Marsh's "Roofed Lizard"

When I was growing up, and the dinosaur bug had well and truly embedded itself into my system, there was only one purpose to explain the dermal plates of Stegosaurus – they were purely a defensive shield to help protect the animal from its arch enemy, Allosaurus. As a kid this all made sense to me and many a restoration of Stegosaurus was constructed in this fashion. The dinosaur with a brain the size of a walnut, who had a second “brain” in his hip region to help him even exist, needed all the help he could get not to be eaten.

Things have moved on somewhat over the years but Stegosaurus remains an iconic and fascinating dinosaur and the function of the plates is still the subject of intense study. The animal was discovered and named in 1877 by O.C. Marsh who originally thought that the plates covered the back of the animal in an overlapping fashion hence the name – “roofed lizard”.

At the time this was a reasonable assumption and, even as recent as 1975, Halstead also ascribed to this theory. However it soon became obvious, as more specimens were recovered, that the plates were very much vertically positioned and situated over the back in two parallel rows. It was also considered that the plates may have been positioned in one single continuous row but, as the plates came under closer scrutiny, it was apparent that they were likely to be covered in some form of sheath, which would have prevented this formation and may have actually forced the staggered alignment we recognise today.

Nowadays, most palaeontologists accept the now general perception of Stegosaurus with the two parallel rows of plates along the back embedded into the skin. So what about the function of these plates?  Well, as I’ve already mentioned, defence was amongst the first theories that was suggested and this was universally accepted for many years.  However, the plates do not really offer any significant protection when you think about it. The wide flanks of the stegosaur are still fully exposed and even the head and neck are still vulnerable.

That interaction between Stegosaurus and Allosaurus actually occurred is not doubted since there are stegosaur bones displaying bite marks and it must be noted that there is at least one plate with allosaurs bite marks (Carpenter et al 2005). Perhaps the most contradictory evidence against the plates- as-defence theory is the fact that they appear to be of a relatively weak construction as indicated by their histological structure and it would seem likely that a defensive plate would have been much sturdier and constructed of solid bone.

It was James Farlow et al (1976) who suggested an alternative theory about the plates  after the first histological studies revealed that they were permeated by a vast vascular network, both inside and out,  leading to the first suggestions that the plates may have served a thermoregulatory role. The analogy used was that of an elephant’s ear which does indeed perform such a function. However, further studies (eg Main et al 2005) show that the blood vessels on the exterior of the plate do not lead anywhere particular and that the apparently large blood pipes, in the base of the plates, did not appear to supply the volume of blood required for the plates to form a heat exchange system as postulated (Buffrénil et al 1986).

Perhaps the heavily vascularised plates were used for display and intraspecific purposes, just as the heavily ornamented frills of ceratopsians are generally believed to be nowadays.  The plates may have been flushed with blood to attract a mate or, equally, to warn off predators and competitors. However, there is no evidence of sexual dimorphism to back up the attraction theory since animals do tend to be dimorphic when such display structures are utilised and, although this may be changing (eg Redelstorrf and Sander 2009), there is a persistent lack of evidence for dimorphism in Stegosaurus. The defensive “flushing” may warrant consideration, however, and all those blood vessels in the plates must have been doing something.

Perhaps the plates were multi-functional performing a combination of tasks. It’s interesting  that, although the plates are not strong enough as a physical barrier to attack by theropods, they still may have acted as form of deterrent since an adult Stegosaurus, with heavily coloured plates and a swinging tail would have have been an extremely imposing sight. The same technique may have been used for attracting a mate and sorting out territorial disputes – there are so many variables.

It is still possible that the plates may have indeed performed a thermoregulatory function and Farlow et al (2010), using continually improving and sophisticated sectioning and CT scanning, have again suggested  so. Hayashi et al (2012) also agree with these conclusions although still favour display as the primary function. In the end it may simply be something as simple as species recognition.

References

Buffrénil, V. de, J. O. Farlow, and A. J. de Ricqlès. 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.

Carpenter, K., F. Sanders, L. McWhinney, and L. Wood,. 2005. Evidence for predator-prey relationships: Example for Allosaurus and Stegosaurus. Pp. 325-350 in Carpenter, K. (ed.) The Carnivorous Dinosaurs. Indiana University Press, Bloomington.

Farlow, J. O, C. V. Thompson , and D. E. Rosner. 1976. Plates of the dinosaur Stegosaurus: forced convection heat loss fins? Science 192 (4244):1123–1125.

Farlow, J. O, Hayashi, S. and Tattersall, G. 2010. Internal vascularity of the dermal plates of Stegosaurus (Ornithischia: Thyreophora). Swiss Journal of Geosciences, 103, 173–185. 

Halstead, L. B. 1975. The Evolution and Ecology of the Dinosaurs. Peter Lowe, London. 

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

Redelstorff, R., & Sander, P. M. (2009). Long and girdle bone histology of Stegosaurus: implications for growth and life history. Journal of Vertebrate Paleontology, 29, 1087–1099.