CHANGING PHYLOGENIES THROUGH TIME: A REVIEW OF THE SPHENODONTIA WITH INSIGHTS FROM SUPERMATRICES AND SUPERTREES

Sphenodontians are a lepidosaurian taxon, represented by the extant genus Sphenodon and its extinct relatives. The group diversified in the late Triassic and declined in the Cretaceous period. Despite a record of approximately 50 species, fewer than half of these have been included in cladistic analyses. A historical review of sphenodontian phylogeny was carried out, using positional congruence analyses. In early analyses there was considerable variation in the taxa used, and many of the relationships defined were found to be unstable in the light of subsequent discoveries. More recently the phylogeny has gained some stability, although this is probably the result of one prominent author currently working on this clade.

A “supermatrix” was constructed using all the data available from existing studies. This was compared with a number of supertrees built with the Matrix Representation using Parsimony (MRP) method. Supertrees were constructed in different ways: using the strict consensus trees from each study, all the most parsimonious trees (MPTs), and all the MPTs differentially weighted by the number of trees produced in each study. The supermatrix differed greatly from the input trees and the supertrees, and the supertrees varied in their representation of relationships depending on whether they were differentially weighted with respect to the input trees.

Supertrees most accurately express a consensus of the input trees, in that the most common relationships are expressed. However, a supermatrix is capable of detecting relationships previously obscured through conventional consensus methods, and this has great potential for further insight into existing phylogenies.