Sexy stegosaurs?

Plate function in stegosaurs has been much debated, with early suggestions that they were defensive structures superceded by alternative explanations that have included use as convective fins for heat gain and loss and/or as visual signals for display and species recognition. A study just published in PLoS ONE by Evan Saitta of Bristol University adds a new twist to this discussion, suggesting that plate shape in Stegosaurus (= Hesperosaurus) mjosi differed between male and female individuals.

Determining the sex of a dinosaur skeleton has proved almost impossible in the vast majority of cases. Although living reptiles (including birds) do exhibit many obvious differences between males and females, a phenomenon termed sexual dimorphism, most of these differences relate to body size (with one sex markedly larger than the other), behaviour (e.g. different display strategies) and soft tissue anatomy (e.g. reproductive organs, skin patterning, feather types, etc.). By contrast, very few features of the skeleton  differ between male and female reptiles (except in terms of overall proportions) and applying these size and shape differences to fossil skeletons is difficult. For example, although sexual size differences are common, size also changes during growth - so are the size differences seen between fossils the result of growth differences, sexual differences, or even differences between adults of very similar species? Although there are methods for eliminating some of these issues, these problems are often so intractable that there are effectively no features of the skeleton that can be used to sex an extinct reptile reliably. There are only two exceptions to this pessimistic statement: both apply only to females. If intact eggs are found inside a dinosaur, and feeding can be ruled out (e.g. position of the eggs in the body, lack of evidence for digestion), you can be pretty sure it's a girl. In addition, female dinosaurs produce a special type of bone, medullary bone, which forms a reservoir for egg-forming minerals just before egg laying commences. If you slice open a dinosaur limb bone and see this bone type you have a female. However, if eggs or medullary bone are absent it just means that you either have a male or a female that wasn't about to lay eggs. Not very helpful, sadly. To date we know only of a handful of dinosaur skeletons that can be confirmed as female - none of the rest can be sexed at present.

Identification of sexual dimorphism in Stegosaurus plates would, therefore, be interesting and important: a feature that could plausibly identify male and female dinosaur skeletons consistently and easily for the first time. This new study is based on a spectacular new discovery: a group of at least five Stegosaurus individuals whose remains were preserved in a small area. Stegosaurus is usually regarded as a rare and solitary animal, so finding a group of these animals together would be newsworthy in itself. Following his study of these remains, Saitta (2015) concluded that these individuals all belonged to the same species (i.e. none belonged to the other species of Stegosaurus, S. stenops), they might have been a social group that died in the same event, and that the plates preserved with these skeletons fell into two distinct types. One of these plate morphs is taller than it is long, forming a narrow pointed triangle in side view (interpreted as female); the other is longer than it is tall, with a much larger area, forming a low rounded triangle (interpreted as male due to its larger size). This conclusion followed measurements of the preserved plates and comparisons to plates from other skeletons of S. mjosi and S. stenops in museum collections around the world. Alternative explanations, such as differences in growth, were considered but rejected by the author.

Although an interesting hypothesis, I'm thus far unconvinced by the arguments set out in the paper, for a variety of reasons.

1. Little relevant information is provided on the age of the individuals in the group. Although there is some work on the histology of the plates to look for growth rings, growth rings in armour do not record growth records in the same way as the widely used growth rings found in limb bones. As a result, it's unclear if there are differences in age between the different individuals in the quarry. As we know very little about shape change in stegosaur plates through growth, this is an important consideration.

2. Only 11 plates are preserved in the quarry, nine of which were complete enough to include in the study. This is a small proportion of the total number of plates that might have been preserved if the whole plate array was present for each individual (up to around 90 plates could plausibly have been present originally if the total count of plates was similar to that in S. stenops - but almost all of the plates seem to have been lost during the fossilisation process). As all of the remains in the quarry are mixed and jumbled it's not clear if the plates came from one of the five individuals present, or more than one individual. It could be that the two different types of plate belonged to one individual that possessed mixed plate types along the plate row, rather than from different individuals with different plate morphologies, or they might have come from individuals of different ages. The lack of intermediate plate types in the quarry, which was used as evidence to suggest the two types were very distinct and thus dimorphic, is perhaps unsurprising given the very small sample of plates available for study.

3. No previously described specimen of S. mjosi possesses a full compliment of plates – all are incompletely preserved with many parts of the skeletons missing. As a result, we have no roadmap to show what a complete set of S. mjosi plates should look like or how shape could have varied along the row.

4. Although all of the individuals were identified as S. mjosi, anatomical details for each specimen have yet to be published, so the possibility that this might be a mixed group of S. mjosi and S. stenops (or as as yet unknown species) has not yet been ruled out definitively. Although the identification of each individual might be solid, the evidence to support this was not fully set out in the paper so other scientists are unable to confirm this suggestion at present. 

5. No details were provided on the completeness or preservation of the plates used in the study and how this might have affected the results. Plates in many of the specimens used for comparison are at least partially restored and reconstructed, altering their sizes and shapes and no data is presented to show that only complete undistorted plates were used for comparison.

Given the above, it's possible that younger individuals of S. mjosi might have had different plate shapes from older ones or that in S. mjosi the plates differed in shape and size depending upon their position within the plate row (or between individuals). Neither of these possibilities was thoroughly tested in the paper: indeed, one of these possibilities can only be tested by finding a new complete skeleton of this species. By comparison, plate shape is known to vary somewhat along the body in Stegosaurus stenops and it's plausible that it varied in a different way in S. mjosi to incorporate two plate types (this would then become an additional way of distinguishing these two species). So, although a sexual difference is plausible, growth differences, individual differences and/or species differences are also equally likely on the basis of current evidence.

Although I remain skeptical, it would be really neat if future work on this assemblage could dismiss some of these concerns: it would be great to finally have a good example of clear sexual dimorphism in a non-avian dinosaur, even though it's likely that such obvious dimorphism would be relatively rare across the group as a whole (given we've not been able to identify in dinosaurs for which we have large sample sizes). The announcement of stegosaur sexual dimorphism seems a bit premature, but I would be very happy to be proved wrong. 

Reference

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 ONE 10(4): e0123503.