Evolution of cetaceans 875148 223663959 2008-07-05T04:07:06Z Thijs!bot 1392310 robot Adding: [[es:Evolución de los cetáceos]] [[Image:Cetaceans.svg|thumb|right|250px|The approximately 80 modern species in the order ''cetacea''.]] The [[cetacean]]s ([[whale]]s, [[dolphin]]s and [[porpoise]]s) are descendants of land-living [[mammal]]s. Their terrestrial origins are '''specifically''' indicated by: *the fact that they must breathe air from the surface; *the bones of their fins, which resemble the jointed hands of land mammals; and *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 leviathans had 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. ==Earliest ancestors== [[Image:Stamboom2.JPG|right|thumb|400px|left|The family tree, including Ungulates.]] The traditional theory of cetacean evolution was that whales were related to the [[mesonychid]]s, an extinct order of [[carnivorous]] [[ungulate]]s (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. 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 [[hippopotamid]]s, deeply buried within artiodactyls.<ref name=science_news_3>{{cite web | url = http://www.sciencedaily.com/releases/2005/02/050205103109.htm | title = UC Berkeley, French Scientists Find Missing Link Between The Whale And Its Closest Relative, The Hippo | accessdate = 2007-12-21 | author = University Of California, Berkeley (2005, February 7) | publisher = ScienceDaily }}</ref> The strong evidence for a [[clade]] combining cetaceans and artiodactyls is further discussed under the entry [[Cetartiodactyla]]. The recent discovery of ''[[Pakicetids|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 [[perissodactyl]]s 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 [[dinosaur]]s). ==Pakicetids: the earliest cetaceans?== [[Image:Pakicetus BW.jpg|thumb|left|thumb|220px|Reconstruction of ''[[Pakicetus]]'']] The [[pakicetid]]s are hoofed-mammals that are sometimes classified as the earliest whales.<ref>{{cite journal|author = P. D. Gingerich & D. E. Russell|year = 1981|title = ''Pakicetus inachus'', a new archaeocete (Mammalia, Cetacea) from the early-middle Eocene Kuldana Formation of Kohat (Pakistan)|journal = Univ. Mich. Contr. Mus. Paleont|volume = 25|pages = 235–246}}</ref><ref name=science_news_2>{{cite web | url = http://www.sciencedaily.com/releases/2007/12/071220220241.htm | title = Whales Descended From Tiny Deer-like Ancestors | accessdate = 2007-12-21 | author = Northeastern Ohio Universities Colleges of Medicine and Pharmacy (2007, December 21) | publisher = ScienceDaily }}</ref> They lived in the early [[Eocene]], around 53 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.<ref>{{cite journal|author = J. G. M. Thewissen, E. M. Williams, L. J. Roe and S. T. Hussain|title = Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls|journal = Nature|volume = 413|pages = 277–281|year = 2001|doi = 10.1038/35095005}}</ref> According to Thewissen, the teeth of ''Pakicetus'' 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.<ref>[http://darla.neoucom.edu/DEPTS/ANAT/whaleorigins.htm Whale Origins]</ref> [[Image:Indohyus BW.jpg|thumb|right|thumb|210px|Reconstruction of ''[[Indohyus]]''.]] Thewissen has since found the same ear structure in fossils of a small deer-like creature, ''[[Indohyus]]'', which lived about 48 million years ago in [[Kashmir]].<ref name=science_news_2/> About the size of a raccoon or domestic cat, this herbivorous creature shared some of the traits of whales, and showed signs of adaptations to aquatic life, including a thick and heavy outer coating to bones which is similar to the bones of modern creatures such as the [[hippopotamus]],<ref name=science_news_1>{{cite web | url = http://www.sciencedaily.com/releases/2001/09/010920072245.htm | title = New Fossils Suggest Whales And Hippos Are Close Kin | accessdate = 2007-12-21 | author = University Of Michigan (2001, September 20) | publisher = ScienceDaily }}</ref><ref name=science_news_3/> and reduces buoyancy so that they can stay underwater. This suggests a similar survival strategy to the African [[chevrotain|mousedeer]] or [[Hyemoschus aquaticus|water chevrotain]] which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.<ref name=TheLoom>{{cite web |url=http://scienceblogs.com/loom/2007/12/19/whales_from_so_humble_a_beginn.php |title=The Loom : Whales: From So Humble A Beginning... |accessdate=2007-12-21 |author=Carl Zimmer |authorlink=Carl Zimmer |date=2007-12-19 |publisher=[[ScienceBlogs]] }}</ref><ref name=GuardianIndohyus >{{cite web |url=http://www.guardian.co.uk/science/2007/dec/19/whale.deer?gusrc=rss&feed=networkfront |title=Whales may be descended from a small deer-like animal | Science | Guardian Unlimited |accessdate=2007-12-21 |author=Ian Sample |date=2007-12-19 |work= |publisher=[[Guardian Unlimited]] }}</ref><ref name=PharyngulaIndohyus>{{cite web |url=http://scienceblogs.com/pharyngula/2007/12/indohyus.php |title=Pharyngula: Indohyus |accessdate=2007-12-21 |author=PZ Myers |authorlink=PZ Myers |date=2007-12-19 |work=[[Pharyngula (blog)|Pharyngula]] |publisher=[[ScienceBlogs]] }}</ref> ==Ambulocetids and remingtonocetids== [[Image:Kutchicetus BW.jpg|thumb|left|220px|Reconstruction of ''[[Kutchicetus]]'']] [[Image:Ambulocetus BW.jpg|thumb|right|220px|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 [[otter]]s, [[pinniped|seals]] and [[whale]]s do. It has been speculated that Ambulocetids hunted like crocodiles, lurking in the shallows to snatch unsuspecting prey. 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 otter]]s, hunting for fish in the shallows. ==Protocetids== [[Image:Protocetus BW.jpg|thumb|left|220px|Reconstruction of ''[[Protocetus]]'']] [[Image:Rhodocetus BW.jpg|thumb|right|220px|Reconstruction of ''[[Rodhocetus]]'']] The [[protocetid]]s 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). ==Basilosaurids and dorudontids: fully marine cetaceans== [[Image:Basilosaurus BW.jpg|thumb|left|220px|Reconstruction of ''[[Basilosaurus]]'']] ''[[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 (60 feet) long; dorudontids were within the range of modern cetacean size, about 5m (16 feet) long. [[Image:Dorudon BW.jpg|thumb|right|220px|Reconstruction of ''Dorudon'']] Although they look very much like modern whales, basilosaurids and dorudontids lacked the '[[Melon (whale)|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. ''Basilosaurus'' had two tiny but well-formed hind legs which were probably used as [[clasper]]s when [[mating]]; they are a small reminder of the lives of their ancestors. ==Early echolocation== [[Image:Squalodon BW.jpg|thumb|left|210px|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. ==Early baleen whales== [[Image:Cetotherium BW.jpg|thumb|right|210px|Reconstruction of ''[[Cetotherium]]'']] All modern [[Baleen whale|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 [[Cetotherium|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. ==Early dolphins== [[Image:Bottlenose dolphin hind.jpg|thumb|left|190px|Bottlenose Dolphin with vestigial hind flippers, captured 2006 in Japan. Source [http://www.edwardtbabinski.us/whales/hind_limb_buds/]]] [[Image:Kentriodon BW.jpg|thumb|right|220px|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. See also <em>[[Dolphin#Evolution_and_anatomy|Dolphin, Evolution and anatomy]]</em> ==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]]). [[Image:Ambulocetus four legs.gif|frame|center||''Ambulocetus'']] [[Image:Basilosaurus hind limb.gif|frame|center||''Basilosaurus'' with well developed hind limbs, though reduced in size]] [[Image:Sperm whale limb rudiment.gif|frame|center||Sperm whales retain hind limb atavisms]] 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:Nasal drift.gif|frame|center||Nasal Drift in Whales]] 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 (whale)|melon organ]]', a pad of fat, for [[echolocation]]. [[Image:Evolution of echolocation.gif|frame|center||Evolution of Echolocation]] == See also == {{cetaceaportal}} *[[Archaeoceti]] *[[Aquatic adaptation]] *[[Indohyus]] *[[Transitional form]] *[[Evolution of sirenians]] *[[Evolution of mammals]] *[[List of extinct cetaceans]] == References ==<!-- Palaeontology48:653 --> {{reflist}} == External links == *[http://news.bbc.co.uk/1/hi/sci/tech/1974869.stm BBC: Whale's evolution] *[http://www.stephenjaygould.org/ctrl/gould_leviathan.html Hooking Leviathan by Its Past] by [[Stephen Jay Gould]] *[http://darla.neoucom.edu/DEPTS/ANAT/whaleorigins.htm Whale Origins], Thewissen Lab, Northeastern Ohio Universities College of Medicine *[http://www.neoucom.edu/DLDD/ Digital Library of Dolphin Development], Thewissen Lab *[http://www-personal.umich.edu/~gingeric/PDGwhales/Whales.htm Research on the Origin and Early Evolution of Whales (Cetacea)], Gingerich, P.D., University of Michigan *[http://www.edwardtbabinski.us/whales/evolution_of_whales/ Evolution of Whales] Adapted from National Geographic, November 2001, <em>Revised 2006</em> Dr. J.G.M. Thewissen * [http://deepblue.lib.umich.edu/handle/2027.42/48501 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 * [http://www.nature.com/nature/journal/v413/n6853/abs/413277a0.html 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 * [http://collections.tepapa.govt.nz/exhibitions/whales/Segment.aspx?irn=161 Evolution of Whales segment from the Whales Tohorā Exhibition Minisite of the Museum of New Zealand Te Papa Tongarewa] [[Category:Cetaceans]] [[Category:Evolution by taxon|Cetaceans]] [[ca:Evolució dels cetacis]] [[es:Evolución de los cetáceos]] [[fr:Histoire évolutive des cétacés]] [[it:Evoluzione dei cetacei]] [[fi:Valaiden evoluutio]]