Both systems have their champions and detractors. At the moment, cladistics is the dominant paradigm, though we still use Linnaean binomial nomenclature at a species level. And the names for larger groups, as they've been around for a while, are still used to describe high-level clades. The pursuit of a single, clear system and naming convention is ongoing. I won't dig myself any deeper into what feels like a huge hole now, and we'll just get to the point of this series. How do dinosaurs fit in? What makes a dinosaur a dinosaur?
Well, there isn't any way to define a dinosaur without getting into some anatomical nitty-gritty. First of all, dinosaurs are diapsids, the main branch of reptiles (minus turtles), who get their name from the two holes they have on either side of their skulls.
Petrolacosaurus, the earliest known diapsid. By Arthur Weasley.
Within the diapsids are a group called the "ruling reptiles," or archosaurs. This is one of the "smaller divisions" I mentioned in my post on Linnaean taxonomy, falling somewhere between a class and an order. Anyhow, the most important thing here is that archosaurs have these distinctive characteristics:
- Additional holes in the skull between the eyes and the nostrils
- Holes in the lower jaw
- Teeth set in sockets
- An extra knob serving as a fourth place for muscle attachment on the femur
This last bit was likely key to the origin of the dinosaurs, because it is the starting point for where dinosaurs distinguished themselves from the crocodiles and other members of the archosauria. All other reptiles had, and have, a sprawling leg orientation. But dinosaurs had a couple other unique features that set them apart.
Those are the structural details which formed the basis for the multitude of forms dinosaurs would take over their nearly two hundred million years of existence. I'll stop here and then we'll pick up with explaining why certain other ancient beasties are definitely not dinosaurs.
EDIT 9-2-09: Cleaned up a bit for clarity.