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Ancient Mitochondrial Sequences Place Extinct Glyptodonts in Armadillo Family Tree

NEW YORK (GenomeWeb) – A new genetic analysis suggests an extinct group of large animals called glyptodonts, fossilized creatures discovered in South America during Darwin's Beagle expedition in the 1800s, were most closely related to armadillos.

"The data sheds light on the familial relations of an enigmatic creature that has fascinated many but was always shrouded in mystery," senior author Hendrik Poinar, director of McMaster University's Ancient DNA Centre, said in a statement.

As they reported in a correspondence article today in Current Biology, Poinar and colleagues from Canada, France, New Zealand, the US, and Argentina used ancient extraction and mitochondrial DNA sequencing to test samples from Doedicurus, a large glyptodont species suspected of weighing as much as one ton.

The team analyzed the ancient Doedicurus mitochondrial genome in the context of sequences from present-day members of the xenarthran superorder, a placental mammalian group that includes armadillos, anteaters, extinct ground sloths and other animals. The analysis placed the glyptodonts in more or less the middle of the armadillo family tree and did not branch off especially early from other armadillo groups.

"While our results are based strictly on the comparison of mitogenomes, the global congruence observed with previous nuclear-based phylogenies as well as molecular dating analyses provides convincing evidence for the proposed xenarthran evolutionary history," Poinar and co-authors wrote.

Though much larger than living armadillos, glyptodonts have been classified as xenarthrans based on physical features such as heavy, armored shells and tails. Even so, their place in the xenarthran tree has been murky.

To take a molecular look at these relationships, the researchers started by using in silico reconstruction of ancestral xenarthran mitogenome sequences to reconstruct the mitochondrial genome to come up with bait sequences needed to capture ancient DNA from 12,000-year-old Doedicurus shell fragments found in Argentina.

Using sequences generated with Illumina instruments, they put together a nearly complete mitochondrial genome for Doedicurus, covered to a depth of about 76-fold, which was then compared with mitochondrial genome sequences for living xenarthran animals.

The team found that the group split off before some lineages leading to more modestly sized armadillos and after others. The most closely related clade includes fairy armadillos, giant armadillos, and three-banded armadillos, the study's authors noted.

More broadly, the results suggest that glyptodonts' size and the solid piece of carapace armor covering them were likely derived features that were not present in shared ancestors of glyptodonts and present-day armadillos.

"Glyptodonts should probably be considered a subfamily of gigantic armadillos," first author Frédéric Delsuc, an evolutionary science researcher with the French National Centre for Scientific Research (CNRS), said in a statement. "We speculate that the peculiar structure of their unarticulated carapace might have evolved as a response to the functional constraint imposed by the size increase they experienced over time."