Osteoderm histology of the Pampatheriidae (Cingulata, Xenarthra, Mammalia): Implications for systematics, osteoderm growth, and biomechanical adaptation

Abstract

Pampatheres are extinct, large-bodied cingulates, which share morphological characters with both armadillos and glyptodonts but are considered to be more closely related to the latter. The osteoderm histology of six pampathere taxa was examined and compared to the histology of other cingulate osteoderms. This study investigates the development and functional adaptation of pampathere osteoderms as well as the phylogenetic relationships of the Pampatheriidae within the Cingulata. We found that pampathere osteoderms share a uniform histological organization based on a basic diploe-like structure. After initial stages of intramembranous growth, metaplastic ossification, that is, the direct incorporation and mineralization of pre-existing protein fibers, plays an important role in osteoderm development and provides information on various kinds of soft tissue otherwise not preserved. The latest stages of osteoderm growth are dominated by periosteal bone formation especially in the superficial cortex. Movable band osteoderms show regular arrangements of incorporated fibers that may increase the resistance of particularly weak areas against strain. The histological composition of pampathere osteoderms is plesiomorphic in its basic structure but shows a number of derived features. A unique array of Sharpey's fibers that are incorporated into the bone matrix at sutured osteoderm margins is interpreted as a synapomorphy of pampatheres. The arrangement of dermal fibers in the deep and superficial cortexes supports the close relationship between pampatheres and glyptodonts.

A new cingulate (Mammalia: Xenarthra),Pachyarmatherium brasiliensesp. nov., from the Late Pleistocene of Northeastern Brazil

“…Together with previous work (Hill 2006;Wolf 2007;Krmpotic et al 2008;Wolf et al 2011), our study shows that osteoderm microstructure may hold relevant information regarding the relationships of cingulates, and provide charac-ters diagnostic at the species level. Our results may support the exclusion of Pachyarmatherium from glyptodonts, based on the absence of histological features present in both basal (Propalaehoplophorus) and derived glyptodonts (extensive bone remodeling, fibers arranged in large bundles, and undeveloped layers of compact bone relative to the region of trabecular bone).…”

“…3B). In contrast to the extensive bone remodeling typical of derived glyptodonts (Wolf 2007;Wolf et al 2011), bone remodeling in Pachyarmatherium is limited, as also seen in dasypodids. Finally, Pachyarmatherium differs from pampatheres in lacking collagen fiber bundles organized into broad, decussating strands at the margin of the osteoderm (see Wolf 2007).…”

“…With the exception of MCC 1089V, which represents a carapace fragment of P. jaguaribensis, they comprise uncatalogued, isolated, eroded osteoderms found in association with catalogued material (other osteoderms and, in some cases, postcrania) held at MCC. We decided to restrict our analysis to Brazilian glyptodontids and Pachyarmatherium because their histological features have not yet been described, while both Dasypodidae and Pampatheriidae were the subject of recent studies (Hill 2006;Krmpotic et al 2008;Wolf et al 2011) The histological sections were deposited at the fossil mammal collection of the Departamento de Geologia (UFRJ) as UFRJ-DG 491-M (Panochthus cf. greslebini), UFRJ-DG 492-M (Pachyarmatherium brasiliense), UFRJ-DG 493-M (Glyptotherium sp.…”

“…Osteoderms show a high degree of morphological and histological diversity (Hill 2006;Wolf 2007), and their external morphology has repeatedly been used in cingulate systematics (Ameghino 1889;Hoffstetter 1958;Paula Couto 1979;Carlini and Scillato-Yané 1996). By contrast, osteoderm microstructure had until recently received relatively little attention (but see Fernandez 1931), although the past five years have been marked by an increased interest in their histology (e.g., Hill 2006;Wolf 2007;Chavez-Aponte et al 2008;Krmpotic et al 2008;Wolf et al 2011), which has provided important insights into cingulate ontogeny and phylogeny (Chinsamy 1997;Krmpotic et al 2008).…”

Los Peltephilidae (Mammalia, Xenarthra) de la Formación Collón Curá (Colloncurense, Mioceno médio), Argentina

“…Para Ciancio (2010), Peltephilus constituye un clado junto con Machlydotherium . Esta posición basal de Peltephilus también es apoyada por los resultados del estudio paleohistológico de sus osteodermos (Wolf, 2007).…”