Speculative Palaeoart: Pterosaur Embryos

This micro-project was born out of one of those Facebook art challenges which I tried (and failed) to ignore, and for which I was nominated by Bob Art Models's Bob Follen (remember him from this post?) and Palaeoplushies's Rebecca Groom. The 'rules' of this particular Facebook challenge were pretty simple: produce three pieces of art for five days. That was it. It didn't specify whether they were to be new pieces, or whether they had to be posted over five consecutive days.

It started well enough as I paved the way to its inevitable non-completion with the usual good intentions. In fact, it's not abandoned, just hard to complete, what with everything else I've got going on. Given that I've got a fairly sizeable pterosaur project in the works, I figured it would make sense to stay close to this subject, and I was inspired to try my hand at portraying pterosaur embryos as close as possible to how extant animal embryos are.

It started with a quick mooch around Google, looking at photos of embryos of familiar animals, to get an idea of general bauplans. Most people are used to the proportions of young animals, with their disproportionately large eyes and heads, and comparatively short bodies and small limbs, but embryonic animals, depending upon their stage of development, can look altogether different to how they will appear at the time of their birth. Indeed, in the early stages, many disparate lineages' embryos may look broadly similar to one another. The diagram below shows representatives of the major vertebrate groups.

A selection of early-stage embryos, representing the major vertebrate groups. (Copyright © 2015 Gareth Monger)

You'll no doubt have seen versions of this line-up. I've opted to cut out most of the internal detailing so common to this style of diagram since I'm only dealing with external features in the pterosaur embryos I'm illustrating. It's easy to appreciate the broad similarities in this selection. The embryos, although perhaps not quite the same age, are at a very similar developmental stage, which shows how vertebrates follow a similar pattern of development in their earliest stages before they begin to specialise. As you can probably appreciate, there's not much point in producing a speculative, days-old pterosaur embryo to add into the above line-up as it won't look significantly different enough to add anything to the subject. The extant maniraptor in that diagram doesn't exactly scream 'DINOSAUR!', or at least not to this non-embryologist.

Today, the only flying vertebrates are birds and bats, and since only bats and pterosaurs share a membranous wing, I chose to look at bat embryos. Of course, bats and pterosaurs are not particularly closely related, having both developed flight apparatus independently, separated by an enormous chunk of time. (View this illustrated lineage at Phylopic to see just how distantly-related they are.) The framework on which they support their wing membranes is different, too, with pterosaurs using an enormously-elongated fourth finger and an internal network of aktinofibrils to stiffen the wing, while bats support a comparatively loose membrane on an enormous, five-fingered hand. Presumably this is the trade-off: pterosaurs have a skeletal scaffold reduced to a single spar, but there's more going on in the membrane and the skeletal mechanism by which they fold the wing away may not be as effective as that of at least some bats.

Embryos of the black mastiff bat (Molossus rufus), showing various stages of forelimb development. Compare 1's short manual digits with those of 2 and 3. (Copyright: Dorit Hockman. Used with permission)

This photo of three black mastiff bat embryos, courtesy of award-winning Dorit Hockman, a junior research fellow at Oxford, shows clearly the embryo pups' 'hands' and dactylopatagia (interdigital wing membranes) which are relatively small compared to those of newborn pups. Roll back several days in these animals' development, and at some point the animals' limbs will appear somewhat unspecialised, without any suggestion of the flighted animal to come. It was this intermediacy which I wanted to explore in my pterosaur embryo illustrations.

There's probably not too much that bat embryology tells us about pterosaurs; after all, bats are altricial and, although relatively well-developed at birth, are unable to fly until they are several weeks old, so there is a period of development and growth during this period. Some exceptionally well-preserved pterosaur fossils suggest that they are precocial, to the point that they can probably even fly within a very short time of hatching. It's reasonable to expect this to be reflected in the anatomy of their embryos and that their embryos go through a stage of rapid development before hatching, at which point they are essentially miniature adults. Unfortunately, early-stage embryo skeletons comprise a lot of cartilage which doesn't often fossilise well anyway, hence the speculative nature of illustrating pterosaur embryos.

A relatively-quick digital 'sketch' of a pterodactyloid embryo, with large head, closed eyes and stubby wings. 10a scalpel blade for scale. (Copyright © 2015 Gareth Monger)

It's difficult to make any scientific claims as to the accuracy of these illustrations, given the lack of direct evidence, which is why I tend towards filing these under 'speculative palaeoillustration'. That's also why I don't feel comfortable tying these down to too specific a taxon, leaving it as loose as 'pterodactyloid'. Other than full-term pterosaur foetuses illustrated to support fossil finds, there doesn't appear to be much in the way of palaeoart for prenatal pterosaurs. It's probable that, owing to the lack of any physical evidence, and the fact that the embryonic/foetal stage is comparatively brief, it's simply not important enough to necessitate producing illustrations of an animal at a point in its development where nothing would see it anyway, particularly when the adult reconstructions are being reviewed and refreshed as frequently as they are.

Another generalised pterodactyloid pterosaur embryo, with wing-digit and brachiopatagium beginning to develop. (Copyright © 2015 Gareth Monger)

So, in short, I don't think this is necessarily a critical aspect of palaeoart, but as science and scientific art, including photography, creeps further and further into our lives, whether that be online or through television or in print, the previously-hidden lives of vertebrates become more familiar to us. It makes some sense for palaeoartists to let themselves be influenced by this.

Biggest of thanks go to Dorit Hockman for letting me use her photograph of the bat embryos, which greatly influenced the final look of my pterosaur embryos. View her professional profile here.