Regular visitors will no doubt have noticed the failure of my promise to post a picture a day. Well, alas, I’m going to have to take a much more relaxed approach, as it has proved impossible to find time even for that. So, I might post a new picture every day on the blog, and I might not. My weekend was taken up with various social events, and since then there just hasn’t been the time to add new stuff… other than the sheep article of course. And what about my promise to, err, stick only to the things I’ve nearly finished writing but have yet to complete? Well, stuff that too. Today, continuing the theme initiated with Ten things you didn’t know about sloths, we’re going to embark on a quick tour of the fascinating, no-holds-barred, pedal-to-the-metal world of THE ARMADILLOS, concentrating in particular on some of the more remarkable fossil forms …although my original plan to write ten things about armadillos got whittled down to five for the usual reasons of space and time…
Armadillos are xenarthrans: part of the same clade of placental mammals as sloths and anteaters. They are, of course, best known for their armour. It is composed of interlocking polygonal dermal ossifications (termed scutes, ossicles or osteoderms) that form continuous pectoral and pelvic bucklers, separated in the middle by flexible bands (numbering from three to 13 depending on the species) formed by rectangular osteoderms. The bands allow the animal to twist and bend and, in some species, to roll into a ball for protection. Armadillos are supreme generalists and occur from rainforests to deserts, and everything in between. They are predominantly insectivorous, and some are specialised myrmecophages (ant- and/or termite-eaters). Others are omnivores. Powerful limbs equipped with large curved claws allow them to be good diggers and burrowers, and the fairy armadillos or pichiciegos Chlamyphorus [pictured above] are mole-like fossorial forms. The word armadillo means ‘little armoured one’ (not ‘little tank’ as I’ve read in a few places).
There are about 25 living armadillo species, so they are quite a speciose group. However, their diversity was greater in the past and we know of several armadillo clades, as well as many, many species, that are now entirely extinct. In fact a 1980 catalogue listed over 100 fossil species of armadillo. So, here we go… (oh, and, no offence intended to the informed readers to whom these things will not be ‘things you didn’t know’)…
1. The biggest living armadillo is the Giant armadillo Priodontes maximus [shown in adjacent image: pic from here]. It can reach 1.5 m in total length and may weigh over 50 kg. This is huge. But it’s not so huge when we compare it with the biggest armadillos ever, the giant armadillos or pampatheres. Properly called pampatheriids (formerly chlamydotheriids), giant armadillos are best known from the Miocene and were still alive late in the Pleistocene (they may even have survived into the early Holocene in Brazil). Giant armadillos participated in the Great American Biotic Interchange and were present in North America during the Pliocene and Pleistocene. The biggest giant armadillo was Holmesina, a rhino-sized animal about 2 m long, 1 m tall at the shoulders, and with a skull about 30 cm long. I’ve been unable to find a precise mass estimate in the literature, but it would have weighed several hundred kg at least. How did these armadillos live? We’ll come back to that in a minute. Incidentally, the giant armadillo Pampatherium was one of the first fossil mammals to ever be described from South America, having been named in 1839… though back then it was known as Chlamytherium… which was a mistake and was changed in 1841 to Chlamydotherium… which proved to be preoccupied, hence the change in 1891 to Pampatherium. Incidentally, we are not talking about the glyptodonts: while armadillo-like, they are not true armadillos. But then, neither are pampatheres actually (read on).
2. The Nine-banded armadillo Dasypus novemcinctus is slowly (or rapidly, take your pick) taking over the United States. Reported from Texas during the 1850s, by the 1920s it had spread to Louisiana. It got into Mississippi, Alabama and Oklahoma during the 1930s and during the 1940s expanded into Arkansas, Missouri and Kansas. By the 1970s it had been reported from Georgia, Tennessee, South Carolina, Colorado and Nebraska. The species has been present in Florida since an introduction made during WWI, and another in 1924. By the 1950s the animals were reportedly common throughout the state [map showing spread of armadillos borrowed from here]. Given their obligate production of quadruplets (read on), excellent swimming ability, general fearlessness and robust constitution, and ability to eat anything and thrive in diverse habitats, they are excellent colonists and would be difficult to eradicate (should anyone try). They may be bad news for ground-nesting birds. Besides humans, the Nine-banded armadillo is apparently the only mammal that carries leprosy, but this is apparently a good thing as they have been utilised in medical research. Why this species has spread so far so quickly is not entirely clear. Possibilities include the spread of poor-quality, predator-free habitat and progressive climatic amelioration (McBee & Baker 1982). Given that an extinct species of Dasypus, D. bellus, occurred across most of the same area during the Pleistocene, one might argue that armadillos are actually meant to be where they are now, and are merely reclaiming the area after a temporary absence.
3. The Nine-banded armadillo is, clearly, the best known and most studied armadillo, but it is only one of six extant long-nosed armadillo (Dasypus) species. All long-nosed armadillos are particularly interesting from the point of view of reproductive biology in that they exhibit obligate polyembryony: females produce a single fertilized egg that then divides into four embryos, all of which are (of course) genetically identical. While suspected as early as 1909, obligate polyembryony was not really confirmed until the 1990s (Prodöhl et al. 1996, Loughry et al. 1998). Long-nosed armadillos are the only vertebrates that exhibit polyembryony every time they reproduce. They also delay implantation of embryos, with the 5 months between June/July and November/December being the usual period. However, exceptional cases are on record where implantation has been delayed for an amazing three years (although I’m sure I recall reading of a much longer delay somewhere – more like 12 years, though I can no longer locate this in the literature!).
4. Several groups of fossil armadillos, including giant armadillos and eutatines, appear to have been specialised herbivores. Actually, whether giant armadillos and eutatines are really armadillos or not is a subject of minor debate, as some experts have argued that – within the armoured xenarthran clade Cingulata – both are more closely related to glyptodonts than to armadillos. Anyway, jaw and tooth morphology shows that giant armadillos were grazing herbivores: the palaeoenvironments and cranial morphology of the different species indicate that some ate fibrous plants growing in dry habitats, while others ate softer vegetation in wetter habitats (De Iuliis et al. 2000, Scillato-Yané et al. 2005, Vizcaíno et al. 1998) [adjacent image shows skull of giant armadillo Holmesina]. Eutatines, best known from the Oligocene, Miocene and Pliocene of Argentina, were shown by Vizcaíno & Bargo (1998) to exhibit jaw and tooth morphology, and a reconstructed muscular anatomy, strongly indicating a herbivorous diet consisting of leaves, buds and perhaps grasses. Perhaps most bizarrely, the absence of anterior incisiform teeth and of any evidence for a flexible snout or lips led these authors to suggest that Eutatus, the largest eutatine (it was about equal in size to the living Giant armadillo), may have used a long and flexible tongue as a food-gathering organ.
5. While some living armadillos are omnivores and incorporate quite a lot of vertebrate prey in their diet (as much as 30% of the summer diet of Chaetophractus is made up of small vertebrates), none of the living species are dedicated carnivores. It used to be thought that the peltephilines of Eocene, Oligocene and Miocene Argentina and Bolivia were cursorial carnivores or scavengers, although the reasons for this were always shockingly poor. These highly unusual armadillos are notable for their short, wide snout, enlarged, tall cranium and horns [skull of Peltephilus shown in adjacent image]. Yes, horns (making them convergently similar to mylagaulid rodents). A recent reanalysis showed that peltephilines are likely to have been fossorial animals, eating tubers and other hard subterranean vegetation (Vizcaíno & Fariña 1997). The good news however is that at least one extinct armadillo really was a horrible rapacious predator: Vizcaíno & De Iuliis (2003) showed that Macroeuphractus from Miocene and Pliocene Argentina and Bolivia, well known for its large size (c. 100 kg) and big, conical caniniform teeth, was well suited for carnivory. Given that Macroeuphractus was a euphractine – a close relative of the living Chaetophractus, Euphractus and Zaedyus – its carnivorous habits can perhaps be regarded as ‘an extreme position in the carnivorous-omnivorous feeding behaviour of euphractines’ (Vizcaíno & De Iuliis 2003, p. 123). In other words, it perhaps wasn’t so exceptional among the group. But its large size meant that it could do things that other euphractines couldn’t, or can’t. A fairly horrific reconstruction of a hungry toothy Macroeuphractus tunnelling its way into the den of a group of happy little cute hare-like rodents decorates p. 135 of Vizcaíno and De Iuliis’ paper.
And on that, I say goodbye.
PS – happy Gigantoraptor day! If I had the time I would definitely write a post about it. An oviraptorosaur the size of a tyrannosaur. Swoon.
Refs – –
De Iuliis, G., Bargo, M. S. & Vizcaíno, S. F. 2000. Variation in skull morphology and mastication in the fossil giant armadillos Pampatherium spp. and allied genera (Mammalia: Xenarthra: Pampatheriidae), with comments on the systematics and distribution. Journal of Vertebrate Paleontology 20, 743-754.
Loughry, W. J., Prodöhl, P. A., McDonough, C. M. & Avise, J. C. 1998. Polyembryony in armadillos. American Scientist 86, 274-279.
McBee, K. & Baker, R. J. 1982. Dasypus novemcinctus. Mammalian Species 162, 1-9.
Prodöhl, P. A., Loughry, W. J., McDonough, C. M., Nelson, W. S. & Avise, J. C. 1996. Molecular documentation of polyembryony and the micro-spatial dispersion of clonal sibships in the nine- banded armadillo, Dasypus novemcinctus. Proceedings of the Royal Society of London B 263, 1643-1649.
Redford, K. H. & Wetzel, R. M. 1985. Euphractus sexcinctus. Mammalian Species 252, 1-4.
Scillato-Yané, G. J., Carlini, A. A., Tonni, E. P. & Noriega, J. I. 2005. Paleobiogeography of the late Pleistocene pampatheres of South America. Journal of South American Earth Sciences 20, 131-138.
Vizcaíno S.F. & Bargo, M. S. 1998. The masticatory apparatus of the armadillo Eutatus (Mammalia, Cingulata) and some allied genera: paleobiology and evolution. Paleobiology 24, 371-383.
– . & De Iuliis, G. 2003. Evidence for advanced carnivory in fossil armadillos (Mammalia: Xenarthra: Dasypodidae). Paleobiology 29, 123-138.
– ., De Iuliis G. & Bargo M.S. 1998. Skull shape, masticatory apparatus, and diet of Vassallia and Holmesina (Mammalia: Xenarthra: Pampatheriidae): when anatomy constrains destiny. Journal of Mammalian Evolution 5, 291-322.
– . & Fariña, R. A. 1997. Diet and locomotion of the armadillo Peltephilus: a new view. Lethaia 30, 79-86.