Acerosodontosaurus piveteaui, an aquatic Tangasaur from the Late Permian (Sakamena Formation) of Madagascar, and a good representation of a typical primitive diapsid (or "eosuchian"). Yet even the fossil remains of this rather unspectacular lizard-like creature were instrumental in overturning the Younginiformes hypothesis and showing that these early Diapsids didn't form a simple monophyletic lineage after all. Illustration by Nobu Tamura (Wikipedia)
This section covers a set of taxa which bridge the gap in reptile history between the amniote radiation, some time in the middle of the Carboniferous, and the flowering of the archosaur and lepidosauromorph clans in the Triassic.
Although the early Eureptilia (or "Romeriida") are rather poorly known, they represent a set of taxa so wildly diverse that we can safely conclude that nothing quite like this extended family has fit under so small a roof in the entire domain of vertebrate phylospace. Many were vaguely lizard-like terrestrial forms, such as Araeoscelis and Youngina. At the other extreme, there were the ichthyosaurs, which were the most completely
water-adapted tetrapods ever to evolve (however, more recently some phylogenies place the ichthyosaurs as more derived diapsids). In between, there are many shades
of amphibiousness represented by Spinoaequalis, the Tangasaurid Younginiformes (see above illustration), Claudiosaurus and their cousins. There were even eureptiles which experimented with flight -- or at least gliding -- among the coelurosauravids. ATW, MAK101002
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