doi: 10.1371/journal.pone.0004366. Epub 2009 Feb 4.
New protocetid whale from the middle eocene of pakistan: birth on land, precocial development, and sexual dimorphism
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- PMID:19194487
- PMCID: PMC2629576
- DOI: 10.1371/journal.pone.0004366
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New protocetid whale from the middle eocene of pakistan: birth on land, precocial development, and sexual dimorphism
Philip D Gingerich et al. PLoS One.2009.
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Abstract
Background: Protocetidae are middle Eocene (49-37 Ma) archaeocete predators ancestral to later whales. They are found in marine sedimentary rocks, but retain four legs and were not yet fully aquatic. Protocetids have been interpreted as amphibious, feeding in the sea but returning to land to rest.
Methodology/principal findings: Two adult skeletons of a new 2.6 meter long protocetid, Maiacetus inuus, are described from the early middle Eocene Habib Rahi Formation of Pakistan. M. inuus differs from contemporary archaic whales in having a fused mandibular symphysis, distinctive astragalus bones in the ankle, and a less hind-limb dominated postcranial skeleton. One adult skeleton is female and bears the skull and partial skeleton of a single large near-term fetus. The fetal skeleton is positioned for head-first delivery, which typifies land mammals but not extant whales, evidence that birth took place on land. The fetal skeleton has permanent first molars well mineralized, which indicates precocial development at birth. Precocial development, with attendant size and mobility, were as critical for survival of a neonate at the land-sea interface in the Eocene as they are today. The second adult skeleton is the most complete known for a protocetid. The vertebral column, preserved in articulation, has 7 cervicals, 13 thoracics, 6 lumbars, 4 sacrals, and 21 caudals. All four limbs are preserved with hands and feet. This adult is 12% larger in linear dimensions than the female skeleton, on average, has canine teeth that are 20% larger, and is interpreted as male. Moderate sexual dimorphism indicates limited male-male competition during breeding, which in turn suggests little aggregation of food or shelter in the environment inhabited by protocetids.
Conclusions/significance: Discovery of a near-term fetus positioned for head-first delivery provides important evidence that early protocetid whales gave birth on land. This is consistent with skeletal morphology enabling Maiacetus to support its weight on land and corroborates previous ideas that protocetids were amphibious. Specimens this complete are virtual 'Rosetta stones' providing insight into functional capabilities and life history of extinct animals that cannot be gained any other way.
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Figures
(A, B)–Dorudon atrox (5.0 m; 36.5 Ma) based on UM 101222 and 101215 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.
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.
Map showing the Kunvit area in the southwestern part of Lakha Kach syncline, northwest of the town of Rakhni. Red diamonds mark type localities forArtiocetus clavis (GSP-UM 3458),Rodhocetus balochistanensis (GSP-UM 3485) ,Protosiren eothene (GSP-UM 3487) ,Makaracetus bidens (GSP-UM 3570; from younger beds of the overlying Domanda Formation) , andMaiacetus inuus (GSP-UM 3475a, b, female, fetal skeleton). Localities are in the upper part of the Habib Rahi Formation of early Lutetian age . The red circle marks the locality of the referred specimen ofMaiacetus inuus (GSP 3551, male), yellow circles show localities of other archaeocete specimens, and white circles show localities of other vertebrate specimens.
Red rectangle highlights the platy limestones and marls of the upper part of the marine Habib Rahi Formation. NannoplanktonSphenolithus spininger andHelicosphaera bramlettei indicative of NP zones 14–15 have been reported from the Habib Rahi Formation . The age ofMaiacetus inuus is about 47.5 Ma. Figure from .
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.
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.
Open areas are reconstructed.Abbreviations:C, cuneiform;L, lunar;M, magnum;P, pisiform;S, scaphoid;Tr, trapezium;Trd, trapezoid;U, unciform.
(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.
Skeleton as preserved in left lateral view. Gray shading is marl matrix (see Figure 1C, D for skeletal restoration).
Pelves are viewed ventrally. Sexes differ in the shape of the notch separating inferior rami of left and right pubes. (A)– FemaleDama dama has the more U-shaped pubic notch labeled in red. (B)– MaleDama dama has the more V-shaped pubic notch labeled in red. (C)– MaleMaiacetus inuus, GSP-UM 3551, has the V-shaped pubic notch labeled in red. Drawings ofDama dama are from . Part of the right pubic ramus of GSP-UM 3551 is restored from the left side.
(A)–Astragalus and cuboid ofArtiocetus clavis (GSP-UM 3458, type, reversed). (B)–Ankle ofMaiacetus inuus (GSP-UM 3551). (C)–Ankle ofRodhocetus 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 ofM. 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.
(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, whereasDorudon 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 .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.
Maiacetus inuus is similar toRodhocetus species, but lacks elongation of the manus, hind limb, and pes characteristic of the latter.Maiacetus is about equally aquatic (PC-II) toRodhocetus 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 .
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]).
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 , . 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) .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 .
References
- Kellogg R. A review of the Archaeoceti. Carnegie Institution of Washington Publications. 1936;482:1–366.
- Gingerich PD, Zalmout IS, Haq M-u, Bhatti MA. Makaracetus bidens, a new protocetid archaeocete (Mammalia, Cetacea) from the early middle Eocene of Balochistan (Pakistan). Contributions from the Museum of Paleontology, University of Michigan. 2005;31:197–210.
- Gingerich PD, Haq M-u, Zalmout IS, Khan IH, Malkani MS. Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan. Science. 2001;293:2239–2242. - PubMed
- Gingerich PD. Land-to-sea transition of early whales: evolution of Eocene Archaeoceti (Cetacea) in relation to skeletal proportions and locomotion of living semiaquatic mammals. Paleobiology. 2003;29:429–454.
- Fordyce RE. Whale evolution and Oligocene southern ocean environments. Palaeogeography, Palaeoclimatology, Palaeoecology. 1980;31:319–336.
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