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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.

A complete skeleton of Darwinopterus as viewed from beneath. From the Proceedings of the Royal Society B paper.

The discovery of new kinds of feathered dinosaurs regularly makes the news these days, but it is important to remember that modern vertebrate paleontology encompasses much more than the search for the origin of birds. Indeed, this week scientists described an equally-spectacular fossil that fills in an important gap in our understanding of ancient life. Dubbed Darwinopterus modularis, this creature from the Middle Jurassic (over 160 million years old) rock of China connects two groups of pterosaurs that have long been divided by a lack of fossil evidence.

Described by Junchang Lu, David Unwin, Xingsheng Jin, Yongqing Liu, and Qiang Ji in the Proceedings of the Royal Society B, Darwinopterus fits snugly between the two major groups of pterosaurs that flew in the sky while dinosaurs ran about on land. The early pterosaurs were characterized by having long tails, short necks, and a separate nasal opening in the skull (among other traits). The later group, called the pterodactyloids, had short tails, long necks, and nasal openings combined with another opening in the skull in front of the eye (technically called the antorbital fenestra). From these forms paleontologists have long predicted that there were creatures of intermediate form between the two groups, and they finally have a good example of such a creature in Darwinopterus.

To put it simply, Darwinopterus possessed a mix of traits from both the earlier and later groups. Its body was like that of the early pterosaurs, including a long tail, but its head was more similar to the pterodactyloids. It had a long snout bearing an array of spiky teeth and had the single nasal/antorbital fenestra opening. Had only the head been found it would have probably been grouped with the pterodactyloids, and had only the body been found the scientists would have said that it was closely related to early pterosaurs, but all together Darwinopterus is an evolutionary mosaic that bears characteristics of both groups.

This means that pterosaurs like Darwinopterus were not evolving as a whole towards any kind of evolutionary goal, a common misconception about how evolution works. Instead different parts of the body were modified to greater or lesser degrees during the evolution of the group, thus why you see a “new” head on an “old” body type. Darwinopterus was not the ancestor of all later pterodactyloids—that is something that we cannot know right now—but its body helps us understand the type of animal the later pterosaurs evolved from. This is why we call it a “transitional form” rather than “ancestor” or “missing link:” it exhibits characteristics that help us understand how a particular group of organisms evolved even if we cannot identify direct ancestors or descendants.

Such a nuanced understanding is missing in most of the popular accounts of Darwinopterus that were published yesterday, and one of the worst offenders was the U.K.’s Daily Mail. “The terrifying flying dinosaur that could unlock the mystery of human evolution” squealed the headline, and it only gets worse from there. The piece states that Darwinopterus could explain why humans evolved so quickly after the last ice age, claiming that the new fossil

… dispels Darwin’s theory that small body parts such as a finger nail or tooth change[d] gradually and could explain how humans developed so quickly from mammals.

There is so much that is wrong here that it is difficult to know where to start. First, Darwinopterus was not a dinosaur. It was a pterosaur, a distinct group of flying reptiles nested within the Archosauria, or the evolutionary group that also contains dinosaurs and crocodiles. Second, the authors say nothing about recent human evolution in the paper. That is entirely the invention of the anonymous writer of the piece.

Furthermore, the new discovery does not do anything to “dispel” Darwin’s theory of evolution by natural selection. If anything, it gives Darwin’s conception of evolution increased influence. The scientists behind the paper consider that the body of Darwinopterus, like other vertebrates, was made up of modules (i.e. skull, neck, back). The form of these parts are regulated by genes that become active during the development of the organism, and slight changes in these regulatory genes could precipitate changes in the body “modules” and thus provide more variation for natural selection to act upon. Also, when we’re talking about evolution, “gradual” means step-by-step, not “slow,” and Darwin knew that rates of evolutionary change differ.

Media misrepresentations aside, Darwinopterus is a wonderful fossil. Not only is it a beautiful transitional fossil represented by multiple specimens, but it provides some key insights into how evolution works. Even better, the discovery of Darwinopterus suggests that there are other transitional pterosaurs out there waiting to be found.

For more on Darwinopterus see Tetrapod Zoology and Archosaur Musings.

The track of a small theropod dinosaur, known as Minisauripus. From the Journal of Cretaceous Research.

Even though some of the largest dinosaurs get the most attention, dinosaurs actually came in a variety of sizes. This past week paleontologists announced two discoveries that emphasize just how large, and how small, some dinosaurs were.

From Plagne, France came the announcement of the largest dinosaur footprints yet discovered. Naturalists Marie-Hélène Marcaud and Patrice Landry found the tracks back in April, at which point paleontologists Jean-Michel Mazin and Pierre Hantzpergue from the Université Claude Bernard Lyon went out to have a look. They discovered that the 150-million-year-old tracks were made by enormous sauropod dinosaurs, the largest probably stretching over 85 feet and weighing more than 40 tons.

Contrast that with a find announced yesterday in the Korea Herald. The newspaper reported that a resident of one of South Korea’s southern provinces discovered one of the smallest dinosaur footprints ever found: the track of a theropod dinosaur measuring only half an inch long. According to Kim Gyeong-su of Chinju National University of Education, the track is about 100 million years old and is consistent with a previously-known track type called Minisauripus.

I wonder how many of the tiny theropods could have fit into just one track made by the enormous sauropod!