Article Review: Evidence for Sexual Dimorphism in the Plated Dinosaur Stegosaurus mjosi

Evidence for Sexual Dimorphism in the Plated Dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA

Evan Saitta of the School of Earth Sciences at University of Bristol in the United Kingdom recently published an article that has evidence supporting sexual dimorphism  in the  Stegosaurus mjosi. Stegosaurus were found in the Upper Jurassic in the western United States (and recently recovered in Portugal). They were herbivores with dermal armor along its back with plates oriented vertically. Most commonly depicted with 17 plates in two staggered rows, with no plate being the same size or shape. Two morphs exist of the plates, where one morph the plates are 45% larger in surface area compared to the other morph. Most important to note the previously discovered specimens in which were isolated only possess only morph of the plates

A) The largest wide morph plate B) The largest tall morph plate

In general studying sexual selection in dinosaurs is difficult due to small sample size. Previous researchers have proposed a hypothesis where dinosaur ornamentation of the back plates were not sexually dimorphic and the differences was for species recognition. Previous claims of sexual dimorphism in stegosaurus stemmed from two types of proximal-end femur shapes, however due to not testing alternate hypothesis such as niche partitioning or ontogenetic change, made the results inconclusive about sexual dimorphism in Stegosaurus. S.mjosi is unique among others in its genus due to the teardrop shape tips of the neural spines on the vertebrae. Due to this uniqueness, it was used as the study specimen in this article.

Methods & Results

The plates used in this study were all specimen located and housed at Wyoming Dinosaur Center & Judith River Dinosaur Institute. Forty S. mjosi plates were analyzed in shape and size and were compared to three other species of Stegosaurus for comparison. Measurements were taken by hand, and X-ray computed tomography was also used to analysis the plates. A principal component analysis was conducted to test morphospaces.

Diagrammatic depiction of the measurements

Results from the PCA

The results concluded that there are two distinct morphs of the plates, where one of the morphs the plates are oval and wider compared to their length, whereas the other morph the plates are taller than they are wide and come to a point at the apex. The wider morph has a surface area of 45% greater than the other morphs do. The histology analysis revealed that within the plates there are large vascular pipes entering through the base of the plate (what’s connecting the plate to the back of the dinosaur). The tall morphs had deeper and more numerous vessels compared to the wider plates. The vascular pipes appeared to be empty or filled with sediment or high-density iron (perhaps to increase the strength). Another interesting result of the histology report is that the tall morph had a larger set of markers of maturity compared to the wide plates.

Histology of plates; A) thin section of the tall morph B) bone surface is towards the left of the image C) CT scan of the tall morph, with the red arrows indicating vascular piping which is a sign of sexual maturity.

Conclusions

Previous research on sexual dimorphism has been deemed inclusive due to lack of disproving alternate hypotheses. The research concluded with alternated hypothesis being: non-sex related individual variation,  or one individual possessing both morphs of the plates. The wide and tall morphs would be present however there was no evidence of a specimen containing both morphs and no intermediate plates were present. Interspecific variation is another hypothesis, however the results concluded that both morphs co-existed in social groups and therefore were likely at the time of death. A final alternative hypothesis is ontogenetic variation, once again due to the results this hypothesis has been disproved because all the specimen in the study were adults and sexually mature. With all the alternative hypothesis ruled out, sexual dimorphism is the most likely the key to the observed differences in the plates. The results from this article is the first support for sexual dimorphism that has not been throw out immediately in non-avian dinosaurs. The morphs of the dermal plates occurs in the ornamentation which was likely a secondary sexual characteristic.

Hypothetical Silhouettes of males & females

Without a specimen with preserved eggs inside the body cavity, it is difficult to assign sexes to the morphs with certainty. It is suggested by the author that the wide morph would belong to males, whereas the tall morph would represent the females. The logic of this statements is based on the fact that males are expected to invest more energy into growing and maintaining their ornamentation and because the wide morph are 45% larger in surface area, it would be more energetic. The orientation of the plates would most likely be driven by female choice rather than male-male competition.