Placentation in sloths

Placenta and fetal membranes of Hoffmann's Two-toed
Sloth (Choloepus hoffmanni) from Turner 1872

Two recent papers cast light on the evolution of sloths. Delsuc and colleagues obtained mitochondrial genomes from living and extinct sloths (here) while Presslee and colleagues used proteomics to describe Type I collagen - likewise from extant and extinct species (here). They agree that notions of sloth evolution based on morphology need revision. 

It was confirmed that two-toed and three-toed sloths are not closely related and group with different forms of ground sloths. A third group with a suspensory life style was found to have diverged even earlier. These sloths crossed a land bridge to the Greater Antilles and went extinct just a few thousand years ago.

Interhaemal barrier of the pale-throated three-toed sloth
(Bradypus tridactylus) Courtesy of Allen C. Enders

Together with anteaters and armadillos, sloths belong to the ancient superorder Xenarthra. Armadillo placenta has been studied in some detail. It is villous haemochorial (discussed here). Anteater placentation is rather similar (previous post). Sloths are different. As shown by Turner, the placenta is lobulated in appearance. It is labyrinthinre and endotheliochorial (here).

Given current opinion on the relations between sloths (Folivora), anteaters (Vermilingua) and armadillos (Cingulata), parsimony dictates that their common ancestor would have had a villous haemochorial placenta. Thus the sloth placenta represents a derived state. The most recent common ancestor of two-toed and three-toed sloths lived >25 million years ago whereas sloths diverged from anteaters >50 mya. 

South American fossil ungulates are related to modern horses

Macrauchenia - an extinct South American native ungulate
Drawn by Kobrina Olga (Wikimedia Commons CC BY 3.0)

South American mammals either arrived recently during the Great American Interchange, came to South America by rafting (previous post), or have a shallow fossil record (xenarthrans). In contrast, South American native ungulates (SANU) have a deep fossil record. Though the lineage is now extinct, genera such as Macrauchenia (above) and Toxodon survived into the Late Quaternary. The conditions under which these fossils were conserved did not favour survival of their DNA. However, a paper just out in Nature (here) shows how proteomics can be used to probe their phylogenetic affinities.

The authors used MALDI-TOF mass spectrometry to determine the amino acid sequences of the alpha chains of type I collagen extracted from fossil bones. They then performed a phylogenetic analysis. This showed that Macrauchenia and Toxodon form a monophyletic group. Interestingly, this resolved as the sister group to modern Perissodactyla, i.e. horses, tapirs and rhinoceroses. Together they form a taxon named Panperissodactyla.

The modern species have an epitheliochorial placenta so perhaps this type was present in the sister species Macrauchenia  and Toxodon.