What were some early thyreophorans?
Some early thyreophorans were the Scutellosaurus and Scelidosaurus. The Scutellosaurus an Early Jurassic period dinosaur whose remains were discovered in western North America was small and bipedal (two-footed), similar to the Lesothosaurus. The Scutellosaurus had numerous small bony skin plates (although not as large as later thyreophorans) and was among the smallest of the armored dinosaurs, growing from just over a foot (around a half meter) to 3 feet (1 meter) in length. The Scelidosaurus was an Early Jurassic period dinosaur living in western Europe and England about 180 million years ago. It was a quadruped (four-footed) and was much heavier than Scutellosaurus, growing to a length of approximately 13 feet (4 meters). It had heavy, bony plates, and hoof-like claws.
What were the stegosaurs?
The stegosaurs (Stegosauria) were one of the two groups comprising the thyreophorans (although some classifications group them with the anklyosaurs in the thyreophorans division). These quadruped, plated dinosaurs first appeared in the Middle Jurassic. They were medium in size, from 13 to 26 feet (4 to 9 meters) long, with a small, long skull with simple teeth, and a long toothless beak. They also had armor consisting of two rows of vertical bony plates or spines along their tail, back, and neck. Fossils of stegosaurs each species with a unique arrangement of spines and plates are found in Jurassic and Cretaceous periods rock layers around the world, but they were most abundant in the Jurassic.
Why were stegosaurs originally thought to have had a second brain?
The dinosaurs that made up the Stegosauria, such as Stegosaurus and Kentrosaurus, had extremely tiny brains. For example, the Stegosaurus, an animal that weighed approximately 1 to 2 tons (0.9 to 1.8 metric tons), had a brain only weighing 0.15 to 0.18 pounds (70 to 80 grams) and thought to be about the size of a walnut. Paleontologists also found an enlarged part of spinal cord in fossils of the stegosaur sacrum. This swelling, larger than the brain cavity, was originally thought to be a second brain. Currently, scientists believe this area is associated with nervous and connective tissue and fat, not a second brain.
What were the ankylosaurs?
The ankylosaurs (Ankylosauria) were one of the two divisions that made up the thyreophorans (the stegosaur was the other). Paleontologists divide the ankylosauria into two more families the Nodosaurids and Ankylosaurids based on differences in skulls, shoulder blades, and armor.
The ankylosaurs were medium-sized quadruped dinosaurs found in many parts of the world. In fact, the first dinosaur ever discovered in Antarctica was the ankylosaurian Antarctopelta; the fossils were recovered from Ross Island in 1986. Ankylosaurs were short-legged and squat, with long, wide bodies. Their heads ranged from long and narrow to wide with broad muzzles. All of these dinosaurs had bony plates of armor over their bodies, often with spines, spikes, or studs projecting outward. Some even had bony clubs on the ends of their tails. Armor consisted of plates of bone, known as scutes, embedded in the skin; some dinosaurs even had their heads covered with this armor. They were the tanks of the dinosaur world and were able to fight off almost any predator.
Unfortunately, very little is known about these dinosaurs, since so few complete fossils have been found. The ankylosaurs first appeared during the Early to Middle Jurassic periods, but they became most numerous and diverse during the Cretaceous period. Most of the ankylosaurs belonged to the Nodosaurid subgroup early in the Cretaceous period (although one genus, Sarcolestes, is from the Jurassic); the subgroup Ankylosaurid were more prevalent in the latter Cretaceous period, and were distinguished by their broad heads, spikes in the back of their skulls, and club-like tails.