Now, in the latest paper to be published about findings from the Hell Creek Formation, Montana State University doctoral candidate John Scannella and three co-authors reveal new insights into the evolution of Triceratops, based on more than 50 specimens that have been collected in recent years.
By recording precise stratigraphic information for each Triceratops, and analyzing the morphological details of the skull, it appears possible to see evolutionary trends in Triceratops, the researchers said in the June 30 issue of the scientific journal, Proceedings of the National Academy of Sciences (PNAS). Stratigraphy is the study of the layer of rocks.
Over one to two million years at the end of the Cretaceous Period, Triceratops went from having a small nasal horn and long beak to having a long nasal horn and shorter beak. The dinosaur with a small nasal horn and long beak is a Triceratops horridus. It was only found lower in the Hell Creek Formation. The dinosaur with a long nasal horn and shorter beak is a Triceratops prorsus. It was only found near the top of the Hell Creek Formation. Skulls found in the middle of the Hell Creek Formation showed characteristics of both Triceratops horridus and Triceratops prorsus.
"This study provides a detailed look at shifts in the morphology of a single dinosaur genus over time," Scannella said.
Several other institutions in the United States joined in an extensive survey of the Hell Creek Formation that was spearheaded by MSU's Museum of the Rockies in 1999. The goal of the Hell Creek Project was to learn everything they could about the geology, flora and fauna of the formation so they could accurately reconstruct the environment at the end of the Cretaceous Period and the lives and evolution of the creatures that once roamed there.
Over the course of the project, the researchers found that Triceratops is the most common dinosaur in the Hell Creek Formation. The Museum of the Rockies now houses one of the largest collections of Triceratops in the world.
"Most dinosaurs are only known from one or a handful of specimens," Scannella said. "Some dinosaurs are known from a large number of specimens, but they're often found all in one place – on a single stratigraphic horizon. The great thing about Triceratops is that there are a lot of them, and they were found at different levels of the Hell Creek Formation.
"So we can compare Triceratops found at different levels," Scannella said. "When you have a larger sample size, you can learn much more about variation, growth and evolution."
Scannella was lead author of the PNAS paper. Co-authors were MSU graduate student Denver Fowler, paleontologist Mark Goodwin from the University of California Museum of Paleontology, Berkeley, and Regents Professor of Paleontology Jack Horner of MSU. All have written or co-authored previous papers about their discoveries in the Hell Creek Formation near Jordan and the Fort Peck Reservoir.
Horner and Goodwin discovered previously that the skull and horns of Triceratops changed shape as it grew from a baby to an adult. In 2010, Scannella and Horner suggested that the skull of Triceratops underwent an even more dramatic transformation than had been suspected and that when fully mature, the Triceratops became what had previously been thought to be a distinct genus of horned dinosaur, the Torosaurus.
"The new study finds evidence that not only did Triceratops change shape over the lifetime of an individual, but that the genus transformed over the course of the end of the age of dinosaurs," Scannella said.
The Hell Creek survey located so many new Triceratops that bones are still being removed from field jackets and prepared for study every day at the Museum of the Rockies. The new Triceratops range from small juveniles to animals with heads the size of a car. Some were found complete and intact. Others were found shattered into countless pieces.
The researchers collected as many specimens as possible in order to put together the most complete picture of the evolution of this famous dinosaur, named by famed Yale paleontologist O.C. Marsh in 1889, Scannella said.
"The study emphasized how important it is to know exactly where dinosaur fossils are collected from," Scannella said. "A beautiful Triceratops without detailed stratigraphic data cannot answer as many questions as a fragmentary specimen with stratigraphic data."
Provided by Montana State University
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