A new large-bodied Pliocene seal with unusual cutting teeth

Abstract

Today, monachine seals display the largest body sizes in pinnipeds. However, the evolution of larger body sizes has been difficult to assess due to the murky taxonomic status of fossil seals, including fossils referred to Callophoca obscura , a species thought to be present on both sides of the North Atlantic during the Neogene. Several studies have recently called into question the taxonomic validity of these fossils, especially those from the USA, as the fragmentary lectotype specimen from Belgium is of dubious diagnostic value. We find that the lectotype isolated humerus of C. obscura is too uninformative; thus, we designate C. obscura as a nomen dubium. More complete cranial and postcranial specimens from the Pliocene Yorktown Formation are described as a new taxon, Sarcodectes magnus . The cranial specimens display adaptations towards an enhanced ability to cut or chew prey that are unique within Phocidae, and estimates indicate S. magnus to be around 2.83 m in length. A parsimony phylogenetic analysis found S. magnus is a crown monachine. An ancestral state estimation of body length indicates that monachines did not have a remarkable size increase until the evolution of the lobodontins and miroungins.

Early monk seals (Monachinae: Monachini) from the late Miocene–early Pliocene of Australia

“…The entotympanic tubercles of Homiphoca and Acrophoca are also anteroposteriorly thin, but not at thin as NMV P160399. The posterolateral surface of the mastoid of NMV P160399 and the stem-monachines (except Sarcodectes; Rule et al 2020d) are all concave, differing from other monachines where the surface is convex, or not posteriorly facing. Overall, the tympanic bulla of NMV P160399 is longer mediolaterally than anteroposteriorly, whereas in the stem-monachines it is longer anteroposteriorly.…”

“…There are also some similarities between NMV P160399 and the stem-monachines Homiphoca capensis (Hendey & Repenning 1972;Muizon & Hendey 1980;Govender 2015), Piscophoca pacifica (Muizon 1981), Acrophoca longirostris (Muizon 1981), and Hadrokirus martini (Amson & de Muizon 2014), Sarcodectes magnus (Rule et al 2020d), and Monotherium? wymani (Ray 1976b).…”

“…However, the phylogenetic characters underlying the mastoid lip have been revised, calling into question the derived nature of the mastoid lip (Rule et al 2020d). Other fossil monachines outside of the 'fossil lobodontins' (Monotherium?…”

“…A multitude of taxa have been referred to the extant subfamilies Phocinae (Cozzuol 2001;Koretsky 2001Koretsky , 2003Koretsky & Ray 2008;Koretsky & Peters 2008;Koretsky et al 2012Dewaele et al 2017a, b) and Monachinae (Van Beneden 1876;Ray 1976b;Muizon & Bond 1982;Koretsky & Ray 2008;Koretsky & Domning 2014;Rahmat et al 2017;Dewaele et al 2018a, b, c); with some even being referred or allied to modern tribes such as Cystophorini (Koretsky & Rahmat 2013), Monachini (Koretsky & Grigorescu 2002), Lobodontini (Muizon & Hendey 1980Muizon 1981;Hendey & Repenning 1972;Amson & de Muizon 2014) and Miroungini (Van Beneden 1876; Koretsky & Ray 2008). The referral of some extinct taxa (such as material previously known as 'Callophoca') to modern subfamilies and tribes has been questioned by a few researchers (Berta et al 2015a;Boessenecker & Churchill 2016), but the doubt of these referrals has not gained much traction (Dewaele et al 2018c). Very few taxa have been described as archaic phocids (Koretsky & Holec 2002;.…”

True seals achieved global distribution by breaking Bergmann's rule

“…Maximum and minimum total body lengths for each specimen were taken from the literature (Stirling 1971;King 1983;Modig 1996;Andersen et al 1999;Bininda-Emonds and Gittleman 2000;Samaranch and Gonzalez 2000;Lindenfors et al 2002;Laws et al 2003;Rogers 2009;Churchill et al 2015;Valenzuela-Toro et al 2016;Dewaele et al 2017;Rule et al 2020bRule et al , 2021a; Tables A3, A4) or, where unavailable (for 12 out of 17 extinct taxa), maximum total body length was estimated following Rule et al (2020b;Tables A1, A2). All length data were then log 10 transformed prior to analysis.…”