C is for Clade

It’s all about the family trees. Today’s dinosaur-themed post is about how the dinosaur world is organized.

I know some of you are thinking, geez, don’t you know the names of any dinosaurs? Why isn’t A to Z going to be about Ankylosaurus to Zupaysaurus? I will have a few posts dedicated to specific dinosaurs, especially my favorites. But you can look up tons of dinosaur lists A to Z. There are kid’s alphabet books that do that. I promised to give you “all about dinosaurs,” not all about 26 dinosaurs. We need to round out this paleontological survey a bit more in order to achieve that goal.

How paleontologists organize the dinosaurs is quite important because it helps us understand how dinosaurs did what they did. As I mentioned in post “A,” the ankle bone structures differing from those of crocodiles urged scientists to think about why, and why was because dinosaurs stood upright. Hip-bone differentiation helped identify the two big groups — wait until letter “H”! And then… the birds. But we’ll get to that.

This Ain’t Your Mom’s Biology

If you took biology or some sort living thing-ology way back when, you might have learned Linnaen taxonomy. Translated, that means you might have had to learn the old organization system that was developed under this Swedish guy, Carolus Linnaeus. Maybe you learned the mnemonic “King Philip Came Over For Green Sugar” or something like it. I still remember it from 7th grade: Kingdom Phylum Class Order Family Genus Species. Remembering the organization for human beings always worked for me: Animal (Kingdom) Vertebrate (Phylum) Mammals (Class) Primates (Order) Man ( Family) Homo (Genus) Sapiens (Species).

The Linnaean system (I keep having to look up how to spell that, his name was not organized very well!) … worked for a while, given that Swedish guy didn’t have computers, DNA, other scientists to pester him, and all the technology and baggage that exists today. He was grouping like things, and it’s not impossible to think that feathered things are different from scaly things which are different from furry placenta-based air breathing tool users.

But, over time, scientists have noticed a lot more animals, plus have thought about including ones from the past. The system of organization–that’s what a taxonomy is, the “order of things”–needed to be more inclusive. The new system is called cladistics and it uses clades. Clades are really just tree structures. They can be shown up, down, or sideways. Everything below a tree branch has the same thing. The Linnaean KPCOFGS system can be organized in a tree-like view. Notice in that “common ancestor” diagram above how crocodiles and birds are more similar to mammals than they are to amphibians and fish because mammals and reptiles use amniotic eggs.

Follow Branches on the Tree of Life

The clade view also organizes things around a common ancestor as well as common traits, rather than “only” what things are like today. It’s important with clades to get the trees correct. Anything that can be viewed as all-inclusive on a branch is a clade. So in the view above, you can’t group things together from different branches, and you can’t include things on a single branch from your clade.

Here’s a clade system for humans again. Notice that you can use clades only for modern creatures, just as you can use them for extinct creatures. The more you examine these clades, the more you can understand how grouping works. Humans are a little like mice, treeshrews and bushbabies, but they’re a lot more like chimpanzees.

Clades Evolve

Cladistics emerged from the changes in ideas plus technology. Darwin (and Alfred Wallace) both developed the concept of natural selection. You can think of it as survival of the fittest if you like, but it’s important to think more about adaptation than “winning.” The environment changes. Organisms that adapt to it will be the ones that survive. But the environment can change again. Dinosaurs “won” … for a time. They were the fittest at one point. But then the environment changed again. So don’t think about evolution as progress; think about it as adaptation. (Makes me crazy when people say Darwin=survival of the fittest because what Darwin did was talk about how change was not=progress.)

One way to see the difference between adaptation and similar organisms is to think about what’s in an animal’s body vs. what it does. Different animals developed flight. Bats, pterodactyls, and birds all fly. But bats don’t lay eggs, and pterodactyls don’t have hip bones like birds. They are all from significantly parts of the clade, based on the design of their bodies–even though all three developed adaptations in their hands and arms that led to wings that led to flight. Pterodactyls split off from dinosaurs pretty early in the tree structure. Both come from the same ancestor as a crocodile, but that’s about all they have in common. As Kristi Rogers, my expert guide for this dino journey, puts it “there are different ways of being the same.”

Natural selection–adaptation of bodies and bones to the environment–was an important idea to include in clades because it allowed scientists to expand the things to include in the tree. If you only include modern animals, your tree will be a lot smaller than if you try to include everything that every lived.

But now the data set starts to get really big. Enter computers. Now you can have tons of animals–extinct and modern–and every little bit of data you can capture about what their bones were like. As scientists have found more and more examples of dinosaur fossils especially, they can make the trees a lot more complicated.

You may be realizing why I keep circling around the birds thing. Birds have a ton of things in common with a T rex. They’re both part of group fairly far down the tree branch of dinosaurs, called the Theropods. In a future post, I’ll talk a bit more about why that’s the case, but sorry to rip away that security blanket. Birds don’t have their own group–Linnaeus gave them their own Class (Aves), but this view suggest he was wrong — the ancestors of birds were reptiles.

Two final points about clades. First, when someone is talking about a clade, they might put up all the known branches of a tree, or just a few. These two diagrams are both accurate. The one on the right just has more branches of a tree.

The second point is that more data means more changes. The clades may shift around over time. Especially when looking at branches early on the tree, the branches may need to split. For instance, forever… meaning since I was in college… dinosaurs have been split into the two groups, Saurischian and Ornithischian. Hip-bones (letter “H” I promise). But recent scientists think that they need to be split into three groups.

Baron, Norman, and Barrett in a 2017 article in Nature make a pretty strong argument, with lots of new data, that suggests splitting dinosaurs into a third group.

Some scientists agree with this split of three, but this idea is only a few years’ old. It will take a lot more arguing and debate and computer analysis for this to be accepted. How you build out the entire tree is going to be subject to change and controversial.

It’s kind of like music. Everybody knows you can split Classical from Rock music. But within Rock, do you split classic rock from metal? What counts as “classic rock” now — does that include Sheryl Crow as well as Led Zeppelin? Does rap have its own group or is it a sub-category of something else like R&B? If it’s a sub-category, then did both classic rock and rap “evolve” out of the blues? I know you’ve had these arguments–if you have, then you understand taxonomy.

And you could build your own clade.