Thyreophora - "shield bearers"
- a diverse group of armored, primarily quadrupedal ornithischians of the Jurassic and Cretaceous with worldwide distribution
Key evolutionary novelty -
presence of one or more rows of armor plates in skin alongside vertebral column
Primitive Thyreophorans
- among the most primitive ornithischians
a) Scutellosaurus (from scutes - armor plates)
- lower Jurassic of Arizona
- somewhat similar to Lesothosaurus
- relatively small ( approx. 1.2 m long)
- cheek teeth positioned on jaw margins
- short neck and long tail
- hind limbs only slightly longer than forelimbs
- extensive covering of bony plates in skin
b) Scelidosaurus
- lower Jurassic of Great Britian
- medium sized ( 4 m long )
- simple leaf-shaped teeth to tip of snout
- limbs massive and apparently of nearly equal length ( but no fore-limbs known )
- skull lacked armor
- back covered by numerous bony plates
very similar to ankylosaurs
- extremely broad sacrum
Two major groups of Thyreophorans:
1) Stegosaurs
2) Ankylosaurs
Stegosauria - "plated lizards"
- medium to large ( up to 9 m )
- quadrupeds
- herbivorous ornithischians
- small heads
- short massive fore-limbs
- long columnar hind-limbs
- short stout feet with hooves on the ends of toes
Key evolutionary novelty:
- vertical bony plates and spines arranged in single or double rows along the back, neck and tail
a) Huayangosaurus
- middle Jurassic of China
- most primitive known member of Stegosaur clade
- 4.3 m long
- spike-shaped armor along midline of body
- additional rows of small armor plates along each side of the row of spikes
- rather deep (tall ) skull with short snout
- eye sockets located relatively far forward above posterior cheek teeth
- Huayangosaurus smaller and less massive then members of Stegosaurid family
- Huayangosaurus had fore- and hind limbs of almost equal length
Family Stegosauridae
- relatively low skulls
- long snouts
- posteriorly located eye sockets
- large size and massive skeletons
- relatively long hind limbs
Stegosaurus
- midline row of armor plates, but arrangement of plates controversial
- 1 to 2 tons
- low, small, slender head
- narrow, toothless beak
- cheek teeth leaf shaped with small denticles
- small brain ( 2.5 - 2.9 oz. )
- shoulder blade and humerous with thick bony crests for massive shoulder and upper arm muscle attachments
- semi-sprawling gate??
- armor included numerous small, knob-like plates distributed in skin over most of body
Anklyosauria Clade ( "fused lizards" )
- medium to large ( up to 9 m )
- quadrupedal, herbivorous ornithischian
- small heads with leaf-shaped, non-interlocking teeth similar to stegosaurids
- broadly arched ribs of ankylosaurs formed a very wide body covered by small round or square armor plates which produced a continuous shield
- some had spikes or spines on sides of body armor
- others had club on end of tail
- limbs robust with fore-limbs 2/3 to 3/4 length of hindlimbs
- short, stout feet had hooves
Evolutionary novelties:
- low skulls
- closure of fenestrae in front of eye and on top of skull
- added dermal bones to skull
- extensive body armor
- mainly Cretaceous
Two families of Ankylosaurs
a) Nodosauridae
b) Ankylosauridae
a) Nodosauridae
- narrow skulls lacked armor and horns at posterior corner
- presence of spines in their armor
- lack ossified tail tendons or tail club
b) Ankylosauridae
- wide armored heads
- long triangular horns at posterior corners of skull
- no spines
- last few tail vertebrae braced by ossified tendons