Evolution of sirenians
Sirenia is the order of placental mammals which comprises modern "sea cows" and their extinct relatives. They are the only extant herbivorous marine mammals and the only group of herbivorous mammals to have become aquatic. Sirenians are thought to have a 50-million-year-old fossil record, they attained modest diversity during the Oligocene and Miocene, but have since declined as a result of climatic cooling, oceanographic changes, human interference. Two genera and four species are extant: Trichechus, which includes the three species of manatee that live along the Atlantic coasts and in rivers and coastlines of the Americas and western Africa, Dugong, found in the Indian and Pacific oceans. Sirenians, along with Proboscidea, group together with the extinct Desmostylia and the extinct Embrithopoda to form the Tethytheria. Tethytheria is thought to have evolved from primitive hoofed mammals along the shores of the ancient Tethys Ocean. Tethytheria, combined with Hyracoidea, forms. Paenungulata and Tethytheria are among the least controversial mammalian clades, with strong support from morphological and molecular interpretations.
The ancestry of Sirenia is remote from that of Cetacea and Pinnipedia, although they are thought to have evolved an aquatic lifestyle around the same time. The first appearance of sirenians in the fossil record was during the early Eocene, by the late Eocene, sirenians had diversified. Inhabitants of rivers and nearshore marine waters, they were able to spread rapidly; the most primitive sirenian known to date, was found in Jamaica, not the Old World. The first known quadrupedal sirenian was Pezosiren from the early Eocene; the earliest known sea cows, of the families Prorastomidae and Protosirenidae, are both confined to the Eocene, were about the size of a pig, four-legged amphibious creatures. By the time the Eocene drew to a close, came the appearance of the Dugongidae; the last of the sirenian families to appear, Trichechidae arose from early dugongids in the late Eocene or early Oligocene. The current fossil record documents all major stages in hindlimb and pelvic reduction to the extreme reduction in the modern manatee pelvis, providing an example of dramatic morphological change among fossil vertebrates.
Since sirenians first evolved, they have been herbivores depending on seagrasses and aquatic angiosperms for food. To the present all have remained tropical and angiosperm consumers. Sea cows are shallow divers with large lungs, they have heavy skeletons to help them stay submerged. Eocene sirenians, like Mesozoic mammals but in contrast to other Cenozoic ones, have five instead of four premolars, giving them a 126.96.36.199 dental formula. Whether this condition is a primitive retention in sirenians is still under debate. Although cheek teeth are relied on for identifying species in other mammals, they do not vary to a significant degree among sirenians in their morphology, but are always low-crowned with two rows of large, rounded cusps; the most identifiable parts of sirenian skeletons are the skull and mandible the frontal and other skull bones. With the exception of a pair of tusk-like first upper incisors present in most species, front teeth are lacking in all, except the earliest sirenians. Evolution of cetaceans Domning DP.
"Sirenian Evolution". In Perrin WF, Würsig B, Thewissen JG. Encyclopedia of Marine Mammals. San Diego: Academic Press. Pp. 1083–1086. ISBN 978-0125513401. Berta A. "5. Diversity and Adaptation of Sirenians and Other Marine Mammals". Return to the Sea: The Life and Evolutionary Times of Marine Mammals. Berkeley: University of California Press. Pp. 127–149. ISBN 978-0-520-27057-2. Berta A, Sumich JL, Kovacs KK. "5. Sirenian and Other Marine Mammals: Evolution and Systematics". Marine Mammals: Evolutionary Biology. Academic Press. Pp. 89–110. ISBN 978-0-12-369499-7. Marsh H, O'Shea TJ, Reynolds JE III. "3. Affinities and diversity of the Sirenia through time". Ecology and Conservation of the Sirenia: Dugongs and Manatees. Conservation Biology. 18. Cambridge: Cambridge University Press. Pp. 35–77. ISBN 978-0-521-71643-7. Appendices Velez-Juarbe J, Domning DP, Pyenson ND. "Iterative Evolution of Sympatric Seacow Assemblages during the Past ~26 Million Years". PLoS ONE. 7: e31294. Bibcode:2012PLoSO...731294V. Doi:10.1371/journal.pone.0031294.
PMC 3272043. PMID 22319622. Origin of Sirenians Macro-evolution at its finest Christina Reed, Geotimes December 2001 Evolution of the Sirenia by Caryn Self-Sullivan at Sirenian International Manatee and Elephant Fossils
A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, petrified wood, coal, DNA remnants; the totality of fossils is known as the fossil record. Paleontology is the study of fossils: their age, method of formation, evolutionary significance. Specimens are considered to be fossils if they are over 10,000 years old; the oldest fossils are around 3.48 billion years old to 4.1 billion years old. The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils; the development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host. There are many processes that lead to fossilization, including permineralization and molds, authigenic mineralization and recrystallization, adpression and bioimmuration.
Fossils vary in size from one-micrometre bacteria to dinosaurs and trees, many meters long and weighing many tons. A fossil preserves only a portion of the deceased organism that portion, mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates. Fossils may consist of the marks left behind by the organism while it was alive, such as animal tracks or feces; these types of fossil are called trace ichnofossils, as opposed to body fossils. Some fossils are called chemofossils or biosignatures; the process of fossilization varies according to external conditions. Permineralization is a process of fossilization; the empty spaces within an organism become filled with mineral-rich groundwater. Minerals precipitate from the groundwater; this process can occur in small spaces, such as within the cell wall of a plant cell. Small scale permineralization can produce detailed fossils. For permineralization to occur, the organism must become covered by sediment soon after death, otherwise decay commences.
The degree to which the remains are decayed when covered determines the details of the fossil. Some fossils consist only of skeletal teeth; this is a form of diagenesis. In some cases, the original remains of the organism dissolve or are otherwise destroyed; the remaining organism-shaped hole in the rock is called an external mold. If this hole is filled with other minerals, it is a cast. An endocast, or internal mold, is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull; this is a special form of mold formation. If the chemistry is right, the organism can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it. If this happens before significant decay to the organic tissue fine three-dimensional morphological detail can be preserved. Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, USA, are among the best documented examples of such mineralization.
Replacement occurs. In some cases mineral replacement of the original shell occurs so and at such fine scales that microstructural features are preserved despite the total loss of original material. A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite. Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues. In this case the fossil consists of original material, albeit in a geochemically altered state; this chemical change is an expression of diagenesis. What remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression. However, the phytoleim is lost and all that remains is an impression of the organism in the rock—an impression fossil. In many cases, however and impressions occur together. For instance, when the rock is broken open, the phytoleim will be attached to one part, whereas the counterpart will just be an impression.
For this reason, one term covers the two modes of preservation: adpression. Because of their antiquity, an unexpected exception to the alteration of an organism's tissues by chemical reduction of the complex organic molecules during fossilization has been the discovery of soft tissue in dinosaur fossils, including blood vessels, the isolation of proteins and evidence for DNA fragments. In 2014, Mary Schweitzer and her colleagues reported the presence of iron particles associated with soft tissues recovered from dinosaur fossils. Based on various experiments that studied the interaction of iron in haemoglobin with blood vessel tissue they proposed that solution hypoxia coupled with iron chelation enhances the stability and preservation of soft tissue and provides the basis for an explanation for the unforeseen preservation of fossil soft tissues. However, a older study based on eight taxa ranging in time from the Devonian to the Jurassic found that reasonably well-preserved fibrils that represent collagen were preser
Evolution of mammalian auditory ossicles
The evolution of mammalian auditory ossicles was an evolutionary event in which bones in the jaw of reptiles were co-opted to form part of the hearing apparatus in mammals. The event is well-documented and important as a demonstration of transitional forms and exaptation, the re-purposing of existing structures during evolution. In reptiles, the eardrum is connected to the inner ear via a single bone, the columella, while the upper and lower jaws contain several bones not found in mammals. Over the course of the evolution of mammals, one bone from the lower and one from the upper jaw lost their purpose in the jaw joint and were put to new use in the middle ear, connecting to the existing stapes bone and forming a chain of three bones, the ossicles, which transmit sounds more efficiently and allow more acute hearing. In mammals, these three bones are known as the malleus and stapes. Mammals and birds differ from other vertebrates by having evolved a cochlea; the evidence that the malleus and incus are homologous to the reptilian articular and quadrate was embryological, since this discovery an abundance of transitional fossils has both supported the conclusion and given a detailed history of the transition.
The evolution of the stapes was an distinct event. Following on the ideas of Étienne Geoffroy Saint-Hilaire, studies by Johann Friedrich Meckel the Younger, Carl Gustav Carus, Martin Rathke, Karl Ernst von Baer, the relationship between the reptilian jaw bones and mammalian middle-ear bones was first established on the basis of embryology and comparative anatomy by Karl Bogislaus Reichert and advanced by Ernst Gaupp and this is known as the Reichert–Gaupp theory. In the course of the development of the embryo, the incus and malleus arise from the same first pharyngeal arch as the mandible and maxilla, are served by mandibular and maxillary division of the trigeminal nerve....the discovery that the mammalian malleus and incus were homologues of visceral elements of the "reptilian" jaw articulation... ranks as one of the milestones in the history of comparative biology.... It is one of the triumphs of the long series of researches on the extinct Theromorph reptiles, begun by Owen, continued by Seeley and Watson, to have revealed the intermediate steps by which the change may have occurred from an inner quadrate to an outer squamosal articulation...
Yet the transition between the "reptilian" jaw and the "mammalian" middle ear was not bridged in the fossil record until the 1950s with the elaboration of such fossils as the now-famous Morganucodon. There are more recent studies in the genetic basis for the development of the ossicles from the embryonic arch, relating this to evolutionary history."Bapx1 known as Nkx3.2, is the vertebrate homologue of the Drosophila gene Bagpipe. A member of the NK2 class of homeobox genes...", this gene is implicated in the change from the jaw bones of non-mammals to the ossicles of mammals. Others are Dlx genes, Prx genes, Wnt genes; the earliest mammals were small animals nocturnal insectivores. This suggests a plausible evolutionary mechanism driving the change. Natural selection would account for the success of this feature. There is still one more connection with another part of biology: genetics suggests a mechanism for this transition, the kind of major change of function seen elsewhere in the world of life being studied by evolutionary developmental biology.
The mammalian middle ear contains three tiny bones known as the ossicles: malleus and stapes. The ossicles are a complex system of levers whose functions include: reducing the amplitude of the vibrations; the ossicles act as the mechanical analog of an electrical transformer, matching the mechanical impedance of vibrations in air to vibrations in the liquid of the cochlea. The net effect of this impedance matching is to increase the overall sensitivity and upper frequency limits of mammalian hearing, as compared to reptilian hearing; the details of these structures and their effects vary noticeably between different mammal species when the species are as related as humans and chimpanzees. Living mammal species can be identified by the presence in females of mammary glands which produce milk. Other features are required when classifying fossils, since mammary glands and other soft-tissue features are not visible in fossils. Paleontologists therefore use a distinguishing feature, shared by all living mammals, but is not present in any of the early Triassic therapsids: mammals use two bones for hearing that all other amniotes use for eating.
The earliest amniotes had a jaw joint composed of the quadrate. All non-mammalian amniotes use this system including lizards, crocodilians and therapsids, but mammals have a different jaw joint, composed only of the squamosal. In mammals, the quadrate and articular bones have evolved into the incus and malleus bones in the middle ear. Here is a ver
Paleobotany spelled as palaeobotany, is the branch of paleontology or paleobiology dealing with the recovery and identification of plant remains from geological contexts, their use for the biological reconstruction of past environments, both the evolutionary history of plants, with a bearing upon the evolution of life in general. A synonym is paleophytology. Paleobotany includes the study of terrestrial plant fossils, as well as the study of prehistoric marine photoautotrophs, such as photosynthetic algae, seaweeds or kelp. A related field is palynology, the study of fossilized and extant spores and pollen. Paleobotany is important in the reconstruction of ancient ecological systems and climate, known as paleoecology and paleoclimatology respectively. Paleobotany has become important to the field of archaeology for the use of phytoliths in relative dating and in paleoethnobotany; the emergence of paleobotany as a scientific discipline can be seen in the early 19th century in the works of the German palaeontologist Ernst Friedrich von Schlotheim, the Czech nobleman and scholar Kaspar Maria von Sternberg, the French botanist Adolphe-Théodore Brongniart.
Macroscopic remains of true vascular plants are first found in the fossil record during the Silurian Period of the Paleozoic era. Some dispersed, fragmentary fossils of disputed affinity spores and cuticles, have been found in rocks from the Ordovician Period in Oman, are thought to derive from liverwort- or moss-grade fossil plants. An important early land plant fossil locality is the Rhynie Chert, found outside the village of Rhynie in Scotland; the Rhynie chert is an Early Devonian sinter deposit composed of silica. It is exceptional due to its preservation of several different clades of plants, from mosses and lycopods to more unusual, problematic forms. Many fossil animals, including arthropods and arachnids, are found in the Rhynie Chert, it offers a unique window on the history of early terrestrial life. Plant-derived macrofossils become abundant in the Late Devonian and include tree trunks and roots; the earliest tree was thought to be Archaeopteris, which bears simple, fern-like leaves spirally arranged on branches atop a conifer-like trunk, though it is now known to be the discovered Wattieza.
Widespread coal swamp deposits across North America and Europe during the Carboniferous Period contain a wealth of fossils containing arborescent lycopods up to 30 meters tall, abundant seed plants, such as conifers and seed ferns, countless smaller, herbaceous plants. Angiosperms evolved during the Mesozoic, flowering plant pollen and leaves first appear during the Early Cretaceous 130 million years ago. A plant fossil is any preserved part of a plant; such fossils may be prehistoric impressions that are many millions of years old, or bits of charcoal that are only a few hundred years old. Prehistoric plants are various groups of plants. Plant fossils can be preserved in a variety of ways, each of which can give different types of information about the original parent plant; these modes of preservation are discussed in the general pages on fossils but may be summarised in a palaeobotanical context as follows. Adpressions; these are the most found type of plant fossil. They provide good morphological detail of dorsiventral plant parts such as leaves.
If the cuticle is preserved, they can yield fine anatomical detail of the epidermis. Little other detail of cellular anatomy is preserved. Petrifactions; these provide fine detail of the cell anatomy of the plant tissue. Morphological detail can be determined by serial sectioning, but this is both time consuming and difficult. Moulds and casts; these only tend to preserve the more robust plant parts such as seeds or woody stems. They can provide information about the three-dimensional form of the plant, in the case of casts of tree stumps can provide evidence of the density of the original vegetation. However, they preserve any fine morphological detail or cell anatomy. A subset of such fossils are pith casts, where the centre of a stem is either hollow or has delicate pith. After death, sediment forms a cast of the central cavity of the stem; the best known examples of pith casts are in cordaites. Authigenic mineralisations; these can provide fine, three-dimensional morphological detail, have proved important in the study of reproductive structures that can be distorted in adpressions.
However, as they are formed in mineral nodules, such fossils can be of large size. Fusain. Fire destroys plant tissue but sometimes charcoalified remains can preserve fine morphological detail, lost in other modes of preservation. Fusain fossils are delicate and small, but because of their buoyancy can drift for long distances and can thus provide evidence of vegetation away from areas of sedimentation. Plant fossils always represent disarticulated parts of plants; those few examples of plant fossils that appear to be the remains of whole plants in fact are incomplete as the internal cellular tis
Evolution of the eye
Many researchers have found the evolution of the eye attractive to study, because the eye distinctively exemplifies an analogous organ found in many animal forms. Simple light detection is found in bacteria, single-celled organisms and animals. Complex, image-forming eyes have evolved independently several times. Complex eyes appeared first within the few million years of the Cambrian explosion. Prior to the Cambrian, no evidence of eyes has survived, but diverse eyes are known from the Burgess shale of the Middle Cambrian, from the older Emu Bay Shale. Eyes are adapted to the various requirements of their owners, they vary in their visual acuity, the range of wavelengths they can detect, their sensitivity in low light, their ability to detect motion or to resolve objects, whether they can discriminate colours. In 1802, philosopher William Paley called it a miracle of "design". Charles Darwin himself wrote in his Origin of Species, that the evolution of the eye by natural selection seemed at first glance "absurd in the highest possible degree".
However, he went on that despite the difficulty in imagining it, its evolution was feasible:...if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is the case. He suggested a stepwise evolution from "an optic nerve coated with pigment, without any other mechanism" to "a moderately high stage of perfection", gave examples of existing intermediate steps. Current research is investigating evolution. Biologist D. E. Nilsson has independently theorized about four general stages in the evolution of a vertebrate eye from a patch of photoreceptors. Nilsson and S. Pelger estimated in a classic paper that only a few hundred thousand generations are needed to evolve a complex eye in vertebrates. Another researcher, G. C. Young, has used the fossil record to infer evolutionary conclusions, based on the structure of eye orbits and openings in fossilized skulls for blood vessels and nerves to go through. All this adds to the growing amount of evidence.
The first fossils of eyes found to date are from the lower Cambrian period. The lower Cambrian had a burst of rapid evolution, called the "Cambrian explosion". One of the many hypotheses for "causes" of the Cambrian explosion is the "Light Switch" theory of Andrew Parker: It holds that the evolution of eyes started an arms race that accelerated evolution. Before the Cambrian explosion, animals may have sensed light, but did not use it for fast locomotion or navigation by vision; the rate of eye evolution is difficult to estimate, because the fossil record of the lower Cambrian, is poor. How fast a circular patch of photoreceptor cells evolve into a functional vertebrate eye has been estimated based on rates of mutation, relative advantage to the organism, natural selection. However, the time needed for each state was overestimated and the generation time was set to one year, common in small animals. With these pessimistic values, the vertebrate eye would still evolve from a patch of photoreceptor cells in less than 364,000 years.
Whether the eye evolved once or many times depends on the definition of an eye. All eyed animals share much of the genetic machinery for eye development; this suggests that the ancestor of eyed animals had some form of light-sensitive machinery – if it was not a dedicated optical organ. However photoreceptor cells may have evolved more than once from molecularly similar chemoreceptor cells. Photoreceptor cells existed long before the Cambrian explosion. Higher-level similarities – such as the use of the protein crystallin in the independently derived cephalopod and vertebrate lenses – reflect the co-option of a more fundamental protein to a new function within the eye. A shared trait common to all light-sensitive organs are opsins. Opsins belong to a family of photo-sensitive proteins and fall into nine groups, which existed in the urbilaterian, the last common ancestor of all bilaterally symmetrical animals. Additionally, the genetic toolkit for positioning eyes is shared by all animals: The PAX6 gene controls where eyes develop in animals ranging from octopuses to mice and fruit flies.
Such high-level genes are, by implication, much older than many of the structures that they control today. Eyes and other sensory organs evolved before the brain: There is no need for an information-processing organ before there is information to process; the earliest predecessors of the eye were photoreceptor proteins that sense light, found in unicellular organisms, called "eyespots". Eyespots can only sense ambient brightness: they can distinguish light from dark, sufficient for photoperiodism and daily synchronization of circadian rhythms, they are insufficient for vision, as they cannot distinguish shapes or determine the direction light is coming from. Eyespots are found in nearly all major animal groups, are common among unicellular organisms, including euglena; the euglena's eyespot, called a stigma, is located at its anterior end. It is a small splotch of red pigment. Together with th
Paleontology or palaeontology is the scientific study of life that existed prior to, sometimes including, the start of the Holocene Epoch. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments. Paleontological observations have been documented as far back as the 5th century BC; the science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, developed in the 19th century. The term itself originates from Greek παλαιός, palaios, "old, ancient", ὄν, on, "being, creature" and λόγος, logos, "speech, study". Paleontology lies on the border between biology and geology, but differs from archaeology in that it excludes the study of anatomically modern humans, it now uses techniques drawn from a wide range of sciences, including biochemistry and engineering. Use of all these techniques has enabled paleontologists to discover much of the evolutionary history of life all the way back to when Earth became capable of supporting life, about 3.8 billion years ago.
As knowledge has increased, paleontology has developed specialised sub-divisions, some of which focus on different types of fossil organisms while others study ecology and environmental history, such as ancient climates. Body fossils and trace fossils are the principal types of evidence about ancient life, geochemical evidence has helped to decipher the evolution of life before there were organisms large enough to leave body fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allow radiometric dating, which provides absolute dates that are accurate to within 0.5%, but more paleontologists have to rely on relative dating by solving the "jigsaw puzzles" of biostratigraphy. Classifying ancient organisms is difficult, as many do not fit well into the Linnaean taxonomy classifying living organisms, paleontologists more use cladistics to draw up evolutionary "family trees"; the final quarter of the 20th century saw the development of molecular phylogenetics, which investigates how organisms are related by measuring the similarity of the DNA in their genomes.
Molecular phylogenetics has been used to estimate the dates when species diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend. The simplest definition of paleontology is "the study of ancient life"; the field seeks information about several aspects of past organisms: "their identity and origin, their environment and evolution, what they can tell us about the Earth's organic and inorganic past". Paleontology is one of the historical sciences, along with archaeology, astronomy, cosmology and history itself: it aims to describe phenomena of the past and reconstruct their causes. Hence it has three main elements: description of past phenomena; when trying to explain the past and other historical scientists construct a set of hypotheses about the causes and look for a smoking gun, a piece of evidence that accords with one hypothesis over the others. Sometimes the smoking gun is discovered by a fortunate accident during other research. For example, the discovery by Luis and Walter Alvarez of iridium, a extra-terrestrial metal, in the Cretaceous–Tertiary boundary layer made asteroid impact the most favored explanation for the Cretaceous–Paleogene extinction event, although the contribution of volcanism continues to be debated.
The other main type of science is experimental science, said to work by conducting experiments to disprove hypotheses about the workings and causes of natural phenomena. This approach cannot prove a hypothesis, since some experiment may disprove it, but the accumulation of failures to disprove is compelling evidence in favor. However, when confronted with unexpected phenomena, such as the first evidence for invisible radiation, experimental scientists use the same approach as historical scientists: construct a set of hypotheses about the causes and look for a "smoking gun". Paleontology lies between biology and geology since it focuses on the record of past life, but its main source of evidence is fossils in rocks. For historical reasons, paleontology is part of the geology department at many universities: in the 19th and early 20th centuries, geology departments found fossil evidence important for dating rocks, while biology departments showed little interest. Paleontology has some overlap with archaeology, which works with objects made by humans and with human remains, while paleontologists are interested in the characteristics and evolution of humans as a species.
When dealing with evidence about humans and paleontologists may work together – for example paleontologists might identify animal or plant fossils around an archaeological site, to discover what the people who lived there ate. In addition, paleontology borrows techniques from other sciences, including biology, ecology, chemistry and mathematics. For example, geochemical signatures from rocks may help to discover when life first arose on Earth, analyses of carbon isotope ratios may help to identify climate changes and to explain major transitions such as the Permian–Triassic extinction event. A recent discipline, molecular phylogenetics, compares the DNA and RNA of modern organisms to re-construct the "family trees" of their
Evolution of cetaceans
The evolutionary history of cetaceans is thought to have occurred in the Indian subcontinent from even-toed ungulates 50 million years ago, over a period of at least 15 million years. Cetaceans are aquatic marine mammals belonging to the order Artiodactyla, branched off from other artiodactyls around 50 mya. Cetaceans are thought to have evolved during the Eocene or earlier, sharing a closest common ancestor with hippopotamuses. Being mammals, they surface to breathe air. Discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea; the two modern parvorders of cetaceans – Mysticeti and Odontoceti – are thought to have separated from each other around 28-33 million years ago in a second cetacean radiation, the first occurring with the archaeocetes. The adaptation of animal echolocation in toothed whales distinguishes them from aquatic archaeocetes and early baleen whales; the presence of baleen in baleen whales occurred with earlier varieties having little baleen, their size is linked to baleen dependence.
The aquatic lifestyle of cetaceans first began in the Indian subcontinent from even-toed ungulates 50 million years ago, over a period of at least 15 million years, however a jawbone discovered in Antarctica may reduce this to 5 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 mesonychids, an extinct order of carnivorous ungulates that resembled wolves with hooves and were a sister group of the artiodactyls. This hypothesis was proposed due to similarities between the unusual triangular teeth of the mesonychids and those of early whales. However, molecular phylogeny data indicates that whales are closely related to the artiodactyls, with hippopotamuses as their closest living relative; because of this and hippopotamuses are placed in the same suborder, Whippomorpha. Cetartiodactyla is a proposed name for an order containing both 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, the first known whale ancestor, appeared during the Early Eocene, implying the two groups diverged well before the Eocene. Since molecular analysis identifies artiodactyls as being closely related to cetaceans, mesonychids are an offshoot from Artiodactyla, cetaceans did not derive directly from them, but that the two groups may share a common ancestor; the molecular data are supported by the discovery of the earliest archaeocete. The skeletons of Pakicetus show. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychids. Archaeocetes retained aspects of their mesonychid ancestry which modern artiodactyls, modern whales, have lost; the earliest ancestors of all hoofed mammals were at least carnivorous or scavengers, today's artiodactyls and perissodactyls became herbivores in their evolution.
Whales, retained their carnivorous diet because prey was more available and they needed higher caloric content in order to live as marine endotherms. Mesonychids became specialized carnivores, but this was a disadvantage because large prey was uncommon; this may be why they were out-competed by better-adapted animals like the hyaenodontids and Carnivora. Indohyus was a small chevrotain-like animal that lived about 48 million years ago in what is now Kashmir, it belongs to the artiodactyl family Raoellidae, is believed to be the closest sister group of Cetacea. Indohyus is identified as an artiodactyl because it has two trochlea hinges, a trait unique to artiodactyls; 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, the diagnostic characteristic of any cetacean. 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 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 digitigrade hoofed mammals that are thought to be the earliest known cetaceans, 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 in the early Eocene fluvial deposits in northern Pakistan and northwestern India. Based on this discovery, pakicetids most 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 included land animals that approached water for drinking or some freshwater aquatic organi