Oldest record of monk seals from the North Pacific and biogeographic implications

Abstract

True seals (crown Phocidae) originated during the late Oligocene–early Miocene (approx. 27–20 Ma) in the North Atlantic/Mediterranean region, with later (middle Miocene, approx. 16–11 Ma) dispersal events to the South Atlantic and South Pacific. Contrasting with other pinnipeds, the fossil record of phocids from the North Pacific region is scarce and restricted to the Pleistocene. Here we present the oldest fossil record of crown phocids, monachines (monk seals), from the North Pacific region. The specimens were collected from the upper Monterey Formation in Southern California and are dated to 8.5–7.1 Ma, predating the previously oldest known record by at least 7 Ma. This record provides new insights into the early biogeographic history of phocids in the North Pacific and is consistent with a northward dispersal of monk seals (monachines), which has been recognized for other groups of marine mammals. Alternatively, this finding may correspond with a westward dispersal through the Central American Seaway of some ancestor of the Hawaiian monk seal. This record increases the taxonomic richness of the Monterey pinniped assemblage to five taxa, making it a fairly diverse fossil assemblage, but also constitutes the oldest record of sympatry among all three extant pinniped crown clades.

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

“…From here, they reinvaded Australasia, before spreading to the North Atlantic around 7.1 Ma, and finally to the Mediterranean around 3 Ma. This scenario corroborates a relatively long history of monachines in the Pacific [59], and questions earlier suggestions that monk seals dispersed westwards from the Atlantic via the Central American Seaway [53,59]. Instead, they either migrated in the opposite direction, or entered the Atlantic by dispersing around the southern tip of South America (figure 4b).…”

“…The best-supported biogeographic model (DIVALIKE + J) implies that stem phocids and monachines crossed the equator eight times over the past 15 Ma (figure 3). This figure is far higher than previously assumed [2][3][4][5][6][7], and yet may still be an underestimate given the recent description of North Pacific monachine remains resembling fossils from Peru [59]. Even today, southern elephant seals have been noted north of the equator [60].…”

Bridging two oceans: small toothed cetaceans (Odontoceti) from the Late Miocene Chagres Formation, eastern Caribbean (Colon, Panama)

“…[37] Similarly, the Late Miocene part of the Monterey Formation in California shares several taxa with the Pisco Fm. (Figure 2), namely the presence of Piscolithax , Atocetus , kogiids, raptorial sperm whales, small herpetocetines, balaenopterids and a monachine seal [5,50]. The overall similarities of the Chagres fauna with Pacific faunas suggest that there was an interchange of shallow waters between the Caribbean and the Pacific during the Late Miocene even though the deep marine connections had already been closed [51].…”

True seals achieved global distribution by breaking Bergmann's rule

“…Unlike fur seals, sea lions, and walruses—all of which remained restricted to the North Pacific for much of their evolution—true seals have long enjoyed a global distribution (Berta et al. 2018; Velez‐Juarbe and Valenzuela‐Toro 2019). Broad temperature tolerances may help to explain this pattern, with true seals being able to invade new environments relatively easily.…”