Breaking Down (a Part of) The Large Reptile Tree

There are various tests one can administer to a tree. One of them is to permit one or more extra steps beyond the minimum to see where the weaknesses and strengths lay.

The following four illustrations show the Archosauria and their ancestors from fully resolved (Fig. 1) to what happens when 1, 2 and 3 extra steps are permitted (Figs 2-4).

segment of the large reptile tree.

Figure 1. A fully resolved segment of the large reptile tree. 1265 steps.

segment of the large reptile tree

Figure 2. Adding one extra step produces lack of resolution at weaker nodes.

Shortest tree plus two extra steps.

Figure 3. Shortest tree plus two extra steps. More breakdown occurs and this is expected, but could be repaired with more characters specific to these clades.

segment of the large reptile tree

Figure 4. The shortest tree with three extra steps permitted. Here the loss of resolution equals or exceeds the retained resolution. No further testing was attempted. Strong clades defined by unique characters retain resolution. Partial specimens also help break down trees. Many of these are skull-only taxa or skull-less taxa.

Some large clades are retained while closest kin lose resolution, as expected. More characters specific to certain clades would help them keep resolution. Note that the major clades retained their distinction from other major clades.

Only more taxa can increase the possibilities for novel nestings by offering more possible nesting sites. More characters cannot provide more nesting opportunities. More characters can refine nestings within a limited range and/or they can strengthen established nestings by finding more synapomorphies (shared traits).

Any other insights would be gratefully appreciated.


1 thought on “Breaking Down (a Part of) The Large Reptile Tree

  1. This is a very interesting test. As I’ve said in relation to results by other authors, I think we need to start paying attention to how well supported each relationship is instead of just what the most parsimonious trees say. Normally I’d say that clades that break down in three steps or less (or even up to 5 steps) are quite likely to change given more data, but as you say, incomplete specimens help break down tests like this. We saw before that Phytodinosauria needed a lot more than 3 steps to break down for instance, so your highly unresolved tree is probably due to a fragmentary taxon like Pseudolagosuchus jumping around and not to weakness of dinosaurian subclades per se. The same phenomenon makes bootstrap tests on analyses with incomplete taxa useless.

    I disagree with your statement that more characters can’t provide different nesting opportunities though. New characters can potentially join taxa which are distantly related in the original tree, and this in turn can change other relationships.

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