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The incus is a mammalian middle ear ossicle linking the malleus with the stapes in the sound-transmission chain. It is homologous to the ancestral quadrate. This historical relationship connects a component of jaw articulation with a later auditory function.

The malleus is a mammalian middle ear ossicle coupled to the tympanic membrane and the incus. It is homologous to the ancestral articular. Comparing those positions and developmental relationships helps reconstruct the jaw-to-ear evolutionary transition.

The four names identify distinct pathways: chorda tympani, the sensory buccal nerve, the radial nerve formerly called musculospiral, and the obturator nerve. Origin must include the parent nerve or nerve plexus, while the function must follow the actual fibre supply rather than the name of a structure crossed.

Chorda tympani. This branch leaves the facial nerve in its descending facial canal before the stylomastoid foramen. It enters the tympanic cavity from behind, passes forwards between the malleus and incus close to the inner surface of the tympanic membrane, and leaves through the petrotympanic fissure. In the infratemporal fossa it joins the lingual nerve, a branch of the mandibular division of the trigeminal nerve. It then travels with that carrier towards the tongue and submandibular ganglion.

The chorda tympani carries taste from the anterior two thirds of the tongue, with sensory cell bodies in the geniculate ganglion and central connections to the nucleus of the solitary tract. It also carries preganglionic parasympathetic fibres from the superior salivatory nucleus towards the submandibular ganglion; postganglionic fibres supply the submandibular gland and sublingual gland. It does not provide the general touch supply of that part of the tongue; that travels through the lingual nerve's trigeminal nerve pathway. Taste and tactile testing can consequently distinguish two functions that briefly share one peripheral trunk.^[ref]

Buccal nerve. In the usual anatomical meaning intended by this list, this is the long buccal nerve from the anterior division of the mandibular trigeminal nerve. It commonly emerges between the heads of the lateral pterygoid muscle, descends near the anterior border of the mandibular ramus and the temporalis muscle insertion, and passes towards the cheek. Its branches reach cheek skin, the buccal mucosa, and the buccal gingiva beside the lower molars. It is principally sensory. The motor supply of the buccinator muscle comes from the facial nerve, whose buccal branches are a different set of nerves. Thus the ability to hold the cheek against the teeth and sensation in the cheek are not tests of the same efferent pathway.^[ref]

Radial nerve, historically musculospiral. This is the major continuation of the posterior cord of the brachial plexus, usually containing fibres from C5–T1. It passes from the axilla into the posterior arm, accompanying the profunda brachii artery in the radial groove of the humerus. It pierces the lateral intermuscular septum and reaches the anterior compartment between brachialis and brachioradialis. Near the lateral elbow it divides into a superficial sensory branch and a deep motor branch. The latter passes through the supinator muscle and continues as the posterior interosseous nerve.

The radial nerve supplies triceps brachii and anconeus, then brachioradialis, the radial wrist extensors and, through its deep continuation, the principal finger and thumb extensors and associated posterior forearm muscles. Its sensory branches supply posterior arm and forearm regions and part of the dorsolateral hand. A lesion's level matters: an arm lesion can impair wrist and finger extension, while an isolated superficial branch lesion is sensory and does not itself cause a wrist drop. Conversely, a posterior interosseous nerve lesion chiefly affects motor function. The fingertip pads are not the characteristic autonomous sensory territory of the superficial radial nerve.^[ref]

Obturator nerve. It arises from the anterior divisions of the anterior rami of L2–L4 in the lumbar plexus. It emerges from the medial border of psoas major, crosses the pelvic side wall and leaves through the obturator canal with the obturator vessels. Its anterior and posterior divisions characteristically run on opposite sides of adductor brevis, although the level of division varies. The anterior division supplies adductor longus, gracilis and commonly adductor brevis; the posterior division supplies obturator externus and the adductor portion of adductor magnus, with variable additional contributions. The hamstring portion of adductor magnus has a different supply. Cutaneous supply to a variable medial-thigh patch and articular branches to the hip and knee supplement its principal motor role in thigh adduction. Obturator internus is supplied by its own sacral-plexus nerve, not by the obturator nerve merely because the names resemble one another.^[ref][ref]

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. ^[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. ^{[ref] [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. ^[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. ^[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. ^[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. ^[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. ^[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. The strongest reconstruction combines nested anatomical characters with independently dated fossils, while treating soft-tissue and behavioural reconstructions as hypotheses.

A skull bone forming the upper component of the ancestral jaw joint, homologous to the mammalian incus.