Herrerasaurus

Herrerasaurus was a relatively light biped predator with a long tail and a rather small head . The skull of adults reached 56 cm with a relative body length of 6 m ^[6] and an estimated weight of about 650 kg ^[10] . Smaller individuals were about half as large; the length of the skull reached approximately 30 cm ^[11] .

Herrerasaurus was completely bipedal . The femur is relatively short, on the contrary, the tibia is quite long, which indicates the possibility of a fairly quick run. The foot had five fingers , but only three middle ones (II, III, and IV) were fully developed. The other two (I and V) did not carry the load from the body, are shifted to the sides and have only reduced claws . The tail was strengthened by the processes of the vertebrae and played the role of a balancer when walking and running ^[11] . The length of the forelimbs is less than half the length of the hind limbs. The humerus and forearm are relatively short, while the hand is elongated. On the first three fingers, large bent claws, probably intended to capture and hold prey. The fourth and fifth fingers are small and devoid of claws ^{[6] [12]} .

Herrerasaurus possesses traits characteristic of various groups of dinosaurs, as well as traits inherent in archosaurs not related to dinosaurs. The main features that are not characteristic of dinosaurs are various anatomical aspects in the structure of the bones of the hind limbs. The pelvis is similar to the pelvis of dinosaurs, but the acetabulum is partially covered by a bone crest. The ilium is supported only by two sacral vertebrae , which is a basal symptom ^[11] . However, the pubic bone is directed back, which is a progressive sign inherent in birds and dromaeosaurids . In addition, the end of the pubic bone is elongated anteriorly from the posterior side, which is similar to the shape of the same bone in representatives of Avetheropoda ; the vertebral body has an hourglass shape similar to the vertebrae of a later allosaurus ^[10] .

The skull of Herrerasaurus is long and narrow, devoid of almost all the specializations characteristic of later dinosaurs and resembling the skulls of more primitive archosaurs, such as Euparkeria ^[13] . There are seven pairs of openings in the skull: a pair of mandibular openings, a pair of temporal windows, a pair of overhanging windows, a pair of eye sockets , a pair of nostrils, a pairantorbital holes and a pair of small (about 1 cm in length) slit-like proxillary holes ^[14] .

Herrerasaurus possessed a movable jaw joint, which made it possible to make sliding movements back and forth by the lower jaw ^[13] . This feature is unusual for other dinosaurs, but independently developed in some lizards ^[15] . In the back of the lower jaw is the mandibular foramen. The teeth are large and serrated, adapted for eating meat; the neck is slender and flexible ^{[13] [16]} .

Fernando Novas (1993) distinguishes the genus on the basis of the following features ^[17] : the presence of the mandibular foramen, the infratemporal tapers back so that the posterior part is less than a third of the size of the anterior, the presence of a crest on the lateral surface of the zygomatic bone , the supratemporal canal runs along medial sorbital process, the lower posterior process of the scaly bone has a lateral recess, the square-zygomatic bone overlaps the posterior face of the square bone , the pterygoid process of the square bone carries a cavity on the ventral edge, the posterior dorsal process of the lower jaw is thin and ribbed, the suprangular bone has a bifurcated appendix for connection with the posterior dorsal outgrowth of the lower jaw, the internal tuberosity of the humerus is concentrated in the region of the joint with the forearm and is separated from the head by a deep notch (a trait characteristic of Coelophysoidae ), the hands are elongated and extended, accounting for about 60% of the length of the rest forelimb entepicondyl crest- shaped , with anterior and posterior hollows, the posterior border of the outgrowth of the ilium forms a right angle with the upper part of the bone shaft on the ilium.

Previously, it was believed that the herrerasaurus belongs to the basal lizards, close to the common ancestors of theropods and sauropodomorphs . Some characteristics of this dinosaur - large claws, teeth, and features of the pelvic bones - indicate its belonging to theropods. Many paleontologists, however, did not agree with such a classification and pointed to the absence of the key, in their opinion, signs of theropods in the herreraosaurus (for example, vertebral cavities intended for air sacs ) ^[18] . However, a study of the Tawa hallae fossils finally establishes a direct link between theropods and the Herreraosaurus .

A closely related, but smaller stavricosaurus ( Staurikosaurus pricei ) was found in Brazilian deposits of the same age with the Herrerasaurus. The Hindesaur ( Chindesaurus bryansmalli ) from the Carnian - Norian tier of Texas also belongs to herrerasaurids.

Cladogram

• https://web.archive.org/web/20160804223609/http://dinogosti.ru/errerazavr/
• Dinosaur World - Herrerasaurus ischigualastensis - triassic dinosaur
• https://web.archive.org/web/20070711145402/http://www.palaeos.com/Vertebrates/Units/340Theropoda/340.100.html (unavailable link from 11/09/2013 [2208 days])
• https://web.archive.org/web/20100722075630/http://www.palaeos.com/Mesozoic/Triassic/Carnian.600.htm#Herrerasaurus (unavailable link from 09/11/2013 [2208 days])