Cetaceans: The Evolution of Whales

The term "cetaceans" (from the Greek ketos, whale) refers to the most diverse group of marine mammals. Currently, two large groups of cetaceans are recognized: the Mysticetes or cetaceans with baleen (the true whales), and the Odontocetes or cetaceans with teeth (dolphins, porpoises, the sperm whale). There are 11 species of baleen whales known, while odontocetes are more numerous, as there are about 76 species.

Fossil evidence indicates that cetaceans originated more than 50 million years ago during the Eocene. Many of the most primitive species became extinct during the evolutionary process without reaching the present day. These species are the "Archaeocetes", which paleontologists named with exotic names such as Pakicetus, Basilosaurus and Ambulocetus.

Toothed and barbed cetaceans diverged from a common archaeocete ancestor about 35 million years ago. The fossilized remains of those ancient whales show that they were originally land animals, which gradually evolved into an aquatic existence.

The terrestrial mammals evolutionarily most closely related to whales are the current ungulates, that is, animals as familiar to us as the horses, deer, pigs and hippos. The detail of the evolutionary relationships between these animal groups, as well as within the cetacean group itself, is still a source of scientific debate.

The adaptation of cetaceans

Certain anatomical characteristics of cetaceans are excellent examples of the complete adaptation of these animals to aquatic life. For example, its body receives thermal insulation thanks to a thick layer of fat under the smooth and delicate skin that promotes hydrodynamics, lacking the typical mammalian fur to protect itself from the cold.

The hind limbs have completely disappeared from the external anatomy of current species, although there are vestiges of the bones of the pelvic girdle embedded in the body musculature and without connection to the spinal column. This reduction of the hind legs and waist was a gradual evolutionary process.

For example, Ambulocetus had large hind legs, while basilosaurus It had small hind limbs that were perhaps used to move in shallow water or also to facilitate copulation.

The main organ of locomotion of cetaceans is the caudal fin, which lacks a skeleton, maintaining its rigid shape thanks to the dense fibrous tissues that compose it. Unlike fish, which move their tail laterally to move through the water, whales and dolphins move through vertical movements of their tail fin. Control of the direction of movement as well as maintenance of stability is achieved thanks to the dorsal fin (absent in some cetaceans such as right whales) and through movements of the pectoral fins.

The pectoral fins do have bone structure connected to the vertebral column, and show a unique evolutionary characteristic among vertebrates. The "fingers" (embedded within the fins and not visible) exhibit "hyperphalangeal", that is, a greater number of phalanges (the little bones of the fingers) than usual, reaching up to 15 in the whale's second finger. pilot.

These and other anatomical characteristics that evolved in parallel with behavioral and physiological attributes unique to cetaceans, make whales and dolphins the group of mammals best adapted to continuous life in the sea. Their conquest of the ocean as a permanent habitat is a wonderful example of the evolutionary capacity that living beings can reveal when time is not a limiting factor for their potential adaptive changes.

Source consulted:
"Marine mammals: evolutionary biology".
A. Berta and JL Sumich. Academic Press. 1999.

*Mariano Sironi
Researcher
Whale Conservation Institute