One aspect of vertebrate palaeontology that often gets overlooked is how much individuals can vary from the ‘type’. Most people focus on interspecific variation – that is, difference between species, but just as important is intraspecific variation – the differences seen within species. In a great many cases (and most especially when it comes to dinosaurs) the fossil record of an animal can be limited to one good specimen with various other bits of others that may be badly distorted, incomplete, broken, or with elements that do not overlap with others. As such you can end up with a rather fixed view of what a taxon looks like if you only take one specimen into consideration (or can only use one).
However, biology deals with a continuum, not absolutes – species are not the same as elements or atoms in their definition or reliability of diagnosis. Individuals vary – you look like your parents and siblings, but not *exactly* like them (even identical twins usually have some distinguishing features). If you look at humans across the earth (admittedly a big sample size and with more variation than many species) you can find individuals of all manner of sizes and body proportions, with no clavicles, extra or fewer fingers and toes, all manner of skin, eye and hair colours (or no hair at all) and more. Some of these are the result of odd occasional defects (like the extra fingers) but can still be hereditary and in large numbers (like missing clavicles) and in both of these cases at least, would of course show up in the fossil record. This is therefore a pretty important consideration when trying to define species and separate out differences between closely related groups.
Obviously we typically lack the sample sizes needed to try and establish this kind of variation in dinosaurs (for example) but we *can* look at patterns we see in extant creatures to see what we might expect in fossils. Does size vary a lot among living populations? Oh yeah! Right, don’t include absolute size as a criterion for defining taxa unless the differences are consistent and very big. Do the number of digits (unless obviously duplicated by a generic error) usually vary? No – so this is a viable indicator of a taxonomic difference. Does the number of teeth vary? In mammals, generally not, but in reptiles, yes (at least a little). So best not use the gain or loss of a single tooth as a factor, but the loss or gain of several is better evidence. And so on, pretty much ad infinitum, though of course just a couple of concepts (relative sizes for example) can cover an awful lot of ground and most of these are pretty obvious.
Sadly, despite the general simplicity of the concept and its application this is not always so vigorously applied. There are plenty of species (and dinosaurs especially) out there that are characterised by being a bit bigger than an otherwise identical taxon, or because they have one more tooth, or one fewer tail vertebrae. A great deal of taxonomic research is, frustratingly and time consuming-ly, a clean-up of old (and even new) bad taxonomic identifications, assignments and descriptions (aside from the normal disagreements). Obviously the field has moved on over the decades (and yes, centuries) but it is amazing how much poor stuff still comes out regularly and how much stuff has never been revised. As I have noted elsewhere, taxonomy is absolutely crucial for basic biology and we are running out of taxonomists and behind on taxonomic progress. I don’t want to be too pessimistic, but the situation is far from bright and the more good taxonomy and less bad that is produced the better off near every field in biology will be.
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