Prehistoric Life/Dinosaurs/Primitive Dinosaurs

Starry dinosaurs: Herrerasaurus and Staurikosaurus

• Thanks to documentaries like Walking with Dinosaurs, several people has 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. 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: Herrerasaurus and Staurikosaurus 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, it’s 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 rhynchosaurs, basal synapsids and small-sized dinosaurs, but retreated against the giant prosauropod Riojasaurus or the 7 m-long Postosuchus-relative Saurosuchus. Discovered in 1970, Staurikosaurus shared the same body-structure 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 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. Together, these two dinosaurs have long disputed the title of “the first/most primitive dinosaur ever appeared on Earth”. Among the numerous hypoteses, 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.

Dawn terror: Eoraptor

• Discovered in Argentina in 1993, Eoraptor (“dawn robber”) 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). Eoraptor has been the most celebrated among all the supposed “first dinosaurs”, in part because was discovered just at the time Jurassic Park came to audiences - and the fact scientists 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, its unspecialized 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. Good-bye, “first-terror”.

All-eating and pleasure-loving: Guaibasaurus, Saturnalia, and Panphagia

• Giving the coup-de-grace to Eoraptor, its common-ancestor-of-all-dinosaurs title is now contended by other dinosaurs found in the 2000s. Astonishingly, they all come from South America, which could be renamed “the cradle of the dinosaur kind” at this point. The most enigmatic ones 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 "prosauropod", not deceptively theropodian like herrerasaurians or Eoraptor. The namesake Guaibasaurus was the first discovered and initially considered a possible basal theropod; then, the mythical-named Saturnalia ^[1] 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. 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”.

Boar-Bird: Heterodontosaurus

• Among basal Ornithischian dinosaurs, there were curious things as well. Heterodontosaurus, for example, might be renamed the boar-bird. Living in Early Jurassic South Africa 190 million years ago, Heterodontosaurus superficially resembled the ornithopod Hypsilophodon with its slender, bipedal body, but was even smaller (1.20 m/4 ft long), more robust and with longer forelimbs. Discovered only in the sixties, its name means “lizard with different teeth”, and with reason: no other dinosaur had such a diversified dentition, with three kinds of teeth surprisingly similar to those found in mammals. The most noticeable are two pairs of canine-like “tusks” visible when the mouth closed, giving it a vaguely boar-like look; behind, molar-like teeth to grind up tough vegetation; in front of them, small peg-like teeth only on the tip of its upper jaw. With this dentition, Heterodontosaurus was probably a mostly herbivorous omnivore, eating insects other than vegetation, while the tusks could have been used for display and/or competiton. Some scientists suspect only males did have the large canines, but there is no evidence. Other close relatives, like Abrictosaurus, are devoid of tusks: their skull could either pertain to females, or, more probably, to totally tusk-less species. All these animals make together their own ornithischian family, Heterodontosaurids. Once thought ornithopods or ancient relatives of ceratopsians and pachycephalosaurs, now they are regarded as very basal ornithischians. Despite their primitiveness, heterodontosaurs not only flourished in the Early Jurassic, but also managed to survive until the Early Cretaceous, with species such as the poorly-known Echinodon.

Size doesn't matter (just for once): Lesothosaurus, Scutellosaurus, and Pisanosaurus

• When talking about Ornithischians, we can find the same issues of Saurischians: in the Triassic/Early Jurassic they were all so-similar each other, it’s hard task to classify them accurately. Nonetheless, they are extremely important animals for scientists, no matter their often tiny size. Other than Heterodontosaurus, we have several other examples. Lesothosaurus, Scutellosaurus and Pisanosaurus have traditionally been the most relevant. Lesothosaurus in particular was once considered the forerunner of all bird-hipped dinos, and thought not to belong to any great ornithischian group; recent research suggest it could be a very basal Thyreophoran, thus ancestor of Stegosaurs and Ankylosaurs. Also from Early Jurassic Southern Africa, its name derives from the Kingdom of Lesotho (a small South African enclave) where its remains were dug out in 1978. Merely 3 ft long, the bulk of a Compsognathus, Lesothosaurus seems not to have any specialization in its anatomy. Its mouth had simple teeth and small cheeks, its forelimbs short and five-digited, its hindlimbs apt for running. It was said that Lesothosaurus resembles a lizard more than any other dinosaur. Fragmentary remains that have been named "Fabrosaurus" may be synonymous with this taxon; since they were named before Lesothosaurus, in old textbooks this dinosaur is often referred “Fabrosaurus”. Scutellosaurus: has traditionally been the most primitive thyreophoran. Discovered only in the 1980s, was also a small bipedal animal with a similar look, but slighty bigger, longer-tailed, more robustly-built than the lesothosaur, and with a light armor made by small bony plates placed in rows upon its torso, similar to that of the bigger Scelidosaurus. Like the scelidosaur, Scutellosaurus lived in Early Jurassic, but was found in Arizona, where the popular double-crested Dilophosaurus lived (and could have been its predator). Also found in the last decades of the XX century, the Argentinian Pisanosaurus lived with the alleged “first theropods Eoraptor and Herrerasaurus, and still remains the most ancient ornithischian known to science. Sadly, is known only from one incomplete fossil, but was arguably similar to Lesothosaurus in shape and size. One curious thing is that some Triassic non-dinosaurian archosaurs were once considered basal ornithischians as well: "Technosaurus" from Texas is one example.

Two great little discoveries: Eocursor and Tianyulong

• Like the basal saurischians above, basal ornithischians as a whole are known only since the 1960s, and still aren’t well-understood. So, every recent discover could be very significative. Eocursor and Tianyulong in particular, are fairly gaining more and more consideration in scientific field because of their objective importance. Found in 2007, Eocursor (“dawn runner”) was discovered in South Africa like Heterodontosaurus and Lesothosaurus, and its name is cleary inspired from that of Eoraptor (“dawn robber”). Its relevance is due to the fact that it’s the only Triassic ornithischian known so far from a complete skeleton (while the Pisanosaurus one is only partial); this gives us precious information about the deepest ornithischian roots, and also could better explain the relationship between bird-hipped dinosaurs and the saurischians. According to the most accepted classification, ornithischians are divided in two main lineages: Thyreophorans and Cerapods. The former are, as is known, Stegosaurs+Ankylosaurs+some basal forms (Scelidosaurus, Scutellosaurus). Cerapods include almost all the other ornithischians, furthermorely divided in Ornithopods (duckbills, Iguanodon, Hypsilophodon etc) and Marginocephalians (ceratopsians+pachycephalosaurs). Indeed, Cerapods is just a Portmanteau made of “Cera[topsian]” and “[Ornitho]pod”. About Tianyulong (guess which country it comes from?): this is a heterodontosaurid from the Late Jurassic found in 2009 in the same Liaoning site from which the Jurassic troodont Anchiornis was discovered, Tianyulong, like the latter, has preserved some sort of proto-feathers around its body. The thing is, this is the first time that unequivocally feather-like structures have been found in a non-theropod dinosaur (not counting the quills of Psittacosaurus of course) see the useful notes about dinosaurs to understand the revolutionary implications of this discovery.

1. ↑ “Saturnalia” is a plural Latin name for a kind of feast made by Ancient Romans to celebrate Saturn, the goddity who protected their crops.