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The “Morphotype 1″ tunnel complex: points marked “a” represent tunnels, and points marked “b” signify vertical shafts. From Colombi et al., 2012.

Dinosaurs never cease to surprise. Even though documentaries and paleoart regularly restore these creatures in lifelike poses, the fact is that ongoing investigations into dinosaur lives have revealed behaviors that we never could have expected from bones alone. Among the most recent finds is that dinosaurs were capable of digging into the ground for shelter. Burrows found in Australia and Montana show that some small, herbivorous dinosaurs dug out cozy little resting places in the cool earth.

But when did dinosaurs develop burrowing behavior? The distinctive trace fossils found so far are Cretaceous in age, over 100 million years after the first dinosaurs evolved. That’s why a new PLoS One paper by paleontologist Carina Colombi caught my eye. In the Triassic rock of Argentina’s Ischigualasto Basin, Columbi and coauthors report, there are large-diameter burrows created by vertebrates that lived approximately 230 million years ago. Archaic dinosaurs such as Eoraptor and Herrerasaurus roamed these habitats–could dinosaurs be responsible for the burrows?

Colombi and colleagues recognized three different burrow forms in the Triassic rock. Two distinct types–differentiated by their diameter and general shape–were “networks of tunnels and shafts” that the authors attributed to vertebrates. The third type showed a different pattern of “straight branches that intersect at oblique angles” created by the burrowing organism and the plant life. The geology and shapes of the burrows indicate that they were created by living organisms. The trick is figuring out what made the distinct tunnel types.

In the case of the first burrow type, Colombi and collaborators propose that the structures were made by small, carnivorous cynodonts–squat, hairy protomammals. In the other two cases, the identities of the burrow makers isn’t clear. The second type included vertical shafts that hint at a vertebrate culprit. Dinosaurs would have been too big, but, Colombi and coauthors suggest, other cynodonts or the bizarre, ancient cousins of crocodiles–such as aetosaurs or protosuchids–could have created the burrows. Unless remains of these animals are found associated with the burrows, it is impossible to be sure. Likewise, the third type of trace might represent the activities of animals that burrowed around plant roots, but there is no clear candidate for the trace-maker.

As far as we know now, Triassic dinosaurs didn’t burrow. Even though they were not giants, they were still too large to have made fossils reported in the new research. Still, I have to wonder if predatory dinosaurs such as Herrerasaurus, or omnivores like Eoraptor, dug poor little cynodonts out of their burrows much like the later deinonychosaurs scratched after hiding mammals. There’s no direct evidence for such interactions, but, if small animals often sheltered from heat and drought in cool tunnels, perhaps predators tried to nab prey resting in their hiding places. One thing is for sure, though: we’ve only just started to dig beyond the surface of Triassic life.

References:

Colombi, C., Fernández, E., Currie, B., Alcober, O., Martínez, R., Correa, G. 2012. Large-Diameter Burrows of the Triassic Ischigualasto Basin, NW Argentina: Paleoecological and Paleoenvironmental Implications. PLoS ONE 7,12: e50662. doi:10.1371/journal.pone.0050662

Did Deinonychus and other “raptors” use their foot claws to restrain prey? Art by Emily Willoughby, image from Wikipedia.

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.

Left hind foot of Deinonychus antirrhopus. Image from Wikipedia.

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.

Reference:

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

The arms of Therizinosaurus–as yet, the rest of the dinosaur is missing. Photo by FunkMonk, image from Wikipedia.

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.

References:

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.

A restoration of Nyasasaurus in its Middle Triassic habitat, based on the known bones and comparisons to closely related forms. Art by Mark Witton.

For the past twenty years, Eoraptor has represented the beginning of the Age of Dinosaurs. This controversial little creature–found in the roughly 231-million-year-old rock of Argentina–has often been cited as the earliest known dinosaur. But Eoraptor has either just been stripped of that title, or soon will be. A newly-described fossil found decades ago in Tanzania extends the dawn of the dinosaurs more than 10 million years further back in time.

Named Nyasasaurus parringtoni, the roughly 243-million-year-old fossils represent either the oldest known dinosaur or the closest known relative to the earliest dinosaurs. The find was announced by University of Washington paleontologist Sterling Nesbitt and colleagues in Biology Letters, and I wrote a short news item about the discovery for Nature News. The paper presents a significant find that is also a tribute to the work of Alan Charig–who studied and named the animal, but never formally published a description–but it isn’t just that. The recognition of Nyasasaurus right near the base of the dinosaur family tree adds to a growing body of evidence that the ancestors of dinosaurs proliferated in the wake of a catastrophic mass extinction.

In March of 2010, Nesbitt and a team of collaborators named a leggy, long-necked creature from the same Triassic rock unit in Tanzania they named Asilisaurus kongwe. This creature was a dinosauriform–a member of the group from which the first true dinosaurs emerged–and, even better, appeared to to be the closest known relative to the Dinosauria as a whole. The find hinted that the dinosaur lineage had probably split off from a common ancestor by this time, meaning that the most archaic dinosaurs may have already existed by 243 million years ago. Roughly 249-million-year-old footprints of dinosauriforms found among Poland’s Holy Cross Mountains, described by different researchers later the same year, added evidence that the dinosauriforms were diversifying right from the beginning of the Triassic–not long after the catastrophe that decimated life on earth at the end of the Permian, around 252 million years ago.

Nyasasaurus is another step closer to the first true dinosaurs, and is just as old as Asilisaurus. To find an animal with such distinctive, dinosaur-like traits in the Middle Triassic indicates that dinosaurs already existed, or their ancestral stem was already established. Either way, Eoraptor and kin from South America can no longer be considered as the first dinosaurs, but rather a later radiation of forms. Even though our knowledge of Nyasasaurus is only fragmentary–the dinosaur is represented by a right humerus and a collection of vertebrae from two specimens–the dinosauriform nonetheless marks an additional 12 million years of dinosaur time that paleontologists are only just starting to explore.

Whether or not we ever achieve a more complete view of Nyasasaurus depends on the luck and the caprices of the fossil record. In the new paper, Nesbitt and coauthors point out that the rare, fragmentary nature of the remains found so far reflects that dinosauriforms–and early dinosaurs–were marginal parts of the ecosystems they inhabited. Dinosaurs did not dominate from the very start. They were  relatively meek, small animals that lived in a world ruled by archosaurs more closely related to crocodiles. It was only in the Late Triassic and Early Jurassic, when their archosaurian competition was diminished, that dinosaurs became dominant. That means the earliest dinosaurs and their ancestors are few and far between in the Triassic record.

Still, when I asked Nesbitt what Nyasasaurus might have looked like, he cited other dinosauriforms and early dinosaurs as templates to constrain our expectations. Nyasasaurus may have looked quite like Asilisaurus–a leggy animal with an elongated neck–although Nyasasaurus may have been bipedal. Future finds will test this idea, but the fact remains that paleontologists are closing in on what the very first dinosaurs were like. As paleontologists uncover more early dinosaurs and dinosauriforms, the dividing line between the two disappears–scientists are starting to smooth out the evolutionary transition between the first dinosaurs and their ancestors. What role Nyasasaurus played in that transformation isn’t yet clear, but the creature is a signal that over 10 million years more of uncharted dinosaur history remains in the rock.

References:

Nesbitt, S., Sidor, C., Irmis, R., Angielczyk, K., Smith, R., Tsuji, L. 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464, 7285: 95–98. doi:10.1038/nature08718

Nesbitt, N., Barrett, P., Werning, S., Sidor, C., Charig, A. 2012. The oldest dinosaur? A middle Triassic dinosauriform from Tanzania. Biology Letters. http://dx.doi.org/10.1098/rsbl.2012/0949

The arms of the huge ornithomimosaur Deinocheirus. How did such herbivorous theropods get to be so big? Photo by Eduard Solà, image from Wikipedia.

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.

Reference:

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