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The giant sauropod Futalognkosaurus (at left) with some of its Cretaceous neighbors. Art by Maurilio Oliveira.

Which was the biggest dinosaur ever? We don’t know. Even though the size-based superlative draws a great deal of attention, paleontologists have uncovered so many scrappy sauropod skeletons that it’s difficult to tell who was truly the most titanic dinosaur of all. But, among the current spread of candidates, Futalognkosaurus dukei is one of the most complete giant dinosaurs yet found.

Discovered in 2000, and named in 2007 by Universidad Nacional del Comahue paleontologist Jorge Calvo and colleagues, Futalognkosaurus was one of many dinosaurs found in an exceptionally rich, roughly 90-million-year0old deposit in northwest Argentina. From fossil plants to pterosaurs, fish and dinosaurs, the one site entombed vestiges of a vibrant Cretaceous ecosystem. And, on that landscape, no dinosaur was as grand the newly named titanosaur.

Contrary to what you might expect given their skeletal sturdiness, the biggest sauropods are often found as partial skeletons. Our knowledge of Argentinosaurus, Puertasaurus, Supersaurus, Diplodocus hallorum and other giants is frustratingly incomplete, and figuring out how large they truly were relies on estimation from more complete representatives of other species.

The lack of complete tails from these dinosaurs makes the matter even more problematic. Dinosaur tails varied in length from individual to individual, and different subgroups had proportionally longer or shorter tails. In the case of Diplodocus hallorum, for example, a great deal of the dinosaur’s estimated  100-foot-plus length comes from the fact that other Diplodocus species had very long, tapering tails.

We don’t really know how long Futalognkosaurus was because, with the exception of a single vertebra, the dinosaur’s tail is entirely missing. Nevertheless, the sauropod that Calvo and coauthors described is remarkable for encompassing the entire neck, back and associated ribs, and the majority of the hips. Together, these elements represent over half the skeleton and comprise the most complete giant sauropod individual yet known.

Even if skeletal incompleteness keeps us from knowing exactly how big Futalognkosaurus was, the collected bones can leave no doubt that this was a truly enormous dinosaur. Calvo and coauthors estimated that the whole animal stretched between 105 and 112 feet in length, which would put it in the same class as the more famous (and less complete) Argentinosaurus. As the paleontologists at SV-POW! said when they posted images of Futalognkosaurus bones next to Juan Porfiri, who helped describe the dinosaur, there’s no doubt that the sauropod was “darned big.” The challenge is finding and filling in the parts of the dinosaur’s body that have not yet been found. There will undoubtedly be other challengers for the title of biggest dinosaur, but, for now, Futalognkosaurus remains our most detailed representative of the biggest of the big.

References:

Calvo, J., Porfiri, J., González-Riga, B., Kellner, A. 2007. A new Cretaceous terrestrial ecosystem from Gondwana with the description of a new sauropod dinosaur. Anais da Academia Brasileira de Ciências. 79, 3: 529-541

Calvo, J., Porfiri, J., González-Riga, B., Kellner, A. 2007. Anatomy of Futalognkosaurus dukei Calvo, Porfiri, González Riga, & Kellner, 2007 (Dinosauria, Titanosauridae) from the Neuquen Group, Late Cretaceous, Patagonia, Argentina. Arquivos do Museu Nacional 65, 4: 511–526.

Novas, F. 2009. The Age of Dinosaurs in South America. Bloomington: Indiana University Press. pp. 201-202

Fossil swim tracks indicate that theropods similar to this Megapnosaurus at least occasionally swam in prehistoric lakes and rivers. Art by Dmitry Bogdanov, image from Wikipedia.

Paleontologist R.T. Bird inspected many dinosaur trackways while combing Texas for the perfect set to bring back to the American Museum of Natural History. During several field seasons in the late 1930s, Bird poked around in the Early Cretaceous rock in the vicinity of the Paluxy River for a set of sauropod footprints that would fit nicely behind the museum’s famous “Brontosaurus” mount. Bird eventually got what he was after but not before poring over other intriguing dinosaur traces. One of the most spectacular seemed to be made by a swimming dinosaur.

Known as the Mayan Ranch Trackway, the roughly 113-million-year-old slab is almost entirely made up of front foot impressions. The semicircular imprints were undoubtedly left by one of the long-necked sauropod dinosaurs. But towards the end of the trail, where the dinosaur’s path makes an abrupt turn, there was a single, partial impression of a hind foot.

At the time Bird and his crew uncovered this trackway, sauropods were thought to be amphibious dinosaurs. Other than their immense bulk, what defense would they have had but to trundle into the water, where theropods feared to paddle? Under this framework, Bird thought he knew exactly how the Mayan Ranch Trackway was made. “The big fellow had been peacefully dog-paddling along, with his great body afloat, kicking himself forward by walking on the bottom here in the shallows with his front feet,” Bird wrote in his memoir. The great dinosaur then kicked off with one of its hind feet and turned.

With the exception of well-defended dinosaurs such as the ceratopsids and stegosaurs, many herbivorous dinosaurs were thought to be at least semi-aquatic. There seemed to be only two options for Mesozoic prey species–grow defenses or dive into the water. In time, though, paleontologists realized that the sauropods, hadrosaurs and other herbivores didn’t show any adaptations to swimming. Our understanding of the ecology of these dinosaurs was based on false premises and faulty evidence.

In the case of the Mayan Ranch Trackway, for example, there’s no indication that the sauropod that made the trackway was swimming. A more likely scenario has to do with evolutionary changes among sauropods. While the sauropods that dominated the Late Jurassic of North America–such as Diplodocus, Apatosaurus and Barosaurus–carried much of their weight at the hips and left deeper hindfoot impressions, the center of mass shifted among their successors–the titanosaurs–such that more of the weight was carried by the forelimbs. Hence, in some trackways, the deeper impressions made by the forefeet are more likely to stand out than those made by the hindfeet, especially if some of the top layers of the rock are eroded away to leave only “undertracks.” What seemed to be evidence of swimming sauropods instead owes to anatomy and the characteristics of the mucky substrate the dinosaur was walking on.

As far as I’m aware, no one has yet found definitive evidence of swimming sauropods or hadrosaurs–the two groups previously thought to rely on water for safety. Stranger still, paleontologists have recently uncovered good evidence that theropod dinosaurs weren’t as bothered by water as traditionally believed. In 2006, paleontologists Andrew Milner, Martin Lockley and Jim Kirkland described swim tracks made by Early Jurassic theropods at a site that now resides in St. George, Utah. Such traces weren’t the first of their kind ever discovered, but the tracksite was one of the richest ever found.

Small to medium-sized theropods made the St. George swim tracks–think of dinosaurs similar to Megapnosaurus and Dilophosaurus. Even better, the large number of smaller-size swim tracks hints that whatever dinosaurs made these tracks were moving as a group as they struggled against the current in the lake shallows. The larger dinosaurs, on the other hand, were a bit taller and able to wade where their smaller cousins splashed around.

A different team of researchers announced additional evidence for swimming theropods the following year. Paleontologist Rubén Ezquerra and co-authors described dinosaur swim traces from Early Cretaceous rock near La Rioja, Spain. Based on the details of the track and their direction, the theropod was swimming against a current that pushed the dinosaur diagonally. Along with other theropod swim tracks, the researchers noted, the discovery meant that paleontologists would have to revise their ideas about the kind of habitats theropods lived in and what carnivorous species would do. Theropod dinosaurs were not so hydrophobic, after all.

Does this mean that dinosaurs like Dilophosaurus were adapted to an amphibious lifestyle? Not at all. As Ezquerra and co-authors pointed out, the swimming strokes of these dinosaurs were exaggerated walking motions. The way the dinosaurs moved on land allowed them to be adequate swimmers while crossing rivers or lakes, but, compared with semi-aquatic animals such as crocodiles and otters, no known dinosaur shows traits indicative of a primarily waterlogged existence. (And dinosaurs found in marine sediments don’t count as evidence, as these were washed out to sea prior to burial. I can’t imagine ankylosaurs taking to life among the high seas, in any case.) Some dinosaurs could swim, but that doesn’t mean that they made the water their home. Still, thanks to special prehistoric traces, we can imagine packs of Megapnosaurus fighting to get ashore, and Dilophosaurus strutting into the shallows, aiming to snatch whatever fish were foolish enough to swim into the carnivore’s shadow.

References:

Bird, R.T. (1985). Bones for Barnum Brown, edited by Schreiber, V. Forth Worth: Texas Christian University Press. pp. 160-161

Ezquerra, R., Doublet, S., Costeur, L., Galton, P., Pérez-Lorente, F. (2007). Were non-avian theropod dinosaurs able to swim? Supportive evidence from an Early Cretaceous trackway, Cameros Basin (La Rioja, Spain) Geology, 40 (10), 507-510 DOI: 10.1130/G23452A.1

Milner, A., Lockley, M., Kirkland, J. (2006). A large collection of well-preserved theropod dinosaur swim tracks from the Lower Jurassic Moenave Formation, St. George, Utah. New Mexico Museum of Natural History and Science Bulletin, 37, 315-328

Tail vertebrae from a previously known Alamosaurus specimen (A), compared with a newly-discovered Alamosaurus tail vertebra (B) and a tail vertebra from the large titanosaur Futalognkosaurus (C). From Fowler and Sullivan, 2011.

Alamosaurus was an unusual sauropod. What makes it so remarkable is not so much its appearance—the dinosaur seems to be a fairly typical member of a group called titanosaurs—but when and where it lived. Even though North America once hosted multiple, coexisting genera of sauropods during the Late Jurassic, that diversity was eventually lost until, about 100 million years ago, there were none left on the continent. By this time the horned dinosaurs and hadrosaurs were the primary herbivores on the landscape. Then, after a 30 million year absence, sauropods returned to what is now the southwestern United States in the form of Alamosaurus. A new study suggests this dinosaur may have been one of the biggest ever.

Among the various dinosaur superlatives, the title of “biggest dinosaur” is always the most hotly contested. It’s also among the most difficult to assign. Despite their size, many of the contenders for biggest dinosaur, whether they’re going for the all-time title or just in a particular slice of prehistory, are known from only partial remains. Some degree of estimation is often required to envision the whole animal. Then there’s the problem of what characteristics make a dinosaur the largest of its kind—is it just length? Or do weight and height also get figured in? Many of the contenders for largest sauropod ever are estimated to have been about 90 to to 110 feet long, and so far, no dinosaurs have exceedingly outstripped that range. (I am not counting the supposed giant Amphicoelias since the original material was lost long ago and no additional remains have been confirmed.) This range may represent some kind of upper limit for sauropod body size due to constraints of structure or biology.

According to the new paper by paleontologists Denver Fowler from the Museum of the Rockies and Robert Sullivan of the State Museum of Pennsylvania, newly-discovered fossils from the latest Cretaceous of New Mexico may indicate that Alamosaurus reached the top tier of titanosaur size. The material in question consists of two partial vertebrae and a partial femur found at different field sites, and the assignment of these bones to Alamosaurus is based upon the fact that this sauropod is the only one presently known from the Late Cretaceous deposits where the remains were found. (Though it should be noted that the fragmentary nature of this specimen and others from around the same time makes comparisons between dinosaurs and estimates of actual diversity difficult.) The fact that the bones were found at different field sites means that the three bones came from different individuals of different sizes and ages, but comparison of these bones with those of other sauropods can provide a rough idea of how large Alamosaurus grew to be.

The two vertebrae described by Fowler and Sullivan appear to indicate that Alamosaurus could reach roughly the same size as the titanosaurs Futalognkosaurus and Puertasaurus. Both of these dinosaurs from South America are estimated to have been within the roughly 90 to 110 foot size range that many other big sauropods fall into, although the fact that paleontologists have not yet found complete skeletons of either dinosaur means we can’t be really sure just how big they got. This makes estimating the actually upper size range of Alamosaurus problematic. The vertebrae Fowler and Sullivan describe are certainly larger than those previously assigned to Alamosaurus, but accurately estimating the size of the dinosaur requires reliance on previous estimates of other partially known specimens. Alamosaurus appears to have been in the upper sauropod size class—and was probably among the biggest ever to have lived in North America—but additional fossil material from this dinosaur and other giants will be needed to figure out exactly how enormous they became.

There was one other wrinkle to this story that caught my attention. A Montana State University press release about the study stated that a tyrannosaur tooth was found near another Alamosaurus vertebrae that was being excavated by the same team. Whether this is an indication of predation or scavenging by the tyrannosaur remains to be seen—teeth are resilient and easy to transport—but the association is a further confirmation that Alamosaurus shared its habitat with Tyrannosaurus rex. The two dinosaurs have been found in the same deposits before, such as Utah’s North Horn Formation, and the occurrence of the two dinosaurs in New Mexico makes me wonder exactly how a large tyrannosaur would go about hunting an enormous sauropod. Clashes of titanic dinosaurs were not restricted to the Late Jurassic of North America or the Cretaceous of South America. At the close of the Cretaceous, prehistoric New Mexico may have been the setting for confrontations between the largest herbivore and carnivore ever to live in North America.

References:

Fowler, D., & Sullivan, R. (2011). The first giant titanosaurian sauropod from the Upper Cretaceous of North America Acta Palaeontologica Polonica DOI: 10.4202/app.2010.0105

Part of a sauropod trackway from the Teruel, Spain tracksite. Image from Castenera et al., 2011.

Sometime between 145 million and 140 million years ago, in the vicinity of what is now Teruel, Spain, a small herd of sauropod dinosaurs traveled together near a shallow, sandy bay. We know this because they left their footprints in the rock record, and paleontologist Diego Castanera and colleagues have just released an in-press report about these significant trackways in Palaeogeography, Palaeoclimatology, Palaeoecology.

Fossilized sauropod footprints have been described from the region before, but trackways—especially those of multiple individuals—are rare. Moreover, trackways record prehistoric behaviors that we can’t observe from our 21^st century perspective, so a collection of tracks left by several sauropods can offer insights into how the animals moved as well as their social lives.

Naturally, knowing the exact genus or species of dinosaur that created the tracks is impossible. Tracks don’t come with labels, and unless an animal literally dies in its tracks, determining the specific creature that created the traces is fraught with uncertainty. Nevertheless, the anatomy of tracks often allows paleontologists to narrow down the list of suspects to particular dinosaur subgroups. In this case, sauropods are the best fit for the kidney-shaped tracks left by the front feet and the roughly triangular prints left by the hind feet, especially given their distance from one another.

What kind of sauropods left the tracks? That’s difficult to say, but Castanera and co-authors propose that small titanosaurs might be the best fit. This widespread sauropod group—which included the gargantuan Argentinosaurus and the dwarf genus Magyarosaurus—was partly characterized by having wide chests, which gave their trackways a “wide gauge”—or a wider gap between the left and right limbs—that matches the pattern seen in the Teruel tracks.  The problem is that the bones of titanosaurs are virtually unknown from the appropriate place and time period, so the trackways could have been left by another sort of sauropod which moved in a similar way.

Regardless of what sort of sauropod left the tracks, though, the most significant aspect of the site is that it preserves the tracks of six individual animals moving in the same direction, nearly parallel to each other. This pattern is typical of other trackways where groups of dinosaurs were moving together. The tracksite represents a herd and not simply a collection of unassociated tracks.

These sauropods were relatively small. The hind foot prints are between nine inches and a foot in length—these animals were not earth-shakers. Frustratingly, though, it is presently impossible to tell whether the track-makers were juvenile animals or just small sauropods. If all the animals were juveniles, then the trackway would throw support to the idea that young sauropods stuck together in small herds after they left the nest, but if the dinosaurs were dwarfed then the tracks may indicate a peculiar, isolated environment where isolated lineages of big dinosaurs evolved into small dinosaurs. Such tiny sauropods have been found in Romania, and represent a widespread but poorly understood phenomenon in which island habitats change organisms in strange ways. Whether the tracks found near Teruel represent another case of nano-sauropods remains to be seen.

References:

Castanera, D., Barco, J., Díaz-Martínez, I., Gascón, J., Pérez-Lorente, F., & Canudo, J. (2011). New evidence of a herd of titanosauriform sauropods from the Lower Berriasian of the Iberian Range (Spain) Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2011.07.015

Two views of the skull of Tapuiasaurus. From Zaher et al, 2011.

Sauropod skulls are rare. As big as impressive as these long-necked giants were, they often lost their heads after death. There were decades of confusion over what the skull of Apatosaurus looked like. This makes the discovery of any complete sauropod skull cause for celebration, and I was delighted to hear that an international team of paleontologists has just described the noggin of a previously unknown sauropod from Brazil.

Described in PLoS One, the new dinosaur is named Tapuiasaurus macedoi. A member of the sauropod subgroup called titanosaurs, its bones were discovered in the 125- to 112-million-year-old strata of Brazil. Vertebrae, parts of the shoulder, the radius, and a femur from this dinosaur were found, but the most significant discovery was a complete skull. In overall form the skull resembles those of titanosaurs from other places—such as Rapetosaurus from Madagascar and Nemegtosaurus from Mongolia—in having an elongated snout, a nasal opening at the level of the eyes, and narrow tooth crowns.

But Tapuiasaurus lived long before these dinosaurs. Even though Rapetosaurus and Nemegtosaurus were found to be the closest known relatives of the new dinosaur, they lived about 30 million years after Tapuiasaurus. In terms of evolutionary history, this means that the skull shape of the Late Cretaceous titanosaurs evolved much earlier than was previously thought, and there are probably many other related dinosaurs yet to be found in the gap between Tapuiasaurus and its later cousins. With any luck, more skulls will turn up to fill out the evolutionary history of these dinosaurs.

References:

Zaher, H., Pol, D., Carvalho, A., Nascimento, P., Riccomini, C., Larson, P., Juarez-Valieri, R., Pires-Domingues, R., da Silva, N., & de Almeida Campos, D. (2011). A Complete Skull of an Early Cretaceous Sauropod and the Evolution of Advanced Titanosaurians PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0016663