Basilosaurus isis

A photomosaic of the WH 10001 Basilosaurus isis skeleton as it lies in the field is illustrated in Fig 3. Late Eocene B. isis skeletons in the overlying Birket Qarun Formation are often partially to fully articulated, but here in the Gehannam Formation bones of WH 10001 are disarticulated and somewhat scattered. CGM 42195 and UM 97526 show that the vertebral formula of B. isis was 7:16:19:4:20, representing counts of cervical, thoracic, lumbar, sacral, and caudal vertebrae, for a total count of 66 vertebrae. In the field we identified 4 cervicals, 10 thoracics, 28 ‘lumbars,’ no sacrals, and no caudals preserved in WH 10001 (but see comment below). Some cervical and thoracic vertebrae of WH 10001 are missing from the site, as are all of the smaller caudals. It appears that much of the caudal part of the skeleton weathered out and was lost to erosion before the cranial part of the skeleton emerged.

Posterior thoracics resemble lumbars in size and proportions, but on close examination thoracics can be identified by the presence of facets for rib articulations. Lumbar, sacral, and anterior caudal vertebrae can only be distinguished and identified reliably to position when they are found in sequence, which is not the case here. Thus our count of 28 ‘lumbar’ vertebrae for WH 10001 no doubt includes a number of posterior thoracics, sacrals, and anterior caudals. It was not possible to identify all of the vertebrae of WH 10001 reliably in situ in the field. However, we can say that the WH 10001 Basilosaurus is an adult skeleton based on co-ossification of most vertebral epiphyses and based on tooth wear of the permanent teeth (see below).

The longest Basilosaurus isis vertebrae in the WH 10001 excavation are lumbars that are 350–360 mm in length. For comparison, the longest lumbar vertebrae in the CGM 42195 adult skeleton of B. isis are 330 mm long, and both lumbar length and femur length indicate that the CGM 42195 skeleton is female. The longest vertebrae in the UM 93231 adult skeleton of B. isis are 355 mm in length, and both lumbar length and femur length indicate that this skeleton is male. We infer from lumbar length that the WH 10001 adult skeleton of B. isis is also male.

Taphonomy of Basilosaurus isis. One key to understanding the taphonomy of vertebrates is to evaluate the quantity and quality of the specimens found [34]. Much information can be deduced from the completeness, disarticulation, degree of scattering and relative positions, and from modifications of the skeletal elements [35]. Minimum convex polygons (not shown) were constructed to enclose elements representing different parts of the B. isis skeleton. These show that the skeleton is relatively complete, but also completely disarticulated. We assessed the completeness of WH 10001, following [29], by calculating the proportion of bones of the skeleton represented at the site. Our excavation was carried out at the size scale of the skull, vertebrae, ribs, and long bones of the limbs. The sediment was well indurated, and a search for smaller hand and foot bones was beyond what we could attempt. Based on the skull, vertebrae, ribs, and long bones of the limbs, we estimate that the WH 10001 skeleton is 77/115 or 67% complete. Cranial bones and the dentaries of WH 10001 were concentrated in the north center of the excavation, but one broken nasal was found near the southern margin. Forelimb bones were found in the northwestern quadrant of the excavation, but one scapula was found in the southwestern quadrant. All four of the expected seven cervical vertebrae identified in WH 10001 were found in the northwestern quadrant of the excavation. Most thoracic vertebrae, ribs, and sternebrae, which together formed the thorax of WH 10001 in life, were found in the northern half of the excavation. However, there are a few thoracics and ribs scattered as far as the southern edge of the excavation, and there is one sternebra in the southwestern quadrant. Vertebrae identified as lumbars are limited to, and concentrated in, the southern half of the map. These are the largest vertebrae of the skeleton and were connected to each other in life by fibrocartilage of the successive intervertebral disks. Observation of many skeletons in the field indicates that vertebrae of the lumbar series of Basilosaurus are usually the last to separate when connective tissue decomposes, and their large size means they are less likely to be moved by ocean currents or by scavengers feeding on the decomposing body. The lumbars of WH 10001 are completely disarticulated, but they remain close to each other in a relatively small area. Bones of WH 10001 were clearly moved on the sea floor, but all were found in a restricted area. This is typical for a whale-fall assemblage [36], where a carcass falls slowly to the sea floor in relatively quiet water. Whale skeletons have a particular bauplan and taphonomic predisposition, which leads to a characteristic pattern and sequence of disarticulation of skeletal elements [37, 38]. Because of their length, Basilosaurus carcasses characteristically produce an s-shaped arrangement of vertebrae articulated on the sea floor, which may be modified, up to complete disarticulation, depending on exposure time, scavenging, and water currents [39]. WH-10001 shows that most bones are no longer located in anatomically correct position along the anterior-posterior-axis, and yet their former positions can be roughly traced. WH 10001 also shows that none of the bones remain in articulation. The combination of high completeness and great disarticulation suggests that WH 10001 fell to the sea floor as a complete carcass (possibly missing distal limbs, which may have been scavenged before the descent). The carcass was then destabilized by an accumulation of gases in the abdominal region, a breakdown of connective tissue, and disturbance by scavengers before burial. Burial was slow and took place in a sheltered environment maintaining close spatial proximity of the bones. This accords well with deposition of the carcass in a sediment-starved offshore environment at Wadi Al Hitan and with the interpretation of the enclosing greenish glauconitic sand as a condensed horizon [39]. We interpret the disarticulation and scatter of bones observed in WH 10001 as indicating disturbance by scavengers and movement by water currents during a relatively long time of exposure on the sea floor. Extant cetaceans have bones that are less dense than those of land mammals [40] but Eocene archaeocetes have bones retaining the density of land mammals [41, 42]. Decay, microbial activity, and bone diagenesis can also affect density [36, 43]. Higher density decreases and lower density increases the chance of bones being moved by currents on the sea floor. Concentrations of elements representing different parts of the skeleton are evidence of the original resting place for each part. The cranium and dentaries probably came to rest in what is now the north center of the excavation, cervicals are concentrated to the north and west of the cranium, thoracics, ribs, sternebrae, and forelimb elements are concentrated west of center, and lumbars are concentrated south and east of center. We envision a skeleton that came to rest with the head near the center, and the cervix, thorax and forelimbs, lumbus, and tail arrayed in a counter-clockwise spiral relative to the head, a spiral that expanded away from the head. We attribute the scatter of elements relative to their original resting place to scavengers, probably principally sharks both small and large (see below). However, the exceptions are also interesting. One cranial element, a broken nasal, was found near the southern margin of the excavation. Several smaller thoracics, some large and small ribs, one sternebra, and the right scapula were found south and east of most of their counterparts. We postulate a bottom current flowing from northwest to southeast to explain these unidirectional displacements. The duration of exposure of the WH 10001 skeleton on the sea floor is difficult to estimate. As indicated above, disarticulation, movement of bones, and deposition on a glauconitic sand favor long exposure, but the paucity of epibionts (oysters, barnacles, bryozoans, etc.) suggests more limited exposure.