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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

Today’s turkeys are living dinosaurs, snoods and all. Photo by Yathin S Krishnappa, image from Wikipedia.

Tonight, at dinner tables all around the country, families are going to dine on dinosaur. If you dissect your holiday theropod just right, the ancient nature of the tasty avian is strikingly evident–right down to the wishbone. But what kind of dinosaur is a turkey, anyway?

Birds are dinosaurs. That’s a fact. But birds are really just one kind of dinosaur. Indeed, we call Triceratops, Euoplocephalus, Futalognkosaurus, Allosaurus and their ilk non-avian dinosaurs because these lineages fell outside the bird subgroup at greater or lesser distances. Birds are a distinct form of dinosaur, nested within a great group of fuzzy and feathery forms.

Let’s start from the bottom up. The dinosaur family tree is divided into two major branches–the ornithischians (the ceratopsids, hadrosaurs, stegosaurs and their relatives) and the saurischians. The saurischian side is made up of the long-necked, big-bodied sauropodomorphs and the bipedal, often-carnivorous theropods. The theropod subset is further subdivided into various groups, one of the major ones being the coelurosaurs. This subset includes the the famous tyrannosaurs, ostrich-like ornithimomosaurs, odd-looking oviraptorosaurs, sickle-clawed deinonychosaurs and birds, among a few others. Every lineage within this group contained at least one representative with feathers, and many of these dinosaurs were quite bird-like both anatomically and behaviorally.

Now here’s where things get tricky. For decades, numerous anatomical characteristics seemed to link the earliest birds, represented by Archaeopteryx, with deinonychosaurs similar to Velociraptor and Troodon. But some paleontologists have questioned this hypothesis. Last year, a controversial Nature paper suggested that the resemblance was because Archaeopteryx wasn’t actually a bird but a non-avian dinosaur more closely related to Deinonychus, while the first birds evolved from feathered dinosaurs akin to Oviraptor or the enigmatic Epidexipteryx. Rather than being deadly hypercarnivores, these alternative candidates for avian ancestry were oddball omnivores that often sported flashy tail feathers.

Not everyone agrees with the new proposal. For now, Archaeopteryx is still widely regarded to be at the base of the bird family tree, recently branched off from a deinonychosaur ancestor. Nevertheless, the argument underscores the point that many traits thought to be exclusively avian evolved much earlier in dinosaurian history than we previously expected. The more dinosaurs we find, the smaller the difference between the earliest avian dinosaurs and their non-avian ancestors. I know the pudgy kid in Jurassic Park called Velociraptor as “six foot turkey” as a put-down, but the comment isn’t too far of the mark. When you pick at the bird on your plate tonight, you’re devouring the dressed remains of a distant Deinonychus cousin.

Not only was Ornithomimus feathered, but the dinosaur’s fluffy coat changed as it aged. Lovely art by Julius Csotonyi.

Another week, another feathery dinosaur. Since the discovery of the fluffy Sinosauropteryx in 1996, paleontologists have discovered direct evidence of fuzz, feather-like bristles and complex plumage on over two dozen dinosaur genera. I love it, and I’m especially excited about a discovery announced today. In the latest issue of Science, University of Calgary paleontologist Darla Zelenitsky adds another enfluffled species to the dinosaurian ranks. Even better, the specimens raise hopes that many more dinosaurs might be preserved with their feathery coats intact.

Zelenitsky’s downy dinosaurs are not newly discovered species. Ornithomimus edmontonicus was initially described by famed bone hunter C.H. Sternberg in 1933, and it is one of the characteristic Late Cretaceous species found in Alberta, Canada’s fossil-rich Horseshoe Canyon Formation. In Sternberg’s time, these dinosaurs were thought to be scaly, but recent finds of so many feathery dinosaurs has raised the likeliehood that the “ostrich mimic” dinosaur was at least coated in some sort of dinofuzz.

A family tree of Saurischian dinosaurs, showing lineages within this group with direct evidence for feathers. From Zelenitsky et al., 2012.

The prediction of fluffy Ornithomimus came from the spread of feathers on the coelurosaur family tree. The Coelurosauria is a major dinosaur group that encompasses tyrannosaurs, compsognathids, ornithomimosaurs, alvarezsaurs, oviraptorosaurs, deinonychosaurs and birds. To date, evidence of feathers has been found in every coelurosaur lineage except one–the ornithomimosaurs. The spread of feathers hinted that some sort of plumage was present in the common ancestor of all coelurosaurs and therefore should have been inherited by the ornithomimosaurs, but, until now, no one had found direct evidence.

A trio of Ornithomimus skeletons have finally confirmed what paleontologists expected. Zelenitsky enthusiastically explained the details to me by phone earlier this week. In 1995, when Zelenitsky was a graduate student, paleontologists uncovered an articulated Ornithomimus with weird marks on its forearms. No one knew what they were. But in 2008 and 2009 a juvenile and an adult Ornithomimus turned up with preserved tufts of filamentous feathers. “When we found these specimens,” Zelenitsky said, “we made the link to the 1995 dinosaur.” All those strange marks on the arms of the previously discovered Ornithomimus, Zelenitsky and colleagues argue, are traces of longer, shafted feathers.

Even though paleontologists expected feathery Ornithomimus, the discovery was still a surprise. “I was in disbelief,” Zelenitsky said. “They’re the first feathered dinosaurs from the Americas, and the first ornithomimosaurs with feathers, as well. It was shocking to say the least.”

But there’s more to the find than simply adding another species of fluffy dinosaurs to the list. The fact that the adult and juvenile animals had different kinds of plumage adds new evidence that coelurosaurs changed their fluffy coats as they aged. “The one juvenile was completely covered in filamentous type feathers,” Zelenitsky said. What the adults looked like comes from the two other specimens. One adult skeleton, lacking forearms, preserves fuzzy feathers, and “the second adult had markings on the forearm.” Together, the specimens indicate that adult Ornithomimus were mostly covered in fuzz but developed more complex arm feathers by adulthood.

Sex is probably behind the plumage change. “We infer that because these wing feathers are not showing up until later in life, they were used for reproductive purposes,” Zelenitsky said. Perhaps adult Ornithomimus used flashy arm feathers to strut their stuff in front of potential mates. Then again, based upon the resting and brooding postures of other theropod dinosaurs, adult Ornithomimus could have used their proto-wings to cover their nests. We don’t know for sure, but the developmental change appears to be another example of dinosaurs undergoing significant changes as they approach sexual maturity. This discovery, and others like it, will undoubtedly play into the ongoing discussion about the role of sexual selection in dinosaur biology and evolution.

Best of all, the new study indicates that paleontologists may soon find more feathered dinosaurs in unexpected places. The Ornithomimus skeletons were found in prehistoric river deposits composed of sandstone. Since almost all feathered non-avian dinosaurs have been found in fine-grained sediment–such as those around Liaoning, China–paleontologists thought that coarser-grained sandstone deposits were too rough to record such fine details. Now we know better. “That’s the really exciting part of it,” Zelenitsky says. If traces of dinosaur feathers can be preserved in sandstone, the twist opens up the possibility that paleontologists might find fluff and feathers with a greater array of dinosaurs–including the tyrannosaurs, deinonychosaurs, therizinosaurs and other coelurosaurs of North America. The trick is recognizing the traces before they’re destroyed during excavation and preparation. Rock saws and airscribes can all too easily obliterate the delicate fossils. A word to researchers–keep your excavation tools sharp, and your eyes sharper.

Reference:

Zelenitsky, D., Therrien, F., Erickson, G., DeBuhr, C., Kobayashi, Y., Eberth, D., Hadfield, F. 2012. Feathered non-avian dinosaurs from North American provide insight into wing origins. Science. 338, 510-514

When paleontologists at the Argentine Museum of Natural Science in Buenos Aires threw the curtain back on the new dinosaur Bicentenaria argentina last month, they showed off a beautiful mount of tussling dinosaurs. But I couldn’t help but wonder about the reconstruction. Just how much of the dinosaur had been found, and was there any direct evidence that these dinosaurs fought each other?

Frustratingly, I couldn’t obtain immediate answers. The press event preceded the actual paper describing Bicentenaria. But last night I finally got my claws on the description of this archaic, peculiar dinosaur and its possible behavior.

Although Bicentenaria is new to science, the dinosaur’s remains were first discovered years ago. In 1998, during a drop in the water level at Argentina’s Ezequiel Ramos Mexía Reservoir, Rauel Spedale discovered and collected the disarticulated, scattered remains of several Bicentenaria from a small quarry. There was no single complete skeleton, but the quarry contained multiple skull and postcranial bones from several animals. The largest of these dinosaurs would have been about 10 feet long.

According to the analysis of the accumulated bones by paleontologist Fernando Novas and colleagues, Bicentenaria was an archaic form of coelurosaur. This is the major group of theropod dinosaurs that includes tyrannosaurs, the fluffy compsognathids, the sickle-clawed deinonychosaurs, the utterly strange therizinosaurs and birds, among other disparate lineages. Bicentenaria didn’t belong to any of these subgroups but was near the base of the coelurosaur family tree.

Yet, despite its old school anatomy, Bicentenaria was definitely not the ancestral coelurosaur. Not even close. Coelurosaurs were already a diverse group by the Late Jurassic, meaning that they started to proliferate before 150 million years ago. Yet Bicentenaria lived around 95 million years ago during the Late Cretaceous. It was over 55 million years too late to be a true ancestor of the other coelurosaur groups.

Bicentenaria can still help paleontologists visualize the anatomy early coelurosaurs, though. Based on the evolutionary analysis in the new paper, Bicentenaria preserved features seen in much, much older dinosaurs that were at the base of the coelurosaur family tree. While not an ancestor of coelurosaurs, the skeleton of Bicentenaria can help scientists figure out what the actual progenitors of the group were like.

The study also speculated about the dinosaur’s social life. Since the small quarry yielded multiple individuals, Novas and collaborators concluded that these dinosaurs must have been socializing when they died. More than that, the paleontologists tie in other theropod bonebeds to suggest that a gregarious lifestyle was the ancestral condition of theropod dinosaurs, “if not Dinosauria as a whole.”

I’m not so sure. The fact that multiple dinosaurs of the same species died in the same place, by itself, isn’t evidence that the animals lived together. It is only evidence that the dinosaurs were buried together. Even though there have been many claims of “dino gangs” and “dueling dinosaurs” based upon associated skeletons, we need to know the details of how those animals died and became buried before we can accurately reconstruct their behavior. Just because we find dinosaurs buried together doesn’t necessarily mean they were socializing before they perished. Some bonebeds really do seem to contain dinosaurs that were in a social group when they perished, while others represent assemblages of individuals that died at different times and were later washed together. The geologic and taphonomic context is critical.

In this case, unfortunately, Spedale did not take any notes on the arrangement of the bones or the context in which they were found. That data is lost. But one quarry block indicates that the bones of the dinosaurs were transported by water and stirred together. The dinosaurs died elsewhere and only parts of them ultimately became preserved in the same spot. This complicates the social Bicentenaria hypothesis. Did all the dinosaurs in the quarry die together, or did their bodies accumulate in a particular place–perhaps due to a drought or other event–over time before being washed together? We don’t know. Bicentenaria very well could have been a social dinosaur, but the evidence isn’t strong enough to say for sure, much less hypothesize that a gregarious lifestyle was the ancestral condition for all theropods. There’s a lot that we can learn about dinosaur lives from their bones, but the intricacies of their social lives remains obscured by the quirks of the fossil record.

Reference:

Novas, F., Ezcurra, M., Agnolin, F., Pol, D., Ortíz, R. 2012. New Patagonian Cretaceous theropod sheds light about the early radiation of Coelurosauria. Rev. Mus. Argentino Cienc. Nat., n.s. 14(1): 57-81 (PDF)

The skeleton of Sciurumimus, seen under UV light. You can see traces of protofeathers alon the dinosaur’s tail. Photo by Helmut Tischlinger.

On Monday, the world met yet another fuzzy dinosaur. The little theropod – named Sciurumimus albersdoerferi – is beautifully preserved in a slab of roughly 150 million year old limestone found in Germany. (These deposits have also brought us Archaeopteryx and the also-fluffy Juravenator.) And, with a little evolutionary context, Sciurumimus hints that filament-like protofeathers were more common among dinosaurs than we previously expected.

Birds – the only surviving lineage of dinosaurs – are covered in plumage. No surprise there. But since 1996, paleontologists have identified about 30 genera of non-avian dinosaurs with feathers. Most of these dinosaurs are coelurosaurs – the major group of theropod dinosaurs that contains tyrannosaurs, the switchblade-clawed deinonychosaurs, the truly weird therizinosaurs, and, among others, birds. As the discoveries accumulated, it seemed that feathers originated at the base of this group, and were inherited by birds. And feathers were not only present an small, especially bird-like dinosaurs. As the recently-described Yutyrannus shows, even 30-foot-long tyrannosaurs were fluffy.

Up until a few years ago, birds and their closest non-avian relatives were the only dinosaurs known to have feathers. Simple enough. But then two ornithischians crashed the party.You see, the dinosaur family tree is split into two halves – the saurischians on one side, and the ornithischians on the other. The split goes back about 230 million years or so, nearly to the origin of the very first dinosaurs.

The feathery coelurosaurs belong to the saurischian side of the tree, but paleontologists have also discovered dinosaurs on the other side – on the ornithischian branches – with feather-like structures. In 2002, paleontologists discovered that the archaic ceratopsian dinosaur Psittacosaurus had a brush of bristle-like structures jutting from its tail. And in 2009, another team discovered Tianyulong – another ornithischian dinosaur with a row of similar filaments running down its back. The bristles were not just like the fuzz and feathers seen among the coelurosaurs, but they were structurally similar.

Paleontologists were left with two possibilities. Either protofeathers evolved multiple times in different dinosaur lineages, or simple “dinofuzz” was an ancestral dinosaur feature that was later lost in some lineages. We don’t have enough fossils yet to know for sure, but the discovery of Sciurumimus is a significant clue that most, if not all, dinosaur lineages were at least partially decorated with protofeathers.

Even though Sciurumimus is a theropod dinosaur – part of the saurischian side of the family – it isn’t a coelurosaur. Sciurumimus is a megalosauroid, which is a lineage of dinosaurs that’s closer to the base of the theropod group. In other words, Sciurumimus is a relatively archaic theropod that isn’t very closely related to birds, yet it still has dinofuzz.

Paleontologist Thomas Holtz helped provide some context on Twitter shortly after the new dinosaur was announced. Before Sciurumimus, only coelurosaurs were known to have fuzz. (What the bristles on Psittacosaurus and Tianyulong actually are is still unclear, but no one calls their filaments “fuzz.”) After Sciurumimus, fuzz has been moved down a branch to a group called the Carnosauria.

We are still left with two possibilities. The fuzz on Sciurumimus could have originated independently. But as paleontologists add fuzz to lineages of dinosaurs only distantly-related to birds, it seems less and less likely that protofeathers evolved from scratch in each and every lineage. It’s looking more and more like feathers were a common, ancestral feature of dinosaurs. In this case, Sciurumimus indicates that simple feathers were an early, common theropod trait that evolved close to the origin of the group. The diminutive dinosaur also fits in the wide gap between coelurosaurs and their very distant ornithischian dinosaurs, bringing us a little closer to the idea that dinofuzz was an early, widely-shared dinosaur feature.

And there’s something else. Pterosaurs – the flying archosaurs with leathery wings stretched over elongated wing fingers – were the closest relatives to the Dinosauria as a whole. They had fuzzy body coverings, too. No one knows for sure, but this might mean that wispy plumage was present in the last common ancestor of dinosaurs and pterosaurs, and those simple body coverings were subsequently modified or lost in different lineages as both groups evolved.

We need more fossils to test the idea that dinosaurs started out feathery. Additional fossils preserving fuzz – fluffy baby sauropods, maybe? – would help us understand the spread of feathers and their precursors among dinosaurs. And, even then, we’d still need to find exceptionally-preserved specimens of the earliest dinosaurs to see if they had any kind of filament-like body covering. The trouble is that the high-definition deposits that would even have a chance of preserving feathers are rare. It may be a very long time before we ever know for sure.

Nevertheless, there’s still a possibility that all dinosaur lineages had some kind of bristly or feathery body covering. It’s a hypothesis that needs testing, but not an unreasonable one. Think about this for a moment. Imagine a Stegosaurus with patches of long, stiff filaments covering its body, or a Ceratosaurus with a little splash of brightly-covered fuzz on its already well-decorated head. And I think a huge sauropod – like Apatosaurus – with a partial covering of dinofuzz would look absolutely spectacular. These visions are wholly different than the scaly dinosaurs I grew up with, but they are not so fantastic as to be fiction. We are only just beginning to understand how fuzzy dinosaurs were.

For more on Sciurumimus, see my Nature News article and paleontologist Oliver Rauhut’s blog post about the discovery.