Fig 1.
Skeletons of Basilosaurus isis (A; CGM 42195) and Dorudon atrox (B; CGM 42183 and UM 97512, 100146, 101215, 101222) from Wadi Al Hitan, Egypt, as exhibited at the University of Michigan.
Both are adult, fully grown, and illustrated at the same scale (scale bar equals 1 meter). CGM 42195 shows a cast of a 15 meter long B. isis specimen.
Fig 2.
Map showing the location of the Wadi Al Hitan UNESCO World Heritage Site in the Western Desert of Fayum Province, Egypt.
Fig 3.
Photomosaic of Basilosaurus isis WH 10001 from the Gehannam Formation of Wadi Al Hitan.
B. isis skeletons in the overlying Birket Qarun Formation are often partially to fully articulated, but here in bones are disarticulated and scattered. Note that thoracic vertebrae, ribs, sternebrae, and forelimb elements predominate in the northwestern quadrant of the map, whereas lumbar vertebrae are concentrated in the southeastern quadrant. Most cranial elements (dark gray) are near the center, but B. isis teeth are distributed more widely (Fig 4). Disarticulation and scatter observed here suggest disturbance by scavengers and possibly long exposure on the sea floor.
Table 1.
Elements recovered from the WH 10001 Basilosaurus isis excavation in Wadi Al-Hitan, Egypt.
Fig 4.
Map of Basilosaurus isis WH 10001 with locations of 13 permanent teeth superimposed.
The eight B. isis teeth studied here, illustrated in Fig 5, are identified by the last three digits of their CGM museum number (CGM 60552, etc.). Outlines of B. isis cranial remains are filled with darker shading, and outlines of B. isis forelimb elements are filled with lighter shading.
Fig 5.
Permanent teeth of adult Basilosaurus isis WH 10001.
The position of each tooth in the WH 10001 B. isis excavation is shown in Fig 4, identified by the last three digits of its CGM number. A, CGM 60552, left M1 or M2. B, CGM 60570, anterior half of right P4 or posterior half of left P4. C, CGM 60568, right I1. D, CGM 60558, right M2. E, CGM 60554, posterior half of right P4. F, CGM 60579, left C1. G, CGM 60561, right I2. H, CGM 60565, right I1. Note heavy wear, especially on premolars, consistent with predation on large animals and relative maturity of the WH 10001 B. isis itself. Abbreviations: C1, first lower canine; I1, first upper incisor; I1-2, lower incisor 1–2; M2, lower molar 2; M1-2, upper molar 1–2; P4, lower premolar 4; P4, upper premolar 4.
Table 2.
Measurements of teeth recovered from the WH 10001 Basilosaurus isis excavation in Wadi Al-Hitan, Egypt.
Fig 6.
Map of Basilosaurus isis WH 10001 with locations of other vertebrate specimens superimposed.
Juvenile D. atrox specimens, identified by the last three digits of their CGM museum number (CGM 60553, etc.), are illustrated in Figs 7 and 8. Pycnodus mokattamensis (CGM 60564) and Carcharocles sokolowi (CGM 60557) specimens are illustrated in Fig 9. Note the wide scatter of D. atrox remains, similar to the scatter of B. isis permanent teeth shown in Fig 4. Outlines of B. isis cranial remains are filled with darker shading, and outlines of B. isis forelimb elements are filled with lighter shading.
Fig 7.
Cranial remains of juvenile Dorudon atrox.
The position of each element in the WH 10001 B. isis excavation is shown in Fig 6, identified by the last three digits of its CGM number. A, CGM 60567, right dentary with crowns of deciduous milk teeth dP3–4 in place and the crown of M1 beginning to erupt (specimen is in ontogenetic stage 2 of 13 tooth eruption and dental wear stages of Uhen [29]). B, UM 83902, right dentary with alveoli for anterior premolars, crowns of dP3–4 in place, roots of M1, and the crown of M2 beginning to erupt. Uhen [29] classified this specimen, from the overlying Birket Qarun Formation in Wadi Al-Hitan, in his ontogenetic stage 4). C, CGM 60562, supraorbital process of a right frontal. D, CGM 60569, supraorbital process of a second slightly smaller right frontal. E, CGM 60577, right squamosal. F, CGM 60574, left tympanic bulla. Note that the CGM 60567 dentary is missing the anterior portion and mandibular condyle, and all of the other cranial elements show evidence of breakage or disarticulation. Abbreviations: c, mandibular condyle; Dent, dentary; dP3-dP4, lower deciduous premolar 3–4; Fr, frontal; M1-2, lower molar 1–2; pgp, postglenoid process; pop, postorbital process; ppt, posterior process of tympanic; punct, puncture bite mark on frontal; sp, sigmoid process; Sq, squamosal; Ty, tympanic; zp, zygomatic process.
Fig 8.
Postcranial remains of juvenile Dorudon atrox.
The position of each element in the WH 10001 B. isis excavation is shown in Fig 6, identified by the last three digits of its CGM number. A, CGM 60553, centrum of a posterior thoracic vertebra. B, CGM 60578, centrum of a posterior thoracic vertebra. C, CGM 60559, posterior thoracic vertebra. D, CGM 60560, posterior thoracic vertebra. E, CGM 60566, possible centrum of a thoracic vertebra. F, CGM 60556, vertebral epiphysis. G, CGM 60571, left anterior rib. H, CGM 60576, midshaft of a rib of unknown position. Note that all vertebral centra all lack epiphyses. Abbreviations: cap, rib capitulum; nc, neural canal; prz, prezygapophysis; tp, transverse process; tub, rib tuberculum.
Table 3.
Measurements of Dorudon atrox vertebral centra recovered from the WH 10001 Basilosaurus isis excavation in Wadi Al-Hitan, Egypt.
Fig 9.
Remains of large fishes associated with WH 10001 Basilosaurus isis.
A, left prearticular and crushing pavement-toothed lower dentition of the mollusk-eating bony fish Pycnodus mokattamensis. B, tooth of the large predatory lamniform shark Carcharocles sokolowi attached to a rib of B. isis. Abbreviations: l, lateral; s, symphyseal.
Fig 10.
Skull widths as a size indicator for cetacean apex predators in relation to the size of contemporary baleen whales.
For comparisons among whales in terms of their ability to feed on large prey, skull widths are shown with red diamonds for the late Eocene apex predator Basilosaurus isis (CGM 42195); late Miocene apex predators Livyatan melvillei, Zygophyseter varolai, Acrophyseter deinodon, and A. robustus (top down) [67, 69]; and extant apex predator Orcinus orca [70]. These are compared to the increasing limit (dashed line) for mysticete baleen whale sizes (blue circles) through Cenozoic time reported by [72]. Note that apex predators B. isis in the late Eocene and the physeteroid L. melvillei in the late Miocene are larger than contemporary mysticetes, but some late Miocene macroraptorial sperm whales (Z. varolai, A. deinodon, and A. robustus) and extant O. orca are smaller than most contemporary mysticetes. Large size is a common characteristic of apex predators, but large size is neither necessary nor sufficient: the largest mysticetes today are sometimes attacked by O. orca hunting in groups. Note large gaps in our understanding of the history of cetacean apex predators.