Mystacodon selenensis, the earliest known toothed mysticete (Cetacea, Mammalia) from the late Eocene of Peru: anatomy, phylogeny, and feeding adaptations

Abstract

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.

Evolutionary Models for the Diversification of Placental Mammals Across the KPg Boundary

“…The most extreme example is Liu et al (2017a) where the estimated divergence date for sperm whale [a toothed whale (Odontoceti)] to minke whale [a baleen whale (Mysticeti)] is only 2.9 Ma. This estimated date is more than an order of magnitude younger than the age of the oldest mysticete fossil ( Mystacodon , 36.4 Ma) (Gatesy and Springer, 2017; Lambert et al, 2017a; de Muizon et al, 2019) and is also younger than numerous extinct mysticete and physeteroid (sperm whale) genera ( Figure 3 ). By contrast, McGowen et al’s (2009) timetree for Cetacea accommodates all of these fossils without any zombie lineages ( Figure 3 ).…”

Extensive Diversity and Disparity of the Early Miocene Platanistoids (Cetacea, Odontoceti) in the Southeastern Pacific (Chilcatay Formation, Peru)

“…Following other works already published by us on the platanistoids from the Chilcatay Formation [24][25][26], the fossils here described further support the great diversity and morphological disparity of this clade. On the whole the Eocene-Pliocene sedimentary succession of the East Pisco Basin represents one of the most significant marine vertebrate Lagerstätte of the Cenozoic Era due to the exceptional preservation and the elevated concentration of fossils [21,22,[40][41][42][43][44][45][46] referred to cetaceans (archaeocetes [47][48][49]; odontocetes [24,25,[50][51][52][53][54][55][56][57][58][59][60][61][62]; mysticetes [10,[63][64][65][66][67][68]), pinnipeds [69,70], marine birds [71][72][73][74], marine turtles [75], marine sloths [76][77]…”

A basilosaurid archaeocete (Cetacea, Pelagiceti) from the Late Eocene of Oregon, USA

“…Basilosaurid morphological affinities : The size of the vertebrae does not exclusively determine whether the vertebrae belong to a basilosaurid or a stem mysticete. While most of the earliest mysticetes such as Coronodon ( Geisler et al, 2017 ) and Mystacodon ( Muizon et al, 2019 ) are too small to have had thoracic vertebrae the size of those of NWMNH 2151, Llanocetus denticrenatus ( Mitchell, 1989 ) is quite large, and may have had thoracic vertebrae similar in size to those of NWMNH 2151 ( Fordyce & Marx, 2018 ). That said, Llanocetus is the only early mysticete known that could possibly be the size of NWMNH 2151, and none are known from the entire Northern Hemisphere.…”

“…Differential identification: The age (Priabonian), environment of deposition (deep marine), and overall construction of the vertebrae (mammalian) indicate that the vertebrae came from a cetacean. During the Priabonian, with the exception of a single protocetid from the earliest Priabonian of Egypt ( Gingerich, Antar & Zalmout, 2019 ), the only known cetaceans are either members of the Family Basilosauridae; stem Neoceti (e.g., Kekenodontidae) currently understood to be represented by a ghost lineage in the Priabonian ( Hernández-Cisneros & Tsai, 2016 ; Uhen, 2018 ); and stem Mysticeti ( Muizon et al, 2019 ; Fordyce & Marx, 2018 ). No Odontoceti are currently known from the Priabonian, and those from the earliest Oligocene are small.…”