Dinosaurs evolved from their smaller ancestors in just a few million years and not the 10 million years or more scientists had suspected, according to a new study. The work, based on radioactive dating of rocks sandwiching the earliest fossils of those predecessors, suggests that paleontologists have long misjudged the overall pace of dinosaur evolution.
The fossils analyzed in the new study are among the earliest of a broad group of creatures called dinosauromorphs. That group includes all dinosaurs but also includes their earlier predecessors and their subsequent kin, which had the same general body plan but didn’t have distinctive anatomical features in their hip bones that all true dinosaurs shared.
The earliest known dinosauromorphs lived in parts of the ancient supercontinent Pangaea that are now South America and southern Africa. But those fossils aren’t well-dated. Scientists have estimated the arrival of these early members of the dinosaur family tree by trying to match them up to the putative ages of the fossils of reptiles and other animals appearing in the same rocks. That technique is known as biostratigraphy, and it is typically much less accurate than methods such as uranium-lead dating. “There’s a lot of uncertainty in the ages of those rocks,” says Kenneth Angielczyk, a vertebrate paleontologist at the Field Museum of Natural History in Chicago, Illinois, who wasn’t involved in the new study. That’s particularly true because the rocks from southern regions of Pangaea don’t include occasional layers of marine sediments, which offer a different range of creatures to help correlate ages.
Now, Randall Irmis, a vertebrate paleontologist at the University of Utah in Salt Lake City and his colleagues have used uranium-lead radioactive dating to estimate the age of what are suspected to be the earliest dinosauromorphs. Unearthed from rocks in northwestern Argentina, the creatures were previously thought to have lived anywhere between 237 million and 247 million years ago. But uranium-lead dating, in which researchers estimate the age of a rock by comparing its concentrations of radioactive uranium and the lead it decays into, tells a different story.
A younger volcanic deposit lying in the rock above these fossils includes zircons, tiny bits of silicate mineral that often contain trace amounts of uranium. Those zircons crystallized about 234 million years ago, the team’s analyses suggest. And older sediments below the fossils contain zircons that crystallized about 236 million years ago, the researchers report online today in the Proceedings of the National Academy of Sciences. So, the fossilized creatures must have lived between those two dates. That means that the earliest known dinomorphs lived anywhere from 3 million to 5 million years later (or even more) than previously estimated, Irmis says. That, in turn, means that dinosaurs must have evolved faster than previously thought.
By shifting the earliest dinosauromorphs' first appearance toward the present, the findings suggest that they appeared only after ecosystems had largely recovered from the end-of-the-Permian mass extinctions, says Paul Olsen, a paleontologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York. Those die-offs, which occurred about 252 million years ago, killed off more than 90% of all species on Earth, and their ecological effects were long-lasting.
The shorter time needed for true dinosaurs to evolve suggests that the shift in ecosystems associated with their appearance was smoother than previously thought. The earliest dinosaurs and their predecessors, the earliest dinosauromorphs, “were only slightly different players,” says Sterling Nesbitt, a vertebrate paleontologist at Virginia Polytechnic Institute and State University in Blacksburg. The creatures in those two groups looked and behaved pretty much the same, but “dinosaurs were slightly larger overall,” he notes. So, an overall mix of creatures dominated by early dinosaurs didn’t really look that much different from earlier ones where dinosauromorphs were predominant, he notes.
The question now, Nesbitt says, is why early dinosauromorphs eventually faded away while dinosaurs as a whole rose to dominance for almost 170 million years.