The potential utility of postnatal skeletal developmental patterns in squamate phylogenetics

Postnatal patterns of skeletal development, including the sequence of appearance of ossification centres and the distribution of sesamoids, appear to be highly conserved species-level phenomena in squamates. As such, they are a potential source of characters for phylogenetic inquiry. These patterns, from 21 species representing 14 crown squamate clades, form the basis for two analyses. In the first, the sequence of postnatal skeletal events is coded as characters using the sequence unit approach. This analysis reveals that the sequence of postnatal skeletal events might be useful for determining relationships at or above the level of crown clades, but not among them. The second analysis utilizes discrete data from postnatal skeletal development, such as the presence/absence of sesamoids and the number of secondary centres in epiphyseal cartilages. These discrete data appear capable of recovering the deeper divergences within Squamata, but evolve too slowly to be informative at the level of crown clades. Thus, patterns of postnatal skeletal development have the potential to help illuminate relationships throughout the squamate tree. Further progress in this area will require the examination of additional squamate species, the exploration of alternative coding schemes for developmental sequences, and comparable postnatal data for Sphenodon.