This clade's characteristic structure contains the allantois and chorion, which allows for gas exchange.
It's not a class, but a puzzle about the phylogeny of this clade partially hinges on the lack of genetic information on the genus Sphenodon. In members of this clade, the pronephros and mesonephros appear during embryonic development, but the metanephros is the final adult kidney. Unlike ancestors which used buccal pumping, this clade evolved costal respiration. They generally possess at least two pairs of sacral ribs. This clade first appeared in the Carboniferous, and they later diverged into Anapsids, Diapsids, and Synapsids. This clade's characteristic structure contains the allantois and chorion, which allows for gas exchange. Their eggs generally adapted to have hard shells to be better suited to life on land. For 10 points, name this clade which includes birds, reptiles, and mammals and whose name derives from a membrane in their eggs which encases the embryo and a certain "fluid".
It's not a class, but a puzzle about the phylogeny of this clade partially hinges on the lack of genetic information on the genus Sphenodon.
amniotes [prompt on tetrapods; accept diapsids until mention; prompt afterwards]