Pectoral evolution: from placoderms to pterosaurs

Once again,
a picture (Fig 1) tells the story better than text can.

When I gave placoderms tetrapod homologies,
(Fig 1, e.g. clavicle, interclavicle, coracoid and scapula), I meant they were indeed tetrapod homologies, not just convenient shorthand for vaguely similar structures.

Here are pectoral elements from placoderms
compared to descendant taxa with homologous pectoral elements colored the same (Fig 1).

The missing element: the sternum,
whether paired (in archosaurs) or single (in lepidosaurs and therapsids by convergence) is a novel ossification in all three cases.

In therapsids the sternum begins as an extra ossification at the base of the interclavicle. These multiply in derived taxa producing a long line of sternal elements, often with the medial ends of the dorsal ribs attached.

In lepidosaurs (Fig 1) the sternum is broader and dorsal to the interclavicle, but still anchoring the medial ends of dorsal ribs.

In archosaurs paired sternae appear in dromaeosaurids, sauropods and other dinosaurs. Sternae disappear in Solnhofen birds. A new single flat sternae with elongate posterior processes appears in most Cretaceous birds. Following a slightly different pattern, new sternae fuse to form deep ventral keels with posterior processes in ichthyornithids and crown birds by convergence.

Plastron and gastralia
appear to be dermal elements arising in belly skin, also by convergence.

Gastralia appear in the earliest reptilomorph, Tulerpeton, originally as a series of belly scales in a chevron pattern. Since reptiles carry larger eggs and lose the lumbar ribs (primitively) to allow for lateral torso distension, the appearance of precursor gastralia at abut the same time might be more than just a coincidence.

Hard-shell and soft-shell turtle ancestors among the small horned and herbivorous pareiasaurs lack gastralia, but hard-shell and soft-shell turtle descendants both develop a robust plastron independently and in parallel.