SEXUAL DIMORPHISM IS A DERIVED CONDITION IN THE EVOLUTION OF HORNED DINOSAURS (ORNITHISCHIA: NEOCERATOPSIA): EVIDENCE FROM GROWTH SERIES OF PACHYRHINOSAURUS LAKUSTAI AND PROTOCERATOPS ANDREWSI

In this study, the growth series of two species of horned dinosaur (Protoceratops, Pachyrhinosaurus) were reconstructed using quantitative cladistic methods. 34 hypothetical growth characters were identified from the primary literature for the snout region of the ceratopsid Pachyrhinosaurus lakustai. The analysis of 10 specimens yielded 3 equally most parsimonious trees with a CI of 0.95. In the growth series sexual dimorphism was recovered; males develop a distinct rostral comb, whereas this structure is absent in females. This approach was applied to Protoceratops andrewsi, a small basal neoceratopsian, which has been hypothesized to be sexually dimorphic using a noncladistic approach. The objective of my study was to test the sexual dimorphism hypothesis through a cladistic reconstruction of the growth series. Using the primary literature, I found 96 hypothetical growth characters among 7 specimens. The data were analyzed in PAUP with all characters unordered and equally weighted. The analysis recovered one most parsimonious tree, with a length of 73 steps and a CI of 0.71. The strict consensus tree recovered a continuous linear growth series, where progressively mature specimens are placed sequentially away from the root. The ontogenetic tree did not split into two distinct groups, as is seen in the growth series of P. lakustai; instead the presumed females are positioned between the juveniles and the presumed adult males. These results demonstrate that ontogenetic variation is a more parsimonious hypothesis than sexual dimorphism for the variation in morphology that is seen in P. andrewsi. These results indicate that sexual dimorphism is a condition that evolved later than the common ancestor of Neoceratopsia, and so it is not a synapomorphy for that clade.