I, Priodontes, the tatuasu

As always, at least a few people got yesterday's picture correctly identified: it was indeed a Giant armadillo or Tatuasu Priodontes maximus, and specifically the animal's right hand and lower arm. I photographed it at the National Museum of Ireland (Natural History) during SVPCA 2008. A stuffed specimen was on display next to the skeleton, so the scaly leg you could see in the background was indeed a pretty big clue. Armadillo skeletons - like those of all xenarthrans - are so weird and unfamiliar to us euarchontoglirans that it would be easy to write thousands of words on their weirdness. In an effort to keep things short, here I'm only going to talk about those amazing hands...

Oh, if you feel the need for a primer on the diversity and natural history of armadillos, go read Five things you didn't know about armadillos.

The hand of Priodontes is so modified that, in the digits, it's kind of difficult to work out where some of the metacarpals and phalanges are as, excepting in digits I and II, all have become proximodistally compressed, block-like elements similar to the wrist bones (carpals). Xenarthrans in general have proximodistally compressed first phalanges: in armadillos, while this is true of digits III-V, the proximal phalanges of digits I and II are elongate (McDonald 2003). Digit V is absent in Priodontes (manual digit number varies from three to five in armadillos). What wasn't so obvious from the cropped image I posted is that the super-robust digit with the enormous claw is digit III, and that an elongate digit I and digit II are both positioned medial to it. You can hopefully appreciate this from the larger image shown here.

Of course, what really makes the hand stand out is the immense size of the claw on digit III. It's over 200 mm long along its curve and is a pick-like tool that the animal uses to dig with (all armadillos are what is known as scratch-diggers). The relatively slender digits I and II almost look out of place next to this monster (this is also true of anteaters, where digit I is both super-gracile and short, or even reduced just down to its metacarpal). I was interested in finding out how much power there is in the Priodontes forelimb, and of what it's capable of but, surprise surprise, virtually no work of any sort has ever been done on armadillo functional anatomy, ever (will someone tell me just what the hell neontologists do with their time?). Having said that, Vizcaíno et al. (1999) and Vizcaíno & Milne (2002) looked at function in armadillo limbs and used an Index of Fossorial Ability (IFA, calculated by dividing olecranon length by the difference between ulnar length and olecranon length) to compare the fossorial ability of the different armadillos. Priodontes had the highest IFA among armadillos after Chlamyphorus, the mole-like pichiciegos or fairy armadillos. It digs for ants and termites, and also constructs burrows despite its size (maximum total length 1.5 m, maximum weight 30 kg [the 60 kg sometimes given was probably based on an overweight zoo specimen]).

What I find really bizarre is that the digit III claw has also become exapted as a support structure used in walking. In other words, in the hindlimb these animals walk on the soles of their feet, but in the forelimb they support their weight just on the tips of their claws: they do not (and cannot) place the palm on the ground, nor do they fold the claws under the hand to walk on their knuckles. This is pretty amazing but very little remarked on, and it isn't unique to Priodontes: the other tolypeutine armadillos* and the fairy armadillos do it too. Pangolins have convergently adopted the same solution. I think this should be called 'claw-supported walking'. Mammals can only adopt claw-supported walking when most of their weight is carried on their hindlimbs, and armadillos and pangolins are built like this because they are facultative bipeds, supporting their weight with the hindlimbs and tail when digging with the forelimbs. Pangolins have in fact even developed proper bipedality, and can look like low-slung, heavy-tailed theropods [adjacent image of claw-supported walking Priodontes from birdtours.co.uk].

* The clade Priodontini consists of Priodontes and the naked-tailed armadillos (Cabassous) and, together with the three-banded armadillos (Tolypeutes), these armadillos form Tolypeutinae (Delsuc et al. 2003).

Note also that the base of the ungual on digit III is surrounded by a sort of collar-like sheath. This is called the ungual crest and helps anchor the keratinous covering of the claw onto the ungual: it's present in some sloths and anteaters too, and outside of xenarthrans is seen in cats. The wrist bones (or carpals) of armadillos are weird, because the scaphoid is small while the lunate and unciform are particularly big. This is difficult to discuss without labelled diagrams though, so I'll stop there.

On that note, I need to get back to work.

For previous Tet Zoo articles on xenarthrans see Ten things you didn't know about sloths and What was that skull? (on glyptodonts). Will do anteaters some time.

Refs - -

Delsuc, F., Stanhope, M. J. & Douzery, E. J. P. 2003. Molecular systematics of armadillos (Xenarthra, Dasypodidae): contribution of maximum likelihood and Bayesian analyses of mitochondrial and nuclear genes. Molecular Phylogenetics and Evolution 28, 261-275.

McDonald, H. G. 2003. Xenarthran skeletal anatomy: primitive or derived? Senckenbergiana biologica 83, 5-17.

Vizcaíno, S. F., Fariña, R. A. & Mazzetta, G. 1999. Ulnar dimensions and fossoriality in armadillos and other South American mammals. Acta Theriologica 44, 309-320.

- . & Milne, N. 2002. Structure and function in armadillo limbs (Mammalia: Xenarthra: Dasypodidae). Journal of Zoology 257, 117-127.