Discovery
We have only a single specimen of Staurikosaurus ("Lizard of the Southern Cross"), recovered from the Santa Maria formation in Rio Grande de Sul, southern Brazil. The name refers to the star constellation "The Southern Cross", only visible in the southern hemisphere - when Staurikosaurus was found in 1970, it was unusual to find dinosaurs in the southern hemisphere. It was first described by Edwin H. Colbert, working at the American Museum of Natural History.
Description
Staurikosaurus was a small theropod from the Middle Triassic, 231 to 225 million years ago - specifically the Carnian age. It is one of the earliest dinosaurs we know of. At just two metres in length, 80cm tall and weighing just thirty kilos, Staurikosaurus was tiny in comparison to later theropods like Megalosaurus. Although its teeth and posture suggest it was a carnivore, some paleontologists prefer to classify Staurikosaurus as a sauropod like the later Diplodocus due to its prosauropod-like skeleton. It seems to represent a transition period as one of these sub-orders evolved from the other. However, another fossil (as yet unnamed) was found in 1984 in Arizona's Painted Desert that was such a typical prosauropod that it seems that that group evolved before Staurikosaurus. Newer research seems to confirm that Staurikosaurus and the related Eoraptor and Herrerasaurus are definite theropods and evolved after the sauropod line had split from theropoda.
We have a very incomplete fossil record of Staurikosaurus. We have most of the spine, the legs and the large lower jaw. However, dating from such an early period in the dinosaurs' history and being otherwise so primitive, we can restore most of Staurikosaurus' other features as being primitive also. For example, Staurikosaurus is usually depicted with five toes and five fingers - very simple features of an unspecialised dinosaur. However, since we do have the skeletal structure of the legs, we can see that Staurikosaurus was a quick runner for its size. It also had just two vertebrae joining the pelvis to the spine, a distinctly primitive arrangement. The tail would have been long and thin to balance the border - later sauropods had larger, shorter tails relative to their weight.
The recovered mandible suggests that sliding joint of the jaw allowed it to move backwards and forwards, as well as up and down. Thus smaller prey could be worked backwards towards Staurikosaurus' throat, along its small and backwards-curving teeth. This feature is common in theropods of the time, but disappears in later theropods who presumably had no need for efficiency in eating smaller prey.
Classification
Since we have only one specimen of Staurikosaurus, we evidently only have one species! That is Colbert's original S. pricei. This is named for Colbert's fellow paleontologist Llewellyn Ivor Price. However, there are some related staurikosaurids, such as Chindesaurus bryansmalli, named by Murray & Long in 1985. This was from a similar time period and has been found in Arizona and New Mexico. This suggests that staurikosaurids spread widely across Central Pangaea.
I apologise for the relative brevity of this write-up. There is simply very little known about this dinosaur. The fact that most of the research on it is in Portuguese didn't exactly help my cause! | 
