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The Stegosaurus Plate Controversy - You live in the Jurassic and you've evolved giant, diamond-shaped bone plates that stick out the top of your neck, back and tail. Why, evolution, why??

July 22 , 2016

by Darren Naish

Stegosaurs are a familiar group of quadrupedal ornithischian dinosaurs, the best known member of which is the Late Jurassic Stegosaurus of the USA, Portugal and perhaps China. A few basic things about Stegosaurus are known to just about everyone with a modicum of general knowledge: it’s large (7 m long or so), has a series of large plates arranged along its neck, back and tail, and has a cluster of spikes at the tip of its tail. As per usual, I will add the caveat that Stegosaurus is not a typical stegosaur: they weren’t all like this; many were smaller, and spikier, with smaller plates or a far lower number of plates. For more on the group see the links below.

In this article I want to recycle and update various observations on the plates of Stegosaurus, since they’ve always been the topic of debate and the focus of competing hypotheses on stegosaur biology and evolution. Much of this text originally appeared at Tet Zoo ver 2 back in 2011.

It has – so far as I know – always been assumed that stegosaur tail spines were sheathed in horn. However, things have been less clear when we come to the plates, and their surface texture has led some workers to propose that they were covered with skin alone (I really should find out when and where this idea originated). The idea that the plates were sheathed in horn seems more likely, predominantly because big bony things that stick out of tetrapod bodies are virtually always encased in this material – the antlers of deer and ossicones of giraffids being remarkable exceptions. De Buffrénil et al. (1986) considered this issue and regarded it as most likely that a thick dermis, not a horny covering, covered the plates. They even likened the hypothesised skin covering to the velvet of deer antlers.

Confirmation for the presence of a horn sheath was finally provided by Christiansen & Tschopp (2010) who reported a continuous, sheath-like covering on one of the plates of a Morrison Formation specimen they referred to Hesperosaurus (a taxon since argued to be synonymous with Stegosaurus). Interestingly, this covering is decorated with fine, closely spaced ridges. Because the fossil is a mold, these ridges actually represent structures that would have been grooves in life. This would seem to confirm once and for all that horn did, indeed, sheath the bony cores of the plates: prediction confirmed, right?

Stegosaur plates for thermoregulation, revisited, again. One of those things that everyone seems to know about stegosaurs – or, rather, about Stegosaurus in particular – is that the plates may have functioned in thermoregulation. Given that most organs that stick out from the body can absorb and radiate heat (the most oft-mentioned ‘facultative thermoregulatory structures’ are the horns of bovids), we might say with some confidence that stegosaur plates and spines had some thermoregulatory role. But were they specialised organs whose distinctive shape had evolved under selection for such a function?

I admit to being sceptical of such explanations because it seems to me that people try too hard to find ‘functional’ explanations when confronted with flamboyant biological structures. The latter could just as well be flamboyant because they’ve evolved under selection for, well, being flamboyant. On the other hand, you could make the argument that – limited as we are, and lacking in time machines – we should seek to test the possible ‘functions’ of flamboyant structures to see if they might be good for something other than display before concluding that this was their probable role.

Farlow et al. (1976) famously suggested that the plates could have been efficiently deployed as “forced convective heat loss fins, [constituting] a physiologically effective thermoregulatory adaptation” (p. 1124). Their article made the cover of Science, as you can see from the image here. De Buffrénil et al. (1984, 1986) drew attention to the presence of vertical ‘pipes’ within the plates and implied that a thermoregulatory role for the plates was more robustly supported than were other roles.

Not for thermoregulation, but for display. Popular books have created the impression to lay readers that the themoregulatory hypothesis is the one mostly endorsed by palaeontologists. But this is not really the case when you look at how other workers have responded to the idea.

Carpenter (1998) noted that, were the plates really specialised thermoregulatory organs, they would most likely be ubiquitous across Stegosauria. They aren’t: Stegosaurus-sized stegosaurs inhabiting similar environments to Stegosaurus typically had smaller, differently shaped plates. Carpenter’s argument is suggestive and certainly not conclusive (counter-argument: Stegosaurus was uniquely odd, doing something not possible in other stegosaurs). Carpenter’s favoured hypothesis was that the plates functioned primarily in visual display, and that it was this which had driven the evolution of their size and shape.

Main et al. (2005) later argued that the gross and microstructural features of the plates were merely typical growth features of thyreophoran scutes (internal ‘pipes’ like those of Stegosaurus plates are present in ankylosaur armour). Very similar results were also reported by Hayashi et al. (2012). Main et al. (2005) concluded that there was no clear evidence for a special thermoregulatory role. Rather, they argued that the plates were ‘extreme’ display structures used in species recognition… ah yes, ‘species recognition’. Regular readers might know that the ‘species recognition hypothesis’ is not considered an especially good one by some of us in the palaeozoological community (more on this in a future article: see Hone & Naish 2014). Regardless, their primary argument was that the form of Stegosaurus’ plates should be seen as part of a pattern of scute elaboration present across thyreophoran phylogeny.

But the thermoregulatory hypothesis hasn’t completely gone away. Farlow et al. (2010) reported CT-scan data from Stegosaurus plates and alligatoroid osteoderms and used this data to look anew at a possible thermoregulatory role for the plates. The scans showed that those internal ‘pipes’ formed a branching structure, connected by a ‘main channel’ that extends along each plate’s base [shown here, from Farlow et al. (2010)]. They suggested that, whatever its homology or origin, this network could well have worked as a “vascular distributary system” (p. 179). Some work (including thermal imaging reported in the paper) implies that crocodylians use their scutes to transfer heat and warm themselves, and sub-vertical canals in the osteoderms (reminiscent of those ‘pipes’ in the stegosaur plates) may well play a role in transferring this heat.

In the end, their conclusion was that the plates were likely multi-functional and had the potential to play a thermoregulatory role (Farlow et al. (2010) mention bovid horns here). So, if the plates worked well as heat-shedding fins, this was plausibly facultative rather than adaptive. This seems reasonable, but note that saying this is not the same as actually finding support for a thermoregulatory function. If you want to be cynical, you could say that (1) people are only trying to find evidence for a thermoregulatory role because, historically, this is the role that’s often been favoured, (2) the fact that crocodylians apparently use their scutes to help collect solar heat doesn’t tell you much about stegosaurs, especially since the main thermoregulatory role for stegosaur plates has been that they were good at shedding heat, not collecting it, and (3) no direct evidence supports the thermoregulatory hypothesis.

Whatever role stegosaur plates had in thermoregulation, you’ll have gathered from this article that the primary role supported by most workers is that the plates evolved within the context of visual display. And it’s this topic we’ll look at soon.

While I’m here, it’s timely to announce that a dinosaur book I co-authored with Paul Barrett - the Natural History Museum’s/Smithsonian’s Dinosaurs: How They Lived and Evolved - is now available for pre-order. It’s out and on sale in October. And yes yes yes... the cover, the cover. I know.

For previous Tet Zoo articles on stegosaurs, see...

A most atypical stegosaur
Stegosaur Wars: the SJG stegosaur special, part I
Heinrich’s digital Kentrosaurus: the SJG stegosaur special, part II
Life as a stegosaur: the SJG stegosaur special, part III
Dinosaurs and their exaggerated structures : species recognition aids, or sexual display devices?

Refs - -

Buffrénil, V. de, Farlow, J. O. & de Ricqlès, A. 1984. Histological data on structure, growth and possible functions of Stegosaurus plates. In Reif, W.-E. & Westphal, F. (eds) Third Symposium on Mesozoic Terrestrial Ecosystems, Short Papers. Attempto Verlag (Tübingen), pp. 31-36.

Buffrénil, V. de, Farlow, J. O. & de Ricqlès, A. 1986. Growth and function of Stegosaurus plates: evidence from bone histology. Paleobiology 12, 459-473.

Carpenter, K. 1998. Armor of Stegosaurus stenops, and the taphonomic history of a new specimen from Garden Park, Colorado. Modern Geology 23, 127-144.

Christiansen, N. A. & Tschopp, E. 2010. Exceptional stegosaur integument impression from the Upper Jurassic Morrison Formation of Wyoming. Swiss Journal of Geosciences 103, 163-171.

Farlow, J. O., Hayashi, S. & Tattersall, G, J. 2010. Internal vascularity of the dermal plates of Stegosaurus (Ornithischia, Thyreophora). Swiss Journal of Geoscience 103, 173-185.

Farlow, J., Thompson, C. & Rosner, D. 1976. Plates of the dinosaur Stegosaurus: forced convection heat loss fins? Science 192, 1123-1125.

Gilmore, C. 1915. A new restoration of Stegosaurus. Proceedings of the United States National Museum 49, 355-357.

Hayashi, S., Carpenter, K., Watabe, M. & McWhinney, L. A. 2012. Ontogenetic histology of Stegosaurus plates and spikes. Palaeontology 55, 145-161.

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.

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.



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