Digital cranial endocast ofPseudoplohophorus absolutus(Xenarthra, Cingulata) and its systematic and evolutionary implications

Abstract

Glyptodonts are a group of extinct xenarthrans with several anatomical features that make them one of the most bizarre groups of mammals. By the late 19th century, some authors began to analyze the brain of Pleistocene glyptodonts using natural endocranial casts. These studies revealed the small size of the brain of the large Pleistocene forms. However, the evolution of the brain in glyptodonts and how it fits in a phylogenetic context has not been analyzed. In order to evaluate the evolution of the brain in this group, we described the first digital endocranial cast of the late Miocene glyptodont Pseudoplohophorus absolutus and compared it with digital endocranial casts of the Pleistocene glyptodonts Glyptodon, Doedicurus, and Panochthus and the extant armadillos Dasypus, Euphractus, Chaetophractus, and Zaedyus. The endocast morphology of P. absolutus is similar to that of Pleistocene glyptodonts: large olfactory bulbs, a small cerebrum with a single neocortical sulcus, and a large cerebellum. However, the relative brain size is larger than in the Pleistocene forms, with values of the encephalization quotient (EQ) close to that of extant armadillos. A comparison between xenarthrans orders shows that Cingulata (glyptodonts and armadillos) have lower EQ values than Pilosa (sloths and vermilinguas). This could possibly be related to certain restrictions and benefits imposed by the presence of the carapace in cingulates. Furthermore, because the carapace restricts the development of the cervical musculature that supports the skull, the small size of the brain in glyptodonts could be a trade-off (along with others) to reduce the weight of the skull.

Digital endocranial cast of Pampatherium humboldtii (Xenarthra, Cingulata) from the Late Pleistocene of Uruguay

“…These measures give a length/width ratio of 1.1 and a length/ height ratio of 1.2, which represent a rather anteroposteriorly elongated and dorsoventrally compressed cerebrum, which generally reflects the overall proportions of the skull. In the Late Miocene glyptodont Pseudoplohophorus absolutus, the length/width ratio is 0.9 and the length/height ratio is 1.1, while in the Pleistocene glyptodonts Glyptodon sp., Panochthus tuberculatus and Doedicurus clavicaudatus, the length/width ratio range is 0.9-1.1 and the length/ height ratio range is 1.3-1.5 (Tambusso and Fariña 2015). In the extant armadillos, the length/width and length/height ratios have ranges of 0.7-0.9 and 0.9-1.1, respectively (Tambusso and Fariña 2015).…”

“…In the Late Miocene glyptodont Pseudoplohophorus absolutus, the length/width ratio is 0.9 and the length/height ratio is 1.1, while in the Pleistocene glyptodonts Glyptodon sp., Panochthus tuberculatus and Doedicurus clavicaudatus, the length/width ratio range is 0.9-1.1 and the length/ height ratio range is 1.3-1.5 (Tambusso and Fariña 2015). In the extant armadillos, the length/width and length/height ratios have ranges of 0.7-0.9 and 0.9-1.1, respectively (Tambusso and Fariña 2015). The ratios of P. humboldtii are close to those of Pleistocene glyptodonts ( Table 2).…”

“…Comparison with glyptodonts (Glyptodon sp., Panochthus tuberculatus, Doedicurus clavicaudatus, Pseudoplohophorus absolutus) and extant armadillos (Dasypus novemcinctus, D. hybridus, Zaedyus pichiy, Chaetophracthus villosus, Euphractus sexcinctus) follow Tambusso and Fariña (2015).…”

“…The actual brain size (E i ) is the measured volume of the brain (or endocast), and the expected brain size (E e ) is the expected volume of the brain (or endocast) in a mammal of the same body mass. For the expected brain size, we used the values obtained by Tambusso and Fariña (2015) from a linear regression on 796 extant mammals: E e = 0.050 m 0.751 , and that obtained only for xenarthrans: E e = 0.123 m 0.606 , were m is the body mass in grams. The body mass was estimated assuming geometric similarity with the pampathere Holmesina occidentalis (Vizcaíno et al 2006) from total skull length and occipital height, respectively, TSL and OCH (Janis 1990; see also Fariña et al 1998).…”

“…They include some Pleistocene glyptodonts (Serres 1865;Gervais 1869;Dechaseaux 1958;Tambusso and Fariña 2009;Vílchez Barral et al 2013) and an Eocene armadillo (Dozo 1998). Recently, Tambusso and Fariña (2015) described the endocast of a Miocene glyptodont and compared it with those of several Pleistocene glyptodonts and extant armadillos. Their results show that the cerebrum of glyptodonts has a simple pattern of cortical sulci both in the Miocene and Pleistocene forms and that extant and extinct cingulates have the smallest brain size relative to body mass within xenarthrans.…”