I made this illustration for a top-secret project I’m working on in collaboration with Matt Wedel (more on that later). I also submitted it as an entry in the All Yesterdays Contest for speculative reconstructions of extinct creatures, hosted by the authors and illustrators of the book ‘All Yesterdays’.
Subadult Diamantinasaurus licking minerals in a cave illuminated by bioluminescent cave glow worms of the family Mycetophilidae.
The main idea behind this illustration is that in the 120 million years that dinosaurs ruled the earth, at some point they most likely entered caves, and perhaps even large dinosaurs entered them. As discussed on SV-POW, modern elephants enter caves to exploit their mineral resources, and it was Matt Wedel’s suggestion that perhaps sauropods also did this at some point in their lengthy natural history.
Here are some close ups of their heads and necks showing speculative scale patterning and armor, based partially on Saltasaurus, and partially on modern reptiles. The spines along the bottom of their necks and abdomens are meant to be keratinous, serving both display and defense functions, much like modern Australian Bearded Dragons (Pogona), Thorny Devils (Moloch) and the Horned Lizards of western North America (Phrynosoma). Also featured are highly reflective scales on the head and neck, similar to the modern skink Proctoporus shrevei , which may serve as adaptations for species recognition or to startle predators in low-light environments such as dark forests or caves (also, to look rad). Another speculative feature of the integument are elongated hair-like scales on the snout, which are meant to be a sensory adaptation, similar to a mammalian whisker, to help these large animals navigate in dark environments.
As I was thinking about doing this illustration, it occurred to me that it would be really cool if the cave was lit by bioluminescence, and after a little research I learned that the glow worms that light up caves are from the genus Arachnocampa and are native to Australia, New Zealand and Tasmania. I shot an email to my brother (who is currently working on a masters degree in entomology) to find out a little more about the natural history of these glow worms, and he found out that although there isn’t any direct evidence of bioluminescent Arachnocampa in the fossil record, the larger group that they are a part of (the fungus gnats of the family Mycetophilida), are a very ancient group, being “well established and diversified by the Cretaceous period at the latest (wikipedia)”. Considering how many times bioluminescence has evolved independently in the history of living things, and considering that much of the flora and fauna of Australia and (especially) New Zealand is derived from common ancestors isolated there shortly after the end of the Cretaceous, it seemed reasonable enough to speculate that perhaps bioluminescent cave glow worms like the Arachnocampa of today illuminated caves that the dinosaurs of mesozoic Australia wandered into…
Based on that I decided to make the dinosaurs in the illustration Diamantinasaurus, because they lived in ancient Australia, and because they were possibly armoured like their close relatives Saltasaurus. I thought it would be cool to draw a speculatively armored sauropod because, 1) armor looks rad, and 2) armor requires a lot of minerals (especially calcium) to grow. In the case of saltasaurus, the armor is a series of boney knobs called osteoderms, which start as keratinous scales, that are reinforced with bone as the animal grows. That inspired me to speculate that perhaps during the phase in their life where they are rapidly growing and hardening their integument, they made a habit of visiting caves to boost up on calcium and other important trace minerals not easily acquired elsewhere. This caused me to begin speculating as to what other adaptations dinosaurs who frequently ventured into caves might’ve developed as they became more and more specialized for dark environments. If indeed, venturing into caves became an essential part of these animals’ life cycle, perhaps then they would have evolved such features as the whisker-like scales on their faces and the reflective display scales I depicted them with on their heads and necks. It seems to me that if the diversity of dinosaur integuments seen in the fossil record is to inform how we imagine the skin features we can’t see in the fossil record, we should imagine dinosaur skin texture and function as diverse and highly adaptable. Arguably it was the adaptational flexibility of dinosaurs’ skin and scales that helped make them so successful for so long. Even just within the theropoda we see everything from scales, to osteoderms, horns, fuzz, feathers, quills, exposed bare skin – and often with several of these different features within the same animal.
But who knows… While fossils of large animals are known from ice age caves, I don’t know of any mesozoic fossils coming from cave systems. Perhaps most cave systems are too unstable to last so long as to survive from the mesozoic to the present, or perhaps we just haven’t been looking in the right places. Based on modern animals, I think it’s safe to assume that dinosaurs and other mesozoic monsters colonized and exploited cave environments, perhaps even adapting into obligate cave dwellers, whose bizarre remains lie undiscovered in some hidden cavity in the geological bowels of the earth…