OnProphocaandLeptophoca(Pinnipedia, Phocidae) from the Miocene of the North Atlantic realm: redescription, phylogenetic affinities and paleobiogeographic implications

Abstract

BackgroundProphoca and Leptophoca represent the oldest known genera of phocine seals, dating from the latest early to middle Miocene. Originally, Prophoca rousseaui and Prophoca proxima were described based on fragmentary remains from the Miocene of Belgium. However, several researchers contested the union of Prophoca rousseaui and Prophoca proxima into one genus, without providing evidence. The stratigraphic context of Prophoca remained poorly constrained due to the lack of precise data associated with the original specimens collected in the area of Antwerp (north of Belgium).MethodsProphoca and Leptophoca are redescribed and their phylogenetic position among Phocidae is reassessed using PAUP. Dinoflagellate biostratigraphy has been carried out on sediment samples associated with specimens from Prophoca and Leptophoca to elucidate their approximate ages.ResultsWhereas the species Prophoca rousseaui is redescribed, Prophoca proxima is considered synonymous to Leptophoca lenis, with the proposal of a new combination Leptophoca proxima (Van Beneden, 1877). Sediment samples from specimens of both taxa have been dated to the late Langhian–early Serravallian (middle Miocene). Following a reinvestigation of Leptophoca amphiatlantica, characters from the original diagnosis are questioned and the specimens of Leptophoca amphiatlantica are considered Leptophoca cf. L. proxima. In a phylogenetic analysis, Prophoca rousseaui and Leptophoca proxima constitute early branching stem-phocines.DiscussionLeptophoca proxima from the North Sea Basin is younger than the oldest known find of Leptophoca proxima from North America, which does not contradict the hypothesis that Phocinae originated along the east coast of North America during the late early Miocene, followed by dispersal to Europe shortly after. Morphological features of the appendicular skeleton indicate that Prophoca rousseaui and Leptophoca proxima have archaic locomotory modes, retaining a more prominent use of the fore flipper for aquatic propulsion than extant Phocidae.

Reappraisal of the extinct seal “Phoca”vitulinoidesfrom the Neogene of the North Sea Basin, with bearing on its geological age, phylogenetic affinities, and locomotion

“…While most Pliocene Phocidae from Europe have also been found in North America, only the larger Miocene L. proxima has a trans-Atlantic range (Koretsky & Ray, 2008; Dewaele, Lambert & Louwye, 2017). All other European and Paratethyan phocids are strongly endemic and have so far not been found in North American deposits or elsewhere (Koretsky, 2001; Koretsky & Rahmat, 2013).…”

“…Because no middle Miocene phocines other than L. proxima are known from the east coast of North America, and because the diversity of middle and late Miocene Phocinae is high in the North Sea Basin and the Paratethys (Van Beneden, 1876, 1877; Koretsky, 2001; Koretsky & Peters, 2008; Dewaele, Lambert & Louwye, 2017), it can be argued that crown Phocinae originated in Europe during the middle to early late Miocene. Although the current analysis is the first to determine the phylogenetic position of N. vitulinoides as the last stem phocine to branch off before the crown group (Fig.…”

Mandibular Morphology of the Mid-Miocene Seal Devinophoca claytoni (Carnivora, Phocidae, Devinophocinae)

“…Recently, Dewaele et al (2017) declared that Leptophoca lenis (middle Miocene, ~16 Ma; Koretsky, 2001: fi g. 42) is a junior synonym of L. proxima. We feel that more work needs to be done to support or dismiss this nomenclature change.…”

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

“…Th e occurrence of Leptophoca on both sides of the Atlantic Ocean, in Th e Netherlands and Maryland and Virginia (USA), supports geological and morphological evidence, and shows that Leptophoca amphiatlantica (Koretsky et al, 2012) originated in Western Europe (Holland,, crossed the Atlantic Ocean, and settled on the western shore of the North Atlantic (fi g. 1), fi rst during deposition of the Calvert Formation (20.0-14.2 Ma), and later spreading southward during the time of the St. Marys Formation (10.0-8.0 Ma). Fossil evidence of the geologically older L. amphiatlantica from Europe and the geologically younger L. lenis from USA contradicts the reclassifi cation of Leptophoca and Prophoca by Dewaele et al (2017).…”

“…Leptophoca lenis (Phocinae) from the Calvert Formation (~18 Ma), renamed Leptophoca proxima by Dewaele et al (2017), is one of the most primitive representatives of true seals, is widely known to researchers, and has been collected at numerous locations along the western shore of the Chesapeake Bay (fi g. 2). Recently, an abundance of fossil material of this species has been collected (deposited primarily in the USNM and CMM), which is very unusual for any fossil seal taxon.…”