Sexual selection in the fossil record

journal.pone.0123503.g001

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.

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Stegosaur plate histology through thin sectioning and CT scanning

Abstract

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.

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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, The Guardian, ABC, The Daily Mail, Popular Science, Phys.org, Live ScienceNBC, Princeton University, and the University of Bristol.

Some researchers have criticized claims of sexual dimorphism in non-avian dinosaurs and some recent papers challenging the studies supporting sexual dimorphism have included discussion of my stegosaur work. However, the critiques are based on an incomplete analysis of my full research and some use inappropriate statistical tests that not only lack statistical power but are not testing for the signal of dimorphism they wish to examine. I also believe that such positions derive from bias with regards to non-avian dinosaurs, leading to false conclusions that non-avian dinosaurs had some unusual aspect of their biology in relation to sexual selection compared to other fossil taxa. I am currently preparing a manuscript about how to approach sexual dimorphism in the fossil record and non-avian dinosaurs in particular that will also present additional analyses of my stegosaur data. I will be presenting 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.

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Stegosaur wide morph model