Sloths. Were there predatory sloths? Sloths that lived in the sea? Sloths that dug immense tunnels? Sloths on Antarctica? Sloths so keen to get to the US of A that they didn’t wait for the land bridge, but swam the way instead? Well, let’s see…
Today I was asked a question about sloths. Sloths are among my favourite mammals, and hence I feel particularly guilty in not having blogged about them, though I did publish a review article on them fairly recently (Naish 2005). Not only does this distraction allow me to delay yet further those promised posts on blood-feeding birds and bats, gigantic feral cats, rhinogradentians, and Triassic crurotarsans, it also occurred to me that if I write a brief list of ‘things you didn’t know about sloths’, then that would be quicker than completing a normal, essay-type post. That was the theory. Let’s get on with it. If you already know a lot about sloths, then none of this will be new to you, and I apologise for insulting your intelligence [adjacent photo of a two-toed sloth borrowed from here].
1. Sloths are entirely American, right? Most of them are, but the oldest fossil sloth (dating to the Eocene) is from Seymour Island, Antarctica. This at least suggests that sloths (and probably other xenarthrans*) were originally common to both South America and Antarctica, though right now we can’t say any more than that (and yes I do know about Eurotamandua and the ernanodontids).
* The placental mammal clade that includes sloths, armadillos and anteaters.
2. Sloths only invaded North America once the Panamanian land bridge formed about 2.5 million years ago. Oh really? While it seems that some sloths did get into North America this way (McDonald & Pelikan 2006), hardly ever mentioned is the fact that two sloths (Thinobadistes and Pliometanastes) appeared in North America before the land bridge formed (McDonald 2005). Presumably they swam and island-hopped to North America. Apparently they weren’t the only tetrapods that did this, given recent evidence indicating that the phorusrhachid Titanis also colonised North America prior to the formation of the land bridge. More on this in a future post (for previous writings on phorusrhacids go here and here).
3. The sloths that got into North America didn’t necessarily idle around in Mexico or the southern states (though some did). Some Pleistocene sloths occurred north of the Arctic Circle: Megalonyx jeffersonii (often called Jefferson’s sloth, and I’m sure you know why) is known to have occurred as far north as Alaska and the Northwest Territories. At this time the climate was a few degrees warmer than it is today, but even so it would have been cool (McDonald et al. 2000). Sloths have therefore not been restricted to tropical or temperate climates for the whole of their history. Incidentally, wouldn’t it be neat if they had actually managed to cross the Bering land bridge at some stage?
4. Perhaps the most startling recent discovery from the world of sloths is that there was once a group of marine, amphibious sloths. First described for the Peruvian species Thalassocnus natans (Muizon & McDonald 1995), we now know of five of these sea-going Pacific sloths. The older species probably lounged around on the beaches, and waded or swam out to feed on marine plants. The youngest and most anatomically specialised member of the group, T. yaucensis, had phenomenally stretched out, spatulate jaw tips, and limb bones shaped like those of a sealion. All Thalassocnus sloths exhibit features of the skull, hindlimbs and tail vertebrae showing that they were specialised for swimming, and for feeding in the water, but T. yaucensis may have been about as aquatic as a sirenian or pinniped (Muizon et al. 2004a, b). Err, gosh.
5. One of the biggest and best known fossil sloths is Megatherium from the Pliocene and Pleistocene of South America. Multiple species are known. Like other sloths, megatheres have generally been imagined as folivores, reaching up into trees and shrubs and using their strong arms and large claws to manipulate branches. However, after reconstructing the forelimb muscles of the best known species, M. americanum (shown in adjacent photo), Farina & Blanco (1996) argued that this animal was a predator, specialised for stabbing and lifting large herbivores. This radical idea has not been widely accepted among other sloth workers. It is mostly based on the assumption that the ability to strike rapidly with the forelimbs correlates with a predatory habit.
6. Sloths didn’t just use their arms and hooked hand claws to manipulate foliage or climb with. Some mylodontid sloths have forelimb anatomy and bone strength matching that of living mammals that regularly dig (Bargo et al. 2000). Furthermore, the wide, straight and relatively flat claws of these sloths match the idea that they were diggers [adjacent photo shows the mylodontid Paramylodon, borrowed from wikipedia's entry on sloths].
7. Confirmation of this digging idea comes from Argentinean Pleistocene burrows apparently made by these sloths. Some of these burrows are more than 1 m tall, 2 m wide and more than 20 m long. Their sides and roofs are scored with scratch marks matching mylodontid claw anatomy.
8. Sloths are phenomenally diverse in hand anatomy. Some fossil sloths had the full compliment of five fingers (albeit sometimes with the thumb and/or fifth finger strongly reduced). Others lacked the thumb, and others lacked the thumb and digit II (so much for the theory that digits tend to be lost from the edges of the hand first). Bradypus (three-toed sloth) possesses only digits II-IV on the hand, and the megalonychid Choloepus only has digits II and III. Several sloth groups exhibit fusion of various manual phalanges, including of both phalanges in the thumb (in Eremotherium) and of the two phalanges at the base of the third digit (in Thalassocnus), as well as fusion of metacarpals to carpals.
9. Historically, ground sloths have only just become extinct: some sloth remains from mainland North America are only about 8600 years old while some of the Caribbean sloths, like Synocnes comes and Parocnus serus from Haiti, were apparently alive just 500 years ago. Ground sloth survival is rumoured in Amazonia but as yet unsupported by any good evidence.
10. While it used to be thought that the living tree sloths were closely related to one another, phylogenetic work indicates quite convincingly that the two living genera (Bradypus and Choloepus) are about as distantly related as they could possibly be. Bradypus (the three-toed sloth: adjacent image – from here – shows how Bradypus moves on the ground) is archaic and represents the most basal sloth lineage that we know of, while Choloepus (the two-toed sloth) belongs to a more recently evolved clade, Megalonychidae. This has a few really interesting ramifications. Firstly, the basal position of Bradypus means that this taxon currently has a ghost lineage extending back to the Eocene at least. Secondly, the fact that this basal sloth is arboreal raises the possibility that arboreality is primitive for sloths, rather than derived. Thirdly, the many anatomical and behavioural similarities shared by Bradypus and Choloepus are convergences, in which case ‘this taxonomic arrangement … surely presents[s] one of the most striking examples of convergent evolution known among mammals’ (Gaudin 2004, p. 255). One more thing: there is no longer a clear dichotomy between ‘tree sloths’ and ‘ground sloths’ (some so-called ground sloths were actually capable climbers), nor were all fossil sloths bigger than the living species (Neocnus toupiti from Hispaniola was the smallest sloth ever).
There’s a lot more I could say, but that’ll do. I didn’t mention the armour, their bizarre vertebral and pelvic anatomy, the stuff we know about their hair, their physiology, their highly unusual feet, the whole Cueva de Milodon story, the DNA work, the combustible dung piles, their extreme ‘tenacity to life’, the screams, the fangs, and the scariest goddam book I’ve ever seen. At least I can rest, safe in the knowledge that I have finally blogged something on xenarthrans…
PS: pdf of Naish (2005) available to anyone who asks.
Refs – –
Bargo, M. S., VizcaÃno, S. F., Archuby, F. M. & Blanco, R. E. 2000. Limb bone proportions, strength and digging in some Lujanian (Late Pleistocene-Early Holocene) mylodontid ground sloths (Mammalia, Xenarthra). Journal of Vertebrate Paleontology 20, 601-610.
Farina, R. A. & Blanco, R. E. 1996. Megatherium, the stabber. Proceedings of the Royal Society of London B 263, 1725-1729.
Gaudin, T. 2004. Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence. Zoological Journal of the Linnean Society 140, 255-305.
McDonald, H. G. 2005. Paleoecology of extinct xenarthrans and the Great American Biotic Interchange. Bulletin of the Florida Museum of Natural History 45, 313-333.
– ., Harington, C. R. & de Iuliis, G. 2000. The ground sloth Megalonyx from Pleistocene deposits of the Old Crow Basin, Yukon, Canada. Arctic 53, 213-220.
– . & Pelikan, S. 2006. Mammoths and mylodonts: exotic species from two different continents in North American Pleistocene faunas. Quaternary International 142-143, 229-241.
Muizon, C. de & McDonald, H. G. 1995. An aquatic sloth from the Pliocene of Peru. Nature 375, 224-227.
– ., McDonald, H. G., Salas, R. & Urbina, M. 2004a. The youngest species of the aquatic sloth Thalassocnus and a reassessment of the relationships of the nothrothere sloths (Mammalia: Xenarthra). Journal of Vertebrate Paleontology 24, 387-397.
– ., McDonald, H. G., Salas, R. & Urbina, M. 2004b. The evolution of feeding adaptations of the aquatic sloth Thalassocnus. Journal of Vertebrate Paleontology 24, 398-410.
Naish, D. 2005. Fossils explained 51: sloths. Geology Today 21 (6), 232-238.