Abstract
BackgroundBalaenopterid 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.MethodsThe 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.ResultsNehalaennia 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.
Highlights
Rorquals and humpbacks belong to the family Balaenopteridae, the most successful group among living baleen whales
Different approaches have been followed to develop a hypothesis of phylogeny for balaenopterid whales resulting in conflicting results
As detailed in the Discussion section, morphology-based, molecule-based and total evidence analyses failed in finding a consensus about (1) the phylogenetic relationships of Balaenopteridae and other mysticetes and (2) the intra-family relationships of Balaenopteridae
Summary
Rorquals and humpbacks belong to the family Balaenopteridae, the most successful group among living baleen whales. One of the key differences between molecule-based and morphology-based works consists in that the fossil record may be included only in the latter ones or in total evidence studies. 8.7–8.1 Ma based on dinoflagellate cysts but, surprisingly, shows a number of synapomorphies of living balaenopterids As it is beautifully preserved, it represents an invaluable source of character states helping the reconstruction of the early evolution of the balaenopterid whales. The discovery of this new taxon has the potential to resolve the questions related to the monophyly of Balaenoptera and Megaptera, and those related to the inclusion of the gray whale within Balaenopteridae or not. Apart from resolving the phylogenetic relationships of the new taxon, we performed a study of the past diversity within Balaenomorpha focusing on the whole clade and on Balaenopteridae and compared our results with those provided by works of other authors
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