Returning to the theme of Archaeopteryx in its 150th anniversary it seemed a good opportunity to mention type specimens. I’ve generally steered clear of these in the past since I suspect most readers have a general idea of how things work and what types actually are and because I really didn’t want to sink deep into the depths of lectotypes and so on. However, there is a more common problem that Archaeopteryx can illustrate well so let’s crack on with that.
Type specimens in general are given special recognition in taxonomic work – these are if you like the ‘definitive’ specimens: the ones that stand out as being the recognised ‘identity’ of a species. As discussed in the ‘morphological species concept’ post, species can have several different definitions and there are various ways of defining things. In order to make sure everyone is talking about exactly the same thing, type specimens are erected to provide that literal physical basis of identity. Among types, the holotype is the most important. This single specimen is the one and all references to species identification should ultimately come back to the holotype.
For most of biology this is fine. You go out and find a new bird species say, collect some specimens, sort through them and when you describe it you name a holotype and maybe a few paratypes or whatever. You have the luxury of a whole set of specimens to pick through and can make sure your holotype really is typical and complete and contains every bit of information you think in should.
In palaeontology of course you generally don’t get a choice. Even if you are lucky enough to discover multiple specimens when finding something destined to be a new species, it’s unlikely that you’ll get a single and nice complete adult animal in good condition. Holotypes are almost inevitably incomplete, or crushed, or have key parts not clearly visible or who knows what and of course some are really based on very little material indeed – as little as a single bone. There’s nothing wrong with this really since if that’s all you have, that’s all you have. You can’t assume you’ll ever find another specimen of the same species (and that may not be any better than what you have already) so you have to go with what’s there and if it’s distinct and diagnosable then it should be named.
However, things can be distinct and diagnosable at the time and later loose that title as new discoveries show that what had appeared to be unique turn out to be common. On occasion though things can be pretty much undiagnostic to begin with. Enter Archaeopteryx stage left.
When it was first named, Archaeopteryx was just a feather (photo here on Pick and Scalpel). For all the famous specimens that attract all the attention, the original name was erected for a single bit of integument. Now arguably this was diagnostic in that there were no such things as Mesozoic birds at the time. I’m inclined to disagree with this since a separation of time and space is helpful to help separate out species, but hardly concrete evidence of genuine difference. In any case the rapid discovery of ‘real’ specimens of Archaeopteryx, and other birds, means that the feather is undiagnostic. There’s not really anything there that can be genuinely shown to be different to any other fossil feathers, or indeed those of many living birds. As a side problem, not only is the feather not diagnostic, but it’s also the holotype specimen.
That actually means that in theory at least, we don’t know what Archaeopteryx is. Or to be more specific, we don’t have an official holotype that is diagnostic. In practice of course we have important and complete specimens like those of London and Berlin that everyone accepts are members of this species and are distinct and diagnostic, so while this does need to be sorted out (and indeed the wheels are very much in motion on this) it’s not a huge problem. But it does provide an obvious illustration of these problems. I doubt there’s anyone reading this who doesn’t have a good mental image of what Archy really is, but I doubt that is just a single small feather, though technically, it should be.
My thanks to Paul Barrett for some info and fact checking on the status of the feather as holotype, Paul is part of the petition to get the London specimen designated as the new holotype.