Pterosaur Ontogeny

Yes it's another reposted piece from the Musings (and with some good comments and discussion) but I could hardly not post it here, given the extreme relevance to pterosaurs in general, and a paper I have coming out shortly to be more specific. We have touched on these various issues before, but here's a crack at being much more explicit about the changes in form of pterosaurs as they age.

Not too long ago, Matt Wedel had an SV-POW! post that talked about ways of diagnosing an adult vs non-adult sauropod. Inspired by this and the fact that I have recently been playing around with issues of ontogeny in pterosaurs, I decided to write something similar for the non-avian Mesozoic fliers. If you have a pterosaur specimen in front of you, just how do you know if it’s an adult or not?

Obviously there are some general indicators that are pretty good for vertebrates as a whole that will get you quite a long way (even if this is a new species). Size is obviously rarely a great indicator, but if you have a pterodactyloid with a 20 cm wingspan then it’s going to be a juvenile, and likewise if you have a rhamphorhynchoid coming in close to the 2 m mark it’s very unlikely to be anything but a big adult. Young animals (and especially very young animals) tend to have big heads compared to their body and especially very big eyes compared to the size of the head. A bunch of fusions are absent in young pterosaurs that are present in adults too, just as you’d expect for most animals. The sutures between the centrum and neural arch of the vertebrae will be open in juveniles and closed in adults, and similarly the elements of the pelvis and sacrum, and the scapula and coracoid will be separate in young animals and fused together in adults.

Pterosaurs also have some characters of ontogenetic change that are rather more peculiar to them than vertebrates in general. Very young pterosaurs also tend to have a very grainy texture to the surfaces of their longbones, despite the fact that even embryonic pterosaurs have a pretty ossified set of bones (unlike many young animals). Smaller pterosaurs also tend to have various parts of the skeleton being less ossified and rather amorphous compared to those of adults. The tarsals are often not well ossified and can be missing (well don’t preserve) and if present may be very simple shapes. The carpals tend to look more ‘blobby’ and lack the detailed morphology seen in adults and will be separated into multiple elements whereas in adults the wrist will primarily be formed of just two massive elements (plus the pteroid). Finally, while obviously you would expect skulls to fuse up during ontogeny, pterosaurs do tend to take it one step further than most. Rather like birds, in adult pterosaurs the sutures all but disappear, or even go entirely, such that the skull looks like a single smooth piece of bone. Also as in some birds, bigger pterodactyloids have a notarium and this only fuses up and fully develops in adults. Similar to the point above about absolute size, the presence and development of some form of head crest is indicative, but not a great indicator of age. Yes a massive and elaborate crest in an animal is indicative that it’s an adult, but there could be a fairly well developed crest in an animal that is close to becoming and adult and of course there are taxa without crests and in at least once case it appears that females don’t have crests.

As in mammals, but unlike dinosaurs and birds, pterosaur also have epiphyses. The growing plates at the ends of the long bones physically separate the main shaft of the bone from the proximal and distal ends, so things like the femur can appear to be in three pieces. Obviously as growth slows towards maturity these epiphyses slowly disappear as they fuse into the single element that you would expect to see.

So in short, something that is small, with grainy textured bones, a big head, with big eyes, unossified tarsals, amorphous carpals, no crest, clear sutures in the skull, no notarium, and separated scapulocoracids, pelvis, epiphyses and neurocentral sutures is going to be a young juvenile. And the close these various features get to the opposite condition the closer the animal is likely to be to adulthood.

As ever with such things these are not absolutes, but merely guides. Good guides, certainly – you simply won’t see a notarium in a very young pterosaur, or open neurocentral arches in a big, old adult. However, in terms of determining more subtle difference in age it will be tricky – one animal may have fused up the notarium, but may have incompletely ossified tarsals and another could have the reverse. Although at least some characters do seem to have a bit of a pattern (the scapulocoracoid seems to fuse pretty early in most things) a general lack of numerous specimens of different ages makes it hard to do any more detailed analysis. Still, in terms of gross age (hatchling – young - adolescent - adult) even for a specimen of a previously unknown species with no obvious close relatives, it should be relatively easy to determine the approximate age of the animal.

The Beauty of Big

One rather obvious trait of pterosaurs, compared to other flying animals, is that they had a tendency to get rather large.  There has been much to do about how they could get so large (as most readers of this blog know, I and quite a few others now prefer the explanation that pterosaurs were quadrupedal launchers).  However, it's a bit trickier to tackle the problem of why some of them became so large. 

What might select for giant size in pterosaurs?  It's probably not something that can be answered definitively, but there are some plausible options.  One of them relates to the issue of long-range travel.  I talked a bit about this potential advantage here. The gist is this: being large makes long-distance travel more feasible for most animals, particularly flyers.  Some reasons this happens include:

1) Large flyers can carry more fuel.

2) Large flyers travel at higher speeds, on average.

3) Large flyers are less affected by adverse weather conditions.

4) Large flyers use less fuel per unit body mass, per unit time.

This means that, on average, a big flying animal can go longer between stops on long migrations, and gets to their destination more quickly, than a small flyer.  This might be particularly important for a flyer which feeds on resources that are patchy in their distribution.  This, in turn, might suggest (very tentatively) that animals like azhdarchids had a tendency to travel long distances between food sources.

Of course, there are a host of other advantages to being large, as well: big animals are harder for predators to kill, can eat larger prey, and can (in some cases) better defend young.  For egg-laying animals, large size often improves fecundity.  So there is no way to say for certain exactly why some pterosaurs grew quite large, but it does raise questions of general biological interest.

The Amazing Pterosaur Pelvis

There is a new paper out in Acta Palaeontologica Polonica on the anatomy and diversity of pterosaur pelvses by Hyder, Witton, and Martill.  It's open access, so check it out: http://app.pan.pl/article/item/app20111109.html

Here's the abstract:
Pterosaur pelvic girdles are complex structures that offer a wealth of phylogenetic and biomechanical information, but have been largely overlooked by pterosaur anatomists. Here, we review pterosaur pelvic morphology and find significant differences that correlate well with pterosaur clades identified in some phylogenetic analyses. We find that the length and orientation of the iliac processes, position of the acetabulum, extent of the ischiopubic plate and presence of supraneural fusion in adult individuals are taxonomically informative. Ontogenetic changes in pelvic morphology dictate that osteologically mature specimens are required to assess the development of many of these characteristics. We suggest that pelvic characters can readily be incorporated into pterosaur phylogenetic analyses and may assist in resolving the controversial interrelationships of this group. Distinctive pterosaur pelvic morphotypes suggest considerable differences in stance, locomotory kinematics and hindlimb functionality across the group.

Winglets

I did a quick post over at Aero Evo on winglets here.  The issue of winglet structures in pterosaurs came up and some followers here might find it interesting (see comments section).

Cheers!