The discovery of Coelophysis at Ghost Ranch is a good example of the role of serendipity in paleontological research. According to the Oxford English Dictionary serendipity is a word “coined by Horace Walpole upon the title of a fairy-tale The Three Princes of Serendip, the heroes of which were always making discoveries, by accidents and sagacity, of things they were not in quest of.” Exactly so, at Ghost Ranch.
—EDWIN H. COLBERT, THE LITTLE DINOSAURS OF GHOST RANCH, 1995
SERENDIPITY!
In the summer of 1947, paleontologist Edwin Harris “Ned” Colbert (figure 11.1) was planning a field season at the Petrified Forest of Arizona to look for Triassic fossils. He had spent summers during the war years teaching paleontology at the University of California at Berkeley. While he was there he frequently looked through their collections and found many intriguing fossils that had been collected by Berkeley field trips over the years. Once the war ended in 1945 and wartime restrictions over tires and gasoline had ended, he and other paleontologists were eager to get back in the field and collect new fossils. Colbert had gone through the arduous process of getting a permit from the National Park Service to collect in the Petrified Forest National Monument, which took months of getting signatures in the federal bureaucracy—all the way to the top, with the signature of the Secretary of the Interior himself.
Figure 11.1
Edwin Harris “Ned” Colbert proudly holding up one of his discoveries. (Image #334947, courtesy of the American Museum of Natural History Library)
Ned Colbert was one of the more important figures in the history of paleontology in the twentieth century. Born in Clarinda, Iowa, in 1905, he grew up in Maryville, Missouri. As a young student at the University of Nebraska, Lincoln, he worked for Edwin H. Barbour of the Nebraska State Museum. There he got his first taste of working in museums, collecting fossils in the field, and preparing them for display. This led him to get his Ph.D. at the American Museum of Natural History and Columbia University in New York, where he was trained by Osborn’s colleagues and protégés, including the great anatomist and paleontologist William King Gregory. His thesis work was not on dinosaurs at all but on Miocene mammals from the Siwalik Hills of Pakistan, which had been collected by the legendary Barnum Brown (see chapter 21) for the American Museum back in the 1920s but never studied or published. As he was finishing his thesis, Barnum Brown retired, and Colbert was named to replace him and study the fossil reptiles in the museum (mostly collected by Brown). Meanwhile, he married Margaret Matthew, the daughter of legendary American Museum paleontologist William Diller Matthew.
A modest, soft-spoken man in a field dominated by outsized personalities like Cope, Marsh, and Osborn, he was a curator at the American Museum for the next 40 years. There he worked on a wide variety of fossil reptiles and amphibians, wrote many popular books on dinosaurs, and helped design the dinosaur halls that were the highlight of the American Museum until they were updated in the early 1990s. In 1969, he capped his career with the discovery of fossils of the dog-sized protomammal Lystrosaurus in Antarctica. This creature was already known from Africa, South America, and India. His discovery confirmed that Antarctica was part of Pangea during the Permian and Triassic (as the new field of plate tectonics had predicted).
Colbert retired from the American Museum in 1969, and then moved to Flagstaff, Arizona, where he was an honorary curator at the Museum of Northern Arizona. He lived to the ripe old age of 96, passing away in 2001. His grandson, Matthew Colbert, is also a paleontologist. He is named after two distinguished paleontologists: his great-grandfather William Diller Matthew and, of course, his grandfather Ned Colbert.
(On a personal note, when I came to the American Museum in 1976 for my own graduate education, Ned Colbert often came back to visit. I met him several times and had nice chats with him about the legendary paleontologists he had known: Osborn, Matthew, Brown, Gregory, Barbour, and so many more. He was once Osborn’s assistant as well, and Osborn met and shook hands with Cope, Marsh, Darwin, Huxley, Haeckel, and the great naturalists of the late 1800s, so I’m only two degrees of separation from them.)
As he reached the field in the summer of 1947, Colbert met with fellow American Museum scientist George Gaylord Simpson, then they took the train to Albuquerque and picked up Army-surplus jeeps to begin their field season in the Southwest. Simpson was working on early Cenozoic mammals of the San Juan Basin of northwestern New Mexico, and Colbert intended to drive west and begin his season in the Petrified Forest. However, before he left New Mexico he wanted to check out a locality just north of Abiquiu, which had yielded fossils he saw in the Berkeley collections. It was known as Ghost Ranch (figure 11.2).
Figure 11.2
Panoramic view of Ghost Ranch, New Mexico, showing the cliffs of Lower Jurassic dune deposits known as the Navajo Sandstone (first bench midway up the cliff), underlain by slopes of the brick-red floodplain shales of the Upper Triassic Chinle Formation, which yields early dinosaurs and many other fossils. Above the Navajo Sandstone are the Lower Cretaceous Dakota Sandstone and Mancos Shale. (Courtesy of S. Lucas)
Ghost Ranch today has a very popular conference center, with a spectacular view of the scarlet and tan and brick red and brown cliffs of the amphitheater. It was once the home of famous American artist Georgia O’Keefe, who used the spectacular cliffs in the background in several of her paintings. According to the Pack family, who ran the property for many years, Ghost Ranch got its nickname back in the days of the Spanish. One of the sons of the original ranch family, the Archuletas, had killed his brother over a dispute, and his ghost still wanders in the lonely canyons. According to the legend, the “curse of Cain” seemed to have haunted the descendants of that family, and many others who have lived there since. Whatever the truth, the place was certainly not cursed for Ned Colbert. In fact, it gave him extraordinary luck instead.
Colbert and his crew drove up to Ghost Ranch, and the owners let them live in one of the comfortable spare ranch houses. His crew consisted of George Whitaker, a young preparator at the American Museum, and Tom Ierardi, a professor at City College in New York and an enthusiastic fossil collector who had worked with Colbert before. On June 19, 1947, they began prospecting across the badlands exposures of brick-red mudstones of the Upper Triassic Chinle Formation, finding bone scraps and a few better Triassic fossils, such as the crocodile-like archosaurs known as phytosaurs. On June 22, they were still prospecting when a very excited George Whitaker came running over the hill to get Colbert and Ierardi to come see what had found. As Colbert wrote:
As he scrambled up the slope toward us it was evident that he was very excited. He had a right to be; in his hands he had some tiny bone fragments, including a well-preserved claw. They were the fossil remains of Coelophysis, a small Triassic dinosaur—at least that was my identification of these fragments, and I was pretty confident I was correct…. We all scrambled down the talus slope where George had joined us, crossed an intermittent rivulet—a tributary of Arroyo del Yeso—and climbed up the opposite talus slope to the level at which George had found his bone fragments. There we found small pieces of fossil bone scattered in moderate abundance.
It seems that paleontologists often make important discoveries just at quitting time, often on the last day of the field season. George found these fragments just before lunch time. So we went back to camp, about a quarter of a mile away, for some nourishment and rest after a long morning, and particularly for some discussion of the discovery that he had made. Needless to say, we returned in short order to the discovery site. We soon found, almost completely covered by loose talus, the stratum of rock that contained the fossils. So we cleared away the soft sand of the slope and began to carefully dig back into the hill to uncover the fossil layer, using small awls and other hand tools. It was clearly not at this stage an operation calling for picks and shovels. The more we dug, the more fossils we found, so that before the afternoon was over we were beginning to realize that here was a most unusual concentration of dinosaur bones—not the huge bones of giant dinosaurs of song and story but rather the tiny bones of ancestral dinosaurs, dinosaurs that lived before the giants.
The excavation continued a few more days until it became clear that it was a huge deposit with many intact skeletons. Colbert realized that Whitaker had made an incredible discovery that would require all their resources, yet he still had the hard-won permit to go work in the Petrified Forest. What to do? He was worried about annoying all the people in the Department of Interior who had approved his permit, only to find out that he did not use it—but the Ghost Ranch find was the opportunity of a lifetime. On June 30, 1947, Colbert got Simpson to come over from his San Juan Basin camp and inspect the new site, and get his opinion. Simpson was immediately impressed, and according to Colbert:
George most generously declared that “this was the greatest find ever made in the Triassic of North America.” Perhaps his statement was hyperbolic—made in the heat of the moment when were all excited by the nature and scope of the discovery. But he did say that—for I made note of it in my fieldbooks at the time.
Simpson and Colbert then put their heads together and made a plan. They sent for another American Museum lab preparator, Carl Sorenson, to help work the big quarry. Colbert, Whitaker, Ierardi, and Sorenson then worked through the rest of the 1947 summer field season, and several of them returned for the next few years until they had all the fossils they could remove. Meanwhile, Colbert had to contact the people at the Park Service and Interior Department and apologize for not using their permit.
(On a personal note, I knew both George Whitaker and Carl Sorensen well when I was a student at the American Museum in 1976–1982. They were both about to retire, but they told me many tales of their exploits when I used to hang out in the lab with them. Whitaker, in particular, was a genuine hero. In 1956, he was with George Gaylord Simpson on an expedition up the Amazon. In camp one day, a crew member cut down a tree, which fell on Simpson and crushed him and broke many bones in his body. Whitaker did his best first aid with splints and bandages, then spent a week canoeing down the Amazon to rush him to the nearest town where he could be flown out to a real hospital. Without George Whitaker, the legendary paleontologist George Gaylord Simpson would have died in the Amazon.)
Sometimes serendipity strikes and you have to snatch the opportunities in front of you rather than doggedly stay with your previous plans. It happens all the time in science. The Bell Lab engineers who were trying to get the background noise out of their microwave antennas accidentally discovered the cosmic background radiation from the Big Bang. Walter Alvarez, doing analyses of rocks from the end of the Cretaceous in Gubbio, Italy, accidentally discovered the evidence for the asteroid impact. Alfred Nobel accidentally found how to put together two chemicals to make a more stable explosive, dynamite. The Raytheon engineers, stuck with excess magnetrons after World War II ended, discovered that they melted a chocolate bar in their lab coats—and the microwave oven was born.
Figure 11.3
The Coelophysis bone bed at Ghost Ranch: (A) an articulated Coelophysis skeleton in a death pose, with the neck drawn back by the contraction of the nuchal ligament; (B) one of the fossil blocks being jacketed in burlap and plaster by Carl Sorensen; (C) the Coelophysis quarry as it looks today, just a scooped out bench in the Chinle slope in the center lower left part of the photo. ([A] Photograph by the author; [B] courtesy of the American Museum of Natural History Library; [C] courtesy of S. Lucas)
The Coelophysis bone bed at Ghost Ranch turned out to be a truly unique discovery. Several thousand bones were found, making up many dozens of articulated skeletons (figure 11.3A). All of these were found before the American Museum had reached its limit of time and money to excavate the bone bed. The quarry floor eventually measured 9–12 meters (30–40 feet) wide and was dug 5 meters (15 feet) into the hillside. The crew had the ranch foreman Herman Hall use a bulldozer to dig a road to the quarry, then scrape away the overburden. They left about 30 centimeters (one foot) of sediment above the bone layer intact so they could carefully dig down and expose the bones without risking damage from a bulldozer blade. The delicate specimens had to be doused with shellac and other hardeners as soon as they were exposed, then covered with rice paper, and finally placed in plaster bandages (figure 11.3B), making large blocks about 1.5 by 1.5 by 0.6 meters (5 feet by 5 feet by 2 feet) in dimension and covered in plaster jackets that would protect the fossils during transport. As the work proceeded and the heat increased that summer, they even built a temporary wooden roof over the quarry to protect the bones from the rain and the workers from the blistering sun. Once the blocks were completed, they were flipped with a chain hoist, and the workers waited with bated breath, hoping that they would not crumble as they were flipped. The newly exposed surface was covered with additional plaster jackets, then allowed to dry in the sun. Then Hall used his bulldozer to drag the blocks down the road he had dug, where they were loaded onto a flatbed truck for their trip back to New York. The driver of that truck said it was one of the heaviest loads he had ever transported.
“SMALL AND TENDER” DINOSAURS
How did Colbert recognize the scrappy specimens as Coelophysis? He had done his homework and studied the Triassic fossils already known to occur in the area. The dinosaur and its name actually go back to Cope and the earliest expeditions in New Mexico. Cope had been collecting in New Mexico for a year or two when Marsh hired local collector David Baldwin to collect from Cope’s localities and send them to Marsh. But Cope soon got wind of this, promised to pay Baldwin better than Marsh did—and Baldwin was happy to jump ship because Marsh had shown no appreciation for those fossils. In 1881, Baldwin was working out of Abiquiu near what would become Ghost Ranch. Somewhere in the area, Baldwin found a bunch of small bones, which he sent to Cope with the following information on the label: “Label Sack 2 Box 1 Prof E.D. Cope Contains Triassic or Jurassic bones all small and tender. All in this sack found in same place about four hundred feet below gypsum strata ‘Arroyo Seco’ Rio Arriba Co New Mexico February 1881. no feet—no head—only one tooth. D. Baldwin—Abiquiu.” Another collection was sent later in the same year, labeled thus: “Box 2. Contains sack 3. Part of fossil dug out Gallina Canyon. April 12–May 1. Three reptile teeth. Triassic or Jurassic. 400 feet below gypsum horizon. 180 feet above grey sandstone.”
Cope discovered that Baldwin had sent him vertebrae, leg bones, pelvis bones, and rib fragments. He finally published a brief note on the fossils without illustrations in 1887. Based on the fragments, at first he called the specimens Coelurus, a small theropod from Marsh’s Morrison localities. Instead of honoring the discoverer Baldwin, however, he named it Coelurus bauri, honoring one of Marsh’s collectors, Georg Baur (who became a noted vertebrate zoologist later in his career). This was his subtle way of flattering Marsh’s unhappy minions and possibly luring them away from their miserable employment, while needling Marsh at the same time. Later that same year, Cope redescribed the same fossils, referencing them to the bizarre giraffe-necked reptile Tanystropheus from the Triassic of Europe. At this point, he recognized not only the species bauri but another he called longicollis (long-necked), and a third species named after yet another of Marsh’s assistants, Samuel Wendell Williston.
Finally, in 1889 Cope realized that the fossils didn’t belong to either of these existing genera but was a new genus, which he dubbed Coelophysis. The name means “hollow shape” in Greek, in reference to the hollow ends of the vertebral centra. The fossils were then forgotten and ended up in the American Museum when Osborn purchased Cope’s collection. Friedrich von Huene (see chapter 6) redescribed Cope’s original fossils in 1915 and illustrated them as well, but still very little was known of the mysterious Coelophysis. In 1912, Williston and Ermine Cowles Case (later the founder of the University of Michigan Museum of Paleontology) relocated the site of Baldwin’s fossils, but they could not find any new specimens. So the “small and tender” fossils were a mystery until 1947 when Colbert and Whitaker hit the bonanza. Some paleontologists think that the name Coelophysis is also invalid because the original Baldwin material is so poor and nondiagnostic. They have recommended that the Ghost Ranch fossils be named Rioarribasaurus because it is not clear to them that the Ghost Ranch fossils and Cope’s original crummy specimens from Baldwin are the same animals. Most paleontologists were horrified of the idea of dumping the familiar name Coelophysis for the Ghost Ranch fossils. In the end, they petitioned the International Commission on Zoological Nomenclature to suppress the name Rioarribasaurus and officially designate the Ghost Ranch fossils as the type specimens of the genus Coelophysis, and the commission agreed.
Setting the dispute on names aside, the Coelophysis quarry samples were truly amazing in their quantity and state of preservation. Most of the specimens were complete and articulated individuals in death poses (figure 11.3A). Nearly all had their necks drawn backward in an arc over their backs. This is a common effect caused when the nuchal ligament (which pulls the neck and head up) starts to shrink and pull backward after the animal dies. Full-grown adults of Coelophysis were about 3 meters (9.8 feet) in length and varied in weights from 15–20 kilograms (33–44 pounds). They were lightly built with long running legs and a long tail with interlocking vertebrae that kept the tail rigid and sticking straight out behind the animal (figure 11.4).
Figure 11.4
Mounted skeletons of Coelophysis in the Denver Museum of Nature and Science. (Courtesy of Wikimedia Commons)
Even though it looks superficially similar to the small bipedal fossils of Eoraptor and Herrerasaurus from the Late Triassic of Argentina (see chapter 5), Coelophysis is a much more advanced dinosaur. The shoulder girdle is fully theropod in its anatomy, and it is the earliest dinosaur known to have its collarbones fused into a furcula, or Y-shaped “wishbone.” (At one time, some scientists argued that birds were not dinosaurs because dinosaurs seemed to have lost their collarbones, and therefore birds would not have a wishbone. It turns out that these collarbones are delicate and are rarely preserved. Many dinosaur skeletons with wishbones are now known.) Unlike more advanced theropods, Coelophysis still had four fingers on its hand (although it used only three, and the fourth was tiny and embedded in tissue). By contrast, most theropods had only three fingers, and some just two. The feet have only three toes, with a tiny vestigial fourth toe, and were configured like the classic theropod foot.
Coelophysis had a long narrow head that was lightly built with thin struts of bone. The eyes faced forward, so it clearly had good stereovision for running and catching prey. Combined with the bony ring in the eye to protect it (sclerotic ring), the large eyes suggest that Coelophysis was mostly a daytime predator. Further research showed that its vision was much better than that of most lizards, and more like that of an eagle or hawk.
Coelophysis had dozens of small, sharp, recurved teeth with serrated edges on the leading and trailing edge of the tooth, showing that it was a vicious predator that could rip open smaller prey. Together with its well-developed front limbs with a wide range of motion, it clearly could reach out and grab fast prey of many sizes.
One of the surprises of the Ghost Ranch was the presence of small bones of another animal in the gut cavity of Coelophysis. Originally it was thought to be a skeleton of a juvenile Coelophysis, suggesting the adults were cannibals like some crocodilians, Komodo dragons, and other reptiles. However, more recent study of these remains suggests that these bones were not from Coelophysis at all but from a small crocodile relative known from the same beds.
Dinosaurs that resemble Coelophysis are known from more locales than Ghost Ranch. A number of similar fossils of a delicate theropod were found in the Upper Triassic beds of Rhodesia (now Zimbabwe). Named Syntarsus rhodesiensis by Raath in 1969, that name had to be abandoned when someone found it had already been used for a beetle exactly a century earlier. That’s against the rules of priority of the International Code of Zoological Nomenclature, so the name Syntarsus cannot be used for a dinosaur. Normally, one would expect the correction to be made by the original author once it is noticed. However, a group of scientists thought that Raath was dead and gave the African fossils the name Megapnosaurus instead. In 2012, paleontologist and blogger Mickey Mortimer wrote, “Paleontologists might have reacted more positively if the replacement name (Megapnosaurus) hadn’t been facetious, translating to ‘big dead lizard.’” In 2004, Raath reanalyzed the fossils and concluded that in fact the African fossils were the same as Coelophysis, so no new name (neither Syntarsus nor Megapnosaurus) was needed after all. Since then most scientists have concurred with Raath and regarded the African fossils as a closely related species, Coelophysis rhodesiensis.
To some paleontologists, the large number of individuals from the same bone bed suggests that Coelophysis lived in large packs and probably hunted that way as well. Other scientists are not convinced of this and think that the large number of specimens just happened to get trapped in the same spot, possibly around a drying water hole. They clearly died slowly and had time to dry out so their necks were pulled back before they were buried, possibly in a flash flood. However, there is a locality in Zimbabwe where over 30 individuals of Coelophysis rhodesiensis are found together in similar circumstances. This suggests that their large numbers are not a coincidence but are indications of true pack behavior.
Whether they lived and hunted together or not, the large number of complete Coelophysis specimens at Ghost Ranch provides paleontologists with an unusual window into the growth and development of a population. By analyzing the dimensions and growth lines in the thighbone, paleontologists have discovered that Coelophysis babies grew very rapidly during their first year of life. The sample had four distinct size clusters that represented separate growth stages, with one-year-olds, two-year-olds, four-year-olds, and adults more than seven years old. In addition to the differences caused by growth, both juveniles and adults seemed to show two different distinctive forms, one with more robust forelimbs and the other with gracile forelimbs. This is thought to be due to sexual dimorphism, where the males and females differ in some parts of their body. This is also suggested by the fact that the robust and gracile forms are in a roughly 50:50 ratio, typical of many populations of animals with slight differences between males and females.
Coelophysis and other dinosaurs of the Upper Triassic Chinle Formation lived in a subtropical environment about 200 million years ago. Analysis of the sediments, especially in the Petrified Forest, suggests that the environment was a broad floodplain with distinct wet and dry seasons. In addition to the Ghost Ranch bone bed, other deposits suggest that many Chinle animals died and became fossilized when they were trying to survive as the waterholes vanished in the dry season. The vegetation was dominated by primitive conifers, cycads, and ferns, and flowering plants did not exist yet. The giant logs that gave the Petrified Forest its name come from primitive conifers related to the modern genus Araucaria, which today includes trees such as the Norfolk Island pine and the monkey-puzzle tree.
The lakes and rivers of that time harbored a wide variety of aquatic animals, including gigantic flat-bodied amphibians and the crocodile-like phytosaurs. Early crocodilians are known from these same beds, but they are small and delicate with long limbs for running on hard ground; only later, when phytosaurs vanished, did they take over the niche of large aquatic predators.
On the land, a variety of reptiles formed a complex community, including large predatory archosaur relatives of crocodiles known as erythrosuchids or rauisuchians (see chapter 5) and herbivorous armored archosaurs known as aetosaurs (see figure. 5.5). The largest herbivores, however, were cow-sized herbivorous animals known as dicynodonts. They are not “mammal-like reptiles” as they say in outdated books but protomammals distantly related to living mammals. In addition to Coelophysis, a number of dinosaurs lived in the Chinle forests, including the herrerasaur Chindesaurus, the coelophysid Camposaurus, the small theropod Daemonosaurus, and a dinosaur called Gojirasaurus (“Godzilla lizard,” using the Japanese spelling of that movie monster).
The American Museum finished its work in the late 1940s, but several groups have been back to reopen the Whitaker Quarry. However, none have found the wealth of perfect skeletons that the American Museum first recovered. Colbert had so many specimens that he gave many away to other museums in exchange for specimens that the American Museum wanted. This was a generous gesture, but it hampers researchers who want to restudy the fossils. Instead of being all in the same museum, as most large collections from a single quarry are stored, they are spread across the globe. Researchers must travel to a lot of different museums at great time and expense rather than completing their research on specimens that are conveniently next to each other for direct comparison. A few were given to institutions that didn’t take good care of their gift, and they have been damaged or destroyed.
The study of Coelophysis goes on and serendipity can strike at any time. There were so many specimens that many of the blocks remain unprepared and sitting in the American Museum basement. Colbert himself didn’t get around to finishing a complete monograph on Coelophysis until 1989, almost 40 years after the first specimen was collected. In 2005, American Museum graduate student Sterling Nesbitt was poking around through these unopened field jackets, looking for some more Coelophysis to study. He opened one cast that had sat in storage for almost 60 years and immediately realized that the specimen inside was not another Coelophysis. Instead, it was primitive relative of the crocodilians. However, it had a long neck and toothless beak, as well as a bipedal build with long hind legs, so its body form converged on that of the ornithomimid dinosaurs, 80 million years before that group had ever evolved. Nesbitt and his advisor, American Museum dinosaur paleontologist Mark Norell, wrote up and published a study of the fossil in 2006, which generated a lot of publicity in the media, including a mention on the Comedy Central TV show The Colbert Report. (There is no relation between Ned Colbert, the paleontologist, and Stephen Colbert, the comedian, so far as I know.) Nesbitt and Norell named the fossil Effigia okeefae. The genus name Effigia is from the Latin word for “ghost” (as in Ghost Ranch), and the species is named in honor of Georgia O’Keefe, who had spent so many years at Ghost Ranch painting its amazing landscapes.
You never know what you might find in a museum basement. The best fossil hunting isn’t always in the field. Almost every museum is full of unstudied or poorly understood fossils that have just been sitting there for decades, waiting for a paleontologist who understands what he or she is looking at to come along. Some of them are still in their field jackets.
FOR FURTHER READING
Colbert, Edwin. A Fossil Hunter’s Notebook: My Life with Dinosaurs and Other Friends. New York: Dutton, 1980.
——. The Little Dinosaurs of Ghost Ranch. New York: Columbia University Press, 1995.
——. Men and Dinosaurs: The Search in the Field and in the Laboratory. New York: Dutton, 1968.
Farlow, James, and M. K. Brett-Surman. The Complete Dinosaur. Bloomington: Indiana University Press, 1999.
Fastovsky, David, and David Weishampel. Dinosaurs: A Concise Natural History, 3rd ed. Cambridge: Cambridge University Press, 2016.
Holtz, Thomas R., Jr. Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages. New York: Random House, 2011.
Naish, Darren. The Great Dinosaur Discoveries. Berkeley: University of California Press, 2009.
Naish, Darren, and Paul M. Barrett. Dinosaurs: How They Lived and Evolved. Washington, D.C.: Smithsonian Books, 2016.
Wilford, John Noble. The Riddle of the Dinosaur. New York: Knopf, 1985.