Reversing the hallux

The reversed hallux has long been an important aspect of the discussion of dinosaur habits, and more specifically the origin of flight though it might not seem so at first glance. The hallux is basically the technical term for what most people would call the big toe, that is, the first digit on the foot and its reversal (or otherwise) basically relates to the orientation it holds – does it point forwards with the other toes, or sideways, or has it reversed to point behind?

The reason this point is so often contested or debated lies in two simple truths. Firstly, birds with a reversed hallux are mostly perching birds – those that live in trees and perch on branches and by extension are generally decent fliers. Secondly, the orientation of the hallux is incredibly difficult to determine in flat 2-D fossils such as those we generally get for things like early birds and close relatives in the non-avian dinosaurs.

The result as you might have guessed is that palaeontologists are incredibly interested in reversed halluces since this may allow us to determine if early birds and (perhaps more importantly) their likely ancestors were hanging around (so to speak) in trees. However between the tendency for the hallux to come free from the rest of the toes and the fact that compression can lead to it lying in a misleading orientation on a slab means that reconstructing the original position accurately is difficult at best.


It will be no surprise therefore to learn that this has been controversial at best and for things like Archaeopteryx with multiple good, and conflicting, specimens know the debate has been long and remains (largely) unresolved. The picture here is of a birds from Liaoning that I saw recently and clearly shows the hallux facing behind on each foot with no obvious signs of dislocation. Convincing is this case, but unhelpfully this is rarely in the more interesting taxa.

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10 Responses to “Reversing the hallux”

  1. 1 Jaime A. Headden 20/08/2009 at 6:00 pm

    A major solution to this issue was handled by Middleton in both his thesis and in subsequent publications, where he showed that twisting of the hallucial metatarsal produces a reversed hallux position in birds, and how some fossil birds, but not Archaeopteryx, lack this twist. If the posture of the digit does not reflect positional veracity, then metatarsal flexion (proscribed by a ridge running the length of the bone in a semi-spiral) has a lot more bearing on it, and is irrelevant to metatarsal 1 position relative to the metatarsus (posterior instead of medial, distal instead of proximal, etc.) and shape (straight, J-shaped, etc.).

  2. 2 Michael 21/08/2009 at 1:35 am

    I rmember reading in Predatory Dinosaurs of the World about how most articulated non-avian theropod specimens show unreversed halluxes (correct plural for “hallux”?), while many footprints show a partly-to-fully-reversed hallux. GSP’s explanation for this was that perhaps metatarsal 1 was not fixed firmly to metatarsal 2, but was instead mobile(!)with some sort of tendon that could pull it back, allowing the partial or full reversal of the hallux we see in said footprints – as well as allowing juveniles to climb trees.

    What do you think, Dave?

    • 3 David Hone 21/08/2009 at 9:17 am

      I think that’s wrong to be quite honest. I don’t want to steal Corwin’s thunder as he is working on hallux reversal in theropods and birds (which is also why the above post was so brief and un-detailed even compared to my normal levels of simplicity), but at the very least in many taxa there is basically an articular facet on metatarsal II which shows where the hallux would be placed. In some (though by no means all) cases it is certainly fused on here, but even if it were not, it would seem very odd that there would be point of articulation on one bone for it to fit, but it could be pulled anything up to 90 degrees out of position by some unknown tendon for no apparent reason.

      • 4 Michael 21/08/2009 at 10:11 am

        Ah, that all makes sense. Thanks! I wonder what’s really going on with the footprints that show a reversed hallux, then.

      • 5 David Hone 21/08/2009 at 12:57 pm

        Sorry I didn’t make that clear – this articualr facet *is* on the rear, and thus this is the likely position of the hallux (in the animals in which I have a) looked for it and b) found it). So the footprints would match the anatomical data.

  3. 6 Nathan Myers 21/08/2009 at 3:51 am

    Jaime: I couldn’t follow after “If the posture…”. Can you explain further?

  4. 7 Mickey Mortimer 21/08/2009 at 5:23 am

    You say the specimen (which is Sapeornis, btw) “clearly shows the hallux facing behind on each foot with no obvious signs of dislocation”, but the tibiotarsus is exposed anterolaterally, the tarsometatarsus is laying with its anterior surface upward, and the main digits are all exposed laterally. Technically, if the first digit was assumed to not be dislocated from the tarsometatarsus in this specimen, it would be facing laterally and attached to metatarsal IV(!), not reversed. But metatarsal I only preserves the very distal tip, so we can’t say how it articulated with the tarsometatarsus.

    The other (upper right in the photo) tarsometatarsus is exposed slightly anteromedially and this time the main digits are exposed medially. In this case as well, digit I is definitely disarticulated because you can see metatarsal I laying on the mediodistal surface of metatarsal II, while the phalanges are on the other side of the tarsometatarsus contacting the distal end of metatarsal IV. I can’t see the details of metatarsal I well enough to determine if it’s twisted, however.

    As Jaime noted, Middleton showed the only way bird halluces become reversed is by metatarsal I twisting, so that’s the way to determine if a fossil taxon had a reversed hallux in life.

  5. 8 Mickey Mortimer 21/08/2009 at 5:26 am

    Oops! I meant the second tarsometatarsus is exposed POSTEROmedially. Sigh.

  1. 1 We shall call it ‘Mini-hallux’ « Dave Hone's Archosaur Musings Trackback on 02/05/2011 at 9:04 am
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