From Land to Water: the Origin of Whales, Dolphins, and Porpoises

Abstract

Cetaceans (whales, dolphins, and porpoises) are an order of mammals that originated about 50 million years ago in the Eocene epoch. Even though all modern cetaceans are obligate aquatic mammals, early cetaceans were amphibious, and their ancestors were terrestrial artiodactyls, similar to small deer. The transition from land to water is documented by a series of intermediate fossils, many of which are known from India and Pakistan. We review raoellid artiodactyls, as well as the earliest families of cetaceans: pakicetids, ambulocetids, remingtonocetids, protocetids, and basilosaurids. We focus on the evolution of cetacean organ systems, as these document the transition from land to water in detail.

Development of the Skull of the Pantropical Spotted Dolphin (Stenella attenuata)

“…This morphology is present in the earliest whales, back to pakicetids, and is characteristic of cetacean ears (Nummela et al, 2007;Thewissen et al, 2009). The small tympanic ring is a U-shaped ridge of bone located on the thin lateral wall of the tympanic bone.…”

“…This is directly and functionally related to the environment in which these animals live (Miller, 1923;Howell, 1930;Klima, 1995;Rommel et al, 2009). The external bony nares moves its position from the tip of the rostrum to the top of the forehead (Klima, 1995;Thewissen et al, 2009). The dorsal movement of the external bony nares appears gradually in evolution in protocetids and basilosaurids and throughout modern whales (Thewissen et al, 2009).…”

“…The external bony nares moves its position from the tip of the rostrum to the top of the forehead (Klima, 1995;Thewissen et al, 2009). The dorsal movement of the external bony nares appears gradually in evolution in protocetids and basilosaurids and throughout modern whales (Thewissen et al, 2009). As the premaxilla and maxilla extend dorsally, the external bony nares is pushed dorsally to the top of the cranium leaving a thin sliver of frontal bone exposed in adult odontocetes (Miller, 1923).…”

“…Lancaster (1990) made theoretical predictions of the position of the middle ear ossicles in transitional cetacean ears based on the fossil record. Thewissen and Hussain (1993), Thewissen et al (2009), and Nummela et al (2004 documented transitional morphologies in fossils such as pakicetids, remingtonocetids, protocetids, and basilosaurids.…”

Correcting some common misrepresentations of evolution in textbooks and the media

“…The reconciliation came with the paleontological discovery that hippos seem to have evolved from a more basal group of mammals called anthracotheres, which are found as far back as the Eocene, and could be closely related to whales (Figure 6; Thewissen et al 2009). These anthracotheres were not aquatic or especially large, and they did not resemble whales at all.…”