The Prerequisites

Over the last two posts, I've given a basic idea of the different approaches of Linnaean taxonomy and cladistics. Carolus Linnaeus' system creates groups of organisms based on physical characteristics: how they're built, how they move around, how they reproduce, and so on. Each large group contains organisms with basic similarities. Mammals are a class containing fur-bearing, milk-producing, warm blooded creatures who give birth to live young. Within class mammalia, we have orders such as the artiodactyls, which claim deer, hippos, and sheep as members. Each of those three then have their own families, and so on. Cladistics is a system which attempts to clear up any inconsistencies and confusion caused by grouping organisms together in such a way, creating charts of evolutionary relationships based on shared ancestry.

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.

Cladding About

My last post in this short series about the classification of dinosaurs, I wrote about the Linnaean system of classification, which uses physical characteristics to arrange organisms into clans. It's the source of binomial nomenclature, the convention that gives us those familiar "scientific names" for organisms; a wolf is known as Canis lupus.

Now we'll deal with cladistics, a method of biological classification which uses evolutionary relationships to create a sort of family tree of life. It was developed in the middle of the last century by German entomologist Willi Hennig. He's the fellow there to the right (and if you know a good way to caption right- and left-justified images on blogger, fill me in).

Anyway, a graphic representation of this tree is called a cladogram. I'll use a cladogram of the dinosauria for an example; it comes from the useful, easily-navigated, longtime bastion of dinosaur knowledge on-line, Jeff Poling's omnipedia at dinosauria.com. The page for the cladogram is here; follow this link or click the image to see it full size.

Cladogram copyright Jeff Poling

What you see here is a tree- or bush-like shape. Each intersection, or node, between branches represents a shared ancestor for all branches that follow the split. The cladist is not interested in building a "top-down" system with large, grandly named categories containing a huge number of organisms. The major unit of classification is the clade. This means that every node signifies a clade, and that traditional terms such as "reptile" don't make as much sense as they used to. Since a clade is defined as "All organisms derived from the most recent common ancestor of organisms X and Y," descendants with radically different morphological traits cause a bit of a problem when "translating" traditional Linnaean groups to cladistics.

For instance, we might try to say that "reptiles" are the first reptile and all of its descendants on a cladogram. Assuming that birds are descendants of small Jurassic theropod dinosaurs, and that the original dinosaurs derived from Permian reptiles, birds are actually reptiles. The Linnaean method, using physical traits, clearly states that reptiles are scaly, egg-laying, cold-blooded tetrapods. Birds are bi-pedal, warm-blooded, egg-laying, and covered in feathers.

Dinosaurs represent an interesting puzzle, then. As does reconciling the fat and happy Linnaean system with lean, mean cladistics. But we'll save that for the next post.

As a side note, I really cannot say enough nice things about Dinosauria.com. It is a great example of the way the internet worked before advertising became so prominent. Nothing flashy. Everything loads quickly. Well-written, clearly explained information. Accessible. Mega-props to Mr. Poling.