The archaeocetes

The term archaeocete groups together all fossil cetaceans that are neither odontocetes or toothed cetaceans (dolphins, killer whales, narwhals, sperm whales) nor mysticetes or baleen plated cetaceans (baleen whales). They are distinguished by numerous characteristics of their teeth, ears and post-cranial skeleton.

Archaeocetes have been found in North America, South America (Peru), Egypt, India and Pakistan. They display various degrees of adaptation to aquatic life, from terrestrial or semi-aquatic forms that use their limbs to swim, to forms that are totally confined to the aquatic environment and which use only their tail to propel themselves. Basilosaurids are the most derived archaeocetes and make up the group that produced modern cetaceans.

Cynthiacetus peruvianus
Martínez-Cáceres & Muizon, 2011
MNHN.F.PRU10
Mammalia, Cetacea, Archaeoceti, Basilosauridae
Late Eocene, 38 to 35 million years ago
Otuma Formation, Department of Ica, Peru

This skeleton of a young adult archaeocte is the first to be discovered in South America. 

Cynthiacetus is one of the largest known basilosaurids and the Cynthiacetus peruvianus species has the greatest number of thoracic vertebrae and ribs observed among cetaceans. Basilosaurids have hind limbs that are less specialised than those of modern whales and fore limbs that are reduced in size and functionless but which are nonetheless almost complete and visible on the exterior of the body. They are the first cetaceans to be completely tied to an aquatic environment. The oldest modern cetaceans, with baleen plates (mysticetes) and with teeth (odontocetes), appear respectively at the end of the Eocene (36 million years ago) and in the Lower Oligocene (30 million years ago).

The deformation of the rostrum (beak), with its tip veering to the left, can be observed in numerous Eocene archaeocetes. This asymmetry has been interpreted as a first step in the directional perception of ultrasound among cetaceans (a perception that subsequently disappeared among mysticetes). In this specimen, the natural torsion of the rostrum has been amplified by a post-mortem deformation that occurred during the fossilisation process.

Pakicetus attocki
Ginerich & Russel, 1990
MNHN.F.PAK2624
Mammalia, Cetacea, Archaeoceti, Pakicetidae
Early Eocene, 50 million years ago
Kuldana formation, Pakistan

Pakicetus is the oldest known cetacean. With hooves and long slender limbs, it was a running digitigrade animal. Related to the artiodactyls (bovines, ovines, antelopes, camels, pigs, giraffes, hippopotamus), it also had an astragal (an ankle bone that articulates with the tibia) with a double pulley that made it flexible and efficient for running.

It was also partly aquatic as evidenced by its thick ear bones, a characteristic of all cetaceans which gives them the capacity for directional hearing under water, and its upward pointing orbits which allowed it to see out of the water without totally exposing its head. 

It lived in an environment of rivers and estuaries at the beginning of the Cenozoic but was undoubtedly no more aquatic than terrestrial. Its powerful teeth made it a formidable predator feeding on fish, amphibians and crustaceans.

Cast of a composite and partly reconstructed skeleton.

Ambulocetus natans
Thewissen et al., 1996
MNHN.F.PAK2625
Mammalia, Cetacea, Archaeoceti, Ambulocetidae
Early Eocene, 48 million years ago
Kuldana Formation, Pakistan

Ambulocetus natans, or the “walking whale which swims”, is one of the first truly amphibious archaeocetes. It is the perfect intermediate species between the terrestrial mammals and the first walking cetaceans, such as Pakicetus, on the one hand and modern cetaceans that are totally confined to the aquatic environment on the other.

This large mammal, measuring 4 m in length, was a quadruped. Its well-developed hind legs and long feet made it an excellent swimmer. It was capable of moving about on dry land even though it was probably less at ease there. Like modern otters, it swam by vertically undulating its hind limbs, pelvis and tail.

Ambulocetus natans inhabited the rivers and estuaries of the Tethys. A formidable predator, it fed on fish and crustaceans in the water, but like modern crocodiles, it could also ambush any unlucky small terrestrial mammals that came to drink on the river bank.

Cast of a composite and partly reconstructed skeleton of the original holotype.