Prehistoric Life/Dinosaurs/Sauropod Predecessors

At a first glance, prosauropods (“before the sauropods”) seem simple variations of a single dinosaur: always with the mixed theropod/sauropod shape that characterize the stock species, Plateosaurus. Actually their resemblance was not necessarily a real sign of relationship: their bodyplan was simply a primitive condition, shared by all most archaic sauropodomorphs. According to recent classifications, the prosauropods in traditional sense can be divided in three ensembles: “core prosauropods”, “near sauropods” and “really basal sauropodomorphs”. The first were typical "prosauropods", large to medium-sized bipedal dinosaurs intermediate between more basal sauropodomorphs and proper sauropods. The second ones were more closely related to sauropods and gave rise to them, and were usually larger than other "prosauropods" as well as being capable of some quadrupedality. The third ones were more primitive than both “core prosauropods” and “near sauropods”. As none of these are natural groupings, there isn’t any external trait which allows to distinguish these groups: both “core prosauropods” and “near sauropods” included small and big animals, for example. Even their period is not indicative: both Late Triassic and Early Jurassic were inhabited by “core prosauropods” and “near sauropods”. However, for the sake of convenience, we put here all the examples of traditionally-intended "prosauropods" together, specifying for each its possible placement in the evolutionary tree.

The little cousin: Massospondylus

• The musically-named Massospondylus: is the most well-known "prosauropod" after Plateosaurus. Almost a hundred individuals have been discovered so far in Southern Africa, but some doubtful remains from North America are also known. Massospondylus was a “core prosauropod” (like Plateosaurus) according to modern classifications. 4-5 m long, smaller than Plateosaurus, Massospondylus had a smaller head, slimmer neck, nimbler limbs and shorter trunk, in short, was a bit like a Plateosaurus which has undergone a weight-reducing diet. Massospondylus lived after Plateosaurus, in the Early Jurassic, and was one of the last "prosauropods". Recently, many nests and hatchlings have been discovered in South Africa from this dinosaur, making Massospondylus one of the dinosaurs we know the most about. The lacking of teeth among the youngest nestlings has surprised scientists: this makes a concrete proof about active parental care from their parents, since these youngsters couldn't feed on their own with their toothless jaws. Adult Massospondylus were probably vegetarian, even though they could have also caught some insects occasionally: some gastroliths (gizzard-stones) found in the ribcage of some individuals seem to confirm a mainly herbivorous diet. Like all "prosauropods", Massospondylus used to be depicted as a quadruped in old paleo-art: now we think it was totally bipedal like Plateosaurus – even though newborn Massospondylus appear to have been quadrupeds. Massospondylus has yet to appear in documentaries despite being very well-understood, and probably will never appear in fiction, not being striking enough to attract pop-writers.

An old story: Anchisaurus

• Remember Dino, The Flintstones’ pet dinosaur? Its shape is very reminding of a prosauropod; more in particular, with its short limbs, it recalls this one: Anchisaurus - this doesn’t means Dino is really an Anchisaurus… considering the writers’ great love for research he’s more probably a funny sauropod. Despite its scarce fossil record, Anchisaurus is one of the most famed sauropod predecessors, thanks to its historical importance. It was the very first dinosaur ever discovered in North America (1818, six years before Megalosaurus). But was not recognized as a dinosaur at the time: this happened only during the Bone Wars sixty years later. An Early Jurassic animal like Massospondylus, Anchisaurus was even smaller (only 9 ft long) and one of the most unsauropod-like “prosauropods”, with its rather short neck and limbs. Talking about its modern classification, several scientists now think Anchisaurus was a “near sauropod”. The larger-sized Ammosaurus is a less-known relative of Anchisaurus from the same habitat, although it could simply be another species of Anchisaurus.

Mousesaur: Mussaurus

• Discovered in 1979 in Triassic Argentina, the unusually short-named Mussaurus (“mouse reptile”) was a “near sauropod” whose only remains are from newborns the size of a rat (hence the name), which died just after being hatched. Curiously, many popular books have reported Mussaurus as the “smallest dinosaur” ever (or at least the smallest herbivorous one) and depicted the adult form with the same large head and short neck of the hatchlings. It should be remembered that dinosaurs were not like the distantly-related snakes and lizards, whose youngster are miniaturized images of the adults; the dinosaurs’ nestlings were more like those of the closerly-related birds and crocodilians, both with “childlike”, cuteness-inspiring traits which get lost in adults. Since adult skeletons of Mussaurus have never been discovered so far, we don’t know how big the adult was: but almost certainly it was at least as large as a human and had the classic small head and long neck of all "prosauropods".

Ancestor or Non-ancestor: Melanorosaurus and Riojasaurus

• Riojasaurus was the polar opposite of the “mouse reptile”. Its Spanish-sounding name reveals it also lived in Triassic Argentina, but was bigger. More than 10 m long, was one of the largest land animals of the Triassic, even bigger than Plateosaurus itself; its success is shown by its abundant record (more than 30 individuals). At a first glance, Riojasaurus resembles more a sauropod, with the same size of many “small” sauropods, massive limbs and stocky body. However, the structure of its feet with distinct digits is typically "prosauropodian"; ironically, Riojasaurus seems to be a “core prosauropod”. It had also one twin in South Africa, Melanorosaurus. Contemporary to Riojasaurus, the two dinosaurs maybe could have met in life, since Africa and South America were reuned in the “Pangea” at the time. Both the same size, with the same robust sauropodian shape but "prosauropod"-like feet, they were perhaps mainly quadrupedal. Both were candidates for the title of “the ancestor of all sauropods”: recent research has shown Melanorosaurus was indeed a "near sauropod", and the riojasaur’s similar frame was obtained by convergent evolution. However, some “core prosauropods” did live alongside Melanorosaurus in South Africa: among them, Euskelosaurus was only slightly smaller than Melanorosaurus.

Mailing With Dinosaurs: Lufengosaurus

• Lufengosaurus is the most well-known early dinosaur from China, with about 40 skeleton discovered. Lufengosaurus was a “core prosauropod” related to Massospondylus but bigger. It actually looked more like Plateosaurus - to the point it could even be considered its eastern twin, only smaller (6 m long). One of the last-surviving prosauropods, Lufengosaurus lived from Early up to the Middle Jurassic, enough to encounter the first true sauropods. Even though this detail is little-known, Lufengosaurus detains the peculiar record to have become in the 1940s the first dinosaur ever portrayed in a postage stamp - made in China just to celebrate Lufengosaurus, which was one of the first-discovered dinosaurs there. Its less-known relative Yunnanosaurus lived in the same epoch/places: one recently-found Yunnanosaurus species is 13 m long, and now is the biggest "prosauropod" known.

A deceptive name: Thecodontosaurus

• “Thecodont” is a now-abandoned name for basal non-dinosaurian Triassic archosaurs. Thus, it could seem that Thecodontosaurus was one of them: actually was a true dinosaur, albeit one of the most primitive known. Triassic and European just like Plateosaurus, Thecodontosaurus was also one of the first-discovered dinosaurs. Only 2 m long, it was the most theropod-like among the prosauropods in traditional sense; in modern taxonomy, is considered a “basal sauropodomorph”. Long considered the most archaic sauropodomorph, the thecodontosaur has recently lost is record (see “Small Theropods”). Another slighty more derived “basal sauropodomorph” from Europe is Efraasia, which was once thought to be the young of another European dinosaur, "Sellosaurus", which in turn is now often thought to be a simple species of Plateosaurus. Science Marches On frequently with the sauropod predecessors: maybe because they are usually less-studied than other groups.