Facial osteoderms of a new specimen of Panochthus sp. (Deparment of General Guido, Buenos Aires)

Abstract

The aim of this work is to describe new facial osteoderms for the genus Panochthus. The specimen (MCHP-001) comes from Chelforó river (36° 55’ 47.8’’ S; 58° 08’ 24.9’’ W), department of General Guido, Buenos Aires Province. The sediment that housed the specimen is Guerrero Member of the Luján Formation (Lujanian Stage, Upper Pleistocene). During the preparation of the skull of MCHP-001, several osteoderms were found. These elements are compressed or spherical and shows smooth or scalloped margins, similar to those published for Glyptodon munizi. We define two morphotypes of osteoderms based on the morphology, size and the position where these was found. The osteoderms of the morphotype 1 are hexagonal to square in shape. These osteoderms are moderately big in size (between 1-3 cm of diameter). During preparation, such elements were found in the area posterior to the orbit and separated of the skull by approximately 10-15 centimeters. The morphotype 2 is characterized by irregular and ameboidal elements and, in some cases, with small projections or processes. The size of these osteoderms is smaller than morphotype 1 (between 0.5-1.5 cm). These osteoderms were found close to the snout and adjacent to the surface of the bone. On the other hand, a few elements were similar to morphotype 1 but were slightly bigger in size and exhibits a weak rosette morphology, typical of glyptodonts. The position of these latter is not certain, but we interpreted as slightly posterior to morphotype 1. We observed at least one intermediate osteoderms between two morphotypes, being big in size but irregular in shape. The position of this element is uncertain. We assign all these elements to facial osteoderms giving the position where these was placed. Also, we discard that these can pertain to the cephalic shell due to the preservation of a complete one in MCHP-001. In this sense, morphotype 1 was found separated from the bone and placed posterior to the orbit. Interestingly, this area houses large masticatory muscles resulting in a deep space separating bone from skin. Similarly, morphotype 2 was adjacent to the bone and placed close to the snout. This is related with the lack of big muscles packs close to snout, resulting in a thin separation between skin and bone. All these reasons show us that in MCHP-001 all these osteoderms follow the contour of the skin of the individual, similar to the reported for Glyptodon.