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The traditional view of ankylosaur feet represented by an Edmontonia reconstruction (left) and Senter's new proposal for a semi-tubular foot in Peloroplites (right - numerals indicate the five metacarpals). From Senter, in press.

Last Friday I wrote about a new study by paleontologist Phil Senter that revised the arrangement of bones in the front feet of Stegosaurus. Despite being only a distant relative of the sauropod dinosaurs, Stegosaurus had convergently evolved a semi-circular pattern of bones which would have given it semi-tubular forefeet similar to that of sauropods like Omeisaurus. Stegosaurus did not splay out its toes as depicted in many reconstructions.

Towards the end of the paper Senter suggested that ankylosaurs, too, may have had sauropod-like forefeet. If correct, this condition may have been shared among the armored dinosaurs, though Senter stated that further research was required to investigate this idea. That research—conducted by Senter himself—has recently been posted as an in-press paper at Acta Palaeontologica Polonica.

As with stegosaurs, the forefeet of ankylosaurs have been traditionally portrayed with the metacarpals—the bones of the forefoot just behind the fingers—being configured in a shallow arc shape. This would have spread out the fingers and suggested the presence of a pad of flesh to help support the weight of the animal. In rare, articulated ankylosaur skeletons, however, the forefeet have the semi-tubular arrangement seen in some sauropod dinosaurs, and the bones actually have to be articulated incorrectly to give the forefeet a splayed appearance.

A study of the forefeet of the Lower Cretaceous ankylosaur Peloroplites cedrimontanus from Utah’s Cedar Mountain Formation confirmed Senter’s hypothesis. When articulated naturally, the bones formed a semi-tube which would have made the metacarpals, rather than the fingers, the main weight-bearing bones. Furthermore, Senter cites the skeleton of the Late Cretaceous ankylosaur Saichania chulsanensis from Mongolia as being found articulated in the rock with metacarpals in a semi-tube shape and therefore supporting the idea that this was a natural configuration.

Senter’s findings have implications for the evolution of the armored dinosaurs, as well. Stegosaurs and ankylosaurs were sister groups and, together with their closest early relatives, composed a group called the Thyreophora. Linked by common ancestry, stegosaurs and ankylosaurs were more closely related to each other than other kinds of dinosaurs. This presents two alternatives. Either this forefoot arrangement evolved independently in each group, or it was a characteristic inherited from the last common ancestor of the two.

Frustratingly, however, we don’t know very much about the early history of armored dinosaurs. Perhaps the best known early form is the approximately 200-million-year-old Scutellosaurus. The trouble is that this dinosaur had forelimbs that were shorter than its hindlimbs, and so it was probably not regularly walking on all fours. If the semi-tube arrangement of metacarpals was an adaptation to support the bulk of these animals, then the characteristic may have been absent in Scutellosaurus.

If Scutellosaurus can be taken as representative of what the last common ancestor of stegosaurs and ankylosaurs was like, then I have to wonder if the semi-tubular metacarpal pattern evolved in each group due to anatomical constraints present in that common ancestor. Rather than inheriting the semi-tubular arrangement directly, perhaps there was something about the forefeet of the last common ancestor that constrained the way the bones could articulate when early stegosaurs and ankylosaurs began walking on all fours. Evolution is not entirely open-ended, and the characteristics of ancestral species place limits on the ways in which their descendants can be adapted.

Furthermore, in the diagram provided by Senter in the paper, the metacarpal arrangement of the ankylosaur Saichania does not form as much of a semi-tube as in Stegosaurus or Pelorolites. Perhaps Saichania, despite being geologically younger than these other dinosaurs, preserves a more archaic condition hinting at independent modification of the forelimb. One data point isn’t enough to know for sure, though. Forelimbs from other ankylosaur species, as well as those of the earliest armored dinosaurs, must be found and studied to investigate the evolution of this trait.

References:

Senter, P. (2010). Evidence for a sauropod-like metacarpal configuration in ankylosaurian dinosaurs Acta Palaeontologica Polonica DOI: 10.4202/app.2010.0041

A restoration of Pteranodon taking off by Mark Witton, lead author of the new PLoS One paper. Image from Mark Witton's Flickr page.

Earlier this week paleontologists Mark Witton and Michael Habib published a new study in PLoS One on how pterosaurs—particularly large forms such as Quetzalcoatlus—took to the air. Rather than pushing off the ground with their legs, pterosaurs used their arms in a pole-vault type of motion to launch themselves skyward. Interesting stuff, but I quickly became irritated by some of the popular coverage of the new research.

Whenever a story about pterosaurs makes it into mainstream news outlets, it is almost inevitable the flying archosaurs are going to be mistakenly called “dinosaurs” by at least one source. In this case the British newspaper the Telegraph and the venerable BBC were two of the main offenders, each declaring that pterosaurs were dinosaurs in their headlines.

It might be easy to brush off my complaint as a case of paleo-pedantry, but word choice matters. “Dinosaur” is a word for a specific group of creatures united by shared characteristics and which had their own evolutionary history—it is not a catch-all term for anything reptilian and prehistoric. Calling a pterosaur a dinosaur is an error of the same order of magnitude as saying that our species is a marsupial, but to understand why we need to flesh out the evolutionary relationships of these animals.

A close up of the relationship between dinosaurs and pterosaurs. Pterosaurs are underlined in red, and dinosaurs are underlined in green. Image modified from Wikipedia.

Let’s start from the bottom and work our way up. The Archosauria is a diverse group of reptiles which contains two major subsections: crocodiles and their close relatives (collectively called crurotarsans or pseudosuchians) are on one side of the split, and dinosaurs, pterosaurs, and their closest relatives (called avemetatarsalians) on the other. For our purposes here, we’re concerned with the second group.

Looking at the Avemetatarsalia (see the diagram above), a major split is apparent at the base of this group. On the one side are the dinosaurs and their closest relatives, and on the other are pterosaurs and animals more closely related to them than dinosaurs. Both pterosaurs and dinosaurs are distinct groups that shared a common ancestor, and so to call a pterosaur a dinosaur is to ignore this major divergence in the evolution of both groups. A pterosaur is no more a dinosaur than a goldfish is a shark.

There is no reason for news sources to keep applying the word “dinosaur” to pterosaurs. We have known about this distinction for a long time, and I bet that your average 10-year old paleo fan would know not confuse the groups. With even just a tiny bit of an evolutionary perspective, the distinction becomes clear.

To learn more about pterosaurs, visit Pterosaur.net, which was recently created by a team of pterosaur experts including Witton and Habib.

The cover for Mark Schultz's collected Xenozoic Tales series.

Regular readers know that I was underwhelmed by IDW’s efforts to take on the Jurassic Park franchise—I’ll have a wrap-up review coming soon—but fortunately for dinosaur comic fans, several forthcoming releases should provide a higher-quality dino fix.

Next February, Image Comics will release a one-shot story called simply Tyrannosaurus rex. Naturally, the story pits the formidable predator against our own species, and it draws its inspiration from the old “cavemen vs. dinosaurs” flicks of the 1970s. Young earth creationists might consider the tale to be based on a true story, but for the rest of us it looks like a fun throwback to b-movies like When Dinosaurs Ruled the Earth.

After a long hiatus, Dark Horse comics has revamped the Turok: Son of Stone series. The new story is a mish-mash of Native Americans, Aztecs, dinosaurs, “Panther People”, and weird prehistoric beasts, but, given the various incarnations of the comic hero, who would expect anything less? The first story arc started last month and runs through February.

Another classic dinosaur title is also being polished up for re-release. Dark Horse will soon release the entire run of Ricardo Delgado’s Age of Reptiles (which includes the latest story arc, “The Journey”), one of the few dinosaur series with nary a human in sight. If you liked the visuals of the Disney film Dinosaur, but couldn’t stand the chattering herbivores, then Age of Reptiles is for you.

The news I am most excited about, though, is that Flesk Publications has just released the collected run of Mark Schultz’s excellent Xenozoic Tales in the single volume Xenozoic. Set in a future in which dinosaurs have returned in the wake of human-caused ecological catastrophe, Schultz’s series remains the acme of dinosaur comics, with each story standing on its own as well as fitting into a larger—and still incomplete—story.

So there you have it. Despite some recent so-so titles, the next few months should be chock full of dino comic goodness.

An updated restoration of Tyrannosaurus rex, "with a tail of appropriate beefiness", by Scott Hartman. From Persons and Currie, 2010.

Almost everyone has a pretty good idea of what Tyrannosaurus rex looked like. The massive head, scrawny arms, and the bird-like posture are all iconic parts of prehistory’s most famous dinosaur, but its tail would probably be tacked on as an afterthought.

You can’t have a good Tyrannosaurus without a tail, but our focus has traditionally been on the business end of the animal. In a new Anatomical Record paper, however, scientists W. Scott Persons IV and Philip Currie have taken another look at the caudal portion of this animal and found it to be a bit more beefy than has been previously thought.

Except in cases of truly exceptional, three-dimensional preservation, we usually can’t directly study the muscles of dinosaurs. More often, scientists must rely on muscle scars visible on bones and the musculature of extant animals to reconstruct details of soft anatomy. This is not quite as straightforward as it sounds.

Birds and crocodiles are the closest living relatives of non-avian dinosaurs, but many dinosaurs were significantly different from both in their anatomy. In the case of tails, especially, birds do not have the long, muscular tails of dinosaurs, and while crocodylians do possess long tails, their posture and mode of life are very different from those of dinosaurs. This uncertainty has led to the reconstruction of dinosaur tails as relatively thin structures which, Persons and Currie state, “appear altogether emaciated when compared to the tails of modern reptiles.”

Yet, despite being evolutionary cousins with a very different natural history, crocodylians may be good proxies for understanding dinosaur tail and leg anatomy after all. As Persons and Currie point, one of the primary reasons for this association is a muscle called the M. caudofemoralis. This is a tail muscle that inserts onto the top of the femur and helps to retract that upper leg bone while walking. Its presence in dinosaurs has been noted for over 150 years, but this same muscle was reduced or lost in many birds during their evolution. This large retractor muscle is present and remains important in living reptiles such as crocodiles, however, meaning that these animals are more useful in reconstructing the tail anatomy of dinosaurs.

To better understand the role of this muscle in reptile anatomy, Persons and Currie dissected the pelvic and post-pelvic muscles of a brown basilisk, spectacled caiman, veiled chameleon, green iguana and Argentinean black and white tegu to see how the muscles in this area corresponded to the tail anatomy of the theropod dinosaurs Gorgosaurus, Ornithomimus and Tyrannosaurus. What they found was that the dinosaurs had scars related to the important M. caudofemoralis muscle stretching back to around the 12th to 14th tail vertebra in each dinosaur, but the question was how thick this muscle was at the base of the tail.

In crocodylians, the M. caudofemoralis muscle creates a thick bulge just behind the hips, and it is likely that it did the same in dinosaurs. By combining anatomical measurements from the modern reptiles with the known anatomy of the dinosaurs, Persons and Currie used computer modeling to recreate dinosaurs with thick, crocodile-like tails, and the scientists argue that this arrangement is supported by a subtle anatomical feature.

In many theropod dinosaurs, the three to four tail vertebrae behind the hips have wings of bone called transverse processes, and these flattened structures are angled upwards. As reconstructed by Persons and Currie, this arrangement would have provided expanded space for the M. caudofemoralis muscle, although they note that the transverse processes of both Gorgosaurus and Tyrannosaurus were not oriented in the same upward diagonal fashion. Nevertheless, given how many theropod dinosaurs had this expanded space near the base of the tail, it is possible that a large M. caudofemoralis muscle was a common feature of these dinosaurs stretching all the way back to early forms such as the circa 228-million-year-old Herrerarasaurus.

This new reconstruction of dinosaur tails has some important implications for how these animals moved. As a prominent retractor of the upper leg, especially, the M. caudofemoralis would have been one of the primary muscles involved in locomotion. Yet larger muscle size did not necessarily translate to greater speed. Persons and Currie found that this muscle would have been relatively larger in Tyrannosaurus than in the juvenile Gorgosaurus they examined, but the overall anatomy of Tyrannosaurus indicates that it would have been a slower runner than its more slender relative. The larger size of the M. caudofemoralis muscle in Tyrannosaurus may have been the result of being a much bigger animal and requiring more muscle power to get around. Still, Persons and Currie argue that the size of this muscle may have allowed Tyrannosaurus to achieve speeds towards the higher end (more than 10 meters per second) of what has been estimated for it, and future tests will have to incorporate the new anatomical data in order to better understand how this dinosaur moved.

Persons and Currie ask that paleoartists take note, too. Even though theropod dinosaurs are often restored with thin, “athletic” tails, the new study suggests a different sort of shape in which the tail is thick and almost square near the base, is tall and thin in the middle, and then tapers into a circular shape at the tip. Even though this arrangement enlarges the posteriors of these dinosaurs, it actually makes them more powerful runners than the thin restorations. We should expect to see more big-bootied tyrannosaurs in the near future.

References:

Persons, W., & Currie, P. (2010). The Tail of Tyrannosaurus: Reassessing the Size and Locomotive Importance of the M. caudofemoralis in Non-Avian Theropods The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology DOI: 10.1002/ar.21290

The remains of two new dinosaurs from Asia. On the left, the hand of the oviraptorid Machairasaurus, and on the right, the partial skull of the troodontid Xixiasaurus. From Longrich et al., 2010 and Lü et al., 2010, respectively.

Paleontologists are discovering dinosaur species at a dizzying pace. These days it seems that a new species is announced almost every other week. Many of these new dinosaurs are being found in China, and two different teams of scientists have recently described a pair of unique species from two locations within the country.

As described by paleontologists Nicholas Longrich, Philip Currie and Dong Zhi-Ming in the journal Palaeontology, the dinosaur Machairasaurus leptonychus was discovered in the 84- to 75-million-year-old rock near the village of Bayan Mandahu in Inner Mongolia (itself a part of northern China bordering the country of Mongolia). There was not much left of this dinosaur. All that remained was a partial right forelimb, parts of the left arm, and a few toe bones, but the lower arm bones, fingers and claws were distinctive enough to identify this as a new type of oviraptorid dinosaur.

More specifically, Machairasaurus appears to have been a small animal most similar to a subgroup of the oviraptorids called the Ingeniinae, and according to Longrich and co-authors, Machairasaurus and its close relatives had relatively robust hands that were not well-suited to grasping. Instead, the forelimbs of this dinosaur appear to have been better suited to “scratching, tearing or, conceivably, digging” than grabbing prey, and the anatomy of their mouths hints that they may have included a large amount of plant food into their diets. As paleontologists have discovered through the study of other Cretaceous dinosaurs, theropod dinosaurs can no longer be cast as a group of entirely carnivorous dinosaurs—multiple lineages of theropods switched over to plant-eating during the Cretaceous.

The second new theropod was described by a team of Chinese scientists led by Junchang Lü in Acta Palaeontologica Polonica. Named Xixiasaurus henanensis, this small animal was a troodontid dinosaur found in the circa 83-million-year-old strata of China’s Henan Province. Represented by a partial skull, lower jaw fragment and a few other bits from its lower arms, Xixiasaurus resembled other troodontids, such as Byronosaurus, in having a set of unserrated teeth which were small and closely-packed in the front of the jaw but larger and recurved in the back of the jaw. As with the forelimb specializations of Machairasaurus, the unique teeth of Xixiasaurus, Byronosaurus, and their closest relatives, Lü and colleagues suggest, may be related to a more cosmopolitan diet that included plants, but more than tooth anatomy alone will be required to investigate this hypothesis.

References:

LONGRICH, N., CURRIE, P., & ZHI-MING, D. (2010). A new oviraptorid (Dinosauria: Theropoda) from the Upper Cretaceous of Bayan Mandahu, Inner Mongolia Palaeontology, 53 (5), 945-960 DOI: 10.1111/j.1475-4983.2010.00968.x

Lü, J., Xu, L., Liu, Y., Zhang, X., Jia, S., & Ji, Q. (2010). A New Troodontid Theropod from the Late Cretaceous of Central China, and the Radiation of Asian Troodontids Acta Palaeontologica Polonica, 55 (3), 381-388 DOI: 10.4202/app.2009.0047