Cladistics Illuminate other Sciences
I. The Extant Phylogenetic Bracket: Understanding the distribution of known characters on a cladogram allow us to make inferences about sharacter states in fossil taxa for which the characters, themselves, are not preserved. The strength of that inference depends on the distribution of the characters in two extant (or well known fossil) outgroups to the taxon in question, thus bracketing it. We recognize three general types of inference:
- Type I inference: Both outgroup "brackets" possess the derived character of interest. Thus we assume all to be descended from a last common ancestor with the feature. Example: Did Tyrannosaurus rex have feathers? Fossils of tyrannosaurids haven't told us, however we could infer from the presence of feathered critters above (birds, etc.) and below (Sinosauropteryx) on the tree that they did. This inferrence was recently confirmed by the discovery of Dilong paradoxus a feathered early Cretaceous tyrannosaurid. (Reconstruction)
- Type II inference: Only one outgroup "bracket" possess the derived character of interest. This is a much weaker inferrence. Example: Did Allosaurus have feathers? Again, fossils of allosaurids haven't told us, however we know that some reasonably close relatives of Allosaurus (coelurosaurs, including birds, raptors, ovraptorosaurs, tyrannosaurids, etc.) did. On the other side of Allosaurus on the tree, we have no evidence, however. Thus, inferring that it had feathers, while not crazy, is distinctly speculative.
- Type III inference: Neither outgroup "bracket" possess the derived character of interest. Thus, the most parsimonious explanation for the presence of the derived character is that it would have evolved convergently. This would bee a very weak inferrence. Example: Did the primitive horse Eohippus have feathers? Fossils of Eohippus don't preserve integument of any kind, however the complete absence of feathers in any close relatives argues strongly that it didn't.