I was helping out on an undergraduate sedimentology fieldtrip on the Isle of Wight today. If you’re into sediments, the southwest coast of the Isle of Wight is the place to be, showing more cross-bedding, bioturbation, lag-deposits and reactivation surfaces than you can shake an Estwing geological hammer at. The Wessex Formation, world famous for its dinosaur splendour, is particularly hot sedimentary property thanks to its showcase of ancient river channels, psychedelically mottled clays and enough sedimentary structures to make respectable sedimentologists skip around like school girls and use words like ‘cool’. Next to this depositional extravaganza is Isle of Wight Pearl, part of a chain of peal retailers that, today, provided a convenient carpark for a coach full of students and their lecturers. Now, because some of our students had bought things in their cafeteria, I figured they could pay the university back, indirectly, by letting me quickly use their facilities before the long coach ride home. Despite having been to that part of the Isle of Wight more times than I remember (I camped there for a month during 2004 for my dissertation project), I’d not stepped foot inside Isle of Wight Pearl until today. It’s a nice place, too: well kept, very neat and, yes, they are indeed a retailer of fine pearls. Their loos, my main concern this afternoon, were also top-notch in the hygiene stakes: very clean and tidy, but, alas, their visual attractiveness was not matched by their air quality. Not for the reason you might think, either: no, the atmosphere was instead thick with an over-abundance of toilet freshener. You could taste it: too many little smelly spongy things in the urinals, too much air freshener... simply not pleasant at all. I mean, it’s nice to have a whiff of freshner in your public loos to demonstrate that someone has, at some point that day, made sure things are up to scratch – and its certainly preferable to some other smells you’ll find in some facilities – but there’s a difference between a faint whiff of cleanliness and something so concentratedly acidic that it feels like it would necrotise open wounds. Suffice to say, I held my breath, did my business quickly and efficiently, and left as soon as I could.
Now, believe it or not, the world of pterosaur research has something in common with the over-fragrant aromas of public loos. Y’see, there is also something of a smell in pterosaur research that, while undeniably reflecting some need, is about as welcome and difficult to stomach as over-abundant air-freshener. Yes folks, we’re back to the sadly familiar and entirely artificial smell that all pterosaurs were shore-dwelling dip-feeders, eating nothing but fish and spending most of their time gliding through the sky. Pterosaur ecology shouldn’t smell of this at all: rather, it’s a fragrant aroma akin to that of a fine wine, rich in diverse lifestyles being acted out in a variety of environments. You can bet that Jilly Goolden from Food and Drink would love the smell of pterosaur ecology. So why then, why oh why, is there a picture up there of that most hateful of pterosaur ecological hypotheses, a pterosaur gliding over an ancient ocean, dip-feeding to grab a fish from the water surface? Surely this funk smells bad enough already? Thing is, y’see, just like air fresheners take nice, pleasant smells and concentrate them to the point where they want to make you gag, the dip-feeding pterosaur hypothesis is not without merit, but it’s been sprayed around so much that it’s pleasantness is over-bearing. Here: put this clothes peg on your nose and let me explain.
The view that pterosaurs were Mesozoic seabird analogues has persisted for years, inspired by the occurrence of most pterosaur fossils in marine sediments, many decades spent with a very limited knowledge of pterosaur diversity and thoughts that pterosaurs were completely inept when grounded. Palaeontologist after palaeontologist labelled pterosaurs as marine fishers, and this idea has become ingrained in scientific papers as well as the popular press (although, in the former, things are changing. A bit). This is particularly well demonstrated by Peter Wellnhofer’s otherwise excellent Encyclopaedia of Pterosaurs , that states - with very, very few exceptions, that pterosaurs ate fish on the wing using dip-feeding and skim-feeding. This idea has become ingrained in the mindsets of palaeontologists of every generation, from bouncing baby palaeontologlets to gerontic, senile palaeontological professors, and I know several people who, to me at least, are prepared to bend the laws of biology and physics to keep this idea entrenched.
So, yes, the idea of pterosaurs as aerial fishers is, in my view, totally overflogged, but that doesn’t mean that there isn’t some truth behind it. The chappie depicted up there is (probably) the shining light of truth behind it all: meet Anhanguera, an ornithocheirid pterosaur from the Lower Cretaceous of Brazil that has relatives occurring, well pretty much everywhere. Ornithocheirids must be the most geographically widespread pterosaur group on the planet: you can find their fossils on every single continent – yes, including Antarctica (Antarctica, for pity’s sake). In fact, ornithocheirids appear to have been a common component of any sub-aerial Lower-to-mid Cretaceous vertebrate fauna, occurring in a range of sedimentary environments from continental deposits to those laid down out to sea. The taxonomy of ornithocheirids is, unfortunately, a real mess and I’m not going to pretend that I really know what’s going on with it: there are countless nomenclatural schemes for the group and unweaving it all properly could probably fill several PhD theses. Ornithocheirid anatomy has been known for donkey’s years from some really, really kick-ass specimens: the sort of three-dimensional, articulated and complete skeletons that palaeontologists dream of. They were also, along with the closely related pteranodontians and istiodactylids, the best known pterosaurs for many, many years. This gave the impression that ornithocheiroids, the broader group that includes pteranodontians, istiodactylids and ornithocheirids, had ‘standard’ pterosaur anatomy, but, in actuality, compared to other pterodactyloid pterosaurs (see here for a discussion of this), ornithocheiroids are freakish. High shoulder joints, weird, blocky-looking humeri with strangely-warped deltopectoral crests (the bit that anchors the pectoral muscles), robust wrist bones, abnormally-long wing fingers, short legs, unusually flexible or strong necks and tiny little bodies: if they weren’t so familiar to us, we would surely class them as the strangest looking of all pterosaurs. These anatomical quirks, of course, aren’t just for show: rather, they are probably a series of adaptations to a far more volant lifestyle than other pterosaurs.
Y’see, the ornithocheiroid body plan (and particularly that of ornithocheirids and pteranodontians) was essentially that of two long, narrow wings with a big head bolted on the front. Their small bodies and stunted legs means that these critters were unusually lightweight for their size and, with very efficient wing shapes, were probably excellent fliers. Long, narrow wings are particularly good for gliding flight (think of long-distance soaring seabirds), so these chaps could’ve probably flown long distances without much difficulty. Their high shoulder girdles would also render them quite stable in the air, suspending their bellies below their wings means to lower their centre of gravity and making them less prone to rolling when turning and whatnot. However, because their hindlimbs were so much shorter than their forelimbs, ornithocheirids and pteranodontians were probably a bit rubbish at moving around on the ground. What’s more, their long wings were probably a bit crappy for take-off, requiring relatively high velocities – not to mention room to stretch out the wings - to become airborne. Thus, it appears that ornithocheirids and pteranodontians were better suited to a life in the skies than any other pterosaur. Their wing shapes suggest that they were better placed for flight in uncluttered environments where they could exploit wind and other air movements to the full, but note that they were poorly adapted to climbing thermal updrafts. They’re glide efficient, sure, but long wings also give their owners broad turning circles. Because ornithocheirids and pteranodontians have the longest, narrowest wings of any pterosaurs, they were probably the most poorly-suited of all the gliding pterosaurs to exploiting thermals.
Now, just being adapted for long-distance flight in open settings doesn’t mean you can’t ever land to feed. Some of the most glide-adapted birds we have today – albatrosses and petrels – do not feed on the wing, preferring to dive, alight on the water surface or land. They take-off again just fine, and I’m really quite confident that some long-winged pterosaurs did the same. However, at least some ornithocheirids shows some adaptations to feeding from water bodies without landing. For one thing, ornithocheirids have teeth of various proportions that seem to project at every conceivable angle from the jaw and demonstrate a range of shapes and sizes along the jawline. Those at the front are either small or large recurved tusks, while those at the back are considerably shorter, more widely spaced and relatively robust. This suggests an entirely different function for the jaw tip to that of the posterior tooth row, with the jaw tip apparently better suited to seizing soft prey items and the back teeth – also slightly recurved – well adapted for ensuring that foodstuffs make it from the jaw tips to the throat. The lack of seizing dentition at the back of the jaw suggests that only the front teeth were employed in grasping prey, which may be expected from an animal trying to maximise distance between its aloft body and the water surface. The lengthening of the jaw, of course, also helps in this regard. That said, you see similarly elongate jaws and dental arrangements in some modern crocodiles, so these are clearly not adaptations reserved for aerial feeders alone.
Similarly, flexible necks are essential to aerial fishers. The aerial predator is always going to be moving much, much quicker than its submerged prey and, therefore, the jaws grabbing it will move beneath and backwards relative to the rest of the predator when lowered into the water to secure its meal. Ornithocheirids do have pretty flexible necks, permitting the front of the neck to point at a perpendicular angle to the long-axis of the body. Thing is, this trait is not strictly indicative of aerial fishing alone: lots of feeding methods benefit from a flexible neck. Thus, what’s needed is a really diagnostic feature of aerial fishers, something that other foraging methods simply wouldn’t require. Thankfully, there may be one: it seems to me that any animal sticking its jaw-tip into the water will have to counteract drag forces when pulling their head out again. Aerial fishers need strong neck muscles attaching to the skull – the complexus musculature – to yank their jaws and prey out from the water against momentum and drag forces pushing their heads further backwards. Such muscles can be detected through relatively swollen and complex anterior neck vertebrae and, in contrast to most other pterosaurs, Anhanguera bears these features. Other pterosaurs simply don’t show this: pteranodontians and other ornithocheiroids lack strong anterior neck vertebrae, suggesting they lacked pulling power when it came to aerial fishing. In fact, they typically show the exact opposite, with the neck vertebrae decreasing in size towards the skull. Such traits suggest these guys weren’t aerial fishers in spite of their strong volant adaptations, and probably foraged by alighting on the water surface or something like that.
So, the take home message: all this funk about aerially-fishing pterosaurs is not a bad smell that should be vented and forgotten. Rather, it’s an excellent hypothesis that has merely been over-sprayed to encompass far more than it needs to: it simply needs some refinement to make it a far more credible idea. I should point out that the thing about the neck vertebrae being diagnostic of aerial-fishing is, as far as I know, virtually un-discussed in pterosaur or zoological literature, suggesting that the bulk of the refinement waits to be done. Unfortunately, the clock has just turned midnight and I’m meant to be helping out in a class at 9:00, so this really isn’t the time to start. Best be off, then. ‘Night.