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

Adams²,

Marx

et al. 2020

Proc. R. Soc. B.

Living true seals (phocids) are the most widely dispersed semi-aquatic marine mammals, and comprise geographically separate northern (phocine) and southern (monachine) groups. Both are thought to have evolved in the North Atlantic, with only two monachine lineages—elephant seals and lobodontins—subsequently crossing the equator. The third and most basal monachine tribe, the monk seals, have hitherto been interpreted as exclusively northern and (sub)tropical throughout their entire history. Here, we describe a new species of extinct monk seal from the Pliocene of New Zealand, the first of its kind from the Southern Hemisphere, based on one of the best-preserved and richest samples of seal fossils worldwide. This unanticipated discovery reveals that all three monachine tribes once coexisted south of the equator, and forces a profound revision of their evolutionary history: rather than primarily diversifying in the North Atlantic, monachines largely evolved in the Southern Hemisphere, and from this southern cradle later reinvaded the north. Our results suggest that true seals crossed the equator over eight times in their history. Overall, they more than double the age of the north–south dichotomy characterizing living true seals and confirms a surprisingly recent major change in southern phocid diversity.

Section: Results and Discussion (A) Fossil Record Of True Sealsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

First monk seal from the Southern Hemisphere rewrites the evolutionary history of true seals

Adams²,

Marx

et al. 2020

Proc. R. Soc. B.

“…The ‘ecomorphotype hypothesis’ states that various bone morphologies belong to ‘ecotypes’, allowing referral of otherwise unassociated fossils to a type specimen. However, a recent quantitative analysis found no support for this hypothesis [ 11 ], and several authors have placed doubts on its validity [ 9 , 15 , 57 ]. Apart from a loose association of ecology and morphology, the ‘ecomorphotype hypothesis' is in essence indistinguishable from the early work of Van Beneden [ 7 , 50 ].…”

Section: Discussionmentioning

confidence: 99%

A new large-bodied Pliocene seal with unusual cutting teeth

Adams²,

Rovinsky³

et al. 2020

R. Soc. open sci.

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.

“…This is the most exclusive clade assignment for any Neogene fossil pinniped from Australia. The only other fossils from the Neogene, either from the Sandringham Sandstone (late Miocene-early Pliocene) of Beaumaris (Rule et al 2020a) or from the Whalers Bluff Formation (Pliocene) of Portland (Victoria) (Fitzgerald 2005;Rule et al 2020c), are either diagnostic to Monachinae, Phocidae or Pinnipedia. The fossils from Beaumaris and Hamilton not only represent the oldest known pinnipeds from Australia, but the two ear regions represent the oldest described monachins in the world (Fig.…”

Section: Biogeographymentioning

confidence: 99%

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

Journal of Systematic Palaeontology

Adams

Fitzgerald

2021

Despite decades of research, the systematics of extinct true seals (Phocidae) is still overly reliant on morphological data from extant taxa. As a result, monk seals (Monachini) have been interpreted as 'archaic' despite an absence of fossil data to support this hypothesis. This has affected systematic hypotheses for extinct phocids, including fossils from Australasia. Recent finds from New Zealand indicate that the first seal fossils described from Australia, two temporal bones from the late Miocene-early Pliocene (6.24-4.35 Ma), need to be revisited. Here we re-describe these temporal bones and find them to represent monk seals. This places the oldest known fossils of this group in the Southern Hemisphere, implying monk seals had a longer history at southern latitudes. Our ancestral state estimation of the temporal bone morphology of monachines indicates that monk seal temporals may be derived, rather than plesiomorphic as has previously been assumed. This suggests that an over-reliance on the morphology of extant true seals may be obscuring the true diversity of both stem-and crown-phocids, and indicates that a rethink of the morphology underlying the taxonomic assessments of fragmentary true seal fossils is needed.