November 28, 2012
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.
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
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 20, 2012
Dinosaur giants are among the most famous Mesozoic celebrities. Yet the dinosaur growth spurt didn’t start just as soon as Eoraptor and kin evolved. For most of the Triassic, the first act in their story, dinosaurs were small and gracile creatures, with the first relatively large dinosaurs being the sauropodomorphs of the Late Triassic. Even then, Plateosaurus and kin didn’t come close to the truly enormous sizes of their later relatives–such as Diplodocus and Futalognkosaurus. Discerning when dinosaurs started to bulk up is difficult, however, and made all the more complicated by a set of enigmatic bones found in England.
The fossils at the heart of the in-press Acta Palaeontologica Polonica study, as described by University of Cape Town paleontologist Ragna Redelstorff and coauthors, have been known to researchers for a long time. During the mid-19th century, naturalists described at least five large, incomplete shafts found in the Late Triassic rock of southwest England’s Aust Cliff. Two of these fossils were later destroyed, but, drawing from the surviving specimens and illustrations of the lost bones, paleontologist Peter Galton proposed in 2005 that they came from large dinosaurs that lived over 200 million years ago. In particular, two of the bones resembled stegosaur bones, which would have extended the origin of the armored dinosaurs further back than previously thought.
Not everyone agreed with Galton’s proposal. The bone shafts could be from as-yet-unknown sauropods, some paleontologists argued, while other researchers pointed out that the lack of distinctive features on the bones were unidentifiable beyond the level of “tetrapod” (the major group of vertebrates descended from fish with limbs, similar to Tiktaalik). The bones came from big creatures–possibly more than 20 feet long, based on comparisons to other fossils–but the identity of the Aust Cliff animals is unknown.
Since the outside of the bone shafts provide so little information about their identity, Redelstorff and collaborators looked to the microstructure of two specimens for new clues. While the histological evidence appears to show that the sampled bones belonged to the same species, the authors argue, each individual shows different growth strategies. One bone shaft came from a slightly bigger, rapidly growing individual, while the smaller bone represents an older animal that regularly experienced temporary halts in growth (visible as lines called LAGs in the bone). Why this should be so isn’t clear, but Redelstorff and coauthors suggest individual variation, differences between the sexes or ecological factors as possible causes.
But what sort of animals were the Aust Cliff creatures? When the researchers compared their sample with three kinds of dinosaurs–sauropods, archaic sauropodomorphs and stegosaurs–and Triassic croc cousins called pseudosuchians, the pseudosuchians seemed to be the closest match. Indeed, while the researchers concluded that the “Aust Cliff bones simply do not offer a good match with any previously described histologies,” the specimens appeared to share more in common with those of croc-line archosaurs than with dinosaurs.
This isn’t to say that the Aust Cliff animals were definitely large psuedosuchians, like the recently named Smok. As the researchers point out, the specimens contained a type of bone tissue not previously seen in pseudosuchians–either these animals were not pseudosuchians, or these pseudosuchians were a previously unknown histology. And, Redelstorff and collaborators point out, the bones might be attributable to a sauropodomorph named Camelotia that is found in the same deposits. Studying the bone microstructure of Smok and Camelotia for comparison would be a logical next step in efforts to narrow down the identity of the Aust Cliff animals. Until then, this early “experiment” in gigantism–as Redelstorff and colleagues call it–remains an unresolved puzzle.
Still, the study highlights the importance of building a deep database of paleohistological samples. Had the researchers sampled just one bone, they may have come to the conclusion that all bones of that type would exhibit the same life history–either rapid, continuous growth or a stop-and-go pattern, depending on which they studied. Together, the bones show variations in the natural history of what is presumably the same species, which brings up the question of how quirks of environment, biology and natural history are recorded in bone. If we are going to understand the biology of dinosaurs and other prehistoric animals, we need to cut into as many bones as we can to understand how variable and biologically flexible the creatures truly were.
Redelstorff , R., Sander, P., Galton, P. 2012. Unique bone histology in partial large bone shafts from Aust Cliff (England, Upper Triassic): an early independent experiment in gigantism. Acta Palaeontologica Polonica http://dx.doi.org/10.4202/app.2012.0073
November 16, 2012
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.
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
November 15, 2012
Xenoceratops was a gnarly-looking ceratopsid. There’s no doubt about that. Much like its horned kin, the dinosaur sported a distinctive array of head ornaments from the tip of its nose to the back of its frill. But that’s hardly the entire story behind this newly named dinosaur.
Contrary to many news reports that focused almost entirely on the dinosaur’s appearance, the real importance of Xenoceratops is in its geological and evolutionary context. The dinosaur is the first identifiable ceratopsid from the relatively unexplored Foremost Formation in Canada, and the creature appears to be at the base of a major horned dinosaur subdivision called centrosaurines. While the dinosaur’s name is certainly aesthetically pleasing, Knight Science Journalism Tracker watchdog Charlie Petit rightly pointed out that the ceratopsid isn’t really any more or less fantastic-looking than close cousins such as Styracosaurus, Spinops and Pachyrhinosaurus. The real importance of the dinosaur–a new data point in an ongoing investigation of a little-known part of the Cretaceous–was obscured by a narrowed focus on the dinosaur’s spiky headgear.
Dinosaurs are perpetually struggling to find context in news reports. Indeed, Xenoceratops is just the latest example and not an anomaly. Theropod dinosaurs are often introduced as Tyrannosaurus rex relatives, even when they’re not particularly closely related to the tyrant king, and journalists had such a fun time giggling over calling Kosmoceratops the “horniest dinosaur ever” that the clues the ceratopsid offered about dinosaur evolution in western North America were almost entirely overlooked. Reports on newly discovered dinosaurs usually contain the vital statistics of when the animal lived, where it was found, how large it was and whatever feature strikes our immediate attention, but the tales dinosaurs have to tell about life, death, evolution and extinction are rarely pulled out by journalistic storytellers.
Fossils don’t divulge their stories all at once, though. Paleontologists spend years drawing paleobiological secrets from dinosaur bones–who was related to whom, grand evolutionary patterns and rates of faunal turnover, and how the animals actually lived. These slowly emerging lines of evidence don’t often receive the same degree of attention. The discovery of a new bizarre species immediately garners journalistic attention, but once the dinosaur has been added to the roster, details about the animal’s life are often forgotten unless the creature earns a new superlative or has been found to have some tenuous connection to T. rex.
Rather than just gripe, though, I want to highlight how discovering and naming a dinosaur is only the initial step in paleontology’s effort to reconstruct prehistoric life. Consider Einiosaurus procurvicornis, a dinosaur I’m selecting here for no other reason than I promised a friend that I’d write about the dinosaur soon.
In 1995, paleontologist Scott Sampson named Einiosaurus from remains of multiple individuals strewn through two bonebeds discovered in Montana’s Late Cretaceous Two Medicine Formation. A geologically younger relative of Xenoceratops by about 4 million years, adults of this ceratopsid species are immediately recognizable by a forward-curved nasal horn, a pair of long, straight spikes jutting from the back of the frill and a suite of more subtle cranial ornaments.
Even before Einiosaurus had a name, though, researchers knew that the collected bones of this dinosaur presented a rich fossil database. Five years before Sampson’s paper, paleontologist Raymond Rogers drew on the two ceratopsid bonebeds to argue that multiple individuals of the species had died in prehistoric droughts. Rather than being places where the bodies of solitary animals accumulated over time, Rogers proposed, the rich assemblages recorded mass mortality events which claimed young and old ceratopsids alike.
The bone assemblages and their geological context outline many tragic dinosaur deaths. But clues about dinosaur lives are preserved inside those bones. For her master’s work at Montana State University, paleontologist Julie Reizner examined the bone microstructure of 16 Einiosaurus tibiae from a single bonebed to reconstruct how these dinosaurs grew and outline their population structure.
The research is still awaiting publication in a journal, but according to Reizner’s 2010 thesis and a poster she presented at the annual Society of Vertebrate Paleontology meeting last month, the histological evidence indicates that these horned dinosaurs grew rapidly until about three to five years of age, when their growth significantly slowed. The dinosaurs did not cease growing entirely, but, Reizner hypothesizes, the slowdown might represent the onset of sexual maturity. Additionally, all the dinosaurs in her sample were either juveniles or subadults–there were no infants or adults (or dinosaurs that had reached skeletal maturity and ceased growing). Even among the two groups, there doesn’t seem to be a continuum of sizes but instead a sharper delineation between juveniles and subadults. If this Einiosaurus bonebed really does represent a herd or part of a herd that died at about the same time, the age gap might mean that Einiosaurus had breeding seasons that occurred only during a restricted part of the year, thus creating annual gaps between broods.
Other researchers have drawn from different bony indicators to restore what the faces of Einiosaurus and similar dinosaurs would have looked like. While the underlying ornamental structures are still prominent in ceratopsid skulls, the horns, bosses and spikes would have been covered in tough sheaths. Thus, in 2009, Tobin Hieronymus and colleagues used the relationship between facial integument and bone in living animals to reconstruct the extent of skin and horn on ceratopsids. While the preservation of the Einiosaurus material frustrated their efforts to detect all the skin and horn structures on the skull, Hieronymus and colleagues confirmed that the nasal horn was covered in a tough sheath and that Einiosaurus had large, rounded scales over the eyes. Artists can’t simply stretch skin over the dinosaur’s skull in restorations–the bone itself shows the presence of soft tissue ornamentation that rotted away long ago.
As with most dinosaur species, we still know relatively little about the biology of Einiosaurus. We are limited to what is preserved in the rock, the technologies at our disposal and the state of paleontological theory. All the same, Einiosaurus is much more than a pretty face. The dinosaur was part of a rich, complex Cretaceous ecosystem, and one in a cast of billions in earth’s evolutionary drama. To me, at least, that is the most entrancing aspect of paleontology. We have only barely begun to plumb the depths of dinosaur diversity, and researchers will continue to introduce us to new species at a breakneck pace, but the true wonder and joy of paleontology lies in pursuing questions about the lives of animals we’ll sadly never observe in the flesh.
Hieronymus, T., Witmer, L., Tanke, D., Currie, P. 2009. The facial integument of centrosaurine ceratopsids: Morphological and histological correlates of novel skin structures. The Anatomical Record 292: 1370-1396
Reizner, J. 2010. An ontogenetic series and population histology of the ceratopsid dinosaur Einiosaurus procurvicornis. Montana State University master’s thesis: 1-97
Rogers, R. 1990. Taphonomy of three dinosaur bone beds in the Upper Cretaceous Two Medicine Formation of northwestern Montana: evidence for drought-related mortality. PALAIOS 5 (5): 394–413.
Sampson, S. 1995. Two new horned dinosaurs from the Upper Cretaceous Two Medicine Formation of Montana; with a phylogenetic analysis of the Centrosaurinae (Ornithischia: Ceratopsidae). Journal of Vertebrate Paleontology 15 (4): 743–760.