Dino of the Week: Riojasaurus

Type Species: Riojasaurus incertus

Classification: Dinosauria - Saurischia - Sauropodomorpha - Prosauropoda

Time Period: Late Triassic

Location: South America

Diet: Herbivore

While a dearth of prosauropods are found in what was Laurasia (the northern half of the rifting Pangaea), peculiar ones are found in Gondwana (the southern half), and the “Rioja Lizard” makes an excellent Gondwana specimen. Named by Jose Bonaparte in 1967, this herbivorous Upper Triassic prosauropod could reach a massive 36 feet long. Though greater sizes would develop in the Jurassic, Riojasaurus is a wonder for any paleontologist: how mysterious for a dinosaur of such massive size to evolve so early!

Around twenty skeletons representing different stages of growth have been found in Argentina. The earliest skeletons lacked skulls, and scientists originally deduced that Riojasaurus had been a predator, since pointed teeth were found among the bones. More specimens—with skulls this time!—showed up, and the leaf-shaped teeth showed it to be herbivorous. The pointed teeth among the earliest skeletons probably belonged to either meat-eating dinosaurs or those of early crocodilians feeding on the carcasses; the Triassic’s trophic (or top-of-the-food-chain) predators weren’t theropod dinosaurs but dinosaur-like archosaurs that would send most early dinosaurs fleeing in terror. 

Riojasaurus’ limb bones were large, dense, and heavy; combined with over-sized shoulder- and hip girdles, not to mention forelegs that were only slightly smaller than the hind legs, it seems a safe bet to assume that Riojasaurus was fully quadrupedal, completely restricted to life on all fours. (In all fairness, a 2016 study by Scott Hartman shows that Riojasaurus may have been able to not only rear up on its hind legs but also walk in a bipedal fashion; more research has yet to be done.) If Riojasaurus were fully quadrupedal, it puts a curious twist in prosauropod evolution: it’s been assumed that prosauropods started off bipedal, branching away from bipedal archosaurian ancestors, but Riojasaurus shows up “large and in charge” early on in prosauropod history. The curtain has barely drawn back when Riojasaurus ungainly limbers on stage, blinking stupidly at the lights (and because prosauropods were among the dumbest of dinosaurs, stupid blinking isn’t a figure of speech). 

Riojasaurus’ backbone was punctured by the presence of hollow, air-filled cavities that kept the backbone from being too heavy (no one likes to be crushed by their own spine). These hollow spaces are rare in prosauropods but predominant in sauropods. These hollow cavities continued up through the neck, helping to keep minimize the weight so that Riojasaurus could lift its long neck to reach higher vegetation. The presence of these air cavities have prompted a number of scientists to wonder if Riojasaurus’ relationship with sauropods is closer than that of “second cousins.”