What was that skull?

Well done and thanks to everyone who had a go at identifying the mystery skull, and congrats to TJ, Jorge Velez-Juarbe, Mark Lees and others: it was indeed a glyptodont, specifically a glyptodontine glyptodontid and, most specifically of all, Glyptodon clavipes. So much for posting the answer on Sunday night - I've been busy but, if you feel in need of a good, lengthy Naish article you can always nip over to SV-POW! and read the piece I recently produced on theories about sauropod pneumaticity. There's an awful lot that you could say about glyptodonts: their armour, tails, limbs, and limb girdles are all pretty amazing, and some neat studies have looked at glyptodont biomechanics, their possible bipedal behaviour, and their fighting style. They were also diverse and abundant, with (according to McKenna & Bell 1997) a ridiculous 65 genera grouped into five 'subfamilies'. In keeping with that photo, here all I want to do is say just a few things about the glyptodont skull...

One of the most obvious things about the glyptodont skull is that it's incredibly deep, and also very short anteroposteriorly. Glyptodonts lacked incisors and canines and their remaining, molariform teeth were highly distinctive: they're columnar, set in a closely-packed battery, and tri-lobed, and they grew throughout life. Their distinctive appearance earned the group its name: glyptodont means 'carved tooth' as Richard Owen, who named Glyptodon in 1839, evidently thought its teeth to be very special objects. As in all (or nearly all*) other xenarthrans, glyptodont teeth lacked enamel.

* It has been reported that the Eocene armadillo Utaetus possessed enamel, but this is doubtful (McDonald 2003).

These long-rooted, constantly-growing teeth were housed in incredibly deep maxillae and dentaries. One of the most distinctive features of the glyptodont skull is the massive ventral jugal flange, descending down below the eye and below the level of the teeth in the lower jaw. A similar structure is also present in sloths (one is also present in diprotodontid marsupials), but in glyptodonts it's flattened anteroposteriorly (from front-to-back) whereas in sloths it's compressed mediolaterally. The superficial masseter muscles, rather than being attaching to the side of the ventral flange, must have attached to its posterior surface in glyptodonts. Why? This is a specialisation allowing powerful anteroposterior jaw movement: glyptodonts didn't grind food up by transverse or vertical jaw motions, but by sliding the lower jaw backwards and forwards (a style of jaw movement called propaliny). This is demonstrated by the wear they have on their teeth. Elsewhere among placental mammals, propaliny and reliance on the masseter muscles is also seen in capybaras, and indeed their teeth are quite similar to those of glyptodonts.

The glyptodont skull roof is rarely figured, and in life was covered by a cephalic shield: a small cap made of osteoderms, protecting the frontals and parietals. We can see that, in Glyptodon, the occipital-parietal region bears numerous foramina, and so far as I can tell this is typical for all glyptodonts (Melton 1964, Gillette & Ray 1981). We know that, perhaps surprisingly, the cephalic shield was not totally predator-proof as a specimen of Glyptotherium texanum was bitten (and presumably killed) by a large cat which pierced both the cephalic shield and skull roof with its upper canines. However, this unlucky glyptodont was a juvenile, so Gillette & Ray (1981) assumed that its cephalic shield had not fully formed (though I'm not entirely sure what they mean by this: that it wasn't fully hardened?). Glyptodonts may not have relied on this cephalic armour alone to protect their heads, as it's also been suggested that they could partially retract their heads, turtle-style, beneath the carapace. Lacking the sort of hyper-mobile neck that turtles have, they couldn't retract it fully of course, but could they, at least, duck the neck and head under the overhanging edge of the carapace?

Finally, we come to the nasal region. Glyptodonts possessed reduced premaxillae and nasal bones, and in fact the premaxillae are so small that they are often missing from fossils. They overhung a wide, deep narial opening. Gillette & Ray (1981) proposed that glyptodonts had a short proboscis, apparently because they couldn't see how these short-necked herbivores could reach the ground with their mouths. I think that they might have underestimated the flexibility of the glyptodont neck, and I am also sceptical of the idea of trunked glyptodonts because trunks and their musculature are associated with strongly retracted nasal bones, and while the nasals of glyptodonts aren't that big, they aren't retracted. However, trunked mammals also possess scars and muscle attachment sites around the nasal cavity, and you'll note in Glyptodon the rugose laterodorsal patches that I mentioned previously. These look to be in the right place for the attachment of particularly well developed levator labii superioris muscles, and it so happens that these form part of the trunk musculature in at least some trunked mammals (Witmer et al. 1999). So what's going on here? I totally don't know: the answer might be in the literature, but it might also be that no-one has looked at this. You might have noticed that at least some glyptodonts possess shallow fossae on the skull roof. Are these anything to do with muscles for a trunk? I don't think so: I'm not sure what they are, but it's more likely that they mark an attachment area for the cephalic shield [life restoration below of the glyptodontine Glyptotherium - with trunk! - from Gillette & Ray (1981), by Bonnie Dalzell].

Finally on the nasal region, some sclerocalyptine glyptodonts had particularly large, strongly pneumatised fronto-nasal sinuses that gave their nasal regions a massively bulbous appearance (this is particularly prominent in species of the Pliocene-Holocene sclerocalyptine Sclerocalyptus). Zurita et al. (2005) argued that this was an adaptation for the cold, arid conditions experienced by these animals, particularly by the Middle and Late Pleistocene Argentinean forms. This idea of glyptodonts living alongside llamas and other cold-adapted mammals in cool, dry climates is a marked contrast to the idea promoted by some that they were animals of humid wetlands, but as is so often the case it seems that glyptodonts were more diverse, and more adaptable, than stereotype allows. Incidentally, the bulbous nasal regions of the Sclerocalyptus species really don't look at all consistent with the presence of a trunk.

So there you go. Can you now go around saying that you know everything about glyptodont skull? Can you hell, but hopefully you know a bit more than you did a few minutes ago.

Glyptodonts were previously mentioned on Tet Zoo here, and among other xenarthrans there's stuff on sloths here, armadillos here.

Refs - -

Gillette, D. D. & Ray, C. E. 1981. Glyptodonts of North America. Smithsonian Contributions to Paleobiology 40, 1-255.

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

McKenna, M. C. & Bell, S. K. 1997. Classification of Mammals: Above the Species Level. Columbia University Press, New York.

Melton, W. G. 1964. Glyptodon fredericensis (Meade) from the Seymour Formation of Knox County, Texas. Papers of the Michigan Academy of Science, Arts, and Letters 49, 129-146.

Witmer, L. M., Sampson, S. D. & Solounias, N. 1999. The proboscis of tapirs (Mammalia: Perissodactyla): a case study in novel narial anatomy. Journal of Zoology 249, 249-267.

Zurita, A. E., Scillato-Yané, G. J. & Carlini, A. A. 2005. Paleozoogeographic, biostratigraphic, and systematic aspects of the genus Sclerocalyptus Ameghino, 1891 (Xenarthra, Glyptodontidae) of Argentina. Journal of South American Earth Sciences 20, 121-129.