by Brian Switek
The contemporary, iconoclastic paleontologist Bob Bakker ran with this idea in an article entitled “The Superiority of Dinosaurs.” Dinosaurs were not the mental and physical sluggards they had always been portrayed as. Our mammalian bias colored our vision of the past and caused us to underestimate dinosaurs, Bakker argued. During the Triassic, our relatives—including the heavy-bodied Placerias as well as small, shrew-like creatures—were sprawlers while early dinosaurs were efficient runners. Thanks to natural selection, dinosaurs were simply better engineered, and their skeletal architecture hinted at other features—such as active metabolism and complex behavior—that made them superior to our own forebears. By the time the first mammals evolved during the Jurassic, dinosaurs had already won the day. Dinosaurs got the jump on mammals by millions of years, and Bakker argued that mammals were runners-up in life’s race for so long because they “were competitively inferior to dinosaurs during the day and were forced to seek secure diurnal shelters in trees and burrows to escape the great reptiles.”
When paleontologists discovered dinosaurs near the ultimate root of the group’s family tree, the idea of dinosaur tactical superiority received a major boost. In 1993, just as Jurassic Park stimulated dinomania to a fever pitch, the paleontologists Paul Sereno, Catherine Forster, and their colleagues named Eoraptor—the “dawn thief”—which they found in roughly 231-million-year-old rocks in Argentina’s Valley of the Moon. This was the earliest and most archaic dinosaur ever found, and it sure looked like a consummate predator. Though it topped out at only about three feet long, Eoraptor had grasping, clawed hands and a mouth set with pointed teeth. Its contemporary Herrerasaurus, discovered in the same valley years before, was even more fearsome. This ten-foot-long carnivore had a boxy skull full of recurved teeth well suited to slicing flesh. Dinosaurs, in their earliest guise, were fast, flesh-rending bastards that totally dominated that landscape.
Soon after these discoveries, the late paleontologist and Mormon bishop William Sill called the upright limb posture of these early beasts their “secret weapon” in the war between dinosaurs and other forms of Early Triassic life. Or, as I recall the narrator of the sensationalized documentary series PaleoWorld explaining, “Dinosaurs didn’t invent killing; they perfected it.” At the outset, Sill and other paleontologists said, dinosaurs were meat eaters that leaped out of the dawn and tore apart whatever they could catch. Only later, once they had fully subdued the competition, did some lineages take up a more peaceful, plant-munching lifestyle.
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The cherished tale of a rapid dinosaur coup lives on at the New Mexico Museum of Natural History and Science. I took the ten-hour drive down to the paleo-centered museum to see the imposing mount of the sauropod Diplodocus hallorum defending itself against an assault by Allosaurus maximus. (These giants—among the biggest dinosaurs of their kind—were once called Seismosaurus and Saurophaganax, but turned out to be larger versions of already known genera.) To get to the large-scale battle scene, though, you’ll need to take a chronological tour through the Triassic exhibit, and about halfway through—in a dark cul-de-sac framed by low couches—an animated short film lays out the basic principles of Triassic life. Natural selection not only explains the dramatic changes in prehistoric life, the narrator’s soothing voice relates, but also makes sense of why some creatures—such as lungfish—are so similar to their prehistoric counterparts. There is no inevitable ladder of progress that all species must climb. Natural selection drives nature’s most astonishing transformations, but if there’s no impetus for change, it can also maintain some forms for millions of years.
As the narrator affably describes, dinosaurs fall into the category of transcendent change. Preceded by platoons of squat, waddling creatures, dinosaurs were a major evolutionary improvement that quickly overtook the world. As a cartoon dinosaur stomps on one of its awkward and outmoded ancestors, the narrator explains that “one thing that makes a dinosaur a dinosaur … is the fact that the limbs are upright, directly under the body.” Standing up straight gave dinosaurs their edge, the traditional story goes, and so early dinosaurs slashed and bit their way to ultimate dominance.
But previous generations of researchers gave dinosaurs too much credit. As paleontologists have uncovered new evidence and reconsidered the old, they’ve discovered that posture alone couldn’t have been the secret to dinosaur success. In fact, dinosaurs were not the only creatures to walk tall. And this change has undermined the traditional understanding of what a dinosaur truly is. An upright posture was thought to be an easy-to-spot symbol of dinosaur identity as well as an explanation for why the creatures became so successful. Now we know that’s no longer true, thanks in part to the work of the early archosaur expert Sterling Nesbitt. I asked Sterling to help me as I struggled to parse the story of the earliest dinosaurs.
I wanted to know what separated dinosaurs from the various other stripes of archosaurs. Since posture is no longer the key factor, Sterling said, we have to move on to other clues to figure out what makes dinosaurs distinct from all their various archosaurian relatives. If you really want to separate the earliest dinosaurs from creatures that are only superficially dinosaur-like, he said, you have to get down to the anatomical nitty-gritty.
There’s no stark characteristic that the untrained dinosaur fan can settle on to tell a dinosaur from a cousin croc-like archosaur with upright limbs like Postosuchus. One of the key features that can help us determine what’s what, Sterling pointed out, is a large flange on the upper arm bone where some of the chest musculature is attached—it’s larger in dinosaurs relative to other archosaurs. There are a few other subtle characteristics as well, but as distinct as later dinosaurs were, the earliest ones lived in a world full of evolutionary copycats. Similar features and natural histories evolved multiple times among early archosaurs.
It’s counterintuitive, but new fossil discoveries can complicate our attempt to solve the twin mysteries of dinosaur identity and success. As paleontologists fill out the base of the dinosaur family tree and uncover the closest relatives of early dinosaurs, Sterling said, “most of the morphological gaps between dinosaurs and other archosaurs disappear discovery by discovery.” Even though the most famous dinosaurs—Tyrannosaurus, Apatosaurus, and their ilk—were distinct animals that were clearly different from everything else at their time, the earliest dinosaurs were not very different from their ancestors. In a way, we see dinosaurs as unique only because extinction claimed all the similar, closely related archosaurs. This is a fabulous yet intricate confirmation of evolution’s grand pattern, but it makes pinpointing the start of dinosaur history maddeningly difficult. There is no isolated, dramatic feature that determines membership in the Dinosauria.
In fact, dinosaurian uniqueness took a major hit when Sterling found an unexpected dinosaur mimic. The cryptic specimen wasn’t a fresh discovery, but a forgotten relic extracted from one of the richest Triassic localities in the world. That site is at Ghost Ranch in northern New Mexico.
When you pull up to Ghost Ranch, the place doesn’t exactly look like one of the most important dinosaur sites in North America. A quick turn off a scenic road near Abiquiu, New Mexico, the old haunt of desert artist Georgia O’Keeffe is now a retreat center run by the Presbyterian Church. The locale is strewn with bungalows and campsites. For paleontologists, working Ghost Ranch is about as luxurious as it gets—the bathhouses have laundry machines and hot showers. A paleontologist could get spoiled in a place like this.
The first time I visited Ghost Ranch, I spent almost a week picking away at the gray, bone-filled sediment of the Hayden Quarry across the road from the main entrance. But the site that made Ghost Ranch world famous among paleontologists lies deeper within the Triassic hills. The University of Utah paleontologist Randall Irmis led me there, along with the rest of a summer field crew, on a low-key afternoon near the end of our 2011 expedition. In a gully flanked by towering orange rock faces, a flow of sandy sediment cascaded away from a damaged
stone wall. Flecks of white plaster left over from excavations completed long ago dotted the loose sand. The fossils are gone now. The site was excavated for all it was worth, and a significant portion still remains in plaster jackets, waiting to be cleaned and studied, but the signs of valiant effort remain. This was a mass graveyard of Coelophysis.
The Coelophysis bonebed is what made Ghost Ranch famous among paleontologists. In 1947, Edwin Colbert scoured Ghost Ranch for fossils. The expedition was supposed to be a quick stop for Colbert’s crew on their way to the Petrified Forest, but George Whitaker, a member of Colbert’s team, found a few intriguing dinosaur fragments and encouraged them all to stay. As the group carefully picked away at the earth with awls and small tools, even more bones appeared. They had stumbled upon an immense treasure trove of tangled dinosaur skeletons.
Colbert’s team stayed at the site from June through September, just enough time to scratch the surface. The dinosaurs were laid down so thick that collecting individual specimens was pointless. Huge chunks of dinosaur-bearing rock were extracted from the quarry (one enormous doughnut-shaped block still sits in the Ghost Ranch paleontology museum, still in the process of preparation). Hundreds of Coelophysis specimens were recovered from this one spot. No one knows why the dinosaurs accumulated here in such numbers.
But the abundant Ghost Ranch dinosaurs also fell victim to what is both the persistent curse and the blessing of fossil hunting. Field expeditions often collect more than can be carefully prepared and studied, and such was the case with the Ghost Ranch fossils. Well-preserved and complete dinosaurs were prepped for study and display, but there were plenty of plaster jackets that were shelved for analysis in the future. Some sat in storage at the American Museum of Natural History for so long that no one could recall what was actually inside them.
Then Sterling came along. While working as a graduate student at the AMNH, Sterling riffled through the detailed bonebed maps Colbert had drawn in 1947. Sterling was looking for additional specimens of Coelophysis he could carefully prepare in the lab to investigate the dinosaur’s anatomy. But something else caught Sterling’s eye. The map cited a few blocks labeled “phytosaur,” and these aquatic predators were relatively uncommon in the quarry. Sterling tracked down the block and began to pick away at the sediment. Only he didn’t find a phytosaur. Peeking out of the rock was the hip and foot of a very unusual archosaur.
The creature looked similar to a poorly known archosaur called “Chatterjea” that had been described from the Triassic of Texas years before, but Sterling needed more to be sure what he was looking at. To his frustration, however, the jacket containing the front half of the animal—itself preserving the telltale clues to the archosaur’s identity—was nowhere to be found. “Another two months went by,” Sterling explained, “and the collections manager … told me that there was a Coelophysis block that had been found hiding among mammoth skulls in the fossil mammal collections.”
He wasted no time finding out what the mystery jacket held. “I raced over there and there it was, a partially prepared front half of the ‘Chatterjea’-like animal,” he recalled. “I could even see an upside-down skull.” Not only did the skull resolve some questions about the relationships of croc-line archosaurs, but the skeleton of this new animal—ultimately represented by four specimens—was extremely dinosaur-like.
In 2006, Sterling and his advisor Mark Norell named the animal Effigia okeeffeae.
The true identity of Effigia was given away by the archosaur’s ankle. Dinosaur ankles are dominated by a large, triangle-shaped bone—the astragalus—and have a very small accessory ankle bone called the calcaneum. Their ankles look like a simple hinge. But crocodile-like archosaurs have a large ankle bone that locks together into a complex unit where the connection between the ankle and foot has an S-shaped divide. This is the kind of ankle Effigia had.
Dinosaurs weren’t the only archosaurs to walk tall. Creatures more closely related to crocodiles, such as Shuvosaurus—seen here—independently evolved an upright, bipedal posture that allowed them to swiftly run around the landscape. (Illustration by Jeffrey Martz)
The articulation with the hip was also key. In dinosaurs, the head of the femur juts inward to a hole in the pelvis. But the femur in Effigia articulated with the pelvis in a different way, in a fashion similar to croc-line archosaurs rather than to dinosaurs. The evidence was clear. Though Effigia was undoubtedly a crocodile cousin, the animal walked with an upright, bipedal posture like the early dinosaurs it lived alongside. Standing on two legs, this toothless archosaur had a long neck, tiny arms, and a body counterbalanced by a long tail. The deadly and über-efficient skeletal design of dinosaurs was not unique, after all.
One of the few ways to tell a croc-line archosaur from a dinosaur is by the construction of their ankles. While the pseudosuchians had a complex joint with a large calcaneal tuber, analogous to the human heel, sticking out the back, dinosaurs—such as the ornithischian Lesothosaurus here—had a simpler hinge-like arrangement. (Illustration by Jeffrey Martz)
Effigia wasn’t an evolutionary fluke. The creature was quite similar to another animal from roughly the same time period, named Shuvosaurus. And while both Effigia and Shuvosaurus were toothless bipeds, a lovely skeleton of their close relative Poposaurus shows that there were sharp-toothed varieties, too. And all three were offshoots of a line of frightening creatures called rauisuchians—terrestrial predators like Postosuchus with deep skulls and limbs held beneath their bodies in dinosaur-like fashion.
So the upright posture of dinosaurs wasn’t a unique invention that made them an unstoppable force. “This has always been a funny argument to me,” Sterling said. Not only is it “nearly impossible” to recognize evolutionary competition among prehistoric lineages, “Effigia and Poposaurus show that dinosaurs were not the only game in town, at least when talking about stance.” Posture alone wasn’t the deciding factor. Why dinosaurs ultimately succeeded, and why Effigia and kin didn’t leave descendants, might have come down to the fact that dinosaurs “had a unique combination of characters” that somehow gave them an evolutionary advantage.
Some undecipherable factor in the dinosaurian mosaic favored Coelophysis and kin over the croc-line archosaurs. By the start of the Jurassic, about 199 million years ago, dinosaurs emerged from the sidelines and took over the ecological roles previously filled by a different cast of creatures. This wasn’t the outcome of an unbroken contest between ancestral dinosaurs and other animals in head-to-head combat. Extinction, the often-underappreciated flip side to evolution, may have created a world where dinosaurs could flourish. Even though the non-avian dinosaurs were ultimately undone by extinction, they got their shot thanks to a pair of ecological catastrophes.
Before 250 million years ago, there were no archosaurs. Landscapes the world over were filled with synapsids—the diverse array of creatures more closely related to you and me than to reptiles. Barrel-bodied, tusked dicynodonts grazed in huge herds; gorgonopsians—saber-fanged predators built like excessively muscular dogs—hunted down large prey; and the precursors of the first true mammals—the small, shuffling cynodonts—burrowed and snuffled their way around the Permian world. Then everything went to hell. A mass extinction more devastating than any before or since rapidly winnowed down Earth’s biodiversity. More than 90 percent of the known species in the oceans were lost, as well as more than 70 percent of species on land. By the dawn of the Triassic, complex and thriving ecosystems were replaced by degraded crisis communities populated by just a few species.
Exactly what triggered the Permian extinction is only known in outline. Rapid, intense atmospheric fluctuations and rampant global warming, related to massive amounts of greenhouse gases released into the atmosphere from their geological prisons, are the most likely culprits. Atmospheric oxygen plummeted, the oceans acidified, and the land baked, though not so thoroughly that life was extinguished. There were survivors, and the lineages that did make it through were faced with a world both ba
rren and full of possibility. Now that the complicated, interacting ecologies were swept away, surviving lineages were relatively unfettered and could adapt and radiate into new forms. Evolution favored the archosaurs in this new world.
The very first archosaurs originated after the effects of the Permian mass extinction ebbed. While the precise origins of the group are as yet unclear, by 244 million years ago archosaurs were already stalking across the world. Some of the earliest known archosaurs, such as Arizonasaurus, Xilousuchus, and Ctenosauriscus, looked like crocodilian renditions of a greyhound with prominent sails on their backs. And, according to the growing record of these creatures and revisions of how they are related to each other, it seems that archosaurs diversified very quickly after their origin. While Arizonasaurus was strutting through what would one day become the American Southwest, dinosaur forerunners were walking through the area which now houses Poland’s Holy Cross Mountains. The bodies of the animals in question have not yet been found, but the AMNH paleontologist Stephen Brusatte and his coauthors have presented evidence from early Triassic footprints that dinosauromorphs were there. The anatomy of the footprints matches the bone structure of the feet of these animals—not true dinosaurs, but slender long-legged creatures that comprised the group from which the first dinosaurs would later diverge. Dinosaurs, in a definitive sense, wouldn’t come onto the scene until later, but the lineage of archosaurs to which they belonged almost instantly split away from other groups in the wake of the Permian catastrophe. Recent discoveries such as Asilisaurus—a graceful Middle Triassic dinosauriform with slender legs and an extended neck—at least partly represent the sort of animal from which the first dinosaurs evolved. The defeat of the synapsids at the close of the Permian cleared the way for the progenitors of dinosaurs.
