Evolution of cetaceans
- Their need to breathe air from the surface;
- The bones of their fins, which resemble the limbs of land mammals
- The vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish.
The question of how a group of land mammals became adapted to aquatic life was a mystery until discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.
- 1 Earliest ancestors
- 2 Indohyus
- 3 Pakicetidae
- 4 Ambulocetidae
- 5 Remingtonocetidae
- 6 Protocetidae
- 7 Basilosauridae and Dorudontinae
- 8 Early echolocation
- 9 Early baleen whales
- 10 Early dolphins
- 11 Skeletal evolution
- 12 See also
- 13 References
- 14 External links
The traditional theory of cetacean evolution was that whales were related to the mesonychids, an extinct order of carnivorous ungulates (hoofed animals), which resembled wolves with hooves and were a sister group of artiodactyls (even-toed ungulates). These animals had unusual triangular teeth similar to those of whales. This is why scientists long believed that whales evolved from a form of mesonychid. But more recent molecular phylogeny data suggest that whales are more closely related to the artiodactyls, specifically the hippopotamus. The strong evidence for a clade combining cetaceans and artiodactyls is further discussed in the article Cetartiodactyla. However, the anthracothere ancestors of hippos do not appear in the fossil record until millions of years after Pakicetus, the first known whale ancestor.
The molecular data is supported by the recent discovery of Pakicetus, the earliest proto-whale (see below). The skeletons of Pakicetus show that whales did not derive directly from mesonychids. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychids. Proto-whales retained aspects of their mesonychid ancestry (such as the triangular teeth) which modern artiodactyls have lost. An interesting implication is that 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. By contrast, whales retained their carnivorous diet, because prey was more available and they needed higher caloric content in order to live as marine endotherms. Mesonychids also became specialized carnivores, but this was likely a disadvantage because large prey was not yet common. This is why they were out-competed by better-adapted animals like the creodonts and later Carnivora which filled the gaps left by the dinosaurs.
Indohyus was a small deer-like creature, which lived about 48 million years ago in Kashmir. It belongs to the artiodactyls family Raoellidae, and is believed to be the closest sister group of Cetacea. About the size of a raccoon or domestic cat, this herbivorous creature shared some of the traits of whales, most notably the involucrum, a bone growth pattern which is the diagnostic characteristic of any cetacean, and is not found in any other species. It also showed signs of adaptations to aquatic life, including a thick and heavy outer coating and dense limb bones that reduce buoyancy so that they could stay underwater, which are similar to the adaptions found in modern creatures such as the hippopotamus. This suggests a similar survival strategy to the African mousedeer or water chevrotain which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.
The pakicetids were hoofed mammals that were the earliest whales, with Indohyus being the closest sister group. They lived in the early Eocene, around 50 million years ago. Their fossils were first discovered in North Pakistan in 1979, located at a river not far from the shores of the former Tethys Sea. After the initial discovery, more fossils were found, mainly in the late-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. By using stable oxygen isotopes analysis, they were shown to drink fresh water. Their diet probably included land animals that approached water for drinking or some freshwater aquatic organisms that lived in the river. The elongated cervical vertebrae and the four, fused sacral vertebrae are consistent with Artiodactyla, making Pakicetidae one of the earliest fossils to be recovered from the period following the Cetacean/Artiodactyla divergence event. 
Pakicetids were classified as cetaceans mainly based on the structure of the auditory bulla, which is formed from the ectotympanic bone only. The shape of the ear region in pakicetids is highly unusual and the skull is cetacean-like, although a blowhole is still absent at this stage. 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 whales. They have dorsal orbits (eye sockets facing up), which are similar to crocodiles. This eye placement helps submerged predators observe potential prey above the water. According to Thewissen et al., the teeth of pakicetids also resemble the teeth of fossil whales, being less like a dog's incisors, with a serrated triangular shape, similar to a shark's tooth, which is another link to more modern whales. It was initially thought that the ears of pakicetids were adapted for underwater hearing, but, as would be expected from the anatomy of the rest of this creature, the ears of pakicetids are specialized for hearing on land. However, pakicetids were able to listen underwater, by using enhanced bone conduction, rather than depending on tympanic membrane like general land mammals. This method of hearing does not give directional hearing underwater.
Pakicetids have long thin legs, with relatively short hands and feet which suggest that they were poor swimmers. 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. According to a morphological analysis by Thewissen et al., pakicetids display no aquatic skeletal adaptation; instead they display adaptations for running and jumping. Hence pakicetids were most likely an aquatic wader.
Ambulocetus natans, which lived about 49 million years ago, was discovered in Pakistan in 1994. It was probably amphibious, and resembled the crocodile in its physical appearance. In the Eocene, ambulocetids inhabited the bays and estuaries of the Tethys Ocean in northern Pakistan. The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and littoral molluscs. Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed a range of water with different degree of salinity, with some specimens having no evidence of sea water consumption and others that did not ingest fresh water at the time when their teeth are fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations. Hence, ambulocetids represent the transition phase of cetacean ancestors between fresh water and marine habitat.
The mandibular foramen in ambulocetids had increased in size, which indicates that a fat pad was likely to be housed in the lower jaw. In modern whales, this fat pad in the mandibular foramen extends posteriorly to the middle ear. This allows sounds to be received in the lower jaw, and then transmitted through the fat pad to the middle ear. Similar to pakicetids, the orbits of ambulocetids are on the dorsal side of the skull, but they face more laterally than in pakicetids.
Ambulocetids had relatively long limbs with particular strong hind legs, and they retained a tail with no sign of fluke (the horizontal tail fin of modern cetaceans). The hindlimb structure of Ambulocetids shows that their ability to engage in terrestrial locomotor activity was significantly limited compared to that of contemporary terrestrial mammals. The skeletal structures of the knee and ankle indicate that the motion of the hindlimbs were restricted in one plane. This suggests that, on land, propulsion of the hindlimbs was powered by extension of dorsal muscles. Although they could walk on land, as well as swim, it is clear that they were not fast in either environment. It has been speculated that Ambulocetids hunted like crocodiles, lurking in the shallows to snatch unsuspecting riparian prey and fish. They probably swam by pelvic paddling (a way of swimming which mainly utilizes their hind limbs to generate propulsion in water) and caudal undulation (a way of swimming which uses the undulations of the vertebral column to generate force for movements), as otters, seals and whales do.  This is an intermediate stage in the evolution of cetacean locomotion, as modern whales swim by caudal oscillation (a way of swimming similar to caudal undulation, but uses energy more efficiently).
Remingtonocetids lived in middle-Eocene South Asia, about 49 to 43 million years ago. Compared to family Pakicetidae and Ambulocetidae, Remingtonocetidae was a diverse family found in north and central Pakistan, and also western India. Remingtonocetids were also found in shallow marine deposits, but they are obviously more aquatic than ambulocetids. This can be seen from the recovery of their fossils from a variety of coastal marine environments, including near-shore and lagoonal deposits. It is shown that most remingtonocetids did not ingest fresh water, and had hence lost their dependency on fresh water relatively soon after their origin.
The orbits of remingtonocetids face laterally and are small. This suggests that vision is not an important sense for them. The nasal opening, which will later evolve to become blowhole in modern cetaceans, is located near the tip of the long snout. The position of the nasal opening had remained unchanged since pakicetids. One of the notable features in remingtonocetids is that the semicircular canals, which are important for balancing in land mammals, had decreased in size. This reduction in size had closely accompanied the cetacean invasion of marine environments. According to 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.
Compared to ambulocetids, remingtonocetids had relatively short fore and hind limbs. Based on their skeletal remains, remingtonocetids were probably amphibious whales that are well adapted to swimming, and likely to swim by caudal undulation only.
The protocetids form a diverse and heterogeneous group known from Asia, Europe, Africa, and North America. They lived in the Eocene, approximately 48 to 35 million years ago. The fossil remains of protocetids were uncovered from coastal and lagoonal facies in South Asia; but unlike previous cetacean families, their fossils uncovered from Africa and North America also include open marine forms. Hence they were probably amphibious, but more aquatic compared to remingtonocetids. Protocetids were the first whales to leave the Indian subcontinent and disperse to all shallow subtropical oceans of the world. There were many genera among the family Protocetidae, and some of these are very well known (e.g., Rodhocetus). Great variations in aquatic adaptations exist among them, with some probably able to support their weight on land, whereas others could not. Their supposed amphibious nature is supported by the discovery of a pregnant Maiacetus, in which the fossilised fetus was positioned for a head-first delivery, suggesting that Maiacetus gave birth on land.
Unlike remingtonocetids and ambulocetids, protocetids have large orbits and are oriented laterally. Increasingly lateral-facing eyes might be used to observe underwater prey, and are similar to the eyes of modern cetaceans. Furthermore, the nasal openings are large and are now halfway up the snout. The great variety of teeth suggests diverse feeding modes in protocetids. In both remingtonocetids and protocetids, the size of mandibular foramen had increased. The large mandibular foramen indicates that the mandibular fat pad was present. However the air-filled sinuses that are present in modern cetaceans, which function to isolate the ear acoustically to enable better underwater hearing, are still not present. The external auditory meatus (ear canal) which is absent in modern cetaceans is also present. Hence, the method of sound transmission present in them combines aspects of pakicetids and modern odontocetes (toothed whales). At this intermediate stage of hearing development, the transmission of airborne sound was poor due to the modifications of ear for underwater hearing; while directional underwater hearing was also poor compared to modern cetaceans.
Some protocetids had short, large fore- and hindlimbs that are likely to be used in swimming, but the limbs give a slow and cumbersome locomotion on land. It is possible that some protocetids had flukes. However, it is clear that they are adapted even further to an aquatic life-style. In Rodhocetus, for example, the sacrum (a bone that in land-mammals is a fusion of five vertebrae that connects the pelvis with the rest of the vertebral column) was divided into loose vertebrae. However, the pelvis was still connected to one of the sacral vertebrae. The ungulate ancestry of these early whales is still underlined by characteristics like the presence of hooves at the ends of the toes in Rodhocetus.
The foot structure of Rodhocetus shows that protocetids were predominantly aquatic. Gingerich et al hypothesized that Rodhocetus locomoted in oceanic environment in a similar manner to how Amublocetids did: pelvic paddling supplemented by caudal undulation. Terrestrial locomotion of Rodhocetus was very limited due to their hindlimb structure. It is thought that they moved in a way similar to how eared seals move on land. 
Basilosauridae and Dorudontinae
Basilosaurids were discovered in 1840 and initially mistaken for a reptile, hence its name. Together with dorudontids, they lived in the late Eocene around 41 to 35 million years ago, and are the oldest known obligate aquatic cetaceans. They were fully recognizable whales which lived entirely in the ocean. This is supported by their fossils usually found in deposits indicative of fully marine environments, lacking any freshwater influx. They were probably distributed throughout the tropical and subtropical seas of the world. Basilosaurids are commonly found in association with dorudontids. In fact, they are closely related to one another. The fossilised stomach contents in one basilosaurid indicates that it ate fish.
Although they look very much like modern whales, basilosaurids and dorudontids lacked the 'melon organ' that allows their descendants to use echolocation as effectively as modern whales. They had small brains; this suggests they were solitary and did not have the complex social structure of some modern cetaceans. The mandibular foramen of basilosaurids and dorudontids now cover the entire depth of the lower jaw as in modern cetaceans. Their orbits face laterally, and the nasal opening had moved even higher up the snout, closer to the position of blowhole in modern cetaceans. Furthermore, their ear structures are functionally modern, with the major innovation being the insertion of air-filled sinuses between ear and skull. Unlike modern cetaceans, basilosaurids retain a large external auditory meatus.
Both basilosaurids and dorudontids have skeletons that are immediately recognizable as cetaceans. A basilosaurid was as big as the larger modern whales, up to 18 m (60 ft) long; dorudontids were smaller, about 5 m (16 ft) long. The large size of basilosaurids is due to the extreme elongation of their lumbar vertebrae. They had a tail fluke, but their body proportions suggest that they swam by caudal undulation and that the fluke was not used for propulsion. In contrast, dorudontids had a shorter but powerful vertebral column. They too had a fluke, and unlike basilosaurids, they probably swam similarly to modern cetaceans, by using caudal oscillations. The forelimbs of basilosaurids and dorudontids were probably flipper-shaped, and the external hind limbs were tiny and are certainly not involved in locomotion. Their fingers, on the other hand, still retain the mobile joints of their ambulocetid relatives. The two tiny but well-formed hind legs of basilosaurids which were probably used as claspers when mating; they are a small reminder of the lives of their ancestors. Interestingly, the pelvic bones associated with these hind limbs were now no longer connected to the vertebral column as they were in protocetids. Essentially, any sacral vertebrae can no longer be clearly distinguished from the other vertebrae.
Ancestry of modern Cetacea
Both basilosaurids and dorudontids are relatively closely related to modern cetaceans, which belong to suborders Odontoceti and Mysticeti. However, according to Fordyce and Barnes, the large size and elongated vertebral body of basilosaurids preclude them from being ancestral to extant forms. As for dorudontids, there are some species within the family that do not have elongated vertebral bodies, which might be the immediate ancestors of Odontoceti and Mysticeti.
Toothed whales (Odontocetes) echolocate by creating a series of clicks emitted at various frequencies. Sound pulses are emitted through their melon-shaped foreheads, reflected off objects, and retrieved through the lower jaw. Skulls of Squalodon show evidence for the first hypothesized appearance of echolocation. Squalodon lived from the early to middle Oligocene to the middle Miocene, around 33-14 million years ago. Squalodon featured several commonalities with modern Odontocetes. The cranium was well compressed, the rostrum telescoped outward (a characteristic of the modern suborder Odontoceti), giving Squalodon an appearance similar to that of modern toothed whales. However, it is thought unlikely that squalodontids are direct ancestors of living dolphins.
Early baleen whales
All modern mysticetes are large filter-feeding or baleen whales, though the exact means by which baleen is used differs among species (gulp-feeding with balaenopterids, skim-feeding with balaenids, and bottom plowing with eschrichtiids). The first members of some modern groups appeared during the middle Miocene. These changes may have been a result of worldwide environmental change and physical changes in the oceans. A large scale change in ocean current and temperature could have initiated the radiation of modern mysticetes, leading to the demise of the archaic forms. Generally it is speculated the four modern mysticete families have separate origins among the cetotheres. Modern baleen whales, Balaenopteridae (rorquals and humpback whale, Megaptera novaengliae), Balaenidae (right whales), Eschrichtiidae (gray whale, Eschrictius robustus), and Neobalaenidae (pygmy right whale, Caperea marginata) all have derived characteristics presently unknown in any cetothere.
During the early Miocene (about 20 Ma), echolocation developed in its modern form. Various extinct dolphin-like families flourished. Early dolphins include Kentriodon and Hadrodelphis. These belong to Kentriodontidae, which were small to medium-sized toothed cetaceans with largely symmetrical skulls, and thought likely to include ancestors of some modern species. Kentriodontids date to the late Oligocene to late Miocene. Kentriodontines ate small fish and other nectonic organisms; they are thought to have been active echolocators, and might have formed schools. Diversity, morphology and distribution of fossils appear parallel to some modern species.
Today, the whale hind parts are internal and reduced. Occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs (known as atavism).
Whereas early cetaceans such as Pakicetus had the nasal openings at the end of the snout, in later species such as Rodhocetus, the openings had begun to drift toward the top of the skull. This is known as nasal drift.
The nostrils of modern whales have become modified into blowholes that allow them to break to the surface, inhale, and submerge with convenience. The ears began to move inward as well, and, in the case of Basilosaurus, the middle ears began to receive vibrations from the lower jaw. Today's modern toothed whales use the 'melon organ', a pad of fat, for echolocation.
- Aquatic adaptation
- Dolphin, Evolution and anatomy
- Evolution of mammals
- Evolution of sirenians
- List of extinct cetaceans
- Transitional form
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- Evolution of Whales segment from the Whales Tohorā Exhibition Minisite of the Museum of New Zealand Te Papa Tongarewa