December 7, 2012
When paleontologist John Ostrom named Deinonychus in 1969, he provided the spark for our long-running fascination with the “raptors.” Similar dinosaurs had been named before–Velociraptor and Dromaeosaurus were named four decades earlier–but the skeleton of Ostrom’s animal preserved a frightening aspect of the dinosaur that had not yet been seen among the earlier finds. The assembled remains of Deinonychus included the dinosaur’s eponymous “terrible claw”–a wicked, recurved weapon held off the ground on the animal’s hyperextendable second toe. Combined with the rest of the dinosaur’s anatomy, Ostrom argued, the frightening claw indicated that Deinonychus must have been a active, athletic predator.
But how did Deinonychus and its similarly-equipped relatives use that awful toe claw? The appendage looks fearsome, but paleontologists have not been able to agree on whether the claw was using for slashing, gripping, pinning, or even climbing prey. Some researchers, such as Phil Manning and collaborators, have even argued that the claws of Velociraptor and related dinosaurs were best suited to scaling tree trunks–a conclusion consistent with the contentious hypothesis that the ancestors of birds were tree-climbing dinosaurs.
All this assumes that the claws of deinonychosaurs correspond to a special behavior, but can foot claw shapes really give away the habits of dinosaurs? That’s the question posed by a new PLoS One study by zoologist Aleksandra Birn-Jeffery and colleagues.
Based on observations of living animals, researchers have often tied particular claw shapes to certain behaviors–relatively straight, stubby claws likely belong to an animal that runs on the ground, while tree-climbing species have thin claws with small, sharp points. But nature isn’t quite so neat as to have a single, tell-tale claw shape for perchers, ground-runners, climbers, and predators. Even then, researchers don’t always interpret claw shapes the same way–depending on who you ask, the foot claws of the early bird Archaeopteryx either indicate that it was a climber or could only run on the ground.
To parse this problem, Birn-Jeffery and co-authors studied the geometry of the third toe claw–on dinosaurs, the middle toe claw–in 832 specimens of 331 species, together representing different lifestyles of birds, lizards, and extinct dinosaurs. The claw shapes didn’t strictly conform to particular behaviors. In the climber category, for example, the frill-necked lizard has lower claw curvature than expected, and, among predatory birds, the common buzzard, secretary bird, and greater sooty owl has less sharply recurved claws that anticipated for their lifestyle.
When the dinosaur data was dropped into the mix, the deinonychosaurs didn’t seem to fit in any single category. The sickle-clawed carnivores fell into the range shared by climbers, perchers, predators, and ground dwellers–these dinosaurs could be said to be anything from wholly terrestrial runners to perchers. And even though the researchers identified a general claw shape that corresponded to walking on the ground–deeper claws with less curvature–the dinosaurs did not strictly fit into this category alone.
Some dinosaurs, such as Microraptor, had claws that might have been suited to climbing. However, dinosaurs that we might regard as behaviorally similar showed differences–Velociraptor seemed to best fit the ground-dweller category, while the larger Deinonychus seemed to have claws more akin to those of predatory birds. This doesn’t mean that Microraptor was definitely a climber, or that Velociraptor wasn’t a predator. As the authors show, the different behavioral categories are not so easily distinguishable as previously thought, and saying that an animal definitely engaged in a particular behavior because of claw shape alone tempts oversimplification.
No wonder there has been such a range of interpretation about dinosaur foot claws! While the new study focused on the third toe claw rather than the famous, second deinonychosaur toe claw, the point of the analysis still applies. Claw geometry alone is not a reliable indicator of behavior. That’s to be expected–as the authors point out, claws are multi-functional, are are unlikely to represent just one type of behavior or habitat. Birds that use their claws to perch may also use them to kill prey, or birds that primarily live in the trees may also forage on the ground. Claw shape is constrained by different aspects of natural history, and reflect flexibility rather than strict adherence to a particular lifestyle. Deinonychosaur claws definitely hold clues to the natural history of dinosaurs, but drawing out those clues is a difficult, convoluted process.
Birn-Jeffery, A., Miller, C., Naish, D., Rayfield, E., Hone, D. 2012. Pedal Claw Curvature in Birds, Lizards and Mesozoic Dinosaurs – Complicated Categories and Compensating for Mass-Specific and Phylogenetic Control. PLoS ONE. 7,12: e50555. doi:10.1371/journal.pone.0050555
December 6, 2012
The most famous set of arms in the history of dinosaurs belong to Deinocheirus–eight foot long appendages from a huge ornithomimosaur that roamed Mongolia around 70 million years ago. But the immense ostrich-mimic wasn’t the only giant omnivore of its time, nor the only one made famous by its imposing arms. About 20 years before the discovery of Deinocheirus, a joint Soviet-Mongolian expedition found extremely long, tapering claws and a few other bones from a gigantic reptile. The identity of this animal took decades to untangle.
Paleontologist Evgeny Maleev described the paltry remains in a 1954 paper. Based on rib fragments, a bone from the hand, and three claws, Maleev believed that he was looking a gargantuan turtle. He named the creature Therizinosaurus cheloniformis–roughly, the “turtle-like scythe lizard.”
The animal’s claws played a key role in the identification. No terrestrial animal had such claws, he argued. Such armaments “may have been originally used by the animal for cutting aquatic vegetation or for another function, constrained by movement and acquiring food.” And even though Maleev only had pieces to work with, he proposed that Therizinosaurus was about 15 feet long with claws at least three feet long. This aquatic, apparently armor-less turtle lived in a time of hadrosaurs, tyrannosaurs, and sauropods.
Therizinosaurus wasn’t recognized as a dinosaur until 1970. In that year, paleontologist Anatoly Konstantinovich Rozhdestvensky published a re-evaluation of Maleev’s fossils that found the rib to be from a sauropod dinosaur, but the hand bone and the claws to be from some as-yet-unknown theropod. This recognition only spawned a new mystery–what sort of theropod dinosaur was Therizinosaurus, and what was the creature doing with such fearsome claws?
More complete forelimb and shoulder material described by Rinchen Barsbold in 1976 showed that Therizinosaurus had extraordinarily robust arms–quite a departure from the trend seen in large carnivorous dinosaurs, in which the arms seemed to become smaller as skulls became more heavily-built. At a time when theropod was generally considered to be synonymous with “carnivorous dinosaur”, it’s not surprising that experts speculated that Therizinosaurus was a monstrous predator who used claws, rather than teeth, to slice up the hadrosaurs and sauropods of its time. That’s the way I encountered the dinosaur in the books I read as a kid–a little-known, Cretaceous hadrosaur-shredder.
What researchers didn’t recognize was that Therizinosaurus represented an entirely new variety of theropod dinosaur. More complete skeletons of related forms such as Segnosaurus, Erlikosaurus, Alxasaurus, and Beipiaosaurus revealed the presence of a previously-unknown group of dinosaurs with long necks, beaked mouths, fat bodies, and stout arms tipped with ludicrously-long claws. These were omnivorous or herbivorous dinosaurs, not carnivores, although paleontologists didn’t immediately agree on what lineage they belonged to. Some thought they might be aberrant ornithischians–on the opposite side of the dinosaur family tree from theropods–or strange variations on the sauropod theme. By the mid-90s, however, paleontologists recognized that these truly were theropods, and ones belonging to the maniraptoran group that also encompasses the strange alvarezsaurs, beaked and crested oviraptorosaurs, the sickle-clawed deinonychosaurs, and birds. This group of tubby, feathery dinosaurs became known as the therizinosaurs.
Although Maleev didn’t recognize it when he named Therizinosaurus, he had found one of the most spectacular dinosaurs of all time–a giant, fluffy, omnivorous dinosaur that challenged what we thought we knew about theropods. Still, our image of Theriziniosaurus relies on the skeletons of more complete, closely-related dinosaurs. So far, we only really know what the arms of this dinosaur looked like, and the hindlimb elements described in the 1980s may or may not belong to another creature. We’re still waiting for the true nature of this undoubtedly bizarre dinosaur to come into focus.
Barsbold, R. 1976. New data on Therizinosaurus (Therizinosauridae, Theropoda) [translated]. In Devâtkin, E.V. and N.M. Ânovskaâ (eds.), Paleontologiâ i biostratigrafiâ Mongolii. Trudy, Sovmestnaâ Sovetsko−Mongol’skaâ paleontologičeskaâ kspediciâ, 3: 76–92.
Maleev, E.A. 1954. “New turtle−like reptile in Mongolia [translated].” Priroda, 1954, 3: 106–108.
Zanno, L. 2010. A taxonomic and phylogenetic re-evaluation of Therizinosauria (Dinosauria: Maniraptora). Journal of Systematic Palaeontology. 8, 4: 503–543.
November 29, 2012
When I was first becoming acquainted with dinosaurs in the mid 1980s, “theropod” was synonymous with “carnivorous dinosaur.” Large or small, from Tyrannosaurus to Compsognathus, every theropod I knew of sustained itself on the flesh of other organisms. But it was just about that time that new discoveries and analyses revealed that many theropod dinosaurs were omnivores, or even herbivores. The ostrich-like ornithomimosaurs, beaked oviraptorosaurs and utterly bizarre therizinosaurs, in particular, embodied a switch from an ancestral meat-filled diet to one more reliant of fruit and foliage. Not only that, but these herbivorous theropods grew almost as large as the biggest carnivores–the ornithomimosaur Deinocheirus, the ovriraptorosaur Gigantoraptor and Therizinosaurus were all enormous Cretaceous dinosaurs. But why did these plant-chomping dinosaurs become giants?
In the latest of a spate of papers considering herbivorous theropods, paleontologists Lindsay Zanno and Peter Makovicky paired evolutionary trees with mass estimates derived from femora lengths and a bit of number crunching to see if there was any distinct evolutionary pattern that might explain why Deinocheirus and similar herbivorous theropods grew to such large sizes. Were these Late Cretaceous dinosaurs just the culmination of an evolutionary trend towards ever-larger body size–called Cope’s Rule–or was something else at work?
Zanno and Makovicky didn’t find any sign of directional selection for larger body size. Even though the earliest representatives of the ornithomimosaurs, oviraptorosaurs and therizinosaurs in Asia were much smaller than their Late Cretaceous relatives, the paleontologists point out that this signal has probably been biased by preservation. The 125-million-year-old deposits that contain small members of these groups seem to be skewed towards “mid-sized vertebrates,” the authors point out, and don’t seem to preserve larger dinosaurs that might belong to the same lineages. Indeed, therizinosaurs of about the same age from North America, such as Falcarius, were larger than species in Asia, meaning that herbivorous dinosaurs might have occupied a range of body sizes and evolved larger body sizes at multiple intervals. There was no simple, straight-line trend of bigger and bigger bodies through time.
Nor did a herbivorous lifestyle alone seem to account for gigantism among these dinosaurs. Even though big herbivores gain particular benefits from their size in terms of breaking down tough, low-quality foods more efficiently, Zanno and Makovicky doubt that this relationship drove the evolution of increased body size in the dinosaurs. Instead, they favor “passive processes” that might be tied to ecology and whether these dinosaurs were omnivores more than herbivores. And, as the paleontologists stress, the pattern relies on how complete we think the dinosaur record is. Some ecosystems might be preferentially preserving larger or smaller dinosaurs, which has the potential to skew the big picture. While Zanno and Makovicky ruled out some possibilities, we still don’t really know what accounts for the multiple herbivorous theropod growth spurts.
Post-Script: After four years working with Smithsonian magazine’s wonderful crew, and over 1,000 posts about various aspects of dinosauriana, it’s time for me to move on. I’ll be leaving Dinosaur Tracking next month. Don’t fret, I’ll still be digging into dinosaur science, but I’ll be at a new blog elsewhere on the web (stay tuned for details). I am deeply indebted to my editors Brian Wolly, Sarah Zielinski and, of course, Laura Helmuth (now doing a great job at Slate), as well as the rest of the Smithsonian staff for inviting me to come here and geek out about dinosaurs every day. And many thanks to all of you–the readers and commenters who have helped make this blog a success. You have all made blogging for Dinosaur Tracking an absolute pleasure.
Zanno, L., Makovicky, P. 2012. No evidence for directional evolution of body mass in herbivorous theropod dinosaurs. Proceedings of the Royal Society B. 280. doi: 10.1098/rspb.2012.2526
November 21, 2012
Last week, I wrote about attempts by paleontologist Phil Bell and colleagues to extract biological secrets from fossilized traces of dinosaur skin. Among the questions the study might help answer is why so many hadrosaurs are found with remnants of their soft tissue intact. Specimens from almost every dinosaur subgroup have been found with some kind of soft tissue preservation, yet, out of all these, the shovel-beaked hadrosaurs of the Late Cretaceous are found with skin impressions and casts most often. Why?
Yale University graduate student Matt Davis has taken a stab at the mystery in an in-press Acta Paleontologica Polonica paper. Previously researchers have proposed that the abundance of hadrosaur skin remnants is attributable to large hadrosaur populations (the more hadrosaurs there were, the more likely their skin might be preserved), the habits of the dinosaurs (perhaps they lived in environments where fine-resolution fossilization was more likely) or some internal factor that made their skin more resilient after burial. to examine these ideas, Davis compiled a database of dinosaur skin traces to see if there was any pattern consistent with these ideas.
According to Davis, the large collection of hadrosaur skin fossils isn’t attributable to their population sizes or to death in a particular kind of environment. The horned ceratopsid dinosaurs–namely Triceratops–were even more numerous on the latest Cretaceous landscape, yet we don’t have as many skin fossils from them. And hadrosaur skin impressions have been found in several different kinds of rock, meaning that the intricate fossilization occurred in multiple types of settings and not just sandy river channels. While Davis doesn’t speculate about what made hadrosaurs so different, he proposes that their skin might have been thicker or otherwise more resistant than that of other dinosaurs. A sturdy hide might have offered the dinosaurs protection from injury in life and survived into the fossil record after death.
Still, I have to wonder if there was something about the behavior or ecology of hadrosaurs that drew them to environments where there was a greater chance of rapid burial (regardless of whether the sediment was sandy, silty or muddy). And the trouble with ceratopsids is that they have historically been head-hunted. Is it possible that we’ve missed a number of ceratopsid skin traces because paleontologists have often collected skulls rather than whole skeletons? The few ceratopsid skin fossils found so far indicate that they, too, had thick hides ornamented with large, scale-like structures. Were such tough-looking dinosaur hides really weaker than they appear, or is something else at play? Hadrosaurs may very well have had extra-sturdy skin, but the trick is testing whether that characteristic really accounts for the many hadrosaur skin patches resting in museum collections.
Davis, M. 2012. Census of dinosaur skin reveals lithology may not be the most important factor in increased preservation of hadrosaurid skin. Acta Paleontologica Polonica http://dx.doi.org/10.4202/app.2012.0077
Osborn, H. 1916. Integument of the iguanodon dinosaur Trachodon. Memoirs of the American Museum of Natural History. 1, 2: 33-54
Sternberg, C.M. 1925. Integument of Chasmosaurus belli. The Canadian Field Naturalist. XXXIX, 5: 108-110
November 19, 2012
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.
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