World of Dinosaurs/How We Organize Life

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What is a clade?Edit

A clade refers to one bushy branch of an evolutionary tree.

  • If we consider any two animals, we can guess what features were common to their last common ancestor.
  • If we consider this last common ancestor, and ALL of its descendants, we are considering one whole clade.

A clade can be small or large.

  • A biologist studying squid might be interested in one particular clade of squids related to the giant squid.
  • A paleontologist studying birds might be interested in a clade so broad that it even includes crocodiles!

If we intentionally exclude a sub-branch, we are no longer talking about a whole clade.

  • For example, the clade dinosaurs includes the last common ancestor of a Triceratops and a pigeon, and alllllllll of its descendants.
  • If a person says, "Well, birds don't really count, because they are birds!" that's ok. Just know they are no longer talking about the WHOLE clade.
  • There are LOTS of different terms for the ways folks break up or combine parts of clades, which anyone can learn about in advanced classes, books, online, etc.
  • In this class, we just want to ask, "Hmmmm, are the animals I'm considering constituting a whole clade? Am I excluding anyone?"

This book will focus on a set of clades into which we can divide dinosaurs and non-dinosaurs.

What is a cladogram?Edit

A cladogram illustrates how a person interprets relationships between different clades of animals.

For example, the clade of dinosaurs fits within a larger clade of animals with bones.

There are lots of formal rules about how to draw and understand cladograms. In this class, we keep it very simple:

  • Names of animals are placed at the edges of the diagram (often the top edge), and each name is a tip – the end of a branch.
  • Every spot branches split apart, we try to draw a simple y – just two branches spreading out (not three or five!).

These illustrations represent specific interpretations.

  • Any two animals ought to have a common ancestor.
    • How different it looked, how long ago, and how far away can totally vary - depends on the clade you're considering.
    • We do NOT need to know exactly what the last common ancestor looked like.
    • We do NOT need to know exactly how many descendants have lived since that common ancestor did, or what they looked like, etc.
  • When we create a y branch to represent the splitting apart of two lineages of decent, we mark that junction with a node.
    • This node basically represents the mysterious last common ancestor.
    • This node is an easy way to recognize the base of a clade.
  • If we see two branches split away from a node, we should mark at least one of those branches with a trait.
    • A trait is an anatomy feature that we show arising within animals along that branch.
    • We will assume all animals along that branch path still have the trait, unless we show them loosing it.
    • If we take a trait away from a subsequent branch of life, we need to mark this too.
  • Some cladograms are built by illustrating the tree that requires as few traits as possible.
    • But traits are so complex, from a genetic and developmental perspective, that this strict rule doesn't work all the time.
    • A trait that actually has really different underlying anatomy is not good for a cladogram.
      • For example, butterflies and birds each have wings.
      • But their wings are constructed in totally different ways!
      • Instead of marking a cladogram with a very complex anatomy feature like "wings", try to observe and include anatomy more fundamental and specific:
        • bones
        • the shape, size, or arrangement of the bones

PhylogenyEdit

Phylogeny refers to a set of evolutionary relationships between animals.

We can consider this as an absolute concept. For example, "We may never know the entire phylogeny of Cretaceous birds, but we have learned a lot more about it in the past decade."

We can also use the term to refer to a specific interpretation of the evolutionary relationships. For example, "Dr. Smith and Dr. Kunungo present conflicting phylogenies for fossil crabs." We'll need more observations to decide which best resembles their actual history.

A cladogram illustrates a phylogeny.

How do we read cladograms?Edit

Cladograms DO illustrate:

  • Traits that arose as lineages of animals diverged and became more and more different from each other.
    • Example: Stumpy tailbones arise on branches of birds as they diverge away from flying dinosaurs that still had long tails.
  • Traits that arose as lineages of animals that were really different acquired similar anatomy to meet an environmental challenge.
    • Example: Penguins have flippers, and dolphins have flippers, but the last common ancestor of a penguin and a dolphin did NOT have flippers.
    • This is called convergent evolution and it's so rad.
  • Clades nest within each other:
    • Parrots are a clade of birds.
    • Birds are a clade of dinosaurs.
    • Dinosaurs are a clade of vertebrates (animals with a spinal chord).

Cladograms do NOT illustrate:

  • When, why, or how the branches split.
  • What the common ancestor looked like, exactly.
  • Which animals are "most evolved"!
    • Dr. Winkelman studies squid, so she'll make a cladogram all about the nuanced differences between calimari squid and giant squid.
    • Dr. Melstrom studies crocodiles, so he'll make a cladogram all about the nuanced differences between alligators and crocodiles.
    • Dr. Yacobucci studies ammonites (extinct shelled squid monsters), so she'll make a cladogram about different ammonites.
  • Because each person studies different animals, they pay attention to different traits. If the animal they study is extinct, or smart, or fancy, doesn't make it more or less evolved. We will explore the relative terms "basal" and "derived" in class during week two.

Why are cladograms important for this class?Edit

Once you get the hang of it, cladograms are easy and fun to read.

  • It's like a road map for evolution.
  • Every cladogram shows an interpretation.
  • Most interpretations can be improved with more observations!

Paleontologists can use cladograms to:

  • Predict that an animal with a certain set of traits should exist in the fossil record.
  • Predict that an animal with a certain set of traits should be fossilized in a particular layer or time-period.

When paleontologists find new fossils:

  • The cladograms can be changed to include these new observations.
  • The new tree gives folks ideas for new predictions on where or what to look for.

In World of Dinosaurs, we want to observe the anatomy features that were used to support the phylogeny illustrated in a cladogram. This class will provide special access and focus on fossils – in the Natural History Museum of Utah, as digital models, and as 3D prints – and live animals – at the Tracy Aviary and via video – on which we can SEE the traits marked on the branches.

This class uses claodgrams as puzzles.

  • We will ask students to read, remember, and reconstruct some cladogram features.
  • This is because the cladogram puzzles are useful tools to visualize and talk about our interpretations.
  • This class is NOT about memorizing cladograms as unchanging facts!