Casineria kiddi (Paton, Smithson & Clack 1999) Visean, Mississippean, Carboniferous, ~335 mya was a small basal reptile and the most primitive known taxon in the Archosauromorpha clade. It was derived from a sister to Gephyrostegus and Cephalerpeton. Brouffia was a sister taxon.

Overall smaller than and distinct from Gephyrostegus, the skull of Casineria had no otic notch. See Brouffia for more possible skull details.

The cervicals of Casineria were increased in number but decreased in size. The presacral vertebral count had increased to over 30. Ribs discontinued after #22. Apparently two vertebrae formed the sacrum and were connected to the pelvis.

The pectoral girdle was composed of unfused elements. The humerus had a small hourglass shape. The manus was enlarged.

The ilium had no dorsal process. The femur was more gracile. The pes was reduced, more nearly the size of the manus.

Tiny Casineria lies at the end of a phylogenetic series of decreasing size beginning with Proterogyrinus and continuing with Silvanerpeton and Gephyrostegus among the basal Tetrapoda. Smaller animals generally mature faster and thus reproduce more often, although their lifespans are also typically shorter. The fact that Casineria is so much smaller than Gephyrostegus is a key factor in the origin of the Reptilia (Carroll 1970). The first eggs provided with an amniotic membrane were probably small and laid by small adult females who lived and laid eggs in moist leaf litter, a transitional environment that stayed humid and protected both the adult and the egg from desiccation.

An unknown predecessor to Casineria and its sister, Cephalerpeton, invented the amniotic membrane, which surrounds and protests the developing embryo in all reptiles (including birds and mammals). The eggs of Cephalerpeton and Cassineria were more or less protected from desiccation and so could be laid away from water, but probably in damp leaf litter and moss rather than any hot and arid place. There is no fossil evidence that an eggshell was created at this node.

The term Amniota is commonly and scientifically used to encompass all reptiles, birds and mammals, but in the present tree, the inclusion set of taxa within the Amniota is redundant with the Reptilia. Hence the more recent term, Amniota, can now be dropped from usage.

Casineria is not the oldest reptile in the Archosauromorpha, but it is the most primitive. It shares more traits with Gephyrostegus and Brouffia than any other taxon in that clade.

The new Archosauromorpha, are all those taxa sharing a more recent common ancestor with Archosauria (crocs and birds), than with Lepidosauria (Gauthier 1986).

There is a strong trend to insectivory and carnivory in the earliest archosauromorpha opposite the trend of herbivory in lepidosauromorphs. This is not to say that all were one way or the other without overlap.

Strong wide skulls and broad bodies with short toes in basal captorhinids, caseasaura and milleretids contrast with the canine teeth, tall skulls and long toes found in archosauromorphs.

The tree at left is not updated. The complete updated tree is here.