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

Abstract

Background Basilosaurid archaeocetes are known from the Late Eocene of virtually all coastlines bearing coeval marine rocks except the North Pacific Basin, until now. Here we report on three consecutive posterior thoracic vertebrae of a large, basilosaurid archaeocete from a Late Eocene horizon in the Keasey Formation in Oregon. Methods These vertebrae were morphologically and morphometrically compared to other vertebrae of similar age from around the world. Results The specimens were determined to be different from all currently named species of fossil cetacean, but most similar to those found in the Gulf Coast region of North America. These vertebrae represent the first confirmed specimen of a Late Eocene basilosaurid from the North Pacific. These and other basilosaurids known only from vertebrae are reviewed here in the context of Late Eocene paleoceanography and cetacean evolution.

The pattern of brain-size change in the early evolution of cetaceans

“…Most authors have assumed that fossil basilosaurid cetaceans, like the extant mysticetes, possessed a relatively enlarged cranial rete. Gingerich [ 30 ] subtracted 20% from the endocranial volume of some basilosaurids to estimate brain mass, based on work of Uhen [ 51 ], and Marino et al [ 7 , 14 ] extended this correction to the remaining basilosaurids. Our data indicate that endocranial volumes of terrestrial artiodactyls and cetaceans would need a volumetric reduction of approximately 24 percent (the average for the fossil whales in our dataset) to approximate brain volume directly from endocranial volume, and this correction is without the assumption that an enlarged rete is present.…”

“…The unusual morphology of basilosaurid endocasts [ 33 , 48 ] has been interpreted by most authors as evidence they possessed an unusually large rete mirabile, and most modern researchers have followed Uhen [ 51 ], Gingerich [ 30 ] and Marino et al [ 14 ] in assuming that a rete filled a fixed 20 percent of the basilosaurid cranial cavity. Our data, which does not specifically address the rete in isolation from the adnexa, shows that the percentage of the cranial cavity filled by adnexa and cerebrospinal fluid is not fixed, but rather scales with endocranial volume.…”

“…Uhen [ 47 , 51 ] first estimated the rete in the basilosaurid Dorudon atrox to have occupied approximately 20% of the cranial cavity volume based on a series of D . atrox endocasts in which the volume of the rete was approximated using clay.…”

“…Body masses of extinct mammals have been estimated using various skeletal metrics such as tooth size, cranial dimensions, length and diameter of long-bones, vertebral dimensions, and body length [ 8 , 30 , 53 – 60 ]. In fossil cetaceans specifically, body masses have been estimated using the width between the outer margins of the occipital condyles (OCW) [ 7 , 9 , 61 , 62 ], serial regressions of vertebral dimensions [ 8 , 30 ], and body length [ 8 , 9 , 14 , 32 , 47 , 51 ]. Alternatively, body length has been estimated using multiple or linear regressions of cranial dimensions [ 52 ], or from postorbital width [ 63 ].…”

“…Most studies examining EQs in fossil cetaceans have used a combination of these techniques to estimate body mass. In general, body mass estimates for Eocene whales were derived from serial regressions of vertebral dimensions based on the work of Gingerich [ 8 , 30 ], along with a few mass estimates based on body length [ 14 , 32 , 51 ]. In contrast, post-Eocene body mass estimates are derived almost exclusively from the OCW based estimates published by Marino et al [ 7 ].…”