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Tuesday, June 6, 2017

A new Eocene archaeocete (Mammalia, Cetacea) from India and the time of origin of whales


Himalayacetus subathuensis is a new pakicetid archaeocete from the Subathu Formation of northern India. The type dentary has a small mandibular canal indicating a lack of auditory specializations seen in more advanced cetaceans, and it has Pakicetus-like molar teeth suggesting that it fed on fish. Himalayacetus is significant because it is the oldest archaeocete known and because it was found in marine strata associated with a marine fauna. Himalayacetus extends the fossil record of whales about 3.5 million years back in geological time, to the middle part of the early Eocene [≈53.5 million years ago (Ma)]. Oxygen in the tooth-enamel phosphate has an isotopic composition intermediate between values reported for freshwater and marine archaeocetes, indicating that Himalayacetus probably spent some time in both environments. When the temporal range of Archaeoceti is calibrated radiometrically, comparison of likelihoods constrains the time of origin of Archaeoceti and hence Cetacea to about 54–55 Ma (beginning of the Eocene), whereas their divergence from extant Artiodactyla may have been as early as 64–65 Ma (beginning of the Cenozoic).
Pakicetus and contemporary archaeocetes have long been the oldest whales known as fossils (13). All are from red beds of the lower Kuldana Formation in Pakistan and the upper Subathu Formation in India, which are intercalated in a thicker sequence of Eocene marine sediments. All were deposited in a shallow epicontinental remnant of the Tethys Sea (Neotethys) that formerly separated the Indian subcontinent from the rest of Asia. These red beds yield essentially the same Kuldana and Kalakot freshwater fish and continental vertebrate fauna of early-middle Eocene age (45). When first described, Pakicetus was interpreted as an amphibious initial stage of whale evolution that rested and reproduced on land and entered Tethys opportunistically to feed on fish (1).
We report here a new pakicetid archaeocete from marine strata of the middle Subathu Formation of India. The new pakicetid was found about 100 m lower stratigraphically and 3.5 million years older geologically than the Kuldana–Kalakot-equivalent upper Subathu red bed interval producing Pakicetus elsewhere. This not only extends the fossil record of Cetacea back in time, but also reinforces the idea that whales originated on the margin of Tethys and corroborates interpretation of pakicetids as an initial amphibious stage of cetacean evolution entering Tethys to feed on fish.


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