GEOL 104 Dinosaurs: A Natural History

Fall Semester 2000
Systematics II

Cladograms represent a way of graphically portraying the evolutionary relationships of organisms. How are these relationships discovered?

Anatomical (and behavioral, and genetic) features that are similar between taxa are said to be shared characters. There can be three different reasons these features are shared:

Cladograms are constructed by sorting out the simplest (most parsimonious) arrangement of shared derived characters. For example, below is a cladogram of living terrestrial vertebrates, and a list of characters above. In that list, a red "X" represents the presence of that feature in the taxon, and a black "0" represents the absence of that feature.

So, in the diagram above:

Or, to put it another way:

  • Limbs with digits is a synapomorphy for Tetrapoda, but a symplesiomorphy for all its subclades (e.g., Amphibia, Amniota, Mammalia, Reptilia, etc.)
  • Eggs with amniotic fluid is a synapomorphy for Amniota, but a symplesiomorphy for all of its subclades (e.g., Mammalia, Reptilia, Testudines, etc.)
  • Toothless beaks is a homoplasy (i.e., a convergence) between Testudines and Aves, as the forms intermediate between them (lepidosaurs, crocodilians) retain the primitive condition of having teeth. Another particular kind of character is an autapomorphy, a feature found in only one of the terminal taxa in an analysis.

    Another, simpler way of drawing the above cladogram is to indicate the characters on the braches where they would have originated:

    In the cladogram above, the characters evolved on the branch where the red line indicate:

  • Limbs with digits evolved in the common ancestor of all tetrapods, and is a synapomorphy for Tetrapoda;
  • Eggs with amniotic fluid evolved in the common ancestor of all amniotes, and is a synapomorphy for Amniota;
  • Etc. Note that even the synapomorphies shown here can be modified through additional evolution at any point further up in the cladogram:

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