The Extent of Metaplastic Hard Tissue Within the Appendicular Skeleton of the Nine‐Banded Armadillo (Dasypus novemcinctus)

Abstract

The developmental and functional advantages of vertebrate metaplastic hard tissue is still relatively unclear. Metaplastic tissue forms from the direct transformation of one cell type to another. Specifically, mineralization of preformed dense connective tissue results in intratendinous metaplastic tissue, and has been described in a variety of taxa and elements including turtle shell, reptilian osteoderms, sturgeon scutes, dinosaur cranial and vertebral elements, ossified tendons, portions of bird skeletons, and tendinous entheses in mammalian bone. However, histological analysis of the extent of metaplastic hard tissues, both across Mammalia and within individual mammalian bones, is lacking. Here we serially sectioned both tibiae of a nine‐banded armadillo, Dasypus novemcinctus [Xenarthra: Cingulata], transversely and longitudinally to identify the existence and possible patterns of metaplastic hard tissue. All skeletal elements were processed using standard procedure for the preparation of undecalcified bone. We found evidence of metaplastic hard tissue in the tibial crest, as well as the medial side of the diaphysis and the proximal lateral side of the tibia. The metaplastic tissue in the tibial crest appears interstitially within a primary woven bone matrix. Along the lateral side, metaplastic tissue forms a thin patch in the mid‐cortex superficial to compact coarse cancellous bone. The presence of metaplastic hard tissue seems consistent with tendinous insertions through fibrous entheses along the length of the tibia and tibial crest. The entrapment of metaplastic tissue within the tibia is likely due to appositional bone growth at the enthesis. However, the presence of metaplastic hard tissue on the medial side of the mid‐diaphyseal and proximal portion of the tibia does not appear to be associated with any tendinous insertions. Previously, unassociated metaplastic tissue was found extensively in osteoderms, ankylosaur tail clubs, and dinosaur cranial ornamentation such as the parietal frill of Triceratops. Study of metaplastic hard tissue in modern bone, independent of tendinous insertions, can improve our understanding of its potential biomechanical advantages and thereby possibly provide clues to its use in ornamentation and within the appendicular skeleton. Metaplastic hard tissues have likely been misidentified and understudied in previous investigations of modern hard tissue microstructure. Additionally, because Xenarthra is proposed to be one of the four original clades of placental mammals, analysis of metaplastic tissue in the skeleton of D. novemcinctus could have implications for early mammalian bone development patterns.Support or Funding InformationPartial funding was provided by Montana State University Undergraduate Scholar’s ProgramFigure 1