Charting the course of pinniped evolution: insights from molecular phylogeny and fossil record integration

Abstract

Pinnipeds (seals, sea lions, walruses, and their fossil relatives) are one of the most successful mammalian clades to live in the oceans. Despite a well-resolved molecular phylogeny and a global fossil record, a complete understanding of their macroevolutionary dynamics remains hampered by a lack of formal analyses that combine these two rich sources of information. We used a meta-analytic approach to infer the most densely sampled pinniped phylogeny to-date (36 recent and 93 fossil taxa) and used phylogenetic paleobiological methods to study their diversification dynamics and biogeographic history. Pinnipeds mostly diversified at constant rates. Walruses however experienced rapid turnover in which extinction rates ultimately exceeded speciation rates from 12-6 Ma, possibly due to changing sea-levels and/or competition with otariids (eared seals). Historical biogeographic analyses including fossil data allowed us to confidently identify the North Pacific and the North Atlantic (plus or minus Paratethys) as the ancestral ranges of Otarioidea (eared seals + walrus) and crown phocids (earless seals), respectively. Yet, despite the novel addition of stem pan-pinniped taxa, the region of origin for Pan-Pinnipedia remained ambiguous. These results suggest further avenues of study in pinnipeds and provide a framework for investigating other groups with substantial extinct and extant diversity.

A quantitative test of the “Ecomorphotype Hypothesis” for fossil true seals (Family Phocidae)

“…4 ), especially the eastern North Atlantic, this taxonomic upheaval has serious implications for the evolution of Phocidae. This region is often invoked as the area where Phocidae originated and diversified ( Berta, Churchill & Boessenecker, 2018 ; Rule et al, 2020b ; Park et al, 2024 ), and includes the entirety of the fossil record of the northern phocids (Phocinae). As such, any future research efforts to uncover, describe, and name more complete fossil phocids should also focus on the Miocene of Europe.…”

“…Other more complete taxa might also need revision, such as Devinophoca emryi . Despite the type specimen being a skull, this taxon is known from the same locality and unnamed formation (underlying the Studienka Formation) as the only other member of its genus ( Devinophoca claytoni ), and both are morphologically very similar and usually end up close to each other on phylogenies ( e.g ., Rule et al, 2020b ; Park et al, 2024 ). Another taxon, Histriophoca alekseevi , is likely valid as it is represented by a partial skull, however the referral to the modern genus Histriophoca should be revisited in light of recent work.…”

“…As the original Ecomorphotype Hypothesis was defined for Phocinae ( Koretsky, 2001 ; Koretsky, Alexander & Rahmat, 2020 ), we only used measurements for fossil and extant Phocinae. We excluded Cystophora cristata (hooded seal) as the authors of the Ecomorphotype Hypothesis consider it to be in a separate subfamily (Cystophorinae; Koretsky, Alexander & Rahmat, 2020 ); however, we note that this assignment contradicts the vast majority of morphological and molecular evidence which supports this taxon as a member of Phocinae ( Fulton & Strobeck, 2010 ; Dewaele et al, 2017b ; Berta, Churchill & Boessenecker, 2018 ; Paterson et al, 2020 ; Rule et al, 2020b ; Park et al, 2024 ). We then expanded on this dataset using 36 specimens from the Natural History Museum in London (NHMUK) and a specimen of Phoca largha (spotted seal) in the National Science Museum in Tokyo (NMNS).…”