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.

Pterosaurs and Dinosaurs

Yesterday, I wrote a few kind words about the pterosaurs, the flying reptiles who were the first vertebrates to figure out flight. Pterosaurs are so frequently referred to as dinosaurs that yesterday, I felt the need to clarify that I am definitely not one of the philistines who would make such a mistake. Biology has its own form of the gene that regulates the behavior of the "grammar police," and I bashfully admit that my hackles are raised when any old freaking big dead thing is referred to as a dinosaur. To me, it's as ridiculous as referring to a bear, a mountain lion, and a cow as dogs. Seriously, that's like a toddler level mistake.

But it's more than simple persnickityness. It's the desire to call things by their right names, to set order to the world. Just as a flat piece of paper with colored lines on it can help us get a handle on the world we move around on, classifying organisms puts them in context so we can learn how they interact to create that world. It puts us in context. It's a basic tool, and blithely throwing around names is essentially dumb. It cheats us out of knowledge. It also seems to me that especially when dealing with knowledge that requires some sort of translation for the everyday reader (such as science), using the proper terms and explaining them clearly is really, really important. Crucial, even.

So over the next few posts, I'll try to explain what makes a dinosaur a dinosaur. Since we have plenty of time and plenty of bytes, I'll start with how it is that we classify any living thing at all.

Here's how it breaks down. You're probably familiar with Linnaean classification. Most of us learned this in grade school. The major divisions are Kingdoms, Phylums, Classes, Orders, Familys, Genera, and Species. There are many smaller divisions, as well. For you and I (apologies to any genetically engineered smart-sharks who may be reading this), our basic Linnaean classification breaks down thisaway:

Kingdom Animalia (internal digestion, soft cell walls)

Phylum Chordata (backbones)

Class Mammalia (furry milk-producers)

Order Primates (monkeys, greater and lesser apes)

Family Hominidae (bipedal, tailless apes)

Genus Homo (smart apes)

Species Sapiens (us)

Carl Linnaeus (or Carolus, as he preferred) came up with this system of classification in the eighteenth century, around a hundred years before Darwin published The Origin of Species. Linnaean classification is largely concerned with grouping organisms into large groups and then tying those large groups together. The way organsims are divided into groups, and then into their own species, is by morphology, or physical characteristics. It was a hugely important step in the quest to figure out how all of the stuff in the world is related to each other. And Linnaeus really was concerned with stuff. He applied this system to minerals, as well. Before biological evolution was accepted, anything was on the table. We didn't understand what fossils were - they sure looked like living things, so maybe they were living things, in a different state of development.

After Darwin figured out the mechanism by which organisms diverged into species, it became necessary to classify organisms according to evolutionary relationships. For this, we use a system called cladistics. This is an elegant system which groups organisms into clades, or groups with a shared ancestor. In his day, Darwin wondered about the means of inheritence: how in the heck does my sister end up with blue eyes while mine are brown? We now understand genetics, and we can use DNA and RNA sequencing to establish evolutionary links which crude physical characteristics would never let us know.

But we're going to save cladistics for next time. HAVE A GOOD WEEKEND ALREADY.

Today's post brought to you by the National Commision on Italicization.