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A restoration of Aardonyx. From the Proceedings of the Royal Society B paper.

The sauropod dinosaurs were the largest animals to have ever walked on the earth. They were so incredibly huge, in fact, that they had to move about on four legs—but since the earliest dinosaurs were bipedal, paleontologists have long known that the ancestors of giants like Brachiosaurus and Apatosaurus actually trotted about on two legs. A dinosaur just described in the Proceedings of the Royal Society B sat close to this major transition in sauropod evolution.

Recovered from Early Jurassic (about 183 – 200 million year old) rock in South Africa, Aardonyx celestae was an approximately 20-foot-long dinosaur that combined elements that are both strange and familiar. It had a small head, a long neck, a large body, and a long tail, but it still had relatively short forelimbs compared to its hind legs. While it could occasionally walk on four legs, its limbs indicate that it primarily walked around on two , and an evolutionary analysis that was part of the new study placed it relatively close to the earliest sauropod dinosaurs (thus fitting Aardonyx within the larger category of dinosaurs called sauropodomorphs).

Aardonyx was not actually ancestral to the larger,  four-feet-on-the-floor sauropods—it lived during a time when such dinosaurs already existed—but it preserves some of the transitional features that we would expect to find in the actual ancestor. (Contrary to a headline published by the BBC, it is not a “missing link” and the entire concept of “missing links” is a hopelessly out-of-date idea that was discarded by scientists long ago. The phrase goes back to a time when life was viewed as proceeding from “lower” forms to “higher” ones in a straight line, and scientists have rightly rejected it in favor of a branching bush of evolutionary diversity.)

While not a direct ancestor of dinosaurs like Diplodocus, this new dinosaur will help us better understand how sauropod dinosaurs evolved. If you would like to know more about it check out the blog of the lead author of the new description, Adam Yates, where he summarizes the important details about Aardonyx. It is good to see working paleontologists take a more active role in communicating their discoveries to the public, and I hope that other dinosaur specialists will follow the example made by Yates and others.

The outline of a Tyrannosaurus showing the measurements used in the PLoS One study.

When dinosaurs were first recognized by European naturalists during the early 19th century, they were interpreted as being immense, lumbering reptiles similar to iguanas and crocodiles. Since that time our understanding of dinosaurs has changed substantially; early paleontologists such as Gideon Mantell, William Buckland, and Richard Owen would not recognize dinosaurs as we know them today. The once revolutionary idea that dinosaurs were dynamic creatures is now the standard view, yet the details of dinosaur physiology are still not completely known. A new study published in the journal PLoS One adds to the ongoing debate about dinosaur biology, and it suggests that dinosaurs might have actually inherited the physiology necessary to lead very active lives.

Most of the debate has centered on whether dinosaurs were endothermic like birds (i.e. internally regulated their body temperature through their metabolism) or ectothermic like living reptiles (i.e. had body temperatures that fluctuated more widely according to their surrounding environment). As some scientists have pointed out, it is not necessary to think that dinosaurs were precisely like living birds or reptiles—they could have had a unique physiology all their own—but the broad questions of whether dinosaurs were more like endotherms or ectotherms has remained.

Given that all the non-avian dinosaurs are extinct, though, we can’t simply stick a thermometer into a dinosaur and take their temperature. (Nor would such an activity be necessarily advisable, at least without wearing a protective suit of armor.)  The questions that remain must be approached more indirectly, and  in the new study scientists Herman Pontzer, Vivian Allen, and John Hutchinson looked at how much energy it would take for dinosaurs to walk and run. If they could figure out the cost of moving around, they reasoned, they could determine whether an ectothermic or endothermic metabolism would be able to provide the amount of energy the dinosaur required.

The team estimated the leg length of the bipedal dinosaurs, as this measurement has been used to estimate the cost of walking and running in living animals. They also estimated the volume of the muscles that would have attached to the leg bones based upon the size of muscles required to move the legs of the dinosaurs. These estimates could then be compared to what has been observed in living animals, providing an indirect way to see whether dinosaurs were more like ectotherms or endotherms.

What the scientists found was that the largest dinosaurs in the study (Plateosaurus, Dilophosaurus, Allosaurus, Gorgosaurus, and Tyrannosaurus) would have required an endothermic metabolism to move around, while the smaller dinosaurs, such as Archaeopteryx, fell more within the range expected for ectotherms. This created something of a paradox as the small, feathered dinosaurs are the ones thought to be most bird-like in terms of physiology.

Size might have made all the difference. While the study produced clear results for the larger dinosaurs the results for the smaller dinosaurs were ambiguous. Even though the smaller dinosaurs in the study (such as Archaeopteryx, Compsognathus, Velociraptor, and Microraptor) had anatomical traits suggestive of endothermy, the study placed them into the ectotherm range. What this probably means, the authors argue, is that energy expenditure in these smaller animals might have been different than in the large dinosaurs, but the technique they used could not successfully distinguish between the two metabolic ranges in the smaller dinosaurs.

More certain were the results of the larger dinosaurs. It had been proposed that large dinosaurs could afford to be ectothermic as their large body size would have allowed them to retain heat, thus living a “warm-blooded” lifestyle without actually being endothermic.  If the new analysis is correct, however, then it is more likely that the largest dinosaurs would have to have been endotherms. And since they evolved from small ancestors, that makes it possible that the smaller dinosaurs were also endotherms. The fact that pterosaurs, close relatives of dinosaurs (which were not included in the present study), also have traits that seem to indicate more bird-like metabolic rates suggests that endothermy either evolved multiple times or that it is an ancestral trait for the common ancestor of both pterosaurs and dinosaurs. Determining which scenario is the case, however, will require further study in combination with other lines of evidence from the fossil record.

A restoration of Jane (right) being bitten on the face by a tyrannosaur of about the same size. From the Palaios paper.

Humans youngsters often use their hands and arms to push and shove, but young Tyrannosaurus were obviously a bit different than us. It would take a lot of effort for two of the fighting dinosaurs to get close enough to scrabble at each other with their small arms, and so they employed a different tactic instead: they bit each other on the face. As reported in the journal Palaios, the controversial tyrannosaur skeleton known as “Jane” shows signs of just such an encounter.

For years scientists have debated whether Jane is a juvenile Tyrannosaurus or representative of a hypothetical smaller tyrannosaur genus, Nanotyrannus, but it is not the purpose of the present paper to resolve this issue. Instead paleontologists Joseph Peterson, Michael Henderson, Reed Scherer and Christopher Vittore document the presence of several puncture wounds in the bone around Jane’s snout that could only have been made by another young tyrannosaur. Like living crocodiles and alligators, tyrannosaurs may have bitten each other on the face during confrontations to establish social dominance, and the pattern of damage on Jane’s snout is more consistent with this kind of social interaction than with an attack with an intent to kill her or feed upon her. It was pretty harsh, but face-biting was a way for theropod dinosaurs to keep individuals in line.

Based upon the details of the punctures the two tyrannosaurs appear to have been facing each other when Jane was bitten. Unlike the fragment of Gorgosaurus jaw discussed here last month, Jane’s wounds show signs of healing, and unlike the Tyrannosaurus study suggesting that dinosaurs suffered from a bird disease, there is no indication of infection. She survived the attack and healed.

This does not mean that Jane was totally unaffected by the bite. Bone is a living tissue that is constantly being remodeled as an organism grows, and damage to bones at a young age can affect the way bones grow. As such the punctures in Jane’s skull caused her snout to bend a little to the left during growth. This would not have affected her ability to hunt or bite, but it would have given her a slightly asymmetrical appearance.

A pair of pachycephalosaurs face off. Photographed at the Museum of Ancient Life at Thanksgiving Point, Utah.

If you knew nothing at all about dogs, but you were presented with a lineup of the skeletons of a variety of breeds from chihuahua to bulldog to German shepherd to mastiff, you could be excused for thinking they were different species. Their skeletons seem to be so different, yet we know they are all just varieties of one subspecies, Canis lupus familiaris, that have been created through artificial selection. Paleontologists, on the other hand, do not have breeder’s records and must think carefully about what distinguishes one species of dinosaur from another. A new study by Jack Horner and Mark Goodwin in the journal PLoS One suggests that some dinosaurs previously thought to be separate species, even genera, were really just the growth stages of one species of dinosaur.

The dinosaurs that are the focus of the new study are three “bone-heads,” or pachycephalosaurs: Pachycephalosaurus, Stygimoloch, and Dracorex. These were bipedal ornithischian dinosaurs that had hard bony domes on their heads, often complemented with an array of spikes. Dracorex was small with a relatively flat head with small spikes, Stygimoloch was mid-sized with a small bony dome and huge horns, and Pachycephalosaurus was large with a large bony dome and relatively small horns. Together these dinosaurs appear to represent a growth series from juvenile to adult, all grouped together as Pachycephalosaurus, and the evidence can be found in the makeup of the bones.

Even though bones are hard they are not static things. They are constantly being remodeled; the change may be difficult to see from day to day but bone is still constantly being reabsorbed and laid down. The same processes happened in these dinosaurs, allowing for major modifications of the skull.

Looking at the microscopic structure of the skull bones, Horner and Goodwin found that the horns on the skulls they examined started off small, grew large, and then were reorganized as smaller structures around the edge of the solid dome of the skull. The young dinosaurs were not born with adult ornamentation but grew into it over time. Why large spikes were a juvenile characteristic and a bony dome was an adult characteristic, however, is still unknown.

Extreme changes in skull shape during growth can also be seen in hadrosaurs, where what were considered “small” species turned out to be juveniles of already known species, and in horned dinosaurs. In fact, at this year’s Society of Vertebrate Paleontology meeting, Horner and paleontologist John Scannella proposed that Triceratops is a growth stage of the larger horned dinosaur presently known as Torosaurus. This hypothesis has yet to be fully supported, but it does seem that many Cretaceous ornithischian dinosaurs underwent major anatomical changes during their lifetimes. No doubt this area of research will generate much discussion and debate in the years to come.

A restoration of Fruitadens. From the Proceedings of the Royal Society B paper.

From movies to museums, the most famous dinosaurs are among the largest. We like superlatives, and want to know what the biggest, fastest, and fiercest dinosaurs are. Yet, just like living animals, dinosaurs came in a variety of shapes and sizes, and a team of paleontologists has just announced, in the Proceedings of the Royal Society B, one of the smallest dinosaurs yet discovered

Named Fruitadens haagarorum, this diminutive dinosaur from the 150-million-year-old strata of western Colorado was only about two-and-a-half feet long. It was a heterodontosaurid, or a member of a group of ornithischian dinosaurs that split off early from the family tree and persisted for millions of years. It is the first time a heterodontosaurid dinosaur has been found in North America.

While many other ornithischian dinosaurs like hadrosaurs and horned dinosaurs were herbivores, though, it appears that Fruitadens was an omnivore. Like other heterodontosaurids it had at least three kinds of teeth: peg-like teeth at the front of the jaw, a single large “tusk” or canine-like tooth, and a series of leaf-shaped teeth good for shearing plants. This would have allowed it to eat a variety of foods, including meat, and its small body size probably meant that it had to.

The bodies of small animals are typically more energetically expensive than those of large ones, meaning that small animals have to find high-quality food like fruit and flesh and consume a lot of it. They cannot get by eating only relatively poor-quality food such as leaves. Such is the price of small body size, and thus Fruitadens may have been a late-surviving relic of an early radiation of small, omnivorous dinosaurs that later gave rise to more specialized plant-eating giants.

Part of the Dalton Wells bonebed exacavation. From the Palaeo paper.

It is often assumed that dinosaur paleontologists are interested only in getting the fossils they discover out of the ground as quickly as possible. This is not true. Paleontologists generally take great care to document and catalogue every fossil removed from a dig site, because the position and surroundings of those fossils may say something about where the animal lived and how it died. This can be especially important when multiple skeletons are found together. Were the animals part of a herd? Did they die at the same time? Were their bones washed to the same place by a river? Did scavengers pick at the bones?

Paleontologists studying the Dalton Wells bone beds near Moab, Utah, have grappled with such questions for a long time. Dated to the Early Cretaceous, about 127-98 million years ago, the site contains the remains of at least 67 individual dinosaurs of eight different genera. Bones from sauropods, ankylosaurus, Iguanodon-like herbivores and the predatory Utahraptor are all mixed together, and many of them appear to have been trampled. What happened?

In a new study published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, researchers led by Brooks Britt of Brigham Young University try to envision how the massive bone bed was formed.  As scientists excavated the bone bed, they found not a collection of articulated skeletons, but a heap of bits and pieces jumbled together. This suggested that the dinosaurs did not die all at once in an event that covered up the bones en masse, but that the bodies probably accumulated over a relatively short span of time, maybe as the result of a drought, and were subjected to the elements. The bones show little sign of scavenging by predatory dinosaurs, but they were extensively damaged from being scattered by water, trampled by other dinosaurs and eaten by insects. Eventually, the dinosaur graveyard was covered with sediment and preserved for tens of millions of years.

Given the damage to the bones, it’s surprising that there is a bone bed to study at all. Anyone who has spent a lot of time on the African savanna can tell you that the skeletons of even large animals, such as elephants, can be reduced to splinters within a relatively short time if they are not covered up. Scavengers, insects and the trampling feet of herbviores can soon turn a full skeleton into bone shards. This fact makes every fossil important, and at places like the Dalton Wells bone bed, even heavily damaged bones can provide us with a window into the distant past.

A cast of the skull of Allosaurus, photographed at the Utah Museum of Natural History.

I have always felt a bit sorry for Allosaurus. It was one of the top predators in what would become North America during the Jurassic, but the fearsome tyrannosaurs of the late Cretaceous are much more popular. In fact, the popularity of Tyrannosaurus and its kin has created the impression that the allosaurs dwindled and died out before the end of the Age of Dinosaurs, that they just could not compete with bigger, meaner predators. But a new study published in the journal Naturwissenschaften by paleontologists Roger Benson, Matt Carrano and Stephen Brusatte shows that close relatives of Allosaurus were going strong until the very end.

Over the past several decades, numerous enigmatic theropod dinosaurs have been discovered from Cretaceous rocks outside North America. A number of these, such as the recently described Aerosteon , closely resembled Allosaurus. And Aerosteon was not alone. The authors of the new study have placed it together with the theropods  Australovenator, Chilantaisaurus, Fukuiraptor, Megaraptor, Neovenator and Orkoraptor in a group called the Neovenatoridae.

If these names sound a bit unfamiliar, it’s because most relatively new dinosaurs are quite new—discovered within the last decade or so—and many of them have been hard to categorize. Megaraptor is a good example: at first, researchers thought that it was an enormous “raptor”-type dinosaur, though later studies suggested that its large claws were a sign that it was related to Spinosaurus. Now we know that it was more like Allosaurus in form and was part of a “hidden” radiation of this type of dinosaur throughout the world during the Cretaceous.

As a group, the Neovenatorid dinosaurs were smaller and more fleet of foot than their well-known relatives the carcharodontosaurids. Both groups are closely related to Allosaurus, being parts of the larger group the Allosauroidea, but they represent different sorts of adaptations. They probably played a very different role as predators in the ecosystems in which they lived.

A section of the trackway showing a ornithischian dinosaur (green tracks) walking uphill. From the PLoS One paper.

About 199 million years ago, on a small patch of land that is now preserved in the present-day African nation of Lesotho, there was an inclined slope next to a riverbed. Within hours, days, or even weeks of each other, several different dinosaurs climbed up and down the slope, leaving their footprints behind. Their tracks can still be seen there today, and as reported by paleontologists Jeffrey Wilson, Claudia Marsicano, and Roger Smith in the journal PLoS One, these tracks give us some clues as to how those dinosaurs moved.

Dinosaur footprints are effectively bits of fossilized behavior, and the Lesotho tracksite provides a rare look at how dinosaurs walked when moving up or down inclines. The site preserves the tracks of several ornithischian dinosaurs, which may have been similar to Lesothosaurus, and a single theropod dinosaur, which the researchers compare to Dracovenator. They handled the slippy slope in different ways.

The theropod dinosaur tracks show that it was walking parallel to the riverbank on the top of the slope before veering downwards to descend to the water. When it did so it stayed on two feet but it moved more slowly, as indicated by the shorter length between footprints in the portion where it was going downhill. This dinosaur also appears to have gripped into the ground with its foot claws, steadying itself as it moved downhill.

The ornithischians did something different. One of the ornithischian dinosaurs started on the riverbank and moved up the slope, and as it moved it changed the way it walked. On the riverbed it walked on all fours, holding its limbs out to the side and placing its entire foot on the ground. This was a slow-and-steady posture. As it began to move up the slope, however, the dinosaur moved its limbs closer to the midline of the body and stood on its tiptoes. Only when it reached the top of the slope did the dinosaur then stand up on two legs, keeping the same tip-toed posture.

What these tracks show is that the way dinosaurs handled walking on inclined surfaces was constrained by the type of bodies they had. The ornithischians changed their posture to cope with different obstacles and walked on all fours if they had to. The theropod, by constrast, could not do the same. It probably had arms that were too short to assist it in coming down the hill and thus relied on gripping the ground with its feet to stabilize itself.

At a time when we regularly see dinosaurs walking around on television and in movies this might seem kind of humdrum, but I think this description is still impressive. It provides us with a fleeting glimpse into the lives on animals that have been dead for hundreds of millions of years.