The pterosaur hindwing

Ornithocheirus by John Conway. Notice legs forming a secondary set of wings, which the author even compares to a biplane in his original description.

A common trend in pterosaur paleoart in the mid-2000’s was depicting them with two sets of wings. In these depictions the legs, usually free from the brachiopatagium, would be elevated in flight and, thanks to their own uropatagia running alongside them, form a secondary set of wings. Essentially, a membranous version of the tail feathers of birds like swallows or kites.

This trend seems to have been popularised by John Conway and quickly took hold, before disappearing roughly in the early 2010’s. Nowadays, it is rare to see this type of depiction, at least among serious artists.

The appeal of the pterosaur hindwing is pretty basic. At the time, it was increasingly common to try to reinvent pterosaurs away from more bat-like depictions of decades prior and to compare them functionally to birds, including giving them a more “dynamic” look. It was thought that pterosaurs rose their legs in flight like modern gliding lizards and the extent of the brachiopatagium at the time was less publicized (though known since Pterodactylus specimens were first described). Add in the discovery of four winged dinosaurs like Microraptor, and the idea that pterosaurs had a similar bauplan just sort of clicked, you know.

So what happened?

Lonchodectes by Ceri Thomas. This represents the modern (and ironically conservative) interpretation of pterodactyloid hindlimbs: attached to the brachiopatagium and with no wing shape.

Seeing as this trend died out around the publication of Mark Witton’s book on pterosaurs, more widespread information on pterosaur anatomy can be considered the killer of the pterosaur hindwings. We’ve already known for a while that the hindlegs of pterosaurs were connected to the brachiopatagium and that the cr/uropatagia of pterodactyloids was vestigial at best while that of their earlier cousins formed a blob-like shape not particularly resembling wings, but a few factoids have since shut down this artistic concept:

• The idea that pterosaurs needed tails is not well funded. Modern birds can fly without tails and many extinct bird clades like Enantiornithes and lithornithids lacked tail fans and were still efficient flyers. Likewise, many modern bats like flying foxes are in the same boat as pterodactyloid pterosaurs, with vestigial uropatagia.
• The need to functionally compare pterosaurs to birds has reached an all time low. Most notable is the notion that pterosaurs used their forelimbs to launch; this would make the seperation of the legs from the brachiopatagium less urgent.
• It is not entirely clear if pterosaurs could raise their hindlimbs like gliding lizards do. A 2018 study suggests pterosaurs had such erect hindlimbs that they were just as incapable as birds of raising their hindlegs, but this study has since been criticised for ignoring soft tissues. It seems most likely pterosaurs could still raise their hindlegs, but this probably only helped reduce drag, rather than form a secondary wing set.

A more recent Rhamphorhynchus depiction by John Conway. Note the cruropatagium; non-pterodactyloid pterosaurs had a single large membrane between their hindlegs, but while this might have added additional lift it doesn’t seem to have formed anything wing-like.

The pterosaur hindwing is an interesting hypothesis, and while it doesn’t seem to have survived the test of time it was an interesting flame while it lasted.