Basal reptiles appear do not have sterna. Neither do they have a sternum. Birds have ’em. We (mammals) have ’em. Lizards have ’em. Crocs and turtles don’t. So what’s the story?
I can’t find sterna within the new Lepidosauromorpha before Saurosternon (Fig. 1), a skull-less, but otherwise completed taxon with long fingers and large feet. This arboreal taxon nests at the base of the Lepidosauriformes and has twin sterna that look like posterior extensions to the coracoids (convergent with metacoracoids in therapsids and araeoscelids).
These sterna fuse to become a sternum in Sphenodon and Homoeosaurus (fig. 2 and presumably their last common ancestor, Gephyrosaurus, but it is no preserved), where they create gliding paths for the coracoids to roll upon in most living lizards. The sternum shifts anteriorly in fenestrasaurs then fuses to the interclavicle and clavicles in Longisquama + pterosaurs where this combo becomes known as the sternal complex. Other than here and in the basal lizard, Huehuecuetzpalli, there is no trace of a sternum in other tritosaurs, including drepanosaurs or tanystropheids. Lizards with legs (including the worm-like Bipes) have a sternum. Those that don’t, including snakes, lack a sternum.
Among the new Archosaurmorpha, there are no sterna until one gets to primitive mammals. The sterna appear as segments growing from the posterior of the very much shortened interclavicle (the anteriormost sternal bone that articulates with the clavicles.) The manubrium (anteriormost sternal bones) appear paired in Bienotheroides, but fused in all others.
The next sternum appears in Araeoscelis (Fig. 3), as a central bone or bones above the elongated interclavicle and between the metacoracoids. Altogether these bones create in immobile pectoral girdle. There is no such sternum in Galechirus, a therapsid which includes metacoracoids. Thadeosaurus has paired sterna. Again creating an immobile girdle. No enaliosaurs have a sternum. The giant coracoids do the job.
Prolacertiformes don’t have sterna. Neither do choristoderes. Neither do any of the basal archosauriformes. The sternum reappears in Archaeopteryx and sterna appear in Velociraptor. In more primitive theropods in situ gaps suggest an unossified sternum was present. In both of these birdy taxa the coracoids had transformed into immobile struts, a morphology indicative of flapping.
Alright, so, the sternum, or sternal bones, are not primitive to reptiles, but develop and disappear independently and convergently in several lineages.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.