Nigersaurus taqueti!

When you hear the word "sauropod," what's the first image that comes to mind? For many people it's an immense, dull colored behemoth lumbering across the landscape (or perhaps wallowing in a swamp if you were first introduced prior to the Dinosaur Renaissance), its long neck stretching out to crop conifers from the high branches of nearby trees. Edwin Colbert pins down this classical image of "Brontosaurus" in his book The Year of the Dinosaur (1977);

[The female Brontosaurus] left the forest, crossed the beach, and entered the shallow water in long, splashing strides. With each step her huge feet sank deep into the muddy bottom, leaving impressions as big as washtubs. As each foot pressed down, bearing the great weight which it partially supported, the mud welled up around it, to form a raised rim bordering the depression that was left when the foot was lifted. Within the rims at the front of the hindfoot impressions were deep marks made by the claws, three for each of these tracks.

Thus she waded through water that was too shallow to reach her belly, so that her full weight pressed down on her feet to leave telltale footprints marking her course, yet deep enough so that her tail floated. Consequently there was no groovelike mark of the tail drag between the footprints, as there might have been if her journey had been recorded on dry land.

Her gait was unhurried, for there was no reason for her to try to make haste. Each foot was firmly planted into the muddy bottom; each foot was lifted out of the ooze with a sucking sound. She waded in a straight course through the shallow water, making for the somewhat distant shore which she could see on the other side of the embayment.

The female sauropod is then attacked by an Allosaurus, although she escapes and finds refuge in a swamp, the ensuing narrative recalling the famous visual imagery of a particular Charles R. Knight painting (reproduced above);

She was not alone. In the swamp, which was of considerable extent, there were other sauropod dinosaurs like herself, wading and feeding. Some of them had been there when she first plunged into its protecting environs, as she fled from Allosaurus, and at that instant those established inhabitants of the marsh, alarmed by the charge of the attacking carnosaur, had retreated as far as they could from the scene of conflict. Now, when all was quiet, they had returned to where the wounded giant rested, and as time went on they were joined by others of their kind. Indeed, a sauropod herd had gathered together, to rest and occasionally to feed within the haven of the marsh.

It must not be thought that the herd of sauropods remained stationary within the confines of the swamp. There was constant, widespread motion, so that the opaque water, almost choked in many places by abundant vegetation, was virtually never still. Although the plant life of the swamp grew prolifically, the dinosaurs were big, and consequently they required, individually and as a herd, tremendous volumes of food. Their food was bulky, and great quantities of plants were necessary to produce the energy needed to maintain to proper functioning of the gigantic bodies.

Fortunately the great sauropod dinosaurs have been ushered out of the confines of the swamp (and out of their supposed temporal and geographic restrictions in Jurassic North America), but for many people the "Theory of the Brontosaurus" is still "All Brontosauruses are thin at one end; much, much thicker in the middle; and then thin at the other far end."

This is often paired with the constant reassignment of the highly prized title of the biggest dinosaur, Diplodocus, Barosaurus, Mamenchisaurus, Brachiosaurus, Futalognkosaurus dukei, and Argentinosaurus all being ascribed the notoriety at one time or another, "biggest" meaning anything from longest to heaviest with little consideration of at least both factors together in popular literature. If we just focus on the dinosaurs that loomed large on the landscape, though, we're going to miss out on some truly amazing animals, and the newly announced Nigersaurus taqueti* is one such creature that shocked me the first time I saw its reconstruction.

*The paper describing this new enigmatic sauropod is available for free through PLoS. Please download it to read about this odd dinosaur for yourself!

Nigersaurus, found in the sands of the Sahara, is an Early Cretaceous rebbachisaurid sauropod, the rebbachisaurids being relatively poorly known members of the Superfamily Diplodocoidea, a group that includes the famous dinosaurs like Apatosaurus but also the Family Dicraeosauridae and the Family Rebbachisauridae. Both of these families deviate in important ways from the more familiar forms within the Family Diplodocidae, the rebbachisaurids in their lack of bifurcated nueral spines (which allow for the attachment of neck muscles along the top of the neck) and apparently in the arrangement of their teeth if Nigersaurus is anything to go by. While we will get to the rest of the skeleton in due time, it is the reconstructed skull of Nigersaurus that is so striking, possessing a very broad muzzle packed with teeth that appear to be somewhat convergent with the tooth batteries of some ornithischian dinosaurs, a definite difference from the paltry number of pencil-like teeth in the skulls of diplodocids like Barosaurus.

Reconstructing the niche and lifeways of sauropods have long been controversial and problematic. As seen in Colbert's reconstruction quoted above, many thought that the largest of these dinosaurs would be most comfortable in water (some going so far as to say that they couldn't even leave the swamp as their bodies would collapse on themselves), feeding on soft marsh plants. No matter what they were doing, sauropods would obviously need huge amounts of food regardless of their metabolic rate, but even when they were moved out of the marshes there has been much debate as to whether these animals were feeding low to the ground, off the tops of trees, or even if they could rear up to reach food inaccessible to other, shorter, herbivores. In fact, paleontologist Robert Bakker, rebelling against the image of sluggish dinosaurs munching on algae, took sauropods to the other extreme, writing in his controversial book The Dinosaur Heresies (1986);

Every single one of the giant Late Jurassic dinosaurs was in reality a high browser of some sort. Brachiosaurus's tail was too weak for a tripodal posture, but it compensated with its spectacularly long neck, so that even on all fours it could reach up forty feet. Camarasaurus was the shortest-necked brontosaur found at Como. It could nevertheless stretch up to twenty-five feet with its forelegs on the ground, and much higher if it assumed a tripodal stance. Never before nor since the Late Jurassic has the world witnessed such a profusion of high-feeding plant eaters. This was nothing less than a unique epoch in the history of herbivorous habits. No plant or leaf was safe from a dinosaur's mouth unless it stood over fifty feet above the ground!

Bakker's narrative is attractive but the morphology (especially in the construction of the neck) just doesn't hold up in the 21 years since The Dinosaur Heresies was published, especially in light of the many African, South American, and Asian forms that have become known since that time. I will not go into a long discussion of neck construction here (SV-POW! holds a treasure trove of information on that subject), but there are two main ways in which sauropods supported their long necks (although there's lots of variations on the two themes). One way to support a long, heavy neck is to increase muscle attachment along the ventral side of the neck, usually by extending the bony projections along the side of the neck vertebrae called cervical ribs. While this does allow the neck to get longer and support itself, there is a cost to flexibility, long cervical ribs preventing the neck from bending too far from side to side. This sort of strategy is most often seen in titanosaurids (which probably held their necks up, browsing higher off the ground) and related dinosaurs, but the diplodocids that we're concerned with took a different route. In sauropods like Diplodocus, for example, the neural spine on the top of each neck vertebra is bifurcated, looking almost like a tuning fork sticking up from the central body of the bone. In these dinosaurs the muscles attached to these prongs along the dorsal side of the neck, allowing for more side-to-side flexibility in sauropods that held their heads out in front (the "cantilevered" position) rather than more erect. As I mentioned, though, there is variation (Apatosaurus seems to mix the strategies, for example) and it's hard to figure out where exactly Nigersaurus fits in.

The PLoS paper illustrates the 5th cervical vertebra of Nigersaurus, the vertebra lacking the large neural spines of its diplodocid cousins but having cervical ribs slightly longer than the length of each central body of the vertebra itself, the reconstruction (illustrated above) showing the cervical ribs slightly overlapping along the neck. This is a bit puzzling to me as overlapping cervical ribs would seem to decrease the side-to-side flexibility of the neck, yet this is the mechanical solution seen in this dinosaur which is proposed to have been a grazer. Part of the problem with Nigersaurus, however, is that it has a relatively short neck with an estimated 13 cervical vertebrae (only eight are known between the available material from four specimens used in the reconstruction), so it is difficult to tell whether this Early Cretaceous sauropod retained the basal conditions from which the larger forms are derived or there's been some sort of parallel evolution or reversal; more fossil material is needed to figure this out. Indeed, 13 cervical vertebrae is a low number and is unlikely to be the result of a reversal, but since we can't be sure how many neck vertebrae there are in this animal is makes comparisons difficult.

While the position of the neck and its flexibility (or lack therof) is largely not addressed by the authors, they propose that Nigersaurus held its head with its tooth-packed muzzle pointing nearly straight down at the ground, the next stage of a trend that they claim is visible through the evolution of the diplodicoids (a cladogram featuring Massospondylus, Camarasaurus, Diplodocus, and Nigersaurus is provided in the paper). While it does seem that the head of Diplodocus did angle slightly down (this makes sense for a low-browsing herbivore with a cantilevered neck position), the "head down" position of Nigersaurus seems primarily predicated on the reconstruction of the inner ear of the animal. A brief mention is made of the articulation of the skull with the vertebral column that would allow for the reconstruction proposed, there is no diagram of the position of the foramen magnum (the hole in the back of the skull through which the spinal cord passes) or how the head articulated with the first vertebra. Perhaps this is a minor point, but the support for the position of the head mentioned in the article itself seemed a bit truncated (I'm sure it could have made another paper just by itself).

What is further interesting about the available vertebral material (and even the skeleton as a whole) is how light it is; there are lots of holes to lighten the skeleton and some of the bone is so thin that it's hard to imagine how this animal could support itself. This sort of lightening seems to be extreme compared to larger relatives of Nigersaurus, perhaps suggesting that Nigersaurus isn't as indicative of the ancestral conditions for these animals after all. Not much time is spent on this subject, however, although I think it is an important aspect of this creature for further study. The extra openings in the skull are especially curious; why would a relatively small sauropod have a skeleton that was paper thin and full of holes? The authors of the new paper do not speculate on this matter, and while I am too unfamiliar to be certain that the following is a reasonable idea at all, I wonder if it could be an adaptation to a very hot habitat (if you're big and heavy it takes a lot of energy to move around the landscape, so perhaps the lightening of the skeleton was a response to a habitat where great energy efficiency at a given body size was favored [Suchomimus and Ouranosaurus are said to be known from the area as in the supplementary material, their sails perhaps being another solution to dealing with living in a hot environment]). I find it a bit strange, however, that the authors made no mention of the absolutely huge size of the orbits in this animal. While the dentition of this dinosaur is certainly interesting, it's a bit odd that the large eyes were totally ignored, the size of the eyes compared to skull size seeming more like a juvenile character than an adult one. I'm not suggesting that every time big eyes are present in the fossil record it means we're dealing with a immature representative of the species, but I have to wonder why the authors completely ignored this feature of the skull that is so obviously different from related sauropod dinosaurs. Was Nigersaurus a nocturnal grazer, grazing during the cool of the night in the riparian forests of the early Cretaceous? Such a notion is entirely speculative, but it's difficult not to wonder why the eyes of this animal were so large.

Nigersaurus certainly is an intriguing sauropod and I'm sure we'll be hearing more about it in the future, but I have to say that I'm a bit disappointed by this new paper. The details about where it was found are placed in the supplemental information, little attention being paid to paleoecology or taphonomy. While it seems that at least four individuals were discovered, there is no quarry map or details about the skeletons either, the paper presenting the end result but somewhat obscuring some of the process. Likewise, some of the most interesting parts of the entire animal are passed over or ignored altogether, and while many of these subjects can (and may) provide fodder for many more papers, I don't feel as if any part of this work really received a level of sufficient detail (I imagine a lot of work was put into studying these animals, but it doesn't seem to come out in the paper). Hopefully further studies and fossil finds will tell us more about this strange dinosaur and better resolve its ecological position, and I can only imagine what other strange forms may yet lie in Europe and Africa waiting to be discovered.

Note: It's a big day for new sauropods, indeed. Make sure that you read Darren's discussion of Xenoposeidon (possibly representing an entirely new kind of sauropod!) on Tetrapod Zoology and enjoy the week-long exultation of this dinosaur on SV-POW!

For more information on Nigersaurus, please see the Project Exploration website and their press release, and another press release from National Geographic.

References;

Bakker, R. (1986) The Dinosaur Heresies. Zebra Books, New York.

Colbert, E. (1977) The Year of the Dinosaur. Charles Scribners' Sons, New York.

Matthew, W.D. (1915) Dinosaurs. American Museum of Natural History, New York.

Sereno, P. C.; Wilson, J., A.; Witmer, L.M.; Whitlock, J.A.; Maga, A.; Ide, O.; Rowe, T.A. (2007) "Structural Extremes in a Cretaceous Dinosaur." PLoS One 2(11): e1230. doi:10.1371/journal.pone.0001230