November 4, 2011
Southern Utah sure has changed from how it was during the Late Cretaceous. Today the area known as Grand Staircase-Escalante National Monument is a dry, rocky place where plants are few and far between. But during a swath of time between 90 and 70 million years ago, the area was a lush, swampy habitat near the great interior seaway that cleaved North America in two. Giant crocs and weird dinosaurs lived in this coastal environment, itself just one part of a vast island continent which was once isolated from other parts of the world. This isolation undoubtedly influenced dinosaur evolution. And it’s possible that distinct pockets within the continent itself caused dinosaur evolution in the north and south to play out very differently. During a specialized technical session yesterday at the annual Society of Vertebrate Paleontology meeting, paleontologists gathered to present the fauna of North America’s lost western continent, called Laramidia.
I did not attend the entire session, but I did catch all the talks in the latter half. Together they created a rough picture of just how different the world once was. For one thing, southern Utah was home to some strange and imposing crocs. Paleontologist Randall Irmis from the University of Utah and the Natural History Museum of Utah reviewed the array of prehistoric crocodyliforms found in Grand Staircase-Esclanate National Monument, including the huge, dinosaur-eating “terror croc” Deinosuchus. There are still some mysteries waiting to be resolved, and discoveries are still being prepped out in the lab, but many of the ambush predators found in the area were alligatoroids—creatures more closely related to modern day alligators than to living gharials or crocodiles.
Damaged bones indicate that one of those long-lost crocs once sunk its teeth into a small dinosaur. In fact, the attacking croc even left part of its tooth behind. In the following talk, University of Iowa paleontologist Stephanie Drumheller highlighted bite marks found on the skeleton of a small, bipedal, unnamed herbivorous dinosaur found in the Kaiparowits Formation of southern Utah. Using high-resolution visualization techniques and comparisons with damage to bones created by modern crocodylians when they feed, Dumheller was able to narrow down the list of possible suspects to a roughly three-foot-long crocodyliform. There is more than one potential candidate among animals of this size, but Drumheller’s work showed that some dinosaurs had as much to fear from relatively small crocs as from huge predators such as Deinosuchus.
Of course, there were large, predatory dinosaurs running around in the same area during this time. Natural History Museum of Utah paleontologist Mark Loewen delivered an overview of theropod dinosaurs found in the Late Cretaceous rock of Grand Staircase-Escalante National Monument with a focus on the weird tyrannosaurs found there. These predators, such as the recently named Teratophoneus, had relatively short, deep skulls set with impressive teeth that set them apart for their cousins living during the same time in the northern part of Laramidia. Exactly why these dinosaurs evolved this way is unknown, but the distinct nature of the tyrants and other dinosaurs from the same deposits have led paleontologists to wonder if there was some sort of physical barrier which isolated them and caused them to undergo distinctive changes. As strange as they might look, though, at least one might provide some resolution as to where the ever-popular Tyrannosaurus rex came from. Drawing on a talk on the animal he gave last year, Loewen suggested that a yet-undescribed tyrannosaur from southern Utah’s Wahweap Formation may represent the form of the long-sought Tyrannosaurus ancestor.
But some of the most spectacular dinosaurs of all were the horned dinosaurs of Laramidia. Andrew Farke from the Raymond M. Alf Museum of Paleontology highlighted the rapid rate of discovery in the southwestern United States that is altering our understanding of ceratopsid evolution. While dinosaurs such as Zuniceratops appear to indicate that the earliest ceratopsid dinosaurs—the lineage including horned dinosaurs such as Styracosaurus and Utahceratops—evolved in North America, the exact time and place of their origin is unknown. Furthermore, the relationships among the various ceratopsid dinosaurs discovered in Laramidia to date is mysterious—better resolution is needed to understand how the dinosaurs evolved in space and time. Though we’re quickly adding new ceratopsid genera thanks to some great new fossil finds, we are going to have to wait for future fossil finds and revised analyses to really understand the big evolutionary picture for this group.
The several talks that followed, by paleontologists Caleb Brown of the University of Toronto, David Evans from the same institution, and Terry Gates of the Field Museum, respectively, highlighted other evolutionary and geographical patterns within other dinosaurs and smaller animals in Laramidia. During his talk on hadrosaurs found in the northern part of Laramidia, for example, Evans pointed out that there was at least some interchange between the northern and southern parts of the continent. The recently named hadrosaur Acristavus has been found in both the northern and southern parts, so perhaps barriers between the two areas were not so impenetrable to dinosaurs after all. Likewise, Gates pointed out that we require a much finer picture of what the ancient environments of Laramidia were like and a clearer understanding of which slices of rock correspond in the northern and southern parts of the continent. Better constraints on these issues will allow paleontologists to make the more exact comparisons needed to draw out evolutionary patterns.
The final talk was delivered by Natural History Museum of Utah paleontologist Scott Sampson. He noted that paleontologists had previously thought that many major dinosaur groups of the Late Cretaceous—the hadrosaurids, the ceratopsids and the tyrannosaurids, among others—had evolved in Asia and later invaded North America. Sampson argued the opposite. New evidence may indicate that these groups emerged within Laramidia and then dispersed to Asia after about 70 million years ago (though some groups of dinosaurs that evolved in Asia likely came into North America, too). There may have been a great dinosaur interchange between what is now Alaska and Russia. Though a number of the talks in the session emphasized the need for additional information before we can draw out the patterns, Sampson did make the case that Laramidia was an important center of dinosaur evolution. As discoveries accumulate, and as paleontologists find new ways to analyze the fossil data, the major evolutionary story will come into focus.
Top image from:
Sampson SD, Loewen MA, Farke AA, Roberts EM, Forster CA, et al. (2010) New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism. PLoS ONE 5(9): e12292. doi:10.1371/journal.pone.0012292.g003
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