I have somehow managed to all but avoid mentioning chevrons on the Musings before so it is about time I redressed the balance, though I don’t think even their strongest supporters (if indeed there are any) would claim that they are exceptionally exciting bones. These are the small bones that sit below the tail vertebrae in most archosaurs (they’re missing in birds and some pterosaurs) and reptiles in general. There’s a fair bit of variation in size, shape, and distribution and I’ll not be going into any great detail (as ever) but they are the kind of thing that often gets overlooked and so even covering the very basics will likely be of interest.

Chevrons are absent from the first few vertebrae since this is still part of the main body of the animal as can be seen by the position of the pelvic bones, and so they only arise where the tail starts to be free of the rest of the body (essentially, posterior to the cloaca). It’s been suggested that females might have one or two fewer chevrons than males since they would not want this to interfere with egg laying but there seems to be no correlation of this in extant reptiles and thus where differences occur it’s likely merely a result of intraspecific variation than sexual dimorphism.

Chevrons are paired one to one with the caudal vertebrae they hang beneath (though as noted above, they are usually absent from the first few caudals, and indeed often the last ones as well – this can be quite a number in the case of things like diplodocids). However, they can articulate with either a single vertebra, or at the junction between two vertebra. They typically have some form of Y or U shape when seen from the front or back since between them and the vertebrae runs the caudal artery and vein of the tail. Thus the chevrons provide some protection for these blood vessels rather than having them hang free under the tail (mammals have the ‘haemal arch’ which is effectively formed by the fusion of chevrons right onto the caudals).

The lower part of the chevron really shows the variation though with some having a slight anterior process, many having a long posteriorly facing one, other extend vertically, and on occasion you can get really unusual ones (like in dromaeosaurs tails). There can be quite a bit of variation along the row of even a single tail too, and obviously the chevrons get smaller towards the end and are typically absent for the very distal tail, and there is a general trend for the more distal ones to be less morphologically complex.

I’m pretty much out on chevrons, though I managed a few hundred words more than I expected. Coming next, oh, something on archosaurs I’d imagine.

3 Responses to “Chevrons”

  1. 1 Heinrich Mallison 29/11/2010 at 8:30 am

    Dave, high time you picked this topic up, it is usually not only neglected, but totally ignored🙂

    Chevrons are also a pretty good guide to the size of the caudofemoralis longus muscle, the main limb retractor. Long chevorns = big muscle, short chevorns =small muscle (caution: width of transverse process on caudals must also be taken into account).

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