For me, Pellucidar has always been the most fascinating world created by Edgar Rice Burroughs, in part because AT THE EARTH'S CORE was the first book that I read by the Master of Adventure. It is also because of my interest in dinosaurs, although this is a secondary consideration because there are actually relatively few references to these animals in the series. Given the number of prehistoric animals that lived in Pellucidar, this is perhaps not surprising. Dinosaurs were the dominant animals on the outer crust for about 160 million years. But even at the peak of their diversity 75 million years ago, they were not as common as smaller animals like frogs, salamanders, turtles, lizards, crocodiles, birds and mammals. Large animals are always out-numbered (in both numbers of species and numbers of indi-viduals) by smaller animals.

    In LAND OF TERROR, we learn that the distribution of dinosaurs and other animals is not uniform in Pellucidar. Al-though most of the inner world is a vast melting pot of life where animals from all geological periods coexist, there are regions like the Phelian Swamp where there are countless thousands of dinosaurs of all sizes and varieties. In SAVAGE PELLUCIDAR, there is confirmation that there are lots of dinosaurs in Pellucidar. Abner Perry's balloons (Dinosaur I and Dinosaur II), the fabric on his airplane, and the sails on the ships were made of cured dinosaur peritonea. Although it is never specified which dinosaurs give up their peritonea for this fabric, lidi are one likely source. They were not the only animals hunted, however, because the best peritonea came from large, ferocious and ill-mannered dinosaurs. To acquire enough peritonea for his projects, there must have been many dinosaurs living in the area.

    There may be many dinosaurs in Pellucidar, but only four types of dinosaurs are named and described in detail. A few, like the duckbilled dinosaurs (probably Edmontosaurus) caught by a giant snake in the Phelian swamp, are described but not named.

    At this point, I should clarify that all large, extinct animals are not dinosaurs. The Dinosauria encompasses two major lines of animals. Saurischian (lizard-hipped) dinosaurs include all of the meat-eaters, plus the giant sauropods. Ornithopods (including the duckbilled dinosaurs), armored dinosaurs (stegosaurs and ankylosaurs) and horned dinosaurs (ceratopsians) are the main types of ornithischian (bird-hipped) dinosaurs. All dinosaurs have a common ancestor, which probably looked like one of the earliest meat-eaters, an animal called Eoraptor from the 235 million year old rocks of Argentina. Not all dinosaurs were large, and some were as small as chickens. There are many characteristics in the skull and legs that distinguish dinosaurs and all other animals except birds, which are descended from meat-eating dinosaurs. For the purposes of this article, it is sufficient to define dinosaurs by excluding animals that are not dinosaurs. Flying reptiles are the first cousins of dinosaurs, but they are not dinosaurs. Marine reptiles, including ichthyosaurs, plesiosaurs and mosasaurs, were not closely related to dinosaurs, and are not classified as dinosaurs. In fact, mosasaurs like the fifty foot long Tylosaurus were lizards that had returned to the sea.

    In TARZAN AT THE EARTH'S CORE, we meet the most peculiar dinosaur of the series. The dyrodor is a derivative of either Stegosaurus or one of its kin. It is much larger than any known stegosaur, however, and is 60-70 feet long, and 25 feet high at the hips. Like Stegosaurus, the head is relatively small, and the front legs are much shorter than the hind ones. The tail terminated in a long, horny spine and there was a pair of three foot long spikes on top of the tail near the tip. There were alternating plates of horn-covered bone along the back. The plates were up a yard high and a yard long, although there were only about one inch thick. In spite of its incredible size, it can orientate its plates horizontally, and glide, using its tail as a rudder. It was also capable of running with speed and agility. It is clearly an aggressive animal, and emits loud screams when angry.

    There are many differences between the dyrodor and our concept of what a stegosaur looks like. The largest stegosaurs known are 25 feet long, considerably less than half the length of a dyrodor. Plates along the back were almost a yard high in some specimens, however, so those of the dyrodor are actually relatively small. It has been speculated in the past that stegosaurs could lay their plates down, but this has never been widely accepted. They certainly could not have been used for gliding. In fact, the canals on the outer surface of the plates show that there was an extensive network of blood vessels, suggesting that they may have served as heat exchangers. When it was cold, the animal would turn so that the plates were perpendicular to the direction of the sun's rays to pick up heat. If the animal was too warm, it could turn into the wind and lose heat to the atmosphere. No stegosaur is presently known that has a tail ending in a spine, but all known species have at least two pairs of spikes on the tail. It is possible that the spike at the end of the tail was simply a horny covering on the tail itself. But considering all of the other differences between the dyrodor and known stegosaurs, and the passage of almost 125 million years since the last known stegosaur (Wuerhosaurus), it may well be a new character.

    The hardest thing to understand is how a 60 foot long animal, even with yard long plates on its back, can be a highly maneuverable glider. The largest known flying animals are azdarchid pterosaurs from the Late Cretaceous of about 75 million years ago. Quetzalcoatlus from Texas, Montana, Wyoming and Alberta had a wing span of more than 40 feet. To get to that size, it had to develop an astoundingly light skeleton (even the largest bones are like thin-walled balloons) to support the disproportionately long wings. To glide, even inefficiently, the dyrodor needs hollow, air-filled bones, although stegosaur bones are not hollow. It is hard to imagine this animal being light considering its incredible size. One can assume that the Pellucidarian stegosaur was carnivorous, because plant eaters of that size need huge guts to process food. This is consistent with its aggressive attack on Thoar and Jason Gridley in TARZAN AT THE EARTH'S CORE, but it is not consistent with its ancestry. Clearly the dyrodor is not Stegosaurus, although it might be classified as a unique family of stegosaurs. If a paleontological expedition is ever sent to Pellucidar again, one of the highest priorities should be to study this highly unusual animal.

    Gyor (TARZAN AT THE EARTH'S CORE) live on the Gyor Cors, the great plains of Pellucidar, approximately fifty miles east of the highest region of the Mountains of the Thipdars. There, these ceratopsian dinosaurs feed on the knee-high grasses, something that was not available to them during the Cretaceous Period. Grasses only appeared on the outer crust about 50 million years ago. Fortunately, the dentition of ceratopsians was as well adapted for chewing plants as the most sophisticated mammalian herbivores of today. Related to Triceratops, the gyor has three horns, one over each eye and one over the nose. Like cow horns, the bony cores are covered by a thick layer of epidermally derived horn. The back of the skull is extended over the neck as a bony collar. A gyor is predominately slate gray, with a yellowish belly and face. The blue bands around the eyes and red of the hood are less pronounced than the gryfs of Pal-ul-don, and the bony protuberances along the back were less colorful. A gyor is capable of moving quickly, in spite of its great size. It is about 24 feet long, a little smaller than the largest known Triceratops. It can emit a terrific bellow, and tends to be short-tempered and irritable. There is no mention that it had clawed feet like a gryf, so it presumably had the normal hoofed feet of a ceratopsian. Gyor also seems to have been strict herbivores, unlike the omnivorous gryf. Otherwise, the coloration, and habits suggest it is intermediate between Triceratops and the ceratopsians of Pal-ul-don.

    Lidi are enormous quadrupedal sauropods, first encountered in AT THE EARTH'S CORE. David Innes estimated that their length was 80 to 100 feet, with the small head towering 40 feet above the ground. Although their movements are slow, they actually walk rapidly because of their enormous strides. They can be painfully wounded in the tail, but not be aware of it for a full minute because of the great length of the nervous system. This was once serious speculation for sauropods, although it is now considered likely that the delay was only a few milliseconds. Apparently lidi are closest to a Late Jurassic dinosaur from North American that we call Diplodocus. In Pellucidar, they are only found beside the Land of the Awful Shadow on the Lidi Plains, where they browse on the "grotesque" vegetation and drink from the slow and sullen rivers. They are used by Thorians as beasts of burden. In spite of their small brains, they exhibit the intelligence of an elephant, and therefore make excellent draft animals. Sauropods undoubtedly had the smallest ratio of brain to body size of any known land animal. Yet is difficult to equate intelligence with brain size. We know from looking at dinosaur trackways and bonebeds that sauropods on the outer crust had complex behavioral patterns not expected in small-brained animals. Perhaps the brain size is linked to physiology (warm-blooded versus cold-blooded) rather than intelligence. Or, perhaps bigger brains are not necessarily better brains, an observation that can certainly be made when looking at the evolution of computers. In any case, the intelligence of lidi is confirmation that something is wrong with our perception of the small brains of sauropods and many other dinosaurs. As an aside, it is interesting to note that all dinosaurs were not small brained. Small, meat-eating dinosaurs like Troodon and Velociraptor had brain sizes well within the range of modern birds and mammals.

    J. Allen St. John was criticized for his portrayal of lidi standing upright on their hind legs. Although sauropods were quadrupedal animals, some had special adaptations that would allow them to rear up on their hind legs. The neural spine of a vertebra is a simple, plate-like or rod-like structure in most vertebrates. Diplodocus and its relatives have split neural spines in the neck, and it is assumed that ligaments ran along the bottom of the trough between the paired spines. The ligaments formed a cable for raising the neck, much like the cables in the arm of a hugh mechanical crane. Normally the split spines are only in the neck, but they do extend backwards into the trunk region of some dipodocids, raising the speculation that they may have been able to pull up their bodies in front of the hips. It was for this reason that an enormous Barosaurus skeleton was recently mounted rearing up on its hind legs in the American Museum of Natural History (New York).

    The zarith (BACK TO THE STONE AGE) is known as Tyrannosaurus rex to paleontologists. Baby zarith live in the forests of Pellucidar, whereas the adults live in more open country where they feed on thags and tandors. Up to fifty feet in length, they are obviously formidable hunters with a keen sense of smell. Since the turn of the century, paleontologists have argued about whether tyrannosaurs were hunters of scavengers. There are few large animals that are true scavengers however, and even the hyena, an animal highly adapted for scavenging and poorly constructed for the hunt, must kill a certain percentage of its food. Furthermore, the body build, jaws, and senses of sight, smell and hearing in Tyrannosaurus all suggest it was the most highly adapted large hunter of the Mesozoic. Burroughs' observations about the zarith confirms this viewpoint.

    The juvenile tyrannosaurid that chased von Horst used its taloned arm to club him into unconsciousness. Because the arms of Tyrannosaurus seem so small in comparison with the rest of the body, it was long thought that they were useless appendages. Recent studies show that they were powerfully built, and were much stronger than the arms of even the most powerful human. Furthermore the arms are relatively longer in juveniles, and therefore presumably had a function. The zarith, which appears to be a cold, calculating engine of destruction, shows the kind of intelligence that we now expect dinosaurs to have. It bellowed and roared. As the larynx does not fossilize, it was not known if tyrannosaurids made noises. However, the structure of the middle and inner ear of Tyrannosaurus shows that it had an excellent sense of hearing, leading one to suspect that it may also have vocalized.

    The dyal is the only other true dinosaur described in the books. However, this gigantic Pellucidarian version of Phorusrhacus (Phororhacos) is a dinosaur only in the sense that birds are the direct descendants of dinosaurs, and most modern paleontologists and biologists classify them as a subset of Dinosauria.

    Horibs (Snake People) and trodon (essentially winged kangaroos) might conceivably have evolved from dinosaurs, but they are so different that their ancestry is untraceable with the information we have. An intriguing coincidence involving these animals is a thought experiment conducted by one paleontologist (Dale Russell) in 1982. He wondered what would have happened if dinosaurs had not died out, especially intelligent ones like Troodon (note the similarity to the name "trodon"). The result was a sculpture of a speculative animal he called a dinosauroid, an animal that looks somewhat like ERB's concept of a horib.

    Paleontologists could learn so much about dinosaurs just by going to the heart of the Earth and observing them, even though they have had 65 million years to change since their disappearance on the outer crust. The problem is that if I were the dinosaurologists sent to Pellucidar, I probably would not want to come back!