Fossils explained 46: Ancient toothed whales

Archaeocetes (‘ancient whales’) were amphibious and aquatic mammals that inhabited the estuaries, seas and oceans of the Eocene. Evolving from primitive hoofed mammals, most probably during the Palaeocene, they developed a variety of body shapes and feeding styles, and gave rise to the toothed and baleen whales of today. While the most primitive archaeocetes lacked aquatic specializations and probably only foraged at the water’s edge, later types became committed to an aquatic life. By the end of the Eocene, archaeocetes were fully marine and sometimes enormous in size. Archaeocete diversity is often understated and only in the past two decades have palaeontologists come to appreciate the variation exhibited by this group. New details provided by new types of archaeocete fossils have helped to piece together the complex story of cetacean (i.e. whale) origins and early evolution. The oldest known archaeocetes – the pakicetids Pakicetus, Ichthyolestes and Nalacetus – are from the early Eocene. Himalayacetus, a small Indian form based on a supposedly early Eocene lower jaw, was described in 1998 as the oldest known archaeocete. However, this has been contested and Himalayacetus may be actually middle Eocene in age. All early archaeocetes are unique to India and Pakistan, suggesting southern Asia as a centre of origin for the group. A supposed early Eocene archaeocete scapula from the English London Clay, named Anglocetus beatsoni Tarlo, was later shown to be from a turtle. By the late middle Eocene, seal-like archaeocetes had spread to Africa and North America. This distribution indicates that these forms were now able to disperse from the shallow marine shelf regions inhabited by earlier types and could cross oceans, an idea supported by their morphology and perhaps also by Darren Naish School of Earth & Environmental Sciences, University of Portsmouth, UK