Comparative anatomy and histology of xenarthran osteoderms

Abstract

Reconstruction of soft tissues in fossil vertebrates is an enduring challenge for paleontologists. Because inferences must be based on evidence from hard tissues (typically bones or teeth), even the most complete fossils provide only limited information about certain organ systems. Osteoderms ("dermal armor") are integumentary bones with high fossilization potential that hold information about the anatomy of the skin in many extant and fossil amniotes. Their importance for functional morphology and phylogenetic research has recently been recognized, but studies have focused largely upon reptiles, in which osteoderms are most common. Among mammals, osteoderms occur only in members of the clade Xenarthra, which includes armadillos and their extinct relatives: glyptodonts, pampatheres, and, more distantly, ground sloths. Here, I present new information on the comparative morphology and histology of osteoderms and their associated soft tissues in 11 extant and fossil xenarthrans. Extinct mylodontid sloths possessed simple, isolated ossicles, the presence of which is likely plesiomorphic for Xenarthra. More highly derived osteoderms of glyptodonts, pampatheres, and armadillos feature complex articulations and surface ornamentation. Osteoderms of modern armadillos are physically associated with a variety of soft tissues, including nerve, muscle, gland, and connective tissue. In some cases, similar osteological features may be caused by two or more different tissue types, rendering soft-tissue inferences for fossil osteoderms equivocal. Certain osteological structures, however, are consistently associated with specific soft-tissue complexes and therefore represent a relatively robust foundation upon which to base soft-tissue reconstructions of extinct xenarthrans.

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

“…3A 3 ). In cross section, the osteoderm resembles turtle shells (e.g., Zangerl 1969) in being composed of well-developed superficial and deep layers of compact bone (the inner and outer tables) enclosing a central region of trabecular bone, as previously observed by Hill (2006). The superficial layer of compact bone contributes about 25% of the total thickness of the osteoderm (Fig.…”

“…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.…”

“…While all osteoderms were sectioned and analyzed along both their anteroposterior and transverse axes, only transverse sections were photographed. Nomenclature follows Hill (2006) and Porpino et al (2009). The classification of glyptodonts in the Systematic paleontology section follows Fernicola (2008 (2006) and Porpino et al (2009).…”

“…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).…”

Sauropod dinosaur osteoderms from the Late Cretaceous of Madagascar

“…However, in light of the fragile nature of hollow titanosaur osteoderms, some of the perceived losses of osteoderms in titanosaur evolution are more likely attributable to taphonomy. With regard to potential function, osteoderms are often assumed to have a protective role, but this is unlikely for Rapetosaurus osteoderms, because they do not form the imbricated 'armour' observed in other vertebrates [1][2][3]16,[22][23][24] . Moreover, protective benefits would seemingly decrease in the vacuous, thin-walled adult structures.…”