Time to wrap up on the SJG special – make sure you see part I and part II first. Wow, I never thought I’d end up writing three long articles on this series of papers (hmm, a familiar theme). In the previous articles we looked at stegosaur systematics, and at Heinrich Mallison’s work on the ranges of movement and posture of Kentrosaurus. This time round, we look at the papers that discuss preserved soft tissues, the possible role of the plates, and on feeding behaviour. Oh, and we finish up by looking at some other stuff too…
As I said before, the papers I’m discussing here appeared last year in a special issue of Swiss Journal of Geosciences (SJG from hereon). These papers resulted from a meeting held at the Sauriermuseum Aathal, Switzerland, in June 2009 (Billon-Bruyat & Marty 2010). Thanks to Daniel Marty for his help.
Stegosaur skin and ‘plate sheaths’
New data on stegosaurian soft tissues are described by Nicolai Christiansen and Emanuel Tschopp in their report of integument preserved with a Hesperosaurus specimen (see the Stegosaur Wars article for a discussion of Hesperosaurus and whether it warrants distinction relative to Stegosaurus).
These several skin impression patches show that small, non-imbricating tuberculate scales – of the sort widespread in archosaurs – covered the animal’s body; the scales are approximately hexagonal [one of the skin impressions is shown here, from Christiansen & Tschopp (2010)]. Larger, domed scales (only two were preserved) are present near the top of the animal’s back. These larger scales were surrounded by smaller ones to form rosette-like patterns: as Christiansen & Tschopp (2010) describe, rosettes of this sort seem to have been widespread in dinosaurs. These are not, by the way, the only skin impressions known for a stegosaur: some were also described for the Chinese taxon Gigantspinosaurus sichuanensis* (you can see a photo of them here), and an unpublished Morrison Formation specimen identified as cf. Stegosaurus also preserves some.
* Yeah, I hate the generic name too. It’s one of the dumbest names ever given to an animal.
While it has (so far as I know) always been assumed that stegosaur spines were sheathed in horn, matters have been less clear when it comes to plates. The surface texture of these structures has led some workers to propose that they were covered only with skin (I really should find out when and where this idea originated). The idea that the plates were sheathed in horn seems more likely, mostly 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 are remarkable exceptions). Christiansen & Tschopp (2010) report a continuous, presumably keratinous covering on one of the plates of their Hesperosaurus specimens. Interestingly, this covering is decorated with fine, closely spaced ridges. Because the fossil is a mold, these 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.
Stegosaur plates for thermoregulation, revisited, again
And having mentioned plates… one of those things that everyone seems to know about stegosaurs – or, rather, about Stegosaurus in particular – is that its remarkable 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 can be confident 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, without 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) suggested that the plates could have been efficiently deployed as “forced convective heat loss fins, [constituting] a physiologically effective thermoregulatory adaptation” (p. 1124) [that article made the cover of Science, as you can see from the image above]. De Buffrénil et al. (1984) 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. However, Carpenter (1998) drew attention to the fact 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, but it’s in agreement with the hypothesis that the plates functioned primarily in visual display. Main et al. (2005) 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), and they 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 (a hypothesis supported by the elaboration of scutes across thyreophoran phylogeny).
The new paper – by James Farlow and colleagues – reports CT scanning of Stegosaurus plates and alligatoroid osteoderms, and uses this data to look anew at a possible thermoregulatory role for stegosaur plates (Farlow et al. 2010). The new scans show 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)]. It’s 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) shows that crocodilians 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 distributary role in transferring this heat.
In the end, the conclusion is 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 absolutely 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 crocodilians 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 [image below – showing Stegosaurus battling Ceratosaurus – by John Conway].
Stegosaur palaeobiology is also examined by Miriam Reichel in another of the special issue’s papers. Bite forces were estimated by digitally constructing 3D Stegosaurus teeth and subjecting them to simulated loading. Reichel (2010) concludes that Stegosaurus teeth and jaws were strong enough to bite through small branches and leaves, but that the animal was not able to crush branches more than 12 mm in diameter. This leads to the suggestion that Stegosaurus perhaps “took advantage of the abundance of smaller, fast growing plants” (p. 239) in the Morrison ecosystem.
Given that the relatively slim muzzle and small teeth of Stegosaurus are already suggestive of selective browsing, one would probably not predict the biting and ingestion of twigs and/or branches anyway. Of course there is the caveat that modelling techniques such as those employed here may not necessarily do an accurate job of replicating in-vivo behaviour. Sorry for the cryptic statement, but I cannot elaborate on that at the moment (this article is already too long).
Final musings on behaviour and such
This compilation of papers represents a significant addition to our knowledge of stegosaurs. Combined with Susannah Maidment’s (2010) historical review and phylogenetic overview of the group, it means that we have a nice collection of papers on stegosaur systematics, biomechanics and anatomy.
We still know essentially nothing about stegosaur social behaviour, though the close association of several Kentrosaurus specimens at Tendaguru has led to the suggestion that this taxon at least was a herding animal. Conversely, the fact that most stegosaurs are discovered on their own has caused some to think of them as mostly solitary animals, but arguments like this are hardly conclusive given that even the most social of animals often die alone. Mortality in some Stegosaurus specimens seems to have been drought-related (Carpenter 1998): Robert Bakker has suggested that Stegosaurus was non-migratory and that it stayed, year-round, in close proximity to big rivers and lakes (Psihoyos & Knoeber 1994, pp. 101-102). However, it’s doubtful whether the data is good enough to show which dinosaurs were seasonal residents and which were migrants. The dinosaur fossil record – even that from the Morrison – is not on par with that of animals where migratory habits can be confidently asserted or denied, like Pleistocene bison.
Baby stegosaurs are known (and some eggs have also been identified as stegosaurian), but nothing is known about parental care. Clusters of juvenile ankylosaurs found preserved together suggest that young thyreophorans lived together in pods (a behaviour that may well have been widespread across extinct archosaurs).
And what about mating behaviour? Stegosaur plates and spines were not mobile (no matter what Bakker said in The Dinosaur Heresies), and it seems very difficult to imagine that males could mount females in the manner conventional for quadrupeds. It’s been seriously suggested that stegosaur mated ventro-ventrally (as shown in the adjacent cartoon) or that males had an enormous, laterally flexible phallus, perhaps deployed transversely when the animals stood close to one another. Another idea is that females lay on their sides and were straddled from behind [see illustration below, by Patrick Redman]. Most recently, Isles (2009) suggested that a mating pair might approach end-to-end. I would love it if stegosaurs were parthenogenetic and didn’t mate at all, but this is contradicted by claims of sexual dimorphism in members of the group.
Incidentally, the frequent complaint that palaeontologists never ask awkward questions about dinosaur sex (here’s an example from the Ask A Biologist site) is another one of those generalisations that is simply and absolutely not true. If anything, palaeontologists have done rather too much speculating on dinosaur mating poses and such, and there is a large literature on this subject. Unsurprisingly, most of it has appeared in popular and semi-popular sources… hell, there’s even a whole book called Dinosaur Sex* [hence image at very top].
* Hats off to Michael Brookfield for writing Dinosaur Sex, but I’m afraid it’s not the dinosaur sex book I would have written. It includes hardly any discussion of or speculation on Mesozoic dinosaurs, features really terrible diagrams, and has peculiar and fairly pointless sections on such topics as masturbation, oral sex and fetishism.
Anyway, I sort of went off on a tangent there at the end. With the completion of this little section on palaeobiology, I’m hereby done in writing up my thoughts on the SJG stegosaur special issue. I hope you enjoyed this look at Stegosauria and have a renewed appreciation for this remarkable, enigmatic group of flamboyant dinosaurs.
For previous Tet Zoo articles on stegosaurs, please see…
- A most atypical stegosaur
- Where the scelidosaurs and iguanodontians roam
- Patagonian Mesozoic Reptiles, a book review
- Stegosaur Wars: the SJG stegosaur special, part I
- Heinrich’s digital Kentrosaurus: the SJG stegosaur special, part II
For articles on other thyreophorans, see…
- Ankylosaur week, day 1: Hungarosaurus
- Ankylosaur week, day 2: Tarchia
- Ankylosaur week, day 3: Aletopelta
- Ankylosaur week, day 4: Panoplosaurus
- Ankylosaur week, day 5: Edmontonia
- Ankylosaur week, day 6: Silvisaurus
- Ankylosaur week, day 7: Animantarx
- A month in dinosaurs (and pterosaurs): 3, Minotaurasaurus and giant chasmosaurines
Refs – –
Billon-Bruyat, J.-P. & Marty, D. 2010. Preface: Symposium on Stegosauria proceedings. An international conference on stegosaur finds of the world organized by the Sauriermuseum Aathal (8th and 9th June 2009, Aathal, Switzerland). Swiss Journal of Geosciences 103, 139-141.
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.
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 (4244), 1123-1125 DOI: 10.1126/science.192.4244.1123
Isles, T. E. 2009. The socio-sexual behaviour of extant archosaurs: implications for understanding dinosaur behaviour. Historical Biology 21, 139-214.
Maidment, S. C. R. 2010. Stegosauria: a historical review of the body fossil record and phylogenetic relationships. Swiss Journal of Geosciences 103, 199-210.
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.
Psihoyos, L. & Knoebber, J. 1994. Hunting Dinosaurs: On the Trail of Prehistoric Monsters. Cassell, London.
Reichel, M. 2010. A model for the bite mechanics in the herbivorous dinosaur Stegosaurus (Ornithischia, Stegosauridae). Swiss Journal of Geosciences 103, 235-240.