This is one of the most picturesque spots that I have ever seen. From our tents, we looked down into a vast pink basin, studded with giant buttes like strange beasts, carved from sandstone. One of them we named the “dinosaur” for it resembled a huge Brontosaurus sitting on its haunches. There appear to be medieval castles with spires and turrets, brick-red in the evening light, colossal gateways, walls and ramparts. Caverns run deep into the rock and a labyrinth of ravines and gorges studded with fossil bones make a paradise for the palaeontologist. One great sculptured wall we named the “Flaming Cliffs” for when seen in early morning or late afternoon sunlight it seemed to be a mass of glowing fire. On the floor of the basin, to the north, is an area of old dead sand-dunes covered by a miniature forest of stunted trees, which we first supposed were tamarisk…. To the south, the rolling plain sweeps back to the Gurbun Saikhan, the last of the prominent uplifts of the Altai system.
—ROY CHAPMAN ANDREWS, THE NEW CONQUEST OF CENTRAL ASIA
THE MONGOLIAN GRIFFIN
Most creatures of ancient mythology are clearly not real or even that similar to real animals. There is some evidence that legends about the Indian rhinoceros became distorted into the myth of the unicorn, with its horse-like body and straight horn sticking out of its forehead, as the Indian rhino has a single horn on its skull. Many other mythical beasts have some combination of real animals in them, although they don’t make much biological sense and are not based on anything that actually lived.
One of these mythical creatures is known as the griffin, a beast with the body, tail, and back legs of a lion; the head and wings of an eagle; and eagle talons for its front feet (figure 24.1A). It was a combination of the “king of the beasts” (the lion) and the “king of the birds” (the eagle), so it was doubly royal. The earliest representations of the griffin can be seen in ancient Iranian and ancient Egyptian art dating to 3000 BCE. There are griffin sculptures and images in many of the Bronze Age cultures of the Middle East, especially in Mesopotamia and Persia, as well as griffins in the throne room of the Minoan Palace of Knossos in Crete. The image remained common in Greek and Roman mythology and art, and was carried on through the Middle Ages as part of the fantastic bestiary in which ancient and medieval cultures believed. They appear in Dante’s Divine Comedy, Milton’s Paradise Lost, and many other works of literature, right through the Harry Potter universe, where they are the namesake of Gryffindor (“golden griffins” in French) House in Hogwarts. Griffins also were incorporated into architecture in the Middle Ages and later, so they are common images throughout buildings in Europe.
Figure 24.1
(A) Bronze figure of a griffin from the Roman period, between 50 and 270 CE. (B) Protoceratops skull eroding out of the Mongolian badlands, which emphasizes the beak and eyes; the arch of the frill suggests wings. (Courtesy of Wikimedia Commons)
The late Roman author Flavius Philostratus wrote this about griffins around 244 CE:
As to the gold which the griffins dig up, there are rocks which are spotted with drops of gold as with sparks, which this creature can quarry because of the strength of its beak. “For these animals do exist in India” he said, “and are held in veneration as being sacred to the Sun; and the Indian artists, when they represent the Sun, yoke four of them abreast to draw the images; and in size and strength they resemble lions, but having this advantage over them that they have wings, they will attack them, and they get the better of elephants and of dragons. But they have no great power of flying, not more than have birds of short flight; for they are not winged as is proper with birds, but the palms of their feet are webbed with red membranes, such that they are able to revolve them, and make a flight and fight in the air; and the tiger alone is beyond their powers of attack, because in swiftness it rivals the winds.”
The griffin is well established in mythology, but was there any basis for it in reality? Folklorist and science historian Adrienne Mayor of Stanford University has argued that the griffin myth has been amplified by ancient accounts of Protoceratops. Reading ancient stories, she found that the I Ching (around 1000 BCE) talks about the “dragons encountered in the fields.” Even today fossil bones from Mongolia and other parts of China are called “dragon bones” and sold in Chinese “medicine” as a ground-up powder that can cure most anything. Later accounts from the thirteenth century in China described how travelers feared the “field of white bones” and “heaps of bright white stones like bones” in the areas near the Flaming Cliffs.
About 600 BCE there are extensive accounts and art objects from the Scythian culture, a nomadic culture that originated in what is now Iran. The Scythians were among the first cultures to master mounted warfare, and they were excellent horsemen. They also had trade routes that ran all the way to China (long before the famous “Silk Road” routes followed by Marco Polo). At their peak, their empire stretched from the Carpathian Mountains in Poland to the edge of China, so they traveled through modern Mongolia on a regular basis, bringing gold mined from the Altai, Tien Shan, and Hindu Kush mountains. Their legends say that griffins guarded these gold deposits.
From the American Museum accounts and photos, Mayor noticed that when Protoceratops skulls were first exposed to weathering, they have the prominent beak, the “eagle eyes,” and a large arch of bone behind the head that could be interpreted as “wings” when only partially exposed (figure 24.1B). Some of the comparisons to known specimens are very striking. Given the abundant mention of griffins in Scythian lore, it seems possible that Scythian traders were influenced by seeing the ghostly bones of a Protoceratops weathering out of the rock. Griffins also laid their nests of eggs in the ground, and the abundance of nests full of eggs in the Djadokhta Formation would have fed this part of the legend.
Greek accounts of griffins date to about 675 BCE, which was when they first made contact with Scythian nomads, so it seems likely that they shaped the way Greeks and later cultures viewed the griffin. This does not address why even older cultures have similar concepts of the griffin; there is no clear evidence that pre-Scythian trade routes extended to Mongolia from the Middle East. It is possible that Protoceratops influenced the post-675 BCE versions of the griffin myth, but some version of that myth was already established as early as 3000 BCE.
TO THE FLAMING CLIFFS
The American Museum Central Asiatic Expeditions of the 1920s to Mongolia and China were led by Roy Chapman Andrews and paleontologist Walter Granger (see chapter 16). After spending time in the Mongolian capital Ulaanbaatar, on September 1, 1922, they headed southwest with their entire party in Dodge touring cars, followed by the slower caravan of camels bringing up the supplies. Eventually they reached a place called Shabarakh Usu (now known as Bayn Dzak), where they spotted the amazing sculpted Cretaceous sandstones of the Djadokhta Formation that they named the “Flaming Cliffs” (figure 24.2). As Andrews described it in The New Conquest of Central Asia:
My car was far in advance of the others and I asked Shackelford [the official photographer of the expedition] to stop the fleet while I ran over to the yurts for a conference with the inmates. During the time that I was gone he wandered off a few hundred yards to inspect some peculiar blocks of earth which attracted his attention north of the trail. From them he walked a little farther and soon found that he was standing on the edge of a vast basin, looking down upon a chaos of ravines and gullies cut deep into the red sandstone. He made his way down the steep slope with the thought that he would spend ten minutes searching for fossils, and if none were found, return to the trail. Almost as though led by an invisible hand he walked straight to a small pinnacle of rock on top of which rested a white fossil bone. Below it the soft sandstone had weathered away, leaving it balanced ready to be plucked off. Shackelford picked the “fruit” and returned to the cars, just as I arrived. Granger examined the specimen with keen interest. It was a skull, obviously reptilian, but unlike any with which he was familiar. All of us were puzzled. Granger and Gregory named it Protoceratops andrewsi in 1923. Shackelford reported that he had seen other bones, and it was evident that we must investigate the deposit…. Quantities of white bone were exposed in the red sandstone, and at dark we had a sizable collection. However, Shackelford’s skull still remains the best specimen, with the possible exception of the skull and jaws of a small reptile found by Berkey [one of the expedition geologists]. Granger brought in, among other things, a part of an eggshell which we supposed was that of a fossil bird, but which subsequently was recognized as dinosaurian. It was evident that the formation was Cretaceous and very rich in fossils, but at that time we could do no more than mark it as one of the localities for future work. We could hardly suspect that we should later consider it the most important deposit in Asia, if not the entire world.
Figure 24.2
The American Museum Central Asiatic Expedition in 1923. Middle row, left to right: Walter Granger, Henry Fairfield Osborn (in pith helmet), Roy Chapman Andrews, geologist Frederick K. Morris, and Peter Kaisen, an American Museum preparator who had worked with Barnum Brown in Canada. Back row, third from left: Albert Johnson, another preparator who worked with Brown; third from right, George Olsen, an American Museum preparator. In addition, there are American cooks and mechanics and numerous Mongolian crew members. (Image #251731, courtesy of the American Museum of Natural History Library).
As Andrews mentions, the very first reconnaissance visit to the Flaming Cliffs produced both Protoceratops and eggshell fragments. During the expedition of 1923, they spent much more time at the locality and collected dozens of skulls and skeletons of Protoceratops, ranging from juveniles to adults. Walter Granger and William King Gregory of the American Museum wrote up the first descriptions of this dinosaur in 1923. The name Protoceratops means “first horned face” in Greek, and the American Museum scientists could tell from the skull that it was “the long-sought ancestor of Triceratops” and the other horned dinosaurs, complete with the large frill over the back of the skull, but without any horns on the nose, face, or frill like more advanced ceratopsians.
Even more important, Protoceratops fossils were so abundant that the American Museum retrieved dozens of skulls and skeletons, some of which they swapped with other museums for fossils they didn’t have and needed. The fossils were so well preserved that even the delicate ring of bones around the eyeball (sclerotic ring) was fossilized. In addition, they collected a complete growth series from the smallest juveniles to large adults, one of the very few dinosaurs known from more than a handful of juvenile specimens. These were mentioned by Granger and Gregory in 1923 and formed a striking exhibit in the American Museum that is still there today (figure 24.3). In 1976, Peter Dodson restudied these juvenile specimens and applied modern methods of analyzing growth and development to them. Over the years, more and more Protoceratops have been found by other expeditions that have visited the Flaming Cliffs, including the Soviet expeditions of the 1950s, the Polish-Mongolian expeditions of the 1960s and 1970s, and the return of the American Museum expeditions collaborating with the Mongolian Academy of Sciences in the 1990s.
Figure 24.3
Growth series of Protoceratops skulls on display at the American Museum. (Photograph by the author)
Protoceratops was not a big dinosaur, reaching about 1.8 meters (6 feet) long and 0.6 meters (2 feet) high at the shoulder (figure 24.4). It may have weighed about 180 kilograms (400 pounds), about the size of a large sheep. Like other ceratopsians, Protoceratops had a sharp beak on its snout made of the rostral bone, a feature unique to the ceratopsians and some of their ancestors such as Yinlong. The large frill over the back of the head and neck were perforated with large holes or “windows” (fenestrae in anatomical terms), which made them lighter and may have added attachment points for jaw muscles. The development of this frill from a tiny edge of bone to the broad flaring structure of adults is one of the most striking features of their development. Initially it was thought that the frill was mainly to protect the neck, but more recent analysis has argued that it wasn’t very effective as neck protection. It is more likely that it served as a display structure to communicate with its own herd and advertise its age and status. The fact that the frill grew dramatically from juveniles to adults is consistent with its ability to advertise the age and strength of adults, comparable to the way larger horns or antlers in adult antelopes and deer show who is boss.
Figure 24.4
(A) George Olsen and Andrews in Mongolia in 1925 excavating a nest of dinosaur eggs. (B) Two mounted skeletons of Protoceratops, with a nest of eggs once thought to belong to this dinosaur, on display at the American Museum. ([A] Image #410760, [B] image #324205, courtesy of the American Museum of Natural History Library)
There is good evidence that Protoceratops had a powerful bite and was able to chew tough vegetation, especially with its dental battery packed with small, wedge-shaped teeth (a feature distinctive to ceratopsians as well). It had large eyes, consistent with the idea that it could see well in the dark and may have been nocturnal. We know that it coped with another night dweller, the turkey-sized dromaeosaur Velociraptor, because of the famous fossil of a Protoceratops with a Velociraptor attacking it, then both dinosaurs dying as they fought, and buried in sand (see figure 17.4B).
DINOSAUR EGGS—AND THE SLANDEROUS NAME
In the 1922 season, the American Museum found a few eggshell fragments but left the Flaming Gorge after only a brief visit. When they came back in 1923, they spent a long time collecting there, and they found not only lots of Protoceratops but several nests full of eggs (figure 24.4). Dinosaur eggs from the Cretaceous of the Provence region in France had been collected and described by a Catholic priest, Father Jean-Jacques Pouech, in 1859, but he thought they were just fossil bird eggs. They were not recognized as dinosaurian until much later.
Andrews and the American Museum staff did not know about the Provence discovery, and they were justifiably excited to find intact nests full of oblong eggs, all in the position in which they had been laid.
On July 13, George Olsen [American Museum preparator and field collector] reported at tiffin that he had found some fossil eggs. Inasmuch as the deposit was obviously Cretaceous and too early for large birds, we did not take his story very seriously. We felt quite certain that his so-called eggs would prove to be sandstone concretions or some other geological phenomena. Nevertheless, we were all curious enough to go with him to inspect his find. We saw a small sandstone ledge, beside which were lying three eggs partly broken. The brown striated shell was so egglike that there could be no mistake. Granger finally said, “No dinosaur eggs ever have been found, but the reptiles probably did lay eggs. These must be dinosaur eggs. They can’t be anything else.” The prospect was thrilling but we would not let ourselves think of it too seriously and continued to criticize the supposition from every possible standpoint. But finally we had to admit that “eggs are eggs” and that we could make them out to be nothing else. It was evident that dinosaurs did lay eggs and that we had discovered the first specimens known to science. The eggs which had broken out of the sandstone block are eight inches long by seven inches in circumference. They are red-brown in color and are rather more elongate and flattened than those of modern reptiles; they differ greatly in shape from the eggs of any known birds, living or fossil. The outer surface of the shell is striated, with broken, longitudinal rugosities, but the inner surface is smooth; the shell is about one millimeter thick. (Andrews, The New Conquest of Central Asia, 205)
Granger collected several nests of eggs and brought them home to the American Museum, where Osborn made the biggest publicity splash he could to promote the Museum and its discoveries and to help raise more funds for future expeditions. In the winter of 1923–1924, the American Museum held an auction for one of the eggs as a fundraiser. The highest bidder, Col. Austin Colgate, donated it to the Colgate College Museum (where it still resides). Unfortunately, the Mongolians later saw this as a sign that the American Museum had plundered their country and planned to sell their finds at a profit. After the 1930 expedition, the Mongolian government refused to let the American Museum return to the Gobi Desert, and the Museum did not get another chance to return until the 1990s when the political situation had changed many times.
The eggs themselves led to much further research on dinosaur eggs, and many new discoveries were made, as well as recognition for what the French dinosaur eggs really were. Dinosaur eggs have been found in many different places, such as Jack Horner’s famous “Egg Mountain” localities in Montana or Luis Chiappe’s Auca Mahuevo sites in Argentina. The study of dinosaur eggs is now very well established and has its own specialists.
The nests that had been recovered by the American Museum were put on display and are still on exhibit (figure 24.4B). Because Protoceratops was the most abundant dinosaur in the Djadokhta Formation, Osborn and Granger assumed that it must have been the egg layer. But in the process of excavating one of the nests, the American Museum expedition made another discovery.
In the ledge beside which the eggs lay we could see many bits of shell imbedded in the rock and it was obvious that other specimens might be enclosed in the sandstone matrix. When Olsen brushed away the loose sediment on top of the ledge, he exposed the fragmentary skeleton of a small dinosaur. It proved to represent a toothless type, and Professor Osborn subsequently named it Oviraptor philoceratops. In referring to its habits he remarks, “The generic and specific names of this animal, Oviraptor, signifying the ‘egg-seizer’, philoceratops, signifying ‘fondness for ceratopsian eggs’, may entirely mislead us as to its feeding habits and bely its character. The names are given because the type skull was found lying directly over a nest of dinosaur eggs, the one photographed being actually separated from the eggs by only four inches of matrix. This immediately put the animal under suspicion of having been overtaken by a sandstorm in the very act of robbing the dinosaur egg nest.” (Andrews, The New Conquest of Central Asia, 209)
The dinosaur they recovered, Oviraptor philoceratops, was fragmentary, so not much could be said about its broken skull. It was a weird looking dinosaur, with a toothless beak and a very bird-like skeleton, but Osborn couldn’t make much sense of it based on the broken parts he had in 1924. In the 1980s, Mongolian paleontologist Rinchen Barsbold found a number of much more complete specimens, and the appearance of Oviraptor turned out to be even weirder than anyone had imagined (figure 24.5A). It was built like a large ground-dwelling bird, almost like an ostrich, with a high crest on top of its head and an oddly shaped beak that resembled a broad scoop.
Figure 24.5
(A) The skeleton of the oviraptorid Hagryphus, with the weird crest on its head and broad toothless beak. (B) An intact nest of eggs covered by a female oviraptorid Citipati in brooding position, discovered by the American Museum crews in the 1990s. These showed that the eggs were not from Protoceratops but from the oviraptorid parents. ([A] Photograph by the author; [B] courtesy of Wikimedia Commons)
In the 1990s, the American Museum crews, under the leadership of Mike Novacek and Mark Norell in collaboration with Mongolian paleontologists, got another chance to return to Mongolia and make new collections. One of the most sensational discoveries was another nest of the “Protoceratops” eggs—but this nest was covered by an oviraptorid in brooding position (figure 24.5B)! The new oviraptorid was described in 2001 by Jim Clark, Mark Norell, and Rinchen Barsbold and named Citipati osmolskae. Citipati is Hindi for “funeral pyre lord.” According to Tibetan Buddhist folklore, the Citipati were two monks who were beheaded by a thief while in a meditative trance, and they became a pair of dancing skeletons hovering over a circle of flame (hence the comparison to the specimen, hovering over the ring of the nest). The species name honors Halszka Osmólska for her many years of work on the dinosaurs of Mongolia.
If this oviraptorid was really brooding the eggs, then who was their real mother? Several eggs were X-rayed and then opened, and inside were the embryonic bones of tiny baby oviraptors. So the answer was clear: Oviraptor and Citipati were not stealing the eggs—they were the mothers of the eggs, protecting them against real predators! Thus the name Oviraptor slanders the bearer of the name because it was not the “egg thief” but the “good mother lizard.” Unfortunately, the rules of zoological names are that you cannot change a name, no matter how inappropriate it becomes.
If the abundant eggs that Osborn and Granger and Andrews incorrectly assigned to Protoceratops were in fact from oviraptors, what did the real eggs of Protoceratops look like? A nest of actual eggs and young Protoceratops was discovered in 2011, and this time the babies and eggs seemed to match the true parents. These young dinosaurs appeared to be too weak to have fended for themselves; they were found in the nest, having grown since hatching, suggesting that the parents cared for them (as Jack Horner found with Maiasaura, the “good mother lizard”).
THE “PARROT LIZARD”
Protoceratops gave us a glimpse of an early primitive horned dinosaur with a broad frill but no horns on its nose or forehead or spikes on its frill. But from what kind of dinosaur did it evolve?
Ironically, the American Museum Central Asiatic Expeditions had found that dinosaur too. Among the more common fossils from the Lower Cretaceous (Aptian-Albian) Ashile Formation in the Artsa Bogdo basin (about 100–125 million years old) was a smaller bipedal dinosaur described by Osborn in 1923 as Psittacosaurus mongoliensis. Psittacos means “parrot” and sauros means “lizard” in Greek, so this was the “Mongolian parrot lizard.” Osborn noticed how the short, narrow head with large eyes and the prominent deep beak resembled a parrot’s head, and this was the inspiration for the name. Osborn had abundant material of this genus, including a nearly complete articulated type skeleton (figure 24.6). This specimen suggested an animal about 2 meters (6.5 feet) in length, and weighing about 20 kilograms (44 pounds). Although he gave a brief description of it in 1923, in no place does Osborn mention its similarities to the skulls of ceratopsians, including the Protoceratops from the same expedition. It is not clear from reading the paper that he realized it was a very primitive relative of horned dinosaurs.
Figure 24.6
Psittacosaurus mongoliensis: (A) articulated type skeleton in the American Museum; (B) Osborn’s diagram of the weird-looking skull, with the broad flaring cheekbones. (Courtesy of Wikimedia Commons)
Altogether, more than 75 specimens of this dinosaur and 20 complete skeletons were found in the same area of Mongolia, and hundreds representing numerous species are now known from all over Asia, including China, Siberia, and possibly Thailand as well as Mongolia. One of these skeletons was found near the original type specimen of Psittacosaurus in 1923. Osborn originally named it Protiguanodon mongoliense because it looked like a primitive ornithopod, and its skull was badly weathered and virtually uninterpretable. Subsequent study has shown that it is merely a juvenile of Psittacosaurus mongoliensis.
(On a personal note, I got to know these fossils well. When I was a student at the American Museum in the late 1970s, my officemate Daniel J. Chure was working on psittacosaurs and Protoceratops for his thesis, and he had most of the best specimens in our office so he could study them. Trying to get around these huge flat trays with specimens without damaging the delicate bones collected in 1922 was a constant nuisance. Dan has recently retired after spending his career as the paleontologist for Dinosaur National Monument.)
The large number of Psittacosaurus fossils from many different Lower Cretaceous beds in Mongolia, China, and elsewhere in Asia has led to lots of arguments about how diverse they were. There are about 11 different species currently recognized in the scientific literature, and many more that are no longer considered valid. This makes it the most species-rich dinosaur known (other than birds). Other scientists consider Psittacosaurus to be a highly variable genus, with many differences due to gender and developmental stage, so most of these species are not real biological species, in their opinion.
Because there are so many specimens from so many localities, a lot is known about the biology of Psittacosaurus. The distinctive skull (figure 24.6B) with the parrot-like beak made of the rostral and predentary bone is the most striking feature. The beak also had a horny covering to it, like many reptilian and bird beaks do over their jaw bones. The front of the beak was toothless, but small cheek teeth were self-sharpening and had a prominent crest across the top, presumably for chopping up plants, seeds, and nuts for its herbivorous diet. The type skeleton even preserved a pile of small stones in the gut cavity that were probably gastroliths, grinding stones for use in its stomach or gizzard.
Another striking feature is the flared cheekbones, which stick out almost like small horns on the side of the face. The eyes were rather large, with a ring of bone (sclerotic ring) protecting the large eyeball, suggesting that it might have been nocturnal. There was no large ceratopsian frill, but the back of the skull still has a significant shelf of bone that is true of all the Marginocephala, including pachycephalosaurs. It was long thought to have a small brain, but a 2007 study showed the brain was larger and more advanced than most other herbivorous dinosaurs, with a brain/body size ratio close to that of Tyrannosaurus rex.
Psittacosaurus was a bipedal dinosaur with small forelimbs, less than 60 percent the length of the long hind limbs. Not only were the arms too short for quadrupedal locomotion but it could not rotate its hand to bring the palms flat on the ground, so it was almost certainly bipedal as an adult. However, the limb sizes and bone structure in a study of juvenile specimens indicated a quadrupedal posture in young animals that gradually became fully bipedal as the legs grew faster than the arms. The arms were too short to reach the mouth, so they were only good for two-handed grasping of near objects and for scratching and fighting. Psittacosaurus had only four fingers on the hand, compared to five in most other ornithischians, and four toes on the feet like most other dinosaurs.
There is one complete specimen (figure 24.7) from the Yixian Formation in the Liaoning Province of China in which all the soft tissues and even the melanosomes indicating its color are preserved. The body was mostly covered by scales rather than feathers, but along the back were an array of bristles that glow like feathers under UV light. The melanosomes preserved on this specimen indicate that it was countershaded, with dark on the top and light on its belly, so it was easier for it to hide in the shade of the forests in which it lived. There were patches of color on the face for display as well as around the cloaca and on the membranes of its hind legs.
Figure 24.7
(A) The complete articulated specimen of Psittacosaurus from the Liaoning beds, showing the melanosomes with different color patches and the spine-like feathers along the backbone and tail. (B) Reconstruction of Psittacosaurus. ([A] Courtesy of Wikimedia Commons; [B] courtesy of N. Tamura)
A number of juvenile Psittacosaurus fossils are known, including a hatchling that is only 11–13 centimeters (4–5 inches) long with a skull barely 2.5 centimeters (1 inch) long. Looking at the histology of their bones, three-year-olds weighed only 1 kilogram (2.2 pounds), whereas the nine-year-olds reached 20 kilograms (44 pounds). This is rapid growth for a reptile, but it is slower than that of birds or mammals. Based on the growth rings of the bones of the largest adults, most lived only 9–11 years. There are numerous specimens of juveniles found together, including a block with six young individuals from two distinct age groups, which were buried together in a volcanic mudflow. Even the youngest hatchlings have worn teeth, suggesting that they could fend for themselves once they hatched and needed little parental care (like most modern reptiles). Another specimen had 34 articulated juvenile skeletons of Psittacosaurus closely bunched together, with a larger skull of a six-year-old individual found on top of them. The six-year-old is too young to breed, suggesting that it was babysitting the 34 tiny juveniles before they all died when their burrow collapsed on them.
Finally, we always think of dinosaurs as the rulers of the Mesozoic, and mammals as tiny shrew-sized creatures that hid in the darkness to avoid being eaten by their reptilian overlords. In one instance, however, an opossum-sized Mesozoic mammal called Repenomamus was found in China with chunks of Psittacosaurus babies in its stomach contents. Mammals were not always the victims during the 130 million years of the Late Triassic through the end of the Cretaceous when they shared the world with dinosaurs. Now and then they were the predators rather than the prey!
Along with Protoceratops and Psittacosaurus, a number of primitive Asian dinosaurs (Xuanhuaceratops, Chaoyangsaurus, Liaoceratops, Auroraceratops, Yamaceratops, Helioceratops, Archaeoceratops, Koreaceratops, and Leptoceratops) now provide a bridge between Yinlong and the pachycephalosaurs, on one hand, and the advanced horned dinosaurs, on the other. The final chapter explores the enormous radiation of the horned dinosaurs.
FOR FURTHER READING
Andrews, Roy Chapman. The New Conquest of Central Asia: A Narrative of the Explorations of the Central Asiatic Expeditions in Mongolia and China, 1921–1930. New York: American Museum of Natural History, 1932.
Colbert, Edwin. Men and Dinosaurs: The Search in the Field and in the Laboratory. New York: Dutton, 1968.
Dodson, Peter. The Horned Dinosaurs. Princeton, N.J.: Princeton University Press, 1996.
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.
Kielan-Jaworowska, Zofia. Hunting for Dinosaurs. Cambridge, Mass.: MIT Press, 1969.
Mayor, Adrienne. The First Fossil Hunters: Paleontology in Greek and Roman Times. Princeton, N.J.: Princeton University Press, 2000.
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.
Spaulding, David A. E. Dinosaur Hunters: Eccentric Amateurs and Obsessed Professionals. Rocklin, Calif.: Prima, 1993.
You, Hailu, and Peter Dodson. “Basal Ceratopsia.” In The Dinosauria, 2nd ed., ed. David B. Weishampel, Peter Dodson, and Halszka Osmólska, 478–494. Berkeley: University of California Press, 2004.