Allosaurus, on display at the CEU Museum in Price, Utah. Photo by author.

Big, predatory dinosaurs were well-adapted to stripping flesh from bone. That’s obvious from the shape and size of their teeth. What has been more difficult to determine, however, is how they behaved as they ate. Studying bones scored with the toothmarks of carnivorous dinosaurs is one of the most direct ways to approach questions about how predatory dinosaurs fed. One such bone—a tail vertebra of the sauropod Pukyongosaurus found on the Korean peninsula—shows that at least two different predators each had their shot at the same carcass.

The damaged bone is described in an in-press Palaeogeography, Palaeoclimatology, Palaeoecology paper by In Sung Paik and colleagues. The paleontologists report that at least five parts of the bone show toothmarks, including gouges, V-shaped scores and divot-shaped lesions. Since the bones of the sauropod dinosaur were otherwise in good condition—they did not exhibit cracks that would indicate that the bones had been lying on the surface for a long time—Paik and co-authors propose that the dinosaur was rapidly buried near the site of death, meaning that all these toothmarks were made in a narrow window between death and burial. Whether or not the sauropod was killed by a predator cannot be determined. All that is clear is that the toothmarks were left after the Pukyongosaurus died.

So what sort of carnivorous dinosaurs left the tooth marks? That is difficult to say. Most of what is known about big predatory dinosaurs on the Korean peninsula comes from teeth attributed to dinosaurs akin to Allosaurus and tyrannosaurs. Big theropods were certainly around in the right area at the right time, but they are almost entirely a mystery.

Nevertheless, the patterns of the toothmarks indicate a few things about how the predatory dinosaurs ate. Some of the marks, for example, are arranged in parallel rows which indicate that the feeding dinosaur was nipping or scraping with teeth at the very front of the jaw, perhaps at a time when the rest of the easily-accessible flesh had been stripped off. Additionally, while three sets of marks appear to have been made by a large animal, there are two that appear to have been left by a smaller carnivorous dinosaur at a time when most of the flesh had been removed. Were the two dinosaurs of different species? Could they have been an adult and a juvenile of the same species? How much time passed between when the big dinosaur fed and the little one tore off the remaining scraps? No one knows, but the traces left on the sauropod bone provide paleontologists with a murky window into an ancient encounter between predator and prey.

References:

Paik, I.; Kim, H.; Lim, J.; Huh, M.; Lee, H. (2011). Diverse tooth marks on an adult sauropod bone from the Early Cretaceous, Korea: implications in feeding behaviour of theropod dinosaurs. Palaeogeography, Palaeoclimatology, Palaeoecology : 10.1016/j.palaeo.2011.07.002

Paleontologist Philip Currie poses with a tyrannosaur skull. Photo courtesy Atlantic Productions.

Tarbosaurus, the great tyrannosaur of Cretaceous Mongolia, hunted in packs. That is the exceptional claim made by University of Alberta paleontologist Philip Currie in a press release, and news outlets all over the world have picked up the story. Just imagine rapacious tyrannosaur families tearing over the prehistoric countryside; it is a terrifying notion that the press release heralds as a “groundbreaking” discovery that will forever change paleontology.

But does the actual evidence live up to all the hype? Unfortunately, the answer is no. The proposal of pack-hunting dinosaurs is old news in paleontological circles, and the hard evidence to support the claims about Tarbosaurus has not yet been released.

Packaged under the theme “Dino Gangs,” the media release, book, and cable-network documentary arranged by Atlantic Productions hinge on a Tarbosaurus bonebed found in Mongolia’s Gobi Desert. The site was one of 90 Tarbosaurus localities surveyed by Currie and the Korea-Mongolia International Dinosaur Project, but it is unique in that it preserves the remains of six individual animals of different life stages. How the animals died and became buried is unknown. Even so, the press claims that these dinosaurs were a single family group that hunted together.

There was no scientific paper attached to the release, and I received no reply from Atlantic Productions when I inquired whether a technical description of the site will soon be published. The media release–reporting conclusions without providing evidence–was presented on its own.

This is not the first time tyrannosaurs have been reconstructed as living in packs. In 1997 Currie relocated a rich dinosaur bonebed in Alberta, Canada that had been discovered by the fossil hunter Barnum Brown in 1905. The site was dominated by remains of the tyrannosaur Albertosaurus—at least a dozen individuals of this species were found in this one place. Why one site should contain so many tyrannosaurs was difficult to explain, but in a 1998 paper published in Gaia, Currie proposed that the Albertosaurus were living in a social group and that the site was evidence of gregarious behavior among the dinosaurs. More than that, Currie proposed that there was a “division of labor” within the Albertosaurus packs. Compared with the adults, juvenile Albertosaurus would have been much faster runners thanks to their different leg proportions, and so Currie suggested: “The faster, more agile juveniles may have been responsible for driving potential prey towards the larger, more powerful adult tyrannosaurids.” Currie has suggested the same thing for Tarbosaurus in the “Dino Gangs” press release.

But the idea that young and old tyrannosaurs worked together to tackle prey rests upon the inference that the bonebeds contain social groups. This is not necessarily so. There are many ways to make a bonebed, and the fine geological details of such fossil-rich sites contain essential information about how the bodies of the different individuals became preserved together. Proximity does not always indicate sociality, as Currie himself noted in a paper published with David Eberth last year about the Albertosaurus quarry.

Although the idea that the Albertosaurus quarry indicates complex social interactions among pack-hunting dinosaurs is a sexy hypothesis, Currie and Eberth noted that the animals could have been brought into close association by some kind of environmental catastrophe. “[T]he evidence for a significant storm and associated flooding event at the [Albertosaurus] site and in the surrounding area is well documented,” the scientists wrote, and they suggested that solitary Albertosaurus might have been driven together into a small area by the floodwaters. Pack behavior among the animals could not be taken as a given. The Albertosaurus were together when they died, but exactly how they died and why they were so close to each other remains unclear.

In the 2005 book Carnivorous Dinosaurs, Currie and several co-authors reported on a bonebed found in Montana that contained several hadrosaurs and remains of three tyrannosaurs identified as Daspletosaurus. Though the scientists suggested that the tyrannosaurs might have been interacting socially before they died, how the animals died and became buried was unknown. The same was true of a site in Argentina described by Currie and colleague Rodolfo Coria. The bonebed contained seven individuals of a large predatory dinosaur unrelated to tyrannosaurs named Mapusaurus. Although the site could have represented a social group, Currie and Coria concluded that “It is conceivable that this bonebed represents a long term or coincidental accumulation of carcasses.”

There is no slam-dunk evidence that tyrannosaurs or other large predatory dinosaurs hunted in packs. Even in the case of Deinonychus—a small, sickle-clawed “raptor” traditionally thought to be a cooperative hunter—evidence of multiple individuals in association with prey species has recently been questioned. In the end, trackways that record the footsteps of multiple raptors moving together has provided better evidence that these dinosaurs were sometimes social. No such evidence exists for tyrannosaurs yet. (Only one footprint attributed to a tyrannosaur has been found so far.)

Various processes can bring bones together into a single fossil deposit. A bonebed might represent a social group killed and buried by a flood, scattered bodies or bones that were washed together by water currents, or a natural trap where multiple individual animals died over a long period of time, among other possibilities. How the animals died, how long it took for the fossil deposit to accumulate, and other questions must be answered before hypotheses about behavior can be drawn out. As for the Tarbosaurus bonebed, no technical details of the site have yet been released. There is no science to talk about at this point. The site might record the death of a dinosaur pack, but that is just one of many possibilities that have yet to be ruled out.

The hubbub over the “Dino Gangs” press release is intensely frustrating. No scientific information is available, and the supposedly jaw-dropping findings are almost exactly the same as those proposed on the basis of a different site in 1998. The press release is full of bombastic language about how it is now time to rewrite the dinosaur books and how this discovery will forever change our understanding of dinosaur behavior. None of the information provided so far will do any such thing. The new find is one more discovery that will add to our understanding of dinosaurs, but is not wildly different from what has been discovered or proposed before. If there is something truly exceptional about the Tarbosaurus bonebed, it has yet to be revealed.

A discovery isn’t important simply because a press release says it is. Scientific findings should not be judged by how glitzy a documentary is or how well a book sells. By the sound of it, Currie and his colleagues have found a spectacular fossil site that is brimming with information about prehistoric life. None of the details have been published yet, and, consequently, they have not been submitted to the process of scientific debate, so no one can definitively say how the Tarbosaurus bonebed will affect our understanding of these dinosaurs. The discovery of the fossil site is just one part of the story. The rest, including how the Tarbosaurus lived and died, will take time to draw out.

References:

Coria, R., and Currie, P. (2006). A new carcharodontosaurid (Dinosauria, Theropoda) from the Upper Cretaceous of Argentina Geodiversitas, 28 (1), 71-118

Currie, P. (1998). POSSIBLE EVIDENCE OF GREGARIOUS BEHAVIOR IN TYRANNOSAURIDS Gaia, 271-277

Currie, P., & Eberth, D. (2010). On gregarious behavior in Albertosaurus Canadian Journal of Earth Sciences, 47 (9), 1277-1289 DOI: 10.1139/E10-072

Currie, P.; Trexler, D.; Koppelhus, E.; Wicks, K.; Murphy, N. (2005) An unusual multi-individual, tyrannosaurid bonebed in the Two Medicine Formation (Late Cretaceous, Campanian) of Montana (USA), in Carpenter, K. (ed.), The Carnivorous Dinosaurs. Indiana University Press, Bloomington; Indianapolis: 313-324.

The skull of Deinonychus. Notice the reconstructed sclerotic ring inside the eye opening. From Wikimedia Commons.

Randall Munroe, the creator of the webcomic XKCD, isn’t going to like this one bit. Fear of attack by Velociraptor is a running theme in the science-themed series—lazy computer programmers should be especially wary—and two separate discoveries announced last week gave those with a phobia of raptors good reason to barricade the doors and windows. Not only did Velociraptor have an excellent sense of smell, but they also hunted at night.

We don’t know for sure what dinosaur eyes looked like. The soft-tissue structures rotted away between the time of death and preservation. But there was one feature of the skull that allowed paleontologists Ryosuke Motani and Lars Schmitz to approach the question of whether some dinosaurs were active in the dark—a circle of bones called the sclerotic ring.

Though relatively rare in the dinosaur fossil record, sclerotic rings can give paleontologists a general picture of eye size and shape. This is because the bone surrounded the pupil and the iris of the eye. Birds, lizards, and other vertebrates have this feature, too, and the details of the sclerotic ring are closely associated with the light conditions when an animal is active.

Modern-day nocturnal animals tend to have wide sclerotic rings with a very large aperture in the middle relative to eye size. Animals that are more active during the day (diurnal), on the other hand, have smaller apertures relative to their eye size. By tracking this association, Motani and Schmitz were able to detect that dinosaurs were active during all times of the day.

(The study also included analysis of pterosaurs and other archosaurs, but I am going to restrict my comments to the findings about dinosaurs here.)

As a group, the dinosaurs did not all neatly fall into nocturnal and diurnal groups. Herbivorous dinosaurs, in particular, appear to have been cathemeral—they would have been active over short periods of time during the day and night. Rather than foraging continuously from dawn until dusk, herbivorous dinosaurs such as the hadrosaurs  Corythosaurus and Saurolophus, the small ceratpsian Protoceratops, the sauropodomorph Plateosaurus and the sauropod Diplodocus were probably most active during the early, cool parts of the day and then again around twilight.

Small, predatory dinosaurs were different. Almost all the carnivorous dinosaurs that were examined had sclerotic rings consistent with a nocturnal lifestyle, including Juravenator, Microraptor and—you guessed it—Velociraptor. Based upon the inferred night-hunting habits of Velociraptor and the cathemeral pattern of Protoceratops, Motani and Scmitz suggest that the deadly encounter between the two species immortalized in the “fighting dinosaurs” specimen probably happened at twilight or in low-light conditions.

Not all theropod dinosaurs stalked prey by night, though. The small predator Sinornithosaurus appears to have had the more varied schedule seen among the herbivores, and this was also found for the omnivorous “ostrich mimic” dinosaurs Garudimimus and Ornithomimus. Early birds—the descendants of small, feathered theropods—were different. Every species in the study—Archaeopteryx, Confuciusornis, Sapeornis and Yixianornis—had eyes specialized for daytime activity. Perhaps, during early bird evolution, there was a transition from nocturnal ancestors to flying descendants active during the day.

These findings change our perspective of what Mesozoic life was like. Dinosaurs were thought to be mostly active during the day, with small mammals—including our ancestors and cousins—coming out at night. Now it seems that the Cretaceous nights were not as safe as had been presumed. With so many agile predatory dinosaurs around, mammals would have much to fear during the nighttime hours.

Then again, the idea that Mesozoic mammals scurried through the night is an assumption based upon the idea that dinosaurs were stomping around during the day. Studies of the mammals themselves will be needed to see how their activity overlapped with that of the dinosaurs. Since mammals lack sclerotic rings, though, some other technique will have to be used. Further studies of dinosaurs will be required, too. Conspicuously missing from the study were large-bodied predators akin to Allosaurus and Albertosaurus. When these giants hunted, and when the mammals under their feet were active, awaits future study.

For more, see Schmitz’s own post on the research at his blog and Ed Yong’s report at Not Exactly Rocket Science.

References:

Motani, R., & Schmitz, L. (2011). PHYLOGENETIC VERSUS FUNCTIONAL SIGNALS IN THE EVOLUTION OF FORM-FUNCTION RELATIONSHIPS IN TERRESTRIAL VISION Evolution DOI: 10.1111/j.1558-5646.2011.01271.x

Schmitz, L., & Motani, R. (2011). Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology Science DOI: 10.1126/science.1200043

A reconstruction of Albertosaurus at the Royal Tyrrell Museum in Canada. Image from Wikipedia.

When I was in elementary school, I was told that mammals and reptiles could easily be told apart by their teeth. Mammals had a full, enamel-covered toolkit in their mouths—incisors, canines, premolars, and molars suited to different tasks—while reptiles had only one kind of tooth. The dental differences were presented as one of the ways in which mammals were superior to reptiles, but like a number of other things I was taught in grade school, this wasn’t quite right.

Not all mammals have differentiated sets of teeth. Dolphins, for example, have jaws full of nearly identical, conical teeth. Among reptiles, on the other hand, multiple species have been found with a variety of tooth shapes in their jaws. Pakasuchus, an extinct cousin of modern crocodiles found in the 105-million-year-old rock of Tanzania, had three different types of teeth in its jaws, and even the mighty Tyrannosaurus and Albertosaurus possessed differentiated teeth. What this meant for how the tyrant dinosaurs ate was addressed in a Canadian Journal of Earth Sciences paper by Miriam Reichel last year.

Although the teeth of Albertosaurus and Tyrannosaurus may seem to be all the same, these dinosaurs actually had three different tooth classes. The teeth at the front of the jaw are small and closely packed; those in the middle of the jaw are exceptionally long and curved and those at the back of the jaw are smaller and only slightly recurved. (The differences between the teeth can perhaps best be seen in the skull of the juvenile Tyrannosaurus “Jane“.) What Reichel wanted to know was how these various teeth functioned, and so she created computerized, 3-D models of Albertosaurus and Tyrannosaurus teeth to test how they would have held up to the stresses and strains created by biting.

As might be expected for large predators, the teeth of both tyrant dinosaurs were suited to different tasks. The small and stout front teeth were likely used for pulling large pieces of meat from carcasses, the much larger teeth in the middle of the jaw were adapted to coping with the stresses of struggling prey, and the teeth at the rear of the jaw were positioned to deliver heavy, crushing forces in an arrangement Reichel likened to a clamp.

There was one notable way in which Albertosaurus and Tyrannosaurus differed, though. Albertosaurus had a matching set of upper and lower teeth—their functions were consistent from front-to-back along the jaw—but in Tyrannosaurus the patterns of the upper and lower teeth differed. Specifically, the teeth at the front of the lower jaw in Tyrannosaurus were not adapted to pulling off chunks of flesh, but were instead suited to withstanding forces associated with capturing prey. Perhaps, Reichel suggests, this is because Tyrannosaurus had a slight overbite in which the teeth at the front of the lower jaw were closest to the large, prey-capturing teeth near the middle of the upper jaw, meaning that they changed in function to compensate for the alteration in jaw position.

Lacking live tyrannosaurs to study, paleontologists will surely continue to find ways to model the bites of these famous dinosaurs. It is not an easy task. Teeth, bones, muscles, ligaments, and other aspects of the living animal must all be accounted for and combined to create a picture of the entire dinosaur. We do not have a fully comprehensive understanding of tyrannosaur bites just yet, but the more we discover about their jaws, they more terrifying the tyrants become.

References:

Reichel, M. (2010). The heterodonty of Albertosaurus sarcophagus and Tyrannosaurus rex: biomechanical implications inferred through 3-D models Canadian Journal of Earth Sciences, 47 (9), 1253-1261 DOI: 10.1139/E10-063

SMITH, J. (2005). HETERODONTY IN TYRANNOSAURUS REX: IMPLICATIONS FOR THE TAXONOMIC AND SYSTEMATIC UTILITY OF THEROPOD DENTITIONS Journal of Vertebrate Paleontology, 25 (4), 865-887 DOI: 10.1671/0272-4634(2005)025[0865:HITRIF]2.0.CO;2

A restoration of Platypterygius specimen SAM P14508 showing the location of the injuries on the lower jaw. From Zammit and Kear, 2011.

The prehistoric world was intensely violent. So I believed when I was a kid, anyway. Almost every book I read or movie I saw about now-fossilized creatures showed them as ferocious monsters that were constantly biting and clawing at each other. I spent hours with plastic toys and mud puddles reenacting these scenes myself, never thinking about whether there were any fossil traces of such epic battles.

Finding fossil evidence of ancient conflicts is very difficult. A predator might leave behind traces of feeding—such as toothmarks on bone or undigested muscle tissue in their fossilized dung—but the signs of prehistoric fights are very rare. Sometimes, as in the case of the holes in the frill of the horned dinosaur Nedoceratops, what were thought to be injuries inflicted by fighting with animal turn out to be a different kind of pathology or a strange bone growth pattern. Nevertheless, a few signs of prehistoric conflicts have been found.

A little more than 100 million years ago, the large ichthyosaur Platypterygius australis swam the seas of Cretaceous Australia. It was not a dinosaur—not even close—but a marine reptile belonging to a lineage that had returned to the sea many millions of years before. Thanks to new fossil evidence reported by Maria Zammit and Benjamin Kear in an in-press Acta Palaeontologica Polonica paper, we now have evidence that one of these ichthyosaurs may have had a painful run-in with one of its own kind. SAM P14508, a Platypterygius found in South Australia, has a distinctive set of healed wounds on its lower jaw that were most likely made by another ichthyosaur.

The key to the Platyptergius puzzle was the fact that the animal survived its injuries. Had a predator been feeding on the carcass, Zammit and Kear would have found distinctive toothmarks without any signs of healing. Since the injured bone had grown and remodeled after being damaged, though, the ichthyosaur clearly lived for quite some time after being attacked. The bones were scored rather than deeply punctured or broken; while painful, the injuries would not have debilitated the ichthyosaur.

Naturally, predators aren’t always successful and might injure an animal without killing it, but the pattern of the wounds lead Zammit and Kear to propose the marks were made by another Platyptergius. The only other large predators in the area at the time were the enormous pliosaur Kronosaurus and large sharks, both of which would have left very different bite marks and probably would have attacked part of the body containing vital organs rather than the snout. Zammit and Kear are tentative about their conclusions—without a time machine and some scuba gear, we can’t know for sure what happened—but the wounds on the specimen are consistent with the damage another Platyptergius could have inflicted. “[I]t is tempting to reconstruct the positioning of the marks on the ventral side of the mandible as the result of a restraining bite,” they write, “delivered when another ichthyosaur approached SAM P14508 from below and attempted to neutralize the threat of a counter attack by clamping onto and forcing aside its elongate jaws.”

References:

Zammit, M. and Kear, B.J. (2011). Healed bite marks on a Cretaceous ichthyosaur Acta Palaeontologica Polonica, 5 : 10.4202/app.2010.0117

A large coprolite, probably left by Tyrannosaurus, found in Saskatchewan, Canada. From Chin et al., 1998.

Tyrannosaurus ate flesh. That much is obvious. The reinforced skull and huge, serrated teeth of the tyrant dinosaur and its kin were not adaptations for cropping grass or cracking coconuts. Both predators and scavengers, the tyrannosaurs must have consumed massive amounts of meat to fuel their large bodies, and paleontologists have been fortunate enough to find a few traces of tyrannosaur meals.

Feeding traces are rare in the fossil record. The very act of feeding itself at least partially destroys the organisms being fed upon—see this time-lapse video of an African elephant carcass to see how efficiently a group of carnivores can dismantle a body—and it takes a series of exceptional circumstances for bite-marks to be preserved. In the case of tyrannosaurs, paleontologists have found distinctive bite marks on Triceratops, hadrosaurs, and even other tyrannosaurs, but there is another category of trace fossils that can tell us something about dinosaur diets: gut contents and scat.

In 1998, paleontologists Karen Chin, Timothy Tokaryk, Gregory Erickson and Lewis Calk described the enormous droppings of a theropod dinosaur found in southwestern Saskatchewan, Canada. Technically termed a coprolite, the foot-and-a-half-long mass of fossil feces was left by a large dinosaur a little more than 65.5 million years ago, and two factors confirmed that it had been left by a carnivorous dinosaur. Not only did the coprolite have elevated levels of phosphorous phosphorus—a common chemical feature of carnivore scat—but about thirty-to-fifty percent of the mass was broken fragments of bone from a young dinosaur. The only dinosaur found in the area capable of leaving behind this trace fossil was Tyrannosaurus rex, and the authors of the report concluded “this rare example of fossilized dietary residues helps to refine our understanding of theropod feeding behaviour by providing physical evidence that a tyrannosaur crushed, consumed, and incompletely digested large quantities of bone when feeding on a subadult dinosaur.”

Chin and a different team of researchers reported a second tyrannosaur coprolite in 2003. The highly fractured fossil was found in the approximately 75-million-year-old rock of Alberta’s Dinosaur Park Formation—home to the tyrannosaurs Daspletosaurus and Gorgosaurus—and careful study of the coprolite’s contents revealed small bits of fossilized soft tissue that had not been fully digested. The large amount of bone in the Saskatchewan coprolite and the numerous traces of soft tissue in the Alberta coprolite suggested that the food tyrannosaurs ingested did not remain in their digestive systems long enough for all of it to break down, making their digestive systems unlike those of living crocodiles and snakes. Tyrannosaurs consumed large amounts of flesh and bone, but it passed through their systems relatively rapidly.

Paleontologists may have even found gut contents still inside the body of a tyrannosaur. In 2001, paleontologist David Varricchio described a partial Daspletosaurus skeleton from western Montana’s Two Medicine Formation, and near the partial hips of the predator were parts of the tail and jaw of a juvenile hadrosaur. The young hadrosaur bones appeared to be degraded by acid—they had a spongy and pitted appearance different from typical bone—and the Daspletosaurus body was found near a low-energy, seasonal pond, making it unlikely that the body of a young hadrosaur had been washed in and mixed up with the predator. Citing his find and the coprolite announced by Chin and co-authors in 1998, Varricchio proposed that juvenile and sub-adult dinosaurs may have been common prey for tyrannosaurs, an idea supported by scientists David Hone and Oliver Rauhut in their 2009 review of predatory dinosaur feeding habits. For large predators like tyrannosaurs, juvenile dinosaurs may have been easing pickings.

Frustratingly, the sample size of tyrannosaur coprolites and gut contents is very small. More of these trace fossils will be needed to further investigate what these dinosaurs were regularly eating and how they might have digested their food. Even so, each coprolite is like a time capsule that can tell us something about an animal’s diet and biology—behavior, diet, physiology, and internal anatomy, all wrapped up in a bit of tyrannosaur scat.

References:

CHIN, K., EBERTH, D., SCHWEITZER, M., RANDO, T., SLOBODA, W., & HORNER, J. (2003). Remarkable Preservation of Undigested Muscle Tissue Within a Late Cretaceous Tyrannosaurid Coprolite from Alberta, Canada PALAIOS, 18 (3), 286-294 DOI: 10.1669/0883-1351(2003)0182.0.CO;2

Chin, K., Tokaryk, T., Erickson, G., & Calk, L. (1998). A king-sized theropod coprolite Nature, 393 (6686), 680-682 DOI: 10.1038/31461

Erickson, G., & Olson, K. (1996). Bite marks attributable to Tyrannosaurus rex: Preliminary description and implications Journal of Vertebrate Paleontology, 16 (1), 175-178 DOI: 10.1080/02724634.1996.10011297

VARRICCHIO, D. (2001). GUT CONTENTS FROM A CRETACEOUS TYRANNOSAURID: IMPLICATIONS FOR THEROPOD DINOSAUR DIGESTIVE TRACTS Journal of Paleontology, 75 (2), 401-406 DOI: 10.1666/0022-3360(2001)0752.0.CO;2

A snapshot of the Tyrannosaurus known as "Stan" at the Institut Royal des Sciences Naturelles de Belgique, Bruxelles. Image from Wikimedia Commons.

Of all the organisms scientists have found in the fossil record, Tyrannosaurus rex is the most prominent ambassador for paleontology. No dinosaur hall is complete without at least some fragment of the tyrant dinosaur, and almost anything about the dinosaur is sure to get press coverage. We simply can’t get enough of old T. rex. It was no surprise, then, that a census of Tyrannosaurus specimens from Montana’s Hell Creek Formation published by Jack Horner, Mark Goodwin and Nathan Myhrvold in PLoS One gained wide media coverage, but there was a sub-story that many news outlets missed. Rather than overturning the image of Tyrannosaurus as a predator, as some reports claimed, the conclusions of the new study actually brought Horner’s stance on the iconic dinosaur close to what other experts thought.

The story behind the new PLoS One study began eighteen years ago. The film Jurassic Park had just triggered a wave of dinomania unlike any seen before, and paleontologists were quick to take advantage of the interest that the film had generated. Among them were Gary Rosenberg and Donald Wolberg, who organized 1994′s Dino Fest event at Indiana University-Purdue University at Indianapolis, and one of the invited speakers was Jack Horner. One of the chief consultants on Jurassic Park, Horner had helped bring the film’s terrifying Tyrannosaurus to life, but in his talk he presented a different picture of the dinosaur.

Horner’s lecture was titled “Steak Knives, Beady Eyes, and Tiny Little Arms (A Portrait of T. rex as a Scavenger),” and a transcript of it was printed in the collected proceedings of the conference. With Jurassic Park fresh in the audience’s mind, Horner explained that the real animal probably wasn’t as speedy or ferocious as the film made it seem. “In fact,” Horner said, “I think the only thing that Tyrannosaurus rex would have done in that movie is eat that lawyer.”

In Horner’s view, Tyrannosaurus was built for scavenging. Despite possessing a huge head full of serrated teeth the size of rail spikes, the tyrant dinosaur had puny, rigid arms, and Horner argued that strong arms would have been essential for an active predator to catch and subdue prey. Furthermore, Horner pointed to the apparently small eyes of Tyrannosaurus and the large olfactory lobe of the dinosaur’s brain. Horner asserted his uncertainty about these features—”I don’t know if it’s worth anything,” he said—but hinted that they might be consistent with the idea of Tyrannosaurus as a scavenger that was better at sniffing out carcasses than following live prey. Since the hadrosaurs and horned dinosaurs of the time lived in giant herds, Horner suggested that tyrannosaurs followed them to pick at the carcasses of those that died as the herds trod around the landscape. Horner concluded:

Picture Tyrannosaurus rex. He has no arms, can’t run fast, appears to have a large olfactory lobe and he’s big. Interestingly enough if you think about it, one of the best things to be if you are a scavenger is big so you can chase away anything else around the carcass.

Horner’s book “The Complete T. rex“, published that year with science writer Don Lessem, presented the “obligate scavenger” hypothesis to a wider audience. Similar ideas had been proposed before, but Horner’s public suggestion that Tyrannosaurus was a lazy scavenger stirred immediate controversy. This was not so much an academic debate as a tug-of-war over who would mold the image of Tyrannosaurus.

Among the early responses to Horner’s ideas was a 1997 lecture delivered by Theagarten Lingham-Soliar to the British Association for the Advancement of science (later printed in Geology Today) titled “Guess who’s coming to dinner: A portrait of Tyrannosaurus as a predator.” Tyrannosaurus would have scavenged when the opportunity arose, Lingham-Soliar said, but the reinforced skull and impact-resistant teeth of the dinosaur were clearly well-suited to handling struggling prey. Even juveniles had these features, and given their small size it was probable that they were actively hunting smaller fare instead of relying on scraps from carcasses already obliterated by adults.

Responses like Lingham-Soliar’s did little to quell the debate. The scavenger hypothesis was popularized in books, news reports and documentaries. Horner’s influence even turned Tyrannosaurus into a scavenger during an early scene of Jurassic Park III. Horner hinted that part of his motivation for proposing the obligate scavenger idea was to get scientists and dinosaur fans to think critically about commonly accepted ideas. Despite the amount of attention the idea received, other paleontologists were not convinced.

The ultimate take-down of Horner’s hypothesis was published by tyrannosaur expert Thomas Holtz in the 2008 book “Tyrannosaurus rex: The Tyrant King.” Right at the outset, Holtz pointed out that flesh-eating animals do not break down into neat categories of “scavenger” and “predator.” Spotted hyenas—traditionally believed to be almost pure scavengers—have been found to be active hunters, and even lions, iconic hunters, obtain a significant part of their food through scavenging. Large carnivorous animals both hunt and scavenge food. Tyrannosaurus would not have been different.

Holtz’s paper was the first comprehensive and scientific critique of Horner’s idea. The ideas had been batted around in talks, documentaries and popular books, but Holtz actually put in the scientific legwork to see if the traits Horner associated with scavenging truly indicated that Tyrannosaurus relied almost entirely on carrion.

Holtz’s analysis dismantled what Horner had proposed. The eyes of Tyrannosaurus were not atypically small; the proportions of its legs would have allowed it to run faster than other large theropods (and, more importantly, potential prey species); it had deeply rooted teeth that would have been able to cope with the stresses generated by struggling prey; and its small forelimbs would not have prohibited it from hunting and killing other dinosaurs. Oddly enough, some of the best evidence of tyrannosaur hunting come from two animals that escaped attacks by the dinosaur: an Edmontosaurus with a partially healed bite along its tail and a Triceratops skull showing a similar type of damage. Since Tyrannosaurus was the only gigantic predator known from the habitats in which the injured herbivores were found, it is probable that the dinosaurs were survivors of Tyrannosaurus attacks.

Tyrannosaurus almost certainly scavenged—something that has been supported by the recent discovery of cannibalism and an instance of scavenging by the related Tarbosaurus—but there was nothing about the dinosaur that barred it from being a formidable hunter. “[T]here is no evidence to suggest that tyrannosaurs were radically different in diet from living large-bodied carnivores, which obtain food [by] both predation and scavenging,” Holtz said.

As reconstructed by Holtz, Tyrannosaurus may have been the spotted hyena of its day. The hyenas do not have large claws or muscular arms like lions. Instead, they primarily catch, kill and consume prey with their robust jaws, which is what the tyrant dinosaur would have done as well. Especially after Holtz’s paper, the idea that Tyrannosaurus hunted and scavenged should not have surprised anyone. So why did so many media sources act with astonishment at the statements by Horner and his team in reference to their new PLoS One paper?

The answer — after the jump

The recent publication of a paper that explicitly attacked Horner’s hypothesis set the stage. A few weeks ago, Chris Carbone, Samuel Turvey and Jon Bielby published a study suggesting that smaller meat-eating dinosaurs would have destroyed most of the available carcasses before Tyrannosaurus had a chance to get to them, making it unlikely that the giant dinosaur relied on carrion for food. There were a few problems with the lists of dinosaurs the authors drew up to create their estimates, but the study still made the important point that Tyrannosaurus probably would have competed with numerous other dinosaurs for carrion. Scavenging would not have been as  easy a gig as Horner initially proposed.

The study by Carbone and co-authors cast doubt on the ability of Tyrannosaurus to find—much less consume—dinosaur carcasses. But a little more than a week later, Horner, Goodwin and Myhrvold concluded that the tyrant must have scavenged.

Horner and colleagues based their hypothesis on a census of dinosaurs found in the vicinity of Fort Peck Reservoir in northeastern Montana during the decade-long Hell Creek Project. The goal of this effort is to “create a comprehensive biotic foundation from which paleobiological and geological hypotheses could be tested,” including an understanding of dinosaur abundance at the end of the Cretaceous. The new paper presented some preliminary results from the census, and Tyrannosaurus turned out to be more common than expected.

A pie chart showing the relative abundance of large dinosaurs in the upper part of the Hell Creek Formation around the Fort Peck Reservoir. The blue slice represents Triceratops, the yellow slice represents Edmontosaurus, and the red slice represents Tyrannosaurus. From Horner et al., 2011.

Outcrops sampled by the Hell Creek Project were divided into three sections: lower, middle and upper slices. The top and bottom sections were the focus of the PLoS One report, and within each portion many remains of Triceratops, Edmontosaurus, and Tyrannosaurus were found. Triceratops was the most common in each section, but, surprisingly, Tyrannosaurus was just as common, if not slightly more common, than the hadrosaur Edmontosaurus. In the upper Hell Creek section, for example, the census included twenty two Triceratops, five Tyrannosaurus, and five Edmontosaurus.

(The dinosaurs Thescelosaurus, Ornithomimus, Pachycephalosaurus, and Ankylosaurus were also included in the breakdown, but were relatively rare. Small predatory dinosaurs, such as Troodon, were reported as being rare and are not included in the breakdown.)

The relative number of Tyrannosaurus skeletons seems high for a predatory species. Why would a large predator be about as common as one of its prey species? Perhaps there was some kind of bias in preservation or collection. If Edmontosaurus was one of the main sources of food for Tyrannosaurus, for example, the skeletons of these dinosaurs were probably destroyed on a regular basis and therefore did not enter the fossil record. The census records what was preserved and discovered, but is not a perfect snapshot of local ecology.  Even so, Tyrannosaurus does seem to be abundant in each section of the Hell Creek Formation that was sampled, and the authors of the new paper suggest that this was because the dinosaur was an opportunistic feeder.

Contrary to the conclusions of Carbone and colleagues, the PLoS One study proposes that Tyrannosaurus scavenged regularly. How else could the area have supported so many tyrant dinosaurs? “Tyrannosaurus may have acquired a larger percentage of meat from carrion sources than did smaller theropods,” Horner and co-authors suggested, “therefore filling the role of a more generalized, carnivorous opportunist such as a hyena.”

Profile of a spotted hyena. From Wikimedia Commons.

The conclusion of the new paper corresponds with what Holtz suggested several years ago, but frustratingly, Horner and colleagues do not specify what kind of hyena they imagine Tyrannosaurus as. This is not just a bit of nit-picking. Despite their reputations of scavengers, the large spotted hyenas actually obtain the majority of their prey by hunting. The degree to which spotted hyenas hunt varies from place to place, but carrion may make up as little as five percent of the diet of some populations, such as Kenya’s “Talek clan.” The smaller brown and striped hyenas, by contrast, are primarily scavengers that also take live prey when they can. Horner, Goodwin, and Myhrvold do not specify which species they are talking about—they refer to hyenas in a general sense—and so their exact idea of Tyrannosaurus feeding habits is left unclear.

Significantly, though, the authors of the PLoS One paper note that the feeding habits of individual Tyrannosaurus may have changed as they grew. Young Tyrannosaurus may have been more predatory, whereas the more powerful jaws of adult individuals allowed them to more effectively scavenge, meaning that Tyrannosaurus actually occupied a range of predatory niches throughout its life. Perhaps this is why smaller predatory dinosaurs are relatively rare in the Fort Peck Reservoir deposits: young Tyrannosaurus may have filled the “small predator” role.

That Tyrannosaurus was an opportunistic carnivore that both hunted and scavenged isn’t news. Paleontologists have been saying this in response to Horner’s “obligate scavenging” hypothesis for years, and Holtz specifically drew comparisons to predators like spotted hyenas. What is noteworthy is that Horner appears to have softened his original hypothesis to the point where I was surprised that Holtz’s paper was not cited as a more direct source of support for Tyrannosaurus as an opportunistic feeder. The abundance of Tyrannosaurus in the Fort Peck Reservoir area is a significant surprise, but the paper’s conclusions about the lifestyle of Tyrannosaurus is not as shocking as news reports made them out to be.

References:

Cooper, S., Holekamp, K., & Smale, L. (1999). A seasonal feast: long-term analysis of feeding behaviour in the spotted hyaena (Crocuta crocuta) African Journal of Ecology, 37 (2), 149-160 DOI: 10.1046/j.1365-2028.1999.00161.x

Hayward, M. (2006). Prey preferences of the spotted hyaena (Crocuta crocuta) and degree of dietary overlap with the lion (Panthera leo) Journal of Zoology, 270 (4), 606-614 DOI: 10.1111/j.1469-7998.2006.00183.x

Holtz, T.R. 2008. “A Critical Reappraisal of the Obligate Scavenging Hypothesis for Tyrannosaurus rex and Other Tyrant Dinosaurs.” in Larson, P. and Carpenter, K. (eds) Tyrannosaurus rex: The Tyrant King. Bloomington: Indiana University Press.

Horner, J.R. 1994. “Steak Knives, Beady Eyes, and Tiny Little Arms (A Portrait of T. rex as a Scavenger.” in Rosenberg, G.D. and Wolberg, D.L. (eds) Dino Fest. The Paleontological Society Special Publication No. 7.

Horner, J., Goodwin, M., & Myhrvold, N. (2011). Dinosaur Census Reveals Abundant Tyrannosaurus and Rare Ontogenetic Stages in the Upper Cretaceous Hell Creek Formation (Maastrichtian), Montana, USA PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0016574

LINGHAM-SOLIAR, T. (1998). Guess who’s coming to dinner: A portrait of Tyrannosaurus as a predator Geology Today, 14 (1), 16-20 DOI: 10.1046/j.1365-2451.1998.014001016.x

An Albertosaurus scares off a smaller predatory dinosaur in this Royal Tyrrell Museum diorama. Large tyrannosaurs would have competed with smaller carnivores for carcasses to scavenge. Image from Flickr user mcwetboy.

Was Tyrannosaurus rex a fearsome hunter or a scavenger? The answer is “both.”

In the early 1990s, the paleontologist Jack Horner popularized the idea that Tyrannosaurus fed entirely on carrion. The idea that this dinosaur—the “prize fighter of antiquity“—could not catch or kill other dinosaurs was shocking. Reporters and documentary-makers ate it up, but other paleontologists were quick to respond with evidence that Tyrannosaurus truly was the apex predator of its time. The academic debate over whether Tyrannosaurus was capable of bringing down live prey has been over for years now, and a study published today in the Proceedings of the Royal Society B finds new support for Tyrannosaurus as one of prehistory’s super-predators.

In order for Tyrannosaurus to have made a living as an obligate scavenger, tons of dinosaur carcasses would have to have been scattered over the Cretaceous landscape. If there were enough dead dinosaurs, Tyrannosaurus could have hypothetically gotten by through scavenging, but the trouble is that it was not the only carnivore around. Smaller, more numerous carnivores would have seriously limited its feeding opportunities.

As tabulated by paleontologists Chris Carbone, Samuel Turvey and Jon Bielby in their new study, there were as many as nine other species of meat-eating dinosaurs alongside Tyrannosaurus during the Late Cretaceous of North America. They ranged in size from the large tyrannosaur Albertosaurus down to the six-foot-long “raptor” Dromaeosaurus. (The authors count the supposed “pygmy tyrant” Nanotyrannus on their list, but these specimens are probably juvenile Tyrannosaurus and do not belong to a distinct genus.) Altogether, there was an entire guild of meat-eating dinosaurs that would have competed for carcasses, just as we see mammals of different sizes competing for carcasses on the African savanna today. In order to subsist on carcasses alone, adult Tyrannosaurus would have been in intense competition with multiple, smaller predators, including their own offspring.

A graph showing the relative abundance of carnivores (blue) and herbivores (red) in the ecosystems where Tyrannosaurus rex lived. From Carbon et al., 2011.

After compiling a list of carnivorous species and prey species, Carbone and colleagues used information about the ecology of modern ecosystems to estimate the number of available carcasses on the landscape and the ability of the carnivores to detect them. The carcasses of small herbivorous dinosaurs would have been relatively abundant, but an adult Tyrannosaurus would have had to walk for days to reach a large carcass. In fact, the researchers estimate that an individual Tyrannosaurus would have had to search for nearly a year before finding a five-ton carcass, and it would have had to rely upon more frequent and less-filling meals.

Unfortunately for Tyrannosaurus, more abundant carnivorous dinosaurs probably would have arrived at the carcasses first. Many small mouths can destroy a body faster than one big one. For example, let’s say that a Triceratops weighing about 8,500 kilograms keels over and dies. Based upon the estimates of search time and carnivore abundance used in the new study, about 1,000 Dromaeosaurus-level carnivores could have reached the carcass in the same amount of time that it would take one Tyrannosaurus to find it. There were simply more of them spread over the landscape.

Overall, the best bet for a scavenging Tyrannosaurus would be to find smaller carcasses more frequently, but even these were probably consumed before it could reach them. As the authors of the new study state, “it is extremely unlikely that an adult T. rex could use scavenging as a long-term sustainable foraging strategy.”

Tyrannosaurus was the biggest meat-eating dinosaur within its ecosystem and certainly would have dominated any carcass it came across, but the likelihood of it reaching a carcass before its destruction at the jaws of smaller, faster dinosaurs was low. We know from fossil evidence that Tyrannosaurus cannibalized carcasses of its own species, and that its cousin Tarbosaurus wasn’t above scavenging, but in order to survive the tyrant king had to hunt. That it did so is clear from its anatomy—Tyrannosaurus was well-adapted for delivering devastating bites that would have felled the large herbivorous dinosaurs of its time. The hunting method of this dinosaur, how often it had to hunt, whether it hunted in groups, and other questions remain, but there can be no doubt that Tyrannosaurus was a formidable predator.

References:

Carbone, C., Turvey, S., & Bielby, J. (2011). Intra-guild competition and its implications for one of the biggest terrestrial predators, Tyrannosaurus rex Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.2497

A reconstruction of the skeleton of Linhenykus. The lighter (white) elements represent what was actually discovered. From Xu et al., 2011.

When it was first described in 1993, Mononykus was one of the strangest dinosaurs known. It had the slender, light build of some of the “ostrich mimic” dinosaurs, yet it possessed two stubby, one-clawed hands and a few other subtle characteristics that placed it in a new group called the alvarezsaurs. Since that time, multiple species of alvarezsaur have been found, and the latest discovery has just been announced in the journal PNAS.

Named Linhenykus monodactylus, the new dinosaur is known from a partial skeleton found in the 84- to 75-million-year-old fossil deposits of Inner Mongolia. It was not a very large dinosaur—as Dave Hone commented at Archosaur Musings, “the living animal would probably have been able to s[t]and comfortably in the palm of your hand”—but what makes it stand out are its heavily built forearms.

Like many of its close relatives, Linhenykus had only one functional finger—a single, stout digit tipped with a heavy-duty claw. Where Linhenykus differs, however, is that it lacked any additional fingers. Other alvarezsaurs discovered so far had tiny, vestigial fingers that were still retained alongside the primary finger. Even in Mononykus, where only the functional finger has been found, there were small indentations in the bone of the hand which suggest that it also had two additional, tiny fingers. Not so in Linhenykus. There is a small, second bone of the palm of the hand next to the large finger, and since this small bit of bone could not have supported a finger we can say that Linhenykus is the first one-fingered dinosaur known.

Curiously, however, the loss of the additional fingers in Linhenykus was not the culmination of a long-term evolutionary trend among the alvarezsaurs. When compared to other members of this group, Linhenykus fell out closer to the base of the family tree than species which retained the vestigial fingers. This means that the anatomy of Linhenykus represents a pattern of mosaic evolution: It retained a suite of archaic characteristics seen among early members of the group, but it also had peculiar specializations not seen among later species such as Mononykus. The loss of the vestigial fingers in Linhenykus was a specialization not yet seen among any other alvarezsaurs.

Further discoveries and future analyses will flesh out the evolutionary pattern seen among these dinosaurs, but one of the recurring questions is why alvarezsaurs had such unique forelimbs. How did they evolve, and what were they used for? These are two distinct questions—even if we can determine the function of a particular trait, that does not necessarily explain how that trait evolved in the first place.

At the moment, the favored hypothesis is that Mononykus, Linhenykus and their relatives used their claws for digging into ant and termite nests. As pointed out by Phil Senter in a 2005 Paleobiology study, the forelimbs of Mononykus were modified so that the palms of their hands faced downward and they were capable of scratch digging with their functional fingers. No one had yet found a preserved termite or ant nest that was raided by an alvarezsaur, but, given the similarity of their claws to those of modern anteaters and pangolins, the idea that these dinosaurs feasted on insect colonies remains the most popular explanation for their unique anatomy.

References:

Xu, X., Sullivan, C., Pittman, M., Choiniere, J., Hone, D., Upchurch, P., Tan, Q., Xiao, D., Tan, L., & Han, F. (2011). A monodactyl nonavian dinosaur and the complex evolution of the alvarezsauroid hand Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1011052108

Senter, P. (2005). Function in the stunted forelimbs of Mononykus olecranus (Theropoda), a dinosaurian anteater Paleobiology, 31 (3), 373-381 DOI: 10.1666/0094-8373(2005)031[0373:FITSFO]2.0.CO;2

Bite marks (black and white arrows), likely made by Velociraptor, on bones tentatively attributed to Protoceratops. From Hone et al., 2010.

What did Velociraptor eat? Despite what the Jurassic Park franchise might suggest, the answer is not “tourists and hapless scientists.” Those were in rather short supply during the Mesozoic. Instead, as reported in Palaeogeography, Palaeoclimatology, Palaeoecology last year, recently found fossils confirm that this famous, sickle-clawed dinosaur fed upon the horned dinosaur Protoceratops.

In 1971, a Polish-Mongolian joint expedition made a spectacular discovery: the exquisitely preserved skeletons of a Velociraptor and Protoceratops locked together. These animals—popularly known as the “fighting dinosaurs”—had died in the midst of combat and have often been taken as an indication that Protoceratops was a regular food source for Velociraptor. But while certainly the most fantastic, this is not the only evidence of a predator-prey relationship between these dinosaurs.

During the field seasons of 2008 and 2009, paleontologists collected numerous dinosaur bone fragments from the Cretaceous rock of Bayan Mandahu, Inner Mongolia. Among the lot were the remains of a horned dinosaur and two teeth of a dromaeosaurid dinosaur. Given the scrappy nature of these remains it was impossible to be absolutely certain of their identity, but given their age, anatomy and the place in which they were found, it is likely that the fossils represent Protoceratops and Velociraptor.

Toothmarks on the Protoceratops bones may explain why the skeleton was not found in better condition. At least eight fragments of bone showed clear signs that they had been bitten, and three different toothmark patterns were visible. There were shallow grooves made in the surface of the bone, two deeper punctures, and one piece of bone had toothmarks on both sides. Regardless of whether the specific identification of the dinosaurs turns out to be correct, the bones show that a Velociraptor-type dinosaur fed upon Protoceratops or a very closely related horned dinosaur.

When the Velociraptor fed upon the Protoceratops is another matter. Given the state of the material, it is impossible to tell whether the horned dinosaur was killed by the predator or whether the meat-eating dinosaur was scavenging. In either case, however, the toothmarks left on the bone were made long after the Protoceratops was killed. The teeth and jaws of Velociraptor were not suited to crushing bone, and so it is reasonable to hypothesize that it would have fed on all the available soft tissues first. The toothmarks on the bone mean that there was relatively little flesh left and the feeding Velociraptor was scraping whatever it could off the tattered carcass. From a paleontologist’s perspective, this also accounts for why the Protoceratops skeleton was so scrappy—by the time it was buried, it had already been torn apart.

For more on this research, see this post on Archosaur Musings by one of the study’s authors, Dave Hone.

References:

Hone, D., Choiniere, J., Sullivan, C., Xu, X., Pittman, M., & Tan, Q. (2010). New evidence for a trophic relationship between the dinosaurs Velociraptor and Protoceratops Palaeogeography, Palaeoclimatology, Palaeoecology, 291 (3-4), 488-492 DOI: 10.1016/j.palaeo.2010.03.028

The skull of Sinornithomimus. Its toothless beak - and a "gastric mill" found in its skeleton - suggest that it was an herbivorous dinosaur. From Kobayashi Lü, 2003.

Coelurosaurs were one of the strangest groups of dinosaurs. In addition to the famous predators Tyrannosaurus and Velociraptor, the coelurosaurs included the small, fuzzy Sinosauropteryx; “ostrich-mimics” such as Struthiomimus; the long-necked, sickle-clawed giant Therizinosaurus; the tiny, ant-eating Albertonykus; the bird-beaked oviraptorosaurs like Citipati; and birds. Within the past decade, especially, new discoveries have radically changed our understanding of this group of dinosaurs. Now a study shows that, even though this group contained some of the most famous predators of all time, many of these dinosaurs were herbivorous.

Traditionally, dinosaur diets seemed to break down along neat evolutionary lines. The long-necked sauropods and all the ornithischian dinosaurs (ankylosaurs, ceratopsians, hadrosaurs, etc.) were herbivores, whereas all the theropods were carnivorous. This is no longer the case. Coelurosaurs were theropods, and in a review of their evolution  just published in PNAS by Lindsay Zanno and Peter Makovicky, the Field Museum scientists found that relatively few coelurosaurs had an exclusively carnivorous diet.

Zanno and Makovicky determined the different dietary habits of coelurosaurs by looking for gut contents, fossil feces and other evidence that would indicate whether a particular dinosaur was a strict carnivore or a herbivore. (They are just labels that are useful for categorization, of course. Alligators sometimes eat fruit, and cows sometimes eat other animals, and so even a primarily carnivorous dinosaur could have eaten plants sometimes and primarily herbivorous dinosaurs may have eaten meat on occasion.) These pieces of evidence, paired with what the authors called “putatively herbivorous traits” in the skeleton, allowed them to more rigorously test ideas about which coelurosaurs might have been herbivores. The ornithomimosaur Sinornithomimus, for example, had toothless, beaked jaws, and specimens have been found with evidence of a gastric mill (small stones in the stomach which would have ground up food), confirming that it ate a significant amount of plant food.

Zanno and Makovicky concluded that there is good evidence for herbivory in 44 known coelurosaur species spanning six groups: the ornithomimosaurs, therizinosaurs, oviraptorosaurs, alvarezauroids, several early birds, and the single troodontid Jinfengopteryx. (The carnivorous dinosaurs consisted of the compsognathids, the tyrannosaurs and most of the dromaeosaurs.) In other words, the coelurosaurs appear to have been “dietary opportunists” in which multiple lineages shifted to herbivorous diets or had more varied diets than the tyrannosaurs and raptors. No two lineages made the shift to herbivory in exactly the same way. Even though many herbivorous coelurosaurs shared similar traits, such as toothless beaks and long necks, these traits evolved independently and in different orders, and so this convergence might indicate some evolutionary constraints which shaped the herbivorous coelurosaurs in similar ways.

Viewed as a whole, the coelurosaurs were a diverse group of dinosaurs that also had an array of diets. In fact, most coelurosaur subgroups show adaptations to eating plant food, meaning that, contrary to what we might suppose, the hypercarnivorous species are actually the oddballs among the group. Further study will be required to better resolve the diets of individual species, but for now it is apparent that the coelurosaurs were the most varied group of dinosaurs to have ever evolved.

References:

Yoshitsugu Kobayashi and Jun-Chang Lü (2003). A new ornithomimid dinosaur with gregarious habits from the Late Cretaceous of China Acta Palaeontologica Polonica, 48 (2), 235-259

Zanno, L., & Makovicky, P. (2010). Herbivorous ecomorphology and specialization patterns in theropod dinosaur evolution Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1011924108

Bite marks - indicated by white arrows - along the side of an articulated crocodile tail close to the hips. From Fiorelli, 2010.

The traditional, simplified recipe for how to make a fossil goes something like this: take a dead animal, keep it safe from scavengers, cover it up with sediment, add a heaping dollop of time and presto!, you have a petrified skeleton. The second step is often cited as being especially important—a skeleton can’t enter the fossil record if it is destroyed—but sometimes predator kills and scavenged carcasses do make it into the fossil record. This year alone there has been a report of Tarbosaurus scavenging a hadrosaur carcass and a study confirming that Tyrannosaurus picking at the deceased of its own kind. Now paleontologist Lucas Ernesto Fiorelli has reported on a collection of Cretaceous crocodile bones that may be theropod dinosaur table scraps.

The crocodile in question, described in 1991 but not yet named, was found on the campus of the University of Comahue in Neuquén, Argentina. There was not very much of it left. A few pieces of skull, some vertebrae, a smattering of limb fragments and a nearly complete tail were all that remained. Based on the geology of the area, this animal lived along rivers or streams that skirted along huge sand dunes in a hot, seasonal environment, and its anatomy shows that it belonged to a group of extinct crocs called the peirosaurid crocodyliforms. These animals were more slender than their modern cousins and adapted to a more terrestrial lifestyle.

As described by Fiorelli, there are about 70 punctures and bite marks on the preserved remains of the animal, present on almost every skeletal element except the skull. Particularly noteworthy is the distribution of bitemarks along the preserved tail of the animal, which appears to have been crushed by the powerful bite of a large predator. The question is what left the bite marks.

Fiorelli rejects the hypothesis that this animal was the victim of aggression by another crocodile. When competing for dominance, modern crocodylians display and bite each other, but the amount of trauma indicated by the bite marks on this individual is inconsistent with such behavior. Additionally, while the crocodile was about 10 to 12 feet long, the animal that left the bitemarks appears to have been considerably larger, suggesting that the injuries were probably not caused by a member of the same species.

The idea that the injuries were caused by other crocodile species that have been found from the same deposits were also ruled out by Fiorelli. One, Notosuchus, may have been primarily herbivorous, and Fiorelli states that its contemporary Comahuesuchus just didn’t have the jaw power to do the kind of damage seen on the other crocodile bones. Likewise, even though two other genera of prehistoric crocodiles called baurusuchids were certainly predators, the pattern of bite marks on the victim’s skeleton indicate an animal with a much larger skull. As hypothesized by Fiorelli, a large theropod dinosaur is the most likely culprit, though the specific species of this predator cannot be ascertained. Both abelisaurids and carcharodontosaurids—two groups of diverse theropods common in the Cretaceous of South America—have been found from the geologic formation the crocodile skeleton came from, but so far, no teeth or other remains have been found in proximity to the skeleton to conclusively close the case.

References:

Lucas Ernesto Fiorelli (2010). Predation bite-marks on a peirosaurid crocodyliform from the Upper Cretaceous of Neuquén Province, Argentina Ameghiniana, 47 (3)

Two bite-marked toe bones from Tyrannosaurus rex. Close-up views of the bite-marks are on the right. From Longrich et al., 2010.

For a Tyrannosaurus rex, there was nothing more dangerous than another Tyrannosaurus rex. From a relatively young age these dinosaurs tussled by biting each other on the face—possibly spreading parasitic microorganisms as they did so—and a few fossil scraps have suggested that some tyrannosaurs may have killed or eaten members of their own kind. This latter kind of fossil forensic evidence—bite-marked bones and teeth embedded in skeletons—has been very rare. A study just published in PLoS One presents new evidence that confirms that Tyrannosaurus rex was certainly capable of cannibalism.

As described by paleontologists Nicholas Longrich, Jack Horner, Gregory Erickson and Philip Currie, at least four Tyrannosaurus rex bones bear toothmarks made by a large carnivorous dinosaur. They are several foot bones and an upper arm bone from four different individual animals. The bite traces they bear are not just punctures into the bone, but U- and V-shaped gouges which suggest that the feeding dinosaur was biting onto the body of the Tyrannosaurus and pulling the flesh off the bones. This is consistent with a set of 13 other bones bearing similar toothmarks, including parts of horned dinosaur and hadrosaur skeletons.

That Tyrannosaurus rex is the most likely culprit in each case rests upon the fact that there was no other creature capable of inflicting that kind of damage in each locality from the end of the Cretaceous. The toothmarks were inconsistent with damage done by crocodiles, the predatory lizards in the area were far too small, and the only predatory dinosaur of suitable size to make such bite marks was Tyrannosaurus rex itself.

The collection of bite marks most likely represents feeding rather than combat. The marks are in places and positions that appear to be impossible for fighting animals, and since the bite-marked bones show no evidence of healing it is most probable that the damage was done after the individual animals died. The fact that the bite marks were found primarily on limb and toe bones hints that the feeding Tyrannosaurus was a scavenger that came by after most of the soft tissues had been removed from the dead Tyrannosaurus. There would not have been very much meat on the upper arms and toes of Tyrannosaurus, and so the authors of the new study hypothesize:

Tyrannosaurus therefore seems to have been an indiscriminate and opportunistic feeder, feeding not only on herbivorous dinosaurs, but also on members of its own species. The traces described here likely result from opportunistic scavenging, and were probably made after most of the flesh and organs had been removed from the carcass.

Furthermore, that four traces from different specimens have already been found hints that Tyrannosaurus may have regularly fed upon its own kind. Considering how rare fossils are to start with, and how much rarer carcasses destroyed by predators are, that scientists have found so many traces already suggests that Tyrannosaurus-on-Tyrannosaurus scavenging was relatively common. It is impossible to know whether these Tyrannosaurus were actually victims of predation or died from some other cause—such as wounds from a fight with another Tyrannosaurus—but the damaged bones show that a hungry Tyrannosaurus would not let a good carcass go to waste.

For more on tyrannosaur feeding, see these posts:

Did Giant Predatory Dinosaurs Eat Bones?
Tarbosaurus: A Predator and a Scavenger With a Delicate Bite

References:

Longrich, N., Horner, J., Erickson, G., & Currie, P. (2010). Cannibalism in Tyrannosaurus rex PLoS ONE, 5 (10) DOI: 10.1371/journal.pone.0013419

In order to understand the ecology of any environment, past or present, you must be able to change the scale of your perspective. Large animals are readily apparent, but what about the interactions between the plants they eat, the insects on those plants, the pollen on those insects, the many microorganisms in the habitat and so on? It is practically impossible to keep all these parts of an ecosystem in mind at once, but if we alter the scale of our perspective, we can better appreciate a greater array of interactions that might otherwise go unnoticed.

Artist John Conway has just created a stunning example of the nested levels of interactions between organisms in a new video. The scene is of prehistoric China’s famous 133-million to 120-million-year-old Jehol biota. At first only the dinosaur Jinzhousaurus and a pair of the pterosaur Jeholopterus can be easily seen, but as the camera zooms in the wasp Tanychora beipioensis comes into view, and it is covered with the pollen grains Protoconiferous funarius. The painting is an amazing reminder that there was much more to prehistoric ecosystems than dinosaurs and the plants they ate, but how did Conway create it? In an interview with paleontologist David Hone on the Archosaur Musings blog, Conway briefly explained the method and motivation behind the piece:

It’s a series of paintings done in Photoshop at successively smaller scales, then stitched together and animated in After Effects.

I was looking for a way to get across the sheer breadth of scale in the fossil record, from dinosaurs to pollen in this case. I was also looking for a way to make picture of a biota without having to do a “menagerie” painting, which is otherwise a necessary evil if you want to get a lot of animals in the one scene.

An Allosaurus menaces a Stegosaurus and its offspring. Image from Wikipedia.

In discussions of dinosaur bite mechanics, the heavy forces generated by predatory species often dominate, but it is important to understand how the jaws of herbivores worked, too. The jaws of Stegosaurus might not be as immediately impressive as those of Tyrannosaurus rex, but it is still important to know how they were put to use if we are to understand the paleobiology of the famous armored dinosaur. Now, thanks to computer models created by Miriam Reichel, we can better understand what Stegosaurus was capable of eating.

The teeth of Stegosaurus are almost all the same: a series of rounded, minutely-ridged teeth arranged in straight rows from front to back. To investigate how this dental arrangement would have worked while consuming food, Reichel created 3-D models of the teeth (both with ridges and without) to create a virtual model of Stegosaurus jaws. This digital dinosaur was then set to work on computer-generated cylinders given the properties of different types of plant food, using the muscle attachments seen on the dinosaur’s skull to determine how hard its bite would have been.

As calculated by Reichel, Stegosaurus didn’t have a very powerful bite. Even you and I could bite harder than Stegosaurus. The dinosaur could generate enough force to crunch through twigs and branches under a half an inch in diameter, but anything bigger than that and it would have a difficult time of it. Given its weak jaws, Stegosaurus would have had to rely on soft, fast-growing plants; it is fantastic to think that this large dinosaur could have survived on such a diet!

Then there is the matter of the actual method by which Stegosaurus processed its food. It probably did not chew its food to any great degree, but instead sliced through soft plants before swallowing. Additionally, Reichel proposes that Stegosaurus may have had a tough beak at the front of its jaws which took most of the punishment during feeding. The teeth were left with the lighter work, although, since Stegosaurus jaws were weakest at the front, this might mean that it was only eating the softest, greenest food available. Further study will be required to understand the precise mechanics of how Stegosaurus ate, but, at the very least, Reichel’s work confirms that this dinosaur had to carefully pick out soft Jurassic salads for lunch.

Reichel, M. (2010). A model for the bite mechanics in the herbivorous dinosaur Stegosaurus (Ornithischia, Stegosauridae) Swiss Journal of Geosciences DOI: 10.1007/s00015-010-0025-1