Early Miocene squalodontid, Phoberodon arctirostris (Odontoceti, Platanistoidea), from Patagonia and phylogenetics of the Platanistoidea

R. EWAN FORDYCE; MARIANA VIGLINO; MÓNICA R. BUONO; JOSE CUITIÑO; ERICH FITZGERALD

Congreso; SVP 76th Annual Meeting; 2016

Shark-toothed dolphins (clade Squalodontidae) are pelagic, Late Oligocene - Late Miocene, long-jawed odontocetes with heterodont teeth. Most of the key named species are from shelf strata around the North Atlantic Ocean and Mediterranean. Recent phylogenetic analyses place the group just basal to or in the Platanistoidea. One of the few named squalodontids from the Southern Hemisphere is Phoberodon arctirostris (Gaiman Formation, Burdigalian, Early Miocene) from Patagonia, Argentina. The species, named in the 1920s, includes two informative types with longirostral skulls and postcrania, but lacking earbones and most of the basicrania. No new specimens have since been described, and the species has not been included in published phylogenetic analyses. A new specimen of P. arctirostris (MPEF-PV 10883; Playa Magagna, Chubut), allows a redescription, and the first phylogenetic analysis for Phoberodon. The fossil, from the Gaiman Formation, comprises a partial skull, a partial mandible, vertebrae and ribs, and both scapulae, but no earbones. The 3 Phoberodon fossils were used in a cladistic analysis based on a published morphological matrix totalling 84 taxa and 292 characters. Phoberodon has 49% missing data (particularly earbones, and also including soft tissues). In the resulting strict consensus of 7488 equally-parsimonious trees, Platanistoidea includes Waipatiidae, Squalodelphinidae, and Platanistidae, defined by 6 synapomorphies, 4 of them related to the periotic. Squalodon is immediately basal to Platanistoidea, with Phoberodon in a more-basal polytomy with Prosqualodon and other stem Odontoceti. The iterPCR procedure did not identify Phoberodon as an unstable taxon. An implied weight analysis (K=3) places (Phoberodon+Papahu)+Squalodon in the Platanistoidea; in other published analyses, Papahu is crownward of platanistoids. Results reiterate that the earbones (tympanoperiotics) and basicranium are important in odontocete morphological phylogenetics, and that taxa like Phoberodon which do not fully preserve these structural complexes commonly plot basal to their expected position. Phoberodon is for now a stem Odontoceti, but is likely to move more crownward, clustering with Squalodon, as new material is added to analyses.