A new early Pliocene species of Mesoplodon: a calibration mark for the radiation of this species-rich beaked whale genus

ABSTRACT

Including at least 15 extant species, the beaked whale genus Mesoplodon (Cetacea, Odontoceti, Ziphiidae) is by far the most species-rich cetacean genus. Such a high diversity reflects one or several pulses of radiation, most likely involving a sexual selection mechanism affecting shape, size, and position of mandibular tusks. However, the tempo of this diversification is currently poorly constrained due to the scarce fossil record. Based on the reassessment of five partial skulls discovered in Antwerp (north of Belgium), a new fossil species of the genus Mesoplodon, M. posti, sp. nov., is described. Only the second fossil species of the genus currently recognized, it is characterized (among other features) by: transverse sections of the rostrum higher than wide (a feature proposed to correspond to the presence of lower tusks more posterior than the apex of the mandibles); the presence of a basirostral groove at the rostrum base; pentagonal joined nasals; the posterior projection of the premaxilla in the vertex shorter than the nasal backward; and a strong transverse compression of the frontals behind the nasals. Our phylogenetic analysis results in M. posti being nested among extant species of Mesoplodon. Palynological study of the organic-walled phytoplankton from sediment retained in cranial cavities of several specimens indicates an age between 4.86 and 3.9 Ma (Zanclean, early Pliocene). Constituting the first temporally well-constrained fossil species of Mesoplodon, M. posti provides an upper calibration point for the beginning of the radiation(s), having produced a surprisingly high number of extant species in this genus.

http://zoobank.org/urn:lsid:zoobank.org:pub:7407D183-BC8A-466B-8934-F7BE05611ED6

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Citation for this article: Lambert, O., and S. Louwye. 2016. A new early Pliocene species of Mesoplodon: a calibration mark for the radiation of this species-rich beaked whale genus. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2015.1055754.

ACKNOWLEDGMENTS

We thank G. Bianucci, I. Mijan, and K. Post for fruitful discussions about beaked whale systematics, phylogeny, and paleoecology; M. Bosselaers and S. Goolaerts for information on the stratigraphy and sedimentology of Neogene localities around Antwerp; and G. Avery, C. de Muizon, C. Lefèvre, G. Lenglet, J. G. Mead, I. Mijan, C. W. Potter, A. Rol, H. van Grouw, and K. van Willingh for access to collections. Photographic work was made by the late A. Wauters and W. Miseur (IRSNB). Finally, we thank the reviewers G. Aguirre-Fernández and M. D. Uhen for their pertinent comments and constructive suggestions.

Submitted March 27, 2015; revisions received May 13, 2015; accepted May 24, 2015.

Handling editor: Marcelo Sánchez-Villagra.