There are a great deal of technical words in science that people often dismiss as jargon, but as I have said (perhaps even more than once or twice) science writing is about brevity and clarity and technical terms are useful when properly defined. In stead of writing ‘that odd situation where the middle metatarsal of a foot is compressed proximally’ you can talk about arctometatarsals for example.
And with that horribly contrived introduction and definition under our belts we can move on. Arctometatarsals (sometimes referred to as ‘the arctometatarsalian condition’) are indeed as I described an unusual feature of some theropods whereby the central metatarsal of the three that the animal stands on (and thus number III) is constricted and covered by the flanking bones such that they splay out at the base. (For those who have missed out the metatarsals are the bones on the foot between the ankle and the toes – in humans at least the majority of the foot, though since theropods walk on their toes only, the metatarsal effectively add to the length of the leg).
You can see a nice example of an arctometatarsal here on this not great photo of Tarbosaurus and I’ve done my best to badly ink in the outlines of the other two bones in red. It should be clear that at the bottom of the middle metatarsal appears the same as the others but further up is appears to shrink and disappear behind the others. In fact is disappears *between* the others – it’s not behind them, but stuck between them. In some cases it can flare out a little at the other end, but usually it is reduced to a very fine splint of bone at the upper end.
This condition has actually evolved a number of times and is present in tyrannosaurs, the wonderfully weird alvarezsaurs (of which much more soon I hope), ornithomimosaurs, troodontids and even a couple of oviraptorosaurs. Among the more derived theropods it is then quite common and worth looking out for. Its exact function is not really known, but has been thought to correlate with running (and with the exception of the giant tyrannosaurs these animals are all good runners).
So there you have it – the arctometatarsal. It’s amazing just how much information you can cram into one word really – long live technical terms.
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