The genesis of reptiles and the amniotic egg - This is Gephyrostegus watsoni (Brough and Brough 1967), a basal reptile from the Westphalian (310 mya, with origins in the Viséan, 340 mya). Formerly considered a non-reptile tetrapod, G. watsoni had a deep pelvis, deep enough to lay large amniote eggs. It lacked posterior dorsal ribs, so gravid females could carry a large cache of amniote eggs. And it was found with at least seven large crushed sphere-shapes, perhaps the most primitive examples of amniote eggs (light blue above hips). No eggshell was present here, but each egg had must have had an amniotic membrane, a trait shared by all living amniotes, including reptiles and their descendants the birds and mammals. And the first reptile (amniote) has been identified, the Devonian Tulerpeton.
Welcome to the study of reptile evolution - Here you'll be able to trace the lineage of major clades, including the line that ultimately led to humans. You'll see where and when body parts (= traits) became added, substracted and modified. Most of the evolutionary links have been made and many of the mysteries and mistakes of the past have been solved and corrected with the large reptile tree. The difference, in this case, is that many more genera were included than ever before (originally 224, now 955+), but the current tree can be recovered with as few as 60 taxa). Prior studies assumed certain relationships, several of which turned out to be mistakes when seen in a larger, more complete context. References and links are provided at the bottom of each page.
And then there's the pterosaur tree with 232 pterosaurs and their ancestors.
Every taxon listed here, unless it represents the last of its lineage or still survives, can be considered a transitional taxon. Notably, smaller, plainer specimens are typically found at the bases of major clades. Larger spectacular specimens are usually evolutionary deadends.