Introduction to the Ornithischia – Bird-hip Dinosaurs: Chapter 3
• Useful questions students in class e-mailed to Dr. Erickson (and his answers)
o How have people tested the rates for uranium to lead breakdown?
Using lava flow rates at deep sea ridges and uranium/lead dates of lava away from
o What were the gastroliths (rocks) that the dinosaurs ingested for?
They aided digestion – most reptiles can’t chew, and the rocks churn around in a
muscular crop prior to entering the stomach and do the same job as our molars.
(Crocodiles do this, too.)
• Part 1: Plated Dummies and Walking Tanks
o First dinosaurs had three fingers.
o All of the dinosaurs that fall in these categories are on the left side of the
cladogram diagram (found at the back of the unit textbook).
o Today, we will start looking at the Ornithischia – the bird-hipped dinosaurs. These
constitute about half of all dinosaurs and are composed solely of herbivores.
o All Ornithischians can be identified by –
Bird-shaped hip bones (pubis pointed backward)
Beaks to crop plants
Nut-cracking jaw joint (Don’t need to remember this for test.)
o The first dinosaurs appeared in the Middle Triassic Period about 225 million years
o Like all early dinosaurs, the first ones were small (30-100 pounds) and bipedal.
o NOTALL DINOSAURS WERE GIANT.
o All dinosaurs in each group on the tree started off small (with a small ancestor)
and evolved to become bigger.
o North America’s smallest-discovered dinosaur – Fruitadens haagarorum
o These earliest dinosaurs had simple, leaf-shaped teeth that did not occlude (come
into contact with another top of the opposite jaw) and thus presumably ate
succulent plants, like many lizards today with similar dentitions.
o Amajor group of ornithischians were the Thyreophora – shield-bearers or
armored dinosaurs. o The diagnostic characteristics of the Thyreophora were armor, also known as
“osteoderms,” like that on the backs of alligators.
Early forms like Scuttelosaurus were small and still bipedal.
Later forms of Thyreophorans wre big, more heavily armored, and
• These include the Stegosaurs (plated lizards) and theAnkylosaurs
(the dinosaur tanks)
• Note – all of these dinosaurs were limited in what they could eat
because of their teeth and lack of ability to perform gastrolith
o The Plated-Lizards or Stegosaurs were characterized by –
Plates and spikes along or beside the backbone
o Stegosaurs are the most commonly known members of this group.
The Great Stegosaurus Plate Debate
• For over a century, it has been debated how the plates of
Stegosaurus were arranged, how many there were, and what they
• Theories of orientation ran the gamut of possibilities
o Early renditions following Marsh’s description of the
animal in 1891 showed a single upright row and eight tail
spikes (the animal actually had four).
o But it was found that there were too many plates to do this
– line them all along the back, that is.
o Several other renditions developed
Plates in pairs
Plates on sides of body
Plates all over, on their sides, and many more spikes
All plates paired and in upright rows
Alternating, upright plates
Paired, flat, outward plates
Variable movement of plates o Finally, in the 1980s, Stephen Czerkas thought he had
discovered what the plates had really looked like.
Some early finds showed they were staggered,
which had somehow been ignored.
Single rows would have made the row longer than
the tail; attachment fibers show they were only
attached as bases.
o Arecent, very rare find shows that plates were actually all
staggered, and there were probably a few more than 17
plates – case closed!
o More madness – what were the plates for?
Upright for defense?
Against body for defense?
Flappable for scaring away allosaurs – no way!
Camouflage against a backdrop – no!
Thermoregulators – plates up for sun, down for
cold, or if permanently up, the body was turned to
orient with sun in the morning and turned parallel in
Staggered for heat dissipation in the wind