Reading Cladograms
Read and create simple cladograms (branching diagrams) that show how groups of dinosaurs are related based on shared features, understanding that species sharing more features are more closely related
What a learner can do afterwards
- Explain that a cladogram shows evolutionary relationships based on shared features
- Read a simple cladogram to identify which two dinosaurs share the most recent common ancestor
- Add a new species to a partially completed cladogram based on its listed features
1 · Read
Ever wonder why T. rex feels more like a Velociraptor than a Stegosaurus? A cladogram is a branching diagram that shows how groups of dinosaurs are related to each other. Every branch point on it stands for a shared ancestor, an animal that both branches came from.
Dinosaurs that share more features branch off closer together, near the top. Dinosaurs that share fewer features branch off farther apart, near the bottom.
T. rex and Velociraptor both have sharp teeth, hollow bones, and clawed feet. Stegosaurus has none of those; it has bony plates on its back instead. On the cladogram, T. rex and Velociraptor branch off from a point close together, while Stegosaurus branches off much earlier. If a new dinosaur shows feathers and hollow bones, it joins the branch with the dinosaurs that share those traits.
To add a new dinosaur to a cladogram, look at its features. Find which group already on the diagram matches the most of them, then attach the new branch right next to that group.
A cladogram groups dinosaurs by shared features. The closer two branches meet, the more closely related those dinosaurs are.
More to read
Phylogenetic trees
A cladogram is a simple diagram of who is related to whom. Species sit at the tips of the branches, and each fork marks one common ancestor they share. To judge how closely two species are related, trace the lines back to where their paths meet. A recent meeting point means close cousins.
New species join the tree through their traits. If a fossil shows the same new traits as one branch, it belongs on that branch. One shared trait can move a whole species to a new home on the tree.
Learn: Phylogenetic trees
Every tree starts with one rule: species that share a more recent ancestor are more closely related. Look at any fork in the branches. The two groups above it are sisters, and both are more distant from anything outside that fork. Reading a tree means reading these forks in order.
Pick any two dinosaurs on the tree and find their shared fork. Then pick a third dinosaur and compare. The pair whose fork sits nearest the tips shares the most recent ancestor, so they are the closest pair.
Learn: Building a phylogenetic tree
Scientists build trees from a table of traits. Each row is a species and each column is a trait, marked present or absent. Species that share the most new traits are joined first. Each join creates a branch that stands for one shared ancestor.
Now add a mystery species. Read down its trait list and find the branch where every trait lines up. Place it there and you have grown the tree by one. If two branches both seem to fit, the one with more matching traits wins.
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2 · Watch
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