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Submitted: November 26, 2009
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Eoraptor and Pisanosaurusby ~Albertonykus

Traditional Art / Drawings / Animals

Here, a Pisanosaurus mertii flees from a Eoraptor lunensis. Both of these dinosaurs are among the most basal of the entire dinosaur family tree. Pisanosaurus mertii is an extremely basal ornithischian (bird-hipped dinosaur). In fact, its pubis bone still appears to point forward, not backward like other ornithischians. Eoraptor is probably a very basal theropod ("meat-eating" dinosaur*) a very basal saurischian (lizard-hipped dinosaur), or even a basal sauropodomorph (long-necked "plant-eating" dinosaur)! "Lizard-hipped" refers to the fact that most saurischians have forward-pointing pubes, as do lizards (and mammals and turtles and crocodiles and so on). However, many saurischians, all of them recruits from the ever so weird maniraptors**, actually have backward-pointing pubes. (Actually, some members of one group of non-maniraptor saurischians, the herrerasaurs, have somewhat backward-oriented pubes, but theirs are not as extreme as in the maniraptors.) As you can see, the direction of the pubis isn't always reliable in diagnosing whether a dinosaur is a saurischian or ornithischian. Here's the major difference: saurischians have hollow vertebrae (which hold air sacs), ornithischians don't. Also, ornithischians have a bone on the tips of their lower jaws called the predentary that saurischians don't have.

*As I always emphasize, not all theropods eat meat.
**These recruits include birds. (Which is how ornithischians got their name. Just remember that birds are actually saurischians.)

~SpongeBobFossilPants Dec 15, 2011

Either you've been to the Redpath Museum or this is a very freaky coincidence.

On its plaque of the phylogeny of the Dinosauria, the museum uses these two to represent the basalmost* members of their respective clades.

*Assuming that is actually a word; I saw it in Holtz's syllabus and thought it sounded appropriate.

~Albertonykus Dec 15, 2011

It's a coincidence. XD

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