INFERRING DIET IN EARLY WHALES FROM MORPHOLOGICAL PATTERNS IN EXTANT TAXA: MORPHOMETRIC ANALYSIS OF PREY CAPTURE IN REMINGTONOCETUS

The ancestors of cetaceans (whales, dolphins, and porpoises) include artiodactyls (even-toed ungulates) that show a mosaic of morphologies associated with both terrestrial and aquatic habits. Morphology of the feeding apparatus in basal marine cetaceans, such as Remingtonocetus, is distinct from that seen in closely related terrestrial artiodactyls, but also dissimilar to that seen in extant odontocetes (toothed whales), providing an ambiguous picture of the feeding strategy that accompanied the initial cetacean invasion of marine habitats. To assess possible feeding strategies, we measured morphological variables related to the feeding apparatus in Remingtonocetus and a sample of 18 extant odontocetes and terrestrial artiodactyls. Odontocete predation strategies fall in two general groups, with varying degrees of overlap: snap feeding, in which the animal pushes its mouth through the water to capture prey; and suction feeding, in which the water surrounding the prey is forcibly drawn into the mouth. We coded feeding strategies in extant taxa as snap, suction, or browse/graze. We applied a novel method (phylogenetic redundancy analysis, PRDA), which allows ordination of morphological characters to be constrained by known relationships with behavior or size in extant taxa, while accounting for phylogenetic structure in the data. This method also allows extinct taxa without behavioral data to be placed in the morphospace. The first of the three canonical axes determined by PRDA shows a strong relationship to a proxy of body size. The remaining two canonical axes show separation between feeding categories, indicated by differences in symphysis length (related to snap feeding), the distance between the most mesial mandibular teeth (suction feeding), and the depth of the mandibular ramus (terrestrial browsing/grazing). Within this constrained morphospace, Remingtonocetus clusters within snap-feeding odontocetes, close to the river dolphins. Compared to other cetaceans, Remingtonocetus is placed furthest from extant suction-feeding odontocetes. The results suggest that snap-feeding is likely the primitive feeding mode for early marine cetaceans. This early variant of snap-feeding was likely employed without the moderate degree of suction that occurs in extant snap-feeding cetaceans.