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Archaeopteryx had a wing that was different from that of modern birds, and, as seen here, might have been a glider more than a powered flyer. Art by Carl Buell, courtesy of Nicholas Longrich.

How did feathered dinosaurs take to the air? Paleontologists have been investigating and debating this essential aspect of avian evolution for over a century. Indeed, there have been almost as many ideas as they have been experts, envisioning scenarios of dinosaurs gliding through trees, theropods trapping insects with their feathery wings and even aquatic Iguanodon flapping primitive flippers as flight precursors (I didn’t say that all the ideas were good ones). The biomechanical abilities of bird ancestors and their natural history has always been at the center of the debate, and a new Current Biology paper adds more fuel to the long-running discussion.

At present, hypotheses for the origin of avian flight typically fall into one of two categories. Either bird ancestors accrued the adaptations necessary for flight on the ground and, through evolutionary happenstance, were eventually able to take off, or small tree-dwelling dinosaurs used their feathery coats to glide between trees and, eventually, flapped their way into a flying lifestyle. There are variations on both themes, but feathers and the characteristic avian flight stroke are at the core of any such scenario. In the case of the new paper, Yale University paleontologist Nicholas Longrich and colleagues draw from the plumage of early bird Archaeopteryx and the troodontid Anchiornis to examine how feathers changed as dinosaurs started to fly.

In modern flying birds, Longrich and coauthors point out, the wing arrangement typically consists of “long, asymmetrical flight feathers overlain by short covert feathers.” This pattern creates a stable airfoil but also lets the flight feathers separate a little during the upstroke of a wing beat, therefore reducing drag. When the paleontologists examined the fossilized wings of Archaeopteryx and Anchiornis, they found different feather arrangements that would have constrained the flight abilities of the Jurassic dinosaurs.

Both prehistoric creatures had long covert feathers layered on top of the flight feathers. Anchiornis, in particular, appeared to have an archaic wing form characterized by layers of short, symmetrical flight feathers and similarly shaped coverts. Archaeopteryx showed more specialization between the flight feathers and the coverts but still did not have a wing just like that of a modern bird. As a result, Longrich and collaborators hypothesize, both arrangements would have stabilized the wing at the cost of increased drag at low speeds, making it especially difficult for Anchiornis and Archaeopteryx to take off. As an alternative, the researchers suggest that these dinosaurs might have been parachuters who jumped into the air from trees, which might hint that “powered flight was preceded by arboreal parachuting and gliding.”

The trick is determining whether Anchiornis and Archaeopteryx actually represent the form of bird ancestors, or whether the dinosaurs, like Microraptor, were independent experiments in flight evolution. At the Society of Vertebrate Paleontology conference in Raleigh, North Carolina last month, flight expert Michael Habib quipped that all that was needed to make dromaeosaurs aerially competent was the addition of feathers. If Habib is right, and I think he is, then there could have been multiple evolutionary experiments in flying, gliding, wing-assisted-incline-running and other such activities. There’s no reason to think that flight evolved only once in a neat, clean march of ever-increasing aerodynamic perfection. Evolution is messy, and who knows how many ultimately failed variations there were among flight-capable dinosaurs?

The three-step Anchiornis-Archaeopteryx-modern bird scenario of wing evolution fits our expectations of what a stepwise evolutionary pattern would look like, but, as the authors of the new paper point out, shifting evolutionary trees currently confound our ability to know what represents the ancestral bird condition and what characterized a more distant branch of the feathered dinosaur family tree. We need more feathery fossils to further investigate and test this hypothesis, as well as additional biomechanical and paleoecological information to determine whether such dinosaurs really took off from trees. We must take great care in distinguishing between what an organism could do and what it actually did, and with so much up in the air, the debate on the origin of flight will undoubtedly continue for decades to come.

Reference:

Longrich, N., Vinther, J., Meng, Q., Li, Q., Russell, A. 2012. Primitive wing feather arrangement in Archaeopteryx lithographica and Anchiornis huxleyi. Current Biology DOI: 10.1016/j.cub.2012.09.052

A restoration of Gigantspinosaurus. Art by Conty, image from Wikipedia.

Stegosaurus was a weird dinosaur. We’ve known that for well over a century, but, as Darren Naish has often pointed out, Stegosaurus was strange even compared to its Jurassic relatives. The dinosaur’s arrangement of broad, alternating plates is a departure from the arrangements of smaller plates, back spikes and accessory spines seen on many other stegosaurs, including the perplexingly well-armed Gigantspinosaurus sichuanensis.

Ornamented with a double row of short, narrow plates along its back, the roughly 160-million-year-old Gigantspinosaurus generally resembled other stegosaurs from Late Jurassic Asia, such as Tuojiangosaurus. But, as you might be able to guess from the dinosaur’s name, the feature that immediately sets Gigantspinosaurus apart from similar species is a enormous hooked spine that jutted out from behind the shoulder blade. These striking spikes were found close to their life position on the first skeleton of this dinosaur to be found–erroneously attributed to Tuojiangosaurus, before being redescribed as Gigantspinosaurus in 1992–although their exact orientation isn’t entirely clear. Did the shoulder spikes curve straight backward, or were they tiled slightly upwards? And, more significantly, how did such prominent ornaments evolve? No one knows.

As yet, we know relatively little about the natural history of Gigantspinosaurus. The dinosaur has a name, and skin impressions have helped researchers restore what the stegosaur looked like, but many aspects of the spiky herbivore’s biology remain mysterious. In the grand scheme of stegosaur evolution, though, the ornamentation of Gigantspinosaurus has sometimes been taken as evidence that similar forms had shoulder spikes. In addition to paired spikes along its tail, the Late Jurassic stegosaur Kentrosaurus possessed an extra pair of spikes along its side. These were originally placed over the hips, but, due to the discovery of Gigantspinosaurus, some researchers have argued that the spikes truly belong at the shoulders.

Frustratingly, paleontologists have yet to find a Kentrosaurus skeleton with side spikes in place. But the discovery of Gigantspinosaurus doesn’t necessarily mean that its cousin Kentrosaurus had the same arrangement. Among stegosaurs, the two genera were relatively distantly related, and it’s entirely possible that more than one side spike arrangement evolved. As paleontologist Heinrich Mallison has argued, the hips of Kentrosaurus seem to possess areas where the spikes could have articulated, and this arrangement would be consistent with the dinosaur’s ornamentation pattern–small plates at the front give way to spikes along the stegosaur’s back and tail. Indeed, the side spikes on Kentrosaurus more closely resemble the same structures along the dinosaur’s back and tail and the shoulder spike of Gigantspinosaurus. If Kentrosaurus had plates up front and serially homologous spikes along the back, then why shouldn’t the hip spikes remain a reasonable hypothesis? Together, Gigantspinosaurus and Kentrosaurus might represent different alternatives in the stegosaur armory.

Many Allosaurus bones have been found with fractures and other pathologies, but were any of these injuries caused by falls? Photo taken at the Natural History Museum of Utah by the author.

If an Allosaurus fell in the Jurassic, would it leave a trace fossil? We know that resting dinosaurs can leave body impressions behind, as shown by a theropod trace found in St. George, Utah, but what if a dinosaur lost its footing and fell over onto a mudflat or sand dune? Such events surely must have happened. The question is whether the embarrassing moments ever became set in stone.

A trace fossil would be the obvious mode of preservation for a dinosaur fall. A messy footprint, recording the slip, paired with a body impression would be a gorgeous snapshot of a dinosaur’s tumble. Sadly, no one has yet found such a fossil, but paleontologists have identified a more subtle clue of a dinosaur fall. In 2007, paleontologist Oliver Wings and colleagues described a Middle Jurassic dinosaur tracksite described in China. Among the dozens of tracks was what appeared to be a slip footprint–parallel grooves made when the dinosaur’s foot slipped backward or foreword over the wet mud of the ancient environment.

But tracks and other impressions may not be the only way dinosaur falls might be recorded. When I brought up the idea of a fossilized dinosaur tumble on Twitter yesterday, Sam Barnett brought up Allosaurus gastralia, or rib-like belly bones, that showed signs of fracture due to a fall. I hadn’t heard of these specimens before, so I checked a review of theropod pathologies published by Ralph Molnar in 2001. The broken bones got a nod, with a reference to a more thicker biography of dinosaur injuries called Dinosores published two years before by Darren Tanke and Bruce Rothschild. I kept pulling at the thread, hoping to find something more.

A 1998 New Scientist story by Jeff Hecht called “The deadly dinos that took a dive” outlined the idea. In a preview of research he was getting ready to show off at that year’s DinoFest symposium in Philadelphia, Rothschild mentioned that an Allosaurus specimen showed “exactly the pattern of fractures that would be caused by a belly flop onto hard ground while running.” But I wanted to know more. What, exactly, was it about the breaks that indicated a clumsy fall?

Unfortunately, I wasn’t able to find any more detailed information. I don’t have any doubt that Allosaurus and other dinosaurs suffered fractures from falls. That’s an inevitable interaction between biology, geology and physics when you have animals walking and running around the Mesozoic. The trick is connecting the pathology with the cause. Still, I have to wonder if virtual models that estimate bone stress–such as the finite element analysis models used in bite mechanics studies–might help paleontologists investigate what happened to dinosaurs when they fell. If paleontologists can trip up a virtual Allosaurus and investigate how those computerized bones respond to the stress of a fall, maybe researchers can predict where breaks might occur and compare the models to the fossil record. For now, though, we can do little more than imagine an Allosaurus falling face-first on a mudflat, shaking itself off, and ignoring the pain in its ribs as it tromped off.

[Hat-tip to Heinrich Mallison for pointing me to the trackway study, on which he was one of the coauthors.]

References:

Claessens, L. 2004. Dinosaur gastralia; origin, morphology, and function. Journal of Vertebrate Paleontology 24, 1. 89-106

Molnar, R. 2001. Theropod paleopathology: A literature survey. pp 337-363 in Tanke, D. and Carpenter, K. eds. Mesozoic Vertebrate Life. Bloomington: Indiana University Press.

Rothschild, B., Tanke, D. 2005. Theropod paleopathology: state of the art review. pp 351-365 in Carpenter, K. ed. The Carnivorous Dinosaurs. Bloomington: Indiana University Press.

Tanke, D., Rothschild, B. 2002. DINOSORES: An annotated bibliography of dinosaur paleopathology and related topics—1838–2001. New Mexico Museum of Natural History and Science. Bulletin, 20.

Wings, O., Schellhorn, R., Mallison, H., Thuy, B., Wu, W., Sun, G. 2007. The first dinosaur tracksite from Xinjiang, NW China (Middle Jurassic Sanjianfang Formation, Turpan Basin) – a preliminary report. Global Geology 10, 2. 113-129

Fossil swim tracks indicate that theropods similar to this Megapnosaurus at least occasionally swam in prehistoric lakes and rivers. Art by Dmitry Bogdanov, image from Wikipedia.

Paleontologist R.T. Bird inspected many dinosaur trackways while combing Texas for the perfect set to bring back to the American Museum of Natural History. During several field seasons in the late 1930s, Bird poked around in the Early Cretaceous rock in the vicinity of the Paluxy River for a set of sauropod footprints that would fit nicely behind the museum’s famous “Brontosaurus” mount. Bird eventually got what he was after but not before poring over other intriguing dinosaur traces. One of the most spectacular seemed to be made by a swimming dinosaur.

Known as the Mayan Ranch Trackway, the roughly 113-million-year-old slab is almost entirely made up of front foot impressions. The semicircular imprints were undoubtedly left by one of the long-necked sauropod dinosaurs. But towards the end of the trail, where the dinosaur’s path makes an abrupt turn, there was a single, partial impression of a hind foot.

At the time Bird and his crew uncovered this trackway, sauropods were thought to be amphibious dinosaurs. Other than their immense bulk, what defense would they have had but to trundle into the water, where theropods feared to paddle? Under this framework, Bird thought he knew exactly how the Mayan Ranch Trackway was made. “The big fellow had been peacefully dog-paddling along, with his great body afloat, kicking himself forward by walking on the bottom here in the shallows with his front feet,” Bird wrote in his memoir. The great dinosaur then kicked off with one of its hind feet and turned.

With the exception of well-defended dinosaurs such as the ceratopsids and stegosaurs, many herbivorous dinosaurs were thought to be at least semi-aquatic. There seemed to be only two options for Mesozoic prey species–grow defenses or dive into the water. In time, though, paleontologists realized that the sauropods, hadrosaurs and other herbivores didn’t show any adaptations to swimming. Our understanding of the ecology of these dinosaurs was based on false premises and faulty evidence.

In the case of the Mayan Ranch Trackway, for example, there’s no indication that the sauropod that made the trackway was swimming. A more likely scenario has to do with evolutionary changes among sauropods. While the sauropods that dominated the Late Jurassic of North America–such as Diplodocus, Apatosaurus and Barosaurus–carried much of their weight at the hips and left deeper hindfoot impressions, the center of mass shifted among their successors–the titanosaurs–such that more of the weight was carried by the forelimbs. Hence, in some trackways, the deeper impressions made by the forefeet are more likely to stand out than those made by the hindfeet, especially if some of the top layers of the rock are eroded away to leave only “undertracks.” What seemed to be evidence of swimming sauropods instead owes to anatomy and the characteristics of the mucky substrate the dinosaur was walking on.

As far as I’m aware, no one has yet found definitive evidence of swimming sauropods or hadrosaurs–the two groups previously thought to rely on water for safety. Stranger still, paleontologists have recently uncovered good evidence that theropod dinosaurs weren’t as bothered by water as traditionally believed. In 2006, paleontologists Andrew Milner, Martin Lockley and Jim Kirkland described swim tracks made by Early Jurassic theropods at a site that now resides in St. George, Utah. Such traces weren’t the first of their kind ever discovered, but the tracksite was one of the richest ever found.

Small to medium-sized theropods made the St. George swim tracks–think of dinosaurs similar to Megapnosaurus and Dilophosaurus. Even better, the large number of smaller-size swim tracks hints that whatever dinosaurs made these tracks were moving as a group as they struggled against the current in the lake shallows. The larger dinosaurs, on the other hand, were a bit taller and able to wade where their smaller cousins splashed around.

A different team of researchers announced additional evidence for swimming theropods the following year. Paleontologist Rubén Ezquerra and co-authors described dinosaur swim traces from Early Cretaceous rock near La Rioja, Spain. Based on the details of the track and their direction, the theropod was swimming against a current that pushed the dinosaur diagonally. Along with other theropod swim tracks, the researchers noted, the discovery meant that paleontologists would have to revise their ideas about the kind of habitats theropods lived in and what carnivorous species would do. Theropod dinosaurs were not so hydrophobic, after all.

Does this mean that dinosaurs like Dilophosaurus were adapted to an amphibious lifestyle? Not at all. As Ezquerra and co-authors pointed out, the swimming strokes of these dinosaurs were exaggerated walking motions. The way the dinosaurs moved on land allowed them to be adequate swimmers while crossing rivers or lakes, but, compared with semi-aquatic animals such as crocodiles and otters, no known dinosaur shows traits indicative of a primarily waterlogged existence. (And dinosaurs found in marine sediments don’t count as evidence, as these were washed out to sea prior to burial. I can’t imagine ankylosaurs taking to life among the high seas, in any case.) Some dinosaurs could swim, but that doesn’t mean that they made the water their home. Still, thanks to special prehistoric traces, we can imagine packs of Megapnosaurus fighting to get ashore, and Dilophosaurus strutting into the shallows, aiming to snatch whatever fish were foolish enough to swim into the carnivore’s shadow.

References:

Bird, R.T. (1985). Bones for Barnum Brown, edited by Schreiber, V. Forth Worth: Texas Christian University Press. pp. 160-161

Ezquerra, R., Doublet, S., Costeur, L., Galton, P., Pérez-Lorente, F. (2007). Were non-avian theropod dinosaurs able to swim? Supportive evidence from an Early Cretaceous trackway, Cameros Basin (La Rioja, Spain) Geology, 40 (10), 507-510 DOI: 10.1130/G23452A.1

Milner, A., Lockley, M., Kirkland, J. (2006). A large collection of well-preserved theropod dinosaur swim tracks from the Lower Jurassic Moenave Formation, St. George, Utah. New Mexico Museum of Natural History and Science Bulletin, 37, 315-328

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