THE IMPLICATIONS OF WITHIN VS AMONG GENUS DISPARITY FOR PHYLOGENETIC ANALYSES: AN EXAMPLE USING LATE CRETACEOUS ACANTHOCERATIDAE (AMMONOIDEA)

Workers often conduct phylogenetic analyses at the genus level if only for practical reasons. Typically, the type species or some other exemplar represents the whole genus. A tacit assumption of genus-level studies is that characters varying among genera vary little within genera. We can assess this by using pairwise dissimilarity to contrast disparity among congeneric species and between genera. In principle, if within-genus disparity approaches among-genus disparity, then genus-level analyses probably omit too much history to provide reliable phylogenetic analyses.

Here, I examine within-genus and among-genus disparity for the Acanthoceratidae known predominately from the Western Interior Seaway during the Cenomanian and Turonian. I coded 138 species for 51 characters that include a variety of binary and multistate characters, with ordered multistates used to summarize continuous characters. The most recent published genus assignments were used for each species.

Median within-genus disparity is typically 75% of the median among-genus disparity when compared to the among-genus groups. However, the level of disparity is still nontrivial. Further work should be done to examine the outliers in the genera and whether the amount of disparity within a genus is due to variation between closely related species or due to improper placement in the genus.