New data on archaic homodont odontocetes from the Early Miocene of Peru reveal a second species of Chilcacetus Lambert, Muizon & Bianucci, 2015 and a Southern Hemisphere record for a northeastern Pacific species

The systematic affinities of several homodont odontocetes (toothed whales) in the genera Argyrocetus Lydekker, 1893 and Macrodelphinus Wilson, 1935 from the Early Miocene of the northeastern Pacific and southwestern Atlantic have long been debated. The description of the longirostrine dolphin Chilcacetus cavirhinus Lambert, Muizon & Bianucci, 2015, based on two finely preserved skulls from the Early Miocene of Peru, revealed similarities with part of these taxa, but questions remained about their phylogenetic relationships. Two new specimens from the Burdigalian of the Chilcatay Formation (East Pisco Basin, Peru) and recently rediscovered skull parts for the holotype of C. cavirhinus lead to the diagnosis of a new species of Chilcacetus Lambert, de Muizon & Bianucci, 2015, Chilcacetus ullujayensis n. sp., and the description of cranial regions previously unknown (or poorly known) for that genus, especially the ear bones. Another smaller skull from the Chilcatay Formation is referred here to Amphidelphis bakersfieldensis n. comb., a species previously only known from California whose palaeogeographic distribution is thus expanded across the equator. Rostral morphological features in the new specimen of A. bakersfieldensis n. comb. differ markedly from Argyrocetus patagonicus Lydekker, 1893, the Patagonian species that was earlier recognized as closely related to A. bakersfieldensis n. comb., and from Eurhinodelphinidae Abel, 1901, the family long proposed to have housed these two species. Finally, two other specimens from the Chilcatay Formation are identified as Odontoceti aff. A. bakersfieldensis. New phylogenetic analyses recover Chilcacetus spp. and A. bakersfieldensis n. comb. in a clade of longirostrine to hyper-longirostrine stem odontocetes, also including Eoplatanista Dal Piaz, 1916, Macrodelphinus, and eurhinodelphinids. The new records of A. bakersfieldensis n. comb. and Chilcacetus spp. presented here confirm the predominance of long-snouted species in the Burdigalian odontocete assemblages of the East Pisco Basin.