A new large squalodelphinid (Cetacea, Odontoceti) from Peru sheds light on the Early Miocene platanistoid disparity and ecology.

Giovanni Bianucci,Christian De Muizon,Elisa Malinverno,Mario Urbina,Olivier Lambert,Giulia Bosio,Igor M Villa

doi:10.1098/rsos.172302

Abstract

The South Asian river dolphin (Platanista gangetica) is the only extant survivor of the large clade Platanistoidea, having a well-diversified fossil record from the Late Oligocene to the Middle Miocene. Based on a partial skeleton collected from the Chilcatay Formation (Chilcatay Fm; southern coast of Peru), we report here a new squalodelphinid genus and species, Macrosqualodelphis ukupachai. A volcanic ash layer, sampled near the fossil, yielded the 40Ar/39Ar age of 18.78 ± 0.08 Ma (Burdigalian, Early Miocene). The phylogenetic analysis places Macrosqualodelphis as the earliest branching squalodelphinid. Combined with several cranial and dental features, the large body size (estimated body length of 3.5 m) of this odontocete suggests that it consumed larger prey than the other members of its family. Together with Huaridelphis raimondii and Notocetus vanbenedeni, both also found in the Chilcatay Fm, this new squalodelphinid further demonstrates the peculiar local diversity of the family along the southeastern Pacific coast, possibly related to their partition into different dietary niches. At a wider geographical scale, the morphological and ecological diversity of squalodelphinids confirms the major role played by platanistoids during the Early Miocene radiation of crown odontocetes.

Highlights

Echolocating toothed whales (Cetacea, Odontoceti) experienced an initial significant radiation during the Oligocene, being well documented by a consistent and continuously improving fossil record [1,2,3,4,5,6,7,8,9,10,11]
During the Early Miocene, this extinction phase was followed by the diversification of several groups of crown odontocetes in the marine environment, generally with an origin during the Late Oligocene
Macrosqualodelphis ukupachai is a new species of the extinct platanistoid family Squalodelphinidae based on a well-preserved partial skeleton collected from the Early Miocene fossiliferous beds of the Chilcatay Fm outcropping in the Western Ica Valley

Summary

Introduction

Echolocating toothed whales (Cetacea, Odontoceti) experienced an initial significant radiation during the Oligocene, being well documented by a consistent and continuously improving fossil record [1,2,3,4,5,6,7,8,9,10,11] This broad diversity and morphological disparity was deeply sifted by selection near the Oligocene–Miocene boundary, when most of the stem odontocetes disappeared. During the Early Miocene, this extinction phase was followed by the diversification of several groups of crown odontocetes in the marine environment, generally with an origin during the Late Oligocene This first crown odontocete radiation is characterized by a significant diversification of nearly homodont members of the superfamily Platanistoidea (Allodelphinidae, Platanistidae and Squalodelphinidae), forming a monophyletic group in several recent phylogenetic analyses [13,14,15]. This initial radiation was followed by successive extinction phases during the Middle to Late Miocene, with only the freshwater South Asian dolphin (Platanista gangetica Lebeck, 1801 [16]) surviving today [17,18]

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