Stegosauria traits and diversity

The "plated" dinosaurs, or Stegosauria, were among the first thyreophorans to appear; the others were ankylosaurs. The earliest stegosaur fossils have been found in China and date from the Middle Jurassic, about 170 million years ago. There were two clades of stegosaurs, the Huayangosauridae and the Stegosauridae. All members of the Stegosauria shared several defining traits, including small, narrow heads; heavy, short forelimbs; long, robust hind limbs; and sturdy feet that bore hooflike bones on the ends of the


toes. All stegosaurs bore some form of vertical, bony plates or spikes that were raised up and attached in a double row along the back.

Huayangosauridae. The earliest stegosaurs are represented by only one member, Huayangosaurus ("Huayang lizard"). Huayango-saurus is known from one complete skeleton with a skull and from several partial skeletons; this makes it one of the best-known early ornithischian dinosaurs. Huayangosaurus differed from members of the Stegosauridae in several ways. Measuring about 14 feet (4.3 m) long, it was somewhat smaller than the stegosaurs of the Late Jurassic and Early Cretaceous. Its skull was taller and had a shorter snout, with eyes positioned more forward than in members of the Stegosauridae. Huayangosaurus had seven teeth in its pre-maxilla, the front-most bone of the upper jaw that was toothless in other kinds of stegosaurs. Its armor consisted of a series of small plates and spikes along its back, and a tail equipped with a cluster of four sharp prongs with which to protect itself. One other significant difference between Huayangosaurus and other stegosaurs was that its front legs were nearly the same length as its hind limbs, an anatomical trait not seen in later stegosaurs, which have longer hind limbs.

Stegosauridae. All known stegosaurs other than Huayangosaurus are placed in this group. The Stegosauria consisted of medium- to large-sized plant eaters measuring from 10 to 30 feet (3 to 9 m) in length. Although once categorized by scientists as part of the group of armored dinosaurs that includes the Ankylosauria, the Stegosau-ria have now been distinguished from other armored dinosaurs by their skull morphology, tail weaponry, and lack of extensive body armor.

Stegosaurus is one of the most iconic of dinosaurs. With its distinctive row of back plates, tiny head, and impressive tail spikes, Stegosaurus illustrates many of the now-familiar traits of this unusual group of dinosaurs. When it was first described by Othniel C. Marsh, in 1877, Stegosaurus was an extraordinary discovery and the first well-known representative of the plated dinosaurs. Its fame spread quickly due largely to a magnificent and nearly complete {§} specimen extracted in Colorado by Marsh's fossil collectors in 1886. {§}

The traits seen in the first specimens of Stegosaurus went far in defining the characteristics of this clade:

• Quadrupedal stance with a body that was highest at the hips because the animal had shorter forelimbs than hind limbs.

• Two rows of vertical plates or spikes running from the base of the neck down the back and onto the tail. These varied in size and were largest over the back and hip region. In Stegosaurus, these armor plates consisted of 17 thin, upright plates, no two of the same exact shape or size. The plates were positioned in two alternating rows along the back of the animal. Other stegosaurs had similar arrangements of the back plates, although they were usually smaller and found in different numbers than in Stegosaurus. Tuojiangosaurus (Late Jurassic, China) had a row of robust spines along its back, and Kentrosaurus (Late Jurassic, Tanzania) had a row of narrow plates on the neck and shoulders that turned into long, sharp spikes that continued from the top of its back to the tip of its tail and measured about 2 feet (60 cm) long. In addition, some species, such as Gigantspinosaurus (Late Jurassic, China), had huge, backward curving spikes protruding from the shoulders.

• Stegosaur tails had two pairs of defensive spikes pointing outward to the sides and a bit upward and backward.

• Stegosaur skulls were typically long, squat, and narrow, with toothless beaks and small, triangular, ridged teeth in the cheek regions.

• Stegosaur tails were not particularly as well stiffened as those of other ornithischians. This may have affected the stegosaurs' mobility because a stiff tail enabled some dinosaurs to keep better balance when they ran. The stegosaurs' sturdy, heavy skeleton, short front legs, and inflexible limbs and feet probably gave them a stiffened gait similar to that of an elephant.

{§} • Some stegosaurs had additional armor, in the form of small {§}

bony knobs, or ossicles, protecting their throats and sides. The ossicles were tightly packed, like the chain mail in a suit of armor.

The function of the back plates of stegosaurs has been a source of debate, particularly for Stegosaurus, which exhibits the largest and flattest example of plates of all the stegosaurs. The plates originally were thought to provide protection; at least one early view (formed before a good, articulated skeleton was discovered) was that the plates lay flat on the back like shingles on a roof.

Close examination of the anatomy of Stegosaurus plates reveals that they were not rock solid in real life but rich with blood vessels, both on their surfaces and internally. As a means of armor protection, such plates were no match for the bone-crunching teeth of a large predatory dinosaur and didn't cover much of the body, either. The visually stunning appearance of the plates, however, could have given the stegosaur the appearance of a much larger animal, making it more threatening to a potential attacker. As a means of display, the plates could have played a role in distinguishing one species from another and, within a species, one individual from another— a useful feature when vying for a mate. It is even possible that the rich blood supply in the plates allowed stegosaurs to change the color of their plates at will, much as a chameleon does today, thus suggesting a kind of billboard for attracting a member of the opposite sex.

Aside from their visual appeal, stegosaur back plates were probably excellent conductors of heat. The flow of blood that permeated the plates would have made them an effective heat-exchange system. Heat would have been dissipated from the plates during hot weather and gathered for absorption by the dinosaur in cool weather. It should also be noted that the thermoregulatory function of stegosaur back plates is restricted to Stegosaurus, which had large, flat, vascularized plates. Most stegosaurs lacked such plates and instead were adorned with bony spikes and spines that probably served as visual displays rather than heat conductors. This suggests that the heat transfer function of Stegosaurus plates, if indeed the animal utilized them for this purpose, was a relatively unimportant secondary function that did not evolve in other stegosaurs.

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