This story has a simple moral With which the wise will hardly quarrel; Remember that it scarcely ever Pays to be too bloody clever normal expansion of the nerve cord that passes through this region, giving off nerves to the hindlimbs and continuing backwards to the tail. While this situation was to be expected, the canal - especially at the front of the sacrum - was proportionately larger than might have been expected in order merely to control the legs and tail; that is, it was much too large for just accommodating nerves. Only in living birds is this extra-enlarged situation known to occur, and in these forms there is a structure called a glycogen body that takes up the extra space. The function of the glycogen body is somewhat enigmatic, but it is thought to supply glycogen (one of the carbohydrate reserves of the body) to the nervous system, where it is used in the synthesis of specialized kinds of nervous tissue. By comparison, then, it is possible that the inordinately large expansion of the neural canal of the sacrum in stegosaurs also housed a glycogen body, much as in all modern birds. It did not function as a second brain and in all likelihood did not provide space for more than the usual amount of nerves to the hindlimbs and tail. A glycogen body seems to be the most reasonable interpretation for the enlarged canal in the sacrum of stegosaurs, but what of the function of this body? Time and more research on those living animals that have them - birds -will surely answer this on-going question.

Social lives of the With so much anatomical detail known, albeit some of it somewhat enigmatic controversial, it is perhaps surprising that the social behavior, reproduction, and growth and development of stegosaurs remains enigmatic. Simply put, we don't have much of an idea about the social behavior of stegosaurs, nor much about their life histories. For example, no nests, isolated eggs, eggshell fragments, nor hatchling material is yet known for any stegosaur. In fact, only a few juvenile and adolescent stegosaur specimens are thus far known for Dacentrurus, Kentrosaurus, Lexovisaurus, and one of the Stegosaurus species. Among fully adult individuals, it appears that there is some sexual dimorphism; that is, differences between the sexes. This shows up in, of all places, the number of ribs that contribute to the formation of the sacrum. Whether it is the male or the female that has the greater number of ribs is anybody's guess. Other ways in which the differentiation of the sexes may have been manifested are unknown, but it might be that sexual dimorphism would be found in the size and shape of the spines and/or plates, if only we had better samples. Within species, little is known about the degree of sociality among stegosaurs. The mass accumulation of disarticulated, yet associated Kentrosaurus material from Tendaguru (see Box 11.1) provides us with the notion that - in this stegosaur at least - there was some degree of herding behavior, either seasonal or perennial. In other species, however, we have no such information - they may have been solitary creatures or gregarious. Perhaps the lack of evidence of complex, social behavior is reflected in the lack of large brains in these creatures. The fossil record is simply silent on this issue, so far.

Figure 6.8. The skeleton of Kentrosaurus, a spiny stegosaur from the Late Jurassic of Tanzania. (Photograph courtesy of the Institut und Museum für Geologie und Paläontologie, Universität Tübingen.)

Plates and spines Whether or not stegosaurs engaged in herding, there are still some features of these animals that assuredly reflect their paleobiology: the spines and plates. As we have learned, the majority of stegosaurs have at least one row of osteoderms along the dorsal margin of each side of the body. And these osteoderms generally have the form of spines, either long and drawn-out spikes (as in Kentrosaurus; Figure 6.8) or as blunt cone-like affairs (as in Tuojiangosaurus). Only in Stegosaurus do the majority of osteoderms become shaped into often large, leaf-shaped plates. In all cases, at the end of the tail were pairs of long spines. All of these plates and/or spines, regardless of their position on the body, did not articulate directly with the underlying neural spines of the vertebrae, but instead were embedded in the skin (Figure 6.9).

Once thought to have solely protective and defense importance, the spines and plates of stegosaurs have begun to take on complex behavioral significance, relating not only to these aforementioned functions, but also to display and thermoregulation. On the one hand, enlarged spines and particularly plates would have provided these animals with a much larger, more formidable appearance. On the basis of studies of Stegosaurus, Bakker has argued strongly that the osteoderms were mobile at their bases and hence able to rotate from a folded down to an erect position, giving these animals a greater degree of protection from, and deterrence to, predators. Others, including V. de Buffrenil and colleagues at the University of Paris VII, disagreed. Working on the microscopic structure of the plates of Stegosaurus, they instead suggest that these

Figure 6.9. Diagram of one of the best skeletons of Stegosaurus as it was found in the field. Note that the plates do not articulate directly with the vertebrae.

plates were not particularly mobile, but rose nearly vertically on the back. In this position, the plates would have been quite useful in intraspecific display, as well as in thermoregulation.

As noted by Russian evolutionary biologist L. S. Davitashvili, by N.B. Spassov of the National Natural History Museum of Bulgaria, and most recently by Buffrenil and his colleagues, the shapes and patterns of plates and spines in stegosaurs are nearly always species specific. That is, more often than not we use the shape and size of the plates and spines as taxonomic identification markers (derived features, if you will) for stegosaur species. In all cases, osteoderms were arranged for maximal visual effect and thus have made their greatest impact during lateral display. If this interpretation of one of the functions of stegosaur osteoderms is correct - and wouldn't it be great if we could identify sexual dimorphism in their size, shape, or placement pattern? - then it is likely that individual stegosaurs would have used these structures not only to tell each other apart, but also to gain dominance in territorial disputes and/or as libido-enhancers during the breeding season.

Yet our picture of the functional significance of these osteoderms, at least for plates, is incomplete. Here we turn to work by J. O. Farlow and colleagues. Again arguing that plates would have offered poor protection from predators, these researchers analyzed these structures as heat radiators and/or solar panels, for regulating body temperature. The plates of Stegosaurus are covered with an extensive pattern of grooves, while the insides are filled with a honeycomb of channels (Figure 6.10). These external grooves and internal channels most likely formed the bony walls for an elaborate network of blood vessels.

With such a rich supply of blood from adjacent regions of the body, Farlow and colleagues argued, these plates were well designed to cool the body by dissipating heat as air passed over them, or to warm the body by absorbing solar energy. In this model, the fine control for cooling and heating would be the regulation of blood flow to plates.

As a test of these ideas, a crude model of a stegosaur with plates along its back was placed in a wind tunnel and temperature changes were

Figure 6.10. Lateral view of one of the dermal plates of Stegosaurus. Note the great number of parallel grooves, presumably conveying blood vessels across the outer surface of the plate. (Photograph courtesy of the Royal Ontario Museum.)

monitored by a thermocouple placed inside the "animal." How these plates were arranged on the model becomes critical to their thermoregulatory performance. In the case of this experiment, two patterns were tested.

First, plates were positioned as symmetrical pairs. This arrangement proved to provide more than adequate heat dissipation. When the plates were placed in alternating positions, however, they functioned much better to dissipate internal heat loads. Thus we might expect that, on biophysical grounds, the plates of Stegosaurus were arranged in alternating pairs. Yet is there any evidence as to their actual arrangement, regardless of their relative ability to absorb or dump heat?

Suggestions of how stegosaur plates were positioned date back almost to the inception of studies of the group. For it was Marsh, in 1891, who first advocated not pairs of plates but a single row down the back. This hypothesis lasted only a decade, when F. A. Lucas replaced Marsh's idea with the suggestion that the back of Stegosaurus supported paired but staggered rows of plates. This hypothesis was strongly supported by C. W. Gilmore in his stegosaur monograph. Not to be outdone, R. S. Lull of Yale argued that Lucas and Gilmore were only half right: there were indeed two rows of plates, but they were symmetrically placed (he argued that the two Smithsonian workers had erred in their staggered interpretation, because the plates had slipped after death). Recently, K. Carpenter of the Denver Museum of Natural History described new Stegosaurus material that confirms this interpretation of paired plate arrangement.

One final point on the plates of Stegosaurus. It is very interesting that juveniles appear not to have developed osteoderms on their backs. If we are right in presuming that plates functioned in display and/or thermoregulation, then their absence in small, sexually immature individuals may reflect already adequate ability in dumping or absorbing heat and the relevance of looking big and sexy as maturity is reached.

We are left with the functional significance of the long, pointed parascapular spines and the pairs of terminal tail spikes. In both cases, these appear to be the main means of defense for stegosaurs. How the flanks of the animal were protected - even with the parascapular spines - is far from clear, and perhaps stegosaurs were particularly susceptible to attack on this account. Whatever the effectiveness of the parascapular spines, the tail spikes could surely have been swished from side to side on the powerful tail in order to injure or deter rear assaults by predators. Older reconstructructions show these spikes as pointing primarily upwards; however, more recently Carpenter has produced strong arguments that the spikes actually splayed out to the sides, producing a much more effective defensive weapon.

Plate and spine function can, and should, be integrated with stegosaur phylogeny (see below) to understand their evolutionary history within the group. For example, we might imagine that, basally in stegosaur history, portions of the neck and all of the back and tail of these animals were covered with baclcwardly projecting pairs of spines. This primitive condition may have been tied to a protective function, but it's equally likely that spines across the back and tail may have been part of the visual display complex that began with the small plates situated along the neck. Perhaps these two functions were not mutually exclusive; they may have been used to bluff and establish dominance both intra- and interspecifically.

Whatever the principal function(s) of this primitive stegosaur condition might have been, these large, spiny projections could not help collecting some degree of solar radiation and/or providing an avenue for dumping heat, even if they were initially covered with horn. As osteo-derms became more plate-like down the back and at the same time show more evidence of vascularity, display and thermoregulatory functions become more important during stegosaur phylogeny. Finally, with the acquisition of a full complement of plates in Stegosaurus, there is a commitment of all osteoderms except the terminal spines to both display and thermoregulation.

Seen in this way, the story of stegosaur evolution is one of changing osteodermal patterns and functions. Moving from spiny osteoderms, whose primary functions are defensive and possibly display to a condition where display and thermoregulation are inextricably linked via platelike osteoderms, stegosaurs appear to have mastered the business of looking and feeling hot.

The evolution Stegosauria is a monophyletic clade of ornithischian dinosaurs (Figure of Stegosauria 6-ll)> defined as the common ancestor of Huayangosaurus and Stegosaurus and all of the descendants of this common ancestor. As shown by P. C. Sereno and Z.-M. Dong, the stegosaur clade can be diagnosed on the basis of a number of important features. These include back vertebrae with very tall neural arches and highly angled transverse processes (Figure 6.12), loss of ossified tendons down the back and tail, a broad and plate-like flange (the acromion process) on the forward surface of the shoulder blade, large and block-like wrist


Figure 6.1 I. Cladogram of "higher'Thyreophora, emphasizing the monophyly of Stegosauria. Derived characters at I include: back vertebrae with very tall neural arches and highly angled transverse processes, loss of ossified tendons down the back and tail, a broad and plate-like acromion process, large and block-like wrist bones, elongation of the prepubic process, loss of the first pedal digit, and loss of one of the phalanges of the second pedal digit, and a great number of features relating to the development of osteoderms, and formation of long spines of plates from the shoulder toward the tip of the tail.

I Ocm

Figure 6.12. Front and left lateral view of one of the back vertebrae of Stegosaurus. Note the great height of the neural arch.

I Ocm

Figure 6.12. Front and left lateral view of one of the back vertebrae of Stegosaurus. Note the great height of the neural arch.

bones, several changes in the pelvis including elongation of the prepubic process on the pubic bone, reduction of the toes on the feet (loss of the first digit, and loss of one of the toe bones of the second digit), and a great number of features relating to the development of osteoderms.

The sister group of Stegosauria consists of ankylosaurs, the armored dinosaurs best known from the Cretaceous Period of North America and central and eastern Asia, but also with a modest, but important, fossil record from the Jurassic of England (see Chapter 7). Together, stegosaurs and ankylosaurs make up a monophyletic group known as Eurypoda (eury - broad; poda - feet), a clade established through important work on the phylogeny of Ornithischia by Sereno. Eurypoda is united on the basis of many important shared, derived features, among them special bones that fuse to the margins of the eye socket, loss of a notch between the quadrate (the bone that buttresses the lower jaw against the skull roof) and the back of the skull, and enlargement of the forward part of the ilium (the upper bone of the pelvis). Together with a few other ornithis-chians - Scelidosaurus, Emausaurus, and Scutellosaurus - Stegosauria and Ankylosauria make up a diverse clade called Thyreophora (see Chapter 5).

Stegosaurs have been among the most resistent of dinosaur clades to cladistic analysis and, in the first edition of this book, we presented a very tentative evaluation of the shape of the stegosaur tree. Fortunately, newer research by P. M. Galton and P. A. Upchurch has brought our understanding of stegosaur interrelationships into line with that of other groups of dinosaurs.

Within Stegosauria, the basal split is between Huayangosaurus on the one hand and remaining species on the other (Figure 6.13). This divergence took place sometime before the latter half of the Middle Jurassic. Huayangosaurus itself has a number of uniquely derived features, among them the oval depression between the premaxilla and maxilla, the great number of cheek teeth, and the small horn on the top of the skull roof.




Figure 6.13. Cladogram of Stegosauria, with Ankylosauria and Scelidosaurus as successively more distant relatives. Derived characters include: at I large antitrochanter; long prepubic process, long femur; absence of lateral rows of osteoderms on the trunk; at 2 widening of the lower end of the humerus, an increase in femoral length, an increase in the height of the neural arch of the back and tail vertebrae.




Figure 6.13. Cladogram of Stegosauria, with Ankylosauria and Scelidosaurus as successively more distant relatives. Derived characters include: at I large antitrochanter; long prepubic process, long femur; absence of lateral rows of osteoderms on the trunk; at 2 widening of the lower end of the humerus, an increase in femoral length, an increase in the height of the neural arch of the back and tail vertebrae.

Stegosaurs meet history: a short account of their discovery

Yet Huayangosaurus lacks a number of important derived features that are shared by a more inclusive group of stegosaurs. This group, called Stegosauridae, appears to be monophyletic and diagnosable on the basis of a large antitrochanter, a long prepubic process, a long femur (as compared with the length of the humerus), among other features.

Within Stegosauridae, Dacentrurus represents the most basal form. The remainder of Stegosauridae, termed Stegosaurinae, includes Stegosaurus, Wuerhosaurus, Kentrosaurus, and Tuojiangosaurus. Stegosaurines are united by a variety of modifications of the vertebral column, widening of the lower end of the humerus, and an increase in femoral length.

Unfortunately, it's unclear at present which taxon is more closely related to another within Stegosaurinae. This work, along with the inclusion of other taxa - Monkonosaurus, Lexovisaurus, Chungkingosaurus, and Hesperosaurus, among others - still remains to be done.

The earliest discoveries of stegosaurs were made in England in the early 1870s, but these went largely unrecognized because of the fragmentary nature of the bones. Some were studied by the great anatomist and vertebrate paleontologist, Sir Richard Owen, of the British Museum (Natural History), while others became the subject of papers by Harry Govier Seeley of Cambridge University and Franz Baron Nopcsa of Vienna, in the early twentieth century. Originally given names that have since been changed for a variety of reasons, these first European stegosaurs now bear the monikers Dacentrurus (named in 1902 by F. A. Lucas) and Lexovisaurus (named by R. Hoffstetter in 1957).

Dacentrurus, from Upper Jurassic beds of England, France, and Portugal, is known from only partial skeletons that include portions of the vertebral column, most of the forelimb, parts of the pelvis, a nearly complete hindlimb, and a few osteoderms. These last consist of small, plate-like elements from possibly over the front part of the trunk and two more spike-like elements near the sacrum and onto the tail. Unfortunately, no skull has yet been discovered. The vertebrae of Dacentrurus have relatively low neural arches and transverse processes. The forelimbs are very stocky, with massive distal elements (radius and ulna), yet relatively small, blocky feet. In the pelvis, there is a large process (the antitrochanter) on the lateral face of the ilium, which probably served as an muscle attachment site. In addition, the prepubic process of the pubis, a strip of the pubis that points toward the head of the animal, is long and the openings in the dorsal sacral shield, through which some of the spinal nerves reach the back, are relatively small. Compared with the rest of the hindlimb, the femur is proportionately very long.

The Middle Jurassic Lexovisaurus was a medium-sized stegosaur, ranging upward of 6 m in length. Originally named "Omosaurus" by Owen, it was found that the name was already occupied by another animal. Accordingly, it was renamed Lexovisaurus. As far as we can tell, it bore on its back a series of very large, thin, plate-like osteoderms that look very much like those of Stegosaurus. The position of these osteoderms is somewhat problematic, since they were not found in place with other skeletal remains of Lexovisaurus. Nevertheless, given their size and shape, they were probably positioned toward the posterior portion of the trunk and above the sacrum. At the end of the tail was at least one pair of terminal spines. In addition to these dorsally placed osteoderms, there were elongate parascapular spines over the shoulders; these spines are much larger than those of other stegosaur taxa within the clade.1 From what is known of the vertebral column of Lexovisaurus, the neural arches of the back and tail vertebrae appear to be quite tall. The openings on the dorsal aspect of the sacral shield are small and the anti trochanter on the ilium is large. Finally, the forelimbs of Lexovisaurus are strongly built and the hindlimbs are long and massive.

Meanwhile, as these European stegosaurs were being discovered, across the Atlantic Ocean, things dinosaurian were beginning to brew -better yet, boil. For it was toward the end of the 1870s that a veritable wealth of dinosaur remains - including many virtually complete skeletons - were discovered at Como Bluff, Wyoming, and many other localities in the western USA. Stegosaurus was one of these new dinosaurs, a product of the Great Dinosaur Rush that was responsible for the early explorations of the North American Western Interior by universities and the federal government. These remains - along with trainloads of other kinds of newly discovered dinosaurs - made their way east, either to Othniel Charles Marsh of Yale University or to Edward Drinker Cope of

I The length of these spines, however; seems to have varied.The specimen from France has quite large ones; the one from England does not. Stegosaurus has no parascapular spines at all.

Philadelphia. It was Marsh who bagged Stegosaurus. In Box 6.2, we recount the bizarre story of the Marsh-Cope feud, which fueled the greatest dinosaur rush in history.

Marsh published many papers on the remains of Stegosaurus, having at his disposal a number of specimens including a nearly complete skeleton. We know this North American stegosaur, the largest member of the clade (up to an enormous 9 m in length), from a wealth of material, all of it from the famous Morrison Formation of the North American Western Interior. The Upper Jurassic Morrison Formation has yielded not only Stegosaurus, the State Fossil of Colorado, but also a wealth of other dinosaurs, among them Allosaurus, Ceratosaurus, Camptosaurus, and Diplodocus. Stegosaurus has an elongate, narrow snout, which bears a roughened margin. In life, this was covered by a rhamphotheca. The external nares are relatively large, as is the eye socket. Around the upper rim of this socket are three roughened bones, the supraorbitals, which in life probably supported a horn-covered brow-ridge. Along the side of the face is a relatively small antorbital fenestra (remember, this is one of the shared, derived features of Archosauria). In the jaws are lots of teeth, as many as two dozen, in both upper and lower jaws. These teeth were relatively simple and triangular, with denticles on the fore and aft surfaces of the crown. Were there cheeks in Stegosaurus? We think so, since the tooth rows are deeply set in from the sides of the face.

Postcranially, the body slopes considerably upward from the head to the shoulders, in large part because of the very long hindlimbs (with proportionately long femora) when compared with the forelimbs. In Stegosaurus, as in many other stegosaurs, there are small openings on the dorsal aspect of the sacrum, a large antitrochanter on the ilium, and a long prepubic process. The upsloping of the back is especially accentuated by the ever-increasingly larger, plate-like osteoderms that adorn the dorsal margin of the body. This series decreases in size down the tail, culminating in two pairs of terminal tail spines.

From the 1870s and 80s, until the early twentieth century, little was heard about stegosaurs, either in terms of new discoveries or revisionary studies. In fact, it was not until 1914, more than 30 years after stegosaurs were recognized in Europe and the USA, that an attempt was made to study comprehensively any of these animals. This work, by C. W. Gilmore of the U.S. National Museum of Natural History emphasized the important finds from the western USA, but it also provided some details of the European stegosaurs. We do not know, but perhaps Gilmore had some regrets in publishing his study just when he did, for in the next year new and abundant stegosaur material, which rivalled anything yet found from the USA was discovered in Tanzania. Named Kentrosaurus by E. Hennig (University of Tübingen, Germany - no relation of Willi Hennig, the father of cladistic analysis) in 1915, this stegosaur represented just one of the many spectacular finds from the famous Tendaguru expeditions led by Werner Janensch of the Humboldt Museum of Natural History in Berlin in the early 1910s (Box 11.1). At 5 m long, Kentrosaurus is known from hundreds of bones, most of them found

BOX 6.2 —----------------------------------------

Nineteenth century dinosaur wars: boxer versus puncher

One of the strangest episodes in the history of paleontology was the extraordinarily nasty and personal rivalry between late nineteenth century paleontologists Edward Drinker Cope and Othniel Charles Marsh (Figure B6.2.1). In many respects, it was a boxer versus puncher confrontation: the mercurial, brilliant, highly strung Cope versus the steady, plodding, bureaucratic Marsh.Their rivalry resulted in what has been called the "Golden Age of Paleontology," a time when the richness of the dinosaur faunas from western North America first became apparent; when the likes of Allosaurus, Brontosaurus, and Stegosaurus were first uncovered and brought to the world's attention. But the controversy had its down side, too. Who were these men, and why were they at each other's throat?

Cope was a prodigy, one of the very few in the history of paleontology. By the age of 18, he had published a paper on salamander classification. By 24, he became a Professor of Zoology at Haverford College, Philadelphia. Blessed with independent means, within four years he had moved into "retirement" (at the grand old age of 28) to be near Cretaceous fossil quarries in New Jersey. He quickly became closely associated with the Philadelphia Academy of Sciences, where he amassed a tremendous collection of fossil bones that he named and rushed into print at a phenomenal rate (during his life he published over 1,400 works). He

Figure B6.2.1. The two paleontologists responsible for the Great North American Dinosaur Rush of the late nineteenth century, (a) Edward Drinker Cope of the Philadelphia Academy of Natural Sciences; and (b) Othniel Charles Marsh of the Yale Peabody Museum of Natural History. (Photographs courtesy of the American Museum of Natural History.)

Figure B6.2.1. The two paleontologists responsible for the Great North American Dinosaur Rush of the late nineteenth century, (a) Edward Drinker Cope of the Philadelphia Academy of Natural Sciences; and (b) Othniel Charles Marsh of the Yale Peabody Museum of Natural History. (Photographs courtesy of the American Museum of Natural History.)

was capable of tremendous insight, made his share of mistakes, and was girded with the kind of pride that did not admit to errors.

Marsh, 9 years older than Cope, was rather the opposite, with the exception that he, too, eventually rushed his discoveries into print almost as fast as he made them (some thought faster) and that he, too, did not dwell upon his mistakes. Marsh's own career started off inauspiciously; with no particular direction, highly disarticulated. What we know of the skull is limited to portions of the braincase and skull roof. Hence we know nothing about the size or shape of the antorbital fenestra or external naris. For the rest of the skeleton, however, there is a wealth of information. The vertebrae all have high neural arches that supported as many as six pairs of plate-like osteoderms over the neck and anterior region of the back. Beyond, there were approximately five pairs of large spine-like osteoderms over the pelvis and down the tail. In addition, over each shoulder was a long, parascapular spine. Like Stegosaurus, the shoulder blade of Kentrosaurus is extremely large and is thought to have supported large muscles that helped to flex and extend the rest of the equally powerful forelimb. Muscular though they may have been, these forelimbs are relatively small when compared with the hindlimbs, as in all stegosaurs.

he reasoned that if he performed well at school he could obtain financial support from a rich uncle, George Peabody. This turned out to be perhaps the most significant insight in Marsh's life: Marsh persuaded Peabody to underwrite a natural history museum at Yale University (which to this day exists as the Yale Peabody Museum of Natural History), and, while he (Peabody) was at it, an endowed chair for Marsh at the Museum.

The careers of the two paleontologists moved in parallel: Marsh slowly publishing but acquiring prestige and rank, Cope frenetically publishing paper after paper At first, there was no obvious acrimony, but this changed when Marsh apparently hired one of Cope's New Jersey collectors right out from under him. Suddenly, the fossils started going to Marsh instead of Cope.Then, in 1870, Cope showed Marsh a reconstruction of a plesiosaur; a long-necked, flippered marine reptile.The fossil was unusual to say the least, and Cope proclaimed his findings in the Transactions of the American Philosophical Society. Marsh detected at least part of the reason why the fossil was so unusual: the head was on the wrong end (the vertebrae were reversed). Moreover; he had the bad manners to point this out. Cope, while admitting no error; attempted to buy up all the copies of the Journal. Marsh kept his.

Cope sought revenge in the form of correcting something that Marsh had done.The rivalry ignited, and the battle between the two spilled out into the great western fossil deposits of the Morrison

Formation. Both hired collectors to obtain fossils, the collectors ran armed camps (for protection against each other's poaching), and between about 1870 and 1890, eastbound trains continually ran plaster jackets back to New Haven and Philadelphia. There Marsh and Cope rushed their discoveries into print, usually with new names.The competition between the two was fierce, as each sought to "out-science" the other. Discoveries (and replies) were published in newspapers as well as scholarly journals, lending a carnival atmosphere to the debate. Because Philadelphia and New Haven were not that far apart by rail, it was possible for one of the men to hear the other lecture on a new discovery, and then rush home that night and describe it and claim it for himself. Because many of the fossils in their collections were similar; it was easy to do and each accused the other of it.

Both Cope and Marsh eventually aged, and, in Cope's case, his private finances dwindled. Moreover; a new generation of paleontologists arose that rejected the Cope-Marsh approach, believing, not unreasonably, that it had caused more harm than good. Both men ended their lives with somewhat tarnished reputations. History has viewed the thing a bit more dispassionately, and it is fair to state that the result ultimately was an extraordinary number of spectacular finds and a nomenclatorial nightmare that has taken much of the past 100 years to disentangle.1

I A superlative account of this rivalry can be found in A, J.

Desmond's book The Hot Blooded Dinosaurs.

The hindquarters must have been powerful: the anti trochanter is quite large and the prepubic process is long. On the dorsal surface of the sacrum, the openings that formerly accommodated spinal nerves to this region of the back have been obliterated.

Kentrosaurus is not the only stegosaur to have been discovered on the African continent. Originally from South African beds that date from the Late Jurassic-Early Cretaceous transition, there is an elongate snout fragment replete with very simple-looking upper teeth that has been called Paranthodon. This animal has had a rather checkered history. Originally thought to have been another sort of extinct reptile when it and a larger jumble of associated bones was described in 1876, Owen named it Anthodon. All of this material was restudied in 1912 by R. Broom of South Africa and yet again by Nopcsa in 1929. Both of these researchers noted the stegosaur affinities of the snout from the rest of the remains pertaining to Anthodon, and it was Nopcsa who provided the specimen with its final, proper name of Paranthodon.

Although China has proven to be a treasure trove of stegosaurs, it was not until 1959 that the first stegosaur, Chialingosaurus, was discovered and described by China's premier vertebrate paleontologist, C.-C. Young. Known from a partial skull and skeleton, Chialingosaurus is a medium-sized form from the Late Jurassic of Sichuan Province. Its skull is high and narrow, and there are fewer teeth in the jaws than in other stegosaurs. The limbs of Chialingosaurus are relatively slender and on the back are small, plate-like osteoderms, but their extent and position are not yet known.

At the time of the discovery of Chialingosaurus, Young did not realize what riches China - particularly central Sichuan - possessed. As it turns out, China has yielded five different species, with probably more on the way. These stegosaurs exploded onto the dinosaur scene in the 1970s and 80s, mostly through the considerable work of Dong Zhiming and coworkers. In close succession, these paleontologists discovered and named Wuerhosaurus from Xinjiang and Tuojiangosaurus, Huayangosaurus, and Chungkingosaurus from Sichuan.

Described first in 1973, the 7 m long Wuerhosaurus is the youngest of the stegosaurs from China (from the Tugulu Group of Xinjiang Autonomous Region; Early Cretaceous in age). We know very little about this stegosaur because of the fragmentary nature of its remains. What we do know, though, is that the forelimb was small, the hips relatively flaring, and the antitrochanter large. The openings on the upper surface of the sacrum are completely closed. A few osteoderms have also been recovered; these plate-like elements are large, long, and low, but their position on the back is unknown.

The late 1970s saw the description of Tuojiangosaurus, another Late Jurassic form from Sichuan Province. Along the neck, back, and forward half of the tail of this stegosaur are 17 pairs of osteoderms. Over the neck and forward region of the back, these elements are circular and platelike. From there back, the osteoderms become quite large (up to 75 cm in height), conical, and even spine-like. Down the tail, these spines become smaller, but at the end are two or four pairs of quite elongate spines. The skull is extremely long and low, with very small external nares. Like Stegosaurus, three supraorbitals rim the upper margin of the eye socket. The front of the upper jaws is edentulous (without teeth) and the pre-maxillary margin is well ridged for attachment of a rhamphotheca. There is no evidence of an antorbital fenestra on the side of the face; it apparently closed over during the evolution of the group. Both upper and lower jaws house as many as 27 cheek teeth, the record for stegosaurs. Further back in the skeleton, the holes in the dorsal sacral shield are relatively small. Both fore- and hindlimbs are massively built, but there is still the great disparity in size between the two. On the ilium is a large antitrochanter and the prepubic process of the pubis is elongate.

The early 1980s saw the discovery and description of Huayangosaurus, a 4 m long, Middle Jurassic stegosaur from Sichuan. This stegosaur has a relatively short, deep skull. Forming the front of the tooth row are seven conical premaxillary teeth, a condition that appears to be absent in other stegosaurs. There is a small, oval depression between the premax-illa and maxilla, behind which are many cheek teeth; more, apparently, than in any other thyreophoran. Above these teeth is a triangular antor-bital fenestra. Surrounding the upper margin of the eye socket are three supraorbitals. Unique to Huayangosaurus among stegosaurs, there is a modest horn on the top of the skull roof just above the eye. A small external mandibular fenestra occupies the lateral face of the lower jaw.

Unlike other stegosaurs, the vertebral column of Huayangosaurus does not appear so strongly flexed upwardly behind the head nor are the neural arches as high. There are large openings on the dorsal surface of the sacrum, the ilium has a very small antitrochanter, the prepubic process is short, all limbs are robustly constructed, and the femur is relatively short (as compared with the remainder of the hindlimb). In addition, the toes of the hindfoot appear to have been slender and somewhat more splayed apart than in other stegosaurs. Finally, running down the back of the neck and along the dorsal aspect of trunk are two rows of osteoderms, each shaped like a blunt, conical spine. There is also a lateral row of oval, keeled osteoderms across the trunk, absent in other stegosaurs but much like those in more primitive thyreophorans. At the tip of the tail, there are two pairs of tall spines, while over each shoulder is a sharp parascapular spine.

Within a year of the description of Huayangosaurus, Dong and colleagues unleashed Chungkingosaurus, one of the smallest stegosaurs yet discovered (3-4 m long). Hailing from the Late Jurassic of Sichuan, the skull of Chungkingosaurus is relatively high, with large external nares. Unfortunately, we have no idea about the nature of the antorbital fenestra because only the front of the snout is preserved. Behind the head, the vertebral column is reasonably complete. The neural arches of these vertebrae are relatively short and the holes in the sacral shield are large, in both cases again more as in Huayangosaurus than in other stegosaurs. The osteoderms over the neck and most of the back are large, thick, and plate-like. These are gradually replaced by spine-like osteoderms on the tail. At the far end of the tail are four pairs of terminal tail spines. The pelvis is somewhat reminiscent of Huayangosaurus in having large openings on the dorsal side of the sacrum. In contrast, however, the lateral face of the ilium bears a medium-sized antitrochanter and the prepubic process is long.

In the midst of this flurry of new stegosaur discoveries in China, P. Yadagiri and K. Ayyasami described a very important early Late Cretaceous stegosaur, Dravidosaurus, in 1979. This, the sole stegosaur from India and the only stegosaur of Late Cretaceous age, is known from extremely fragmentary material. Some scientists have argued that Dravidosaurus is not a stegosaur at all; instead they believe it to be a fully sea-going plesiosaur. Nevertheless, it is still regarded as a stegosaur by P. M. Galton, of the University of Bridgeport, Connecticut, one of the world's foremost specialists on this group. For example, the eye socket is rimmed by two supraorbital bones and there are a few large, triangular, thick-based osteoderms found over the sacrum (which is otherwise very poorly preserved). Dravidosaurus possessed unusually shaped tail spines, each with uniquely expanded mid-shafts.

The most recent stegosaur discovery was reported in 2001 by Carpenter and colleagues. Named Hesperosaurus (hesper - western), this form hails from the Morrison Formation (Late Jurassic in age) from Wyoming and appears to be a basal member of Stegosaurinae.

Although Hesperosaurus marks the last stegosaurian discovery anywhere in the world, its description has not been the last word on stegosaurs. Seventy-five years after Gilmore provided the first comprehensive study of these animals, Galton presented a more modern synthesis of available information on these plated dinosaurs in 1990. Within two years of this work, another important study appeared that analyzed the phylogenetic relationships of Huayangosaurus to other stegosaurs. This research, by Sereno and Dong, is the first to present Huayangosaurus within a cladistic framework of Stegosauria. The same treatment was provided by Carpenter and co-workers for their new stegosaur Hesperosaurus. Finally, comprehensive work by Galton and Upchurch, reviewing the stegosaur fossil record, including anatomy, phylogeny, and paleobiology, constitutes the most recent work on this group. From these sorts of studies, we are beginning to have a much clearer sense of stegosaurs as living, evolving denizens of the Mesozoic.

Important readings Alexander, R. McN. 1989. Dynamics of Dinosaurs and Other Extinct Giants.

Columbia University Press, New York, 167pp. Bakker, R. T. 1986. Dinosaur Heresies. William Morrow, New York, 481pp. Bakker, R. T. 1987. Return of the dancing dinosaur, In Czerkas, S. J. and Olsen, E. C. (eds.), Dinosaurs Past and Present. Natural History Museum of Los Angeles County, pp. 38-69. Buffrenil, V. de, Farlow, J. O. and de Riqles, A. 1986. Growth and function of Stegosaurus plates: evidence from bone histology. Paleobiology, 12, 459-473.

Carpenter, K. (ed.) 2001. The Armored Dinosaurs. Indiana University Press,

Bloomington, IN, 512pp. Davitashvili, L. S. 1961. [The Theory of Sexual Selection]. Izdatel'stov

Akademia Nauk SSSR, Moscow (in Russian), 537pp. Desmond, A. J. 1975. The Hot-Blooded Dinosaurs. The Dial Press, New York, 238pp.

Galton, P. M. and Upchurch, P. A. 2004. Stegosauria. In Weishampel, D. B., Dodson, P. and Osmolska, H. (eds.), The Dinosauria, 2nd edn. University of California Press, Berkeley, pp. 343-362. Giffin, E.B. 1991. Endosacral enlargements in dinosaurs. Modern Geology, 16,101-112.

Gilmore, C. W. 1914. Osteology of the armored Dinosauria in the United States National Museum, with special reference to the genus Stegosaurus. Bulletin of the U.S. National Museum, 89,1-136.

Hopson, J. A. 1977. Relative brain size and behavior in archosaurian reptiles. Annual Reviews of Ecology and Systematics, 8,429-448.

Hopson, J. A. 1980. Relative brain size in dinosaurs - implications for dinosaurian endothermy. In Thomas, R. D. K. and Olson, E. C. (eds.) 1980. A Cold Look at the Warm-Blooded Dinosaurs. AAAS Selected Symposium no. 28, pp. 278-310.

Jerison, H. J. 1973. Evolution of the Brain and Intelligence. Academic Press, New York, 482pp.

Sereno, P. C. 1986. Phylogeny of the bird-hipped dinosaurs (Order Ornithischia). National Geographic Society Research, 2,234-256.

Sereno, P. C. and Dong, Z.-M. 1992. The skull of the basal stegosaur Huayangosaurus taibaii and a cladistic analysis of Stegosauria. Journal of Vertebrate Paleontology, 12,318-343.

Spassov, N. B. 1982. The "bizarre" dorsal plates of Stegosaurus: ethological approach. Comptes rendus de VAcadémie Bulgare Sciences, 35, 367-370.

Thulborn, R. A. 1982. Speeds and gaits of dinosaurs. Palaeogeography, Palaeoclimatology, and Palaeoecology, 38, 227-256.

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