Early Pliocene of Belgium. The new specimen preserves relevant portions of the cranium, periotic and tympanic bulla, and shows a character combination never observed up to now in other balaenid fossils. We compared the new taxon to all the other balaenid species known and included it in a morphology-based phylogenetic analysis. The results suggest that Balaenidae includes a stem group and two main subclades, respectively related to the extant bowhead whale (Balaena mysticetus) and to the right whales (genus Eubalaena). Quasibalaena portisantuerpiae belongs in the former subclade. Body size reconstruction allowed us to infer that the origin of the Balaena-related clade was characterized by a miniaturization process and that gigantism occurred independently in bowhead and right whales.