Tetrapod Zoology

What was that skull?

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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).

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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?

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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].

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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.

Comments

  1. #1 Thylacine
    May 19, 2008

    So is that a sub fossil then?

  2. #2 Mo Hassan
    May 19, 2008

    I didn’t recognise it without its hat on!

  3. #3 Nick P.
    May 20, 2008

    Do the skull bones and muscle attachment scars look consistent with the presence of a particularly powerful and mobile upper lip?

    Then again, I suppose that’s not too far from a trunk…

  4. #4 TJ
    May 20, 2008

    Do the osteoderms grow from separate centers and fuse to form the cephalic shield? Might that account for “not fully formed”?

  5. #5 Allen Hazen
    May 20, 2008

    There’s a glyptodont skull on display at the AMNH in New York a hole (maybe a pair of holes- I don’t remember) in the roof the label says was made by the canine tooth of a jaguar-size cat. It MAY be a juvenile, but it certainly wasn’t a SMALL juvenile: as far as I can remember, the skull was at least the size of a human skull.

  6. #6 Neil
    May 20, 2008

    Ah glyptodonts. I still remember plying with my NHM toy one and using the tail to help it drill for roots. My knowledge of such things has increased since then – if only slightly

    interesting article :)

  7. #7 David Marjanovi?
    May 20, 2008

    a ridiculous 65 genera grouped into five ‘subfamilies’.

    Is there a chance that there’s some individual and sexual variation hidden here, as in the formerly 16 species of Triceratops?

  8. #8 yesapel
    May 20, 2008

    Me gustaría si alguien puede certificar que esta piedra puede ser un calculo renal. Vamos una piedra creada por un riñón prehistórico. Gracias. la foto se puede ver en mi página. Yesapel.com y en foto flickr.com galeria yesapel.

  9. #9 Barn Owl
    May 20, 2008

    If I could see living, breathing glyptodonts roaming the Texas Hill Country, I could die a happy woman. Even if I died from a fatal head injury when my horse spooked at the sight of giant armadillo-like creatures.

    One evening, as I was driving near my friends’ ranch, a Nine-banded Armadillo ran across the gravel road in front of my truck. I hit the brakes and slid to a stop, and noticed that the armadillo was now watching from the side of the road. Since my window was down, I said “Watch where you’re going, Armadillo-next time I might not be able to stop.” At which point the armadillo leapt straight up in the air (as they do when startled), and rushed off into the brush.

  10. #10 Cameron
    May 20, 2008

    What’s with very large Xenarthrans and facultative bipedalism? It sucks that Farina 1995 doesn’t seem to be online anywhere…

  11. #11 Zach Miller
    May 20, 2008

    Okay, now I’m really confused. That skull looks maybe a decade old, not hundreds of thousands of decades old. Where was it found? Also, glyptodonts rule.

  12. #12 Zach Miller
    May 20, 2008

    Crap. Sorry, double-post. It’s really difficult to visualize muscular attachment on a glyptodont skull (or ground sloth) without visuals. Are there any papers or illustrations detailing craial musculature of giant terrestrial xenarthrans?

  13. #13 Nathan Myers
    May 20, 2008

    Zach: I brought that back on my last “trip”. Sorry I couldn’t bring the live animal, the head was all that would fit in… Sorry, I’ve said too much already.

  14. #14 Nathan Myers
    May 20, 2008

    Zach: Seriously, it might be as little as one hundred decades old. One of your own ancestors might have eaten the creature it came from.

  15. #15 Nathan Myers
    May 20, 2008

    Oops, sorry, that’s one thousand decades.

  16. #16 Hai~Ren
    May 20, 2008

    Funny how a group as diverse as the glyptodonts are always represented in the media by only a tiny handful of species: Glyptodon, Doedicurus, Glyptotherium, and for a period of time, the horribly-named Parapropalaehoplophorus. There’s a painting of Panochthus by Todd Marshall, but that’s the only thing I know of this species. And now there’s Sclerocalyptus, that survived into the Holocene???

    Just how many species of glyptodont were there running around in the Late Pleistocene?

  17. #17 Darren Naish
    May 21, 2008

    Yes, there are loads of glyptodonts – but this isn’t because they’re over-split (like the multiple alleged Triceratops species), it’s because there really were lots of them. Though, having said that, a lot of splitting did occur early on because many taxa were named just for armour fragments. And it’s Propalaeohoplophorus, not Parapropalaeohoplophorus. As Simpson explained, Palaeohoplophorus Ameghino, 1883 was named as a possible ancestor of Hoplophorus Lund, 1838, and an older relative of Palaeohoplophorus was then named Propalaeohoplophorus Ameghino, 1887. That last taxon was used by Ameghino as the type for a new subfamily, of course named Propalaeohoplophorinae!

    Propalaeohoplophorus and kin (Metopotoxus, Asterostemma etc.) are relatively small Miocene glyptodonts with flexible shells, and it now seems that neither Hoplophorus nor Palaeohoplophorus are particularly close to it. We still seem to be lacking a modern comprehensive look at glyptodont diversity and phylogeny.

  18. #18 Graham King
    May 22, 2008

    Darren, I’m delighted to find “Giant Armadillos” featured here repeatedly! They are another of my favourite animals
    (er, I go by the name “armadillozenith” in various sites online). There’s a sectioned one in the Royal Scottish Museum in Edinburgh… I mean a lateral half of the carapace is cut away, revealing that side of the skeleton.

    I’m interested in their peculiar front of mouth. Do you think they had horny pads maybe on upper and lower jaws… mobile (divided) lips? Prehensile tongue?
    Those eye sockets seem to look more backward than forward. Do you think they browsed off-ground (?Bipedal!!?!!), or did they tilt head right down to browse substrate (like the amazing Nigersaurus)?

    Perhaps worryingly, Glyptodon is a creature I have been considering for another wearable costume.
    (My list of questions to ask you about my prime aim, the Stegosaurus costume project, is growing. The DinoHorizons (TM) steg skeleton I bought in South Kensington before our Crystal Palace dino-trek raises many questions… orientation of plates, ribcage/scapula… )

  19. #19 David Marjanovi?
    May 22, 2008

    The eye sockets look precisely sideways. They are the little rounded corner in the temporal fenestrae. :-)

  20. #20 Brian
    May 22, 2008

    Is it known what newborn glyptodonts looked like? Should I imagine them looking like miniatures of their parents or would parts of the carapace not have present until a later age?(as in ankylosaurs) I personally suspect the first.

  21. #21 kate
    May 23, 2008

    this is absolutely fascinating. in fact, as someone unfamiliar with paleozoology, one of the more interesting blog posts i’ve read in a long long time.

  22. #22 Allen Hazen
    June 6, 2008

    I had a chance to visit the American Museum of Natural History today. The glyptodont skull with the (probably fatal) punture wounds from predator canine teeth (two holes, and I suppose the spacing would give evidence of the size of the predator: label just said it was probably a large cat, so “jaguar size” was just creative memory on my part) is Glyptotherium texanum from the Pliocene of Arizona (2.5 mya). Only the skull was displayed; comparing it by eyeball with the two more complete glyptodon skeletons (one much larger, one much smaller) in the same case, I’d say the animal it came from might have had a head-and-body length of a metre, more or less, approximately, as a guesstimate. So I don’t know whether any of the American Ice Age predators (Smilodon? Panthera atrox?) could have tackled a full-grown specimen of one of the larger species.

  23. #23 JCP
    September 10, 2008

    If you want to see a great specimen of Panocthus tuberculatus go to the Cenozoic Mammal Hall at the Harvard Museum of Natural History. Lots of other great specimens as well.

  24. #24 SJS
    June 11, 2009

    Awesome read, nice to find such info from learned persons, are there more reads like this on other fossils including Neanderthal and the Paleofelids?

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