Abstract
Sirenians are the only extant herbivorous mammals fully adapted to an aquatic lifestyle. They originated in Africa during the Paleocene from an undetermined clade of afrotherian mammals, and by the end of the Eocene they were widely distributed across the tropical latitudes. Here we introduce Sobrarbesiren cardieli gen. et sp. nov. It is the first adequately-known quadrupedal sirenian from Eurasia and the oldest record of this clade from western Europe. Fossils have been recovered from the middle Lutetian (SBZ15) site of Castejón de Sobrarbe-41 (Huesca, Spain), and comprise many cranial and postcranial remains, including pelvic girdle and hind limb bones, from at least six sirenian individuals of different ontogenetic stages. Sobrarbesiren shows a suite of characters previously considered synapomorphies of different clades of derived sirenians, such as the presence of the processus retroversus of the squamosal and the pterygoid fossa, combined with ancestral characters such as the presence of an alisphenoid canal, a permanent P5, at least two sacral vertebrae, a primitive pelvis and functional femora and fibulae. Sobrarbesiren is recovered as the sister taxon of Dugongidae and represents a transitional stage of adaptation to aquatic life between the amphibious quadrupedal prorastomids and the aquatic quadrupedal protosirenids.
Highlights
Sirenians originated from an undetermined clade of afrotherian mammals in the Paleocene[1]
The Castejón de Sobrarbe-41 sirenian fossil site (CS-41) fossiliferous level is in the uppermost part of the Sobrarbe Formation and has been correlated with nearby sections dated by magnetostratigraphy and benthic foraminifera[21], placing CS-41 within the C19r chron and biozone SBZ15 and making these the oldest sirenian remains in western Europe
Sobrarbesiren is recovered at the base of the clade that includes all sirenians except Prorastomus sirenoides, Pezosiren portelli and the clade Protosirenidae, as the sister taxon of all dugongids and trichechids (Fig. 11)
Summary
Sirenians originated from an undetermined clade of afrotherian mammals in the Paleocene[1]. The new locality found in the central Pyrenees, the site of Castejón de Sobrarbe-41 (CS-41) in the province of Huesca (northeastern Spain), is an exceptional middle Lutetian sirenian bone bed, which has provided more than 300 remains from at least six sirenian individuals, of different ontogenetic stages, assigned to the new taxon. It is the oldest sirenian in western Europe and is represented by cranial and postcranial material including functional hind limbs, constituting the first adequately-known quadrupedal sirenian from Eurasia. Further data about the geological and taphonomic context of Castejón de Sobrarbe-41 can be found in the supporting information
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