Here we've listed the herrerasaurs and other basal saurischians which are (or were once believed) too primitive to be either true theropods or sauropodomorphs. The most commonly-seen in media have been Herrerasaurus & Staurikosaurus (since the 1980s) and Eoraptor (since the 1990s).
A very ancient hero: Herrerasaurus
- Thanks to dino-books and documentaries like those of the Walking with Dinosaurs series, several people have become conscious about the existence of Coelophysis, which has become the forerunner of the dinosaur world. However, some carnivorous dinosaurs lived even before it; but are so ancient, that could not even be real theropods. In Triassic world, dinosaurs still were not so differentiated each other, and the familiar Coelophysis shape was shared by several other animals, obviously with some degree of variation. Herrerasaurus and Staurikosaurus are the two most classic examples. Together, they form their own dinosaur subgroup, Herrerasaurians or Herrerasaurids. Their shape was typically theropodian, but their skeleton was more archaic and less bird-like; for example, they had five digits in their feet, more similarly to sauropodomorphs than to neotheropods (theropods more derived than herrerasaurs), which have only four. Also their pelvis were unique. This bony-puzzle was responsable of many headaches among paleotaxonomists: Staurikosaurus and Herrerasaurus have been variably classified as true theropods, true sauropodomorphs, neither-theropod-nor-sauropodomorph saurischians, or even non-dinosaur dinosauromorphs! Herrerasaurus was the first discovered, in the 1960s. Found in what is now Argentina, its the biggest of the two (4 m /12 ft long), was longer than a Coelophysis and much more robust, with a larger, stronger head and much shorter neck. It was arguably a more powerful predator, hunting relatively large animals such as the beaked rhynchosaurs, basal synapsids and small-sized dinosaurs, but retreated against the giant prosauropod Riojasaurus or the 7 m-long Postosuchus relative Saurosuchus. Some Herrerasaurus remains were once put in their own genuses, "Frenguellisaurus" and "Ischisaurus". The latter's name comes from Ischigualasto, the formation in Argentina in which herrerasaurs have been found — the complete name of this dinosaur is Herrerasaurus ischigualastensis.
Starry dinosaur: Staurikosaurus
- Discovered in 1970 slightly after Herrerasaurus, Staurikosaurus shared the same body-structure of the latter (with some differences) but was only the size of an Ornitholestes: 2 m (6 ft) long, and arguably hunted smaller preys, perhaps young rhynchosaurs or the primitive ornithischian Pisanosaurus. Staurikosaurus was originally put in its own family, the Staurikosaurids; among primitive traits of Staurikosaurus, it had only two vertebrae in its sacrum (like modern reptiles), not four or five like the typical dinosaurs and birds. Staurikosaurus is one of the most poetically named dinosaurs, Southern Cross lizard: it has been for several decades the only dinosaur found in Brazil, and Brazilian flag shows just this constellation. However, the staurikosaur was found in the southernmost part of Brazil, just near the boundary with Argentina. Its complete name is Staurikosaurus pricei; one relative, "Teyuwasu", has recently been reclassified in its genus. Together, Staurikosaurus and Herrerasaurus have long disputed the title of the first/most primitive dinosaur ever appeared on Earth, with some sources preferring one and other preferring the other. Anyway, among the numerous hypotheses made in the eighties, some paleontologists went to claim herrerasaurians were the ancestors of all the other dinosaurs: now this hypothesis is totally discarded, since both animals had their specializations on their own, included some evolved traits (ex the shape of their pelvis). Monotremes (platypus & echidnas) are the most primitive mammals today, but this doesn't mean they are the ancestor of marsupials and placental mammals after all, and they too have specialized traits (electric sensors, for example) that lack in the other living mammals.
The first dinosaur?: Eoraptor
- Discovered in Argentina in 1993 (the same year of the first Jurassic Park film), Eoraptor lunensis means dawn robber from the Moon valley (another astronomical-named dinosaur!). Its discovery made by palaeontologist Paul Sereno suddenly seemed to solve the rivalry between Staurikosaurus and Herrerasaurus for the Whoa, the very first dinosaur ever appeared! title. When was described, it was thought more primitive than both; however newer studies don't always agree with this. 3-4 ft long, the same size of a Compsognathus, Eoraptor shared with herrerasaurians some skeletal features resembling non-dinosaurian archosaurs; it too was thought neither saurischian nor ornithischian, but a more basal animal in the middle between true dinosaurs and other dinosauromorphs such as contemporaneous Lagosuchus (also found in South America twenty years before) and Silesaurus. Eoraptor has been the most celebrated among all the supposed first dinosaurs, maybe because was discovered just at the time Jurassic Park came to audiences - and the fact that Paul Sereno gave to it the now-familiar suffix -raptor could have done its bit, too. Since then, our dawn robber has gained much attention in media, also being object of some degree of sensationalism. Several awesome nicknames were invented, from the first terror to The father of all killer dinosaurs. But Real Life Eoraptor wasn't so fearsome, really: it was a tiny, gracile dinosaur, which could even become a meal for a hungry Herrerasaurus or even a Staurikosaurus. Adding to this, recent research suggested its differentiated teeth were more probably from an omnivorous rather than carnivorous animal. Science Has Marched On Even More as recently as in year 2011, and one study has found Eoraptor to be a very unspecialized sauropodomorph, while other recent researches consider it a proper theropod.
- The study about which dinosaur really deserves the common-ancestor-of-all-dinosaurs title marches on relentlessly, and now there are many other contenders found and/or described in the 2000s and 2010s other than the three most traditionally known above. Astonishingly, many of them come from South America as well, to the point South America could be renamed the cradle of the dinosaur kind at this point. An exception is Nyasasaurus (arguably the very first dinosaur, but initially believed a prosauropod), found in Africa in 1956 but described only in 2012. Other exceptions are the North American Chindesaurus (possibly a herrerasaur), the Indian Alwalkeria (possibly non-dinosaurian and/or a chimaera), and the Chinese Lukousaurus (variably classified from a ceratosaurian to a crocodylomorph). The most basal dinosaur was the common ancestor of Saurischians and Ornithischians, and almost certainly was more similar in anatomy to the former than to the latter: among archosaur groups saurischians are less-derived than ornithischians, at least if you count the structure of the jaws and the pelvis. Some basal saurischians make together the Guaibasaurids. These saurischians were extremely generic and unspecialized dinosaurs, whose external shape was really in the middle between a small theropod and a basal sauropodomorph, not deceptively theropodian like herrerasaurians or Eoraptor. The namesake Guaibasaurus was the first discovered and initially considered a possible basal theropod (which it may actually be); then, the mythical-named Saturnalia note from Brazil, which was initially believed the first prosauropod. In the second half of the 2000s, scientists decided that both dinosaurs were too basal to be either theropods or sauropods, and put together in the same family, guaibasaurids. Now they are often considered very basal sauropodomorphs, though this, too, may still change (for instance, Guaibasaurus itself may be a theropod as said above). With their unspecialized traits, guaibasaurids were almost surely omnivorous creatures; indeed, a third member found in 2009 has received a meaningful name: Panphagia, eat-all. Other very basal dinosauromorphs were once considered basal dinosaurs, such as Spondylosoma and Eucoelophysis, but have recently reclassified as basal dinosauromorphs like Lagosuchus.