Your inner 9-year-old may be in for a shock: A new study gives the long-standing dinosaur family tree an overhaul. Based on analyses of hundreds of fossils, the study strikes down a fundamental split of dinosaurs into "bird-hipped" and "reptile-hipped"; it also shifts the charismatic theropods—the group that includes Tyrannosaurus rex and eventually gave rise to birds—to a new spot on the tree, closer to the bird-hipped dinos. But don't throw out your dog-eared dinosaur books just yet, other researchers caution: This new family tree is likely to be debated for some time to come.
"This paper does have the potential to make us rewrite textbooks and redesign museum exhibits," says paleontologist Thomas Holtz of the University of Maryland in College Park. But, "before the American Museum of Natural History shifts all their dinosaur skeletons around, we have to remember this is just one paper."
For decades, paleontologists have separated the "reptile-hipped" saurischians, which included both herbivorous sauropods such as Brontosaurus and carnivorous theropods such as T. rex, from the "bird-hipped" ornithischians, among them herbivores such as Stegosaurus and Triceratops. But these broad groupings haven't been entirely satisfactory. Researchers have noted that some of the earliest ornithischians are very theropodlike: bipeds that may have had a taste for meat. And no one knew where to put the herrerasaurids—early, carnivorous dinosaurs that look a little like theropods and a little like sauropods.
To trace dinosaur relationships, paleontologists study fossils in minute detail, cataloging every bump and hole, each of which may be a heritable trait. They then analyze these data to construct a family tree. Matt Baron, a Ph.D. student at the University of Cambridge in the United Kingdom, noted that past analyses of dinosaur relationships often gave short shrift to the ornithischians, perhaps because they are so different from other dinosaurs. They possess unique traits such as an extra chin bone and evenly spaced, molarlike "cheek teeth," notes Baron, and previous studies assumed they were "this weird offshoot" that must have split off from other dinosaurs early. But few fossils of ornithischians date to the Triassic, the earliest era of the dinosaurs.
For their study, out this week in Nature, Baron and his co-authors used hundreds of traits gleaned from existing studies and fossils—including "lots of ornithischians that had been overlooked," he says. In many cases they relied on subtle details, from the exact shape of a tiny eye socket bone to the presence or absence of a groove on the femur. The devil is in those details—and in the selection of exactly which species and traits to analyze.
In all, Baron's team analyzed 457 traits in some 74 taxa, including some reptiles considered outside the dinosaur family. They found that despite their reptilelike hips, theropods aren't saurischians at all; instead, they and ornithischians are "sister clades," arising from a common ancestor (see graphic). The team concluded that herrerasaurids also branched off later than thought and, with the sauropods, make up a second pair of sister clades.
This shuffle implies a few things, Baron notes: For one, it suggests that sharp, recurved, and serrated teeth, as well as a hypercarnivorous diet (greater than 70% meat) evolved independently in theropods and herrerasaurids, a result of convergent evolution. And it confirms that "reptilelike hips" are not a reliable sign of shared ancestry. The analysis also supports the prevailing idea that the earliest dinosaur ancestor was a small bipedal animal with grasping hands.
The shuffled tree may upend the picture of where the dinosaurs arose, too. Most researchers have long assumed that dinosaurs were born in the Southern Hemisphere, in part because the oldest dinosaur fossils, including the 231-million-year-old Herrerasaurus and a number of early relatives, have been found in Argentina, South Africa, and other southern spots. But with herrerasaurids now considered further from the base of the dinosaur family tree, Baron argues that enigmatic Triassic fossils from the Northern Hemisphere deserve a second look as candidates for early dino ancestors. If so, Triassic sites in the United States and Europe may be a good place to ramp up the fossil hunt.
The new picture is "plausible, but not a slam-dunk," says Stephen Brusatte, a paleontologist at the University of Edinburgh, noting that the tree shuffle isn't based on new fossils, but on a new analysis of existing specimens. Researchers need to comb through the characters and evaluate the team's choices, he says. "It would be cool if they're right, [but] there's a big burden of proof when you're going against a long legacy in the literature." The authors haven't met that burden, says paleontologist Paul Sereno of the University of Chicago, because they don't explain how their analysis of traits altered the tree so dramatically.
Kevin Padian, a paleontologist at the University of California, Berkeley, says he is intrigued but reserving judgment. "It shows that with just a slightly new analysis you can overturn results," he says. The Triassic was a "crazy time, one big spring break at Cabo [San Lucas]," he adds, with many different animals bursting on the scene at once. Tracing their relationships remains a dinosaur-sized task.