Fun with Cladograms!
In a cladogram, it is the branching relationships which are important, not the "right to left"/"top to bottom" order. As long as two cladograms contain the same branching relationships, and do not have any contradictory branching relationships, they are equivalent:

In the above cases,cladograms 1-3 are all equivalent: they represent the same information. However, cladogram 4 is not equivalent to the other three:

Cladogram 4 says that C and D are more closely related (share a more recent common ancestor) with each other than either do with E, but in the other cladograms D is more closely related to E than to C.

In a cladogram, terminal taxa can be expanded to show the relationships within them, and nodes can be collapsed if we aren't interested in the details within that group.

For example, if we were not interested in reptile evolution, but were more interested in mammalian relationships, we can modify the tetrapod cladogram from last lecture to show the following:

This cladogram was modified from the previous one by:

• Collapsing the Reptilia node
• Expanding the Mammalia node
• Rotating at the Amniota node

• Conveys more information about mammal relationships, but
• Conveys less information about reptile relationships

More on Systematics and Taxonomy
In Lecture 10 we saw how names can be placed on nodes or terminal taxa. Here we look in some more detail about names and cladograms.

Previously named groups of organisms can be evaluated based on cladograms. These groups might be:

• Monophyletic: represent an immediate common ancestor and ALL of its descendants;
• Paraphyletic: represent an immediate ancestor and some but not all of its descendants;
• Polyphyletic: represent two or more independent groups with no immediate common ancestor that is also part of that group.

In modern systematics only monophyletic groups are used: paraphyletic groups are redefined to encompass monophyletic groups, or are abandoned.

Another name for a monophyletic group is a clade; modern systematics is thus called cladistics.

Diagnosis of a taxon: the list of features that are synapomorphies at that node (or autapomorphies, if a terminal taxon). Definition of a taxon (in cladistics): a statement of common ancestry which describes a particular clade.

Definitions can be node-based (all descendants of the most recent common ancestor of taxon X and taxon Y) or stem-based (taxon X and all taxa sharing a more recent common ancestor with taxon X than with taxon Y). The actual ancestors are only very rarely known, but can be inferred by the joining of two branches at a node.

Owen's Dinosauria is now defined (with a node-based definition) as:

(Dinosauria is thus has a node-based definition).

Dinosauria has two major branches, Saurischia and Ornithischia:

We will discuss the diagnoses (synapomorphies) of these groups, and the various subgroups within these clades, throughout the second section of the course.

NOTE: The definition for Dinosauria used here (all descendants of the most recent common ancestor of Iguanodon and Megalosaurus) is, sadly, not the formal scientific definition because of priority. For information on the actual accepted definitions of Dinosauria, S aurischia, and Ornithischia in the scientific literature, click here.

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