Raising the curtain on a new pterosaur

Darwinopterus divebombing the small dinobird, Anchiornis. Illustration: Mark Witton, PhD

Paleontologist and illustrator Mark Witton explains why the Darwinopterus modularis find introduced today is one of the most significant in many years.

I do like old science fiction B-movies. Rather than filling my time with life-enriching literature or culture, I’ve spent far too many evenings watching ‘classics’ like The Giant Claw, Attack of the Killer Leeches and The Brain that Wouldn’t Die, laughing at the often not-entirely-subtle propaganda and wondering why we don’t say things like ‘oh boy!’ and ‘swell!’ as much any more.

Of course, the main reason to watch such flicks are the monsters: gigantic stop motion octopuses, aliens that look like they’re made from stacked flowerpots, fishy-looking prosthetic bodysuits, dogs with oversize shrew masks, claymation reptilian spacemen from Venus. They’re all extremely naff but somehow excellent at the same time. Very often the creatures in such movies are little more than scaled-up versions of modern animals or, occasionally, taken straight from the pages of The Big Boys Book of Prehistoric Life. Other monster designs show a little more originality: the Martians in George Pal’s War of the Worlds, say, or the bizarre flying battleship in the aforementioned Giant Claw are both products of pure imagination (although the latter does look a little like Sesame Street’s Big Bird if he had a hard night of drug abuse).

"The new species published in Proceedings of the Royal Society B: Biological Sciences is probably the most significant find in pterosaur research for many years as it not only brings together two very different branches of the pterosaur family tree, but does so in a wholly unexpected way."Nestling between design inspirations are chimeric critters bolted together from different animals to create new, exciting combinations. You’ve got the monitor lizard with a carnivorous dinosaur’s head in The Beast from 20,000 Fathoms, the man-crocodile thing in 20 Million Miles to Earth and, borrowed from Greek mythology, the likes of centaurs, fauns and griffins in numerous sword-and-sandal Ray Harrehausen flicks. Outside of Tinseltown, such chimeras are seldom seen in real life, and for good reason. You wouldn’t expect our tiny coccyx to evolve into a heavy tail like that of a crocodile, for instance: our pelvic bones and musculature are just too different. Correspondingly, you’d expect correlated developments across the rest of our skeleton: such a large scale change should have knock on effects elsewhere. I mean, you just don’t find animals looking like they’ve swapped entire body parts with other critters like they’re made of Lego, right?

Well, yes, actually. Today, the curtain is being raised on a new pterosaur, a type of extinct reptile that flew around the skies of the Mesozoic Era above the heads of all the non-birdy dinosaurs. The new species, published today in Proceedings of the Royal Society B, is probably the most significant find in pterosaur research for many years as it not only brings together two very different branches of the pterosaur family tree, but does so in a wholly unexpected way.

See high resolution images on Witton's flickr blog.

Simply put, the new critter looks like the head and neck of one pterosaur were bolted onto the shoulders of another, almost like it was assembled from a selection of prefabricated parts ordered from the Ikea catalogue. In fact, my first reaction to the specimen was suspicion: fossil faking is rampant after all, and this new thing looks like the paleontological equivalent of a cut-and-shut car. Thankfully, the discovery appears genuine: there are at least six specimens of this species known, and the authors of the new study supervised their preparation to ensure that no funny business had taken place. Hence, there is little chance that the fossil is a manufactured chimera, and this makes it’s unique combination of features a very big deal. In fact, this one species reveals a truckload of stuff about the evolution of pterosaurs, our understanding of them and even something about the nature of evolution itself. Neat.

All this fuss concerns a new pterosaur species from the fossil rich Liaoning region of China, christened Darwinopterus modularis by the researchers lucky enough to work on it (Lü et al. 2009). It’s a smallish critter, spanning something like 0.7 – 1.0 m across the wings and, if you travelled back in time around 160 million years, you would have seen it zipping around the richly vegetated landscapes that would eventually form the Tiaojishan Formation. 

Its anatomy is, paradoxically, both unimpressive and generalised but also extremely unusual. The skull is long and low with a shallow crest along most of its length, the teeth are isodont (that is, they all bear the same shape and size) and the naris and antorbital fenestra (openings in the skull associated with pneumaticity and holding the nasal tissues) are fused into a single nasoantorbital opening. The neck vertebrae are relatively long and lack ribs, the tail is long and stiff, the metacarpals (the bones in the palm of the hand) are short, while the fifth toes (the equivalent of our pinkie toes) are long and hooked. Take any one of these features on their own and there’s little to write home about, but the combination of these characters has never been seen before and is the cause of so much interest. More specifically, the head and neck of Darwinopterus bear all the hallmarks of one pterosaur group, the Pterodactyloidea, while everything below the neckline is classically basal, or ‘primitive’ pterosaur. It is, therefore, a ‘transitional’ fossil (although that’s a bit of a loaded term, but we don’t really have time to go into it here), one that reveals hitherto unknown details about the evolution of these groups. But wait: we’re getting ahead of ourselves. First, a little background.

Pterosaur researchers have recognised for over a century that there are two broad groupings of pterosaurs: basal forms and pterodactyloids. Basal forms, as their name suggests, lie close to the root of the pterosaur tree and, while once regarded as a monophyletic group – that is, a collection of species that arose from a single, common ancestor – are now recognised as a rag-tag bunch of different pterosaur lineages that, at some point in the Jurassic, gave rise to the more derived pterodactyloids. With few exceptions, basal forms have separate nares and antorbital fenestrae, often bear multiple tooth types, possess ribs on their short, stocky neck vertebrae, short metacarpals, long, stiff tails and elongate fifth toes. Famous basal pterosaurs include Rhamphorhynchus and the weird Anurognathus, a form that, along with a few closely related species, modified the standard basal pterosaur bauplan more than any others. Pterodactyloids, by contrast, are considered to be a monophyletic group of pterosaurs that have typically been characterised by their relatively elongate, ribless neck vertebrae, short tails, confluent nasoantorbital fenestrae, long metacarpals and stunted fifth toes. Celebrity pterodactyloids demonstrating all these features include the most famous pterosaurs of all: the first pterosaur to be named, Pterodactylus, the snaggle-toothed Ornithocheirus and the gigantic, toothless forms Pteranodon and Quetzalcoatlus.

Until now, there has been jack-all information about the morphological transition of pterodactyloids from basal forms: the closest pterodactyloid relatives have some subtleties of pterodactyloid morphology about them, but the lion’s share of evolutionary work had clearly yet to commence. DarwinopterusRhamphorhynchus-like form that, before virtually any changes were incurred on the body skeleton, evolved a bona fide pterodactyloid-grade head and neck. changes this, demonstrating the transition from basal pterosaur to pterodactyloid morphology began with a Rhamphorhynchus-like form that, before virtually any changes were incurred on the body skeleton, evolved a bona fide pterodactyloid-grade head and neck.

This is Darwinopterus Big Deal No. 1: it appears to confirm several analyses of pterosaur interrelationships that critters like Rhamphorhynchus are very closely related to pterodactyloids, which is like a big, warm hug to many researchers across the globe.

Big Deal No. 2 is less comforting: if you were to separate the head and neck of Darwinopterus from its body, you’d have no issues placing the latter amongst derived basal pterosaurs and the former well within Pterodactyloidea. This means, then, that shadowy doubt now looms over the identities of some basal pterosaur fossils with no neck or head material and, conversely, some isolated pterodactyloid neck and skull fossils: we can now only really distinguish derived basal pterosaurs and fragmentary pterodactyloids when we have relatively complete material, so the identity of several pterosaur fossils may now need revision.

This ties in with Big Deal No. 3: after decades of distinguishing pterodactyloids by a suite of characters across their skeletons, we now have to chuck all of those pertaining to their skull and neck in the bin and can only use features like their stunted fifth toes and tails and long metacarpals to exclusively characterise the group.

Darwinopterus has certainly shaken the branches of the pterosaur tree, then, but it has even further reaching consequences well beyond the realms of pterosaur research and even the broader discipline of vertebrate palaeontology. By demonstrating distinct evolution of the head and neck but effectively no changes to the basal pterosaur body, Darwinopterus is good evidence for modular evolution, the idea in which evolution operates on distinct complexes of features rather than gradual, independent transformation of smaller attributes over time. This is one mechanism that may allow evolution of very noticeable features – like complete remodelling of an animal’s skull, say, or loss of a tail – to happen relatively quickly and at different rates to other parts of the body. Thing is, modular evolution has really only been studied at the genetic level: there has never really been much evidence for it in the fossil record. Most evolutionary ‘intermediates’ (such as the dinobird Archaeopteryx, the protowhale Ambulocetus) possess mixtures of derived and basal characteristics across their entire anatomy, demonstrating ‘mosaic’ evolution but not modular. Hence, the distinction in pterodactyloid and basal pterosaur anatomy across the Darwinopterus neckline is something special, providing an intriguing insight into how modular evolution may have been expressed in the evolution of derived pterosaurs. Of course, Darwinopterus is by no means proof that modularity is a regular phenomenon in evolutionary processes by itself: further research into the genetic and morphological workings of modularity, as well as further demonstration in the fossil record, are needed before such conclusions can be drawn.

Still, if nothing else, Darwinopterus provides a good impetus to explore these ideas further.

In all, then, the bizarre amalgamation of basal pterosaur and pterodactyloid body parts in Darwinopterus has proved a rich ground for research already, but is there anything else we can learn from it? Well, almost certainly. Why, for instance, did the pterodactyloid head and neck evolve so completely while the rest of the skeleton remained quite unchanged? Previously, the evolution of many pterodactyloid features were thought to be associated with increased terrestrial capabilities, but the probably lesser-terrestrial abilities of Darwinopterus (basal pterosaurs are generally considered to be less competent at terrestrial locomotion than pterodactlyoids) show that we now need to rethink this idea. And just what are the functional implications for the juxtaposition of a pterodactyloid head and neck being bolted onto a basal pterosaur body?

The folks behind the Darwinopterus study suggest that the long, flexible neck and large head of Darwinopterus are indicative of aerial predation and imagine it plucking gliding dinosaurs, mammals and other pterosaurs from the air (as depicted in the associated image). While this idea is undeniably cool, it needs testing and there may be other possibilities we’ve yet to consider. And there’s more: what do the curved bones in the Darwinopterus fifth toes mean? How come the Darwinopterus skull looks so much like that of Germanodactylus?

Bottom line: we could list things to investigate off the back of Darwinopterus for ages, but we’ve not the time nor space to do that here. Instead, I think it’s time sign off and fire up the B-movietron: I’ve a hankering to see more chimerical monsters do battle with the best science the mid-twentieth century had to offer. Because, oh boy, that sounds swell to me.

Mark Witton, PhD, is a paleontologist at the University of Portsmouth, UK. His illustrations and blog entries can be found on flickr.  

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