Lessons learned about the base of the Reptilia – part 1
Earlier we looked at two professional interpretations of the basal reptile, Brouffia, and attempted a third reconstuction here, combining the most reasonable parts of both drawings (not photos or first-hand access) skewing any decisions largely on the absence of palatal fangs and other reptilian traits. When Brouffia became resolved from two taxa to one in the large reptile tree, it nested at the base of the new Archosauromorpha.There was no shuffling of the taxa at the base of the Reptilia or preceding it. It was not a pre-reptile.
Looking at the tree and the character distribution, a few traits grouped together in clades that seem worth mentioning as they help define who and what we (and our reptilian relatives) were at the very beginning.
1. Both Gephyrostegus specimens, the large one and the small one, had a relatively large skull, at least half as long as the presacrals. No other close relatives had this trait, including Brouffia, which had a long, slender torso.
2. The skull width was 1.2 to 2x the skull height in pre-reptiles and the new Archosauromorpha. The skull was wider than twice its depth in the new Lepidosauromorpha.
5. The ventral naris was bordered subequally by the premaxilla and maxilla in pre-reptiles and the new Archosauromorpha. The pattern often, but not always, changed in various new lepidosauromorphs, whether maxilla dominated or premaxilla dominated.
8. Snout constriction (a pinch as seen dorsally) is found in the new Lepidosauromorpha, but not in Cephalerpeton, the basal taxon.
9. The prefrontal separates, just barely, from the postfrontal in all basal reptiles.
10. In pre-reptiles and Brouffia, the preorbital portion of the skull was longer than the postorbital. That reversed in the new Lepidosauromorpha.
11. The dorsal nasal shape includes parallel lateral rims in pre-reptiles and basal reptiles, but not Gephyrostegus.
13. In lateral view the shape of the rostrum is slightly convex in the new Lepiodosauromorpha and the new Archosauromorpha (basal members only), but angled over the naris then convex in pre-reptiles and Brouffia.
14. 15. The ventral rims of the premaxilla and maxilla form an angle that droops the premaxilla in Silvanerpeton (see the new reconstruction) to Brouffia plus the new Lepidosauromorpha.
30. The orbit is at least equal to the rostrum length in Utegenia (leading toward frogs, salamanders and caecelians), Cephalerpeton and Thuringothyris (at the base of the Captorhinidae).
31. The orbit is smaller than the postorbital skull (at its shortest length) in the new Lepidosauromorpha plus Westlothiana.
39. The pineal opening becomes larger than 20% of the parietal length in the new Lepidosauromorpha and the new Archosauromorpha, but not in Brouffia or Westlothiana among forms at the pre-reptile/reptile transition.
More later.
2012-12-22 12:48:03
Source:
