Macrauchenia: a South American perissodactyl

Figure 1. Macrauchenia museum mount.

Figure 1. Macrauchenia museum mount.

Figure x. Macrauchenia cladogtam. Tapir and Chalicotherium are perissodactyls.

Figure x. Macrauchenia cladogtam. Tapir and Chalicotherium are perissodactyls.

Famous for its assumed
elephant-like proboscis, arising from a dorsal narial opening (Fig 2), Macrauchenia was a long-legged grazing ungulate with three toes on each manus and pes. In the large reptile tree it nests with Chalicotherium, which, in turn, nests with Tapirus, (the tapir) an extant perissodactyl with a short flexible trunk. A recent analysis of collagen sequences (Welker et al. 2015) found the same relationship. Not sure why this needed resolution… tapirs also have a trunk, dorsal narial opening, three hooves per foot AND some still live in South America. I guess that used to be considered ‘convergence.’ Here the LRT calls it ‘homology.’

Discovered by Charles Darwin in 1834,
and published by Richard Owen in 1836, Macrauchenia patachonica (Pliocene 7mya to Pleistocene .02mya; 3m length) was otherwise similar to a camel in proportions with a horse-like skull. Macrauchenia was an herbivore with a full arcade of short teeth in its jaws (Fig. 2). The last premolar looks like a molar, but, like other premolars, it is slightly larger than the other teeth and all sister taxa have 3 molars per side.

Figure 2. Macrauchenia skull in several view (from Owen 1836?) with bones colorized here. Note the dorsal extension of the premaxilla.

Figure 2. Macrauchenia skull in several view (from Owen 1836?) with bones colorized here. Note the dorsal extension of the premaxilla. The fossa posterior to the naris could anchor large proboscis muscles. 6 premolars and 3 molars appear to have been present. Not sure about the palatine here.

Wikpedia reports, “Macrauchenia was a long-necked and long-limbed, three-toed South American ungulate mammal, typifying the order Litopterna.  Early forms are near the condylarths, to such an extent that the litopterns might be considered merely as surviving and diversely specialized condylarths.” The LRT did not nest Macrauchenia with the basal Condylartha, but that is still a monophyletic clade that now includes all hoofed and edentate mammals — along with all the original basal condylarths.

Thanks to reader SBJ
for suggesting a number of South American mammals to add to the LRT. This is number one of several to come.

References
Owen R 1838. Description of Parts of the Skeleton of Macrauchenia patachonica. In Darwin, C. R. Fossil Mammalia Part 1 No. 1. The zoology of the voyage of H.M.S. Beagle. London: Smith Elder and Co.
Welker F et al. 2015. Ancient proteins resolve the evolutionary history of Darwin’s South American ungulates. Nature. doi:10.1038/nature14249. ISSN 0028-0836.
wiki/Macrauchenia

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