Glyptodonts are an extinct group of large mammals which were reasonably common across South (and later, Central) America during the Pleistocene era. These large herbivores resembled ankylosaurs and armadillos, to the latter of which they are close relatives. They could be the size of a small car, with short legs and a fairly squat posture. Much of the body and the back was covered by a large carapace made up of hundreds of bony scutes, with additional plates protecting the tail and a bony plate on the top and back of the head. Of the six major groups, four have particularly large, spine-like scutes around the anterior (head) end of the carapace, as seen in this image. But why are these scutes only found in only these species? It just so happens that the appearance of this new trait coincides with the occurrence of the Great American Biotic Interchange, which peaked around 3 million years ago, at the beginning of the Pleistocene. As the isthmus of Panama was formed by volcanic activity, South and North America became connected and a period of considerable migration began. For the glyptodonts, this meant the appearance of several North American predators, including the sabre-toothed cat, Smilodon. It is thought that the evolution of the neck scutes could have occurred in response to increasing predation, protecting the most vulnerable part of the body which was not covered already by the carapace or cranial shield.
Anterior: An anatomical term referring to the head end of the body, with the tail (if present) at the opposite end of this axis.
Carapace: A shell or hard covering which protects the body. Although this usually applies to crustaceans (crab and lobster shells, for example) it can be used for other shelled animals such as tortoises and spiders.
Pleistocene: A geological age from about 2.7 million years to 10,000 years ago, including most of the recent glaciations or Ice Ages.
Image courtesy of Alfredo Eduardo Zurita.