© 2020 The Linnean Society of London, Zoological Journal of the Linnean Society, 2021, 192, 1237–1252 1237 Zoological Journal of the Linnean Society, 2021, 192, 1237–1252. With 7 figures. Dental enamel structure in long-nosed armadillos (Xenarthra: Dasypus) and its evolutionary implications MARTÍN R. CIANCIO 1,2*, , EMMA C. VIEYTES 2,3 , MARIELA C. CASTRO 4 and ALFREDO A. CARLINI 1,2 1 Laboratorio de Morfología Evolutiva y Desarrollo (MORPHOS) y División Paleontología Vertebrados, Museo de La Plata, CONICET, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Paseo del Bosque s/n, B1900FWA, La Plata, Argentina 2 Cátedra Anatomía Comparada, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Paseo del Bosque s/n, B1900FWA, La Plata, Argentina 3 División Zoología Vertebrados, Museo de La Plata, CONICET, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Paseo del Bosque s/n, B1900FWA, La Plata, Argentina 4 Departamento de Ciências Biológicas, IBiotec, Universidade Federal de Catalão, Avenida Dr. Lamartine Pinto de Avelar, 1120, 75704-020, Catalão, Brazil Received 20 September 2019; revised 29 July 2020; accepted for publication 12 September 2020 Most xenarthrans have a reduced and simplified dentition that lacks enamel. However, the presence of prismatic enamel has been recorded in the Eocene armadillos Utaetus buccatus (Euphractinae) and Astegotherium dichotomus (Astegotheriini). Among extant xenarthrans, the occurrence of enamel has been recognized only in the long- nosed armadillo, Dasypus novemcinctus (Dasypodinae), but its microstructure has never been described. In this contribution, we analyse the enamel microstructure in deciduous and permanent teeth of four Dasypus species. In deciduous molariform teeth of some species, we identify an apical cap of vestigial enamel (without crystalline structure), interpreted as an amorphous ameloblastic secretion. In permanent teeth, a thin layer of true enamel is found in the apical portion of unworn molariforms. The enamel is prismatic in D. novemcinctus, but in Dasypus hybridus, Dasypus sabanicola and Dasypus punctatus it is prismless. Taking into account the Eocene species of armadillos, the ancestral condition of enamel in cingulates could have been more complex (as in other placentals) and undergone progressive reduction, as shown in the Dasypus lineage. In light of previous genetic and developmental studies, we review and briefly discuss the processes that can account for the reduction/loss of enamel in extant and extinct armadillos. The retention of enamel and the fact that this genus is the only living xenarthran with two functional generations of teeth support the early divergence of the Dasypus lineage among living cingulates. This is in agreement with morphological and molecular analyses. ADDITIONAL KEYWORDS: Dasypodidae – development – evolution – prismatic enamel – teeth – vestigial enamel. INTRODUCTION Among placental mammals, reduction and/or simplification of teeth with loss of enamel is recorded in some lineages within major clades, such as Tubulidentata (Afrotheria), Pholidota and Cetacea (Boreoeutheria). However, Xenarthra (i.e. armadillos, sloths, anteaters and their fossil relatives) is a major clade characterized by that condition ( Davit-Béal et al., 2009). Their distinctive dentition is generally characterized by crown simplification and reduction in the number of teeth (McDonald, 2003). Concerning early eutherians, xenarthrans show mostly derived dental features, such as homodonty, euhypsodonty (dentine hypsodonty sensu Koenigswald, 2011) and teeth without enamel, differing markedly from most major clades of extant placental mammals, which are heterodont (I3/3, C1/1, PM4/4, M3/3 = 44 teeth), brachyodont, with complex enamel structure, a tribosphenic pattern of molar cusps and two dental *Corresponding author. E-mail: mciancio@fcnym.unlp.edu.ar Downloaded from https://academic.oup.com/zoolinnean/article/192/4/1237/5999221 by guest on 01 August 2022