Archive for July, 2020

How to grow your dragon – pterosaur ontogeny

Life reconstructions of Rhamphorhynchus on display in Munich.

The giant pelagic pterosaur Pteranodon is probably the most famous, and is certainly the most iconic, of pterosaurs and specimens and casts of this show up in museums around the world. There’s something like 1100 specimens in public collection and plenty more in private hands. Unfortunately though, almost all of them a squashed very flat and they are often rather distorted and worse, the overwhelming majority are very incomplete and often composed of only a few elements. They are also almost all of a good size (‘subadult’ and up) with only one specimen recognised as being something close to juvenile in age. That means that while this is an amazing number of specimens, it’s also really quite hard to work with as the data is limited in lots of ways.

However, if we turn to Rhamphorhynchus we have only a fraction of the number of specimens but pretty much all the other issues are absent. Most specimens are complete or at least have a very healthy amount of the specimen present, they are often flat but show nothing like the distortion of Pteranodon and there are even fully 3D specimens. They also cover a near order of magnitude in size with everything for animals of c 30 cm wingspan up to nearly 2 metres and include everything from putative hatchling-sized animals to a couple of genuine outliers that are much bigger than other known individuals. Thus despite the relatively low numbers they represent and absolutely fantastic resource for studying various aspects of pterosaur biology.

The numbers of course are not tiny, well over 100 good specimens, and that alone would make them an exceptional sample of most terrestrial Mesozoic archosaurs. The legendary Solnhofen researcher Peter Wellnhofer catalogued over 100 of these in his amazing 1975 monograph on them and this dataset has become an industry standard for pterosaur research ever since. However, we are still discovering more and there are plenty sitting in various collections around the world that nave never entered the literature because, well, there’s already 100 of them out there. But even big samples are improved with the addition of more material and so for the last decade I’ve been scouring collections and databases and hunting down every specimen I can to add it to Peter’s data. That takes us from his total of 108 to 129. The ‘real’ total is actually a little lower since several of his were in private hands and two of mine are casts, though of unique specimens, and not all of these are complete. Even so, it represents a hefty increase in the available data and marks the first major increase in the catalogue in 45 years.

Obviously I’m not going to make a dataset like that and sit on it, so this post inevitably marks the publication of an analysis of growth in Rhamphorhyunchus. In a lot of ways, this mirrors Chris Bennett’s fantastic 1995 paper on this genus where he convincingly demonstrated that all specimens belonged to a single species and not multiple ones as previously thought, and part of his arguments for doing this looked at the relationships between various elements based on Wellhofer’s dataset. Chris’ point was that while there were some discreet clusters of specimens (which he attributed to year classes) most of the alleged differences between the putative species vanished when you put them on a graph and the rest were classic ontogenetic traits like the fusion of the pelvis in large individuals of big eyes in small ones. So while he didn’t really deal with growth as such, he was already showing similar patterns to what I and my coauthors confirm now – Rhamphorhynchus was weirdly isometric in growth.

In other words, in the case of the vast majority of their anatomy, young animals are basically just scaled down adults. This is a weird proposition for a terrestrial vertebrate as most undergo some quite notable and even extreme allometry with some parts proportionally growing and others shrinking as they grow. Think of young animals with big eyes, in big heads and large hands and feet, or antelope with especially spindly legs and so on. But in the pterosaurs even the smallest animals are, aside from the eyes, basically carbon copies of the adults.

Rhamphorhynchus

One of the less well preserved Rhamphorhynchus out there, it nevertheless has most elements intact

To put this in context we looked at another group of quadrupedal, powered flying vertebrates with bony spars supporting membraneous wings, the bats. Yes, obviously they are not ideal in terms of their ancestry but functionally they are about the best analogue you could get for a pterosaur. Looking at their development we see that juveniles have proportionally very small wings and right around the time they start to fly and become independent, their wings grow rapidly. This is the pattern we would expect, young animals have only so much they can invest in their development and growing wings that are not being used is what we would expect, exactly as things like sheep (and indeed dinosaurs) don’t grow their horns until they reach sexual maturity, they are not being used before then. We do though, see the bats developing their legs early as they need to grip into cave roofs and their mothers so it’s not a case of overall reduced development of limbs, but clearly selective growth.

Birds are functionally poor analogues of pterosaurs but are much closer phylogenetically and are the only other powered flying tetrapod so we also looked at some existing datasets for them too. Most birds, unsurprisingly have allometric growth of various elements, but like bats the legs develop before the wings with one notable exception, those that are hyperprecocial. Some birds like mallee fowl are capable of flying within days, or even hours of having hatched from the egg. These birds have isometric growth and this immediately then suggests that Rhamphorhynchus at least (as has been suggested before) was precocial and flying while young.

This may sound correct since if you are flying when young and flying when adult you probably want to be the same but that’s not the case. As a flying animal in particular, relying on wings to hold you up you have a problem. If you grow isometrically you wings will get longer and wider but your weight will increase much faster since you as a whole will get longer and wider and deeper. So mass will increase much faster than wing area and that can only have a profound impact on how you fly. There are two things that might offset this, first of all different animals can use different flying gaits at different sizes which might mean that performance is not quite as different as might be predicted from this (though we’d still expect juveniles to be more agile) and secondly, changes in pneumaticity. Birds increase penumaticity as they grow and there’s evidence this is the case in other pneumatic clades too and if so for pterosaurs, then the mass increase in adults would also be offset somewhat by a proportionally lower mass in adults for a given volume than juveniles.

Precociousness has been suggested in pterosaurs before based on the evidence for them flying while young, but it has also been challenged. It suggested that to be flying at that size would require a huge amount of effort and this would leave little energy for growth. That’s largely true, but overlooks that there could be post hatching parental parental care. That is normal for archosaurs (including dinosaurs) and we would expect it for pterosaurs. Being precocial in terms of the ability to move does not mean they have to be independent, things like horses have babies that are capable of running within hours of birth but are still suckled for months, and various ducks take their ducklings out to sea soon after hatching. That’s obviously not the quite same thing as the energetics of flight, but it does show that being a good locomotor is not mutually exclusive with parents protecting and feeding their offspring.

So in short, Rhamphorhynchus is perhaps the best pterosaur for large studies about populations and growth and this genius at least grew isometrically, and this may or may not be the same for other pterosaurs. This then may or may not have some big implications for pterosaur taxonomy which is often based on the ratios of various wing elements. But it does imply that young pterosaur could fly, and fly well and that adults and juveniles were probably flying in different ways to each other and that could then have implications for where and how they foraged and what they ate. This is an incremental step in our understanding of this group (and again, much of what we say has been said before but this firms things up nicely) and hopefully opens up the options for further research on them as living animals.

 

The paper is open access and available here:

Hone, D.W.E., Ratcliffe, J.M., Riskin, D.K., Hermanson, J.W. & Reisz, R.R. 2020. Unique near isometric ontogeny in the pterosaur Rhamphorhynchus suggests hatchlings could fly. Lethaia.


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