Monday, November 19, 2007

Stegosauropod?

My favorite animals are those that are not what they seem. Effigia, for example, is actually a crurotarsian which converged on dinosaurs. Henodus is the placodont answer to turtles, and Nigersaurus predated lawnmowers. And then you’ve got the contrary animals, of which Europasaurus is a member. A dwarf sauropod, can you believe it? While most sauropods were well over sixty feet long, and at least one may have hit the 150-foot mark, little Europasaurus was less than twenty feet long. But at least Europasaurus looks like a miniature sauropod. Aside from its dwarfism, it’s easily identified as a macronarian. Can’t say the same, though, for today’s freak sauropod. Brachytrachelopan mesai, a dicraeosaurid diplodocoid from Patagonia, is not only small overall, but also had an incredibly short neck.

Go figure. One of the defining characteristics of the Sauropodmorpha is a long neck, but Brachytrachelopan (rolls of the tongue, doesn’t it?) bucked that trend and decided to make its own bizarre fashion statement. Although the only well-preserved elements in the Brachytrachelopan type include articulated cervical and dorsal vertebrae (and an ilium) (and the distal end of the femur and proximal end of the tibia), and they are very strange indeed. Moving forward from the sacrum, the neural spines of the individual vertebrae begin to crane forward. The further toward the head you get, the more severe the forward bend. The dorsal/cervical transition is abrupt, and the cervical vertebrae are markedly smaller than the dorsals. Only eight cervicals are preserved, although the authors assume that Brachytrachelopan had the usual dicraeosaur complement of 12. I question this assumption, because the fifth cervical (the last preserved one) is so small that I wonder how far the head was from it.

The ribs also sweep back toward the pelvis, rather than straight down. The ilium itself is flared to allow at least two ribs to pass behind it. Only two and a half sacral vertebrae are preserved, although there may have been more. The neural spines are fused together, forming a solid block of bone above the pelvis. Sadly, the limb elements are not preserved in any informational capacity. The distal femur and proximal tibia pieces are similar to Dicraeosaurus. With those strange ribs, I wonder how the pectoral girdle attached.

Anyway, the cranially-curved neural spines of Brachytrachelopan, in addition to its meager neck, means that it was virtually unable to raise its head beyond a horizontal line, and even that could be stretching it. With its head perpectually facing the ground, perhaps facing it, one surmises that its food must have been gathered in a manner similar to that of Nigersaurus. Unfortunately, no skull material is known from Brachytrachelopan, and closely related Dicraeosaurus from Africa has a skull that's roughly similar to Diplodocus and Apatosaurus. Whether Brachytrachelopan varied wildly from this norm, as Nigersaurus did, remains to be seen.

Brachytrachelopan belongs to a third family of diplodocoids (aside from Diplodocidae and Rebbechisauridae): Dicraeosauridae. Only three genera are recognized. Dicraeosaurus seems to be the largest known member, at 40 feet long. Amargasaurus, the "spike-necked" sauropod from Argentina, was a mere 33 feet long. Brachytrachelopan is thought to have been less than 30 feet long. Dicraeosaurs are characterized by short necks, relatively small bodies (for sauropods), and unusually tall neural spines. Exactly what these sauropods were eating is a bit of a mystery, but in Brachytrachelopan's case, probably vegetation that was between 1 and 2 meters off the ground.

I think Brachytrachelopan looks an awful lot like a stegosaur, what with its robust limbs, tall neural spines, and short neck. Perhaps it was a Patagonian stegosaur mimic!

Rauhut, O. W. M., Remes, K., Fechner, R., Cladera, G. & Puerta, P. (2006). Discovery of a short-necked sauropod dinosaur from the Late Jurassic of Patagonia. Nature 435: 670-672.

2 comments:

Brad said...

That's a very strange forelimb you've drawn. Shouldn't there be more muscle on the humerus?

Zach Miller said...

Yeah, he's kind of missing his bicep, isn't he?