Sexual selection in the fossil record


Stegosaur plate morphs

I am interested in sexual selection theory, particularly in how we approach sexual selection and sexual dimorphism in the fossil record. My work in this area began with the publication of one of the chapters of my undergraduate thesis which rejects several alternate hypotheses for variation in plate shape and size in Stegosaurus mjosi, leaving sexual dimorphism as the most likely explanation. The study involved field work, laboratory analyses, and visits to fossil collections in the US and Switzerland. The paper is open access in PLOS ONE:

Saitta, E. T. (2015). Evidence for sexual dimorphism in the plated dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of western USA. PloS one10(4), e0123503.


Stegosaur plate histology through thin sectioning and CT scanning


Conclusive evidence for sexual dimorphism in non-avian dinosaurs has been elusive. Here it is shown that dimorphism in the shape of the dermal plates of Stegosaurus mjosi (Upper Jurassic, western USA) does not result from non-sex-related individual, interspecific, or ontogenetic variation and is most likely a sexually dimorphic feature. One morph possessed wide, oval plates 45% larger in surface area than the tall, narrow plates of the other morph. Intermediate morphologies are lacking as principal component analysis supports marked size- and shape-based dimorphism. In contrast, many non-sex-related individual variations are expected to show intermediate morphologies. Taphonomy of a new quarry in Montana (JRDI 5ES Quarry) shows that at least five individuals were buried in a single horizon and were not brought together by water or scavenger transportation. This new site demonstrates co-existence, and possibly suggests sociality, between two morphs that only show dimorphism in their plates. Without evidence for niche partitioning, it is unlikely that the two morphs represent different species. Histology of the new specimens in combination with studies on previous specimens indicates that both morphs occur in fully-grown individuals. Therefore, the dimorphism is not a result of ontogenetic change. Furthermore, the two morphs of plates do not simply come from different positions on the back of a single individual. Plates from all positions on the body can be classified as one of the two morphs, and previously discovered, isolated specimens possess only one morph of plates. Based on the seemingly display-oriented morphology of plates, female mate choice was likely the driving evolutionary mechanism rather than male-male competition. Dinosaur ornamentation possibly served similar functions to the ornamentation of modern species. Comparisons to ornamentation involved in sexual selection of extant species, such as the horns of bovids, may be appropriate in predicting the function of some dinosaur ornamentation.


Reconstructed stegosaur plate series of the two morphs

I have presented the study at the 2015 annual meeting of the Palaeontological Association. The video of this talk is available through Palaeocast while the slides are available through my Researchgate page.

The publication was covered by many outlets including BBC, BBC radio (both National and Bristol), The Guardian, ABC, The Daily Mail, Popular ScienceLive ScienceNBC, Princeton University, and the University of Bristol.

I presented a talk on this subject entitled “Approaching sexual dimorphism in non-avian dinosaurs and other extinct taxa” at the 2017 annual meeting of the Palaeontological Society in London. The presentation slides are available through my Researchgate page and video of the talk is available courtesy of Palaeocast. I presented an updated version of this work at the 2019 annual meeting of the Society of Vertebrate Paleontology, as linked on Researchgate.


Stegosaur wide morph model