Early thyreophorans small (1 m long) bipedal animals; later forms much larger and
quadrupedal, with heavier armor
Best known early of these is small bipedal Scutellosaurus (Early Jurassic,
North America).
Scelidosaurus (Early Jurassic, Europe (maybe North America, China). Larger
than Scutellosaurus, about 4 m long. Armor was proportionately much larger.
Heavy armor required it to be an obligate quadruped. May actually be the basalmost
ankylosaur.
Thyreophorans more derived than Scelidosaurus are divided into two major branches:
- Stegosauria, concentrating on more active defense
- Ankylosauria, concentrating on more passive defense
Both appear in the Middle Jurassic, just as more sophisticated predators are appearing.
Stegosaurs:
- First appear in Middle Jurassic, common in Middle and Late Jurassic, rare in Early Cretaceous,
die out before Early-Late Cretaceous boundary
- Reduce the size of most of the scutes on the body relative to Scelidosaurus
- Lost the ossified tendons found in other ornithischians
- 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
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 hips are very wide, and the ilia flare out to form shelf
Ankylosaurs include the Nodosauridae (characterized by an expanded process on the scapula) and
the highly derived Ankylosauridae. Definite nodosaurids are known from the Cretaceous of North America
and Europe. Various Late Jurassic and Early Cretaceous ankylosaurs have been placed at the base of either
Nodosauridae and Ankylosauridae, although some have proposed a third clade: the "polacanthids".
- Hylaeosaurus is an Early Cretaceous European polacanthid; Edmontonia
is a Late Cretaceous North American nodosaurid
- 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
- Developed a bony tail club for active defense against predators (tyrannosaurids)
- Ankylosaurids are known from the mid-Cretaceous of Asia and the Late Cretaceous of Asia and western North America
- Ankylosaurus is the largest ankylosaurid,
ankylosaur, and one of the largest thyreophorans (are some very large stegosaurs, too)
To Next Lecture.
To Previous Lecture.
To Syllabus.
Last modified: 14 July 2006
