Part 1: What is a Dinosaur?
Dinosaurs are an incredibly diverse grouping of animals. Their bodies ranged from the small to the enormous. Some were slow and lumbering herbivores, while others were quick-footed hunters. But what really ties all these creatures together?
To understand that, we first have to understand a bit of how scientists group animals. The most commonly used way of looking at this is called the Linnaean system of classification. Simply, this is a system by which animals are put into groups based not only on similarity but on their distance since a common ancestor. As we move through the groups, the number of animals diminish until we arrive at a solitary species.
Let's look at the tiers of this classification system. Going from the largest group down to the individual, they go:
(A quick and easy way to remember these is DKPCOFGS, or 'Dumb Kids Playing Cards On Freeway Get Smashed')
We can use the example of a grizzly bear to further explain these groups.
The first tier, Domain, is a relatively new grouping, so it is forgivable when it is easily overlooked. Domain is divided into three groups, the three most basic groupings of life on earth. Mostly, it is based on the type of cell that makes up that living thing. The three domains are Archaea (single celled organisms without a nucleus), Bacteria (which consists of a number of microorganisms), and Eukaryota (which is any organism whose cells contain a nucleus and other cell structures, that means us and most other multicellular things).
So our bear is obviously in the Eukaryota domain, next are the kingdoms. The Eukaryota domain can be divided into four kingdoms: Fungi (mushrooms and such), Protista (microorganisms including some molds, plankton, and algae), Plantae (just a fancy way of saying plants), and of course, the kingdom of us and our bear, Animalia (animals).
Continuing to get more refined, our next grouping is Phylum. As we divide up kingdoms into phyla and the phylum into classes, things get pretty darn complicated. But the important idea isthat each time we drop down to the next level, we eliminate creatures that areless alike. Like refining your search on Google, by the end we should end up with only results that fit closely together.
This system can also show us the basic relatedness of creatures. From an evolutionary standpoint, we have to go further back in time to find a common ancestor for everyone in the larger groups. Using our example, we have to go back nearly a billion years ago to find a common ancestor between members of the same kingdom. Making the grizzly and the starfish, very distant relatives. But we only have to go back 100-200 million years ago to find an ancestor connecting most mammals.
So let's just quickly explain the tiers of the grizzly's chart, before we get back to the giant ancient reptiles I know you were wanting to read about (seriously, why am I talking about bears?).
The grizzly is an animal, part of the enormous Animalia kingdom. It has a backbone or spinal cord, so we also place it in the Chordata phylum. It has fur and gives milk to its young, so we place it in the Mammalia class. It eats meat, a distinct order of mammals we call Carnivora. And finally we place all the bears into a nice cuddly family, I would have named them the Barenstains, but we'll use Ursidae instead. A species is usually named after the genus it is in, and the particular species, giving us both parts of the animal's name. So the grizzly comes from the Ursus genus(Latin for bear) and the species name arctos (which is Greek for bear). Latin and Greek together, and we get a species' scientific name. (this is called binomial nomenclature - meaning 'two names').
So you are probably thinking. Okay, that is great, you managed to fill most of my time talking about bears instead of dinosaurs. Yes, I did spend a long time on grizzlies, but with a purpose. We can now use this info and apply it to something we know a little less about.
So what are dinosaurs?
Well we know they were part of the Eukaryota domain, and the Animalia kingdom. That part is easy. We can also tell from fossils that they had backbones, so we can put them in the Chordata phylum. But where do we go from here?
This is where clades come in. Clades are a branching structure on our Linnaean system tree. They often sit in place of the Class grouping, but clade and class are not interchangeable terms. Just as we know that mammals branch off somewhere from the other chordates, we can assume that dinosaurs do too. So we give them a clade under the Chordata group that we call Dinosauriformes, literally things with a dinosaur shape. This may sound a bit generic, and trust me it really does get a lot more complicated, but I am trying to avoid some of that for simplicity.
Dinosauriformes can also be grouped into the Dinosauria clade (like a said, complicated).
So that brings us back to the question: What is a dinosaur?
Well using what we now know of the Linnaean system, we can easily see what is not a dinosaur. But what about all those other ancient reptiles? What about lizards? Dinosaur means "terrible-lizard", right? Well the odd history of that name aside, dinosaurs are not lizards. There were many ancient reptiles, including lizards that existed millions of years ago. But they did not belong to the Dinosauriforme clade and it is a very unique thing that gets you into that exclusive club. And, like using a hoola-hoop, it was all in the hips. The hip socket to be exact. Dinosaurs had evolved a very distinct pelvic structure that made them stand out from other creatures of the time. In fact, it is something that we possess in our own bodies (though we mammals evolved it separately). Called the acetabulum, it is simply a socket in the hip for the leg to connect to. Ours is specifically designed to allow us to walk upright, and it similarly allowed the dinosaurs to put their legs beneath them, rather than to the sides like other reptiles.
So now you know. A dinosaur is an ancient backboned animal with a fancy hip-socket (no wonder old people get called dinosaurs). Okay, so it is more complicated than that. So let's dive further in...
Next time : Part 2 - Dinosaur Groups