Abstract Mammals of the Xenarthra clade show a large number of unusual characters in the skeleton, mainly in the vertebral column. In spite of the importance of the knowledge on the axial skeleton in this group, there are no detailed studies on the morphology of the entire vertebral column. Here we performed a comprehensive study of the vertebral column of Chaetophractus villosus (Desmarest, 1804), a representative of Chlamyphoridae, in order to provide a more reliable comparative framework among armadillos. Morphological description was based on 44 adult postcranial axial skeletons. As a complement to the morphological descriptions of the skeleton we studied the paths followed by blood vessels and nerves in close relationship to the axial skeleton (using 13 fresh adult specimens, six females and seven males, part of the doctoral Thesis of one of the authors). Intraspecific variability in the thoracolumbar number in C. villosus was also evaluated. We identified certain variability in the position of the first vertebral pair that bears a xenarthral facet, ranging from T4-T5 to T6-T7, being T5-T6 and T6-T7 the most frequent positions. The second pair of xenarthrales is set near the thoracolumbar limit, and ranges between T9-T10 and T11-L1 (T10-T11 in most specimens). The variability in the total number of thoracolumbars in C. villosus falls within the range of most mammals and sets controversy about an important morphological feature that groups extant Xenarthra and Afrotheria into the southern placentals (Atlantogenata). In this regard, a more comprehensive study of this character is necessary to elucidate the patterns of distribution of this trait among xenarthrans.

Here we described the ossification sequences of the limbs of Dasypus and Chaetophractus, analyzed them in a phylogenetic framework in order to identify heterochronic events. We used 58 specimens of D. hybridus, C. vellerosus, and C. villosus, and recorded ossification centers of the fore and hind limbs. In addition, we performed a phylogenetic mapping using event-pair cracking to identify sinapomorphic events (movements of ossification events in a sequence along each branch of a phylogeny). To perform this analysis, we included several non-xenarthran extant mammal taxa. The Cingulata clade is characterized by one delayed event (unciform ossification), and three events that moved to earlier positions (calcaneus, ectocuneiform, and cuboid). In Dasypus, 14 autapomorphic events were recorded, standing out the earlier ossification of the distal phalanges. In Chaetophractus six autapomorphic events are recorded, and the distal phalanges also ossified in earlier stages. In the carpal region the largest elements would ossify earlier, while smaller ones would do so in later stages (e.g., pisiform and magnum in Chaetophractus). The pattern is similar in the tarsal and phalangeal regions. In Dasypus the fifth manual digit is highly reduced and ossifies rather late in the ontogeny. Some events in other taxa follow the same pattern (e.g., the fibula of Equus). The autopodium of mammals could be divided in compartments, and the ossification sequence would correlate with size within each compartment, starting the ossification in larger elements.

El objetivo de este trabajo es caracterizar las secuencias de osificación de las extremidades de Dasypus y Chaetophractus y analizarlas en un marco filogenético, con el fin de identificar eventos heterocrónicos en el grupo. Se procesaron 58 especímenes de Dasypus hybridus, Chaetophractus vellerosus y C. villosus y se registraron los centros de osificación de los miembros anteriores y posteriores. Además, se realizó un mapeo filogenético mediante event-pair cracking, para identificar eventos sinapomórficos (movimientos de los eventos de osificación en la secuencia a lo largo de las ramas de una filogenia). Para ello se incluyeron varios taxones de mamíferos actuales no xenartros. El clado Cingulata se caracteriza por un retardo (osificación del unciforme) y tres adelantamientos (calcáneo, ectocuneiforme y cuboides). Los eventos autapomórficos de Dasypus son 14, destacándose los adelantamientos de las falanges distales. En Chaetophractus son seis y también las falanges distales se adelantan en la secuencia. En la región carpal los elementos de mayor tamaño relativo comenzarían a osificar más temprano, mientras que aquellos de menor tamaño lo harían en etapas más tardías (e.g., pisiforme y magno en Chaetophractus). En la región tarsal y las falanges sucede algo similar. En Dasypus el quinto dígito manual está muy reducido y osifica tardíamente. Algunos eventos en otros taxones seguirían este mismo patrón (e.g., fíbula de Equus). En el autopodio de los mamíferos podría existir una compartimentalización, en la que la trayectoria de osificación obedecería esencialmente al tamaño de los elementos cartilaginosos que conforman el compartimiento, comenzando por los elementos más grandes.