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\b[<cetaceans> illustrate a sequence of inherited mammalian features, terrestrial specializations and subsequent aquatic transformations, followed by divergence in feeding and sensory <anatomy>.] This is a branching <phylogenetic tree>, not a procession of named <fossils> that can all be identified as direct ancestors. Here <Cetacea> includes living whales, dolphins and porpoises; <mammals> means the <crown group> containing living <monotremes> and <therians>, whereas <mammaliaforms> also includes close extinct relatives outside that <crown group>. Dates below are approximate millions of years ago, abbreviated Ma.
The starting condition is a terrestrial, non-mammaliaform <cynodont>, itself a <therapsid> within <Synapsida>. Along the mammalian <stem group>, enlargement of the <dentary> and reduction of the other lower-jaw bones accompanied more differentiated <teeth>, increasingly precise <dental occlusion> and elaboration of a <secondary palate>. These changes belong to a feeding and respiratory complex: a larger tooth-bearing bone carries the <mastication> load, differentiated <teeth> process food, and a bony partition separates nasal and oral passages. They were acquired in a mosaic; neither a full modern mammalian <jaw> nor the aquatic body plan appeared at the initial divergence from other <cynodonts>.
Early <mammaliaforms> show how <jaw> support and hearing became reorganized. <Morganucodon>, around the <Triassic>–<Jurassic> transition and Early <Jurassic>, retained the old <articular>–<quadrate> <jaw> joint alongside a <dentary>–<squamosal> joint. In the mammalian condition the <dentary>–<squamosal> articulation supports the <jaw>, while the <articular> and <quadrate> correspond to the <malleus> and <incus>; the <angular bone> contributes to the <ectotympanic>. This is <biological homology> supported by comparative <anatomy> and <biological development>, not merely similarity in function. <Hadrocodium>, about 195 Ma, records further changes in <jaw> attachment and the <braincase> consistent with increased separation of <mastication> and hearing. Its tiny <skull> does not preserve an entire living-style ear apparatus; the reconstruction depends partly on bony attachment evidence. <A new mammaliaform from the early Jurassic and evolution of mammalian characteristics by Zhe-Xi Luo and others>[Science 292, pp. 1535–1540; Hadrocodium skull and jaw comparisons]{ref}
The inherited mammalian package also includes <diphyodonty>, <hair> and <lactation>, although the latter two leave much poorer evidence in these particular <fossils> than do <teeth> and <jaws>. Precise adult <dental occlusion> is easier to maintain when repeated replacement ceases. The lineage subsequently passed through <Theria>, <Eutheria> and the ancestors of living <placental mammals>. These nested relationships matter: a <Cretaceous> <eutherian> is not automatically a member of the living placental <crown group>, and neither the first known <fossil> nor the date of a <tooth> innovation fixes the origin of an entire group.
The immediate terrestrial context of whale evolution is within <Artiodactyla>, broadly circumscribed to include <Cetacea>. Living <hippopotamuses> are the closest living relatives of <cetaceans>, not their ancestors. Some early whale <teeth> suggested relationships with extinct <mesonychians>, but dental resemblance alone was misleading. Early whale skeletons preserve the characteristic artiodactyl double-pulley <astragalus>, providing a powerful independent anatomical test of the relationship inferred from molecular evidence. The relevant comparison is the detailed construction of the ankle, not simply whether an animal has hooves or eats meat. <Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls by J. G. M. Thewissen and others>[Nature 413, pp. 277–281; pakicetid postcranial skeletons]{ref} <Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan by Philip D. Gingerich and others>[Science 293, pp. 2239–2242; protocetid ankle anatomy]{ref}
Early <Eocene> <pakicetids>, roughly 50 Ma, combine a largely terrestrial, weight-bearing skeleton with diagnostic whale ear-region <anatomy>, notably the thickened medial wall of the <auditory bulla> called the <involucrum>. Thus identification as a whale does not imply an obligately marine animal. <Ambulocetus>, roughly 48 Ma, has a different combination: robust limbs still useful around shore, large feet and an axial skeleton suited to substantial movement in water. Reconstruction of amphibious locomotion uses limb proportions, joint surfaces and vertebral <anatomy> together; it does not require assuming that its swimming exactly resembled an otter's. These are successive grades represented by branching relatives, not proof that a particular <pakicetid> gave birth to the known <Ambulocetus> specimen. <From Land to Water: the Origin of Whales, Dolphins, and Porpoises by J. G. M. Thewissen and others>[sections on pakicetids, ambulocetids, protocetids and basilosaurids]{ref}
Middle <Eocene> <protocetids> further document reduction of terrestrial support and increasing commitment to swimming. Changes in the connection between <pelvis> and <vertebral column>, limb proportions and the position of external <nostrils> occurred in different combinations. The ankle of <Rodhocetus> is especially informative about ancestry even while other parts of its skeleton document aquatic specialization. It would be unjustified to draw a preserved <tail fluke> onto <Rodhocetus>: a reconstructed swimming mode must distinguish actual preserved <anatomy> from inferred soft parts. <Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan by Philip D. Gingerich and others>[Science 293, pp. 2239–2242; protocetid ankle anatomy]{ref}
Aquatic hearing changed alongside locomotion. Enlargement of the mandibular sound-reception region and changes in the middle ear support a transition towards receiving underwater sound through the lower <jaw>, with increasing acoustic separation of the ears from the <skull>. Early whale lineages display different intermediate combinations, so the presence of an <involucrum> alone does not imply a modern dolphin hearing system. Reconstructing soft sound-conducting tissues requires comparison with living <anatomy>; the <fossil> preserves their bony context rather than the entire pathway. <Eocene evolution of whale hearing by Sirpa Nummela and others>[Nature 430, pp. 776–778; sound transmission reconstructed from early whale ear and jaw anatomy]{ref}
By the later <Eocene>, <basilosaurids> were fully aquatic. The <forelimbs> functioned as <flippers>, the <vertebral column> and tail supplied propulsion, and the <hind limbs> were far too small to support terrestrial walking. Nevertheless, the reduced limbs of <Basilosaurus> retain recognizable limb and foot elements. Loss of a locomotor role is not equivalent to absence of all function; reproductive or muscular roles are compatible with reduction, although a specific behaviour cannot be read directly from a bone. The transition therefore involved changing proportions and connections as well as losses. <Hind limbs of Eocene Basilosaurus: evidence of feet in whales by Philip D. Gingerich and others>[Science 249, pp. 154–157; reduced pelvis and hind limbs]{ref}
Finally, the living branches acquired different specializations. <Odontoceti> retain <teeth> and evolved specialized sound production and <echolocation>; <Mysticeti> evolved <baleen> and lack functional adult <teeth>. <fossil> toothed <mysticetes> and degraded enamel-related <genes> show that modern toothlessness is derived. Palatal vascular features have been used to infer an intermediate combination of <teeth> and <baleen>, but such foramina do not preserve the <baleen> itself, and the exact sequence of feeding behaviours is less secure than the transition from toothed ancestry. <Morphological and molecular evidence for a stepwise evolutionary transition from teeth to baleen in mysticete whales by Thomas A. Deméré and others>[Systematic Biology 57, pp. 15–37; palatal foramina, tooth characters and enamel-gene pseudogenes]{ref} The resulting history explains why a whale can have a mammalian <jaw> and reproductive system, an artiodactyl ancestry, a radically modified swimming skeleton, and a feeding apparatus that evolved after the aquatic transition. \b[The strongest reconstruction combines nested anatomical characters with independently dated <fossils>, while treating soft-tissue and behavioural reconstructions as hypotheses.]
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