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.


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And, strangest of all - this big, unusual animal with huge range is practically unstudied in the wild and not kept in captivity!

The hand skeleton looks like a Swiss army knife with four different blades... But can it do anything specific with digit I?

I used to wonder about 'claw-supported walking'in derived therizinosaurs, although I suppose that must have been before they were recognised as theropods.

Maybe we should call it "nychogrady"?

By Mark Evans (not verified) on 24 Sep 2008 #permalink

It's called in Brazil "tatu-canastra", meaning something like "basket or trunk-armadillo". "Canastra" is a big kind of basket or trunk. Tatuasu (Tatuaçu or Tatuassu) was the Amerindian name, meaning "big armadillo".

By J.S. Lopes (not verified) on 24 Sep 2008 #permalink

> us euarchontogliran laurasiatheres

Euarchontoglirans are laurasiatheres :-0? I thought Euarchontoglires and Laurasitheria were sister groups?

Euarchontoglirans are laurasiatheres :-0? I thought Euarchontoglires and Laurasitheria were sister groups?

Oops, my bad, now corrected. Thanks :)

CMIIW, but I think it would have to be "onychogrady". Or, better yet, "unguigrady" or "ungulagrady".

Unguligrady... which already exists.

the scaphoid is small while the lunate and unciform are particularly big.

Wait a second. I never remember the mammalian nomenclature for carpals and tarsals. What are the normal equivalents to those names?

By David Marjanović (not verified) on 24 Sep 2008 #permalink

In reference to a comment on the 2007 "five things" article, I would pay cash money to see Matt body-tackle an armadillo, anytime, anywhere. A warthog would be acceptable as a substitute (for the armadillo), as would a wolverine or wombat.

By Nathan Myers (not verified) on 24 Sep 2008 #permalink

That's ridiculously awesome. Actually makes me think that monkey-lizards were doing similar things--look at Megalancosaurus in particular. It has an enormously oversized claw on its second finger. Hmmm...if somebody hasn't already written a paper detailing this convergence, perhaps now would be the time to STRIKE! :-)

Nick Longrich has a new paper out in Cretaceous Research in which he convincingly argues that alvarezsaurids were bug-eaters who cracked wood open with their stumpy little arms and licked up the swarming insects. Turns out alvarezsaurs have many adaptations which suggest such a lifestyle apart from their strong little stump arms.

Expect a post about that paper soon, actually, on my blog. I have to restore an Albertonychus cracking open some fallen tree bark, first.

I was going to suggest unguigrade, distinct from unguligrade which refers to hooves. Onychograde would be an abominable Greek-Latin hybrid.
The best-developed ungual sheath I know of is on the enlarged manual ungual I ('hooded claw') of Thylacoleo carnifex. But in cats, isn't the sheath actually formed by the penultimate phalanx, into which the ungual retracts? I may have this wrong, things with limbs are not exactly my area.
Speaking of Thylacoleo, the most-reproduced reconstruction of this beast is a picture by Peter Schouten showing it perched on a rather gnarly log, first published (I think) in Prehistoric Animals of Australia (1983). In around 1987, at a well-lubricated social event for VPers in Sydney, Peter told me something about that 'log' that I really wish I didn't know. I've never been comfortable looking at the picture since.

By John Scanlon FCD (not verified) on 24 Sep 2008 #permalink

Just been reading the Longrich & Currie paper on Albertonykus (note spelling, Zach), with evidence of termite-chewed wood from the same formation. The size and meandering shape of the borings are compered in the paper to those of a dampwood termite (Termopsidae; fig. 14), but I'd say they're also very similar to those of Mastotermes darwiniensis (Mastotermitidae), the largest and one of the phylogenetically most basal termite species, which happens to be eating its way through most of the trees in my back yard. (In the paper they mention this as 'Darwin's termite', but etymologically it should be the 'Darwin termite' although quite widely distributed across northern Australia.) They inhabit both standing and fallen wood, and I've noted galleries in half-buried railway sleepers (eucalypt of some kind), and live and dead branches and trunks of peppercorn, mandarin, and mulberry, but not mango trees.

By John Scanlon FCD (not verified) on 24 Sep 2008 #permalink

"Onychograde" would be abominable, but would it be abominable enough?

Were the Romans squeamish about adopting Greek loan-words? If they weren't, why should we be? Aren't locomotoronomists all one happy Proto-indo-european family?

By Nathan Myers (not verified) on 24 Sep 2008 #permalink

Right, "-nykus" because of Mononykus. Thanks, John. But yeah, I loved that article. Not so much for the animal described (we've got an arm and a tibia!) but for the investigation into alvarezsaur lifestyles.

Random morphological questions: Where are its eyes? And what if any is the function of the 'bumps' on the forehead? Is the scapula as large and modified as the picture makes it seem? And is that John McCain in the background?