So if you haven't been reading along, thus far we have discussed:
Now, we will look at the first of those two groups, the bird-hipped dinosaurs of the Ornithischia order. This group is generally divided into five subgroups, though there can be a lot more. For the ease of simplicity, we will just look at the five most common groups, the ones that feature many of the dinosaurs everyone knows (especially the ones you can recognize but never remember their names, we have all been there).
These groups are the Ceratopsians, the Ankylosaurs, the Stegosaurs, the Pachycephalosaurs, and the Ornithopods. We will look at each one and what makes them identifiable. Figure this out, and you will be a regular Ornithischian expert (not sure how well this impresses at cocktail parties, people stopped inviting me to cocktail parties).
Ceratopsia means 'horned faces' in Greek (hey, if you aren't learning anything about dinosaurs, you should at least be learning some Greek). They are an iconic grouping of dinosaurs I think we can all easily recognize. Most members sport horns on their skull and typically some sort of boned frill. This isn't always the case as can be seen in some species like the parrot-beaked Psittacosaurus. But if you see a specimen strutting that iconic frill and horns, of any size, odds are it is a ceratopsian of some sort. The most noted member of this group is, of course, Triceratops, but also the Zuniceratops pictured on the right.
Another unique feature is the Rostral bone, that forms the distinct beak-like mouth. This feature is found on ceratopsians and no other animal.
With a name meaning 'crooked lizard', these dinosaurs were bulky armor plated creatures. They kept to all fours, walking on short powerful legs, but it is their armor that made them so distinct. Like a crocodile, Ankylosaurs were covered in bony protrusions. These structures are called osteoderms ('bone skin', learning yet?) and served to protect the animals from predation. It is also this structure that connects them to the next group.
Stegosaurs and Ankylosaurs could be considered members of a subgroup within a subgroup. Together they are part of a subgroup called Thyreophora (armored dinosaurs), but they still remain parts of their own subgroups. Early forms of Stegosauria were probably very similar to the Ankylosaurs, but by their height in the Jurassic period, they had distinct armored scutes along their back. (Fun Fact: that spiky tail structure that Stegosaurus so expertly wielded, has no official name, but has come to be known as a "Thagomizer" named by cartoonist Gary Larson).
One of my favorite groups if only for their bizarreness. Meaning "thick headed lizards", this group contains dinosaurs who sported a hard skull dome and cranial spikes. The jury is still out on whether these creatures used these thick skulls for head-butting competitions or not, as heaps of evidence seems to be piled on either side of the argument. A notable species from this group is Dracorex hogwartsia, meaning 'dragon king of Hogwarts' (see scientists can have fun too). But that isn't the only connection between this group and science-fiction. They came to popularity through the classic SF short story A Gun for Dinosaur, written by L. Sprague de Camp in 1956, featuring time travelling dinosaur hunters.
Meaning 'bird foot', this fifth group of ornithischian dinosaurs started as small creatures that ran on two feet. They became one of the most successful groups of dinosaurs on the planet. Though some earlier forms had four toes, most had three-toed feet that looked very much like the splayed digits of birds (hence their name). Many had thick beak-like mouths that helped them graze and feed. The species that make up this subgroup are numerous. For a while scientists threw any bipedal ornithischians into the pile. But most notable of the ones that have remained over the years are the hadrosaurs, large duck-billed herbivores that were among the most widespread of dinosaurs.
The most common North American example of this group is the Iguanadon.
There are far more groups than this, and really I have simplified much of this so it is far easier to understand. The proper breaking down of the Ornthischia order is a bit more complex. But this should at least give you a better handle on the kinds of dinosaurs that made up the bird-hipped order.
Next time: Part 4 - Saurischian Dinosaurs - Sauropods