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A therapsid belonging to the group containing mammaliaforms and close extinct relatives. Enlargement of the dentary, differentiation of teeth and changes in the palate record a mosaic of feeding transformations.

An early mammaliaform whose jaw preserves both articular–quadrate and dentary–squamosal articulations.

Living cartilaginous fishes are specialized descendants, not a surviving anatomical stage before the origin of bony vertebrates. fossil discoveries have shifted several apparently “bony-fish” features toward the ancestry of all living jawed vertebrates. Their absence in living Chondrichthyes can consequently represent character loss, while other shark features remain ancestral or are new specializations. The change is a revision of character polarity, not a claim that every aspect of shark anatomy is reduced.

A simple comparison of living sharks with Osteichthyes can suggest a one-way sequence: an ancestor with a cartilaginous internal skeleton, followed by origin of extensive bone on the bony-vertebrate branch. That inference is insecure because the two living sister lineages can both have changed, and living jawless outgroups can also have lost structures. The fossil record samples extinct combinations near the relevant divergences. To determine whether a structure is a gain or loss, one must place those combinations on a rooted phylogenetic tree and distinguish anatomical absence from poor preservation.

The early gnathostome problem involves four familiar assemblages: placoderms, acanthodians, Chondrichthyes and Osteichthyes. Treating all four as separate complete clades can conceal transitions occurring along the stems of surviving groups. In particular, many newer analyses place acanthodians as successive branches on the chondrichthyan stem. Their name “spiny sharks” is not itself evidence for that placement; cranial, scale, fin and other characters must support it. If this topology is correct, the large difference between living sharks and early armoured jawed fishes includes a history of reduction within the cartilaginous-fish lineage.

The 2012 redescription of the Acanthodes braincase illustrates why anatomical interpretation and tree position must be kept separate. It revealed a mixture of conditions previously assigned to different living groups and argued for shark-like aspects of the last common ancestor of modern jawed vertebrates. Its analysis still placed Acanthodes on the osteichthyan stem, rather than treating every acanthodian as a stem chondrichthyan. Thus a resemblance initially interpreted as an osteichthyan specialization could instead be an ancestral feature retained in more than one lineage. Later hypotheses should not be projected retrospectively onto that study.^[ref]

Entelognathus primordialis, described in 2013 from the Silurian of China, supplied a particularly important association: an armoured placoderm-like body and skull with large marginal jaw bones identifiable as premaxilla, maxilla and dentary. Such bones had been treated as characteristic innovations of Osteichthyes. Placement of this animal near the top of the jawed-vertebrate stem, together with recovery of acanthodians on the chondrichthyan stem, instead supported a macromeric, extensively ossified dermal ancestor. On that reconstruction, the small-scale-dominated covering and absence of those large jaw bones in living Chondrichthyes reflect reduction and reorganization rather than simple retention of a primitively unossified condition.^[ref]

Janusiscus schultzei, described in 2015 from the Early Devonian, reinforced the point using a different anatomical region. Its combination of a bony dermal skull roof and informative internal cranial conditions supported a position outside crown gnathostomes. Braincase anatomy therefore did not align neatly with the traditional picture of a primitive shark head versus a derived bony-fish head. Together, these fossils move apparently osteichthyan-like states closer to the common ancestry of both surviving groups. Their significance comes from character combinations and placement, not from choosing whichever living animal looks most similar.^[ref]

A separate question concerns internal skeletal tissue. dermal bone, perichondral bone, endochondral bone and calcified cartilage must not be conflated. endochondral bone forms during replacement of a cartilaginous template; perichondral bone forms around it; calcified cartilage remains a different tissue despite being mineralized. Therefore a bony dermal skull roof alone cannot establish endochondral ossification in the same animal.

The 2020 description of Minjinia turgenensis, from the Early Devonian of Mongolia, addressed that distinction. micro-computed tomography revealed extensive internal trabecular tissue in the braincase, interpreted as endochondral bone, rather than just an external bony shell. Its recovery close to the crown divergence permits endochondral ossification to be reconstructed before the separation of Chondrichthyes and Osteichthyes, followed by loss or marked suppression along the chondrichthyan lineage. This conclusion depends on identification of the tissue and the animal's position; the absence of a more complete skeleton and uncertainty among early jawed-vertebrate trees constrain how strongly it can be generalized.^[ref]

The resulting inference is conditional but testable. If well-supported stem relatives and the bony-vertebrate branch possess a particular homologous tissue, while living cartilaginous fishes lack it, an ancestral presence followed by loss can fit better than several independent origins. Moving a crucial fossil to another branch, changing a tissue identification or allowing different transition rates can change the ancestral reconstruction. maximum parsimony is useful but is not proof that losses and gains have equal biological probability. Character coding should record unknown preservation as unknown, rather than use an unobserved delicate feature as evidence of absence.

The major revision is from a ladder of increasing ossification to a branching history of gains, retention and losses. It concerns particular tissues and anatomical elements: reduction of large dermal bones need not coincide exactly with loss of endochondral ossification. fossils reveal that apparently simple modern anatomy can be secondarily simplified, and that major living-group differences may reflect transformations on both sides of a divergence. Continued uncertainty over placoderm relationships and individual tissue homologies defines the comparisons still needed; it does not restore living sharks as unchanged proxies for the jawed-vertebrate ancestor.

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 participating in the mammalian dentary–squamosal jaw articulation.