New Miocene Monachinae from the Western Shore of the Chesapeake Bay (Maryland, USA)

Abstract

The Family Phocidae includes four subfamilies (Phocinae, Monachinae, Cystophorinae, and Devinophocinae) consisting of mediumto large-sized mammals that possess distinctive adaptations to semi-aquatic life. In the Miocene of the Chesapeake Group, only two subfamilies of the Family Phocidae were identified: Phocinae and Monachinae. Leptophoca, a representative of the subfamily Phocinae, appears on the eastern shore of the North Atlantic around 16 million years ago. Recently, two new monachine species, the larger Terranectes magnus (n. gen., n. sp.) and the medium-sized T. parvus (n. sp.), were recorded in the Upper Miocene of the Chesapeake Group in the Eastover Formation (7.0–6.0 Ma) and St. Marys Formation (10.0-8.0 Ma). These two distinct subfamilies of seals indicate a well-marked divergence between phocines and monachines, much earlier than 18 million years ago, as previously suggested. The Eastover Formation was deposited in a shallow embayment that covered southern Maryland, the coastal plain of Virginia, and the northeastern corner of North Carolina. The geologically older St. Marys Formation represents a tide-influenced coastal environment, with low-salinity estuaries. There was a sharp temperature decrease in the Late Miocene, indicated by a shift to a cooler-water fish fauna during St. Marys time. The Eastover Formation reflects warmer waters with relatively strong currents, significant shoals, barriers, and varied depths. Fossil evidence of earlier seals suggests that phocids originated in the North Atlantic and otarioids in the North Pacific. True seals diverged from ancient Carnivora in the early Oligocene (or earlier) in the Paratethyan / Mediterranean Basins, spread widely during the Middle Miocene and crossed westward across the Atlantic Ocean, before dispersing in the eastern United States by the Early Pliocene.

A critical revision of the fossil record, stratigraphy and diversity of the Neogene seal genusMonotherium(Carnivora, Phocidae)

“…Apart from M. ? wymani , tympanic bullae have been presented for three other Neogene seals from the North Atlantic: Leptophoca proxima [ 2 ], Terranectes magnus and Terranectes parvus [ 66 ]. However, Dewaele et al .…”

Diversity of late Neogene Monachinae (Carnivora, Phocidae) from the North Atlantic, with the description of two new species

“…Messiphoca mauretanica Muizon, 1981 from the Messinian (upper Miocene) of Algeria [ 19 ] and Terranectes spp. from the upper Miocene of Maryland, USA [ 20 ] have formerly been identified as Monachinae from the North Atlantic realm, with a known humerus, but they are problematic for various reasons. M. mauretanica is represented by an associated left humerus, left ulna, left radius and six dorsal vertebrae (MNHN.F.ORN1, holotype) [ 19 ].…”

“…The genus Terranectes Rahmat, Koretsky, Osborne, Alford, 2017 was recently described, based on isolated material from the Chesapeake Bay Area, Maryland, USA [ 20 ]. Issues regarding the incomparability of both species to each other and to other known taxa make comparison impossible.…”

“…Rahmat et al . [ 20 ] invoke Koretsky's [ 16 ] ecomorphotype hypothesis to support the designation of isolated specimens of different types of bone to different taxa. Koretsky's [ 16 ] ecomorphotype hypothesis is constructed for mandibles, humeri and femora.…”

What do we know about the fossil record of pinnipeds? A historiographical investigation

“…Despite the fragmentary nature of their holotypes (proximal half of the left humerus and a partial femur, respectively), Rahmat et al . [89] erected taxonomic names for these species based on non-overlapping and non-associated cranial, axial and appendicular elements (including some from different stratigraphic levels). Later, Dewaele et al .…”