BSC-1005 Lecture Notes - Middle Triassic, Neuroanatomy, Stegosaurus
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Introduction to the Ornithischia – Bird-hip Dinosaurs: Chapter 3
•Useful questions students in class e-mailed to Dr. Erickson (and his answers)
oHow 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
oWhat 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
oFirst dinosaurs had three fingers.
oAll 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).
oToday, we will start looking at the Ornithischia – the bird-hipped dinosaurs. These
constitute about half of all dinosaurs and are composed solely of herbivores.
oAll 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.)
oThe first dinosaurs appeared in the Middle Triassic Period about 225 million years
oLike all early dinosaurs, the first ones were small (30-100 pounds) and bipedal.
oNOT ALL DINOSAURS WERE GIANT.
oAll dinosaurs in each group on the tree started off small (with a small ancestor)
and evolved to become bigger.
oNorth America’s smallest-discovered dinosaur – Fruitadens haagarorum
oThese 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.
oA major group of ornithischians were the Thyreophora – shield-bearers or
oThe 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 the Ankylosaurs
(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
oThe Plated-Lizards or Stegosaurs were characterized by –
Plates and spikes along or beside the backbone
oStegosaurs 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
oEarly renditions following Marsh’s description of the
animal in 1891 showed a single upright row and eight tail
spikes (the animal actually had four).
oBut it was found that there were too many plates to do this
– line them all along the back, that is.
oSeveral 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