out at the same time as the non-avian coelurosaurs. Morphology Anatomy Tetanurans have two basic skull morphologies. The first skull type, typical in large theropods such as Allosaurus, is common within ceratosaurs and may be primitive for tetanurans. In this type, the skull is about three times longer than tall, with a blunter snout and frequent elaborations such as horns or spikes along the lacrimals, nasals, and frontals. In the second skull type, the skull is lower and longer, with a less elaborated skull roof and a more elongated snout. Shared tetanuran features include the maxillary fenestra (an opening in the antorbital fossa), a pneumatic excavation in the jugal, and the position of the maxillary teeth anterior to the orbit. The posterior skull is little modified in tetanurans, except within Spinosauridae. In the postcranial skeleton, tetanurans transition between the most primitive theropod morphologies in basal tetanurans towards more derived, bird-like states in coelurosaurs. Most tetanurans possess specialized wrist bones, the absence or reduction of the fourth digit of the hand, a strap-like scapula, stiffened tails, and a laminar astragalar ascending process. Advanced tetanurans would have possessed a sophisticated air-sac-ventilated lung system similar to birds, and an advanced circulatory system. In megalosaurids and allosaurids, the orientation of the femoral head is anteromedial such as in ceratosaurs, but in avetheropods this orientation is fully medial. Tetanuran locomotor morphology is relatively generalized, with few variations between taxa. The hands of tetanurans, unlike those of the closely related ceratosaurs, lack a fourth finger. Early tetanurans still possessed metacarpal IV, but it was vestigial and not part of a finger anymore. The tridactyl manus was preserved in birds. Evidence from the basal ceratosaur Saltriovenator indicates the tetanuran digits are I, II and III instead of II, III and IV. Body Size Basal tetanurans were the first theropods to achieve truly giant body sizes, with both megalosauroid and allosauroid taxa weighing over 1 ton. Sequential temporal appearances of large body size in subsequent clades suggest a pattern of size-cycles, with the extinction of incumbent giant forms allowing for replacement with a new, more bird-like theropod group that then also evolved giant body size. It is, however, possible that more than one giant tetanuran existed at a time in the same paleoenvironment, perhaps with feeding habit variations. Within most dinosaur clades, body size tended to increase over time along a lineage according to Cope's rule. Coelurosaurian theropods