Smithsonian.com

Last week, paleontologists at the Argentine Museum of Natural Science in Buenos Aires literally unveiled a new dinosaur. Named Bicentenaria argentina to celebrate the museum’s 200th anniversary and just over two centuries of Argentine independence, the dinosaur was presented in a dramatic mount in which two of the predatory dinosaurs face off against each other.

As yet, there’s not very much to say about the dinosaur. The paper officially describing Bicentenaria has yet to be published. Based on various news reports, though, Bicentenaria appears to be a 90 million year old coelurosaur. This is the major group of theropod dinosaurs that contains tyrannosaurs, deinonychosaurs, therizinosaurs, and birds, among others, and Bicentenaria is reportedly an archaic member of this group that represents what the earliest coelurosaurs might have looked like. It wouldn’t be an ancestor of birds or other coelurosaur groups – by 90 million years ago, birds and other coelurosaurs had already been around for tens of millions of years – but Bicentenaria may have had a conservative body plan that preserved the form of the dinosaurs that set the stage for other coelurosaurs. For now, though, we’re left to admire the impressive skeletal mount until the paper comes out.

The rebbachisaurid Limaysaurus. This sauropod was similar to the ones discovered by Salgado and colleagues in the Patagonian bonebed. Image by FunkMonk, from Wikipedia.

Dinosaur skeletons are marvelous things. The reconstructed bones of Allosaurus, Stegosaurus, Styracosaurus, Barosaurus and the like are beautiful monuments of natural architecture. But what really makes the skeletons so fantastic is that we know they once cradled viscera and were wrapped in flesh. It’s impossible to look at a dinosaur’s skeleton and not wonder about how the animals looked and acted in life.

How social dinosaurs were is one of the most persistent mysteries of their natural history. Rare trackways record the steps of dinosaurs that walked together, and bonebeds containing the bones of multiple individuals of a particular species have sometimes been taken as evidence that the dinosaurs must have been traveling together when they died. But the evidence is never straightforward. Sometimes multiple dinosaurs walked over the same patch of ground at different times, creating trackway slabs that record the independent activities of several dinosaurs rather than a coordinated herd. And just because dinosaurs were preserved together doesn’t necessarily mean that they composed a social group—natural disasters such as drought and flood, as well as transportation of carcasses by water, can create assemblages of animals that didn’t actually flock together in life. Great care is required in piecing together dinosaur lives.

With this in mind, I was curious to read a paper by Leonardo Salgado and colleagues in the latest Journal of Vertebrate Paleontology about possible evidence for social sauropods from Cretaceous Patagonia. While searching for a previously discovered dinosaur quarry in Argentina, Salgado and collaborators stumbled across a small bonebed containing the jumbled remains of three sauropods. The deposit was formed over 100 million years ago.

The largest dinosaur at  the site—presumably an adult—was primarily represented by strings of articulated vertebrae arranged in the classic dinosaur death pose, while two smaller sauropod skeletons were scattered in other parts of the quarry. The dinosaurs are still undergoing study and don’t have a formal identity yet, but they appear to be rebbachisaurids, a group of sauropods that were distant cousins of the more familiar Diplodocus.

The juvenile dinosaurs alone were a significant find—no one had identified juvenile rebacchisaurids before. But the association of those skeletons is the focus of the new paper. Evidence from trackways and bonebeds has hinted that different sauropods had distinct social structures. Some, such as Alamosaurus, seemed to group together in small herds as juveniles and either become solitary as they grew or form age-segregated adult herds. Other sauropods seemed to live in mixed-age herds, where juveniles remained with older individuals. In the case of the bonebed in Argentina, it would seem that juveniles and adults traveled together.

But how do we know these dinosaurs really lived together? The skeletons are incomplete and mostly disarticulated—perhaps they were all washed up to the same spot and buried. Salgado and co-authors present a different interpretation.  The bonebed doesn’t seem to be a trap or mire, and the paleontologists noted that the skeletons show “few signs of transport.” It would seem that the sauropods died all at once. The reason why is a mystery. While they frustratingly do not provide details about this scenario, the researchers speculate that “the death of the adult triggered the death of the two juvenile individuals.”

The fact that the three dinosaurs were preserved in place, without evidence of transport, seems to be fair evidence that this species of sauropod was social. But even that hypothesis brings up a series of other questions. Did individuals stay with the herd from the time they were born? Was there any form of parental care after the babies left the nest? Did these dinosaurs really form large herds, or did the young simply stick with one of their parents? We still have a lot to learn about the lifestyles of the big and extinct.

References:

Myers, T., & Fiorillo, A. (2009). Evidence for gregarious behavior and age segregation in sauropod dinosaurs Palaeogeography, Palaeoclimatology, Palaeoecology, 274 (1-2), 96-104 DOI: 10.1016/j.palaeo.2009.01.002

Salgado, L., Canudo, J., Garrido, A., & Carballido, J. (2012). Evidence of gregariousness in rebbachisaurids (Dinosauria, Sauropoda, Diplodocoidea) from the Early Cretaceous of Neuquén (Rayoso Formation), Patagonia, Argentina Journal of Vertebrate Paleontology, 32 (3), 603-613 DOI: 10.1080/02724634.2012.661004

A skeleton reconstruction of Eoabelisaurus, showing the recovered parts of the skeleton. From the LMU press release.

Some dinosaur lineages are more famous than others. I can say “tyrannosaur” and most anyone immediately knows what I’m talking about: a big-headed, small-armed predator similar to the notorious Tyrannosaurus rex. The same goes for “stegosaur,” and of course it helps that Stegosaurus itself is the famous emblem of this bizarre group. But public understanding hasn’t kept up with new discoveries. In the past two decades, paleontologists have identified various dinosaur lineages vastly different from the classic types that gained their fame during the Bone Wars era of the late 19th century. One of those relatively obscure groups is the abelisaurids: large theropod dinosaurs such as Carnotaurus with high, short skulls and ridiculously stubby arms that make T. rex look like Trogdor the Burninator. And paleontologists Diego Pol and Oliver Rauhut have just described an animal close to the beginning of this group of supreme predators—a dinosaur from the dawn of the abelisaurid reign.

Pol and Rauhut named the dinosaur Eoabelisaurus mefi. Discovered in roughly 170-million-year-old Jurassic rock near Chubut, Argentina, the mostly complete dinosaur skeleton is about 40 million year older than the next oldest abelisaurid skeleton. Eoabelisaurus, placed in context with other theropod dinosaurs of the same era, represents a time when predatory dinosaurs were undergoing a major radiation. Early members of many terrifying Cretaceous predators such as the tyrannosaurs and abelisaurids had already appeared by the Middle to Late Jurassic.

Not all of these Jurassic predators looked quite like their later Cretaceous counterparts. Jurassic tyrannosaurs such as Juratyrant and Stokesosaurus were relatively small predators, unlike their bulky, titanic relatives from the Late Cretaceous. Eoabelisaurus was a little closer to what was to come.

Despite being many tens of millions of years older than relatives such as Carnotaurus and Majungasaurus, the newly described dinosaur displays some tell-tale features that characterize the group. While a significant portion of the dinosaur’s skull is missing, the head of Eoabelisaurus had the short, deep profile seen among other abelisaurids. And this dinosaur already had distinct forelimbs. Much like its later relatives, Eoabelisaurus had a strange combination of heavy shoulder blades but wimpy forelimbs, with a long upper arm compared to the lower part of the arm. The dinosaur’s condition was not as extreme as in Carnotaurus—a dinosaur whose lower forelimbs were so strange that we have no idea what, if anything, Carnotaurus was doing with its arms—but they were still comparatively small and tipped with little fingers good for wiggling but probably useless in capturing prey.

And with a 40-million-year gap between Eoabelisaurus and its closest kin, there are plenty of other abelisaurids to find. The question is where  they are. Is their record so poor that very few were preserved? Or are they waiting in relatively unexplored places? Now that the history of these blunt-skulled predators has been pushed back, paleontologists can target places to look for the carnivores.

Reference:

Pol, D., Rauhut, O. (2012). A Middle Jurassic abelisaurid from Patagonia and the early diversification of theropod dinosaurs. Proceedings of the Royal Society B, 1-6 : 10.1098/rspb.2012.0660

A clutch of sauropod eggs at the geothermal nesting site in Argentina. Eggs are outlined by black dashes. From Fiorelli et al., in press.

Imagine a dinosaur as massive as Apatosaurus sitting on a nest. It doesn’t really work, does it? We know without a doubt that these large sauropod dinosaurs laid eggs, but there is no conceivable way that the gargantuan dinosaurs could have sat on their grapefruit-sized eggs without crushing them all. There must have been some other way that the eggs could have been kept safe and warm enough to develop properly. One special site in Argentina suggests that some sauropods had a geological solution to the problem.

Two years ago, paleontologists Lucas Fiorelli and Gerald Grellet-Tinner announced the discovery of a unique nesting site that sauropods returned to multiple times. During a stretch between 134 million and 110 million years ago, expectant mother sauropods came to this site to deposit clutches of up to 35 eggs within a few feet of geysers, vents and other geothermal features. This basin held naturally heated dinosaur nurseries.

A new, in-press paper about the site by Fiorelli, Grellet-Tinner and colleagues Pablo Alasino and Eloisa Argañaraz reports additional details of this site. To date, more than 70 clutches of eggs have been found across an area spanning more than 3,200,00 square feet in a section of rock about four feet thick. Rather than focusing on the habits of the dinosaurs, however, the new study fills out the geological context of the place as a possible explanation for why the dinosaurs came here.

On the basis of geological features and minerals, the authors suggest that the site may have resembled the Norris Geyser Basin of present-day Yellowstone National Park. A series of underground pipes and tubes fed geysers, hot springs and mud pots scattered across an ancient terrain crossed by rivers. The fact that the egg clutches are consistently found near the heat-releasing features is taken by Fiorelli and co-authors as an indication that parent dinosaurs were seeking out these spots to lay their eggs. And this site isn’t the only one. Fiorelli and collaborators also point out that similar sauropod egg sites have been found in South Korea.

Exactly what happened to preserve so many nests is not immediately clear, but the eggs were buried in sediments at least partly produced by the surrounding geothermal features. The eggs were eroded and thinned by the acidic nature of the entombing sediment. Some eggs were destroyed by these and other processes, but others held out and became preserved in place.

Not all sauropod dinosaurs selected such sites for nests. Particular populations near geothermal features may have received a benefit from the natural heat, but how did other populations and species far removed from these hot spots lay and protect their nests? We still have much to learn about how baby sauropods came into the world.

References:

Fiorelli, L., Grellet-Tinner, G., Alasino, P., & Argañaraz, E. (2011). The geology and palaeoecology of the newly discovered Cretaceous neosauropod hydrothermal nesting site in Sanagasta (Los Llanos Formation), La Rioja, northwest Argentina Cretaceous Research DOI: 10.1016/j.cretres.2011.12.002

A reconstruction of Patagonykus. The newly-described Bonapartenykus was a close relative of this dinosaur. Image from Wikipedia.

Alvarezsaurs are Cretaceous mysteries. These small dinosaurs, a feathered subgroup of coelurosaurs, had long jaws studded with tiny teeth, and their arms were short, stout appendages that some researchers hypothesize were used to tear into anthills or termite mounds. But no one knows for sure. We understand very little about the biology of these dinosaurs, but even as we puzzle over their natural history, more previously unknown genera are being found. The latest is Bonapartenykus ultimus from the Late Cretaceous of Patagonia, and what makes this dinosaur so special is what was found with its bones.

Paleontologists Federico Agnolin, Jaime Powell, Fernando Novas and Martin Kundrát describe the new dinosaur in an in-press Cretaceous Research paper. The alvarezsaur was not in good shape when the researchers found it. While some of the bones, particularly those of the leg, were close to their original articulation, Bonapartenykus is represented by an incomplete set of partially damaged bones, without a skull. In life, the dinosaur is estimated to have been about eight and a half feet long. (Subtle characteristics of the preserved vertebra, shoulder girdle, and hips are what led Agnolin and co-authors to identify this animal as an alvarezsaur despite the paucity of bones.) But there was also something else. Next to the bones were the battered remnants of at least two dinosaur eggs. Could these be fossil evidence of a Bonapartenykus that was protecting its nest?

Determining who laid those eggs is a difficult task. No evidence of embryos has been found inside the egg, so we can’t entirely be sure of what kind of dinosaur was growing inside. The close association between the fossils is the primary line of evidence that the eggs might be attributable to Bonapartenykus. This is the hypothesis favored by Agnolin and co-authors, but they doubt that the small site represents parental care. There is no evidence of a nest. Instead the scientists suggest that the two eggs may still have been inside the dinosaur when it died—a hypothesis based on the previous discovery of an oviraptorosaur from China with a pair of eggs preserved where the dinosaur’s birth canal would have been. When the alvarezsaur perished, the eggs may have fallen out of the body and been preserved with the bones.

Yet I wonder if there might be alternative explanations. Just because fossils are found together does not necessarily mean that the organisms those fossils represent interacted in life. Making connections between organisms found at the same site requires a detailed understanding of taphonomy—what happened to those organisms from the time of death to discovery. In this case, the bones of  Bonapartenykus are scattered and poorly preserved, and the eggs were also partially broken. Did the animal simply fall apart, as the authors seem to suggest, or were the bones and eggs brought together through rushing water? Perhaps the body of Bonapartenykus was carried by a water flow to the location of the eggs, fell apart after the water receded and then was buried again. This is a bit of armchair speculation on my part, and the hypothesis proposed by Agnolin and co-authors is a reasonable one, but we need a detailed understanding of how this little fossil pocket formed if we are to understand the relationship between the eggs and the bones. The geological and taphonomic details of the fossil site are important for framing hypothesis about what happened so many millions of years ago. We may have to wait for more intricately preserved fossils to be sure. A Bonapartenykus preserved on a nest, or a female dinosaur with eggs preserved within her hips, would do nicely.

References:

Agnolin, F., Powell, J., Novas, F., & Kundrát, M. (2011). New alvarezsaurid (Dinosauria, Theropoda) from uppermost Cretaceous of north-western Patagonia with associated eggs Cretaceous Research DOI: 10.1016/j.cretres.2011.11.014