Is the French Monster a Euhelopodid?

Posted by Nima On Thursday, July 10, 2014 2 comments

One of these days we may see a paper come out about this. France, as you may know, contains a dinosaur of truly gigantic proportions (and no, I'm not talking about the Algerian species "Brachiosaurus" nougaredi which seems to have disappeared into a black hole somewhere in Paris). No, this one's a local. A new "titanosaur" known only as the French Monster, or the giant of Angeac, has turned up in the past few years in a bone bed which contains a huge jumble of Early Cretaceous dinosaurs of various sizes - everything from baby abelisaurs to massive hundred-foot sauropods.

Except it's not really a titanosaur, is it?

The French Monster is another one of those legendary or semi-legendary sauropods which should have a name and a formal description, though it's unlikely most of them ever will. Even the pictures evoke something rarely seen in the fossil record.

Mainly the image that sticks in your mind is the very long and oddly blackened right femur, over 2.2m long, with which everyone seems to be "doing the Jensen" though none of them come close to getting it right.

But there is much more to the Angeac-Charente site where this bone was found. There appears to be part of a second femur also recovered at the site, and possible rib fragments. And different individuals of this species have been found, with the fossils in different shades of mineralization.

GIGANTIC tail vertebrae and toe bones. Over a foot in length or diameter. Note the cheesy scaled-up mid-90s "Brachiosaurus" skeleton model designed by Dale Russell and Ely Kish - these things originally came in a screw-cap plastic egg, and were the highlight of every kid's otherwise dull-as-doorknobs trip to T.J. Maxx. Some of you may be too young to remember... The model isn't 6 feet tall like it may appear, it's on the shelf, not the floor!

The heavy toe bones and caudals. These may be from multiple individuals.

Some teeth were found, very well preserved and encrusted with mineral deposits formed in fossilization.

These teeth have a very basal appearance, and so the labeling of this animal as a titanosaur is unlikely. The teeth could easily pass for Brachiosaur teeth. But there is more than just this first indication what what we are seeing may not be a true titanosaur.

The femur and the other pieces were apparently packed and shipped to a museum. It's not clear if this is in Paris or elsewhere. The femur is crushed and snapped in a few places but its total length is still preserved. A cast and wall mount were rapidly made. And this is the peculiar part.

Here you see the femur, fragments of another, and a rib. Apparently this is a part of the full wall mount you see in the background of the previous picture. It's evident both that there was crushing and that very little erosion has happened. The most unusual things about this femur are the very long femoral head, and the odd lower end, with its inner condyle extending lower than the outer one. This is the exact opposite of when you see in derived titanosaurs. But is is a classic femur morph in a group which is close to basal titanosaurs.

Between brachiosaurs and basal somphospondyli (creatures like Chubutisaurus, Ligabuesaurus, and some would say, Paluxysaurus and Sauroposeidon) and true titanosaurs, there was an amazing radiation of transitional forms. These appear to form at least two major families: Euhelopodidae, and Acrofornica (tall-arches). Both are characterized by extremely long necks, high cervical counts, and bifid neural spines in the neck. The Acrofornica are further distinguished by very tall neural arches, high diapophyses, and nearly no neural spine in the dorsals. They tend to have well-separated sacral ribs, whereas those of euhelopodids tend to be extensively fused together. And whereas euhelopodids (or at least some of them) have procoelous tail vertebrae (anticipating derived titanosaurs!) the tails of acrofornicans revert back to simple amphiplatyan tails, as in basal titanosaurs.

After about 3 years of morphometric comparisons between various elements and overlaps between different specimens (some of them very fragmentary) the following family tree slowly began to reveal itself. This is not a complete family tree of titanosauriformes; only some of the more well-known ones intermediate between brachiosaurs and titanosaurs are included here.

And in the comparisons between different elements, specifically femora, the French Monster appears to belong in this transitional "gap" rather than in the more derived true Titanosauria. Previously I had put several hypotheses out there, what the French Monster may have been. Perhaps a lognkosaur, possibly a Malawisaurus-grade stem lognkosaur. Perhaps an oddly slender Antarctosaur of Pellegrinisaurus ilk. Maybe even a nemegtosaurid like Huabeisa- wait a minute, that one's not even considered a nemegtosaur anymore! That's the great thing about finally using digital photos... they clear up all sorts of confusion - which Pang & Cheng had forgotten to do in the original blurry description.

Well it turns out that the French Monster is more like Huabeisaurus than previously realized. Both are closer to Euhelopus and the acrofornicans than to true titanosaurs.

Complicating matters is the fact that some lognkosaurian titanosaurs have a protruding femoral head and a high and prominent lateral bulge, which differs from all other titanosaurs, and converges on that of some euhelopodids and acrofornicans. However the distal end of the femur follows radically different patterns in the two lineages. Here's the comparison of posterior views, you be the judge (not to scale):

So far the morphology of the French Monster's femur (the long and slender femoral head, the tibial condyle being longer and more massive than the fibular condyle, the lateral bulge high on the femur) appear more like a basal somphospondyl than a titanosaur. Most specifically, it resembles Paluxysaurus, and to a lesser degree, Huabeisaurus. The distal end of the Chubutisaurus femur is too eroded to make a good comparison on the condyles. All the same, the basal lineage of this unusual animal look much clearer now. The French Monster points to a radiation of basal sonphospondyli well outside Asia in the early Cretaceous. Angolatitan was the first euhelopodid to be described outside Asia, proving that the clade was not an exclusively Chinese one - and there are rumors of some similar fossils in Europe as well. If it's a chubutisaur, it would be the first to be found outside the Americas.

Perhaps the biggest oddity of all is how slender the French Monster's femur is. The crushing is mostly from front to back, not lateral. So it really was this narrow. There is a possibility then, that this animal was not even close to the maximum size possible. As an adult it may have been more robust. There is no coracoid or scapula material, so the degree of suture fusion in the shoulders (and thus the animal's maturity) is open to speculation.

As far as I can see it, this dinosaur is most likely a chubutisaur (or whatever Paluxysaurus is, seeing as it's more or lesss totally busted as a purported brachiosaur), but may also belong in acrofornica or euhelopodidae depending on how the cladistics stack up. In any case it's huge and unusual, and maybe soon we may get to see a description and some idea of its overall proportions.