Paris, 22 February 2016
Glyptodonts are classified in the Xenarthra superorder, together with armadillos, sloths and anteaters. Yet today's armadillos and glyptodonts differ from all other mammals by their carapace, made up of mobile bands, while that of their prehistoric cousins was in a single piece. Several morphological differences have led glyptodonts to be considered as a group in their own right, representing a sister group2 to the armadillos within the Xenarthra. This traditional interpretation was recently called into question by a study on cranial and dental characters that classified the glyptodonts with the armadillos.
To investigate this issue, researchers studied the DNA from a fragment of carapace thought to be from a 12,000-year-old Doedicurus. This species was one of the largest glyptodonts, with a body mass of 1.5 metric tons and a characteristic spiny club-shaped tail. Sequencing of ancient DNA is difficult for a number of reasons, firstly because this DNA gets greatly fragmented by time and humid conditions. Furthermore, endogenous DNA strands need to be separated from any environmental contamination.
To do this, the researchers had to develop RNA probes capable of recognizing the genetic material of the target species, which is a challenging task when none of the genomes of closely related species are known. To overcome this obstacle, the group modeled plausible ancestral sequences using mitochondrial genomes from present-day Xenarthra (armadillos, anteaters and sloths). These bioinformatic sequences were then used to synthesize suitable RNA probes, which then made it possible to reconstruct the mitochondrial genome of this extinct species almost completely. The phylogenetic analyses then clearly placed the glyptodonts within the cingulates, where they represent a distinct lineage. The last members of this lineage of giant cingulates, which dates back some 35 million years, died out only 10,000 years ago, at the end of the last ice age.
This study illustrates the potential of ancient DNA capture methods based on ancestral sequences reconstructed using bioinformatics, a technical approach used for the first time in this study.
© Peter Schouten
© Museo de La Plata / Sergio Vizcaino
Fossil Doedicurus specimen from the La Plata mueseum (Argentina)
1 The Ancient DNA center at McMacter University in Hamilton (Ontario, Canada)
2 For example, humans and chimpanzees belong to sister groups.
The phylogenetic affinities of the enigmatic glyptodonts. Delsuc F., G. C. Gibb, M. Kuch, G. Billet, L. Hautier, J. Southon, J.-M. Rouillard, J. C. Fernicola, S. F. Vizcaíno, R. D. E. MacPhee, et H. N. Poinar. Current Biology. February 22 2016.
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