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THYREPHORA: STEGOSAURS
     The thyrephorans, or “shield bearers”, were low browsing armored ornithischians.  Most had toothless beaks but a few had teeth.
sCUTELLOSAURUS
     The species of the family Scutellosauridae were semi-armored bipedal/quadrupedal thyrephorans about 1 meter long.  They were covered by rows of little bony plates with hundreds of individual pieces.  They were the basal thyrephorans ancestral to later groups and lived from the Early to Late Jurassic.  Scutellosaurus, “small shield lizard”, is the best known.  Scutellosaurus had a very long tail with 60 caudal vertebrae.  Although it was bipedal, its legs were relatively shorter than other bipedal ornithischians (Lambert, 1990, Fastovsky, 1996; Lucas, 2004).

Scutellosaurus and Scelidosaurus retained features from their fabrosaur ancestors. Although Scelidosaurus was much larger, it lacked any tooth modifications that would have increased its success as a herbivore other than its toothless beak for cropping plants. The Early Jurassic Scelidosaurus could have been ancestral to both ankylosaurs and stegosaurs (Czerkas, 1990).

SCELIDOSAURUS
     The Scelidosauridae were quadrupedal thyrephorans with a covering of rounded bones that could reach 4 meters in length.  Since some fossils have been found in marine sediments, they might have browsed near riverbanks and after death remains were washed to the sea.  In Scelidosaurus, the largest armor plates were positioned along the midline of the back but there were at least two lateral rows as well.  It is possible that they squatted to cover themselves for defense.  They seem to be the ancestors of both the stegosaurs and ankylosaurs--their teeth are like those of the stegosaurs and their plates like those of the ankylosaurs (Sereno, 1986; Lucas, 2004). 

     In stegosaurs, rows of bones called osteoderms run down the back. There are 12 known species, virtually all of which lived from the Middle Jurassic to the Early Cretaceous.  They probably had a slow gait and if they tried to run, the long hindlimbs could overtake the shorter forelimbs.  There were broad hooves on the toes. In at least one site (India), stegosaurs survived into the Late Cretaceous.

   Stegosaurs had a toothless snout and simple blunt cheek teeth.  The narrow snout suggests that maybe they were selective feeders, perhaps choosing fruits and succulent plant parts where possible.  Their neck could not lift the head high off the ground and therefore they were low browsers.  Some have suggested that they could rear on hind legs to reach higher branches. They did not have the dense dental battery of cheek teeth found in many other ornithischians and could not chew well (Lambert, 1990, Fastovsky, 1996).

STEGOSAUR SKULL stegosaurus

     Stegosaurs had extremely small brains.  Their brain was estimated to be .001% of body weight--only 70-80 grams for a 1-2 ton animal.  This is the lowest ratio among dinosaurs except some sauropods and perhaps some ankylosaurs.  Brain endocasts can be made to analyze the brains of fossil animals.  To visualize the cavity inside the skull, latex is used to fill an empty braincase and then is pulled out the foramen magnum (the opening through which the spinal cord passes).  The actual brain volume is less than the volume of the endocast because the brain doesn’t fill the cranium and in living reptiles the volume of the brain is much less than the volume of an endocast.  Given the small size of their brains, stegosaurs did have relatively large olfactory bulbs.

     Did stegosaurs have a second brain in their spinal cord? No.  There is a sacral (hip region) enlargement in the spinal cord of stegosaurs and sauropods.  Birds are the only living animals with such an enlargement; but it stores the glycogen body (whose function isn’t well known but may serve as an energy source for the nervous system) rather than provide extra neural processing.  There seems to be some sexual dimorphism in the number of sacral ribs.

     The fossils of multiple Kentrosaurus individuals at one site suggest that at lest some might have traveled in herds.  Stegosaur fossils have been found in wooded plains habitats (Lambert, 1990, Fastovsky, 1996).

plate
     The most obvious trait of the stegosaurs was the armor plates on their backs.  They were leaf shaped in Stegosaurus (“roof lizard”) and narrower in other genera.  Perhaps they functioned as protection but some researchers feel they wouldn’t have been that effective in defense.  Why did they cover so little of the body?  Given that there are so many blood vessels going to these plates, why send blood to a body part that might get bitten?  Other species have smaller plates and more spikes so perhaps the large plates of stegosaurs evolved from plates that were primarily defensive.  Perhaps the plates also had some display value in making the animals look larger.  The blood vessels that went to might have made plates look redder, serving as a warning signal.  The plates may have been mobile at their bases and could rotate from being folded up to folded down in which case they could offer a greater degree of protection (Fastovsky, 1996; Lucas, 2004).

     It is also possible that the plates functioned in thermoregulation.  There are abundant external grooves and internal chambers through which blood vessels probably passed.  If a stegosaur was facing away from the sun, the plates would lose more heat than they would gain, especially in a wind.   If the plates had an asymmetrical arrangement, they would help lose heat no matter which direction the wind was blowing.  If a stegosaur exposed its body length to sun’s rays, the additional body surface area provided by the plates would allow the animal to warm more quickly.

     How the plates were arranged is not known with certainty.  At first it was thought there was one row of plates, then two rows staggered, then two rows of paired plates.  The evidence supports the model of two staggered rows best.  Recent discoveries seem to confirm the overlapping plates model.  Young stegosaurs may not have had these plates.  In 1992 found that they had disc shaped plates protecting stegosaur hips and bony studs protecting the throat; some stegosaurs had shoulder spines as well (Lucas, 2004).

    Whether or not the plates played a defensive role, the tail spikes were certainly threatening weapons.  Stegosaurs lacked the stiff tails of other ornithischians so that they could swing their tails.

     The family Huayangosauridae, “Huayang lizards”, contains what seem to be the most primitive stegosaurs.  Huayangosaurus possessed teeth in its premaxillary bone (the tip of their upper jaw) which all other stegosaurs lacked. It is known from the Middle Jurassic and reached a length of 4.3 meters (Lambert, 1990, Fastovsky, 1996).  The number of phalanges on its toes were 0-2-3-3-0 as opposed to Stegosaurus and Kentrosaurus whose phalangeal formulas were 0-2-2-2-0.

Huayangosaurus probably possessed a single row of plates along its back. Its plates are similar to spikes, representing the primitive condition of stegosaur armor. One find indicates that stegosaurs had only single row of plates along the back and tail although the plates were paired in the neck region (Czerkas, 1990).

huayangosaurus huayangosaurus skull

Although all stegosaurs possessed legs which were longer than their arms, the difference in limb length was less in Huayangosaurus than in the higher stegosaurs.

 

Ratio of Femur Length to Humerus Length

Huayangosaurus

115

Dacentrosaurus

147

Chialingosaurus

163

Kentrosaurus

163

Lexovisaurus

173

Stegosaurus

180-230

(Weishampel, p. 447)

Interestingly, the most basal members of a number of ornithopod groups, including the Huayangosauridae, are known from Southeast Asia, suggesting that it was the center of considerable ornithischian diversification.

tuojiangosaurus
     The family Stegosauridae,“roof lizards”, contains the later members of the group.  Stegosaurus had tall neural arches of back vertebrae would have provided leverage for tail swinging. 
Some feel that the Stegosaurus species with the largest plates should be classified as the genus Diracodon.  Kentrosaurus is an African stegosaur which had 9 pairs of thin plates from the neck to the back and 5 pairs of spikes over hip and tail.
kentrosaurus tuojiangosaurus skull