Were Dinosaurs Undergoing Long-Term Decline Before Mass Extinction?



The research team, which includes Brusatte; Mark Norell, chair of the Museum’s Division of Paleontology; and scientists Richard Butler of Ludwig Maximilian University of Munich and Albert Prieto-M‡rquez from the Bavarian State Collection for Palaeontology, both in Germany, is the first to look at dinosaur extinction based on “morphological disparity”-the variability of body structure within particular groups of dinosaurs. Previous research was based almost exclusively on estimates of changes in the number of dinosaur species over time. However, it can be very difficult to do this accurately.

“By looking just at trends in taxonomic diversity, you get conflicting answers about the state of dinosaurs prior to extinction,” Brusatte said. “This is because the results can be biased by uneven sampling of the fossil record. In places where more rock and fossils were formed, like in America’s Great Plains, you’ll find more species. We wanted to go beyond a simple species count for this study.”

By looking at the change in biodiversity within a given dinosaur group over time, researchers can create a rough snapshot of the animals’ overall well-being. This is because groups that show an increase in variability might have been evolving into more species, giving them an ecological edge. On the other hand, decreasing variability might be a warning sign of extinction in the long term.

The researchers calculated morphological disparity for seven major dinosaur groups using databases that include wide-ranging characteristics about the intricate skeletal structure of nearly 150 different species.

“People often think of dinosaurs as being monolithic-we say ‘The dinosaurs did this, and the dinosaurs did that,'” Butler said. “But dinosaurs were hugely diverse. There were hundreds of species living in the Late Cretaceous, and these differed enormously in diet, shape, and size. Different groups were probably evolving in different ways and the results of our study show that very clearly.”

The researchers found that hadrosaurs and ceratopsids, two groups of large-bodied, bulk-feeding herbivores-animals that did not feed selectively-may have experienced a decline in biodiversity in the 12 million years before the dinosaurs ultimately went extinct. In contrast, small herbivores (ankylosaurs and pachycephalosaurs), carnivorous dinosaurs (tyrannosaurs and coelurosaurs), and enormous herbivores without advanced chewing abilities (sauropods) remained relatively stable or even slightly increased in biodiversity.

As a complication, hadrosaurs showed different levels of disparity in different locations. While declining in North America, the disparity of this dinosaur group seems to have been increasing in Asia during the latest Cretaceous.

“These disparity calculations paint a more nuanced picture of the final 12 million years of dinosaur history,” Brusatte said. “Contrary to how things are often perceived, the Late Cretaceous wasn’t a static ‘lost world’ that was violently interrupted by an asteroid impact. Some dinosaurs were undergoing dramatic changes during this time, and the large herbivores seem to have been mired in a long-term decline, at least in North America.”

In North America, extreme fluctuations of the inland Western Interior Sea and mountain building might have affected the evolution of dinosaurs in distinct ways from species on other continents. Therefore, the authors say, the North American record might not be representative of a global pattern, if one exists. They also note that there is no way to tell whether a declining dinosaur group would have survived if the asteroid had not struck Earth.

“Even if the disparity of some dinosaur clades or regional faunas were in decline, this does not automatically mean that dinosaurs were doomed to extinction,” Norell said. “Dinosaur diversity fluctuated throughout the Mesozoic, and small increases or decreases between two or three time intervals may not be noteworthy within the context of the entire 150-million-year history of the group.”

Funding for this study was provided by the National Science Foundation through the Division of Earth Sciences, the Division of Biological Infrastructure, a Graduate Research Fellowship, and a Doctoral Dissertation Improvement Grant; the German Research Foundation’s Emmy Noether Programme; the Alexander von Humboldt Foundation; the Charlotte and Walter Kohler Charitable Trust; the American Museum of Natural History; and Columbia University.

Two New Species of Horned Dinosaur Named

ScienceDaily (Mar. 12, 2012) — Two new horned dinosaurs have been named based on fossils collected from Alberta, Canada. The new species, Unescopceratops koppelhusae and Gryphoceratops morrisoni, are from the Leptoceratopsidae family of horned dinosaurs. The herbivores lived during the Late Cretaceous period between 75 to 83 million years ago. The specimens are described in research published in the Jan. 24, 2012, online issue of the journal Cretaceous Research.

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“These dinosaurs fill important gaps in the evolutionary history of small-bodied horned dinosaurs that lack the large horns and frills of relatives like Triceratops from North America,” said Michael Ryan, Ph.D., curator of vertebrate paleontology at The Cleveland Museum of Natural History, lead author on the research. “Although horned dinosaurs originated in Asia, our analysis suggests that leptoceratopsids radiated to North America and diversified here, since the new species, Gryphoceratops, is the earliest record of the group on this continent.”

Unescoceratops koppelhusae lived approximately 75 million years ago. It measured about one to two meters (6.5 feet) in length and weighed less than 91 kilograms (200 pounds). It had a short frill extending from behind its head but did not have ornamentation on its skull. It had a parrot-like beak. Its teeth were lower and rounder than those of any other leptoceratopsid. In addition, its hatchet-shaped jaw had a distinct portion of bone that projected below the jaw like a small chin.

The lower left jaw fragment of Unescoceratops was discovered in 1995 in Dinosaur Provincial Park, a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site by Philip Currie, Ph.D., now of the University of Alberta. Originally described in 1998 by Ryan and Currie, the dinosaur was referred to as Leptoceratops. Subsequent research by Ryan and David Evans, Ph.D., of the Royal Ontario Museum in Toronto, Canada, determined the specimen was a new genus and species. The genus is named to honor the UNESCO World Heritage Site designation for the locality where the specimen was found and from the Greek “ceratops,” which means “horned face.” The species is named for Eva Koppelhus, Ph.D., a palynologist at the University of Alberta and wife of Currie.

Gryphoceratops morrisoni lived about 83 million years ago. It had a shorter and deeper jaw shape than any other leptoceratopsid. Researchers believe the individual was a full-grown adult. Based on unique characteristics of the jaw and its size, the researchers believe that Gryphoceratops was an adult that did not exceed one-half meter in length. This means it is the smallest adult-sized horned dinosaur in North America and one of the smallest adult-sized plant-eating dinosaurs known.

Lower right jaw fragments of Gryphoceratops were discovered in southern Alberta in 1950 by Levi Sternberg while he worked for the Royal Ontario Museum. The genus is named for “Gryphon,” a mythological Greek figure with the body of a lion and the head of an eagle, which is a reference to the animal’s beaked face. The species name honors Ian Morrison, a Royal Ontario Museum technician, who discovered how the bones fit together.

Second author Evans, associate curator of vertebrate palaeontology at the Royal Ontario Museum and assistant professor at the University of Toronto, said, “Small-bodied dinosaurs are typically poorly represented in the fossil record, which is why fragmentary remains like these new leptoceratopsids can make a big contribution to our understanding of dinosaur ecology and evolution.”

Contributing authors are Philip Currie, Ph.D., of the University of Alberta; Caleb Brown of the University of Toronto; and Don Brinkman, Ph.D., of the Royal Tyrrell Museum of Palaeontology.