Palaeos website (to which many links on these pages point to anyway. More info here
During the later phase of the early Triassic, the first thecodonts, such as Protorosuchus, gave rise both to gigantic forms (Erythosuchids) and tiny ones (Euparkerids).
Erythosuchus africanus was five metres in overall length, with a stocky body and an enormous head a metre in length. This creature, known from the Cynognathus Zone of South Africa (similiar relatives have been found in contemporary rocks in European Russia and in China) represented the culmination of early Thecodont evolution. It was obviously the largest creature of its day, being the size of a large crocodile, and must have been a fearsome preditor upon contemporary large herbivores, such as the abundant, bullock-sized Kanneymerid dicynodonts.
There is a strong similarity between the Erythosuchids of the early Triassic and the dinocephalian (primitive Therapsid) Anteosaurs of the Mid-Permian. In both there was a strong growth in body size, but also an even greater corresponding growth in head-size, which became fully half the length of the body excluding the tail. As with the Labyrinthodont amphibians, the head becomes truely enormous relative to the body. But whereas the Labyrinthodonts had weak jaw and neck muscles, and had to support the weight of their great heads by the bouyancy of water, these great carnivores held their heads aloft on massive necks, and their huge teeth and tremendous jaws would have enabled them to feed easily on other animal prey.
Yet these animals, the largest predators of their time, were too heavy and clumbsy to be able to actively pursue and run down prey. Like the modern crocodile or komodo-dragon lizard, they would have been ambush-preditors, lying in wait for prey behind thick vegetation, then rushing out in a short burst of speed, and quickly overwhelming the helpless victim with their tremendous jaws.
ERYTHOSUCHUS AFRICANUS
Family
Formation: Cynognathus Zone; South Africa
Length: 4.5 - 5 metres Weight:
Habitat: Possibly semi-aquatic
Food: Other large animals
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