Silesauridae

Silesauridae is an extinct family of early dinosauriforms which lived during the Triassic Period. Their fossils have been found in Europe, North America, South America, and Africa, reaching peak diversity early in the Late Triassic. The exact affinities of silesaurids are debated, and various studies come to different conclusions regarding the relationship between silesaurids and early dinosaurs.

Some studies regard silesaurids as a clade of non-dinosaur dinosauriforms, as well as the sister group of dinosaurs. In other words, all silesaurid species originated from a single common ancestor which evolved adjacent to, but not within, the group Dinosauria.

Other studies argue that most or all silesaurids (a.k.a. "silesaurs") belong within Dinosauria, specifically as long-sought Triassic representatives of the ornithischian dinosaurs. A few silesaurs may still comprise an exclusive clade within Ornithischia, but most correspond to a paraphyletic grade (a series of species increasingly close to Jurassic-Cretaceous "traditional" ornithischians).

Most silesaurid species are based on fragmentary fossils, but a few are known from partial skeletons. They have a consistent lightly-built body plan, with a fairly long neck and legs. Their forelimbs are notably long and slender compared to other Triassic dinosauriforms, so many silesaurids may have been primarily quadrupedal. Silesaurids occupied a variety of ecological niches. Early examples such as Lewisuchus were small carnivores with knife-shaped teeth. Many later taxa (such as Kwanasaurus) were specialized herbivores with leaf-shaped teeth and a beak at the tip of the lower jaw. As indicated by the contents of referred coprolites, Silesaurus may have been insectivorous, feeding selectively on small beetles and other arthropods.

In general, silesaurids were small compared to most Jurassic and Cretaceous dinosaurs. Silesaurus opolensis, a species with well-understood skeletal anatomy, could reach a total length of around 2.1–2.3 m (6.9–7.5 ft). Considering indeterminate fossils, some silesaurids were the largest dinosauriforms of their time. Isolated femur bones from Tanzania and Zambia belong to silesaurids up to 3.5 m (11 ft) in length. These African fossils were very early in dinosauriform evolution, from the Middle Triassic or the early Carnian stage (earliest Late Triassic). The largest silesaurid fossils from South America are also Middle Triassic in age.

By the late Carnian, the carnivorous dinosaur Herrerasaurus reached a similar size, and theropod and sauropodomorph dinosaurs achieved even greater sizes later on in the Late Triassic. Early Jurassic ornithischians were smaller than most silesaurids, hinting at a miniaturization event across the Triassic–Jurassic mass extinction.

In many regards, silesaurid skulls are similar to early dinosaurs and other archosaurs. All Triassic archosaurs have weight-saving holes on the snout (the antorbital fenestra), the jaw (mandibular fenestra), and the back of the skull (the upper and lower temporal fenestrae). Most unique qualities of silesaurid skulls relate to the areas adapted for processing food.

The skull of Lewisuchus is the most generalized among silesaurids. The snout is slender, with a long series of narrow, recurved teeth bearing fine serrations and a sharp tip. Teeth with this ziphodont (knife-like) shape are adapted for rending flesh. A ziphodont tooth shape was the standard ancestral condition for archosauriforms, including many theropod dinosaurs.