The evolution of cetaceans is thought to have proceeded in modern day Pakistan during the Eocene epoch (56–34 mya), the second epoch of the Paleogene period of the present-extending Cenozoic Era. Specifically, cetaceans are thought to have transitioned from land to water at the tailend of the Paleocene–Eocene transition about 56 Mya, which was marked by increased global temperatures of 5.6°C, warming the earth’s oceans.
Cetaceans are fully aquatic mammals belonging to the order Artiodactyla.Davis, R. W. 2019. Return to the Sea: The Evolution of Marine Mammals. Pages 7–27 in R. W. Davis, ed. Marine Mammals: Adaptations for an Aquatic Life. Springer International Publishing, New York. Molecular and morphological analyses suggest that they share a relatively recent closest common ancestor with hippopotamuses – their sister group, diverging about 55.5 Mya. Cetacea completely branched off from other artiodactyls around 50 mya. Research conducted in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea. 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 ungulata ancestors.Berta, A., J. L. Sumich, and K. M. Kovacs. 2005. Marine Mammals: Evolutionary Biology. Elsevier, Amsterdam.Thewissen, J. G. M., L. N. Cooper, J. C. George, and S. Bajpai. 2009. From land to water: the origin of whales, dolphins, and porpoises. Evolution: Education and Outreach 2:272–288.
The two modern parvorders of cetaceans – Mysticeti (baleen whales) and Odontoceti (toothed whales) – are thought to have separated from each other around 28–33 mya in a second cetacean radiation, the first occurring with the archaeocetes. The adaptation of animal echolocation in toothed whales distinguishes them from fully aquatic archaeocetes and early baleen whales. The presence of baleen in baleen whales occurred gradually, with earlier varieties having very little baleen, and their size is linked to baleen dependence (and subsequent increase in filter feeding).
Early evolution
The aquatic lifestyle of cetaceans first began in the Indian subcontinent from even-toed ungulates 50 million years ago, with this initial stage lasting approximately 4-15 million years. Archaeoceti is an extinct parvorder of Cetacea containing ancient whales. The traditional hypothesis of cetacean evolution, first proposed by Van Valen in 1966, was that whales were related to the mesonychians, an extinct order of carnivorous ungulates (hoofed animals) that resembled wolves with hooves and were a sister group of the artiodactyls (even-toed ungulates). This hypothesis was proposed due to similarities between the unusual triangular teeth of the mesonychians and those of early whales. However, molecular phylogeny data indicates that whales are very closely related to the artiodactyls, with hippopotamuses as their closest living relative. Because of this observation, cetaceans and hippopotamuses are placed in the same suborder, Whippomorpha. Cetartiodactyla (formed from the words Cetacea and Artiodactyla) is a proposed name for an order that includes both cetaceans and artiodactyls. However, the earliest anthracotheres, the ancestors of hippos, do not appear in the fossil record until the Middle Eocene, millions of years after Pakicetus, whereas the first known whale ancestor appeared during the Early Eocene; this difference in timing implies that the two groups diverged well before the Eocene. Molecular analysis identifies artiodactyls as being very closely related to cetaceans, so mesonychians are probably an offshoot from Artiodactyla, and cetaceans did not derive directly from mesonychians, but the two groups may share a common ancestor.
The molecular data are supported by the discovery of Pakicetus, the earliest archaeocete. The skeletons of Pakicetus show that whales did not derive directly from mesonychians. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychians. Archaeocetes retained aspects of their mesonychian ancestry (such as the triangular teeth) which modern artiodactyls, and modern whales, have lost. 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). Mesonychians also became specialized carnivores, but this was likely a disadvantage because large prey was uncommon. This may be why they were out-competed by better-adapted animals like the hyaenodontids and later Carnivora.
Raoellidae
Raoellidae (/reɪoʊˈɛlɪdeɪ/) is a family of extinct semiaquatic artiodactyls from the middle Eocene (about 48 million years ago) that were not cetaceans, but closely related to cetaceans. Raoellidae has been recovered as the family most closely related to Cetacea in multiple phylogenetic analyses. They are known primarily from northern India and Pakistan, although potential remains of the raoellid Khirtharia have been found in northern China. They are relatively small animals; on average, raoellids were the size of a red fox. However, Khirtharia major, at about twice the size of an average-sized raoellid, would have been approximately the size of a coyote. Meanwhile, the smallest raoellid, Metkatius, was roughly the size of a house cat. Raoellidae is of particular importance to the understanding of cetacean evolution due to representing a transitional form between fully-terrestrial artiodactyls and the semi-aquatic pakicetids. The raoellid that has been most thoroughly studied and most heavily contributed to the knowledge of Raoellidae is Indohyus. It showed signs of adaptations to aquatic life, including dense limb bones that reduce buoyancy so that they could stay underwater, which are similar to the adaptations found in modern aquatic mammals such as the hippopotamus. This suggests a similar survival strategy to the African chevrotain or water chevrotain which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.
Pakicetidae
The pakicetids were digitigrade hoofed mammals that are thought to be the earliest known cetaceans, with Raoellidae 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 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, implying that they lived around freshwater bodies. 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 artiodactyls, making Pakicetus one of the earliest fossils to be recovered from the period following the Cetacea/Artiodactyla divergence event.
Pakicetids are classified as cetaceans mainly due to the structure of the auditory bulla (ear bone), which is formed only from the ectotympanic bone. 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 cetaceans. 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 a 2009 study, the teeth of pakicetids also resemble the teeth of fossil whales, being less like a dog's incisors, and having serrated triangular teeth, which is another link to more modern cetaceans. 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 the tympanic membrane like other land mammals. This method of hearing did 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 2001 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 aquatic waders.
Ambulocetidae
Ambulocetus, which lived about 49 million years ago, was discovered in Pakistan in 1994. They were vaguely crocodile-like mammals, possessing large brevirostrine jaws. In the Eocene, ambulocetids inhabited the bays and estuaries of the Tethys Sea in northern Pakistan. The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and littoral mollusks. Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed a range of water with different degrees 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 were fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations. Hence, ambulocetids represent a 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 toothed 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 top of the skull, but they face more laterally than in pakicetids.
Ambulocetids had relatively long limbs with particularly strong hind legs, and they retained a tail with no sign of a fluke. The hindlimb structure of Ambulocetids shows that their ability to engage in terrestrial locomotion was significantly limited compared to that of contemporary terrestrial mammals, and likely did not come to land at all. The skeletal structures of the knee and ankle indicates that the motion of the hindlimbs was restricted into one plane. This suggests that, on land, propulsion of the hindlimbs was powered by the extension of dorsal muscles. 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 modern cetaceans do. This is an intermediate stage in the evolution of cetacean locomotion, as modern cetaceans swim by caudal oscillation (a way of swimming similar to caudal undulation, but is more energy efficient).