Sphenosuchia

Sphenosuchia are considered more primitive representatives of the superorder of crocodilomorphs than other groups assigned to this superorder. They are distinguished by their extremely light skeleton and running way of movement, especially the genus Terrestrisuchus , but it is these signs, probably inherited from their ancestors, that can indicate the primitiveness of sphenozuchia ^[2] .

The suborder is represented by medium-sized forms up to 2 m in size, with one notable exception in the form of a large predator of the redondavenator ( Redondavenator quayensis ) with a skull 60 cm long and a total length of at least 4 m. They had skulls with a relatively short muzzle and a small preorbital window, as well as well developed, as in other crocodilomorphs, a system of aerial sinuses associated with the middle ear cavity. Coracoid ^[3] has a posteriorventral process ^[4] .

Some sphenosuchia may have been quadruped, such as the mid-Triassic Argentinean Pseudhesperosuchus , which had a preorbital window, unusually large for sphenozuchia. Other species, such as Gracilisuchus from the Middle Triassic of South America and the slightly larger Terrestrisuchus from the Upper Triassic of England, up to 0.5 m in size, were apparently capable of bipedal movement ^[4] .

Gracilisuchus

This genus, whose representatives met in South America in the Middle Triassic, is one of the oldest in the suborder. In the 1970s, he was attributed to ornithosuchids , in particular, in the works of the American paleontologist Alfred Romer , but in 1981 Brinkman showed that Gracilisuchus differs from all representatives of the aforementioned family in the structure of the tarsus and buccal region, and suggested its kinship with Sphenosuchus and others childbirth suborder Sphenosuchia .

According to most signs of the skeleton, Gracilisuchus resembles tecodonts. Tarsus of the normal crocodile type, the convex lateral surface of the talus enters the groove of the calcaneus . The fifth finger is reduced, and the proximal end of the fifth metatarsal ^[5] does not retain a primitive hook-like configuration. The pubic bones are long and, apparently, do not participate in the formation of the acetabulum ^[6] .

Coracoids are short, the preorbital window, as with most tecodonts, is large and is surrounded by an extensive recess.

The structure of the buccal region convincingly indicates a relationship with crocodiles. The dorsal end of the square bone is bent forward, and a wide edge of the scaly bone hangs over it. The parietal bones merge behind.

The dermal carapace is located along the entire length of the spinal column. About one of its plates falls on one segment. Elements of two rows are serrated in a midline. The cervical ribs are very similar to those of modern crocodiles: they have two distinctly separated heads and a short, rearward directed rod, overlapped in front by the end of the previous rib. There are seven cervical vertebrae, the next one carries ribs of an intermediate structure.

Despite a number of characteristics similar to crocodiles in the structure of the skull, most features of the skeleton connect Gracilisuchus with tecodonts, however, representatives of this genus cannot be attributed to any of the known families of this group ^[2] .

Sphenosuchus

Representatives of the genus were discovered in the 1960s in the Lower Jurassic deposits of South Africa.

Skull Sphenosuchus - low with an elongated muzzle, closer to the so-called. "Crocodile" type. The square and square zygomatic bones are inclined forward, and the preorbital window is reduced. The skull is akinetic, the frontal bones extend ventrally , connecting with the palatine . The parietal bones are connected at the back, while the posterior orbital and posterior frontal bones remain unfused. Crocodiloid pneumatization of the occipital region is observed, the cochlea of ​​the inner ear lengthens. As in later crocodilomorphs, the lateral sphenoid bone forms the lateral wall of the brain box in front of the anterior bone.

Primitive compared with later crocodilomorphs, structural features of the skull are expressed in the absence of connections between the lateral wall of the brain box and the square bone. The latter reaches the anterior bone in front of the posterior auricle. The superior temporal foramen is long. The palate is probably open, like that of tecodonts, although the internal nostrils are moved backwards, and the maxillary bones form a short secondary palate in front of them.

Along the length of the coracoid, the postcranial skeleton approaches the evolutionary level of later crocodilomorphs, but the clavicle is preserved. A long, forward pubic bone is weakly involved in the formation of the acetabulum, which is perforated in the related form of Terrestrisuchus . As in modern childbirth, the wrist is elongated ^[2] .

Thus, Sphenosuchus differs significantly from the pseudosuchus tecodont in the structure of the square region of the skull, which is characterized by a very high lateral temporal window, and has some fundamental similarities with birds - in the structure of the brain box, palate and system of aerial cranial sinuses, which allowed the English paleontologist A. Walker in 1972, put forward the hypothesis of sphenosuchia as the closest ancestral relatives of birds. Subsequently, however, the hypothesis was invalidated ^[4] .

Hesperosuchus

This late Triassic sphenosuchia from New Mexico was rewritten in 1993. Hesperosuchus , like Sphenosuchus , had an elongated anterior process of the square-zygomatic bone, which extends inward from the posterior process of the zygomatic bone to the base of the postorbital process of the latter. In this case, the aforementioned process crosses the lower temporal window; at the posterior margin of the latter, in the area of ​​the jaw joint, the zygomatic, square-zygomatic and square bones are closed to each other. Fangs are poorly differentiated ^[4] .

Childbirth