A new balaenopterid whale from the late Miocene of the Southern North Sea Basin and the evolution of balaenopterid diversity (Cetacea, Mysticeti)

Background: Balaenopterid mysticetes represent the most successful family-rank group of this clade. Their evolutionary history is characterized by a rich fossil record but the origin of the living genera is still largely not understood. Recent discoveries in the southern border of the North Sea revealed a number of well preserved fossil balaenopterid whales that may help resolving this problem. In particular, skull NMR 14035 shares morphological characters with the living humpback whale, Megaptera novaeangliae and, for this reason, its characteristics are investigated here. Methods: The comparative anatomical analysis of the new specimen formed the basis of a new phylogenetic analysis of the Mysticeti based on a matrix including 350 morphological character states scored for 82 Operational Taxonomic Units. The stratigraphic age of the specimen was determined based on the analysis of the dinocyst assemblage recovered in the associated sediment. We assessed clade diversity in Balaenopteridae by counting the numbers of clades in given time intervals and then plotted the results. Results: Nehalaennia devossi n. gen. et sp. is described for the first time from the late Tortonian (8.7-8.1 Ma) of the Westerschelde (The Netherlands). This new taxon belongs to Balaenopteridae and shows a surprisingly high number of advanced characters in the skull morphology. Nehalaennia devossi is compared to a large sample of balaenopterid mysticetes and a phylogenetic analysis placed it as the sister group of a clade including the genus Archaebalaenoptera. The inclusion of this fossil allowed to propose a phylogenetic hypothesis for Balaenopteridae in which (1) Eschrichtiidae (gray whales) represents a family of its own, (2) Balaenopteridae + Eschrichtiidae form a monophyletic group (superfamily Balaenopteroidea), (3) Cetotheriidae is the sister group of Balaenopteroidea, (4) living Balaenoptera species form a monophyletic group and (5) living M. novaeangliae is the sister group of Balaenoptera. Our work reveals a complex phylogenetic history of Balaenopteridae and N. devossi informs us about the early morphological transformations in this family. Over time, this family experienced a number of diversity pulses suggesting that true evolutionary radiations had taken place. The paleoecological drivers of these pulses are then investigated.