Osteoderm Histology of the Pampatheriidae (Cingulata, Xenarthra, Mammalia): Implications for Systematics, Osteoderm Growth, and Biomechanical Adaptation Dominik Wolf, 1 * Daniela C. Kalthoff, 2 and P. Martin Sander 3 1 Laboratory of Evolutionary Biology, Department of Anatomy, Howard University, Washington, DC 20059 2 Department of Paleozoology, Swedish Museum of Natural History, SE–104 05 Stockholm, Sweden 3 Division of Paleontology, Steinmann Institute, University of Bonn, Bonn 53115, Germany ABSTRACT Pampatheres are extinct, large-bodied cin- gulates, 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 inves- tigates the development and functional adaptation of pampathere osteoderms as well as the phylogenetic rela- tionships 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 incor- poration and mineralization of pre-existing protein fibers, plays an important role in osteoderm development and provides information on various kinds of soft tissue other- wise 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 osteo- derms 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 synapo- morphy of pampatheres. The arrangement of dermal fibers in the deep and superficial cortexes supports the close relationship between pampatheres and glyptodonts. J. Morphol. 273:388–404, 2012. Ó 2011 Wiley Periodicals, Inc. KEY WORDS: Pampatheriidae; systematics; biomechanics; osteoderm growth; Glyptodontidae INTRODUCTION The Pampatheriidae are an extinct group of Eocene to Pleistocene cingulates (e.g., Scillato- Yane ´, 1980; Iuliis et al., 2000; Scillato-Yane ´ et al., 2005) that demonstrate a number of armadillo-like characteristics (especially the movable bands within their carapace) and glyptodont-like features such as osteodentine cores in their cheek teeth (Engelmann, 1985; Gaudin and Wible, 2006; Kalthoff, 2011). The phylogenetic relationships of pampatheres have been controversial. Originally, the group was considered as a subfamily within the Dasypodidae, a taxon comprising all cingulates with an armadillo-like appearance (e.g., Flower, 1882; Simpson, 1945). Patterson and Pascual (1968) linked pampatheres and glyptodonts, mainly based on the morphology of the bony audi- tory region. This was later supported by Engel- mann (1985), who considered movable transverse bands a plesiomorphic feature of all armadillo-like cingulates, because these bands occur in some ba- sal glyptodonts in a rudimentary way (Gaudin and Wible, 2006). The Glyptodonta sensu Engelmann (1985) consequently comprised glyptodonts and pampatheres as well as the extinct eutatines; simi- larly, Carlini and Scillato-Yane ´ (1993) considered the Pampatheriidae and the Glyptodontidae as sis- ter taxa. Recently, Gaudin and Wible (2006) showed that pampatheres and glyptodonts are sis- ter taxa in a monophyletic group, based on sixteen unambiguous synapomorphies. Because an in- depth review of pampathere phylogeny is not available to date, we consider pampatheres a sepa- rate clade (Pampatheriidae) within the Cingulata (Fig. 1), following Edmund (1987, 1996) and Gau- din and Wible (2006). Whereas this view was al- ready widely accepted, taxonomy and relationships among individual pampathere taxa are still dis- cussed (e.g., Edmund, 1985b, 1987; McKenna and Additional Supporting Information may be found in the online version of this article Contract grant sponsor: Synthesys resources (European Commu- nity Research Infrastructure); Contract grant number: AT-TAF- 1220. *Correspondence to: Dominik Wolf, 520 W St. NW, WA 20059. E-mail: dwolf@bison.howard.edu Received 14 December 2010; Revised 4 September 2011; Accepted 12 September 2011 Published online 1 November 2011 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/jmor.11029 JOURNAL OF MORPHOLOGY 273:388–404 (2012) Ó 2011 WILEY PERIODICALS, INC.