Palaeos Vertebrates 320:100 Ornithischia

*Palæos:*		Unit 320: Ornithischia
*The* *Vertebrates*		100: Ornithischia

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Ornithischia

"Fabrosauridae" and Other Basal Ornithischians

The cladistic paradigm has meant these early types have been stuck as "outgroups" with no Linnean status beyond family level. Since these early groups differ at least as much as the later ones I decided to take matters into my own hands and christen a new suborder [ Stanley Friesen's Dinosaur classification follows a similar tack].

Suborder Fabrosauria - small early ancestral forms - active fast-running lightly-built bipeds, paraphyletic and ancestral to all other Ornithischian lineages, Late Triassic to Early Jurassic - length about a metre. This isn't shown on our cladogram for the obvious reason that it isn't a clade. It collectively refers to things like Lesothosaurus and Agilisaurus. These actually may form a monophyletic group, the Fabrosauridae. In addition, it would include the South American Pisanosaurus, which is probably not a fabrosaurid. (MAK000117 & ATW020420)

The most basal, generally recognized clade is Genasauria. Genasauria is defined as the last common ancestor of Thyreophora and Cerapoda and all of its descendants. The thyreophorans are the armored ornithischians: the stegosaurs and ankylosaurs. The cerapods include both the hadrosaur-like forms and the line which includes both Triceratops and the bone-headed Pachycephalosauria. In short, Genasauria includes all of the major Ornithischian groups.

The most important unique characteristic of the group is the inset tooth row. That is, the teeth are not located in the same plane as the sides of the skull. Instead, they are moved in close together, toward the center line. The functional significance of this structure is the advantage it gives to a plant-eater. Herbivores on the model of a sauropod, or even an aetosaur, gulp masses of vegetation. However, leaves, stems and seeds are tough stuff, and chemically hard to digest. This kind of herbivory requires an enormous amount of internal processing, which means carrying around grinding stones in the gut (gastroliths) and a big fermentation vat for chemical decomposition. The simple adaptation involved in putting the teeth close together, combined with more teeth and articulating dentition (teeth which are in contact when the mouth is closed and thus cut or crush food between opposing jaws) allows the animal to cut plant food up into tiny bits and crush the juices out of leaves and fruit. This, in turn allows it to do without gastroliths (intestinal rocks for cutting up bulk vegetation), and also to reduce the size of the fermentation area for chemical digestion. Since the pieces swallowed are small, they have a greater surface area per unit volume and can be attacked more quickly by digestive enzymes. This huge gain in efficiency translates into an ability to be more selective about food, to move faster or to carry armor, to spend more time on mate selection or in rearing young. Quite possibly, this simple anatomical change was ultimately responsible for the worldwide relative decline of the sauropods and the extinction of the prosauropods by the Late Jurassic.

Of course, narrow jaws are not enough. Other changes were important. For example, if the teeth were merely moved inward, plant material might simply fall out the sides as the dinosaur chewed -- hugely wasteful as well as frustrating. The narrowing of the jaw gives an advantage to an animal with cheeks to keep the food in. Thus, the evolution of genasaurs required cheeks as well as jaw structure innovations. But cheeks, in turn, allow further changes: improvements in jaw musculature, the ability to use salivary enzymes to start chemical digestion in the mouth (not to mention the improved etiquette of chewing with the mouth closed!), reduced water loss by evaporation from the mouth, and so on.

In this connection, it might be useful to examine the details of a basal genasaur. It might -- if there were any to examine. As it turns out, there are probably none. The only form that comes reasonably close is Echinodon. Unfortunately, Echinodon is a Late Jurassic (or even later) species, and most of the relevant developments are presumed to have taken place in the Triassic. Worse, there are few remains. The best Echinodon material is still a handful of disarticulated jaw parts from England originally described by Owen in 1861 and more recently by Galton (1978). Other than this, there is almost nothing. A single tooth from Spain has been referred to the genus. Estes & deAvalle (1982). Callison & Quimby (1984) have discussed post-cranial material from an unnamed and undescribed "fabrosaur" from the Jurassic of Colorado. Olshevsky (1994) and others have suggested that this, and accompanying jaw material, is Echinodon. However, without any attempt by anyone at formal publication of the fossils, it seems pointless to speculate.

As if to emphasize the ambiguity of these bits and pieces, there has been little agreement on the position of Echinodon. Galton (1978) assigns it to the Fabrosauridae. Sereno (1991) disputes a number of important details of Galton's re-description, disputes the very existence of the Fabrosauridae, and suggests that Echinodon is a heterodontosaur. Olshevsky (1994) agrees (more or less), but his conclusions about the phylogenetic place of heterodontosaurs are markedly different from those of Sereno. Coombs et al. (1990) have an entirely different understanding, and place Echinodon near Scutellosaurus in the Thyreophora, based partly on the association of the English fossils with dermal scutes of uncertain provenance. Perhaps Sereno would now agree. Sereno (1999) currently places virtually all of the fabrosaur material in the basal Thyreophora, although he omits any discussion of Echinodon itself in the most recent study. We, also, treat Echinodon as a thyreophoran; but the choice is rather arbitrary. Much of the debate centers on the "canine" heterodont teeth of the English fossils; but, based on the photographs in Galton's paper, it is not at all certain that the fossil even has such teeth. There are negative casts which might reflect caniniform teeth. In addition, one specimen bears a disarticulated caniniform tooth which might have belonged to Echinodon, but could also have been left by a scavenger. The Fruita specimen is said to have unambiguous fangs, but the specimen remains undescribed.

This is obviously an unsatisfactory state of affairs. In the last analysis, it appears that Echinodon is simply another one of those fossils "whose charm exceeds their scientific merit." Feduccia (1999), quoting Storrs Olsen. It is particularly discouraging that acute and experienced observers such as Sereno and Galton cannot even agree on a description of the known material, much less on its interpretation. All things considered, this is a genus best tucked back into the deep recesses of the British Museum until something more informative comes along. ATW000514.

Thyreophora

These are small to large specialized quadrupedal armoured forms - Early Jurassic to Late Cretaceous - length one to ten metres. There are three traditional subgroups: the primitive ancestral scelidosaurs, the unusual stegosaurs, and the heavily armoured ankylosaurs. It has been suggested that the Stegosaurs are actually more closely related to the ornithopods, but generally the following three lineages are grouped together, with the scelidosaurs as the basal (paraphyletic) grade from which the other two clades developed.

Suborder Scelidosauridae

Ancestral armoured types - mostly Early Jurassic. Quadrupedal. Includes light running forms and large heavily armed types. Length 1 to 4.5 meters.

Suborder Stegosauria

Plated dinosaurs - mostly Jurassic, spines along the back and tail for protection, plates served a thermoregulatory purpose. Quadrupedal, short forelimbs, slow-moving. Length 4 to 10 meters

The Stegosaurs are a distinctive component of the Jurassic megafauna. These striking dinosaurs are characterized by pairs of bony plates along the back (presumably for the purpose of thermoregulation) and varying numbers of pairs of spikes along the tail and, in some species the back, hips, and even shoulders. Despite their protective spikes, they were never a very diverse group. Reasonably common during the Middle and Late Jurassic, the stegosaurs were almost wiped out by the Tithonian extinction event. Stragglers continued on to the Early or middle Cretaceous, and perhaps even later on the island continent (as it was then) of India.

The stegosaurs are divided into a number of families, typically with only one well-known member each. These may or may not be monophyletic. Stegosaurs are not very diverse, and there is a reasonably good chance that we are looking at ancestor - descendant relationships, rather than separate clades.

The Huayangosaurs were earliest well-known forms, and lived during the Early to Middle Jurassic. They are generally much smaller than the later stegosaurids. The skull is short and high relative to later stegosaurs, and retains the archosaurian antorbital fenestra (opening in front of the eyes). Although Huayangosaurus is the only certain form, it is likely that other earlier types like Tatisaurus and Emausaurus also belong here. An early Cretaceous jaw of uncertain affinities called Regnosaurus northamptoni has been placed in this group, on the basis of its strong resemblance to the lower jaw of Huayangosaurus, but in view of the incomplete nature of the specimen and its much later date (contemporary with the latest advanced stegosaurines) one may remain skeptical.

The Kentrosaurinae are primitive stegosaurids, small to large in size, with generally small spiky plates and numerous spikes along the tail, and sometimes the hips and shoulders as well. A paraphyletic group, they are transitional between the Dacentrurines and the stegosaurines.

The Stegosaurinae are the most specialized stegosaurids, with generally larger plates and spikes that tend to be limited to the end of the tail, although primitive forms still retain additional spines. One could distinguish two tribes: the Tuojiangosaurini, which may be ancestral, and the Stegosaurini. The tuojiangosaurines generally retain the Kentrosaurine pattern of back and hip spines. The stegosaurines are distinguished by large size, no spines on hips or shoulders, only two (or rarely 4) pairs of tail spines, relatively short forelimbs, a small elongate head, and large plates in life covered with skin and blood vessels. This tribe includes only two genera, Stegosaurus and Wuerhosaurus. The controversial dinosaurologist Bob Bakker distinguishes between two types of Stegosaurus - those with long legs and relatively smaller plates (Stegosaurus proper) and a more primitive form with shorter legs and larger plates (Diracodon). Others are critical of proposing a new stegosaur genus on this basis. Also, different sources (both in print and on the Web) give different lists of species; e.g. some are synonymized with others. It may be that some of these species are only subspecies or local variants. MAK990724

Suborder Ankylosauria

The ankylosaurs were heavily armoured dinosaurs, 2 to 10 meters long, quadrupedal, slow moving, and fed on swampy soft vegetation. They are largely known from the Cretaceous. These creatures were equipped with bony plates, studs, spikes, and, in the case of one lineage, a bony tail-club. Despite their formidable defenses, they are never very common in the fossil record, although a large number of different species are known.

The ankylosaurids were the last of the armoured dinosaurs to evolve. They are distinguished by the heavy club-like tails, presumably used as a defensive weapon against predators. The primitive polacanthine group had small tail clubs, and was originally included under the Nodosauridae. (mostly MAK990512)

Thyreophorans as a model of Evolutionary Radiation

Writing about popular dinosaurs is a daunting proposition. So many have written so much and so well that little remains but to be repetitious or wrong. Given the choice between ennui and error, we will choose a little of both by discussing choices -- lineage splitting -- and how these dinosaurs made them.

The armored dinosaurs are an interesting opportunity to study radiations. The earliest dinosaurs all looked rather alike: barrel-bodied, with long necks, long, smallish heads like truncated cones, powerful hind limbs and tails, and surprisingly agile-looking forelimbs with prodigious claws. Examples of this general body plan include Herrerasaurus, Saturnalia, Thescelosaurus, and even Scelidosaurus. These forms represent all of the major groups of the first, Triassic, dinosaurian radiation: Sauropodomorpha, Theropoda, Cerapoda, and Thyreophora, respectively. These transitions can be readily understood -- if at the cost of considerable oversimplification -- as diversification into large-bodied guilds of carnivores, and of high, medium, and low browsers, respectively. It appears that either the dinosaurs possessed overwhelming selective advantages, or they were radiating into an ecological vacuum, since they managed to dominate all of these guilds by the earliest Jurassic without greatly elaborating on the basic dinosaur body plan.

At that point, however, the dynamics of dinosaur evolution began to change. Perhaps this occurred because the only serious competition left was from other dinosaurs. In any case, by the Middle Jurassic, the dinosaurs evolved many new and peculiar body forms which differed radically from the original dinosaur bauplan. These included the big-headed, bipedal tetanurans, the classic long-necked sauropods, the posturally enigmatic, short-bodied, stiff-tailed iguanodonts and their early hadrosaur and marginocephalian relatives, the heavily armored eurypods (ankylosaurs and stegosaurs), and perhaps minor forms we know nothing about as yet. The Jurassic was a poor time for fossils and we are still missing large parts of that story.

The eurypod part of the tale is the part that concerns us. Here, two lineages split: the stegosaurs and the ankylosaurs. Both were elephantine, armored, low-level browsers with wide bodies presumably accommodating a powerful digestive fermentation system. Both groups are relatively uncommon dinosaurs, but both are cosmopolitan in distribution. Unlike the original radiation, these forms do not appear to have split along the fault lines of easily identified ecological guilds. The question (which it has taken us rather long to get to) is "why?" Scelidosaurus suggests that the first thyreophorans looked more or less like every other early dinosaur, with the addition of some osteoscute armor and a wedge-shaped skull. It is understandable that some lineage would diverge into armored, elephantine herbivory. But why two such bizarre and divergent forms?

The question is complicated by the fact that stegosaurs and ankylosaurs flourished at different times. Stegosaurs are known from the Cretaceous, but are primarily creatures of the Jurassic. The opposite is true of the ankylosaurs. However, there is little doubt that the split occurred in the early Jurassic. The ankylosaurs were simply less successful. The possible reasons for this may fall into one or more of the following categories: (1) some environmental condition(s) disfavored the ankylosaurian form, (2) ankylosaurs did not evolve their typical mid-Cretaceous body form for a very long time, or (3) ankylosaurs were marginalized by competition from the stegosaurs.

Reason (1) seems unlikely. Absent conditions amounting to a general mass extinction, it would be hard to imagine a case which would encourage the diversification of stegosaurs, while suppressing the ankylosaurs worldwide, for 100My or so. Condition (2) can also be eliminated. We know almost nothing about Jurassic, or even Early Cretaceous, ankylosaurs. However, it takes only one exception to disprove this hypothesis, and at least two exceptions appear to exist. Dracopelta and Tianchiasaurus are well-enough known to state that both definitive nodosaurids and ankylosaurids, respectively, existed in widely divergent locations during the Jurassic. Therefore, we can conclude that the most important condition limiting the success of the ankylosaurs was most probably competition from their cousins and fellow low-browsers, the stegosaurs.

Based on this somewhat attenuated chain of reasoning, as well as the earlier history of the thyreophoran radiation, we can tentatively identify three stages of a major radiation (or, more cynically, we are going to shoehorn the eurypods into the following theoretical framework):

1. Geographical dispersal: A fairly generalized form radiates geographically without much anatomical change. The innovations introduced by this form, or perhaps some pre-existing mass extinction, creates an environment in which there is little competition. The main constraints are (a) environmental limitations and (b) some rather basic anatomical choices. Thus the dinosaurs radiated without much variation from the basic form, except the marginal exception of certain prosauropods, for much of the Middle and Late Triassic. Their anatomy restricted them from marine environments and probably dictated a basic election between carnivorous and herbivorous habits. However, there were few constraints other than such fundamental anatomical and environmental conditions. Note that, so long as the group is expanding its range, selection will tend to disfavor specialization and favor generalist pioneers.

2. Diversification into Guilds: Following consolidation of their position in an ecosystem, the principle constraints become internal to the group. The advantage shifts from highly generalized pioneers to guild specialists who can take advantage of one particular type of environmental resource or lifestyle more efficiently than other members of the group. Like Darwin's finches, this does not require gross changes in body form. Rather the same basic body form is retained, with the development of specific guild-related tools. In the case of dinosaurs, this involved, over the course of the Latest Triassic and Early Jurassic: top predators with larger heads and jaws, high browsers with long necks, middle browsers with considerable range of head motion and specialized dentition, low-browsers with armor and elephantine quadrupedalism -- all retaining the basic dinosaurian body plan.

3. Intra-guild Competition: At some point, particularly with organisms much larger than finches, further guild specialization is not possible. Finches can continue to subdivide ecospace according to seed types almost indefinitely. By contrast, a dinosaur runs out of any specialized resource, within a reasonable geographical range, fairly quickly. This limits the degree of anatomically "easy" specialization possible, unless the beast has very high mobility and access to other geographical pockets with the specialized resource. Arguably, the iguanodont - hadrosaur - marginocephalian group, the Cerapoda, was capable of more specialization (and hence more coexisting similar forms) because of greater mobility. The thyreophorans, burdened by armor, greater weight, and simple graviportal posture, lacked this option. This ecological trap created the conditions for the intense adaptive pressures (and perhaps geographical fragmentation) necessary for major re-engineering of the body plan. That is, further subspecialization could be achieved only by major anatomical changes because of greater size and limited mobility. Hence the basic thyreophoran body types diverged earlier than the corresponding cerapod types. The same could be said of the sauropodomorphs by contrast to the theropods, and for much the same reasons.

Clearly, if one of the new body plans is sufficiently flexible, and conditions are right, this may start a whole new radiation. The radiation of birds from the theropod stem may be an example of this sort of thing. However, for the most part, only a few, increasingly aberrant and specialized forms will survive -- until the specialization becomes so extreme that a change in environment extinguishes the entire line. ATW 011216

Descriptions

Characters: Cheek teeth with low, subtriangular crowns; teeth foliate with constricted roots; predentary; toothless & rugose snout (probable horn beak); jaw joint below upper tooth row; palpebral; 3 to 5 sacral vertebrae; retroverted pubis; ossified tendons above sacrum; pes V reduced to splint. All had cheeks, horny beaks and were herbivorous. Great variety of "defensive" ornamentation.

Fabrosauridae: Agilisaurus, Fabrosaurus, Lesothosaurus, and perhaps various other poorly-known forms including Alocodon, Gongbusaurus, Lufengocephalus, Nanosaurus, Technosaurus, and Xiaosaurus.

Characters: Generally 1-2 m; very lightly built & clearly bipedal; triangular skull; small antorbital fenestra partially occluded by maxilla (primitive); very large orbits & palpebral; lacrimal inserts into slot in maxilla; premaxillary teeth small, laterally compressed, leaf-shaped incisiforms; dentary and maxillary dentition marginal (primitive); single wea r facets on teeth (vertical tooth action) (primitive); upper and lower teeth occlude alternately; teeth with enamel on medial & lateral surfaces; supra-acetabular buttress over anterior half of acetabulum; femur with head not strongly inturned; femoral shaft slightly bowed anteriorly; fourth trochanter long (caudofemoralis attachment); tibia longer than femur; fibula is reduced to narrow splint; astralagus & calcaneum strongly integrated into tibia and fibula function as part of crus; distal tarsals separate but closely associated with heads of metatarsals; pes tridactyl, with fifth digit lost & first reduced with a slight turn to the rear.

Notes: This likely not a clade, but a paraphyletic collection of very basal ornithischians. The description above largely relates to Fabrosaurus and should not be taken too seriously.

Characters: Tooth rows offset medially, from which existence of muscular cheeks is inferred; mandibular foramen reduced and spout-shaped.

Characters: fairly weak coronoid process [R56]; $ postorbital process of jugal expanded transversely; broadened pelvis; $ keeled scutes on dorsal surface in parallel rows.

Notes: Scutellosaurus is the most basal form known from reasonably good remains[C97]. It looks more or less like any other advanced dinosauriform, i.e. somewhat bipedal, long necked and long-tailed, but stouter and with the beginnings of armor. ATW011201. Another extremely primitive form, and not so well-known, is Tatisaurus oehleri Simmons 1965. Tatisaurus is an isolated dentary from the Dark Red Beds, Lower Lufeng Series, Yunnan, China, and is probably of Late Hettangian- or Early Sinemurian age (Earliest Jurassic). Tatisaurus is the earliest known member of the Thyreophora It is still not very far removed from its fabrosaur-like ancestors, but within a few million years Tatisaurus, or a form like it, evolved into large armoured quadrupeds, the scelidosaurs. MAK990714.

Range: Late Jurassic of England. Additional undescribed material from North America.

Note: Echinodon becklessi Owen 1861 is a small (60 cm), basal thyreophoran from the Tithonian (Late Jurassic) of England. Some North American materials may also be referred to this taxon. The remains and characteristics of Echinodon are discussed in detail in the Overview. ATW020807 MAK020420.

Ornithischia

Abbreviated Cladogram

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