April 12, 2012
You can’t understand dinosaurs without a sense of time. We need to know when a dinosaur lived to comprehend how it fits into what paleontologist William Diller Matthew called “life’s splendid drama.” But we throw around Deep Time estimates, framed in millions of years, so often that it’s easy to become inured to the wider context of life’s history.
The Mesozoic Era, which lasted from about 250 million to 66 million years ago, is often called the Age of Dinosaurs. As a kid, this brought to mind one endless summer when dinosaurs flourished. And many of the books I read picked one environment from three different periods within the era to represent dinosaur life. Little Coelophysis was the canonical Triassic dinosaur; the huge sauropods and theropods of the Morrison Formation represented the Jurassic, and a Cretaceous Tyrannosaurus versus Triceratops face-off ultimately capped off the succession. With the periods juxtaposed this way, millions of years didn’t seem so very long.
But let’s unpack some of that scenery. Diplodocus, Apatosaurus, Allosaurus, Stegosaurus and their neighbors roamed western North America about 150 million years ago. This slice of time falls in the latter portion of the Jurassic. The traditional representatives of the latest Cretaceous scene—Tyrannosaurus and Triceratops—did not evolve until about 67 million years ago. By themselves, these dates are just labels, but think of them falling along evolution’s timeline. About 83 million years separated Apatosaurus from Tyrannosaurus and Allosaurus from Triceratops. The so-called Age of Mammals—which began when the non-avian dinosaurs were wiped out—has been going on for about 66 million years. Less time separates us from Tyrannosaurus rex than separated T. rex from Stegosaurus.
Consider how much life has changed in the past 66 million years. Archaic mammals flourished and ultimately went extinct long before anything like the world’s modern fauna appeared. Saber-fanged, knobbly-headed herbivores such as Uintatherium, lemur-like primates called adapiforms, razor-jawed carnivores known as creodonts and many other strange forms proliferated and disappeared. Even lineages familiar to us today, such as horses, rhinos and elephants, evolved and diversified and are now represented by just remnants of what once existed.
The time between the last Triceratops and now has seen radical evolutionary changes. Now think of the 83 million years between the Jurassic and Cretaceous titans. During that time, the first flowering plants bloomed; the fish-like ichthyosaurs disappeared as plesiosaurs and mosasaurs became the predominant predators of the seas; vast herds of hadrosaurs and ceratopsids occupied places once dominated by sauropods; tiny tyrant dinosaurs transformed into apex predators, and early birds established themselves in ever-greater variety alongside their dinosaurian kin. These are just a few highlights, and that is part of the wonder and frustration of tracking the history of life on earth. We are offered only glimpses of an ever-changing picture, and when viewed separately, it’s easy to forget how those snippets relate to each other. But when we can step back, and consider how all those snippets run together, the long and ever-changing history of life on our planet seems all the more fantastic.
September 20, 2010
Tyrannosaurus rex was an obligatory inclusion in every book and documentary about dinosaurs I saw as a kid. It was the tyrant king of all dinosaurs, the supreme predator of the end-Cretaceous, but for all its majesty no one could explain where it had come from. Along with its kin—such as Albertosaurus and Tarbosaurus—Tyrannosaurus simply seemed to be the culmination of a trend towards larger size and ferocity among predatory dinosaurs, but plotting the succession of giant theropods during the course of the Mesozoic did not seem to provide many clues about the origins of the tyrannosaurs.
As summarized in a new Science review by a team of tyrannosaur experts, however, new discoveries made in the last decade have finally placed Tyrannosaurus in its proper evolutionary context. In the past year alone, no less than six new tyrannosauroids have been either discovered or identified from previously-known specimens, and this growing knowledge of tyrannosaur evolution has confirmed that the largest predators of Late Cretaceous North America started off small. The first tyrannosaurs were not derived from already-large Jurassic predators such as Allosaurus, but instead were relatively small coelurosaurs, with small heads and long arms, which evolved during the Middle Jurassic more than 165 million years ago. Proceratosaurus, a crested dinosaur from England once believed to be closely related to Ceratosaurus and other early theropods, was just recently found to be one of the first tyrannosauroids. At a glance, Proceratosaurus and similar tyrannosaurs would have looked more like “raptors” than like their more famous relatives. Exceptionally-preserved specimens of the Early Cretaceous tyrannosauroid Dilong from China show that, like their relatives among the coelurosauria, these dinosaurs were covered in feathery dino-fuzz.
After almost a century of uncertainty, it was finally confirmed that enormous Late Cretaceous tyrannosaurs evolved from small, elaborately-ornamented coelurosaurian ancestors. The approximately 80 million years between the first tyrannosaurs and the radiation of truly giant forms is still relatively sparsely known, though. The recent discovery of the long-snouted genus Xiongguanlong and the announcement of the miniature tyrant Raptorex have illustrated that there was no single, slow evolutionary march towards the Tyrannosaurus rex body form. Instead there was a radiation of relatively small genera which preceded the development of large body size, and there are probably a number of strange Late Jurassic and Early Cretaceous tyrannosaurs waiting to be found.
There is more to recent tyrannosaur research than just filling out evolutionary trees, though. Not only is Tyrannosaurus rex the most famous of all dinosaurs, but thanks to numerous specimens and decades of scientific study it is also the most extensively studied. Bite forces, brain anatomy, running speed, growth rates, bone microanatomy, biogeography and other aspects of its paleobiology have all been—and continue to be—extensively investigated. The abundant remains of some of its close relatives, such as Albertosaurus, have even allowed paleontologists to see how different the last tyrannosaurs were; paleontologists could hardly wish for better fossilized records of these dinosaurs. Research will continue, and new discoveries will continue to revise our understanding of tyrannosaur evolution, but it is wonderful that a more complete history of the tyrannosaurs is beginning to come together.
Brusatte SL, Norell MA, Carr TD, Erickson GM, Hutchinson JR, Balanoff AM, Bever GS, Choiniere JN, Makovicky PJ, & Xu X (2010). Tyrannosaur Paleobiology: New Research on Ancient Exemplar Organisms. Science (New York, N.Y.), 329 (5998), 1481-1485 PMID: 20847260
June 25, 2010
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
July 29, 2009
Scientists at Utah’s Dinosaur National Monument have been quite busy this summer. At the beginning of the season they were blasting some sauropod skulls out of the rock for collection, and now the Chicago Tribune reports that they have discovered hundreds of tiny footprints in rock about 190 million years old. These tracks were not made by dinosaurs, though, but possibly by mammals.
Even though many people think of the Mesozoic (about 251 to 65 million years ago) as the “Age of Dinosaurs,” there were plenty of other creatures around during that time. The first true mammals evolved around 190 million years ago, about when the Dinosaur National Monument tracks were made, although mammal-like creatures had been around for tens of millions of years. Once the first mammals evolved, the group began to diversify, giving rise to the ancestors of modern groups as well as lineages that have gone extinct.
According to a Chicago Tribune report, it seems that the creatures that made the tracks lived in a dry, desert-like environment. Each dime-sized track differs in preservation, but together they provide a snapshot into the life of a rat-sized creature scurrying about the ancient dunes. Given that the animal did not die in its tracks, however, we cannot be entirely sure whether the tracks were made by a “true” mammal or a creature closely related to the common ancestor of all mammals. Since the details used to tell the difference between mammals and mammal-like animals are skeletal, it may not be possible to determine which sort of animal made the tracks. Still, though, the tracks are a rare find and I cannot wait until they are published in an academic journal so we can all learn more about them.
March 17, 2009
If you visited what is now Alberta, Canada 75 million years ago, you would have to beware of some formidable predators. The large tyrannosaurids Daspletosaurus and Gorgosaurus prowled the landscape while the smaller sickle-clawed killers Dromaeosaurus and Saurornitholestes stalked their prey in the forest. You might be excused, then, if you missed a smaller feathered predator that weighed about as much as a domestic chicken and was named Hesperonychus.
Announced by paleontologists Nicholas Longrich and Philip Currie this week in the journal PNAS, Hesperonychus is the smallest predatory dinosaur yet known from North America (even smaller than the termite-eating Albertonykus, which Currie and Longrich described last year). It still would have been quite large compared to the mammals of its day, however, and it may have been the scourge of our ancient relatives. This fits with the hypothesis that dinosaur predation on mammals kept mammals small, but as Longrich and Currie point out, it could also mean that the occupation of niches by mammals kept dinosaurs from becoming much smaller.
During the Mesozoic, the time when non-avian dinosaurs flourished, there were no large mammals. One of the biggest was Repenomamus, which was about the size of a small dog and lived during the Cretaceous. It was large enough to eat some baby dinosaurs (which fossil evidence has shown it did) but this was unusual. Most mammals were smaller and ate seeds, insects, and fruit. This means that if there were dinosaurs smaller than Hesperonychus they may have come into competition with mammals for food and places to live in the forest. Rather than coming into such direct competition for resources with mammals it seems that the smallest of theropod dinosaurs were just large enough to see mammals as food.
What is even more surprising is that Hesperonychus does not fit in with any other maniraptoran dinosaurs from North America. When Longrich and Currie studied its bones to determine what kind of dinosaur it was, they found that it was most closely related to the microraptorine dinosaurs from China. This group of feathered dinosaurs, which includes Microraptor and Sinornithosaurus, had not been found in North America before. Not only that, but Hesperonychus is about 45 million years younger than the oldest members of this group in Asia. Therefore it extends the range of the microraptorine dinosaurs over both time and geography, hinting at other tantalizing finds yet to be disinterred from the rock.