Before my brief hiatus with a trip to Shandong (very worthwhile, despite the illness that stuck me down) I was talking about convergent characters and the importance of examining all the available evidence and not just a select feature or two. However, that post was written rather quickly and I didn’t have time to expand on it in the way I wished so now I’ll push on with a few more observations.
While last time I mentioned only specific characters like spines or claws that can arise convergently, it is course possible for entire suites of characters to evolve convergently. While a big and specialised ungual will help you break into a termite or ant nest, a well evolved termite eating animal will have modified arms to provide power to that claw, and have a long snout and / or tongue to reach those termites, perhaps specialised sticky saliva, digestion suited to large amounts of formic acid, lost teeth and more. Eating ant is more than just a claw, so animals that specialise for eating ant are likely to have whole sets of characters which all help to increase ant-eating abilities and thus evolve in the same way in multiple lineages – in other words convergence. After all, if you want to run fast, having a long lower leg is great, but having a long metatarsal too, and cutting down on the number of toes, and actually shortening the femur as well are even better.
This takes us nicely to the second point – that when those suites of characters combine in certain parts of the body this can cause taxonomic and systematic problems. Animals that are highly specialised for running often have the characteristics I note above, but what if you recover a fossil that is *just* a pair of legs? If you have an anteater arm then you will spot the big claw and modified ulna, but there are also some other features that should help reveal its true identity – pick up a maniraptoran theropod leg though and things get trickier. It is perhaps no surprise that there were early questions over the phylogenetic position of things like alvarezsaurs and oviraptorosaurs with their specialised cursorial legs – there is quite a lot of convergence there.
Finally, we come to a point about defining characteristics. Things can look very different from a general description or a very detailed one and apparent convergences can vanish in a puff of adjectives (like the flippers of penguins and turtles shown here). Both birds and pterosaurs have wings, but one has feathers and reduced fingers, the other a hugely elongate arm and membraneous wings. Bats can try and muscle birds out the way with their long wingers and ‘skin’-like wings, but again, closer examination shows that pterosaurs rely on a single finger to support their wing, not a whole hands worth. These are, of course, especially coarse examples, but I hope the point gets home.
That largely wraps up convergence, or at least as much as I wanted to say about it for now. Coming up soon, something else. For once I’m a bit bereft of pre-prepared posts so I’m not sure what I’ll get round to finishing up and posting. Stay tuned!
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