Herrerasaurus ischigualastensis (Reig 1963) Ladinian, middle Triassic ~230 mya, 3m in length was derived from a sister to Trialestes and Gracilisuchus at the base of the Archosauria. Herrerasaurus preceded members of the Dinosauria, including Pampadromaeus at the base of the Phytodinosauria and Coelophysis at the base of the Theropoda.
Distinct from Trialestes, the skull of Herrerasaurus had a smaller antorbital fenestra. The naris was small and placed high on the rostrum. The lateral temporal fenestra was more open as the quadrate and squamosal retreated. The mandible had a larger fenestra straighter tip.
Distinct from Gracilisuchus, the skull was taller and narrower posteriorly. The squamosal did not contact the jugal. Note the similarity of the medially meeting pterygoids.
Distinct from Gracilisuchus the cervicals were smaller and the cervical ribs were longer. The neural spines were taller, especially near the pelvis. The caudals tapered distally.
The coracoid and ventral scapula were enlarged. Manual digits 4 and 5 were vestiges.
The pubis was deeper and angled posteriorly with a large "boot." The ilium was taller. The ischium was more gracile. The hind limb was more robust. The calcaneal heel was absent. The metatarsals were shorter. Digit 5 had at least one phalanx.
The premaxilla as reconstructed above is different than the traditional reconstruction.