Osteoderm morphology and development in the nine‐banded armadillo, Dasypus novemcinctus (Mammalia, Xenarthra, Cingulata)

Abstract

Among modern mammals, armadillos (Xenarthra, Cingulata) are the only group that possesses osteoderms, bony inclusions within the integument. Along the body, osteoderms are organized into five discrete assemblages: the head, pectoral, banded, pelvic, and tail shields. The pectoral, banded, and pelvic shields articulate to form the carapace. We examined osteoderm skeletogenesis in the armadillo Dasypus novemcinctus using serial and whole-mount histochemistry. Compared with the rest of the skeleton, osteoderms have a delayed onset of development. Skeletogenesis begins as condensations of osteoblasts secreting osteoid, localized within the papillary layer of the dermis. Osteoderm formation is asynchronous both within each shield and across the body. The first osteoderms to mineralize are situated within the pectoral shield of the carapace, followed by elements within the banded, head, pelvic, and tail shields. In general, within each shield ossification begins craniomedially and proceeds caudally and laterally, except over the head, where the earliest elements form over the frontal and parietal bones. The absence of cartilage precursors indicates that osteoderms are dermal elements, possibly related to the all-encompassing vertebrate dermal skeleton (exoskeleton). The mode of development of D. novemcinctus osteoderms is unlike that described for squamate osteoderms, which arise via bone metaplasia, and instead is comparable with intramembranously derived elements of the skull.

Sauropod dinosaur osteoderms from the Late Cretaceous of Madagascar

“…The shape and surface texture of Rapetosaurus osteoderms indicates that these elements were low in relief and embedded in the integument as in other vertebrates [1][2][3]14,16 . As evidenced from extant taxa, all osteoderms form and reside within the dermis, either adjacent to the interface between the strata superficiale and compactum, or exclusively within the stratum superficiale, and are overlain by the epidermis [1][2][3][14][15][16][17] . The surface texture of Rapetosaurus osteoderms is consistent with the hypothesis that, as in crown group archosaurs, these structures were overlain by epidermal scales [1][2][3] .…”

“…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.…”

“…Osteoclastic remodelling of bone mineral is necessary for blood calcium homeostasis 15,[27][28][29] and increases seasonally 30 , during oogenesis 23 , and with increased age 13 in extant animals. Osteoderms are often associated with complex vascular networks, including numerous small diameter anastomozing arteries and veins within the stratum superficiale 3,14,16,22,25,26,31,32 . These vessels then join larger diameter subcutaneous arterioles and venules 22,25,26,32 .…”

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

“…1). Similarly, the external morphology of the osteoderms of Pachyarmatherium closely resembles that of dasypodine armadillos, such as Dasypus and Propraopus (see Ameghino 1889;Hill 2006;Vickaryous and Hall 2006), although it is also almost identical to that of the basal glyptodont Neoglyptatelus (Carlini et al 1997). Like the armadillos Dasypus (Hill 2006) and Chaetophractus (Krmpotic et al 2008), but unlike glyptodonts and pampatheres, Pachyarmatherium brasiliense is characterized by very thick superficial and deep layers of compact bone (Fig.…”

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

“…La relación entre los elementos epidérmicos (escamas epidérmicas) y los osteodermos, dentro de los Dasypodidae es variable (Simpson, 1948 más de una escama epidérmica por osteodermo, la fi gura central (= fi gura principal o "main fi gure" sensu Oliveira & Pereira, 2009;Porpino et al, 2010) posee su propia escama y las fi guras laterales poseen escamas compartidas por dos, tres o hasta cuatro osteodermos (Holmes & Simpson, 1931;Scillato-Yané, 1982;Wetzel & Mondolfi , 1979;Wetzel, 1985;Vizcaíno, 1994;Carlini et al, 1997;Vickaryous & Hall, 2006;Carlini et al, 2009). Algunos Euphractinae primitivos (i.e.…”