Having covered intraspecific variation, it seemed only appropriate to nail down another oft missed (or misunderstood) aspect of change that relates to taxonomy: ontogeny. This is the term we give to the changes that an individual undergoes as it grows and develops (in humans at least from a single cell to an adult, though much of that early stuff would be of little interest to a vertebrate palaeontologist while there are no bones involved). Obviously this has an incredibly important impact on taxonomy (and again, especially of fossil vertebrates) as juveniles can look quite different to adults and change markedly during their development.
I’ll deal separately with indictors of ontogeny in a future post and try to deal here specifically with what this might mean for taxonomy (for vertebrate palaeontologists at least). To take one obvious pattern as an example though, throughout vertebrates most newborn animals generally have proportionally large heads that get progressively smaller as the animal matures to become an adult. Human babies are an excellent example of this – they are born with heads close to the size of their torso and this of course is far from the case in adults.
If we then take this example over to theropod dinosaurs you might know that tyrannosaurs have proportionally large heads compared to their bodies (and in comparisons to allosaurs for example). If therefore you were a bad taxonomist just looking at a few gross characters of body size proportions you might note that your new theropod had a big head and consider it to be a tyrannosaur, when in fact it was just a young allosaur that had yet to grow out of its ‘baby’ shape. Worse you might think that it was a dwarf species given how small it was and even miss that it belonged to an already described and well known species because you were sticking to sized-based characters or those that changed markedly during ontogeny.
Again, this is something that has happened a lot in the past and causes lots of work for modern taxonomists. While this is certainly something that is less problematic that it once was, it is something that does still crop up. I should mention at this point that there is not necessarily anything wrong with naming a new taxon based on obviously juvenile material *provided* that the characters you are using to define it are not subjected to ontogenetic variation. As with intraspecific variation we can get quite a good idea of this from looking at living taxa, but even in the fossil record we have some good examples of growth series from juvenile to adult for some taxa which gives us a great idea of how some characteristics change (or remain the same) during ontogeny, so it’s not guesswork or intuition but based on observation and deduction. Taxonomy remains inevitably somewhat subjective, but much of that operates at defining exact species as opposed to what can and cannot be used to separate species, an important distinction.
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