Now that I have managed to introduce taxonomy to you all, I can now go and play with some specific areas or examples to expand on the subject. While pterosaurs were always going to feature sooner or later, they genuinely do provide a good example of the difficulties inherent in taxonomy in palaeontology, so hopefully I can make good use of them and cover some real pterosaur issues and ones of taxonomy too.
(If you have not read it before I’d also suggest taking a look at the post on pterosaur origins as various bits of that also fit in here. You can read it first or last and inevitably some bits are a bit repetitive).
Pterosaur taxonomy is basically quite tricky and this stems from a number of issues: partly though them just being pterosaurs and partly because they are known only from fossils, and then from certain kinds of fossils, that make life harder for the average taxonomist. First off, pterosaurs are incredibly highly derived organisms (that is they have a lot of unusual evolutionary history making them very different to their relatives) and very specifically thanks to their evolution of flight they have essentially managed to hide an awful lot of characteristics that would normally be present and allow you to tell them apart from one another. The bones of the skull have all fused up so that you can’t tell where one bone ends and another begins or what shape and size they were, the skeleton as a whole is very conservative (one looks very much like another), and most of them have either reduced or got rid of their teeth (an absolute wealth of characters normally, especially if you compare them to mammals say). This means that characters you would normally be able to use quite happily for dinosaurs (shape and size of teeth, size of skull bones, number of fingers, support struts on the vertebrae etc.) are either absent or just the same.
Next up we have the problems of preservation. Not only are many specimens incomplete (more on this below) but some pterosaurs are preserved flat in 2D on the rock which means that quite simply you can’t see much (the back of every bone for starters) and what you can see might be distorted because it has been crushed. Now some stuff we have in superb 3D preservation, but then it’s very hard to compare a 3D fossil to a 2D one, so even if the information is there for one, since you can’t compare it to the others, it is only of limited value.
Next up is incompleteness, and specifically with very early discoveries of pterosaurs. A great many of the early finds and descriptions were of partial (*very* partial) skeletons, often only half a bone, or commonly the front of a jaw. These were (at the time) correctly identified as being different to everything else and then given a name. Of course once you start finding more complete material it gets more complex. That half a humerus was clearly new when it was found, but now we have four or five animals with an identical humerus, but with the rest of them different. Which one goes with the original piece and thus the original name, and which need new names? And what do we do with the one that has the same humerus as one specimen, and the same jaw as another? Are any synonymous (two things of the same species each given a different name), and if so which? While there are rules in the ICZN and other for dealing with at least some of these problems, it can be incredibly hard to track down all the differences (and similarities) especially when spread out over 150 years of research and few thousand miles of collections (the ornithocheiroids are especially bad in that early descriptions were based on teeth and snouts, but there are good collections in the UK, US, Brazil, China, Germany and north Africa).
Finally there is also some ontogenetic (i.e. growth – most juvenile animals can look rather different to adults) variation going on that until recently was not even really considered. There are now widely recognised to be large numbers of pterosaurs species that were based on descriptions of immature individuals. As a result there is a huge amount of synonymy in some groups that needs revising and of course it is hard to work out exactly which individuals might belong to which species, and if some are indeed different enough to warrant a separate species name.
When you combine these factors together you can see why pterosaur taxonomy is such a tricky subject compared to say, something like the dinosaurs where we have few juveniles to get mixed in with the rest and far more characters for individual 3-D preserved bones. There are of course always issues in palaeontology over skeletons that are only partially or badly preserved and with any well studied group there are issues to be resolved with old descriptions and synonymy, though these are far more acute in some clades than others. As ever there is simply more work to be done and new discoveries await, it is rare that there are not exciting times in taxonomy, even if the actual process is often less than thrilling.