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Photo Credit: Monique Pool, Green Heritage Fund Suriname

Evolution of the Mylodontidae and Arborealism in Sloths

Nuclear and mitochondrial data from Darwin's extinct ground sloth shows close relation to two-fingered sloths and supports multiple origins for arboreality.

May 25, 2018

Named by Richard Owen in honour of Charles Darwin who discovered the first discovered the species' remains during his famous voyages on the Beagle, the evolutionary mystery of Darwin's extinct ground sloth has now been resolved. Researchers at the McMaster Ancient DNA Centre and their collaborators have successfully extracted ancient DNA from a 12,800-year-old Mylodon darwinii bone fragment from Mylodon Cave in Chile. The exceptional preservation of the sample allowed the researchers to obtain a high quality mitochondrial genome and partial nuclear information. These results firmly cement Mylodon darwinii as a member of the distinct lineage (Mylodontidae) closely related to two-findered sloths and suggests that both extant sloth families evolved to living in trees independently from ground-living ancestors.Mylodon darwinii figures from the original description by Sir Richard Owen in 1842.

Resolving the phylogenetic position of Darwin's extinct ground sloth (Mylodon darwinii) using mitogenomic and nuclear exon data

Authors: Frédéric Delsuc, Melanie Kuch, Gillian C. Gibb, Jonathan Hughes, Paul Szpak, John Southon, Jacob Enk, Ana T. Duggan, Hendrik N. Poinar

Proceedings of the Royal Society B: Biological Sciences, Vol. 285, Issue 1878. May 2018. DOI: https://doi.org/

Abstract

Mylodon darwinii is the extinct giant ground sloth named after Charles Darwin, who first collected its remains in South America. We have successfully obtained a high-quality mitochondrial genome at 99-fold coverage using an Illumina shotgun sequencing of a 12 880-year-old bone fragment from Mylodon Cave in Chile. Low level of DNA damage showed that this sample was exceptionally well preserved for an ancient subfossil, probably the result of the dry and cold conditions prevailing within the cave. Accordingly, taxonomic assessment of our shotgun metagenomic data showed a very high percentage of endogenous DNA with 22% of the assembled metagenomic contigs assigned to Xenarthra. Additionally, we enriched over 15 kb of sequence data from seven nuclear exons, using target sequence capture designed against a wide xenarthran dataset. Phylogenetic and dating analyses of the mitogenomic dataset including all extant species of xenarthrans and the assembled nuclear supermatrix unambiguously place Mylodon darwinii as the sister-group of modern two-fingered sloths, from which it diverged around 22 million years ago. These congruent results from both the mitochondrial and nuclear data support the diphyly of the two modern sloth lineages, implying the convergent evolution of their unique suspensory behaviour as an adaption to arboreality. Our results offer promising perspectives for whole-genome sequencing of this emblematic extinct taxon.

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Mylodon cave (Puerto Natales, Chile). Photo by Walter Ferry Dissmann.