Morphological and Molecular Analyses of Phocidae (True Seals); with Classification of New Fossil Material

Abstract

Contentious disagreement surrounding the relationships within pinnipeds as well as their phylogenetic affinities still remains. Molecular-based studies neglect significant morphological data and ignore the entire fossil record, explaining contradictory results from morphology-based studies. Recently, the Paleobiology Department of the Calvert Marine Museum (Maryland, USA) collected a partial skull from the Upper Miocene (Chesapeake Group, St. Mary’s Formation, Tortonian, 8.5 - 7.5 Ma, in Maryland, USA), plus postcranial bones and multiple isolated mandibles from the Meherrin River, Murfreesboro, Hertford County, North Carolina, USA. The formations known to be exposed in the Meherrin River are the Eastover Formation (7.0–6.0 Ma, Miocene) and the Yorktown Formation (5.2– 3.4 Ma, Pliocene). This study reports detailed morphological assessments of the new partial skull, five isolated mandibles, a humerus, a femur, and three innominate bones, with comparisons to representatives of the various phocid subfamilies. Based on morphological assessment and comparison to fossil and modern taxa, the skull assessments permit the description of a new taxon of fossil monachine seal from the St. Mary’s Formation in Maryland, USA. Its possession of unique characters that do not match any known genus of the Subfamily Monachinae leads to the naming of a new genus and species, Magnotherium johnsii. This is now the third taxon of the Subfamily Monachinae described from this locality. Assessment of the mandibles results in the naming of two new monachine species (Homiphoca murfreesi and Hadrokirus gravis) and assignment of two mandibles to known phocine species (Phocanella pumilla and Leptophoca lenis). A fifth phocine partial mandible cannot be accurately identified at the genus or species level. Postcranial analyses identified a humerus as Phocanella pumila (Subfamily Phocinae), a femur as Homiphoca capensis (Subfamily Phocinae), and three innominate bones (Subfamily Monachinae): Homiphoca capensis, Sarcodectes magnus, and a new genus and species Seronectes meherrinensis. This study emphasizes the significance of data from the fossil record, which are completely ignored by those working on molecular phylogenetics. Overall, this study further shows that the origin of pinnipeds cannot be resolved by using only molecular (genetic) approaches and future studies need to integrate both morphology and molecular data.