Evolution of cetaceans

Molecular and morphological analyses suggest Cetacea share a relatively recent closest common ancestor with hippopotami and that they are sister groups.

[3] Being mammals, they surface to breathe air; they have five finger bones (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retain many skeletal features from their terrestrial ancestors.

The earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, and today's artiodactyls and perissodactyls became herbivores later in their evolution.

Whales, however, retained their carnivorous diet because prey was more available and they needed higher caloric content in order to live as marine endotherms (warm-blooded).

[7] Approximately the size of a raccoon or domestic cat, this omnivorous creature shared some traits of modern whales, most notably the involucrum, a bone growth pattern which is the diagnostic characteristic of any cetacean; this is not found in any other species.

[17] After the initial discovery, more fossils were found, mainly in the early Eocene fluvial deposits in northern Pakistan and northwestern India.

Based on this discovery, pakicetids most likely lived in an arid environment with ephemeral streams and moderately developed floodplains millions of years ago.

[1] By using stable oxygen isotopes analysis, they were shown to drink fresh water, implying that they lived around freshwater bodies.

The jawbone of pakicetids also lacks the enlarged space (mandibular foramen) that is filled with fat or oil, which is used in receiving underwater sound in modern cetaceans.

[1] To compensate for that, their bones are unusually thick (osteosclerotic), which is probably an adaptation to make the animal heavier to counteract the buoyancy of the water.

[18] A recent study suggests that ambulocetids were fully aquatic like modern cetaceans, possessing a similar thoracic morphology and being unable to support their weight on land.

According to a 2002 study done by Spoor et al., this modification of the semicircular canal system may represent a crucial 'point of no return' event in early cetacean evolution, which excluded a prolonged semi-aquatic phase.

A 2001 study done by Gingerich et al. hypothesized that Rodhocetus locomoted in the oceanic environment similarly to how ambulocetids pelvic paddling, which was supplemented by caudal undulation.

[33] Basilosaurids and dorudontines lived together in the late Eocene around 41 to 33.9 million years ago, and are the oldest known obligate aquatic cetaceans.

Their orbits faced laterally, and the nasal opening had moved even higher up the snout, closer to the position of the blowhole in modern cetaceans.

However, according to a 1994 study done by Fordyce and Barnes, the large size and elongated vertebral body of basilosaurids preclude them from being ancestral to extant forms.

Filter feeding is very beneficial as it allows baleen whales to efficiently gain huge energy resources, which makes the large body size in modern varieties possible.

[40] It is hypothesized that these mutations occurred in cetaceans already possessing preliminary baleen structures, leading to the pseudogenization of a "genetic toolkit" for enamel production.

[43] Mysticetes are also known for their gigantism, as baleen whales are among the largest organisms to ever have lived; they reach lengths greater than 20 m and weigh more than 100,000 kg.

These anatomical differences suggest that these ancient species may not have necessarily been deep-sea squid hunters like the modern sperm whale, but that some genera mainly ate fish.

After studying numerous fossil skulls, researchers discovered the absence of functional maxillary teeth in all South African ziphiids, which is evidence that suction feeding had already developed in several beaked whale lineages during the Miocene.

[60] Modern cetaceans have internal, rudimentary hind limbs, such as reduced femurs, fibulas, and tibias, and a pelvic girdle.

Their transition from land to water led to reshaping of the skull and food processing equipment because the eating habits were changing.

[64][65] Occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs (atavism).

[73] The timing of this second radiation event is not coincidental, as the following diversification of cetaceans was likely due to new ecological opportunities the change in oceans gave them.

Due to these differences, fitness levels change within the dolphins of a population, which further causes evolution to occur in the long run.

Spongers put sea sponges on their snouts as protection against abrasions from sharp objects, stingray barbs, or toxic organisms.

However, when comparing data from within the West Gulf, the spongers vs. the nonspongers in the deep channels had very different fatty acid results even though they are in the same habitat.

[78] Social structure forms groups with individuals that interact with one another, and this allows for cultural traits to emerge, exchange, and evolve.

[88] In the Northeast Atlantic, specifically, genetic evidence suggests that the bottlenose dolphins have differentiated into coastal and pelagic types.