The arms of the huge ornithomimosaur Deinocheirus. How did such herbivorous theropods get to be so big? Photo by Eduard Solà, image from Wikipedia.

When I was first becoming acquainted with dinosaurs in the mid 1980s, “theropod” was synonymous with “carnivorous dinosaur.” Large or small, from Tyrannosaurus to Compsognathus, every theropod I knew of sustained itself on the flesh of other organisms. But it was just about that time that new discoveries and analyses revealed that many theropod dinosaurs were omnivores, or even herbivores. The ostrich-like ornithomimosaurs, beaked oviraptorosaurs and utterly bizarre therizinosaurs, in particular, embodied a switch from an ancestral meat-filled diet to one more reliant of fruit and foliage. Not only that, but these herbivorous theropods grew almost as large as the biggest carnivores–the ornithomimosaur Deinocheirus, the ovriraptorosaur Gigantoraptor and Therizinosaurus were all enormous Cretaceous dinosaurs. But why did these plant-chomping dinosaurs become giants?

In the latest of a spate of papers considering herbivorous theropods, paleontologists Lindsay Zanno and Peter Makovicky paired evolutionary trees with mass estimates derived from femora lengths and a bit of number crunching to see if there was any distinct evolutionary pattern that might explain why Deinocheirus and similar herbivorous theropods grew to such large sizes. Were these Late Cretaceous dinosaurs just the culmination of an evolutionary trend towards ever-larger body size–called Cope’s Rule–or was something else at work?

Zanno and Makovicky didn’t find any sign of directional selection for larger body size. Even though the earliest representatives of the ornithomimosaurs, oviraptorosaurs and therizinosaurs in Asia were much smaller than their Late Cretaceous relatives, the paleontologists point out that this signal has probably been biased by preservation. The 125-million-year-old deposits that contain small members of these groups seem to be skewed towards “mid-sized vertebrates,” the authors point out, and don’t seem to preserve larger dinosaurs that might belong to the same lineages. Indeed, therizinosaurs of about the same age from North America, such as Falcarius, were larger than species in Asia, meaning that herbivorous dinosaurs might have occupied a range of body sizes and evolved larger body sizes at multiple intervals. There was no simple, straight-line trend of bigger and bigger bodies through time.

Nor did a herbivorous lifestyle alone seem to account for gigantism among these dinosaurs. Even though big herbivores gain particular benefits from their size in terms of breaking down tough, low-quality foods more efficiently, Zanno and Makovicky doubt that this relationship drove the evolution of increased body size in the dinosaurs. Instead, they favor “passive processes” that might be tied to ecology and whether these dinosaurs were omnivores more than herbivores. And, as the paleontologists stress, the pattern relies on how complete we think the dinosaur record is. Some ecosystems might be preferentially preserving larger or smaller dinosaurs, which has the potential to skew the big picture. While Zanno and Makovicky ruled out some possibilities, we still don’t really know what accounts for the multiple herbivorous theropod growth spurts.

Post-Script: After four years working with Smithsonian magazine’s wonderful crew, and over 1,000 posts about various aspects of dinosauriana, it’s time for me to move on. I’ll be leaving Dinosaur Tracking next month. Don’t fret, I’ll still be digging into dinosaur science, but I’ll be at a new blog elsewhere on the web (stay tuned for details). I am deeply indebted to my editors Brian Wolly, Sarah Zielinski and, of course, Laura Helmuth (now doing a great job at Slate), as well as the rest of the Smithsonian staff for inviting me to come here and geek out about dinosaurs every day. And many thanks to all of you–the readers and commenters who have helped make this blog a success. You have all made blogging for Dinosaur Tracking an absolute pleasure.

Reference:

Zanno, L., Makovicky, P. 2012. No evidence for directional evolution of body mass in herbivorous theropod dinosaurs. Proceedings of the Royal Society B. 280. doi: 10.1098/rspb.2012.2526

A reconstruction of Deinosuchus at the Natural History Museum of Utah. Photo by the author.

From the time of their origin around 230 million years ago, to the extinction of the non-avian forms 66 million years ago, dinosaurs ruled the Earth. That’s how we like to characterize the Mesozoic menagerie, anyway. We take the long success of the dinosaurs as a sign of their long-lived and terrifying domination, but, despite our belief that they were the most vicious creatures of all time, there were creatures that even the dinosaurs had reason to fear. Chief among them was Deinosuchus – North America’s “terrible crocodile.”

Between 80 and 73 million years ago, when North America was divided in two by the shallow Western Interior Seaway, the marshes and swamps along the coasts were ruled by Deinosuchus. Fossils of this Cretaceous cousin of modern alligators have been found from Mexico to Montana and in east coast states such as North Carolina and Georgia, tracing the margins of the western subcontinent Laramidia and its eastern counterpart, Appalachia. For the most part, paleontologists have found the bony armor, vertebrae, and teeth of Deinosuchus, but pieces of jaw and partial skeletons found in places such as Texas and Utah indicate that this alligatoroid was a giant, growing over thirty feet in length and approaching forty feet among the biggest individuals.

During the heyday of Deinosuchus, adults of the aquatic ambush predator were among the largest carnivores in their ecosystems. The enormous Tyrannosaurus rex was over five million years off, and the tyrannosaurs of the time were not quite so long or bulky. (Teratophoneus, found in southern Utah among strata that also yield Deinosuchus, was about twenty feet long, and Daspletosaurus from Montana grew to be about thirty feet long.) A fully mature Deinosuchus would have outstretched and outweighed the dinosaur competition, and would have undoubtedly been a deadly apex predator in the water habitats it haunted.

The skull of Deinosuchus testifies to its destructive potential. The alligatoroid’s skull was large, broad, and equipped with an array of teeth deployed to pierce and crush. Indeed, even though there were other giant crocodylomorphs of near-equal size during the Mesozoic (such as the narrow-snouted Sarcosuchus), Deinosuchus appears to be unique in having the anatomical necessities to take down hadrosaurs and other unwary dinosaurs at the water’s edge. And, thanks to tooth-damaged fossils, we know that Deinosuchus truly did dine on dinosaurs. Two years ago, Héctor Rivera-Sylva and colleagues described hadrosaur bones bearing tell-tale Deinosuchus toothmarks from Mexico, and similar finds have been reported from Texas. There may be other candidates in museum drawers elsewhere.

Of course, we don’t know whether the bitten bones record hunting or scavenging. Unless the injuries show signs of healing, toothmarks on bones record feeding rather than hunting behavior. The evidence only takes us so far. Adult Deinosuchus were apparently capable of taking down dinosaurs, but, as yet, there’s no direct evidence of such an incident. Indeed, while images of Deinosuchus chomping on dinosaurs fires our imagination, we actually know relatively little about how this alligatoroid fed and what it ate. Probably, like modern alligators, large Deinosuchus were generalists that snagged fish, turtles, and whatever carrion it happened upon. We don’t know for sure. Nevertheless, dinosaurs in the habitat of this monstrous croc would have been wise to carefully approach the water’s edge, looking for teeth and scutes hiding just beneath the surface.

A reconstruction of Patagonykus, one of South America’s alvarezsaurs. Image from Wikipedia.

If there’s one group of dinosaurs that needs better PR, it’s alvarezsaurs. They’re among the strangest dinosaurs to have ever evolved, yet outside of dinosaur die-hards, few people have ever heard of them. They’re not one of those classic forms–the sauropods, tyrannosaurs, stegosaurs, or ceratopsids–that have been cherished for the past century. Paleontologists only recently began to uncover their bones. Alvarezsaurus itself was named in 1991, but it and its close relatives didn’t quite get swept up in the same wave of dinomania as their other Mesozoic cousins.

Alvarezsaurs weren’t big, toothy, or menacing. That’s part of makes them so special. Alvarezsaurus, Mononykus and their relatives from Cretaceous Asia, South America and North America were small dinosaurs–these feathered dinos ranged from the size of a pigeon to about the size of a turkey. In fact, these dinosaurs were so avian in nature that there was once a debate about whether alvarezsaurs were non-avian dinosaurs or birds that had lost the ability to fly. Since those early debates, numerous studies have confirmed that they were non-avian dinosaurs that were closely related to the strange therizinosaurs and ostrich-like ornithomimosaurs.

But the strangest thing of all is the mystery of what alvarezsaurs ate.

Despite being short, alvarezsaur arms weren’t wimpy. Not at all. Alvarezsaur forelimbs were very stout and included one robust finger tipped in a big claw. (Among these dinosaurs, the total number and development of the fingers varied, but they’re connected by having one finger that was bigger than the others.) In contrast, these dinos often had a reduced number of very small teeth. Paleontologists thought they saw a connection between these traits and a life feeding on social insects. Mammals such as pangolins and ant-eaters also have stout, heavy-clawed arms and are toothless–a functional pairing that goes with a life of tearing into ant and termite nests to slurp up the scurrying insects in their nests.

Could alvarezsaurs have done the same? So far, it’s the most popular hypothesis for their bizarre nature. In a 2005 paper, paleontologist Phil Senter proposed that Mononykus would have been capable of the kind of scratch-digging needed to rip open social insect nests. Then, in 2008, Nicholas Longrich and Philip Currie described the alvarezsaur Albertonykus in deposits that also contained traces of Cretaceous termites. Alvarezsaurs seemed to have the right equipment and live at the right time to be social insect predators.

But we don’t really know. No one has published any direct evidence that Albertonykus or any other alvarezsaur ate ants or termites. The hypothesis is certainly a reasonable one, but we still need a test of the idea. Fossil feces may eventually hold the answer.

If paleontologists eventually uncover dinosaur dung of appropriate size that contains ants or termites and comes from a habitat shared by alvarezsaurs, that discovery would strengthen the ant-eating hypothesis. A cololite would be even better. While coprolites are petrified feces that have already been excreted, cololites are fossil poop preserved inside the prehistoric creature’s body prior to expulsion. If paleontologists found an alvarezsaur with a cololite containing termites, that would be direct evidence that these dinosaurs truly did snarf down hordes of insects. For now, though, we can only hope that some lucky fossil hunter makes such a discovery.

Sinocalliopteryx feeds on the dromaeosaurid Sinornithosaurus (left) and the early bird Confuciusornis (right). Art by Cheung Chungtat, from Xing et al., 2012.

Earlier this week, I got into a snit over the blinkered assertion that feathery dinosaurs are lame. I argued the opposite point–as I wrote at the time “Feathered dinosaurs are awesome. Deal with it.” How fortunate that a new paper this week offers proof of fuzzy dinosaur superiority. The evidence comes in the form of gut contents found within predatory dinosaurs that stalked Cretaceous China around 125 million years ago.

The carnivores in question are a pair of Sinocalliopteryx. These dinosaurs were close cousins of the much earlier Compsognathus, albeit quite a bit larger. While Compsognathus was turkey-size, about three feet long, Sinocalliopteryx grew to be about eight feet long. And this big predator was fluffy. The original description of the dinosaur mentioned the vestiges of simplified dinofuzz around the body of Sinocalliopteryx, and this makes sense given the dinosaur’s relationships. While considerably bigger than its close relatives, Sinocalliopteryx was a compsognathid–a group of theropod dinosaurs that also includes fuzzy forms such as Sinosauropteryx and Juravenator. Big or small, the compsognathids were hunters wrapped in wispy plumage.

And the initial description of Sinocalliopteryx mentioned something else. The skeleton that formed the basis of the original paper contained the leg of an unidentified dromaeosaurid dinosaur in its gut contents. Even though dromaeosaurids have long been cherished as sickle-clawed uber-predators, Sinocalliopteryx had clearly eaten the drumstick of one of the smaller feathered predators. Since then, paleontologists have identified a second Sinocalliopteryx with gut contents, and the two dinosaurs form the basis of a new PLoS One study by University of Alberta paleontologist Lida Xing and colleagues.

Looking back at the first Sinocalliopteryx, Xing and colleagues identified the victim as Sinosauropteryx. The second Sinocalliopteryx specimen had a different menu before it perished–its stomach contains the remains of two Confuciusornis, an archaic bird, and bones from an unidentified ornithischian dinosaur. But these gut contents invoke an aggravating mystery. Did these Sinocalliopteryx hunt their dinosaurian prey, or did they scavenge their meals?

This isn’t the first time paleontologists have puzzled over the meaning of predatory dinosaur gut contents. Earlier this year, Dave Hone and collaborators investigated a pterosaur bone found inside a Velociraptor, and last year Jingmai O’Connor and colleagues described a Microraptor with the remains of a bird in its gut (just to pick two examples of many). Frustratingly, though, it’s difficult to say how the dinosaurs obtained the meat. In the case of the Velociraptor, the researchers could not rule out hunting even though scavenging seemed the more likely option. Likewise, even though O’Connor and co-authors suggested their Microraptor hunted birds in the trees, the non-avian dinosaur could have just as easily scavenged a dead bird that fell to the forest floor. Gut contents tell us about what dinosaurs consumed, but they almost never provide direct evidence of how carnivores obtained flesh and bone to eat.

In the case of Sinocalliopteryx, the PLoS One study concludes that the dinosaur may have been skilled at catching live avian prey. The fact that one Sinocalliopteryx fed on two Confuciusornis in quick succession could mean that the large dinosaur was adept at nabbing early birds. “[T]he evidence of bird predation in Sinocalliopteryx,” Xing and colleagues conclude, “suggests that it was a highly capable stealth hunter.” Then again, the same researchers also note that their scenario “is speculative.” While it may seem improbable, the Sinocalliopteryx in question could have scavenged one or both of those birds, as well as the non-avian dinosaur remains in its stomach. We just don’t know. Like many predators, Sinocalliopteryx most likely hunted live prey and took advantage of carrion. Frustratingly, these fossil gut contents can’t tell us what happened in each case. Sinocalliopteryx may very well have been a skilled bird-slayer. Or perhaps not. The fact is that we don’t know for sure.

Perplexing feeding habits aside, there’s something else about the gut contents of Sinocalliopteryx that can give us a closer look at the dinosaur’s biology. In the dinosaur that ate the two birds and the ornithischian, the bone of the ornithischian dinosaur was corroded by stomach acid. The more delicate bird bones, by contrast, had not been so damaged. This means that the Sinocalliopteryx ate the ornithischian first, followed by one bird and, later, another. More than that, the acid damage indicates that at least some dinosaurs had highly-acidic foreguts where bone was broken down–comparable, but not exactly like, the stomachs of crocodilians and perhaps some bone-eating birds like the bearded vulture.

All of which is to say that Sinocalliopteryx is a great example of a fluffy dinosaur you wouldn’t want to mess with. Even if we can’t discern the backstory of each meaty morsel, the variety of prey in the Sinocalliopteryx stomachs shows that this dinosaur wasn’t a picky eater and may have even been a quick hunter that specializing in snapping up other feathery dinosaurs. For our fuzzy mammalian predecessors, hiding the Cretaceous forests, this would have been one scary dinosaur.

Reference:

Xing L, Bell PR, Persons WS IV, Ji S, Miyashita T, et al. (2012) Abdominal Contents from Two Large Early Cretaceous Compsognathids (Dinosauria: Theropoda) Demonstrate Feeding on Confuciusornithids and Dromaeosaurids. PLoS ONE 7(8): e44012. doi:10.1371/journal.pone.0044012

A reconstructed Acrocanthosaurus at the North Carolina Museum of Natural Sciences. Photo by the author.

Compared to mounted dinosaur skeletons, fossil footprints might seem like mundane objects. They only record one small part of a fantastic creature, and it is harder to envision a whole dinosaur from the ground up than the wrap flesh around a skeletal frame. But we should not forget that dinosaur footprints are fossilized behavior—stone snapshots of an animal’s life. And sometimes, trackways record dramatic moments in dinosaur lives.

In 1938, American Museum of Natural History paleontologist Roland T. Bird traveled to Glen Rose, Texas to investigate rumors of huge dinosaur tracks found in the vicinity of the Paluxy River. Bird found them in abundance, but one site was especially intriguing. Set in 113-million-year-old rock were the footprints of a huge sauropod dinosaur—and it seemed that the long-necked giant was followed. The large, three-toed footprints of a predatory dinosaur, probably the ridge-backed Acrocanthosaurus or a similar theropod, paralleled and eventually converged on the footsteps of the sauropod. And at the point of overlap, the predator seemed to skip a step—a little hop that Bird took to mean that the carnivore had sunk its teeth into the herbivore and was lifted out of its tracks a short distance.

Bird excavated the trackway in 1940. About half of the long trail went to the AMNH and can now be seen stretching out behind the museum’s Apatosaurus mount, despite the fact that Apatosaurus lived millions of years before the tracks were made. The other portion is housed at the Texas Memorial Museum in Austin. Bird’s hypothesis about how the tracks were made has inspired exhibits at other museums, such as the Maryland Science Center and the North Carolina Museum of Natural Sciences. Yet not everyone agrees about what the tracks represent. Do they record an Acrocanthosaurus attack as it happened? Or could the trackway simply be a fortuitous association of tracks from dinosaurs that walked the same ground at different times?

Artist David Thomas and paleontologist James Farlow went back to Bird’s notes and the trackway evidence to reconstruct what might have transpired. The association between the sauropod and theropod tracks seemed too tight to just be coincidence. The predatory dinosaur very closely followed the pathway of the larger herbivore, both moving along a broad left curve. Near the end of the excavated area, both the theropod and sauropod turned abruptly to the right. If the two dinosaurs had passed at different times, then we’d expect that the sauropod or theropod would have continued on in the same trajectory and crossed another set of tracks preserved nearby. Based on the fully reconstructed image, the sauropod and theropod were interacting with each other.

And there’s something else. Just before the enigmatic double-right-footprints made by the theropod, there is a drag mark made by the sauropod’s right hind foot. This might be where the titan was attacked and faltered, or maybe the sauropod threw its weight to avoid being bitten. Frustratingly, we can’t know for sure. And the missing left theropod footprint isn’t a clear sign of an attack, either—all we know is that there’s a missing track right where the animals were in close proximity.

Whether or not the Paluxy River Trackway records a successful Acrocanthosaurus assault is uncertain. But the tight connection between the theropod and sauropod tracks suggests that the carnivore at least stalked the herbivore, and perhaps even took a swipe at it. Specimens like this test our ability to draw brief moments in time from stone. The task is made all the more complicated by the gradual loss of information contained within the rock. While they look sturdy, trackways are actually fragile fossils, and the half of the trackway at the Texas Memorial Museum has significantly deteriorated since it was put on display. The museum is trying to raise a million dollars to properly conserve and house this historically and scientifically significant fossil. If you wish to learn more about their campaign, you can find more information here.

Hadrosaurs have often been called “duck-billed dinosaurs.” You don’t have to look at their skulls for very long to see this analogy is wide of the mark. Not only did hadrosaurs such as Edmontosaurus have shovel-shaped, grooved beaks, but their jaws were lined with rows of cropping, crushing teeth. These dinosaurs didn’t dabble in Cretaceous swamps – they grazed the prehistoric plains. And, up until recently, it was thought that these huge herbivores possessed an evolutionary innovation that made them the dinosaurian equivalent to cows.

In general, dinosaur jaws and teeth were for cutting, plucking, and tearing. Dinosaurs didn’t chew their food, but instead ripped or clipped their morsels, which were then swallowed whole. (Strange as it may seem, this style of eating might have had a role to play in why sauropods were able to maintain such large body sizes.) But hardosaurs were thought to be different.

The idea I encountered as a kid was that when hadrosaurs such as Edmontosaurus opened their jaws, the tooth-bearing bones of their upper jaws – the maxillae – swung inwards. Then, when the lower jaws came back up, the lower teeth met the upper teeth and ground the plant food across the tooth surfaces. This wasn’t chewing like mammalian herbivores do it, but it was an evolutionary alternative that allowed hadrosaurs to better break down their food before swallowing. You can see a visualization of this hypothesis in action in this YouTube video.

But this model of hadrosaur chewing required a great deal of flexibility in the skull to create a complex chewing motion. As a video uploaded by the Canadian Museum of Nature – posted above – shows, hadrosaur jaw movements were probably a great deal simpler. The key to the puzzle is the interlocking group of small bones at the back of the skull. When the virtual Edmontosaurus drops its lower jaw, the movement compresses some of these bones at the back of the skull, which moves the upper tooth rows slightly inward. But the lower jaw doesn’t just drop – a joint at the back of the mandible also allows the lower jaw to extend forward. When the jaws close, the lower jaw moves back in a diagonal motion, and the contact of the upper and lower teeth gently push the maxilla slightly outwards. There is still a lot of movement in the skull, but it’s not quite as dramatic as the swingin’ maxilla version. And this goes to show just how much we still have to learn about dinosaurs. Even though we know more about Edmontosaurus and its kin than ever before, the basics of dinosaur biology remain rich grounds for investigation and debate.

[Hat-tip to Thomas Holtz for sharing this video on Twitter.]

What would life be like if dinosaurs such as this Ceratosaurus (at Ogden, Utah's Eccles Dinosaur Park) suddenly returned? Photo by the author.

I was probably too young for my Dinosaurs Attack! cards. When the Topps set popped up at local convenience stores in 1988, I was only five—a touch on the innocent side as I opened the packs of gratuitous dinosaurian carnage. But maybe my naïveté worked to my advantage. Images of Parasaurlophus munching on babies (!) and Stegosaurus thagomizers dashing people’s eyes from their sockets were so over the top that I wasn’t bothered by them. Dinosaurs were supposed to be fearsome and dangerous, right? The gonzo violence looked more or less like what I imagined during my mock battles with little green army figures and plastic dinosaurs.

If you haven’t seen the cards yourself—that is, assuming you want to see them—the entire set is up at Bob’s Dinosaurs Attack! HomePage. The Monster Brains blog also posted the whole run, along with some of the gory promotional images. Don’t expect scientific accuracy. The tyrannosaur on the ghastly “Entombed!” card was pretty good for its time, but the super-size Gorgosaurus with human hands on “Coasting to Calamity” looks like a rejected B movie creature. Speaking of which, a few celebrity monsters make cameos in the set: Godzilla, Gorgo, the Beast From 20,000 Fathoms and the redundantly named Giant Behemoth all show up. Although my favorite “What the heck?” cards are those featuring herbivorous dinosaurs gone bad, such as the carnivorous ankylosaur in “Heartland Horror” and sauropodomorphs that chew the hair of heavy metal musicians in “Rock Concert Carnage.” These cards were sensationally unscientific, but they reminded me that even plant eaters could be dangerous.

As silly, stupid and just pain gross as the series was, it looked like Dinosaurs Attack! was poised to become a major bit of dinosaur culture. A comic series promised to continue the mayhem, a toned-down animated show was pitched, and rumor had it that a major motion picture was in the works. But that all fizzled. The comic only ran one issue, the cartoon never got off the ground and the impending release of Jurassic Park killed hopes for a film. (Instead we got the awful, unfunny Mars Attacks!, a Tim Burton adaptation of the earlier Topps series that served as a template for the unrelated dinosaurian follow-up.)

Done right, though, I think a Dinosaurs Attack! movie could be bloody fun. There have been a few R-rated dinosaur films—the lackluster Carnosaur series being the most prominent—but all the great examples of dinosaur cinema have been toned down for kids. Maybe it’s time for a dinosaur movie that says “This is not fit for children” and really runs with the idea of what life would be like if packs of Deinonychus roamed the streets and an ornery Styracosaurus decided to graze on the front lawn.

The skull of Tarbosaurus. Photo by Flickr user Jordi Paya.

When I think of Deinocheirus, I think of arms. A few other parts of the dinosaur’s skeleton are known—vertebrae, ribs and most of the hip—but none of those elements are quite as impressive as the immense forelimbs. The arms, tipped with curved claws, measure about eight feet long, and the creature that carried them must have been about as large as the stubby-armed tyrannosaurs that roamed the same habitats in Mongolia around 70 million years ago. The clues from the arms and associated bones hint that Deinocheirus was a gigantic ornithomimid—one of the “ostrich mimic” dinosaurs like Struthiomimus. The trouble is that only the single specimen has been described so far, and so many parts of the skeleton are missing that we don’t wholly know what the gargantuan dinosaur looked like. A new paper, online at Cretaceous Research, suggests that the dining habits of tyrannosaurs might explain why paleontologists didn’t find more of Deinocheirus.

Deinocheirus was discovered in 1965 by the Polish-Mongolian Palaeontological Expedition. To find out more about this dinosaur, in 2008 members of the Korea-Mongolia International Dinosaur Project tracked down the quarry that yielded the single known specimen. The paleontologists hoped that the original excavations had left some bones behind, or that new pieces of the dinosaur’s skeleton might have been exposed in the intervening time.

According to the Cretaceous Research paper by Phil Bell, Philip Currie and Yuong-Nam Lee, the search turned up multiple bone fragments and several gastralia—the “belly ribs” that formed a basket beneath the dinosaur’s ribcage. And those gastralia may explain why so little of Deinocheirus became preserved. Two of the slender, curved bones recorded the bite marks of a large predatory dinosaur. This Deinocheirus was being eaten shortly before burial.

There are many kinds of bite marks. Paleontologists can categorize them, and each pattern of damage corresponds to different biting behavior. The Deinocheirus gastralia exhibited two different kinds of bite marks: tooth scores and parallel striations created as the serrations of the carnivorous dinosaur’s teeth scraped along the bone surface. The minute troughs suggest that a large tyrannosaur, most likely Tarbosaurus, fed on the Deinocheirus. Since the striations record the number and shape of bumps called denticles on the feeding dinosaur’s teeth, they act like a sort of dental fingerprint. Of all the theropod dinosaurs found in the same geologic formation, only Tarbosaurus seems to have had teeth that match the damaged bones.

We can’t know whether the tyrannosaur killed the Deinocheirus or scavenged it. While healed bite wounds record attacks that the victim survived, unhealed bite marks only show that the dinosaur was consumed before burial. In this case, it seems that the tyrannosaur opened up the stomach of the Deinocheirus for access to the viscera inside, but the bite marks record only those brief, violent moments. Whether the tyrannosaur brought down the Deinocheirus or just happened across a rotting carcass is a mystery. But the tyrannosaur also ensured that the particular Deinocheirus would remain an enigma. As the Tarbosaurus feasted, it dismembered the body and scattered the bones of its prey. If paleontologists want a complete look at Deinocheirus, they are going to have to hope for another skeleton elsewhere.

References:

Bell, P.R., Currie, P.J., Lee, Y. (2012). Tyrannosaur feeding traces on Deinocheirus (Theropoda:?Ornithomimosauria) remains from the Nemegt Formation (Late Cretaceous), Mongolia Cretaceous Research : 10.1016/j.cretres.2012.03.018

The chest cavity of Velociraptor MPC-D100/54. The white arrow indicates a broken rib, and the black arrows point to pterosaur bones preserved inside the dinosaur's skeleton. From Hone et al., 2012.

Though only about the size of a turkey, Velociraptor still looked like a formidable predator. With snatching hands, a jaw set with recurved teeth and, of course, a retractable claw on each foot, almost every end of this dinosaur was sharp. But what did this well-equipped Cretaceous killer actually eat?

One of the prime candidates for a Velociraptor entree has been the small horned dinosaur Protoceratops. A truly spectacular fossil cemented the connection between these dinosaurs. In 1971, a Polish-Mongolian expedition to the Gobi Desert found “fighting dinosaurs“—a Velociraptor and Protoceratops preserved in the throes of fatal combat. While the Velociraptor had kicked its deadly foot claw into the neck of the Protoceratops, the little ceratopsian had crushed the right arm of the predator, and the two remained locked together in death. The trouble is that we can’t know why these two dinosaurs were fighting. Was the Velociraptor trying to hunt the Protoceratops? Or was the little predator itself attacked by a territorial Protoceratops? That the dinosaurs battled each other is obvious, but the reason for their combat remains a mystery.

But a recently described fossil confirmed that Velociraptor or a very similar dinosaur ate Protoceratops flesh. In 2010, paleontologist Dave Hone and co-authors reported a set of Protoceratops bones that had been scratched and scored by the teeth of a small predatory dinosaur. How the horned dinosaur died was unclear, but the toothmarks indicated that the carcass had almost been entirely stripped by the time the carnivorous dinosaur came along to pick off the remaining scraps. Since Velociraptor shared the same habitat and was of the right size to leave the bite marks, the dinosaur is a good candidate for being the scavenger.

Another fossil provides an even closer connection between Velociraptor and its prey. In a paper to be published in Palaeogeography, Palaeoclimatology, Palaeoecology, Hone and co-authors Takanobu Tsuihiji, MahitoWatabe and Khishigjaw Tsogtbaatr describe part of a Velociraptor meal preserved inside the dinosaur’s body cavity. Represented by a single bone, the gut contents show the dinosaur had fed upon a pterosaur.

The broken pterosaur bone was probably inside the dinosaur’s stomach when it died. How that bone found its way into the Velociraptor digestive system is another matter. Based on the anatomy of the bone and the pterosaurs that were around at the time, Hone and colleagues hypothesize that the ingested pterosaur was an azhdarchid, one of the long-legged, long-necked pterosaurs that included the largest flying animals of all time.

This particular pterosaur was not a giant by pterosaur standards—Hone and colleagues estimate that the animal probably had a wingspan over six feet across and weighed more than 19 pounds. But it would have been large compared to the relatively small Velociraptor that consumed it. This would have made the sharp-beaked pterosaur “a difficult, and probably even dangerous, target [for] a young dromaeosaur,” Hone and co-authors suggest, and therefore “unless the pterosaur was already ill, infirm or injured, it seems unlikely that this would be a case of predation.” And the fact that the dinosaur consumed a large bone further suggests this might have been another instance of Velociraptor scavenging. If the pterosaur carcass was fresh, the Velociraptor probably would have consumed the available soft tissues first. The fact that the dinosaur ate bone may be an indication that the pterosaur had been picked over and there was only a little meat left clinging to the carcass.

This isn’t the first time evidence of small dromaeosaurs scavenging on pterosaurs has been found. In 1995, paleontologists Philip Currie and Aase Roland Jacobsen reported a partial skeleton of an azhdarchid pterosaur that had been bitten by a small predatory dinosaur. A tooth embedded in the skeleton identified the scavenger as Saurornitholestes, a dromaeosaurid cousin of Velociraptor from Cretaceous North America.

Although Velociraptor is often celebrated as a vicious and cunning predator, the accumulating evidence shows that the dinosaur wasn’t above scavenging. This isn’t surprising. Even highly active predators will regularly scavenge if the opportunity arises. And while I consider the ballyhooed argument over whether Tyrannosaurus rex was primarily a hunter or scavenger to be dead and buried—the tyrant dinosaur was certainly both hunter and scavenger—it is worth noting that even small, apparently highly predaceous dinosaurs at least occasionally scavenged. In outlining his case for “Tyrannosaurus the scavenger,”  paleontologist Jack Horner pointed to Velociraptor as the epitome of what a predatory dinosaur should look like. Yet this new paper, as well as other recently reported indications of dinosaur hunting and scavenging, underscores the fact that the hunting-scavenging dichotomy is too narrow a view on nature. As Hone and colleagues wrote near the beginning of their paper, many carnivores hunt and scavenge. The trick is figuring out which type of flesh-acquisition behavior was more important to a particular species.

Frustratingly, though, we’re more likely to find evidence of dinosaur scavenging than active predation. Relatively small predators like Velociraptor, which may have specialized on even smaller prey, are especially troublesome in this regard. Unless someone is lucky enough to find a small mammal, dinosaur, or other creature in the gut contents of Velociraptor, we  may never know what this dinosaur primarily hunted. When predatory dinosaurs wrenched tattered bits of flesh from denuded carcasses, though, they often left tell-tale signs of damage behind, and these traces are more likely to be preserved than are gut contents. Despite its celebrity, we are still just beginning to put together a picture of how Velociraptor hunted and fed.

For more details on the pterosaur-eating Velociraptor, including some excellent art by Brett Booth, visit Dave Hone’s blog Archosaur Musings.

References:

Currie, P., & Jacobsen, A. (1995). An azhdarchid pterosaur eaten by a velociraptorine theropod Canadian Journal of Earth Sciences, 32 (7), 922-925 DOI: 10.1139/e95-077

Fowler, D., Freedman, E., Scannella, J., & Kambic, R. (2011). The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds PLoS ONE, 6 (12) DOI: 10.1371/journal.pone.0028964

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

Hone, D., Tsuihiji, T., Watabe, M., Tsogtbaatr, K. (2012). Pterosaurs as a food source for small dromaeosaurs Palaeogeography, Palaeoclimatology, Palaeoecology : 10.1016/j.palaeo.2012.02.021

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