Psittacosaurus mongoliensis is an important early horned dinosaur whose growth series has never previously been recovered using cladistic methods. This fossil species has an excellent fossil record that includes many complete skeletons, including skulls. The goal of this study was to cladistically recover its growth series of the skull and jaws based on all of the available data published in the scientific literature.

This data set includes qualitative and quantitative characters that were compiled into a data matrix and analyzed using the software program Phylogenetic Analysis Using Parsimony.  Based on its vector of data, AMNH FARB 6535 was identified as the least mature specimen and was designated as the outgroup analog to allow for optimization of character states. Based on its vector, AMNH FARB 6254 was a priori identified as the most mature specimen.

From this data matrix, on tree of five growth stages was recovered (TL: 16, CI: 1.0, RC: 1.0). Growth changes include: an increase in both maxillary and dentary teeth, the labial median primary ridge of the maxillary teeth shifts from at midlength to offset, a change from ovoid to flat tooth shape, the development of a prominent sagittal crest, increased prominence of the jugal horn, and the opening of rostral and premaxillary skull sutures. The next step is to compare this growth series with that of its closest relatives.