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A restoration of Repenomamus snacking on a young Psittacosaurus. Image by Nobu Tamura, from Wikipedia.

What dinosaurs ate, and how they ate it, is an endless source of fascination. Whether it’s the predatory habits of Tyrannosaurus rex or how sauropods managed to horf down enough food to fuel their bulky bodies, the details of dinosaurs’ paleo diets fuel scientific study and dinosaur restorations alike. If basic cable documentaries have taught me anything, it’s that dinosaurs were all about eating.

But dinosaurs were not invulnerable consumers. Even the biggest and fiercest dinosaurs were food sources for other organisms—from giant crocodylians to parasites and bone-boring beetles that took up residence in dinosaur carcasses. Even mammals sometimes dined on dinosaur.

The most famous case is Repenomamus. Hardly a household name, this critter is the exception to everything I heard about mammals in the Age of Dinosaurs. The classic story is that mammals were so stifled by the dinosaurian reign that our furry ancestors and cousins remained small and hid among the shadows. There is some truth to the notion. Mammalian evolution was influenced by dinosaur evolution, and as Mesozoic mammals diversified, most stayed small and became adapted to burrowing, swimming, gliding and other modes of life in the shadow of the dinosaurs.

Repenomamus, on the other hand, was huge for a mammal of its time. This roughly 130-million-year-old carnivore, found in the rich fossil beds of northeastern China, was a badger-like creature a little over three feet long—bigger than some of the feathery dinosaurs that lived at that same time. Repenomamus was big enough to eat dinosaurs, and we know that the mammal definitely did. In 2005, paleontologist Yaoming Hu and co-authors described a Repenomamus skeleton with the remains of a juvenile Psittacosaurus, an archaic ceratopsian dinosaur, in its gut contents. Based on the way the little dinosaur bones were broken up, the researchers said, “the juvenile Psittacosaurus was dismembered and swallowed as chunks.”

We don’t know whether Repenomamus caught the young dinosaur or scavenged it. Those details aren’t recorded in the fossils. Either scenario is possible—Repenomamus was certainly large enough to catch and kill a juvenile Psittacosaurus, but there’s no reason to think that such a large carnivorous mammal would have passed up a dinosaur carcass. While many Mesozoic mammals might have qualified as dinosaur prey, Repenomamus reminds us that the classic narrative of total dinosaur dominance gives the prehistoric archosaurs too much credit.

Of course, mammals didn’t have to be burly carnivores to eat dinosaurs. Dead dinosaurs were rich food resources on the prehistoric landscape, and mammals took advantage of these bonanzas. In a study I wrote about two years ago, paleontologists Nicholas Longrich and Michael Ryan documented several fossils—including dinosaur limb and rib fragments—that displayed toothmarks made by small mammals called multituberculates. These mammals, often restored in opossum-like garb, had large, pointed incisors that helped them gnaw on tough plant foods but that could also be repurposed to scrape at dinosaur carcasses. Given the chance, mammals made the most of dead dinosaurs.

References:

Longrich, N., & Ryan, M. (2010). Mammalian tooth marks on the bones of dinosaurs and other Late Cretaceous vertebrates Palaeontology DOI: 10.1111/j.1475-4983.2010.00957.x

Yaoming Hu, Jin Meng, Yuanqing Wang, Chuankui Li (2005). Large Mesozoic mammals fed on young dinosaurs Nature, 433, 149-152 DOI: 10.1038/nature03102

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

The dig marks made by a predatory dinosaur, including a claw impression. From the Geology paper.

Dinosaurs overshadowed mammals for most of the Mesozoic, but evidence of actual dinosaur-mammal interactions are very rare. On the mammalian score, a specimen of the relatively large Cretaceous mammal Repenomamus robustus described in 2005 was found with the bones of baby dinosaurs in its stomach—it had apparently fed on young Psittacosaurus shortly before it died. A new set of fossils from southern Utah, though, evens the score for the dinosaurs.

In Utah’s Grand Staircase-Escalante National Monument, within the 80-million-year-old rock of the Wahweap Formation, paleontologists have discovered evidence that small predatory dinosaurs dug down into the soil to reach the burrows of small mammals. As reported in the journal Geology, the vestiges of these events are left behind as traces within the rocks—scratches made by dinosaurs and dens used by mammals—and by looking at them together scientists can replay what might have happened during those Late Cretaceous days at the end of the Mesozoic era.

The first trace fossil type was made by a digging dinosaur, probably a maniraptoran similar in form to Deinonychus and Troodon. At first glance it doesn’t look like much—just a lumpy bit of sandstone—but if you look carefully, a claw impression and numerous downward-arcing grooves can be seen. It appears that the dinosaur was repeatedly sticking its foot into the hole and raking out sediment, a behavior consistent with the idea that these dinosaurs probably did not use their arms to dig because their feathers would have gotten in the way or been damaged.

The second group of traces, found near the claw marks, preserve mammal burrows and dens. Networks of branching, winding burrows connect to bulbous chambers where mammals found refuge, and these underground structures are very similar to those made by small, social mammals living today. Based upon the close association of these structures with the claw marks, and especially the correspondence between deeper burrows and deeper dig marks made by dinosaurs, the researchers hypothesize that the predatory dinosaur was trying to get at the mammals.

Together the scratches and burrows tell of ancient interactions we could only previously infer on the basis of bones. It most have been terrifying for those small mammals, hearing the predatory dinosaur scratching deep into the ground in the hopes of catching them.

Edward L. Simpson, Hannah L. Hilbert-Wolf, Michael C. Wizevich, Sarah E. Tindall, Ben R. Fasinski, Lauren P. Storm and Mattathias D. Needle (2010). Predatory digging behavior by dinosaurs Geology, 38, 699-702 : 10.1130/G31019.1

A comparison of the skull of a multituberculate mammal with tooth marks made on a dinosaur rib. From the Palaeontology paper.

Mammals have long been characterized as the underdogs of the Mesozoic world. They diversified in habitats ecologically dominated by dinosaurs, but, even though most were small, they did not simply cower in their burrows until the non-avian dinosaurs were wiped out 65 million years ago. In fact, Mesozoic mammals were more varied in anatomy and habits than is often appreciated, and, as has just been reported in Palaeontology, some small mammals gnawed the bones of the giant archosaurs.

As described by paleontologists Nicholas Longrich and Michael Ryan, a number of fossil bones from the Cretaceous rock of Alberta, Canada were damaged by bites which could only have been made by mammals. A dinosaur rib fragment, a piece of dinosaur limb bone, a partial lower jaw from the marsupial mammal Eodelphis and a femur from a reptile called a champosaur bear bite marks made by an animal with closely-spaced, paired teeth. This bite pattern matches the tooth placement of an extinct variety of mammal called multituberculates—these mammals had long incisor teeth at the front of their jaw separated from the other teeth by a gap, thus explaining why the only toothmarks on the bones were made by incisors. While other mammals could potentially have been the culprit, the anatomy of the multituberculates make them the best fit.

The multicuberculate-made toothmarks are, at present, the oldest known fossil traces of mammal toothmarks. More than that, the authors suggest that some multituberculates used their incisors to gnaw on hard, resistant food items, meaning that they were perhaps more versatile in their diets than had previously been presumed. From the traces on the bones it appears that these small mammals scavenged dead dinosaurs and other creatures for food (leaving behind the relatively shallow tooth marks on some of the specimens) and sometimes bit into the bone itself, perhaps to obtain minerals like calcium (as seen by the deeper bite marks). Now that these traces have been recognized, perhaps other paleontologists will see similar marks in bones they collect, potentially helping us better understand the lives of the mammals that lived alongside the dinosaurs.

LONGRICH, N., & RYAN, M. (2010). Mammalian tooth marks on the bones of dinosaurs and other Late Cretaceous vertebrates Palaeontology DOI: 10.1111/j.1475-4983.2010.00957.x

The exceptionally-preserved skeleton of Darwinius. From PLoS One.

This week news services were all a-twitter about a 47-million-year-old fossil primate from the famous Messel deposits of Germany. Named Darwinius masillae and described in the journal PLoS One, the lemur-like primate was heralded as being a transitional form between a group of extinct primates called adapids and anthropoid primates (monkeys and apes). As it turns out the fossil may not be all it has been cracked up to be, but it is still a spectacular find that represents one branch of the primate radiation that occurred after the mass extinction that killed off the dinosaurs at the end of the Cretaceous. Creatures like Tyrannosaurus perished, but primates survived.

Tracing the record of the earliest primates is a challenge. Since primates started off small and lived in forested habitats their fossils are extremely rare, and most fossils that are found are teeth. This can make comparisons between these creatures difficult, and the relationships among early primates or primate-like creatures are controversial. The fact that some molecular studies places the origin of primates even further back in the Cretaceous, about 85 million years ago, makes things even more complicated as no verifiable primate fossils have yet been found from that age. Despite these complexities, however, scientists do have a broad outline of early primate evolution.

One of the earliest primate-like creatures was Purgatorius, a tree-shrew-like mammal that lived right around the end of the Cretaceous 65 million years ago. Whether it was one of the first primates or only closely related to the first primates is still controversial, but it does seem to represent what the ancestors of primates were like during the time that dinosaurs were the dominant land-dwelling vertebrates.

After the mass extinction, mammalian evolution exploded. Mammals were no longer under the feet of dinosaurs, and among the groups that diversified were primate-like creatures called plesiadapiformes. Whether these creatures were true primates or just very primate-like is still being debated, but they underwent a boom and bust during the Paleocene (about 65 to 55 million years ago). In many ways these creatures were somewhat squirrel-like, with clawed hands and eyes on the sides of their heads, but at the very least they seem to be the closest extinct relatives to other primates.

The creatures that are regarded as “true” primates flourished during the Eocene (about 55 to 33 million years ago), and can largely be placed into two groups: the adapids and omomyids. The adapids were lemur-like primates, while the omomyids closely resembled living tarsiers, but both had forward-oriented eyes and adaptations to life in the trees. Both these groups are relevant to yesterday’s big announcement.

According to the new paper, Darwinius is an adapid, and many scientists presently regard this group as being more closely related to modern lemurs and lorises than to monkeys or apes. Many paleontologists who study extinct primates favor omomyids and ancient tarsiers as being closer to monkeys and apes, but the authors of the new paper don’t think so. In the paper itself they claim that Darwinius belongs to the same large group of primates, haplorrhines, as tarsiers, monkeys, and apes, thus placing adapids in a position to potentially become our ancestors. This conclusion has caused the scientists involved in the study and the popular media to herald it as a “missing link” that connects us to other primates.

Unfortunately, however, the scientists who wrote the paper did not conduct a detailed evolutionary analysis of the new fossil or its relationships to other primates. The fossil is spectacular, the first fossil primate to be find in such a state of exceptional preservation, but it has been oversold by the History Channel (who organized the media hype) and the scientists involved in the study. They simply did not do the work to support the conclusions they drew from the fossil, and the real relationship of Darwinius to other primates will have to wait for further studies.