Palaeos website (to which many links on these pages point to anyway. More info here
If the tendency to gigantism was represented by Erythosuchus, then the
opposite, tendency towards the small light agile bird-like form was expressed
by its cat-sized relative Euparkeria, which at 65 centimetres (26 inches)
from snout to tail-tip was only an eighth the linear dimensions.
This was a small, long-legged, and agile, with a more erect stance, and
capable of bipedal gait. Euparkeria in turn gave rise, to two evolutionary
branches - the Psuedosuchians and crocodiles on the one hand, and ornithosuchians
and dinosaurs on the other. Both began as small agile forms, but
the Psuedosuchians quickly reverted to the standard lizard-like form, although
these were lizards of great size - e.g. the parasuchia, aetosauria, and
crocodilia, all of which reached 3 metres or more in length. Most
interesting of all were the great Rauisuchians, which although roughly
lizard/crocodile-like in form had a fully erect dinosaurian and mammalian
posture.
Euparkeria
, image by Richard Hammond
Palaeos Page (incorporates some of this material, plus a lot of additional material)
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Kingdoms of Life > 
Animalia > Vertebrata > Tetrapoda > Reptilia/ Sauropsida > Archosauria > Thecodontia