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

Stephen Louwye,Leonard Dewaele,Olivier Lambert

doi:10.7717/peerj.3024

Stephen Louwye, Leonard Dewaele + Show 1 more

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https://doi.org/10.7717/peerj.3024

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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.

Highlights

Compared to many other marine mammal taxa, extinct true seal remains (Carnivora, Phocidae) are not uncommon
At present, only a few areas yielded a significant number of Neogene specimens: (1) the Miocene of the Paratethys region (Koretsky, 2001), (2) the Miocene and Pliocene of the North American East Coast (True, 1906; Ray, 1976a; Koretsky & Ray, 2008), (3) the Miocene and Pliocene of the southern North Sea Basin, including both the Belgian Antwerp area and the Netherlands (Van Beneden, 1859, 1871, 1876, 1877; Koretsky & Peters, 2008; Koretsky, Ray & Peters, 2012; Koretsky, Peters & Rahmat, 2015), and (4) the Miocene/Pliocene Pisco Formation of Peru
Geological and stratigraphical setting The original Van Beneden collection of fossil Phocidae was collected during fortification works around Antwerp, during the 1850s (Van Beneden, 1871, 1876, 1877)

Summary

Introduction

Compared to many other marine mammal taxa, extinct true seal remains (Carnivora, Phocidae) are not uncommon. Dated to the Miocene ‘Anversien,’ the Prophoca genus is considered among the oldest known fossil seal taxa (Demere, Berta & Adams, 2003). Louwye et al (2010) described an innominate and a femoral capitulum from the Berchem Formation at Posthofbrug, Antwerp, Belgium At the time, they identified both specimens as Phocidae aff. Prophoca and Leptophoca represent the oldest known genera of phocine seals, dating from the latest early to middle Miocene. Prophoca rousseaui and Prophoca proxima were described based on fragmentary remains from the Miocene of Belgium. Discussion: Leptophoca 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

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