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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.

Only hindlimb elements of Alnashetri are known so far, but, based on the dinosaur’s relationships, the tiny theropod probably looked something like this Alvarezsaurus. Photo by FunkMonk, image from Wikipedia.

Many dinosaurs have gained fame thanks to their gargantuan size. A creature in the form of a dipldodocid or tyrannosaur would be wonderful at any scale, but the fact that Apatosaurus was an 80-foot-long fern-sucker and Tyrannosaurus was a 40-foot carnivore make their skeletal frames all the more spectacular. Even as an adult, long after my first encounter with their bones at the American Museum of Natural History in New York City, I still feel tiny when I look up at what’s left of the great dinosaurs.

But not all non-avian dinosaurs were gigantic. There were 100-foot giants, like the sauropod Argentinosaurus, but there were also pigeon-sized theropods such as the strikingly-colored Anchiornis. Indeed, a significant part of how we know dinosaurs really ruled the earth is because they occupied such a wide range of body sizes–from the breathtakingly large to the diminutive. And, earlier this month, Field Museum of Natural History paleontologist Peter Makovicky and colleagues added a previously unknown tiny dinosaur to the ever-growing roster of Mesozoic species.

Named Alnashetri cerropoliciensis, the small dinosaur is mostly a mystery. All that we know of it, Makovicky and coauthors report, are a set of articulated hindlimbs from a single animal found in the roughly 95-million-year-old rock of La Buitrera, Argentina. (The dinosaur’s genus name, the paper says, means “slender thighs” in a dialect of the Tehuelchan language.) Yet those appendages contain enough clues about the dinosaur’s identity that the researchers were able to figure out that the specimen represented a new species of alvarezsaur–one of the small, possibly ant-eating dinosaurs recognizable by their short, stout arms and long skulls set with tiny teeth. While the paleontologists acknowledge that their Alnashetri specimen might be a juvenile, Makovicky and collaborators estimate that the dinosaur was comparable to its relative Shuvuuia in size–about two feet long.

How Alnashetri resembled other alvarezsaurs, and where it departed in form, will have to wait for more complete specimens. Further research is also needed to narrow down when this dinosaur lived, but for the moment, Alnashetri appears to be the oldest alvarezsaur found in South America. If only we knew more of this dinosaur! As Makovicky and coauthors conclude, “continued fieldwork and future discoveries hopefully will provide more information on the anatomy of Alnashetri and allow a more definitive evaluation of its affinities and its significance for understanding biogeography and evolutionary trends such as body size evolution within alvarezsaurids.” At least the enigma has a name.

Reference:

Makovicky, P., Apesteguía, S., Gianechini, F. 2012. A new coelurosaurian theropod from the La Buitrera fossil locality of Rio Negro, Argentina. Fieldiana Life and Earth Sciences, 5: 90-98

Dilophosaurus, in a restoration based on an impression found at St. George, Utah. Art by Heather Kyoht Luterman, from Milner et al., 2009.

The Early Jurassic is a mysterious time in dinosaur evolution. In North America, at least, paleontologists have uncovered scores of dinosaur tracks from this critical time when dinosaurs had been handed ecological dominance in the wake of a mass extinction, but body fossils are rare. In the orange sandstone that makes up so much of Arches and Canyonlands national parks in Utah, for example, only a handful of skeletons have ever been found. This formation–called the Glen Canyon, Navajo, Nugget or “Nuggaho” depending on who you ask–preserves immense sand dunes that recorded prehistoric footsteps but rarely bone. The recently described sauropodomorph Seitaad, and a group of as-yet-unnamed coelophysoids, are exceptionally rare finds.

Yet, from Connecticut to Arizona, there is one dinosaur that is constantly presented as an icon of dinosaurs circa 190 million years ago. This is Dilophosaurus–the 20-foot-long, double-crested theropod that gained dubious fame thanks to Jurassic Park. (Contrary to the film, there’s no evidence that this carnivore was a “spitter” with a collapsible neck frill.) At sites where Early Jurassic theropod tracks are found in abundance, Dilophosaurus is invoked as a possible trackmaker. But is this really so?

The remains of what would eventually be named Dilophosaurus were discovered in 1942 by Jesse Williams near Tuba City, Arizona. It took another 12 years before paleontologist Samuel Welles mistakenly attributed the bones to a new species of Megalosaurus –“M.” wetherilli–and the name Dilophosaurus itself wasn’t actually coined until 1970. Despite all this shifting around, though, Dilophosaurus wetherilli became a symbol of top Early Jurassic carnivores. Paleontologists had found plenty of Early Jurassic tracks made by a Dilophosaurus-size dinosaur, and now they finally had a body.

Frustratingly, though, we usually don’t know what dinosaur left a particular trace fossil unless the animal literally died in its tracks. While Dilophosaurus is a good fit for many large-size, Early Jurassic tracks, and may very well have left tracks at places such as St. George, Utah’s megatracksite, there’s no way to know for sure. And it seems unlikely that the same species of dinosaur that left tracks in Early Jurassic Utah also made footprints in the mud of what would become the Connecticut Valley. Who knows how many mid-sized theropods might have stalked lakeshores during this time? We don’t know, and the situation is made all the more irksome since the sediments which preserve tracks often don’t contain body fossils. We know these dinosaurs from the bottom of their feet but little else. Until future discoveries fill out the fauna of North America’s Early Jurassic, Dilophosaurus will remain the most familiar and iconic predator of its epoch.

Reference:

Naish, D. 2009. The Great Dinosaur Discoveries. University of California Press: Berkeley. pp. 94-95

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)