Postnatal skeletal development in squamates: Its relationship to life history and potential phylogenetic informativeness
This is the first systematic survey of skeletal development, throughout postnatal ontogeny and throughout the skeleton, in Squamata. The absolute and relative timing of postnatal skeletal events, including calcifications, ossifications, and terminal fusions, is described in detail from densely-sampled growth series for seven iguanian and 14 scleroglossan species.
Postnatal skeletal development in squamates (and perhaps lepidosaurs generally) is particularly interesting because their epiphyses ossify from secondary ossification centers, and a large number of sesamoid bones occur throughout the skeleton—features known to occur elsewhere only in mammals. Previous investigations into this aspect of squamate development have suffered from imprecise terminology, limited sampling of the postnatal size range, and limited scope in terms of topology and the kinds of postnatal events documented.
Within species the absolute timing of appearance of skeletal events is found to be somewhat variable, thus skeletal maturity does not correlate directly with body size. The relative sequence of appearance of events in postnatal ontogeny is found to be highly conserved within species, yet highly variable among them. There is little evidence that models of tetrapod limb morphogenesis, such as the primary axis and digital arch, influence the postnatal ossification of the skeleton. Interspecific variation in the state of ossification in neonatal squamates is probably as a whole due to phylogeny.
However, skeletally immature neonates may characterize viviparous species. No terminal fusions are found to reliably indicate the onset of sexual maturity in squamates as a clade. Complete fusion of the limb epiphyses and of the basicranium is closely coincident with the achievement of maximum size in all species examined, and thus should reliably indicate skeletal maturity in fossilized squamate individuals.
Phylogenetic analyses indicate that the sequence of postnatal skeletal events should be useful in elucidating relationships within crown squamate clades, whereas discrete characters, such as the number of secondary centers in the epiphyses, are capable of recovering deeper divergences within Squamata. Thus, both types of data derived from postnatal skeletal development should contribute to better resolution of phylogenetic relationships within this clade.