August 11 , 2016

by Darren Naish

Mesozoic dinosaurs of several sorts evolved impressive, sometimes radically extravagant structures: horns, crests, frills, dorsal sails, giant bony plates and so on. Over the years, I’ve probably had cause to use a sentence of this sort maybe 10 or 15 times. Huh, sorry. Anyway… in an article from last month, I discussed how the giant, vaguely diamond-shaped plates of the famous Jurassic stegosaur Stegosaurus have sometimes been interpreted as specialised thermoregulatory structures, their size and shape being linked to the role of the plates as heat-collecting and/or heat-dissipating structures.

Now, don’t get me wrong – like many people interested in palaeobiology I’m more than happy to think that stegosaur plates may well have functioned in thermoregulation somehow, perhaps on a facultative or opportunistic basis. What I think we can doubt, however, is that this possible function ‘explains’ their evolution. In other words, it does not look likely that a role in temperature control was the factor driving the evolution of their size and shape. What seems more likely is that they were visual display devices: that they functioned in signalling of some sort. But what sort of signalling?

Here we come to an issue that’s one of the most debated and rancorous in dinosaur biology: did structures like stegosaur plates evolve within the context of intraspecific sexual display, or were they species recognition devices? That is, were they used, and indeed needed, to recognise conspecifics and differentiate them from heterospecifics? Regular readers will recall that this debate has been covered on Tet Zoo a few times before (see links below). See also Knell et al. (2012, 2013).

There are a few reasons why the ‘species recognition hypothesis’ doesn’t really work as a good explanation for visual display structures (aka ornaments or exaggerated structures). Rather than explain them de novo, I thought I’d be lazy and quote what Dave Hone and I had to say about this issue in our recent technical paper ‘The ‘species recognition hypothesis’ does not explain the presence and evolution of exaggerated structures in non-avialan dinosaurs’, at least in terms of how it relates to stegosaurs (Hone & Naish 2013)…

Padian & Horner (2011a) argued that the presence of exaggerated structures in sympatric, closely related taxa supports their role in species recognition. However, it has been noted that “mating signals of sympatric species often are more distinct from one another than are other signals produced by the same species” and, furthermore, that “species confined to different regions have no possibility of confusing their signals” (both quotes by Wells & Henry 1998). In short, we would expect that if these features functioned in species recognition, they would be more divergent between sympatric species, and less divergent between allopatric ones. However, this is clearly not true for a number of examples in the dinosaur fossil record.

Wuerhosaurus (or Stegosaurus) homheni is the only stegosaur recognized in the Lower Cretaceous Lianmuging Formation of China (Maidment et al. 2008). Given the distinctive bauplan of stegosaurs relative to potential sympatric dinosaurs, it is unlikely that individuals would struggle to identify conspecifics simply because they lacked dorsal plates and tail spikes. This and other examples (e.g. the lone Asian spinosaurine, Ichthyovenator, Allain et al. 2012) render it difficult to interpret species recognition as a viable primary explanation for the evolution of exaggerated structures among these taxa. Main et al. (2005) noted of stegosaur anatomy that while ‘we have no independent evidence of mate competition, we can use the features of their plates to identify species’. However, this is not always true: disagreement continues over stegosaur taxonomy, with variation in plate and spike form being interpreted as within intraspecific variation by some, but exceeding it by others (Maidment et al. 2008). Similar problems exist for other lineages.

If stegosaur plates and other such structures didn’t evolve as ‘species identification badges’ – if, that is, they were more likely involved in intraspecific display – what does this say about their intraspecific role, function and appearance? Well, that’s something that needs further discussion too. I’ll aim to come back to it in time.

Refs - -

Allain, R., Xaisanavong, T., Richir, P. & Khentavong, B. 2012. The first definitive Asian spinosaurid (Dinosauria: Theropoda) from the early cretaceous of Laos. Naturwissenschaften 99, 369-377.

Hone, D. W. E. & Naish, D. 2013. The ‘species recognition hypothesis’ does not explain the presence and evolution of exaggerated structures in non-avialan dinosaurs. Journal of Zoology 290, 172-180.

Knell, R., Naish, D., Tomkins, J. L. & Hone, D. W. E. 2012. Sexual selection in prehistoric animals: detection and implications. Trends in Ecology and Evolution 28, 38-47.

- ., Naish, D., Tomkins, J. L. & Hone, D. W. E. 2013. Is sexual selection defined by dimorphism alone? A reply to Padian and Horner. Trends in Ecology and Evolution 28, 250-251.

Maidment, S. C. R., Norman, D. B., Barrett, P. M. & Upchurch, P. 2008. Systematics and phylogeny of Stegosauria (Dinosauria: Ornithischia). Journal of Systematic Paleontology 6, 367-407.

Main, R. P., de Ricqles, A., Horner, J. R. & Padian, K. 2005. The evolution and function of thyreophoran dinosaur scutes: implications for plate function in stegosaurs. Paleobiology 31, 291-314.

Padian, K. & Horner, J. 2011. The evolution of ‘bizarre structures’ in dinosaurs: biomechanics, sexual selection, social selection, or species recognition? Journal of Zoology 283, 3-17.

Wells, M. M. & Henry, C. S. 1998. Songs, reproductive isolation and speciation in cryptic species of insects. In Howard, D. J. & Berlocher, S. H. (eds) Endless Forms: Species and Speciation. Oxford University Press, Oxford, pp. 217-233.