Yet another Patagonian Giant: Futalognkosaurus dukei

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An artist’s reconstruction, released by the National Museum of Brazil, of the paleoecology inhabited by Futalognkosaurus (left). It is being menaced by Megaraptor, now known to be a tetanuran theropod.

For quite some time it was thought that after the Jurassic period the massive sauropods that roamed North America were all but extinct, a radiation of ankylosaurs, ceratopsians, hadrosaurs, and other ornithischians becoming the primary herbivores in place of the massive long-necked saurischians. At least one genus did hang on until the Late Cretaceous, the titanosaur Alamosaurus sanjuanensis, although it was a dinosaur that seemed to be out of place among the more famous Late Cretaceous North American dinosaur fauna, at least until now. Before plate tectonics the biogeography of dinosaurs was rather enigmatic, the fossil finds of North America seemingly being the standard by which other continents were to be judged by. As other areas in Africa, Asia, and South America were opened up and the mechanisms governing continental drift accepted, though, it became apparent that during the Cretaceous the northern and southern hemispheres had their own distinct dinosaurian fauna. In the northern hemisphere, the landmasses that now make up North America and Asia shared groups like tyrannosaurs, ceratopsians, and hadrosaurs, while recent discoveries in South America have shown that South America and Africa shared Carcharodontosaurid theropods and Titanosaurid sauropods, the continents evolving their own predator and prey animals. Indeed, it is the southern giants that we will be concerned with here, the diverse and massive sauropods of Patagonia.

It appears as if South America had no shortage of unique, large dinosaurs during the Cretaceous, starkly different from the more famous taxa of North America. The Carcharodontosaurid theropod Giganotosaurus carolinii made headlines when it was found that it rivaled Tyrannosaurus rex in size, it’s less well-publicized relatives Tyrannotitan chubutensis and Mapusaurus roseae also pushing the large theropod size limit. The titanosaur sauropods absolutely dwarfed these predators by comparison, the latest one to be announced (but not the last, believe me) being dubbed Futalognkosaurus dukei (I keep wanting to pronounce the tongue-twisting name “Foot-long-o-saurus”). The near unpronounceable genus name is followed by a species name continuing the paleontological tradition of recognizing financiers/land owners/benefactors, in this case the Duke Energy Argentina Company which sponsored the dig.

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Recovered remains of Futalognkosaurus dukei. Note the length of the cervical ribs. From Calvo, J. O., Porfiri, J. D., Gonzalez-Riga, B. J. & Kellner, A. W.A. (2007). A new Cretaceous terrestrial ecosystem from Gondwana with the description of a new sauropod dinosaur. Anais da Academia Brasileira de Ciencias 79(3): 529-541.

As can be seen from the above diagram, the recovered skeleton mostly consisted of vertebrae that made up the neck, back, and sacral (hip) sections of this dinosaur along with most of the hips. Sauropod heads are often missing, but despite the lack of limbs or tail vertebrae there is enough Futakognkosaurus material to diagnose the sauropod as a new kind of titanosaurid that shows a number of affinities to African titanosaur relatives like Malawisaurus and Gondwanatitan, which is especially impressive given the lack of a tail in the holotype. Although there are certainly other ways to identify them, one of the most easily recognized diagnostic characters of titanosaurid sauropods is the existence of ball-and-socket joints between the tail vertebrae. The anterior-facing sides of the centrum (the central body of the vertebrae) are cupped and the posterior-facing side bears a convex “ball,” a feature that was recognized as early as 1877 when Richard Lydekker named Titanosaurus indicus and it is likely that Futakognkosaurus shared this feature.

Speculation about the mysterious missing tail of this animal aside, researchers were fortunate in that they were able to find many of the neck (or cervical) vertebrae, bones that not only serve to discern the taxonomic position of the dinosaur but some of its functional morphology as well. While we may be able to look at reconstructions of sauropods and simply say that they all just evolved long necks (counterbalanced by long tails) there are very important differences in the neck vertebrae of certain groups of sauropods that can give us some clues as to how flexible their necks were and possibly at what level they were cropping vegetation. Indeed, there are two main ways of solving the evolutionary problem of developing a long neck (although there are variations on these themes that exist on a continuum), but before proceeding further I should probably note that all sauropods have tiny heads. While this is an extremely obvious feature and a fact that I’ll be taking for granted through the rest of this discussion, it’s important to note as much of the architecture of the neck deals with supporting its own weight and not a massive head (head size thus being constrained by the physical demands of evolving a long neck).

Regardless of how an organism achieves it, a long neck requires plenty of space for muscle attachment to support the weight of the neck, and one way to do this is to increase the height of the neural spines. This sort of solution was arrived at by sauropods like Diplodocus, showing a sort of forked or bifurcated condition of a split neural spine with two “prongs” close together on the top of the centrum providing muscle support. This sort of “support from the top” is likely indicative of cantilevered necks, the dinosaurs holding their necks out in front of the body rather than erect, the arrangement seemingly occurring in sauropods that had relatively short front legs. This kind of skeletal architecture would allow these dinosaurs a good degree of flexibility side-to-side but they may not have been able to lift their heads up very high, although they probably would have the ability to browse over a range of plant resources from the ground to somewhat above head-level.

The other way to support a long neck is by having muscle support along the bottom of the neck, the neural spine being in a usual, un-bifurcated condition on top of the centrum. The muscles have to attach somewhere, however, and so the sides of the vertebrae bear long cervical ribs, which are elongated spines of bone sweeping backwards and often extending backwards past other vertebrae. This is the sort of condition seen in Futalognkosaurus (estimated being between 32 and 34 meters long) and the largest of sauropod dinosaurs, and it can easily be seen in the diagram of skeletal material above. If you look at the 8th vertebrae back you’ll see one of the cervical ribs illustrated beneath the vertebral column, extending backwards. Other vertebrae had these structures as well, getting longer as the vertebrae got farther back from the head, and these provided plenty of space for muscle attachment. Still, there is a price to pay for such long structures that overlap each other, namely a loss in flexibility. Instead, dinosaurs with elongated cervical ribs could hold their heads higher and erect but did not have snake-like flexibility in their necks, even the aforementioned sauropod Diplodocus having a more flexible neck. Apatosaurus deserves at least a passing mention here as it is a bit strange; it seems to combine the two strategies and might have had the most flexible neck of all sauropods. In terms of our main discussion, however, the two main structural types of sauropod necks could give us clues as to possible behaviors and niches of the animals, different evolutionary and physical constraints resulting in similar forms with differing abilities.

The paper describing the holotype of Futalognkosaurus is significant beyond considerations of giant Patagonian sauropods, however, as the Turonian age (~93.5 mya to ~89.3 mya) fossil beds also bore a number of other fossils that are now undergoing further study. Perhaps most significant of the other fauna at the site was a well-preserved and articulated arm of Megaraptor, the manus of this theropod being so massive that it was previously thought to have been the foot. This realization changes things a bit, booting Megaraptor out of the Coelurosauria and placing it within Tetanurae, it’s formidable digits being somewhat convergent with those of spinosaurid dinosaurs, although the authors of the paper note that the hands and claws of Megaraptor seemed even larger. Hopefully skull material will be found from this theropod as I would be interested to see if it had a lighter or more gracile skull, which would confirm the hypothesis that as theropods evolved heavier and more robust skulls their arms underwent reduction, the possession of large arms and a robust skull making the animal too front-heavy. I guess we’ll just have to wait and see if more of Megaraptor turns up.

The site also contained much more than apex(?) predator and favored prey remains, and the abundance of fossil material allowed the researchers to reconstruct a bit of the paleoecology of the site. During the Late Cretaceous the area appears to have been flat with a meandering river system, perhaps undergoing a cycle of warm and humid conditions punctuated by dry seasons. The plant life by this time was primarily angiosperms, but a few gymnosperms managed to hang on, the plant community providing varying levels of browse for herbivores (small ornithopods were also found at the site, as well as some remains of dromeosaurids that probably fed on them). Crocodiles, turtles, fish, and pterosaurs also are present, corroborating the notion that the site was close to water or at least experienced flooding events, which would be likely if the landscape was as flat as supposed. Thus we can conjure up an image of angiosperm-dominated forests fed by rivers over a flat plain, titanosaurs browsing off the tops of trees and smaller ornithopods feeding lower to the ground and trying to avoid the diversity of predators in the area.

While the site in Argentina that revealed Futalognkosaurus appears to be especially rich in fossils, it was refreshing to read a paper that took into account the ecology of the area. Perhaps it’s my own bias, but I think more effort should be put into placing dinosaurs and other extinct creatures into their proper ecological context as ecology has a large impact of evolution. Indeed, considering dinosaurs outside of the world they inhabited can easily turn into stamp-collecting if we’re not careful, and I hope more researchers follow the lead of Calvo, et al and include more information about environmental context when new finds are described.