New species of protocetid archaeocete whale, Eocetus Wardii (Mammalia: Cetacea) from the Middle Eocene of North Carolina

Journal of Paleontology, May 1999 by Uhen, M D

All of the ribs are osteosclerotic, but only some of them are pachyostotic. Osteosclerosis is the condition of having cancellous bone replaced by compact bone, while pachyostosis is the condition of having an increased cross-sectional area of bone (Domning and de Buffrenil, 1991). Pachyostosis is evidenced in the ribs by the pestle-shape of the distal ends and the mid-shaft pieces. It is difficult to determine what part of the rib cage includes these pestle-shaped ribs, since none of them is associated with proximal ends, which could help determine their position in the rib cage. The single complete rib is not pestle-shaped, suggesting that the pachyostotic ribs are from the anterior part of the rib cage, since the complete rib is single-headed and thus from the posterior part of the rib cage (Fig. 5.1).

The osteosclerosis seen in the ribs is similar to that described in the vertebrae. The broken ribs clearly show the internal structure of the bone (Fig. 5.2). The ribs lack a medullary cavity, but have a core of cancellous bone. This core is off center, being located on the medial side of the rib. An off-center cancellous core of bone is also found in other mammals with osteosclerotic ribs like manatees and basilosaurine archaeocetes. In manatees the cancellous core is found on the medial side of the ribs (Fawcett, 1942), but in basilosaurines, it is found on the lateral side of the ribs (de Buffrenil et al., 1990). Thus, Eocetus is more like manatees in this respect than other archaeocetes, suggesting that the osteosclerosis of Eocetus was not inherited from a common ancestor with other archaeocetes. In addition, the compact cortical bone does not cover the entire medial side and distal end of the rib. The cancellous bone is visible on the medial side of the distal third of the complete rib, and the entire distal tip is uncovered as well (Fig. 5.1).

A portion of the right innominate of Eocetus ward was recovered as part of specimen USNM 310633 (Fig. 6). The fragment presumably includes parts of the ilium, ischium, and pubis, since it includes the entire acetabulum. The dorsalmost edge of the obturator foramen is preserved, as is part of the ilium extending away from the acetabulum, and much of the ischium. Most of the pubis is missing, including the pubic symphysis. Clearly defined landmarks are difficult to identify, as the innominate of Eocetus is quite different from any previously described cetacean.

The acetabulum is roughly circular and is divided into two sections, one narrower and with steeper sides, the other broader with more shallowly sloping sides. These two portions are nested one within the other, but slightly offset. The more narrow portion is offset dorsally, while the broader portion is offset ventrally. The anteroposterior diameter of the narrow portion of the acetabulum is around 30 mm while the anteroposterior diameter of the broad portion is around 45 mm. This larger diameter is more difficult to measure, as the edge of the acetabulum is less sharp in the broader portion. The acetabulum is about 21 mm deep on the shallow side of the dividing line between the two portions, and about 24 mm deep on the deep side.