Description
Parapontoporia is a common Miocene-Pliocene (7.2-1.8 Ma) fossil river dolphin from California and Baja California, Mexico. There are currently three recognized species of Parapontoporia: P. sternbergi, P. wilsoni, and P. pacifica. Though it is abundant in the fossil record, the evolutionary history of Parapontoporia remains largely uncertain. Previous studies have differed in their placement of Parapontoporia. Conflicting evidence shows it to be the sister taxon of the La Plata river dolphin, the Chinese river dolphin, or the South Asian river dolphin. Furthermore, the three currently recognized species of Parapontoporia were established based on qualitative descriptions of cranial morphology and have yet to be verified by modern standards for species delimitation. One goal of this study was to resolve the uncertainty of the Parapontoporia’s phylogenetic placement and to verify the validity of the three purported Parapontoporia species. In order to reveal species-level distinctiveness and phylogenetic placement of Parapontoporia, a character matrix was constructed using 71 cranial morphology characters. The taxon sample included Parapontoporia, seven extant odontocetes, three extinct crown odontocetes, and one stem odontocete. Parsimony analyses were executed using six Parapontoporia specimens as individual operational taxonomic units (OTUs) to examine species level distinctiveness among the specimens. Another analysis included Parapontoporia specimens as combined OTUs to examine the placement of Parapontoporia within Odontoceti. The results supported the presence of two valid Parapontoporia species and the South Asian river dolphin as the sister taxon of Parapontoporia. Another goal of this study was to examine the habitat of ancestral odontocetes to provide an ecological context based on phylogenetic position. Two different phylogenetic hypotheses were used to perform an ancestral state reconstruction, which identified odontocete ancestors that initiated occupation of freshwater habitats. One hypothesis placed Parapontoporia as sister to the South Asian river dolphin and the other placed Parapontoporia as sister to the Chinese river dolphin. In both analyses, the ancestral state reconstruction explained the current diversity of river dolphins as the result of two independent freshwater invasions. However, only the analysis that placed Parapontoporia as sister to the South Asian river dolphin could identify a definitive freshwater ancestor of the South Asian river dolphin.
