Early thyreophorans were small bipedal forms, much like an armored Lesothothosaurus. Best known of these is Scutellosaurus.

• Early Jurassic of western North America
• Armor useful against small predators
• However, not so useful against bigger predators

Selection could have favored descendnats which were either faster with reduced armor (but there already were such dinosaurs present: ornithopods, for example) OR heavier armor. Result was larger, more heavily armored thyreophorans.

Best example of these: Scelidosaurus

• late part of the Early Jurassic of Europe (and possibly western North America, China)
• Larger than Scutellosaurus, about 4 m long
• Armor was proportionately much larger
• As a result, heavy armor required it to be an obligate quadruped

Thyreophorans more derived than Scelidosaurus are divided into two major branches, each specializing in a different mode of defense:

• Stegosauria, concentrating on more active defense
• Ankylosauria, concentrating on more passive defense

Stegosaurs:

• First appear in Middle Jurassic, common in Middle and Late Jurassic, rare in Early Cretaceous, die out at or before Early-Late Cretaceous boundary
• Reduce the size of most of the scutes on the body relative to Scelidosaurus:
  • Many scutes still present on neck, to allow flexibility but protect from attack
  • Some scutes over pelvic region, and possibly along the sides of a few species
  • Reduction in armor allowed them to be faster and more maneuverable
• However, some scutes become much larger and highly specialized:
  • Plates (flat sided) and Spines (round cross-section) in pairs down the the back (derived stegosaurs such as Late Jurassic North American Stegosaurus had alternating plates rather than pairs)
  • Shoulder spines in the more primitive forms
  • Thagomizers (pairs of laterally facing spines) on the end of the tail:
    • Used as an active defense, swung from side to side
    • Named after a Far Side cartoon
• Stegosaurs tend to have narrow pointed skulls, and probably fed as browsers
• Primitive stegosaurus, such as Middle Jurassic Chinese Huayangosaurus, had front legs only slightly shorter than the hindlimbs; later stegosaurs had much reduced front legs
  • Some paleontologists suggest this would allow them to rear up to feed
• Advanced stegosaur vertebrae look "stretched" dorsoventrally: their centra are vertical ovals in anterior view, and the neural archs are very tall

Ankylosaurs:

• Also first appear in Middle Jurassic, are present but rare in Late Jurassic, but become extremely common in Cretaceous
• Are more heavily armored than Scelidosaurus including:
  • Scutes fused directly to skull roof
  • Rings of fused scutes on neck and front of shoulders
  • Fused scutes over hip region
  • Laterally-facing spines in all but the most advanced forms
  • Small triangular hornlets sticking out from the rear of the dorsal surface of the skull
  • Large shoulder spines in some forms
  • Scutes down arms, and even over eyelids and cheeks of some species!
  • Were most likely passive defenders: hunker down and absorb attacks (although probably pushed back with their spine as well)
• Ankylosaur skulls are generally more broad than stegosaur or Scelidosaurus skulls
• Ankylosaur hips are very wide, and the ilia flare out to form shelf

• Nodosauridae
  • Characterized by longer, lower, flatter skulls, very large shoulder spines,
  • Very likely paraphyletic
  • Hylaeosaurus is an Early Cretaceous European nodosaurid; Edmontonia is a Late Cretaceous North American one
• Ankylosauridae
  • Characterized by shorter, deeper, triangular skulls with small triangular hornlets sticking out of the ventral surface of the back and by complex nasal passageways
  • Gastonia is an Early Cretaceous North American primitive ankylosaurid
  • The most advanced ankylosaurids were the Ankylosaurinae of the mid-Cretaceous of Asia and the Late Cretaceous of Asia and western North America
    • Developed a bony tail club for active defense against predators (tyrannosaurids)
    • Ankylosaurus is an ankylosaurine, and the largest ankylosaurine, ankylosaurid, ankylosaur, and one of the largest thyreophorans (are some very large stegosaurs, too)

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