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
September 26, 2011
Variation is one of the basic elements that makes evolution possible. The tiny differences between individuals in a population provide the raw material for natural selection to act upon and cause evolutionary changes. This can readily be seen among living animals, but identifying and understanding variation among dinosaurs is much more difficult. Paleontologists typically have only a handful of specimens, represented by incomplete materials, from a range of sites which may span hundreds of thousands, if not millions, of years. Nevertheless, studying how individual dinosaurs of well-sampled species vary from one another can help researchers investigate details of dinosaur diversity and dinosaur lifestyles. Among the latest dinosaurs to be studied this way is Anchiceratops ornatus, a relatively obscure horned dinosaur from the Late Cretaceous of Canada.
As reviewed by paleontologist Jordan Mallon and colleagues in the recent Journal of Vertebrate Paleontology study, Anchiceratops has had a tangled history. Fossil hunter Barnum Brown named the first species, Anchiceratops ornatus, in 1914, and in 1929 experienced dinosaur excavator Charles M. Sternberg described a second species he called Anchiceratops longirostris on the basis of what he thought was a more gracile, slender skull. The two species were later lumped together into just one, A. ornatus, and despite a lack of rigorous testing, the disparity between the two skulls has been attributed to sexual differences between males and females. (Though sexual dimorphism has often been proposed for dinosaurs, no clear-cut, entirely convincing case has been found.)
But there are more than two Anchiceratops skulls. The trouble with dinosaur discoveries is that additional fossils of already named genera or species often don’t get described unless they are exceptional in some way or are used in a project that requires comparisons between multiple individuals. In the case of Anchiceratops, a total of ten more or less complete skulls have been found that can be attributed to the genus, and these fossils form the basis of the new study. Each of the skulls varied significantly from others in the sample—something that was expected based on big samples of other horned dinosaurs such as Triceratops and Centrosaurus. But did any of the differences hint that some of the dinosaurs belonged to a separate species, or that certain characteristics could be used to distinguish the dinosaur sexes?
Mallon and co-authors used measurements of particular parts of the skull to compare the ten specimens in the sample in what’s called a morphometric analysis. The results of each test plotted the skulls out on a graph that represented the variation in the sample. If there were two different species or sexes, then the scientists would expect to see two distinct clusters of skulls on the graphs. No such pattern was found. Even though the sample size was small, the results indicated that there was no detectable male-female split. Additionally, the anatomy of the skulls and the lack of clustering offered no support to the idea that there was more than one species of Anchiceratops. There appears to have only been one species, Anchiceratops ornatus, preserved in the rocks of the Horseshoe Canyon Formation dating between about 71 million to 69 million years ago. Two million years is a pretty good run compared to the amount of time other horned dinosaur species persisted: In the older Dinosaur Park Formation in the same area, horned dinosaur species appear to have hung on for only about 700,000 years or so.
Why Anchiceratops ornatus was a longer-lived species than geologically older dinosaurs in the same neighborhood is unknown, but Mallon and colleagues offer several hypotheses. Perhaps, due to the lower dinosaur diversity in the Horseshoe Canyon Formation, Anchiceratops had less competition for food from other herbivores and therefore was able to persist for longer. Then again, the shrinking of the Western Interior Seaway during that time may have affected the history of the species. During the days of the Dinosaur Park Formation, the sea may have created fragmented habitats that resulted in the isolation of dinosaur populations which evolved into new species. Since the seaway was receding during the time of Anchiceratops, habitats were less fragmented and those environmental pressures were released, and so fewer species may have taken up the roomier and more continuous habitats. Alternatively, Anchiceratops may have been a relatively hardy species that could better cope with the environmental changes created by the regression of the sea and, as a result, persisted for longer than species that relied on specialized foods or habitats. At the very least, though, Anchiceratops appears to be a small dinosaur success story.
Mallon, J., Holmes, R., Eberth, D., Ryan, M., & Anderson, J. (2011). Variation in the skull of (Dinosauria, Ceratopsidae) from the Horseshoe Canyon Formation (Upper Cretaceous) of Alberta Journal of Vertebrate Paleontology, 31 (5), 1047-1071 DOI: 10.1080/02724634.2011.601484
April 15, 2011
When I opened my email inbox this morning, I was met with a pleasant surprise. Phil Tippett’s exquisite short film Prehistoric Beast has finally been released in its entirety.
I had only seen bits and pieces of Tippett’s stop-motion story as a kid. The short’s dinosaurs – a Monoclonius and a tyrannosaur – had been featured in the 1985 documentary Dinosaur!, but the full film from which those scenes were taken was only seen at animation festivals. Now, after 26 years, Tippett has posted Prehistoric Beast on YouTube for all to see.
When I saw Tippett’s stop-motion dinosaurs for the first time, they embodied everything I imagined the living creatures to be. They still look good. Poorly-animated digital dinosaurs run rampant on television these days, but Tippett’s carefully-crafted stop-motion models have a certain life-like quality missing from modern Jurassic Park knockoffs. The braying of the lone, lost Monoclonius in the depths of the primeval forest looks like a brief moment in the life of a real animal.
Prehistoric Beast was skillfully shot, too. The film contains no dialog at all – The Land Before Time, it’s not – and the entire story is told through the experience of the Monoclonius. Sometimes the viewer is close-up – looking up at the dinosaur’s muzzle as it crops soft plants – and at other times we see the dinosaur from far away, feeling its isolation as it wanders into the dark woods. In one tense scene, the camera pans around the frightened dinosaur as the tyrannosaur stalks it in the background. We can see the predator disappear behind the trees, but the poor Monoclonius cannot.
Above all, though, Prehistoric Beast is impressive for the level of craftsmanship required to make it. We will probably never see such a film again. Dinosaurs can now be easily brought back to life via computers, even if many of them look absolutely atrocious, and so stop-motion dinosaurs have gone extinct. Maybe it’s just childhood nostalgia for the dinosaurs I grew up with, but, for me, Prehistoric Beast beautifully captures a few moments of prehistoric life that are now only represented by the bones and rock of Alberta’s Dinosaur Park Formation. Tippett’s stop-motion creation is about as close as I am ever going to get to actually seeing the lost Cretaceous world.