What is an arctometatarsal?

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).

Arcto496You 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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19 Responses to “What is an arctometatarsal?”


  1. 1 Andreas Johansson 23/10/2009 at 6:46 pm

    Aren’t juvenile tyrannosaurs supposed to be more cursorial than adults? That way they might still fit the pattern.

  2. 2 Thomas R. Holtz, Jr. 23/10/2009 at 8:42 pm

    All theropods (outside of birds) show a marked negative allometry of the distal hindlimb elements: juveniles have longer tibiae and especially longer metatarsi than adults. So if we regard relative metatarsal length as a cursorial-correlate, young tyrants are much more cursorial than adults. Indeed, they have essentially the same limb proportions as ornithomimids.

    Additionally, Raptorex shows the most basal presence of an arctometatarsus in Tyrannosauroidea, and does so at a much smaller body size than Tyrannosauridae. If it was only 3 m or so long as an adult, than it may well be that the tyrant pinched foot evolved in a context where even adults were fully cursorial, and later giant tyrants inherited that trait.

    Trivia time: when searching for a name for this structure, I considered using “Fascis” (the bundled sticks that were the symbol of Rome) as the root for the prefix, but the thought of fascist footed dinosaurs didn’t seem right…

  3. 5 Michael O. Erickson 24/10/2009 at 12:08 am

    “and with the exception of the giant tyrannosaurs these animals are all good runners”

    ????

    The giant tyrannosaurs were good runners too!

    • 6 David Hone 24/10/2009 at 9:27 am

      I’m sorry but I can’t countenance the idea that 7 and 8 ton (probably) graviprotal tyrannosaurs were good runners when compared to ornithomimids and alvarezsaurs. Even compared to typical allosaurs and basal tetanurans its hard to think of them reaching high speeds (and according to a fair bit of research they couldn’t). Now yes there is more to the definition of running than speed, but the work suggests that they were not getting into a suspended aerial phase (i.e. both feet off the ground at the same time) and thus were hardly even running at all so much as walking quite fast.

  4. 7 Mickey Mortimer 24/10/2009 at 4:30 am

    Semantic question- Is it really proper to speak of an arctometatarsal, with that word being a noun? I’ve seen it described as an arctometatarsus, and arctometatarsal and arctometatarsalian being used as adjectives. But to say “here’s an arctometatarsal” would seem to imply only a single metatarsal in the foot was being referenced, perhaps metatarsal III? I guess Tom would be the arbiter here.

  5. 8 Thomas R. Holtz, Jr. 24/10/2009 at 5:00 am

    Actually, in order, I used “arctometatarsalian” as the general condition, and “arctometatarsus” as the noun. I have seen “arctometatarsality” used. I suppose one could use “arctometatarsal” to describe the mtIII, though.

    • 9 David Hone 24/10/2009 at 9:29 am

      Thanks Tom! I guess it’s a question of how the word can be used, some in English serve as both noun and verb or adjective you can bat something with a bat or batter it I suppose. As long as it’s clear what is meant I don’t think it’s likely to be a big issue especially when it’s a newly coined word that people are still exploring the exact meaning of. However, I’m happy to accept that Tom thinks it is OK!

  6. 10 David 24/10/2009 at 6:45 am

    Has “arcto” got anything to do with “arctos” = bear?

  7. 11 Thomas R. Holtz, Jr. 24/10/2009 at 9:13 pm

    The arctus is a variant form of artus, meaning “compressed” or “channeled”. The Greek “arktos” is a different root (from which we get “arctic”, actually: bear country).

    After publishing the name, I did find that Haeckel had used “Arctopoda” for what would become “Prosauropoda” in one of his numerous iterations of taxonomies. (He also used “Dracones” for a monophyletic group of Pterosauria, Dinosauria & Aves).

  8. 15 Thomas R. Holtz, Jr. 26/10/2009 at 8:47 pm

    Andrea,

    In a sense, yes. But the principle of priority doesn’t govern names above the “family group” rank, so it is merely an earlier name for the same group, not a formal senior synonym.


  1. 1 Guest Post: Love the Tyrant, Not the Hype « Dave Hone's Archosaur Musings Trackback on 19/10/2011 at 8:11 am
  2. 2 An articulated alvarezsaur pes « Dave Hone's Archosaur Musings Trackback on 31/01/2012 at 8:11 am
  3. 3 Arctometatarsal origins « Dave Hone's Archosaur Musings Trackback on 08/02/2012 at 2:01 pm

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