A new unusual Monachinae from the Neogene of the Atlantic Coastal Plain (Maryland, USA)

Abstract

ABSTRACT To date, for the family Phocidae, only two fossil genera of the subfamily Monachinae (Monotherium and Terranectes) and two fossil genera of the subfamily Phocinae (Prophoca and Leptophoca) have been described from the Western Shore of the Chesapeake Bay (Maryland, USA). This study presents the first description of a new fossil partial seal skull from the St. Mary’s Formation (late Miocene, 8.5–7.5 MYA) in Maryland, USA. The new skull exhibits both primitive and derived features. Morphological assessment of this new skull supports classifying it within the subfamily Monachinae (based on numerous diagnostic characters). However, the presence of unique characters that do not match with any known genera of Monachinae led 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. Some key characters of the Magnotherium johnsii (sp. nov.) skull include the presence of six incisors (similar to seals of the subfamily Phocinae), a long rostrum, and long concavity of the preorbital area of the maxilla.