Figure 1.
Skeletons of the Eocene archaeocete whales Dorudon atrox and Maiacetus inuus in swimming pose.
(A, B)– Dorudon atrox (5.0 m; 36.5 Ma) based on UM 101222 and 101215 [11] in lateral and dorsal views, respectively. (C, D)– Maiacetus inuus (2.6 m; 47.5 Ma) based on male specimen GSP-UM 3551 in lateral and dorsal views, respectively.
Figure 2.
Adult female and fetal skeletons (type) of the protocetid Maiacetus inuus.
Skull of the adult female (GSP-UM 3475a) is colored beige with brown teeth; her postcranial skeleton is colored red; the fetal skeleton (GSP-UM 3475b) is colored blue with red teeth. Blue dashed lines indicate the contours of the three field jackets and the red dashed line marks the edge of erosion.
Figure 3.
Map showing localities of some Eocene whales in eastern Balochistan (Pakistan).
Map showing the Kunvit area in the southwestern part of Lakha Kach syncline, northwest of the town of Rakhni. Red diamonds mark type localities for Artiocetus clavis (GSP-UM 3458), Rodhocetus balochistanensis (GSP-UM 3485) [3], Protosiren eothene (GSP-UM 3487) [49], Makaracetus bidens (GSP-UM 3570; from younger beds of the overlying Domanda Formation) [2], and Maiacetus inuus (GSP-UM 3475a, b, female, fetal skeleton). Localities are in the upper part of the Habib Rahi Formation of early Lutetian age [12]. The red circle marks the locality of the referred specimen of Maiacetus inuus (GSP 3551, male), yellow circles show localities of other archaeocete specimens, and white circles show localities of other vertebrate specimens.
Figure 4.
Temporal constraints on the age of Maiacetus inuus.
Red rectangle highlights the platy limestones and marls of the upper part of the marine Habib Rahi Formation. Nannoplankton Sphenolithus spininger and Helicosphaera bramlettei indicative of NP zones 14–15 have been reported from the Habib Rahi Formation [13]. The age of Maiacetus inuus is about 47.5 Ma. Figure from [14].
Figure 5.
Skull of Maiacetus inuus (GSP-UM 3475a; female).
Skull in ventral view (A) with interpretive shading and labels (B). Shaded components include skull bones (grey), teeth (brown), hyoids (orange), and postcranial bones (red). Osteological abbreviations: Ba., basihyal; Basicr., basicranium; C, cervical vertebra; Dent., dentary; Exoc., exoccipital; Jug., jugal; L, left; Max., maxilla; R, right, rib; Scap., scapula; Squ., squamosal; St., stylohyal; Stern., sternebrum; T, thoracic vertebra; Th., thyrohyal; Tym., tympanic.
Figure 6.
Skull of Maiacetus inuus (GSP-UM 3475a; female).
Skull in dorsal view (A) with interpretive shading and labels (B). Shaded components include skull bones (grey), teeth (brown), and postcranial bones (red). Osteological abbreviations: Dent., dentary; Front., frontal; Jug., jugal; L, left; Lac., lacrimal; Max., maxilla; Nas., nasal; Par., parietal; Pmax., premaxilla; R, right, rib; Scap., scapula; Squ., squamosal.
Table 1.
Skeletal measurements (cm) of Maiacetus inuus compared to those of the previously known protocetids Artiocetus clavis and Rodhocetus balochistanensis, which are also from the early middle Eocene of Pakistan [3].
Table 2.
Measurements (mm) of teeth in female and male specimens of Maiacetus inuus.
Figure 7.
Left radius, ulna, carpus and manus of Maiacetus inuus (GSP-UM 3475a, female).
Open areas are reconstructed. Abbreviations: C, cuneiform; L, lunar; M, magnum; P, pisiform; S, scaphoid; Tr, trapezium; Trd, trapezoid; U, unciform.
Figure 8.
Fetal skull of Maiacetus inuus (GSP-UM 3475b).
(A)–Photograph showing bones (shaded white) and teeth (shaded brown) in lateral view. (B)–CT image overlain on tooth-bearing part of fetal skull in lateral veiw. Abbreviations: left frontal (L. Front.), left jugal (L Jug.), occipital (Occip.) with left and right condyles flanking the foramen magnum, and left and right dentaries (L dent., R dent.). Teeth are shaded brown, with darker brown representing enamel, and lighter brown representing roots and exposed dentine. White and black labels identify teeth of the left and right sides, respectively. Dotted lines trace outlines of fully formed crowns of left dP3 and dP4. These crowns are visible on the surface (A) where thin bone of the maxilla is pressed over more rigid underlying crowns, and as denser masses in the CT scan (B). Remaining teeth are identified by size and position relative to dP3 and dP4. Note presence of the developing crown of permanent left M1 posterodorsal to the crown of left dP4 (dorsal to the left jugal and posterior to the left frontal). Partial crown of right M1 (unlabeled) is visible just below and posterior to left M1.
Figure 9.
Skeleton of Maiacetus inuus (GSP-UM 3551, male).
Skeleton as preserved in left lateral view. Gray shading is marl matrix (see Figure 1C, D for skeletal restoration).
Table 3.
Vertebral centrum measurements (mm) for adult male Maiacetus inuus (GSP-UM 3551; Figure 9).
Figure 10.
Pelvic morphology in the extant sexually-dimorphic fallow deer Dama dama compared to that of male Maiacetus inuus.
Pelves are viewed ventrally. Sexes differ in the shape of the notch separating inferior rami of left and right pubes. (A)– Female Dama dama has the more U-shaped pubic notch labeled in red. (B)– Male Dama dama has the more V-shaped pubic notch labeled in red. (C)– Male Maiacetus inuus, GSP-UM 3551, has the V-shaped pubic notch labeled in red. Drawings of Dama dama are from [29]. Part of the right pubic ramus of GSP-UM 3551 is restored from the left side.
Figure 11.
Comparison of right ankle bones of Artiocetus, Maiacetus, and Rodhocetus.
(A)–Astragalus and cuboid of Artiocetus clavis (GSP-UM 3458, type, reversed). (B)–Ankle of Maiacetus inuus (GSP-UM 3551). (C)–Ankle of Rodhocetus balochistanensis (GSP-UM 3485, type). Specimens are drawn to approximately the same astragalus+cuboid length, and viewed dorsally. Blue arrow points to distinctive indentation in lateral margin of astragalus of M. inuus; red arrows point to the broad and deep, narrow and intermediate, and narrow and shallow indentations in the calcaneum and lateral margins of cuboids. Abbreviations: Ast., astragalus; Cal., calcaneum; Cub., cuboid; Ecc., ectocuneiform; Enc., entocuneiform; Mec., mesocuneiform; Nav., navicular.
Figure 12.
Proportion-adjusted skeletal profiles of middle and late Eocene archaeocete whales Maiacetus inuus and Dorudon atrox, respectively.
(A)—Maiacetus inuus, a semiaquatic foot-powered swimmer from the Middle Eocene. (B)—Dorudon atrox, a fully-aquatic tail-powered swimmer from the Late Eocene. Baseline is mean length of anterior thoracic vertebrae (stippled box). Sacral vertebrae are enclosed in a second stippled box where sacrals can be identified (e.g., by co-ossification). Maiacetus has a profile more like that of mammals capable of supporting their weight on land, whereas Dorudon has the profile of a modern whale. Interpretation of profiles and the method of median serial-multiple-regression estimation of body weights is explained in [50]. Abbreviations: TCNDSFB, longest tooth length, condylobasal cranium length, narial position, dentary length, symphysis position, mandibular foramen height; and bulla length, respectively; SHRC, scapula, humerus, radius, and Mc-III lengths, respectively; IFTT, innominate, femur, tibia, and Mt-III lengths, respectively.
Figure 13.
Principal components plot of trunk and limb skeletal proportions for Maiacetus inuus and a representative sample of 50 extant semiaquatic mammals.
Maiacetus inuus is similar to Rodhocetus species, but lacks elongation of the manus, hind limb, and pes characteristic of the latter. Maiacetus is about equally aquatic (PC-II) to Rodhocetus but less specialized as a hind-limb swimmer (PC-III). It falls closest to the giant otter (Pteronura brasiliensis) among extant mammals. Background and comparative data for this analysis are documented and explained in [4].
Figure 14.
Head versus tail presentation of near- and full-term calves in a domestic cow and harbor porpoise.
Domestic cow (Bos taurus) calf (blue) in (A) ninth and final month of gestation before ‘turning’ (axially rotating), (B) turned as the birth process begins, and (C) with forelimbs and head partially extruded in the initial stage of birth (from [51], [52]). Harbor porpoise (Phocaena phocaena) (D) with full-term calf (blue) with tail partially extruded in the initial stage of birth (from [18]).
Table 4.
Developmental maturity categories of Langer [21] and mineralization of first molar M1 and other permanent teeth in newborn terrestrial mammals compared to the early whale Maiacetus inuus.
Figure 15.
Mating systems and sexual dimorphism in marine mammals.
Sexual dimorphism of males and females is quantified for 105 species of cetaceans, pinnipeds, sirenians, and the sea otter, expressed as male-minus-female length in natural log units [17], [30]. Territorial and harem systems that involve intense male-male competition and spatial aggregation of females have male-female dimorphism greater than about 0.15 (dashed line) [31]. Maiacetus inuus falls in the group of ‘dispersed’ mating systems with limited aggregation and limited male control of mating. Both cetaceans and pinnipeds span the entire range shown here, although mating systems are known for relatively few species [32].