When I was first becoming acquainted with dinosaurs, the origin of the gigantic Jurassic sauropods seemed pretty straightforward. There was Plateosaurus (see above) from the Late Triassic of Europe, and it almost seemed certain that it was the ancestor to behemoths like Diplodocus and Brachiosaurus that were present in every dinosaur book I owned. What has become apparent, however, is that the evolution of sauropodmorph dinosaurs is not nearly so simple, and the animals once considered to be the ancestors of the later "earth shakers" are more likely a sister group, the Prosauropoda. Within this Infraorder, there are several families of somewhat enigmatic early sauropodmorphs, and in the newest issue of the quarterly journal Acta Palaeontologica Poloncica a new Antarctic sauropodmorph belonging to the Family Massospondylidae has been named; Glacialisaurus hammeri.
Like many of the previous fossil-hunting expeditions in the Antarctic, retrieving the remains of Glacialisaurus was no easy task; found at an elevation of about 13,000 feet on Mt. Kirkpatrick in the Queen Alexandra Range of Antarctica, the partial left femur, ankle, and foot bones were entombed in the cold, hard rock. Glacialisaurus isn't the only fossil to come from Mt. Kirkpatrick, however; other dinosaurs like the famous Cryolophosaurus ellioti have been found there as well (including some material that may be referable to a true sauropod dinosaur). Still, like many fossil specimens Glacialisaurus is fragmentary, making it difficult to determine precisely where it might fit in the branching bush of sauropodmorph evolution, but what has been found is enough to give us a few clues about the range and evolution of sauropodmorph dinosaurs in the Early Jurassic.
Like many early dinosaurs, Glacialisaurus exhibits a mix of ancestral and derived characters that it shares with other prosauropods and even shows some similarities with Herrerasaurus (which was at one time considered to possibly be a basal sauropodmorph itself). Overall, however, the excavated remains of Glacialisaurus are more robust and derived than the somewhat gracile condition seen in the early sauropodmorph Saturnalia, most closely resembling sauropodmorphs like Lufengosaurus and Massospondylus, placing it within the Family Massospondylidae as a sister taxa to Lufengosaurus (Coloradosaurus being a sister group to both Lufengosaurus and Glacialisaurus). As the authors of the paper note, however, there's a lot of moving & shaking going on with prosauropods at the moment, and the fragmentary nature of Glacialisaurus makes this analysis tentative at best (a complete skeleton would greatly help to stabilize the position of this new dinosaur).
Glacialisaurus is significant in a biogeographic context as well; along with the presence of possible sauropod remains, it appears that basal sauropodmorph dinosaurs had a nearly global distribution by the Early Jurassic, and perhaps prosauropods and early sauropods overlapping temporally and geographically. As the authors note, though, this hypothesis has yet to be rigorously pursued and investigated, the biostratigraphy of the Hansen Formation from which Glacialisaurus came being difficult to pin down. It still is an intriguing idea, though, and I would expect our picture of sauropodmorph evolution is going to get even more complex as new fossils are found and the debate continues.
References;
Smith, N.D., and Pol, D. (2007) "Anatomy of a basal sauropodomorph dinosaur from the Early Jurassic Hanson Formation of Antarctica." Acta Palaeontologica Polonica 52 (4): 657-674.
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Well, to be fair, "prosauropoda" is currently considered a paraphyletic term. Traditional "prosauropods" and true sauropods are not sister groups. Rather, prosauropods are an amalgamation of basically unrelated taxa, some forming their own natural groups (like Massospondylidae) while others form a sort of "stepwise" progression (I hate to use the word "progression") from Saturnalia to true sauropods like Isanosaurus and Antetonitrus.
Ironically, the old view that prosauropods are ancestral to sauropods is "truer" than to say that prosauropods form their own natural group! Good summary, though. It's such a shame that Antarctica is covered in ice--I'll bet there are LOADS of primitive dinosaurs down there.
I forgot who was responsible, but an attempt has been made to define "Prosauropoda" as a monophyletic group by limiting it to anything more closely related to Plateosaurus than to sauropods. Most researchers have accepted this redefinition, I think, but have tended not to use it because it's a clade of pretty doubtful content that may only contain Plateosaurus and a few of its closest cronies. It depends on whether or not there is an actual sauropod-less clade containing most of the traditional prosauropods.
The problem is that that prosauropods, as a group, exhibit more or fewer things in common with true sauropods. Melanosaurus, for instance, has been called an advanced prosauropod, a prosauropodian sauropod mimic, and a primtive sauropod. Anchisaurus, a former prosauropod, was recently re-established as a very basal sauropod. It's pretty clear that "prosauropoda" is not monophyletic, although a few of the animals we call "prosauropods" may actually BE monophyletic among themselves (confused yet?).
I know. A horrible mess, isn't it? In the past, we had a bunch of distinct "prosauropod" families - Thecodontosauridae, Anchisauridae, Plateosauridae and Melanorosauridae, to name a few. Now that we know a bit more about early sauropodomorph diversity, things are really getting shaken up.
Just like other basal groups such as "hypsilophodontidae" and "fabrosauridae", which are now paraphyletic. And we don't even know for sure where critters like Herrerasaurus and Eoraptor stand - basal dinosaurs, basal saurischians, or basal theropods?
One thing I find particularly intriguing is the paper that argues pretty convincingly that some of the early sauropodomorphs were obligate bipeds, when most of the time we've seen them as being predominantly quadrupedal and facultatively bipedal. Taking into account that baby Massospondylus were apparently quadrupeds, it does hint that the first sauropods had a reversal of sorts.
Who mounted that sprawly plateosaur? Nopsca? He should have known better. He might have been an excentric, a difficult person to work with, a political adventurer (to say the least) and he had this rather unhealthy obsession with Albanians, but he was a brilliant palaeolontologist.