Overexposure of Stegosaurus, but in a good way

Is it just me, or are stegosaurs not quite as popular as they used to be? Stegosaurs are iconic dinosaur species that, like tyrannosaurs and ceratopsids, have been drawn to death by generations of palaeoartists eager to capture their freakishly weird anatomy, but they don't seem to be quite the mainstay of dinosaur pop culture that they used to be. I could be wrong, but it seems that other dinosaur taxa, primarily feathery, near-birdy things, have take a share of the stegosaur limelight. Perhaps stegosaurs, and particularly Stegosaurus, are just a so  familiar now that we've become a bit blasé about them. I know I certainly have, so I've not sketched or painted one in years. It was only in revisiting them for this piece that it struck me how freakin' weird stegosaurs are, even in this world of therizinosaurs, mononykosaurs and four-winged microraptors. The front of their bodies clearly belong to relatively small or medium-sized animals, but evolution thought it would be fun to bolt them hindquarters borrowed from an large elephant or small sauropod. Supporting the dainty head and neck is a set of seemingly well-engineered but overly-short forelimbs, which force their spinal columns into high, curving arches to span the height discrepancy between each limb set. And then there's the osteoderms, shaped into broad plates or spikes, which sit along their backs and may turn the distal end of the tail into a morning star. Stegosaurs make feathery maniraptorans look positively boring.

For this painting, I wanted to show a stegosaur - specifically the Upper Jurassic Morrison Formation species Stegosaurus stenops - with some real character, looking like it had lived a hard life in an unforgiving climate and surrounded by extreme and frequently dangerous animals. For this reason, I chose a rarely depicted, more or less head on aspect for the painting, thus bringing its tiny head to the fore, allowing us to see its face without forgetting that the body behind it was large and powerful. I imagine that standing next to a big stegosaurs should be like standing next to any big, unfamiliar animal. It may not eat you, but the feeling that we're a small, inferior species, and that the 6 tonne animal next to us has absolute right of way, will never disappear. There also seemed to be a lot that could be done with its appearance. Stegosaurus was a large enough animal that they were probably fairly long-lived, and would accumulate decades of wear-and-tear on their hides. Thus, I depicted his very imperfect skin with an extremely washed-out but high contrast colour scheme, in efforts to enhance his battered appearance. Fossil evidence also came into play with creating a history for this animal, as we have good evidence that stegosaur osteoderms were occasionally subject to extreme damage in life, perhaps because they were bitten by predators or, in the case of their defensive tail spines, winged into the side of assailants with enough force to break their tips (Carpenter et al. 2005). With this in mind, my stegosaur has a number of broken plates along its back, this animal having seen off its fair share of aggressors. At one point, he was also going to be depicted drooling long strands of spittle through heat-stress, but the effect wasn't quite in keeping with his posture, so I took a napkin to his beak and tidied him up (see detail, below).


I also thought it might be fun to play with the scaly depiction of stegosaurs a bit, decking the thagomizer out with a set of long, bright filaments. Excellent skin impressions from Morrison stegosaurs (possibly even from Stegosaurus itself, if the assessment of stegosaur taxonomy by Maidment et al. [2008] is correct) reveal that their bodies were covered, probably mostly, in typically archosaurian pebbly scales (Christiansen and Tschopp 2010). This integument is exactly what we would expect from a large ornithischian in a warm climate. As with many dinosaurs, their scales are of variable size across the body, with long rows of large (20 mm wide) scales stretching across the dorsal regions and smaller scales lining the belly (see image, below). By analogy with modern 'naked' mammals however, I wondered if some scaly dinosaurs would retain small regions of fuzziness from their ancestors for specific functions. The bushy tails, ear tufts and eyelashes of naked mammals are good analogues here. In this case, my stegosaur's thagomizer isn't bristly to swat flies as are the hairy tail tips of mammals, but to advertise its bristling spikes to marauding predators, and make them think twice about attempting an attack. Further analogy can be made here with the striking colours of many poisonous or otherwise well defended animals: camouflage is thrown to the wind in favour of making themselves unmistakeable to predators, letting them know to think twice about attacking them. Additional uses for fuzzy thagomizers may be sociosexual display, dusting hard to reach shelves and corners or, perhaps for defensively tickling they way out of hairy situations (hat tip to Spike Ekins and Simon Clabby for the latter).

Stegosaur skin impressions, probably from Stegosaurus, from Christiansen and Tschopp (2010). Top, belly scales, bottom, large scale surrounded by smaller, satellite scales. Scale bars represent 20 mm (top) and 10 mm (bottom).

And speaking of patchy filament distribution, I also gave this guy eyelashes, but you can't really see them in all the shadow I then layered over the top. Eyelashes may seem very odd things to put on dinosaurs, but they are common features of animals that have fuzzy ancestors. Numerous bird species have specially adapted feathers which are functionally analogous to mammal eyelashes, for instance. At least hornbills, secretary birds, seriemas, parrots, roadrunners and ostriches bear them, which serve to  trigger blinking when touched (as in mammals) and, in some species, shade the eye. Eyelashes are also frequently retained in mammals that have mostly or entirely lost their fur: elephants, rhinos, hippos, you, and others. Thus, it seems quite plausible that many dinosaurs and other ornithodirans had eyelash-like filaments, and that some scaly dinosaurs will have held onto them.

Right, that's a reasonably concise post for these parts, and will have to do for now: I need to get going with a big palaeoart project that will, coincidentally, also require some consideration of ornithodiran eyelashes. If I'm allowed, there may even be some bits of it being posted here before any of us are too much older.

References

• Carpenter, K., Sanders, F., McWhinney, L., and Wood, L. 2005. Evidence for predator-prey relationships: Examples for Allosaurus and Stegosaurus. In Carpenter, K. (Ed). The Carnivorous Dinosaurs. Indiana University Press. pp. 325–50.
• Christiansen, N. A., and Tschopp, E. 2010. Exceptional stegosaur integument impressions from the Upper Jurassic Morrison Formation of Wyoming. Swiss Journal of Geosciences, 103, 163-171.
• Maidment, S. C., Norman, D. B., Barrett, P. M., and Upchurch, P. 2008. Systematics and phylogeny of Stegosauria (Dinosauria: Ornithischia). Journal of Systematic Palaeontology, 6, 367-407.