by Brian Switek
On this clear October morning, I have a date to meet with the Petrified Forest paleontologist Bill Parker.
I was on my way home from a science writing conference in Arizona, but couldn’t resist the opportunity to take the wide detour, chat with Parker in person, and take a peek at the dinosaurs hiding in the Petrified Forest’s collections. It would be a great chance to learn more about the creatures that inspired the awful highway Americana and to piece together two intertwined mysteries—what a dinosaur actually is, and what allowed dinosaurs to thrive for so long.
I had made Bill’s virtual acquaintance a few years before. In the tiny circle of paleontology blogs, Bill’s is a great one. He runs Chinleana, which focuses on his own research and related studies. The blog’s name comes from the Chinle Formation, an expanse of 228- to 200-million-year-old Late Triassic time that’s exposed at Petrified Forest and elsewhere.
By the time I roll up to the park’s gift shop and visitor center, Bill has just a few minutes to spare. An administrative meeting sometimes takes precedence over a dinosaur fanatic. But that’s okay. Even five minutes among the park’s collection of fossils is worth the trip. On our way past the park’s offices to the fossil preparation and storage space, I explain to Bill that I’ve stopped by to learn more about Petrified Forest’s early dinosaurs. Bill knits his eyebrows. “Well, we don’t have that many,” he says. He leads me to the park’s fossil collection to explain why.
Beneath the storeroom’s blue-white fluorescent lights, Bill kneels down and unlocks a squat olive-green safe—the holotype cabinet. This is the secure box where the park keeps important fossil specimens that have been used to establish new species of Triassic life. Bill slides out a drawer containing what seem to be mundane scraps of bone, but those small pieces are the chief representatives of an enigmatic early dinosaur named Chindesaurus. The fossil is distinctive enough to deserve its own name, but so little is known of the animal that it’s difficult to tell exactly what it would have looked like or what its closest relatives were. Scattered fragments, like those resting in the cabinet drawer, are all that paleontologists have found so far. And that is actually typical of most dinosaurs. Contrary to what Steven Spielberg’s adaptation of Jurassic Park depicted, paleontologists usually find only isolated bones and fragments of dinosaurs. Partial skeletons are rare, and complete, articulated dinosaurs are even more elusive.
That’s why paleontology relies on comparative anatomy to reconstruct ancient life. Scientists compare scrappy skeletons to more complete ones from closely related forms to fill out the overall appearance of the whole animal. The trouble is, sometimes there isn’t a particularly close relative to serve as a stand-in, and that’s the case with poor incomplete Chindesaurus. We know a bit more about the only other dinosaur found in Petrified Forest. Called Coleophysis, this was a slender carnivore that had much to fear from bigger, more powerful predatory neighbors. The dinosaur was not anywhere near as big or as gruesome as the terrible models that stand a few miles outside the park.
Until recently, paleontologists thought that there might have been a third dinosaur in Petrified Forest. After locking up the holotype drawers, Bill points out a couple of skull casts sitting on the top of a long row of cabinets running down the room. The reconstructed heads could fit comfortably in the palm of my hand, and they look nothing like that of any animal alive today. The restored cranium looks a bit like a deep alligator skull that has been smushed into a shorter form, with rounded teeth. This is Revueltosaurus, Bill says, the “dinosaur” that wasn’t.
For years, paleontologists assumed that Revueltosaurus was an elusive form of dinosaur. Adrian Hunt named the animal in 1989, when Revueltosaurus was represented by nothing more than a collection of battered teeth. Based on their structure, however, Hunt thought that they were once set in the mouth of a particular form of early ornithischian dinosaur (a precursor of the horned ceratopsians, shovel-mouthed hadrosaurs, and other forms that would come later). This was important. While paleontologists had found definitive skeletons of saurischian dinosaurs in the Triassic of North America (such as Coelophysis, an early relative of carnivores such as Allosaurus and a distant relative of the titanic sauropods), no one had ever found an ornithischian. Up until this point, half the dinosaur family tree appeared to be missing from the continent during Triassic time. All Hunt had was teeth, but those teeth hinted to him that there were skeletons just waiting to be found. But when a skull with teeth was finally found, it turned out that Revueltosaurus wasn’t a dinosaur at all. The teeth fit into the mouth of a very different animal that was more closely related to crocodiles than dinosaurs.
Just as ornithischians and saurischians were kinds of dinosaurs, the entirety of Dinosauria was just one branch in a wider family tree called the Archosauria—the justly named “ruling reptiles.” Today there are only two types of archosaur left—birds and crocodylians. During the past 250 million years, though, there were many, many other types of archosaurs, all of which died out at one time or another.
Birds and crocodiles are our modern guides to this family tree. The Archosauria is typically split into those forms that were closer to birds (the Avemetatarsalia) and those that were closer to crocodiles (the Pseudosuchia). The crocodile side is the one that Revueltosaurus is anchored to, and Parker, with a few colleagues, concluded that it was probably a primitive relative of the aetosaurs. These well-armored archosaurs looked like a crocodylian version of a pig, and a pig decorated with bone plates and spikes at that. And these long-lost croc cousins were just one marvelous form of life peculiar to the Triassic.
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Chinle time was an age of wonderfully fantastic life. If you could travel back to this part of the Triassic you’d see many vaguely familiar creatures, harbingers of what was to come, but they wouldn’t look exactly like anything we see around us today. The dinosaurian ancestors of birds were rare, slender creatures; the closest relatives of the first mammals were either hulking tusked holdovers from an earlier era or tiny shrew-like fuzzballs; and a profusion of bizarre crocodile relatives ruled the land, from armor-encased omnivores to aquatic ambush predators and bipedal dinosaur mimics. The Late Triassic was when reptiles began to rule, but the earliest dinosaurs only hinted at the potential of what was to come. Growing up, I had often heard that the Triassic was the “Dawn of the Dinosaurs,” but the more I’ve learned, the less apt this title seems. Triassic dinosaurs were the prehistoric equivalent of Chekhov’s gun—loaded with potential, but not set to go off until much later in the world’s evolutionary story.
My visit with Bill just makes me anxious to see more fossils, especially the bones of the archosaurs that reigned before dinosaurs dominated the planet. I had come here looking for dinosaurs, but seeing the skull of Revueltosaurus reminded me that there was far more to the Triassic story than my favorite prehistoric animals.
Fortunately, a few of the weird Triassic creatures are featured in another Petrified Forest National Park museum farther down the park’s main roadway. I cruise past the desert hills, thinking about what life must have been like during this time. The crumbled remnants of Triassic trees—red and purple fossil chunks against the gray sediment—have tumbled out of the hills and road cuts to create a surreal vestige of prehistoric forests. Here, the forest primeval has gone to pieces. I want to ditch my car at a turnout and wander among the fossiliferous hills, but that will have to wait for another day. I have only an hour to explore the Rainbow Forest Museum before I need to drive back to Utah.
There aren’t any dinosaur skeletons among the park’s brightly lit displays. The Rainbow Forest Museum makes it abundantly clear that when the fossil-bearing rocks of Petrified Forest were laid down, the croc-line archosaurs ruled the land—as long as you read the signs and don’t assume that any extinct creature with big teeth was a dinosaur. There are a few skeletons that look weird and ancient enough to be dinosaurs, but they actually represent very different animals. Just beyond the small gift shop is a phytosaur skull in
a glass case. The creature’s head resembles that of a crocodile, but from years spent poring over books and technical papers, I know that this isn’t a crocodile at all. The easiest way to tell is by the placement of the nostrils, which are at the tip of the snout in crocodiles, but up near the eyes in this trap-jawed archosaur. And, really, we should call crocodiles “phytosaur-like” rather than the other way around. These aquatic ambush-predators perfected the wait-and-strike aquatic ambush tens of millions of years before crocodiles would do the same.
The vertebrate family tree. Archosaurs—the greater group to which dinosaurs belong—is at the upper right. Crocodylians and dinosaurs (including birds) are evolutionary cousins within the Archosauria. (Illustration by Jeffrey Martz)
I notice a trio of skeletons menacing one another in the next display over. There are no dinosaurs here, either. On the left squats Placerias—a tusked, beaked quadruped more closely related to us than to any reptile. This dopey-looking animal was one of the last of its kind, a holdover from a time when the forerunners of mammals were the dominant creatures on land. This herbivore, technically called a dicynodont, had a skeleton that looks like an osteological barrel with sturdy limbs and a tusked turtle skull attached. This was the big, plodding plant eater of its era, and it was undoubtedly food for the park’s Triassic apex predator, Postosuchus. The terror of Petrified Forest was a deep-skulled, knife-toothed carnivore most closely related to the forerunners of crocodiles. Postosuchus looked like a crocodile’s impression of a tyrannosaur—a bulky animal with legs tucked beneath its body, able to run down prey on two legs as well as four. This is the stuff nightmares are made of.
On the right side of the diorama is Desmatosuchus. This was an aetosaur, or, as those who study these creatures lovingly call them, an “armadillodile.” The name fits. Covered in long sets of bony plates and bearing a set of curved spikes over the shoulders, this blunt-nosed creature was an omnivore that sliced plants and grubbed for food in the ancient dirt. Not all the prehistoric croc forerunners were out for blood.
These Triassic players, and their neighbors, come to life in the Rainbow Forest Museum’s next room. A recently installed mural by artist Victor Leshyk presents the whole Triassic fauna. The painting, simply titled “Late Triassic Life,” hardly contains any dinosaurs at all, but embodies an essential lesson about the Triassic. A phytosaur floats in the foreground, an aetosaur snuffles around on a distant bank, a Postosuchus snags a hapless little archosaur by the tail, and, right in the middle, another phytosaur bursts out of the water to snatch a Coelophysis from the water’s edge. That’s how marginal the dinosaurian presence is at Petrified Forest. The painting doesn’t show ravening hordes of Coelophysis killing and outmaneuvering the competition. At Petrified Forest, and elsewhere, dinosaurs were pipsqueaks who sneaked by in a world haunted by larger, more powerful creatures.
A gallery of Late Triassic creatures from Petrified Forest National Park’s Chinle Formation. The snap-jawed phytosaurs (along the bottom), heavily armored aetosaurs (in the middle), and varied other archosaurs (such as Postosuchus, Effigia, and Poposaurus at left) ruled the land. There were only two dinosaurs here—Coelophysis and Chindesaurus (at top center). (Illustration by Jeffrey Martz)
I don’t like seeing Coelophysis as phytosaur chow. At the time of the restored scene, around 225 million years ago, dinosaurs had already been around for about 5 million years. The Age of Dinosaurs had already started—didn’t the phytosaurs and other would-be predators have enough sense to know that? I know my internal discomfort is silly. From the very start, dinosaurs were parts of complex ecosystems where some acted as predators and even more were prey for other creatures. As I leave the museum and wind my way back through the painted hills toward the interstate, I imagine the scattered pieces of fossil wood reassembling themselves into a dense, dark forest, not unlike the West Coast’s redwood groves. In my mind’s eye, a Postosuchus patrols the forest floor, and off in the background—just visible as a glimmer of fuzz and scales—a dinosaur skitters away.
The denizens of the Triassic forest didn’t bow to the dinosaurs. Coelophysis was like a ghost on the landscape, a dinosaur that didn’t embody the classic image of reigning magnificence. Ultimately, though, the dinosaurs not only survived but thrived as the croc branch of the archosaur family tree was decimated. Something changed the course of life. What made dinosaurs so special? The mystery continued to nag at my mind as I headed north for Utah. Maybe there is some clue in what makes a dinosaur a dinosaur, I thought.
Dinosaurian identity has always been in flux. In 1824, before anyone could conceive that the creatures we now recognize as dinosaurs even existed, the British naturalist William Buckland described a paltry collection of strange bones under the name Megalosaurus (“great lizard”). “Whilst the vertebral column and extremities much resemble those of quadrupeds,” Buckland told his colleagues at the Geological Society of London, “the teeth show the creature to have been oviparous, and to have belonged to the order of Saurians or Lizards.” This poorly known creature wasn’t a modest, squirming reptile, either. Based on the size of the animal’s femur, and with the assumption that the reptile was proportioned like modern lizards, Buckland concluded that Megalosaurus “would have equalled in height our largest elephants, and in length fallen but little short of the largest whales.” The second such reptile to be described (named Iguanodon by Gideon Mantell the following year) was an herbivore of equal size. These animals were strange, enormous versions of crocodiles and iguanas from a time incomprehensibly distant from the modern era.
Then Richard Owen made these bizarre animals his own. In 1842, he published an scholarly survey on British fossil reptiles. Instead of considering Megalosaurus, Iguanodon, and the more recently described armored form Hylaeosaurus as greatly expanded versions of modern lizards and crocodiles, Owen portrayed them as unique, unprecedented reptiles that represented the apex of scaly vertebrate life. United by a unique mosaic of features—from the nature of the spine to the shape of their teeth—these three animals took their place in a new group that Owen coined just for them. They were the Dinosauria, what Owen described as “fearfully great, a lizard.”
Ten years later, Owen received the unique opportunity to bring his vision of these animals directly to the public. In 1852, he was tapped to be the scientific advisor to Benjamin Waterhouse Hawkins, an artist who had been commissioned to create life-size models of dinosaurs and other fossil animals for London’s Crystal Palace Exhibition. Owen outlined his vision of dinosaurs and schooled Hawkins in their anatomy, and the artist executed the designs. The dinosaurs, resurrected life-size, looked like reptilian versions of rhinos and elephants—like putting a crocodile body on a hippo chassis, wrapped in scaly skin. (The sculptures—now grossly dated—still greet visitors to the Crystal Palace’s home at Sydenham Hill.) Owen’s versions of Megalosaurus and kin were not even close to dinosaurs as we know them today, but instead were strange reptile-mammal hybrids.
Like those before him, Owen had only scraps to work with. The knowledge of his time and Owen’s own theories melded together as he tried to create these outlandish forms. But when paleontologists across the Atlantic began to probe the fossil-bearing rocks of New England, they didn’t find anything quite like the strange sculptures Owen and Hawkins had created.
In 1858, just a few years after Owen’s dinosaurs debuted, polymath Joseph Leidy described the partial skeleton of a herbivorous dinosaur pulled from a marl pit in southern New Jersey. He named the ancient herbivore Hadrosaurus. This wasn’t a four-on-the-floor model like Hawkins’s creations, but an even more peculiar dinosaur with short, slender arms and long, robust legs. A good deal of the dinosaur’s skeleton was missing, but, combined with evidence from trackways and other partial skeletons, Hadrosaurus forced naturalists to realize that many dinosaurs were bipedal and more birdlike than they ever expected. The American paleontologists Edward Drinker Cope and Othniel Charles Marsh, as well as their British peer Thomas Henry Huxley, began exto
lling the extremely active, avian nature of dinosaurs. Dinosaurs in artistic depictions still dragged their tails and smiled with a reptilian grimace, but by the 1870s the basic nature of what dinosaurs were had finally been filled out.
Most paleontologists of the time agreed that what set dinosaurs apart was their posture. Whether they towered on two legs or four, they stood erect with their limbs directly beneath their bodies. And for decades, paleontologists thought that feature was the secret of their success. The paleontologist Alan Charig codified this idea in 1972, when he set about figuring out why dinosaurs stood with their limbs erect while crocodiles and their prehistoric relatives, as far as people knew at the time, had arms and legs that sprawled out to the side. He surveyed the hips and legs of various dinosaurs, crocodiles, and their close kin. All of the archosaurs fell into one of three categories. There were the sprawlers (they had legs thrown out to the sides and they crawled, like lizards); animals with a “semi-improved” posture (with upper leg bones held more vertically and placed at a significant angle to the rest of the body, as in crocodiles); and some—like dinosaurs—with limbs held in a column-like fashion directly beneath their bodies. This not only characterized the different archosaur limb postures, but seemed to represent a three-step evolutionary pathway leading to the unique dinosaur posture. Charig’s value-based terminology left no question that dinosaurs were superior to everything that had come before. Dinosaurs were unique in that they had a perfected stance that reduced the amount of energy required for each step, which ultimately made them faster and deadlier than anything else. The most conspicuous feature of dinosaurs gave them a competitive edge in a Triassic world where speed made all the difference.
