A new diminutive fossil ziphiid from the deep-sea floor off northern Chile and some remarks on the body size evolution and palaeobiogeography of the beaked whales

Abstract

The evolutionary history of the beaked whales (Ziphiidae), odontocetes nowadays adapted to deep diving, is well known thanks to a significant fossil record mainly from the deep ocean floors. A partial cranium of a ziphiid recovered from Plio-Pleistocene deep sea deposits (about 1000 m) off the port of Pisagua, northern Chile, during fishing activity is here described and referred to the new species Ihlengesi changoensis. Ihlengesi changoensis differs from the type species Ihlengesi saldanhae, from the sea floor off South Africa, by having a more elongated premaxillary sac fossa and consequently a more anteriorly located premaxillary foramen; dorsal margin of each premaxillary crest sloping markedly ventrolaterally and generating an acute dorsal profile of the vertex in anterior view; less anterolateral extension of the right nasal forming part of the premaxillary crest; lateral margins of the nasals not anteriorly diverging but weakly convex; nasofrontal suture anteriorly pointed. The phylogeny supports a sister-taxon relationship between I. changoensis and I. saldanhae, both members of the crown ziphiids Hyperoodontinae. Ihlengesi changoensis shares with I. saldanhae and other fossil ziphiids a small body size (estimated length 3.5 m) supporting the hypothesis that in the past small beaked whales (<4 m) were more common than today. Such recent shift of the ziphiids towards a larger size could be the result of a progressive change of diet from fish to cephalopods, to the competition with the delphinids, and the predatory impact of the white shark Carcharodon carcharias and/or of the killer whale Orcinus orca. This new Chilean ziphiid further supports the hypothesis that crown beaked whales originated and firstly dispersed in the oceanic waters of the Southern Hemisphere. Their radiation and geographical distribution could have been driven by the southern oceanic circulation and related localized concentration of trophic resources in high productivity upwelling areas.