Background: Xenopus laevis is a widely used model organism in the fields of genetics and development, and more recently evolution. At present, the most widely used staging table for X. laevis is based primarily on external features and does not describe the corresponding skull development in detail. Here, we describe skull development in X. laevis, complete with labeled figures, for each relevant stage in the most widely used staging table.Results: We find skull development in X. laevis is, for the most part, distinct at each of the previously established stages based on external anatomy. However, variation does exist in the timing of onset of ossification of certain bones in the skull, which results in a range of stages where a skull element first ossifies. The overall sequence of ossification is less variable than the timing of ossification onset. Conclusions: While events in skull development vary somewhat between specimens, and in comparison, to external events, this staging table is useful in showing both when bones first appear and for documenting the range of temporal variance in X. laevis skull development more accurately than previously done. Furthermore, when only skull data are available, the approximate stage of a specimen can now be determined.

Background: As the role of Ambystoma mexicanum, or the Mexican axolotl, expands in research applications beyond its traditional use in studies of limb regeneration, a staging table that is more anatomically extensive is required.Here, we describe axolotl skull development as it relates to previously established developmental stages that were based on limb development.Results: We find that most key developmental events in the skull correspond to these previously established stages, creating easily recognizable stages of axolotl throughout skull morphogenesis.