August 3, 2012
Dinosaurs didn’t all live at the same time. Not counting the avian species that have thrived during the last 65 million years, dinosaurs proliferated throughout the world during a span of over 160 million years. As I’ve pointed out before, it’s amazing to think that less time separates us from Tyrannosaurus than separated Tyrannosaurus from Stegosaurus.
Even within specific geologic formations, not all the dinosaurs found in those layers lived side by side. Dinosaur-bearing strata accumulated over millions and millions of years and record both ecological and evolutionary changes. Look closely enough, and you can even see particular communities of dinosaurs give way to different assemblages. In an in-press Palaeogeography, Palaeoclimatology, Palaeoecology paper, Jordan Mallon and colleagues have done just that.
Canada’s Dinosaur Park Formation is one of the most spectacular slices of Late Cretaceous time found anywhere in the world. Spanning approximately 76.5 to 74.8 million years ago, the formation has yielded lovely specimens of dinosaurs such as the crested hadrosaur Corythosaurus, the spiky ceratopsid Styracosaurus, the lithe tyrannosaur Gorgosaurus, the heavy-armored ankylosaur Euplocephalus and many others. Not all of these dinosaurs were neighbors, though. Since 1950, at least, paleontologists have recognized that some kinds of dinosaurs are restricted to certain slices of the formation, and the dinosaur community changed over time. Mallon and co-authors decided to have another look at the dinosaur turnover, focusing on the large herbivores and investigating what might have shook up the dinosaur populations during the time the Dinosaur Park Formation was being laid down.
The paleontologists identified two broad divisions in the Dinosaur Park Formation, which they call “megaherbivore assemblage zones.” Each zone lasted roughly 600,000 years each. There are a lot of names here, so bear with me. In the lower zone, the horned dinosaur Centrosaurus and the crested hadrosaur Corythosaurus are found throughout; other dinosaurs restricted to this half of the formation include the ceratopsid Chasmosaurus russelli, the hadrosaurs Gryposaurus and Parasaurolophus, and the ankylosaur Dyoplosaurus.
Yet there are some dinosaurs that first appear in the lower zone and persist into next one. The ceratopsid Chasmosaurus belli, the ankylosaur Euoplocephalus and the hadrosaurs Lambeosaurus clavinitialis and Lambeosaurus lambei show up in the lower zone but pass through into the second zone as well. And, as with the lower swath, there were dinosaurs that were only found in the second zone. The hadrosaurs Prosaurolophus and Lambeosaurus magnicristatus, as well as the horned dinosaurs Styracosaurus, Vagaceratops and a pachyrhinosaur, are only found in the upper zone.
So the big picture is that the lower zone is characterized by Centrosaurus and Corythosaurus, the upper zone is distinguished by Styracosaurus and Prosaurolophus, and there are some dinosaurs–such as Lambeosaurus and Chasmosaurus–that are smeared across the two. As the researchers note, it’s even possible to break down the two halves into even smaller subsets, although the picture gets a little muddier at these levels.
What does all this evolutionary dinosaur shuffling mean? Other researchers have proposed that the Dinosaur Park Formation represents a series of turnover pulses–after a period of stability, rapid ecological change wiped out some dinosaurs while creating opportunities for a new community. The now-vanished Western Interior Seaway has been invoked as a possible mechanism for this. As this shallow sea, which once split North America in two, expanded and encroached further inland, the area of the Dinosaur Park Formation became a mostly coastal, muddy, swampy habitat. This may have put pressure on some forms of dinosaur while providing opportunities for others. As the seaway fluctuated, the attendant changes would have altered the environment and therefore affected dinosaur populations.
According to Mallon and collaborators, though, there’s no strong evidence for the turnover pulse hypothesis. We simply don’t have the resolution to tell how closely certain dinosaurs were tied to particular habitats or niches, and shifts in ecology would have influenced dinosaur evolution. Other possible influences–such as dinosaurs migrating to the area from elsewhere, or the evolution of one species into another within the formation–are also frustratingly unclear. As the researchers state, “Whether the appearance and disappearance of the megaherbivorous taxa of the [Dinosaur Park Formation] was due to evolution, migration, or to a combination of these factors, is difficult to determine.” We don’t yet know what drove the alterations in the formation’s dinosaur communities.
Aside from the ongoing mystery about what caused the changes between the two zones, the revised look at the Dinosaur Park Formation also raises a few questions about dinosaur ecology. Despite the shifts in dinosaur communities, the paleontologists note, there were about six to eight different megaherbivorous dinosaur species living alongside each other. That’s a lot of big herbivores on the landscape, especially since the hadrosaurs and ceratopsids may have formed huge herds. Such vast, hefty dinosaur communities would have required a large amount of vegetation, and the disparate megaherbivores were in competition with each other for food. In order to live alongside one another, then, we can assume that there was some kind of niche partitioning–the dinosaurs were adapted to have restricted diets or live in particular habitats as a result of their competition for resources. How exactly this happened, though, requires further study into the ecology and evolution of these dinosaurs.
And there was something else that caught my eye. The new study focused on the megaherbivores, but what about the large carnivores? The large tyrannosaur Gorgosaurus was also present in the Dinosaur Park Formation and was rejected by the researchers as a zone marker because this theropod ranges throughout the formation. Think about that for a moment. We can see a significant amount of change and turnover among the big herbivores, but one of the large carnivores stays the same throughout the entirety of the formation. Why should this be so? Perhaps it has something to do with the fact that the ornamentation and headgear of hadrosaurs and ceratopsids changed quite a bit, but their general body plans were conservative–a Gorgosaurus could take down a Corythosaurus just as well as a Lambeosaurus.
Likewise, I wonder if the same pattern might hold true elsewhere. The Kaiparowits formation of southern Utah, laid down around the time of the Dinosaur Park Formation further north, also hosts an array of hadrosaurs, ceratopsids and ankylosaurs, but there seems to be just one large dinosaurian predator, the tyrannosaur Teratophoneus. (The giant alligator cousin Deinosuchus was another megacarnivore in the Kaiparowits.) We need more fossils to be sure, but perhaps, like Gorgosaurus, the short-snouted Teratophoneus remained the same as different large herbivores came and went. If this turns out to be the case, the lack of an arms race between predator and prey would be further evidence that the ornamentation of ceratopsids and other dinosaurs had more to do with decoration and combat among each other than defense.
Indeed, the new study of the Dinosaur Park Formation lays some important groundwork for future studies. Paleontologists are currently investigating and debating why the roughly 75-million-year-old dinosaurs from Alberta are different from the roughly 75-million-year-old dinosaurs from southern Utah. What factors drove the diversity and disparity of these dinosaurs across the latitudes, and who really lived alongside whom? So far, the Dinosaur Park Formation is the best-sampled slice we have, and there is much work to be done. With any luck, and a few more decades of careful sampling, we’ll be able to put together an intricate picture of how dinosaurs lived and evolved during this brief span of Late Cretaceous time.
Mallon, Jordan C., Evans, David C., Ryan, Michael J., Anderson,, & Jason S. (2012). Megaherbivorous dinosaur turnover in the Dinosaur Park Formation
(upper Campanian) of Alberta, Canada Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2012.06.024
June 20, 2012
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.
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
March 16, 2012
Dinosaurs are sexy creatures. They are icons of both evolution and extinction, and are our most beloved fossil celebrities. But we can’t learn everything we need to know from studying the bones of dinosaurs themselves. Charismatic as they were, dinosaurs were only part of ancient ecosystems, and reconstructing the world of the dinosaurs requires additional lines of evidence. In the latest of the Royal Tyrrell Museum’s lecture series, paleontologist Annie Quinney turns to fossil soils to restore what life was like for the hadrosaur, ceratopsids, tyrannosaurs and other dinosaurs that roamed Canada’s Horseshoe Canyon Formation around 70 million years ago.
November 10, 2008
When I think of places to look for dinosaur bones, the badlands of the western United States and the desolate Gobi desert most immediately come to mind. I would never have guessed that Alaska, of all places, would hold a treasure trove of dinosaur bones, yet there they are. North of the Arctic Circle, on Alaska’s North Slope, lie the scattered remains of dinosaurs like the horned Pachyrhinosaurus, the tyrannosaur Gorgosaurus, the hadrosaur Edmontosaurus, and the maniraptorans Troodon and Dromeosaurus. Smithsonian magazine ran a story about polar dinosaurs last year and now the NOVA documentary “Arctic Dinosaurs” tells the story of their life, death, and discovery.
Seventy million years ago, near the end of the Cretaceous period, Alaska’s North Slope was closer to the North Pole than it is today. This means that it experienced nearly four months of darkness every year instead of the six weeks of night of today. The climate was much more temperate then, however, as indicated by the plants that lived at the time. It would not be an unfamiliar setting if we saw it today. The dinosaurs would have lived in a temperate forest like those seen in southern Alaska today, with ferns covering the ground and tall conifers stretching into the air.
Such was the home of many of the great North American dinosaur lineages at the time, but no one knew that they were there until just a few decades ago. In 1961, an oil geologist working for Shell named Robert Liscomb found a large fossil on the North Slope. He sent it back to a Shell warehouse, but he died in a rockslide the next year and his find fell into obscurity. It was not until Shell decided to do some spring cleaning in the 1980’s that the bone was found, sent to the United States Geological Survey, and identified as belonging to a dinosaur.
The location of Liscomb’s initial find was then tracked down, and the documentary picks up with the present efforts of paleontologists Tom Rich and Kevin May to further excavate the site. Such a task is not easy. The weather is harsh and the site is isolated, and the rock is hard and frozen. Where some fossil sites require only a sharp eye and a popsicle stick, the Liscomb bone bed requires dynamite to even get to the bones. Then it takes a combination of power and finesse to remove them from the rock, particularly after the team returns to the site and finds that the floor of their bone mine became covered with several inches of ice during their absence!
Another team working on the North Slope, led by Anthony Fiorillo, did not have to worry about blasting through rock, but the challenges were no less intense. A tough climb to the site and freezing rain were near constant challenges for them, and the weather conditions made moving fossils treacherous. The standard operating procedure for transporting excavated dinosaur bones involves wrapping them in plaster-soaked-burlap, which then hardens and holds the fossil and surrounding rock together. On the North Slope it is so humid and cold that the encasing material does not dry well, and so moving the fossils out of the quarry and back to the museum has its risks.
Yet the risks have paid off.
Paleontologists now know that the North Slope was home to not just one kind of dinosaur, but a whole ecological assemblage. But how did they survive there? It was warmer there in the past, but the four months of night choked off plant communities every year. Could the dinosaurs have migrated southward to better feeding grounds like modern caribou? Possibly, but it is difficult to determine.
The alternative would be that the dinosaurs remained during those harsh months, but how they would have gotten enough food is left unanswered. If herbivores went into a torpor or hibernation they would have been easy prey for predators. It seems more likely that herds of herbivores struck out to whatever patches of green they could get to, followed by the meat-eating dinosaurs, but this hypothesis has yet to be confirmed or refuted.
The intertwined stories of discoveries and an ancient Alaska are compelling, but the poor-quality cgi dinosaurs mar the quality of the show. The models presented at the beginning of the show, in particular, are a far cry from the beautifully rendered creatures of other shows like Jurassic Fight Club and even 1999’s Walking With Dinosaurs. The fact that Gorgosaurus, a close relative of Tyrannosaurus, is depicted with three equally-long fingers instead of the correct number of two adds insult to injury.
Nevertheless, “Arctic Dinosaurs” provides a fascinating look at paleontology in action, from a chance discovery to excavation and reconstruction of an entire “lost world.”