The anterior caudal ribs (aka: transverse processes) of basal tetrapods
are bent back. Their tips point to the end of the tail.
The anterior caudal ribs of derived tetrapods are transverse.
They stick straight out to the sides.
There are exceptions and transitions
(based on taxa recovered in the large reptile tree).
Among the Lepidosauromorpha –
- Eunotosaurus (alone, but many sisters are skull-only taxa)
- Pareiasaurs + Turtles
- Lepidosauriformes – (except Rhyncosaurs as a reversal with curved back ribs)
Among the Archosauromorpha –
- Therapsida
- Mesenosaurus (alone, so far)
- Broomia (alone, so far)
- Marine and Terrestrial Younginiformes (beginning with Galesphyrus) This clade includes all enaliosaurs and archosauriformes.
So what’s going on here?
I don’t know. It’s just important to note the patterns and exceptions. Straight out anterior caudal ribs are shown to be a convergent trait several times over. So are bent back ribs.
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At a guess — backswept is heritage of fish–lateral undulation through trunk and tail, and likely continued as simple unmodified metameric pattern. Transverse in cases you show, probably modifications of femoral muscles [with relative immobility of tail base.]