New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism

Abstract

BackgroundProtocetidae 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 FindingsTwo 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/SignificanceDiscovery 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.

New Middle Eocene Whales from the Pisco Basin of Peru

“…Dorudon is a composite of UM 101215 and UM 101222 (Uhen, 2004). Maiacetus is from specimen GSP-UM 3551 (Gingerich et al, 2009). ,106.9 L11 -FIGURE 6-MUSM 1465, material belonging to Supayacetus muizoni, n. gen. n. sp.…”

“…In addition, the recently described Maiacetus was also added as a new taxon to the matrix. Character codings for the new Peruvian taxa and Maiacetus Gingerich et al 2009 are listed in Supplemental Table 4 (online archives). In the second stage of the analysis, a stratigraphic character was added to the analysis and overall most parsimonious trees were explored in MacClade (Maddison and Maddison, 2000).…”

The relationship of head movements to semicircular canal size in cetaceans

“…One possible explanation is that the head motions of the ancestors of modern cetaceans might have been increased and their canal sizes decreased during their early evolution, with the small size of the canals remaining phylogenetically constrained despite the animals returning later to less agile behaviors. Given the disadvantage of small canals (Spoor et al, 2007) and the finding that a broad tail evolved with stabilizing flippers (Gingerich et al, 2009), this seems unlikely. An alternative proposed explanation is that the cetacean vestibular system has become profoundly hypofunctional to help prevent motion sickness (Ketten, 2000).…”

Case of Cephalic Presentation of Foetus in a Harbour Porpoise Phocoena Phocoena (Cetacea, Phocoenidae), with Notes on Other Aquatic Mammals

“…Suggested cephalic presentation in a member of Protocetidae, an Eocene family of Archeoceti, became the point for a recent discussion (Gingerich et al, 2009;Thewissen and McLellan, 2009): female remnants of Maiacetus inuus (the species described by Gingerich et al, 2009) contained the remnants of a foetus or newborn of the same species. If it was a near-term foetus, the authors concluded, it was an evidence for giving birth on land rather than in water.…”