Odontocetes (toothed whales) rely upon echoes of their own vocalizations to navigate and find prey underwater [1]. This sensory adaptation, known as echolocation, operates most effectively when using high frequencies, and odontocetes are rivaled only by bats in their ability to perceive ultrasonic sound greater than 100 kHz [2]. Although features indicative of ultrasonic hearing are present in the oldest known odontocetes [3], the significance of this finding is limited by the methods employed and taxa sampled. In this report, we describe a new xenorophid whale (Echovenator sandersi, gen. et sp. nov.) from the Oligocene of South Carolina that, as a member of the most basal clade of odontocetes, sheds considerable light on the evolution of ultrasonic hearing. By placing high-resolution CT data from Echovenator sandersi, 2 hippos, and 23 fossil and extant whales in a phylogenetic context, we conclude that ultrasonic hearing, albeit in a less specialized form, evolved at the base of the odontocete radiation. Contrary to the hypothesis that odontocetes evolved from low-frequency specialists [4], we find evidence that stem cetaceans, the archaeocetes, were more sensitive to high-frequency sound than their terrestrial ancestors. This indicates that selection for high-frequency hearing predates the emergence of Odontoceti and the evolution of echolocation.

Although combined molecular and morphological analyses point to a late middle Eocene (38-39 million years ago) origin for the clade Neoceti (Odontoceti, echolocating toothed whales plus Mysticeti, baleen whales, and relatives), the oldest known mysticete fossil dates from the latest Eocene (about 34 million years ago) of Antarctica [1, 2]. Considering that the latter is not the most stemward mysticete in recent phylogenies and that Oligocene toothed mysticetes display a broad morphological disparity most likely corresponding to contrasted ecological niches, the origin of mysticetes from a basilosaurid ancestor and its drivers are currently poorly understood [1, 3-8]. Based on an articulated cetacean skeleton from the early late Eocene (Priabonian, around 36.4 million years ago) of the Pisco Basin, Peru, we describe a new archaic tooth-bearing mysticete, Mystacodon selenensis gen. et sp. nov. Being the geologically oldest neocete (crown group cetacean) and the earliest mysticete to branch off described so far, the new taxon is interpreted as morphologically intermediate between basilosaurids and later toothed mysticetes, providing thus crucial information about the anatomy of the skull, forelimb, and innominate at these critical initial stages of mysticete evolution. Major changes in the morphology of the oral apparatus (including tooth wear) and flipper compared to basilosaurids suggest that suction and possibly benthic feeding represented key, early ecological traits accompanying the emergence of modern filter-feeding baleen whales' ancestors.

is a toothed mysticete that represents the earliest member of the suborder in the current state of knowledge. Its holotype is a relatively complete skeleton from the upper Eocene (early Priabonian, c. 36.4 Ma) Yumaque Member of the Paracas Formation from the southern coast of Peru. The thorough description of this specimen is presented here and reveals numerous similarities with the contemporaneous basilosaurids including the retention of an innominate that originally articulated to the unpreserved hind limb. However, several characters of M. selenensis clearly relate this taxon to the mysticetes, such as the large palate with a dorsoventrally flattened rostrum, the posterior extension of the palate with an infraorbital plate of the maxilla, the shortening of the premaxillary part of the rostrum, the zygomatic process of the squamosal being closely apposed to the postorbital process of the frontal, and the humeral head being oriented more proximally than posteriorly. A parsimony analysis retrieves Mystacodon as the earliest diverging branch of the Mysticeti with no close phylogenetic relationship with Llanocetus the second oldest known mysticete (c. 34.2 Ma). The dental formula of M. selenensis is that of basilosaurids (I 3/3, C 1/1, P 4/4, M 2/3). The anterior teeth (incisors and canine) are distinctly proportionally smaller than in basilosaurids, whereas the cheek teeth are very close in relative length, but are relatively larger than in most other toothed mysticetes (except Coronodon). The large cheek teeth of Mystacodon suggest a raptorial feeding strategy, probably assisted with some GEODIVERSITAS • 2019 • 41 (11) Muizon C. de et al.

No abstract