Aegicetus gehennae, a new late Eocene protocetid (Cetacea, Archaeoceti) from Wadi Al Hitan, Egypt, and the transition to tail-powered swimming in whales.

Philip D Gingerich,Iyad S Zalmout,Mohammed Sameh M Antar,Thierry Smith

doi:10.1371/journal.pone.0225391

Abstract

Aegicetus gehennae is a new African protocetid whale based on a partial skull with much of an associated postcranial skeleton. The type specimen, Egyptian Geological Museum, Cairo [CGM] 60584, was found near the base of the early-Priabonian-age (earliest late Eocene) Gehannam Formation of the Wadi Al Hitan World Heritage Site in Egypt. The cranium is distinctive in having ventrally-deflected exoccipitals. The vertebral column is complete from cervical C1 through caudal Ca9, with a vertebral formula of 7:15:4:4:9+, representing, respectively, the number of cervical, thoracic, lumbar, sacral, and caudal vertebrae. CGM 60584 has two more rib-bearing thoracic vertebrae than other known protocetids, and two fewer lumbars. Sacral centra are unfused, and there is no defined auricular surface on the ilium. Thus there was no weight-bearing sacroiliac joint. The sternum is distinctive in being exceptionally broad and flat. The body weight of CGM 60584, a putative male, is estimated to have been about 890 kg in life. Long bones of the fore and hind limbs are shorter than expected for a protocetid of this size. Bones of the manus are similar in length and more robust compared to those of the pes. A log vertebral length profile for CGM 60584 parallels that of middle Eocene Maiacetus inuus through the anterior and middle thorax, but more posterior vertebrae are proportionally longer. Vertebral elongation, loss of a sacroiliac articulation, and hind limb reduction indicate that Aegicetus gehennae was more fully aquatic and less specialized as a foot-powered swimmer than earlier protocetids. It is doubtful that A. gehennae had a tail fluke, and the caudal flattening known for basilosaurids is shorter relative to vertebral column length than flattening associated with a fluke in any modern whale. Late protocetids and basilosaurids had relatively long skeletons, longer than those known earlier and later, and the middle-to-late Eocene transition from foot-powered to tail-powered swimming seemingly involved some form of mid-body-and-tail undulation.

Highlights

Protocetidae are semiaquatic whales known from middle Eocene strata in Africa, Asia, North America, and South America [1,2,3,4,5,6,7]
Aegicetus gehennae is important for four reasons: (1) A. gehennae extends the stratigraphic range of Protocetidae into the earliest Priabonian age of the geological time scale
(3) A. gehennae groups with other protocetids on a spectrum of semiaquatic mammalian skeletal proportions, but it was more aquatic than most protocetids in lacking any firm connection of the pelvis and hind limbs to the vertebral column

Summary

Introduction

Protocetidae are semiaquatic whales known from middle Eocene strata in Africa, Asia, North America, and South America [1,2,3,4,5,6,7]. Pedicles and laminae for the neural arch rise from the anterior margin of the centrum and extend posteriorly and laterally toward the posterior capitular facets. The base of the neural spine is broad, linking the postzygapophyses It is triangular in cross section through much of its preserved length, narrowing dorsally. Prezygapophyses extend anteriorly from the neural arch, positioned above and lateral to the neural canal, with surfaces facing medially and slightly dorsally. An estimate of 890 kg for Aegicetus gehennae is a little less than estimates for body weights of late Eocene basilosaurids such as Zygorhiza kochii (1,000 kg) and Dorudon atrox (1130 kg [43])

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Discussion

Conclusions