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\b[<Sarcopterygii> is the <clade> comprising the most recent common ancestor of living <coelacanths>, <lungfish> and <tetrapods>, together with all its descendants.] In this crown-based usage, <humans> are <sarcopterygians>: acquisition of <limbs> and loss of a fish-like outline do not remove descendants from a <clade>. Extinct taxa assigned along the stem require an explicitly broader total-group usage. A diagnosis lists characters useful for recognizing a group; a <synapomorphy> is a shared derived character whose position has been inferred on a <phylogenetic tree>. Those are related but not interchangeable statements.
Two particularly instructive traditional diagnostic features are the fleshy, internally supported <paired appendages> and true <enamel> associated with the dentition. The first survives conspicuously in our arms and legs. The second survives in our <tooth> crowns, although modern evidence requires qualification of its traditional status as an exclusively <sarcopterygian> innovation.
\b[The paired-appendage transformation.] A lobed <fin> contains musculature and a substantial <endoskeleton> extending into the appendage. In the relevant ancestral pattern, a single proximal skeletal element connects to the girdle: the <humerus> in the pectoral appendage, and the <femur> in the pelvic appendage. Subsequent elements elaborate the jointed internal axis. The important comparison is historical organization and articulations, not merely that both a <fin> and an arm contain <bone>. <fins> in other lineages also possess internal supporting elements, so the presence of any <fin> skeleton is too broad a diagnosis.
Within the lineage leading to <tetrapods>, the pectoral series can be recognized as <humerus>, <radius bone> and <ulna>, followed by increasingly elaborate distal elements. <Tiktaalik> retains <fin rays> but has a distal jointed skeleton capable of limb-like positioning against a substrate. Its combination of characters shows that parts of a wrist-like support system preceded a fully digit-bearing hand. Comparing several articulated specimens and the geometry of joint surfaces constrains the reconstruction better than matching isolated <bones> by outline.<The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb by Neil H. Shubin and others>[Pectoral anatomy, joint configurations and proposed substrate-supported postures]{ref}
Further changes included <anatomical digits>, loss of external <fin rays> in the <limb>, changes to the girdles and altered force transmission through the trunk. They did not all arise simultaneously. Early digit-bearing <tetrapods> were not uniformly five-fingered terrestrial walkers; aquatic settings and <polydactyly> make it misleading to present the modern <human> hand as the immediate purpose or original endpoint of the transition. <Acanthostega>, with eight <anatomical digits> in its <forelimb>, supplies a concrete counterexample to an ancestrally universal five-digit condition.<Polydactyly in the earliest known tetrapod limbs by Michael I. Coates and Jennifer A. Clack>[Digit counts and implications for the ancestral tetrapod limb]{ref}
Our own upper <limb> retains the proximal single-bone and paired-bone organization, but the <shoulder>, <elbow>, <wrist> and <fingers> are extensively specialized. Rotation of the <radius bone> relative to the <ulna> contributes to turning the forearm; an opposable <thumb> and independently controlled <fingers> support grasping. In the lower <limb>, the <femur>, <tibia> and <fibula> show the corresponding serial organization, while the <pelvis>, <hip>, <knee> and <foot> transmit loads in habitual <bipedalism>. These are derived modifications within <tetrapods>, rather than defining traits of all <Sarcopterygii>. Comparing our arm with a fish pectoral <fin> concerns <biological homology> between organisms; comparing arm and leg also involves <serial homology> within the body.
\b[The dental transformation and its qualification.] <human> <tooth> crowns have an outer layer of <enamel> over <dentine>. <enamel> is highly mineralized and is produced by epithelial <ameloblasts>, whereas <dentine> is a different tissue formed by <odontoblasts>. Identifying a <fossil> surface as <enamel> therefore requires tissue architecture and developmental or molecular comparison, not simply a shiny hard covering. In our <teeth> the coating has specialized organization into <enamel rods> and contributes to wear resistance during biting and chewing. <tooth> classes and crown shapes diversify the ancestral feeding apparatus; they are not themselves basal <sarcopterygian> <synapomorphies>. After eruption, loss of the enamel-producing cellular layer severely limits biological replacement of worn crown <enamel>.
The traditional contrast between <sarcopterygian> true <tooth> <enamel> and other hard coverings needs an explicit historical correction. <Qingming Qu> and colleagues combined <fossil> <histology> with evidence for enamel-matrix <genes> in the skin of <gar>. In <Psarolepis>, external <odontodes> carried <enamel> while the <teeth> lacked an <enamel> coating. Their evidence supported an origin in dermal coverings followed by expansion into the mouth, and an ancestry deeper than <Sarcopterygii> alone.<New genomic and fossil data illuminate the origin of enamel by Qingming Qu and others>[Fossil tissue distribution and gar enamel-matrix gene expression]{ref} Thus <enamel> and its oral deployment must be coded separately; <tooth> <enamel> remains a useful traditional diagnostic comparison, but \b[the tissue itself is not a uniquely <sarcopterygian> <synapomorphy>]. Shared tissue can undergo <heterotopy>, loss and later modification.
The two histories illustrate different forms of transformation visible in our bodies: an internally supported paired <fin> became a jointed, digit-bearing, specialized <limb>, and an inherited mineralized epithelial covering acquired a complex role in our dentition. A rigorous diagnosis states the anatomical level being compared and distinguishes inherited organization from later functional specialization. Neither the modern function nor the external <human> appearance determines membership of <Sarcopterygii>.
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