The Ornithischia is a major monophyletic clade that includes the armored, horned, and duckbilled dinosaurs.  The ornithischians are all herbivorous dinosaurs.

     The pelvis of ornithischians is like that in birds, the opisthopubic condition. From (Anatomical Dictionary at http://www.dinosauria.com/dml/anatomy.htm#1a2)

     Ornithischians are also characterized by the presence of a predentary bone in the mandible that caps the front part of the lower jaw. 
                                         From (Anatomical Dictionary at http://www.dinosauria.com/dml/anatomy.htm#1a2)

     They also have numerous other shared derived characters, including:

• 1.  A palpebral bone - a narrow bone that crosses the outside of the eye socket (probably mobile and set within the upper eyelid.
• 2.  Ossified (bony) tendons above the sacral region for stiffening the backbone and pelvis.

     The oldest known ornithischians (represented by Lesothosaurus) are of Early Jurassic Age.  The primitive thyreophorans, Scutellosaurus and Scelidosaurus are also of Early Jurassic Age.  The Stegosauria, more advanced thyerophorans, have a world-wide distribution and range from Medial Jurassic to Late Cretaceous.

     Perhaps here we should briefly review the position of the continents during the Jurassic.  Certainly the world wide distribution of the Stegosauria is related to the close positons of the continents during the Early Jurassic.  The ancestors of the Stegosauria, something similar to the primitive thyreophorans Scutellosaurus and Scelidosaurus, must have been able to roam the globe fairly easily.  Please view and study the paleogeographic maps of the Jurassic Period at the website given below.

THE MAPS AT THE FOLLOWING WEBSITE ( Jurassic Paleogeography at http://vishnu.glg.nau.edu/rcb/jurpaleo.html)  ARE FROM THE SITE OF DR. RON BLACKEY AT NORTHERN ARIZONIA UNIVERSITY.  THERE ARE MAPS SIMILAR TO THESE FOR OTHER PERIODS OF GEOLOGIC TIME.  AN EXCELLENT SITE!
TO GO TO THE MAIN SITE, CLICK THE FOLLOWING: Stratigraphy and Sedimentology on the Colorado Plateau (http://vishnu.glg.nau.edu/rcb/RCB.html)

ANOTHER GOOD SITE FOR REVIEW OF JURASSIC PALEOGEOGRAPHY: The Jurassic Period - Geography, Climate and Tectonics (http://www.nova.edu/ocean/biol1090/W10A-JURAGEOL.htm)
 
 

   Royal Tyrrell Museum Tour: Stegosaurus (http://www.tyrrellmuseum.com/tour/stego.html)

INTRODUCTION

     Stegosaurus is one of the most familiar dinosaurs to the general public (particular kids).  Stegosaurs and Ankylosaurs are considered close relatives by paleontologists and are placed in the group (order rank by many) Thyreophora (Shield Bearers) (check out this web link  Introduction to the Thyreophora at http://www.ucmp.berkeley.edu/diapsids/ornithischia/thyreophora.html).  The Thyreophora have osteoderms (bones in the skin) as rows on the back or sides.  The stegosaurs have one or more rows of armor plates or spikes in the skin above or alongside the vertebral column.  The thyreophorans are armored, quadrapedal, ornithischian dinosaurs that lived during the Jurassic and Cretaceous Periods.

     The primary derived character shared by the thyreophorans is the development of armored plates (osteoderms on the body).
 

PRIMITIVE THYREOPHORANS

      Scutellosaurus and Scelidosaurus are examples of primitive thyreophorans.  Primitive thyreophorans are also among the most primitive ornithischians.

     Scutellosaurus is a small (about 1.2 meters in length) generalized ornithischian with primitive ornithischian characters as follows:

• 1.  small skull and jaws
• 2.  cheek teeth at jaw margins
• 3.  short neck
• 4.  hind limbs slightly larger than fore legs
• 5.  long tail.

     Scelidosaurus was very different from Scutellosaurus, being much larger with massive limbs and had forelimbs that were about equal in length to the hind limbs.  The back was covered by numerous armored plates.  Scelidosaurus also had a broad sacrum, like the ankylosaurs. 
 
 

STEGOSAURIA

      The Stegosauria are plated dinosaurs with armor plates running down the back.  They were medium sized to large (up to 9 meters) quadrapedal, herbivorous ornithischians.  The Stegosauria had small heads, short and massive fore limbs, long columnar hind limbs, and short, stout feet with hooves on the end of the toes.

      The key derived character for the stegosaurs were the vertical bony plates and/or spines arranged in single or double rows along the neck, back, and tail.

Huayangosauridae

     The most primitive stegosaur is Huayangosaurus of the middle Jurassic of China.  Huayangosaurus is known from complete skeletons, so it is one of the best known stegosaurs.  Huayangosaurus is placed as a single genus in the family Huayangosauridae.

     This dinosaur was 4.3 meters long and had spike shaped armor along the mid-line of the back and additional rows of small armor plates along each side of the row of spikes.  The skull was deep and tall with a short snout.  Huayangosaurus was smaller and less massive than the family Stegosauridae and had fore and hind limbs more equal in size.

Stegosauridae

     Members of the family Stegosauridae had low skulls, long snouts, more posteriorly placed eyes and were of larger size and more massive skeletons than the Huayangosauridae.  They also had relatively long hind limbs as compared to their fore limbs.

     The best known member of this family is the genus Stegosaurus of Upper Jurassic rocks of the Morrison Formation of the western U.S.

Genus Stegosaurus
 

• Had a mid-line row of armor plates.  An alternating offset rows of plates are favored by most paleontologists.
• Low slender head for a two ton animal, with a toothless beak and leaf shaped teeth (as in all ornithischians), but did not have the extensive dental batteries as in the ornithopods.
• Brain very small (estimate of 70 to 80 grams based on endocranial casts).
• May have had a semi-sprawling posture for fore limbs (based on the nature of the shoulder and humerus bones, which have broad processes that probably attached to large muscles).  However, the hind limbs are long and pillar-like, lacking broad processes; which suggests an upright hind limb posture.
• The hind foot had three stout hoofed toes.
• The dorsal plates are not solid sheets of bone.  The surfaces of the plates are covered with grooves and channels, the interior contains large canals.  Presumably the grooves and channels on the surface and the canals on the interior of the plates contained arteries and veins that were filled with blood when the animal was alive.

     What was the function of the dorsal bony plates of Stegosaurus?  Paleontologists used to think that they were for defense.  But they are not solid and they probably were filled with blood, would it be wise on the part of Stegosaurus to let an attacker "chomp" on these plates?  Now it is thought that the plates served as a sexual display (for mating and identification purposes) and as a means to help regulate the metabolism.  The plates may have served to expose blood to the Sun to warm the animal up or to the wind in the shade to cool the animal off (the plates would have exposed a lot of surface area that contained blood filled arteries and veins).

     In the 1970s, James Farlow (then at Yale as a graduate student and now at Indiana University) performed experiments on diamond shaped plates of metal in wind tunnels.  He determined the arrangement of plates that would be most effective as radiators of heat or solar panels for collection of heat.  He concluded that double rows of alternating plates was the most effective for cooling or heating.

     Thermoregulation of plates doesn't work as well for other stegosaurs that had more spike like plates.  Perhaps the earlier stegosaurs evolved spikes along the neck, back, and tail for defense.  They would have probably had some thermoregulatory effect also.  Perhaps these spikes evolved into plates where the main function was thermoregulatory.

     The spikes or plates may have also functioned for display for recognizing members of the same species and for mating purposes.  In fact, there may have been sexual dimorphism with respect to spike and plate size (remember, there have not been large numbers of complete stegosaur skeletons found).
 

STEGOSAUR LIFESTYLE AND EVOLUTION
 

• Plant eaters that cropped vegetation with their horny beaks.  The premaxillary and predentia bones were covered with a horny rhamphotheca (beak + cup or sheath).
• Habitual quadrapeds.  Perhaps stegosaurs could have reared up on their hind limbs to reach higher vegetation (as suggested by Bakker).
• The stegosaurs first appeared in the Medial Jurassic of China.  By Late Jurassic they had a world-wide distribution and reached their zenith of diversity with 8 genera.
• Stegosaurs were among the dominant large herbivores of the Late Jurassic.
• Stegosaurs are known from and common in nonmarine rocks of Upper Jurassic from western North America (the Morrison Formation), China, and Eastern Africa.
• The Cretaceous record of stegosaurs is more sparse, they were past their peak of diversity.
• Perhaps the stegosaurs declined during the Cretaceous due to new kinds of plants (Angiosperms) and the rise of the ornithopods in diversity.
• Upper Cretaceous finds in India (in the 1970s) show that they survived to near the end of the reign of dinosaurs.

ANKYLOSAURIA

  Royal Tyrrell Museum Tour: Ankylosaurus
                                    From: http://www.tyrrellmuseum.com/tour/ankylo.html

     The Ankylosauria are the armored tanks of the Mesozoic Era.  The name means "fused lizards", based on a rod of fused vertebrae in the back.

• Medium sized to large (up to 9 meters).  Similar in size to stegosaurs.
• Quadrapedal, plant eating dinosaurs.
• Similar teeth as in the stegosaurs.
• Broadly arched ribs and very wide body.
• Covered with small round or square armor plates (osteoderms) that formed a fairly continuous shield over the dorsal surface.
• Some had spikes or spines as part of the body armor.
• Some had a club at the end of the tail.
• Limbs robust, forelimbs 2/3 to 3/4 the length of the hindlimbs.
• Short, stout feet that bore hooves on toes.

• 1) Low skulls with the width between the eyes greater than the height.
• 2) Closure of the anteorbital fenestrae.
• 3) Added dermal bones on the skull.
• 4) Extensive body armor.
• 5. Modification of the rib cage and hip girdle to form a base for the armor.

     The Ankylosauria are divided into families:

• Nodosauridae
• Ankylosauridae

• Narrow skulls lacking armor and horns at the posterior corners.
• Prescence of spines in their armor.
• Lack of ossified tendons in the tail and lack a tail club.
• 
• Reconstruction of Gastonia from the Late Cretaceous of Utah.  (From:  CEU: Museum Pictures at http://www.ceu.edu/museum/Dinos/images/gaston.htm).

      Edmontia is another genus that was abundant in Alberta during Late Cretaceous time.
Click here for a nice picture of Edmontia:  Royal Tyrrell Museum Tour: Edmontonia

Ankylosauridae

     The Ankylosauridae are of striking contrast to the nodosaurids.

• Wide armored heads (as wide as long).
• Long triangular horns at the posterior margins of the skull.
• The lateral temporal fenestrae (which are open in nodosaurids) are completely covered with armor.
• Few or no spines present on the body armor.
• Last few tail vertebrae braced by ossified tendons.
• Tip of tail had a club (the fusion of 4 plates of bone).

     Ankylosaurus of the Upper Cretaceous of the U.S. and Canada is another well known and typical genus of the akylosaurids.  Check this link:   Royal Tyrrell Museum Tour: Ankylosaurus (http://www.tyrrellmuseum.com/tour/ankylo.html)
 

Ankylosaur Life Style

    The Ankylosauria are known as the Mesozoic Tanks.  As armored tanks, they had to balance three factors: speed, armor, and firepower.  For an increase in speed, there must be a decrease in armor and firepower (and vice versa).  Ankylosaurs sacrificed speed and firrepower for heavy armor.  Ankylosaurs did not have powerful jaws nor large teeth.  However, some (the ankylosaurids) did have a wicked club on the end of the tail, which might cripple or knock down a Tyrannosaurus.

     The ankylosaurs had a wide rib cage, broad ribs, and a very broad sacrum with huge ilia that hung down from the sacrum.  Heavy boney armour covered most of the body (but it was not sutured, so somewhat flexible).  The Nodosauridae stopped here, thus they had some speed.

     Ankylosaurids went farther.  They had a boney plates covering the head and a tail club for firepower, but they were slower than the nodosaurids.  In fact, they probably did not try to flee from a predator or couter-attack when attacked.  They probably just maintained a low profile and swung at the predator with their tail club.
 

Ankylosaur Evolution

     The oldest ankylosaurs are of Middle Jurassic of Europe.  All Jurassic ankylosaurs known from Europe are from fragmentary fossils.  A recent find of an Upper Jurassic ankylosaur in western Colorado has extended the range in North America.
 

     Two types of Mesozoic tanks evolved, the lighter nodosaurid and the heavier ankylosaurid.  The split between the two probably occurred during Late Jurassic time.  Once each body style evolved, there was very little change druing the rest of their existence.

     The nodosaurids reached their peak of diversity in Early to Medial Cretaceous time, but survived until Late Cretaceous time.  The Ankylosaurids attained peak diversity during the Late Cretaceous, but are not particularly abundant in the fossil record.