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
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