The cetaceans (whales, dolphins and porpoises) are descendants of land-living mammals, and remnants of their terrestrial origins can be found in the fact that they must breathe air from the surface; in the bones of their fins, which look like huge, jointed hands; and in the vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish. The question of how land animals evolved into ocean-going behemoths has been a mystery for a long time, owing to gaps in the fossil record. However, recent discoveries in Pakistan have managed to solve many of these mysteries, and it is now possible to see several stages in the transition of the cetaceans from land to sea. Suborders Mysticeti Odontoceti (see text) The order Cetacea includes whales, dolphins and porpoises. ... This article is about the animal. ... Genera See article below. ... Genera Neophocaena Phocoena - Harbor porpoise Phocoenoides - Dalls porpoise The porpoises are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. ... Subclasses & Infraclasses Subclass †Allotheria* Subclass Prototheria Subclass Theria Infraclass †Trituberculata Infraclass Metatheria Infraclass Eutheria Mammals (class Mammalia) are warm-blooded, vertebrate animals characterized by the production of milk in female mammary glands and by the presence of: hair, three middle ear bones used in hearing, and a neocortex region in... It has been suggested that this article or section be merged into Fossil. ...

Earliest ancestors

The traditional theory of cetacean evolution was that whales were related to the mesonychids, an extinct order of carnivorous ungulates (hoofed animals), which looked rather like wolves with hooves and were a sister group of artiodactyls. These animals possessed unusual triangular teeth that are similar to those of whales. For this reason, scientists had long believed that whales evolved from a form of mesonychid. Mesonychids are an extinct order of even-toed carnivorous ungulates (hoofed animals) which looked like wolves, and were scavengers for carrion and hunters of fish. ... This article deals with meat-eating animals. ... Orders & Clades Order Perissodactyla Eparctocyona Order Arctostylonia (extinct) Order Mesonychia (extinct) Cetartiodactyla Order Cetacea Order Artiodactyla Bulbulodentata (extinct) Family Hyopsodontidae Meridiungulata (extinct) Order Litopterna Notoungulata (extinct) Order Toxodontia Order Typotheria Ungulates (meaning roughly being hoofed or hoofed animal) are several groups of mammals most of which use the tips of... Families Antilocapridae Bovidae Camelidae Cervidae Giraffidae Hippopotamidae Moschidae Suidae Tayassuidae Tragulidae The even-toed ungulates form the mammal order Artiodactyla. ...

However, since the early 1990s analysis of a wide variety of protein and DNA sequence data consistently indicated that whales should be included among artiodactyls. Most probably they are a sister group of hippopotamids, deeply buried within artiodactyls. The strong evidence for a clade combining cetaceans and artiodactyls is further discussed under the entry Cetartiodactyla. Genera Choeropsis Hexaprotodon Hippopotamus Phanourios Hippopotamuses are the members of the family Hippopotamidae. ... A clade is a term belonging to the discipline of cladistics. ... Orders Order Cetacea Order Artiodactyla Humpback Whale breaching. ...

The recent discovery of Pakicetus, the earliest proto-whale (see below) supports the molecular data. The skeletons of Pakicetus demonstrate that whales did not derive directly from mesonychids. Instead, they are a form of artiodactyl (another type of ungulate) that began to take to the water after the artiodactyl family split from the mesonychids. In other words, the proto-whales were early artiodactyls that retained aspects of their mesonychid ancestry (such as the triangular teeth) which modern artiodactyls have since lost. An interesting implication is that the earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, today's artiodactyls and perissodactyls having switched to a plant diet later in their evolution. Whales, due to the readier availability of animal prey and their need for higher caloric content to live as marine endotherms, naturally retained their carnivorous diet, as did mesonychids, who were however out-competed by better-adapted animals like the Carnivora later on (mesonychids became specialized carnivores when the overall availability of large animal prey was still low; thus their adaptation was likely at a disadvantage when new forms had filled the gaps left by the dinosaurs). Genera Gandakasia Ambulocetus Pakicetus Ichthyolestes Pakicetids (formally known as Pakicetidae or Pakicetinae) are a family (or subfamily depending on the author) of extinct mammals that are the earliest known cetaceans. ... Families Suidae Hippopotamidae Tayassuidae Camelidae Tragulidae Moschidae Cervidae Giraffidae Antilocapridae Bovidae The even-toed ungulates form the mammal order Artiodactyla. ... Families Hapalodectidae Mesonychidae Triisodontidae Mesonychia (Middle Claws) are an extinct order of carnivorous ungulate mammals that were closely related to artiodactyls and whales. ... Families Equidae Tapiridae Rhinocerotidae The odd-toed ungulates or Perissodactyla are large to very large browsing and grazing mammals with relatively simple stomachs and a large middle toe. ... Families 17, See classification The diverse order Carnivora (IPA: or ; from Latin carō (stem carn-) flesh, + vorāre to devour) includes over 260 species of placental mammals. ... Orders & Suborders Saurischia Sauropodomorpha Theropoda Ornithischia Thyreophora Ornithopoda Marginocephalia Dinosaurs were vertebrate animals that dominated the terrestrial ecosystem for over 160 million years, first appearing approximately 230 million years ago. ...

Pakicetids: the earliest cetaceans?

An artist's impression of Pakicetus. Illustration by Carl Buell, and taken from [1]

Reconstruction of Pakicetus

The pakicetids are hoofed-mammals that are sometimes classified as the earliest whales.^[1] They lived in the early Eocene, around 52 million years ago. They looked rather like dogs with hoofed feet and long, thick tails. They have been linked to whales by their ears: the structure of the auditory bulla is formed from the ectotympanic bone only. The shape of the ear region in Pakicetus is highly unusual and only resembles the skulls of whales. The feature is diagnostic for cetaceans and is found in no other species. It was initially thought that the ears of Pakicetus were adapted for underwater hearing, but, as would be expected from the anatomy of the rest of this creature, the ears of Pakicetus are specialized for hearing on land, and if Pakicetus is related to the ancestors of whales, underwater hearing must have been a later adaptation.^[2] According to Thewissen, the teeth of Pakicetus also resemble the teeth of fossil whales, which is another link to more modern whales.^[3] Image File history File links Pakicetus. ... Image File history File links Pakicetus. ... Binomial name Pakicetus inachus Gingerich & Russell, 1981 Pakicetus is a genus of extinct cetaceans found in the upper Eocene of Pakistan, whence their name. ... Image File history File links Pakicetus. ... Image File history File links Pakicetus. ... Binomial name Pakicetus inachus Gingerich & Russell, 1981 Pakicetus is a genus of extinct cetaceans found in the upper Eocene of Pakistan, whence their name. ... Genera Gandakasia Pakicetus Nalacetus Ichthyolestes Pakicetids are the members of the family Pakicetidae, sometimes called the subfamily Pakicetinae, of extinct mammals that are the earliest known cetaceans. ... hfajhfiudshfas == == == --24. ...

Ambulocetids and remingtonocetids

An artist's impression of the Remingtonocetid Kutchicetus. Illustration by Carl Buell and taken from [2]

Reconstruction of Ambulocetus natans

The most remarkable of the recent discoveries in Pakistan has been Ambulocetus, which looked like a three-metre long mammalian crocodile. Ambulocetus was clearly amphibious, as its back legs are better adapted for swimming than for walking on land, and it probably swam by undulating its back vertically, as otters, seals and whales do. It has been speculated that Ambulocetids hunted like crocodiles, lurking in the shallows to snatch unsuspecting prey. Image File history File links Kutchicetus. ... Image File history File links Kutchicetus. ... Reconstruction of Kutchicetus An artists impression of the Remingtonocetid Kutchicetus. ... Image File history File links Ambulocetus. ... Image File history File links Ambulocetus. ... Binomial name Ambulocetus natans Thewissen et al. ... Binomial name Ambulocetus natans Thewissen et al. ... Genera Mecistops Crocodylus Osteolaemus See full taxonomy. ... This article is about the carnivorous mammal. ... Families Odobenidae Otariidae Phocidae Pinnipeds (fin-feet, lit. ... This article is about the animal. ...

Reconstruction of the remingtonocetid Kutchicetus

A smaller cousin of Ambulocetus was the remingtonocetid family, which had longer snouts than Ambulocetus', and were slightly better adapted for underwater life. It has been speculated that they lived like modern sea otters, hunting for fish in the shallows. Image File history File links Kutchicetus. ... Image File history File links Kutchicetus. ... Binomial name Enhydra lutris (Linnaeus, 1758) The Sea Otter (Enhydra lutris) is a large otter native to the North Pacific, from northern Japan and Kamchatka west across the Aleutian Islands south to California. ...

Protocetids

Reconstruction of Rodhocetus

The protocetids form a diverse and heterogeneous group known from Asia, Europe, Africa, and North America. There were many genera, and some of these are very well known (e.g., Rodhocetus). Known protocetids had large fore- and hindlimbs that could support the body on land, and it is likely that they lived amphibiously: in the sea and on land. It is unclear at present whether protocetids had flukes (the horizontal tail fin of modern cetaceans). Image File history File links Rodhocetus. ... Image File history File links Rodhocetus. ... Rodhocetus is one of several extinct whale genera that possess land mammal characteristics, thus demonstrating the transition from land to sea that whales went through. ... Rodhocetus is one of several extinct whale genera that possess land mammal characteristics, thus demonstrating the transition from land to sea that whales went through. ...

Basilosaurids and dorudontids: fully marine cetaceans

Basilosaurus

Reconstruction of Basilosaurus

Reconstruction of Saghacetus osiris

Basilosaurus (discovered in 1840 and initially mistaken for a reptile, hence its name) and Dorudon lived around 38 million years ago, and were fully recognizable whales which lived entirely in the ocean. Basilosaurus was a monstrous creature, up to 18m long; dorudontids were within the range of modern cetacean size, about 5m long. Image File history File links Basilosaurus_illustration. ... Image File history File links Basilosaurus_illustration. ... Species Basilosaurus (King Lizard) was a genus of cetacean that lived from 39 to 34 million years ago in the Eocene. ... Image File history File links Basilosaurus. ... Image File history File links Basilosaurus. ... Species Basilosaurus (King Lizard) was a genus of cetacean that lived from 39 to 34 million years ago in the Eocene. ... Image File history File links Saghacetus_osiris. ... Image File history File links Saghacetus_osiris. ... Species Basilosaurus (King Lizard) was a genus of cetacean that lived from 39 to 34 million years ago in the Eocene. ... 1840 is a leap year starting on Wednesday (link will take you to calendar). ... Species Dorudon atrox Dorudon serratus Dorudon was a genus of ancient cetacean that lived alongside Basilosaurus 41 to 33 million years ago, in the Eocene. ...

Reconstruction of Dorudon

Although they look very much like modern whales, basilosaurids and dorudontids lacked the 'melon organ' that allows their descendants to sing and use ultrasound as effectively as modern whales. They had small brains; this suggests they were solitary and did not have the complex social structure of modern whales. Basilosaurus had two tiny but well-formed hind legs which were probably used as claspers when mating; they are a small reminder of the lives of their ancestors. Image File history File links Durodon. ... Image File history File links Durodon. ... The melon is a oily, fatty lump of tissue found at the centre of the forehead of most dolphins and toothed whales. ... Ultrasound is a form of cyclic sound pressure with a frequency greater than the upper limit of human hearing, this limit being approximately 20 kilohertz (20,000 hertz). ... In biology, clasper is a body part of male insects that is used to hold the female during copulation. ... Sevenspotted Lady Beetles mating In biology, mating is the pairing of opposite-sex or hermaphroditic internal fertilization animals for copulation and, in social animals, also to raise their offspring. ...

Early echolocation

Reconstruction of Squalodon

Toothed whales (Odontocetes) echolocate by creating a series of clicks emitted at various frequencies. Sound pulses are emitted through their melon-shaped forehead, reflected off objects, and retrieved through the lower jaw. Skulls of Squalodon show evidence for the first appearance of echolocation. Squalodons lived from the early to middle Oligocene to the middle Miocene, around 33-14 million years ago. A peculiar blend of archaic and modern features characterize Squalodon. The cranium was well compressed, the rostrum telescoped outward, giving an appearance of modern toothed whales. However, it is thought unlikely that squalodontids have anything to do with the ancestry of most living dolphins. Image File history File links Squalodon. ... Image File history File links Squalodon. ... Squalodon is an extinct genus of whales, belonging to the superfamily Squalodontoidea and family Squalodontidae. ... Families See text The toothed whales (systematic name Odontoceti) form a suborder of the cetaceans. ... Squalodon is an extinct genus of whales, belonging to the superfamily Squalodontoidea and family Squalodontidae. ...

Early baleen whales

Reconstruction of Cetotherium

All modern mysticetes are large filter-feeding or baleen whales, though the exact means by which baleen is used differ among species (gulp-feeding with balaenopterids, skim-feeding with balaenids, and bottom ploughing 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 thought the four modern mysticete families have separate origins among the cetotheres, however this is only speculative. 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. Image File history File links Cetotherium. ... Image File history File links Cetotherium. ... Paleo Template Project Cetotherium was a cetacean that looked more like modern whales of today. ... Diversity Around 15 species; see list of cetaceans or below. ... Baleen hair is attached to the baleen plate Baleen (also called whalebone) is a substance made of keratin and is therefore stiff but somewhat elastic. ... Paleo Template Project Cetotherium was a cetacean that looked more like modern whales of today. ...

Early dolphins

Bottlenose Dolphin with vestigial hind flippers, captured 2006 in Japan. Source [3]

Reconstruction of Kentriodon

During the early Miocene, 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. Image File history File links Bottlenose_dolphin_hind. ... Image File history File links Bottlenose_dolphin_hind. ... Image File history File links Kentriodon. ... Image File history File links Kentriodon. ... Reconstruction of Kentriodon Kentriodon The genus Kentriodon is the most diverse of all kentriodontids, which include three named species and five undescribed species. ... Reconstruction of Kentriodon Kentriodon The genus Kentriodon is the most diverse of all kentriodontids, which include three named species and five undescribed species. ...

See also Dolphin, Evolution and anatomy Genera See article below. ...

Skeletal evolution

Today, the whale hind parts are internal and reduced, and they serve as anchor for the muscles of the genitalia. Occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs (known as atavism). A sex organ, or primary sexual characteristic, narrowly defined, is any of those parts of the body (which are not always bodily organs according to the strict definition) which are involved in sexual reproduction and constitute the reproductive system in an complex organism; namely: Male: penis (notably the glans penis... An atavism can mean an organism that is a real or supposed evolutionary throwback; the unexpected appearance of primitive traits; or a reversion to or reappearance of a trait that had been present in a lineage in the past, but which had been absent in intervening generations. ...

Whereas early cetaceans such as the Pakicetus had the nasal openings at the end of the snout, in later species such as the Rodhocetus, the openings had begun to drift toward the top of the skull. This is known as nasal drift. Image File history File links Ambulocetus_four_legs. ... Image File history File links Basilosaurus_hind_limb. ... Image File history File links Sperm_whale_limb_rudiment. ... Binomial name Pakicetus inachus Gingerich & Russell, 1981 Pakicetus is a genus of extinct cetaceans found in the upper Eocene of Pakistan, whence their name. ... A snout is the protruding portion of an animals face, consisting of its nose, mouth, and jaw. ... Rodhocetus is one of several extinct whale genera that possess land mammal characteristics, thus demonstrating the transition from land to sea that whales went through. ...

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. Image File history File links Nasal_drift. ... The melon is a oily, fatty lump of tissue found at the centre of the forehead of most dolphins and toothed whales. ... See: Animal echolocation: animals emitting sound waves and listening to the echo in order to locate objects or navigate. ...

Image File history File links Evolution_of_echolocation. ...

References

External links

• BBC: Whale's evolution
• Whale Origins, Thewissen Lab, Northeastern Ohio Universities College of Medicine
• Digital Library of Dolphin Development, Thewissen Lab
• Research on the Origin and Early Evolution of Whales (Cetacea), Gingerich, P.D., University of Michigan
• Evolution of Whales Adapted from National Geographic, November 2001, Revised 2006 Dr. J.G.M. Thewissen
• Pakicetus inachus, a new archaeocete (Mammalia, Cetacea) from the early-middle Eocene Kuldana Formation of Kohat (Pakistan). Gingerich, P.D., 1981, Museum of Paleontology, The University of Michigan
• Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls, Nature 413, 277-281 (20 September 2001), J. G. M. Thewissen, E. M. Williams, L. J. Roe and S. T. Hussain

See also

• Archaeoceti
• Aquatic adaptation
• Transitional form
• Evolution of sirenians
• List of extinct cetaceans