19 March 2016

Fossil Vertebrate(s) of the Month: Glyptodontinae

Glyptodon clavipes & other South American megafauna
Field Museum
It's an old adage that one of the joys of paleontology is that you never know what you're going to find in the field.  It could be something fantastically bizarre or gorgeous, or it could be something that changes our understanding of a group's evolution.  As paleontology itself evolves into a more analytical field, new and improved research methods lead mean that game-changing discoveries now happen in the lab at least as frequently as they do in the field.  A prime example of this is a subfamily of South American mammals known as the glyptodontines.  Were I writing this a month ago, I'd have referred to them as glyptodontids, members of their own distinct family related to, but distinct from, sloths, anteaters, and armadillos.  Glyptodonts are as far from a recently-discovered taxon as you can get, having been named in 1879 and even before that having been the subject of study by such luminaries as Darwin and Owen.  Likewise, they are anything but obscure, having long been the focus of both scientific and public attention, due in no small part to their size (the largest are often compared to VW Beetles), their massive shells composed of hexagonal plates (the size and bulkiness of which leading to relatively high rates of preservation and a very good fossil record), and their armored (and in some cases weaponized) tails.  And yet, there's still a lot we don't know about even such a seemingly well-understood group.  Until now, I've used glyptodonts as prime examples of convergence, evolving armor similar to that of armadillos despite being from a separate xenarthran lineage.  Last month, though, a study by Frederic Delsuc and colleagues showed that armadillos and glyptodonts are similar not due to convergence, but because they are very closely related.  In fact, using mitochondrial DNA recovered from a 12,000 year old specimen of the spike-tailed glyptodont Doedicurus, Delsuc et al. showed that not only do glyptodonts belong to the same order (Cingulata) as armadillos, they can actually be placed in the armadillo family Chlamyphoridae.  This family does not include armadillos of the genus Dasypus so familiar to Texans and Floridians, but it does include, among other species, the pink fairy armadillo and the giant armadillo (competitor and, sadly, early-round upset victim in March Mammal Madness).  That such a well-studied group can still surprise us as we develop new methods of study is not only a testament to the data-driven, non-dogmatic nature of science, but nicely illustrates why it's so much fun to study fossils: be it on the pampas of Argentina or in a genetics lab in Montpellier, you really never know what you're going to find.

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