Abstract
Here, we describe the odobenid Nanodobenus arandai gen. et sp. nov., based on a nearly complete left mandible from the mid to late Miocene Tortugas Formation in Baja California Sur. Nanodobenus is distinguished among odobenids by displaying a unique combination of plesiomorphic and derived characters, such as narrow mandibular symphysis, well-developed genial tuberosity, bilobed canine and p2 roots, bulbous post-canine teeth with the paraconid, protoconid and hypoconid, and smooth lingual cingula. Moreover, it is characterized by its small adult body length, which is estimated at about 1.65 m. Throughout the Miocene–Pliocene odobenids are characterized by an increase in body size, especially after the extinction of desmatophocids in the late Miocene. The small size of Nanodobenus departs from this trend, demonstrating that there was greater size disparity among odobenids in the mid–late Miocene than previously thought. It is hypothesized that Nanodobenus occupied a niche that was later on occupied by similar-sized otariids, such as Thalassoleon mexicanus, which occurs sympatrically with large odobenids in the overlying Almejas Formation.
Highlights
The evolutionary history of odobenids is intimately tied with the North Pacific, with the group originating in the region during the early Miocene [1,2]
During the late Miocene–Pliocene, odobenids show a marked trend of increasing body size, similar to what is observed in other marine mammals [3,4,7,31]
Nanodobenus arandai seems to be the exception to this trend
Summary
The evolutionary history of odobenids is intimately tied with the North Pacific, with the group originating in the region during the early Miocene [1,2] As such, their fossil record offers unique insight into their ancient diversity and evolutionary patterns over time [1,2,3,4]. Their fossil record offers unique insight into their ancient diversity and evolutionary patterns over time [1,2,3,4] During their early history odobenids occurred sympatrically with otariids and desmatophocids, with the latter having larger body sizes and presumably occupying higher trophic levels [1,2,3,4,5,6].
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