Litopterns are one of those groups of animals that tend to be represented in recent popular media by a single example, which many of you may recognise in the picture above. This was Macrauchenia patachonica, one of the latest surviving litopterns (like many other South American taxa, litopterns did not fare well during the so-called Great Faunal Interchange when South and North America became connected). But as with so many other under-represented groups, the popular exemplar is not necessarily a prime example. Macrauchenia was not only one of the last litopterns, it was also one of the largest, and the litopterns came in a whole range of appearances.
The earliest litopterns are known from the late Palaeocene. The basalmost members of the group are classified as the Protolipternidae, but the members of this family are united by primitive characters only. It is generally accepted that litopterns were closely related to two Palaeocene families of South American 'condylarths', the Didolodontidae and Sparnotheriodontidae, and there is a certain degree of arbitrariness about whether or not these families should also be treated as litopterns. Those who would exclude the didolodontoids from the litopterns do so on the basis of the latter's tarsal morphology, which has become adapted for a more cursorial lifestyle. The protolipternids bridge the gap between didolodontoids and other litopterns in that they possessed a litoptern-like tarsus, but retained teeth more like the didolodontoids. Protolipterna also retained five toes on the feet, while this number was reduced in later litopterns (Bastos & Bergqvist 2007). The other noteworthy feature of protolipternids was that they were not very big. Rose (2009) includes an illustration of partial upper and lower jaws of the protolipternid Asmithwoodwardia whose scale bar indicates that the complete skull must have been less than five centimetres in length, or about the size of a brown rat. Cifelli (1983) suggested on the basis of their small size that these animals may have been more leapers than runners, a suggestion not directly supported but not entirely ruled out by Bastos & Bergqvist (2003).
The remaining litopterns mostly belong to the families Proterotheriidae, Adianthidae and Macraucheniidae, united by specialisations of the dentition and reduction of the number of toes to three (a fifth family, the late Palaeocene Notonychopidae, are represented by dental remains only). The Palaeocene to Pleistocene Proterotheriidae have attracted a reasonable amount of interest in the past because of their convergences with the horses in the Northern Hemisphere. Like horses, proterotheriids centred locomotion on the middle toe only, with the toes on either side being reduced. In the Miocene proterotheriid Thoatherium, the side toes were almost completely lost, reduced to splints even smaller than those of the modern horse (being by this measure more horse-like than an actual horse, Thoatherium has also been a popular subject for books on evolution). In other respects, however, proterotheriids were not so horse-like. With relatively low-crowned teeth, proterotheriids and other litopterns were browsers rather than grazers, and they may have preferred more wooded terrain rather than grasslands. Ecologically, proterotheriids were probably more like deer or small antelopes than horses, and they resembled small antelopes in size. Only one proterotheriid survived into the Pleistocene, Neocaliphrium recens, and only in Uruguay and northern Argentina (Ubilla et al. 2011).
The Adianthidae were small litopterns (though not as small as the protolipternids) known from the Eocene to the Miocene. Most adianthids are known only from dental remains and/or jaw fragments, though some limb bones are known from the Miocene Adianthus godoyi (Cifelli 1991). These indicate a gracile form, probably more similar to proterotheriids than to macraucheniids, though Cifelli noted the similarities to the former were likely related to size rather than indicative of any deeper affinity.
The Macraucheniidae retained three functional toes, with the middle toe not substantially larger than the two side ones. They also differed from the proterotheriids in the development of a longer neck, and have usually been compared to camels in appearance (the name 'Macrauchenia' was originally coined to effectively mean 'big llama', in the mistaken belief that it represented an ancestor of that animal). They have been divided between to subfamilies, the Oligocene to Miocene Cramaucheniinae and the late Miocene to Pleistocene Macraucheniinae, though the latter are undoubtedly descended from the former. The cramaucheniines retain a plesiomorphic anterior nasal opening, but in the Macraucheniinae the nasal bones are reduced and the nasal opening has moved posteriad on the skull (Dozo & Vera 2010). It is this dorsal position of the nasal opening that has lead to the interpretation of Macrauchenia as having some form of proboscis, like that of a tapir. The combination of a long neck and a proboscis is, however, an unusual one, and I've wondered if it may have been more of a prehensile upper lip. The macraucheniids did better in the Pleistocene than the proterotheriids, with three species described from a large chunk of the continent, but eventually they two went the way of the toxodont.
Bastos, A. C. F., & L. P. Bergqvist. 2007. A postura locomotora de Protolipterna ellipsodontoides Cifelli, 1983 (Mammalia: Litopterna: Protolipternidae) da Bacia de São José de Itaboraí, Rio de Janeiro (Paleoceno superior). Anuário do Instituto de Geociências 30 (1): 58-66.
Cifelli, R. L. 1983. Eutherian tarsals from the Late Paleocene of Brazil. American Museum Novitates 2761: 1-31.
Cifelli, R. L. 1991. A new adianthid litoptern (Mammalia) from the Miocene of Chile. Revista Chilena de Historia Natural 64: 119-125.
Dozo, M. T., & B. Vera. 2010. First skull and associated postcranial bones of Macraucheniidae (Mammalia, Litopterna) from the Deseadan Salma (late Oligocene) of Cabeza Blanca (Chubut, Argentina). Journal of Vertebrate Paleontology 30 (6): 1818-1826.
Rose, K. D. 2009. The Beginning of the Age of Mammals. JHU Press.
Ubilla, M., D. Perea, M. Bond & A. Rinderknecht. 2011. The first cranial remains of the Pleistocene proterotheriid Neolicaphrium Frenguelli, 1921 (Mammalia, Litopterna): a comparative approach. Journal of Vertebrate Paleontology 31 (1): 193-201.