Recent studies show Xenarthra to be even more isolated systematically from other placental mammals than traditionally thought. The group not only represents 1 of 4 primary placental clades, but proposed links to other fossorial mammal taxa (e.g., Pholidota, Palaeanodonta) have been contradicted. No unambiguous Paleocene fossil xenarthran remains are known, and Eocene remains consist almost exclusively of isolated cingulate osteoderms and isolated postcrania of uncertain systematic provenance. Cingulate skulls are unknown until the late middle Eocene, and the oldest sloth and anteater skulls are early Oligocene and early Miocene age, respectively; there are no nearly complete xenarthran skeletons until the early Miocene. Ecological reconstructions of early xenarthrans based on extant species and the paleobiology of extinct Neogene taxa suggest the group’s progenitors were myrmecophagous with digging and perhaps some climbing adaptations. The earliest cingulates were terrestrial diggers and likely myrmecophagous but soon diverged into numerous omnivorous lineages. Early sloths were herbivores with a preference for forested habitats, exhibiting both digging and climbing adaptations. We attribute the rarity of early xenarthran remains to low population densities associated with myrmecophagy, lack of durable, enamel-covered teeth, and general scarcity of fossil localities from tropical latitudes of South America. The derivation of numerous omnivorous and herbivorous lineages from a myrmecophagous ancestor is a curious and unique feature of xenarthran history and may be due to the peculiar ecology of the native South American mammal fauna. Further progress in understanding early xenarthran evolution may depend on locating new Paleogene fossil sites in northern South America. Los estudios sistemáticos recientes muestran que, a nivel sistemático, los xenartros están aún más aislados de otros mamíferos placentarios de lo que se pensaba tradicionalmente. El grupo no sólo representa una de las cuatro ramas principales de los Placentalia, sino que también se han refutado las hipótesis previas de posibles conexiones con otros taxones de mamíferos fosoriales (por ejemplo Pholidota, Palaenodonta). No se conocen restos fósiles inequívocos de xenartros del Paleoceno y los restos provenientes del Eoceno consisten casi exclusivamente de osteodermos aislados de cingulados y restos postcraneanos aislados de origen sistemático incierto. No se conocen cráneos razonablemente completos de cingulados hasta finales del Eoceno medio; los cráneos más antiguos de perezosos y osos hormigueros provienen del Oligoceno temprano y del Mioceno temprano, respectivamente; y no existen esqueletos completos o casi completos de ninguno de los 3 linajes hasta el Mioceno temprano. Reconstruimos la ecología de los primeros xenartros basándonos en las especies actuales y lo que se sabe de la paleobiología del Mioceno y de los taxones extintos más recientes. Nuestros resultados sugieren que los primeros xenartros eran mirmecófagos y poseían adaptaciones para cavar y tal vez para trepar. Los primeros cingulados eran cavadores terrestres y probablemente mirmecófagos, pero pronto divergieron en numerosos linajes omnívoros. Nuestras reconstrucciones indican que los primeros perezosos eran herbívoros con preferencia de hábitats boscosos, tal vez exhibiendo adaptaciones tanto para cavar como para trepar. Atribuimos la rareza de restos de los primeros xenartros a varios factores: bajas densidades poblacionales asociadas a hábitos mirmecófagos; falta de dientes duraderos y cubiertos de esmalte; y una escasez general de localidades de mamíferos tempranos de las latitudes tropicales de América del Sur. La derivación de numerosos linajes omnívoros y herbívoros de un ancestro mirmecófago es un rasgo curioso y único de la historia de los xenartros y puede deberse a la peculiar ecología de la fauna de mamíferos sudamericanos. Los nuevos avances en la comprensión de la evolución temprana de los xenartros podrían depender de la localización de nuevos sitios fósiles paleógenos en áreas de tierras bajas poco accesibles del norte de América del Sur.
Armadillos comprise a particular group of armoured animals whose functional morphology of locomotion remains unclear. For the first time, the kinematic patterns of Dasypus novemcinctus are analysed. Eight specimens of nine‐banded armadillos were studied at a research institute in São Paulo State, Brazil. The individuals were induced to cross a horizontal corridor and each gait performed during the time each of them was kept inside this structure was recorded to a detailed analysis posteriorly performed in a computer program. Four parameters regarding speed range were considered: stride frequency (Hz) (1/stride period), stride length (m), speed (ms −1 ) and duty factor (%). A total of 89 strides have been analysed among symmetrical (60.6%) and asymmetrical gaits (39.4%), and six footfall patterns were here reported as follows: lateral sequences (symmetrical), transverse gallop, canter, bound, half‐bound and crutch walk (asymmetrical). This kind of analysis implements our knowledge on the locomotory aspects of these animals, hence contributing to the improvement of our knowledge on this still poorly known group.
Recently, dental microwear analysis has been successfully employed to xenarthran teeth. Here, we present new data on use wear features on 16 molariforms of Orophodon hapaloides and Octodontotherium grande. These taxa count among the earliest sloths and are known from the Deseadan SALMA (late Oligocene). Modern phylogenetic analyses classify Octodontotherium and Orophodon within Mylodontoidea with whom they share lobate cheek teeth with an outer layer of cementum and a thick layer of orthodentine. Similar target areas of 100μm2 were analyzed on the orthodentine surface of each tooth by stereomicroscopic microwear and by SEM microwear. Results were unlike those of extant sloths (stereomicroscopic microwear: Bradypus, Choloepus) and published data from fossil sloths (SEM microwear: Acratocnus, Megalonyx, Megatherium, Thinobadistes); thus, both approaches independently indicate a different feeding ecology for the Oligocene taxa. The unique microwear results suggest that both taxa fed on plant material with low to moderate intrinsic toughness (foliage, twigs) but also proposes intake of tougher food items (e.g., seeds). Frequent gouging of the tooth surfaces can be explained by exogenous influence on microwear, such as possible intake of abrasive grit. We suggest an unspecialized herbivorous diet for Octodontotherium and Orophodon utilizing diverse food resources of their habitat. These interpretations support the reconstruction of (1) Deseadan environments as open habitats with spreading savannas/grasslands and (2) both taxa as wide-muzzled bulk feeders at ground level.
Here, we review population genetic, phylogeographic, and phylogenetic studies on xenarthrans and show how this information fits in current discussions about patterns of diversification within the Neotropics. Specifically, we focus on how the genetic diversity of xenarthrans has been shaped by important historical processes such as Andean uplift, the Great American Biotic Interchange, and fluctuating linkages between the Amazon and Atlantic forests. We also describe latitudinal patterns of differentiation within the Atlantic forest and discuss how these might have been generated. Even with the modest amount of information currently available, our comparative analyses indicate 3 things: the Andes may have promoted events of intraspecific divergence for at least 2 xenarthran species; the biogeographic history of the Neotropical rain forests influenced the divergence of clades in sloths; and inter- and intra-specific genetic patterns reveal a very high diversity in xenarthrans, probably higher than currently recognized from morphological data. Finally, we highlight Xenarthra as an appropriate model for investigating biogeographic patterns in the Neotropics and also point to additional directions to be taken in future studies of this unique mammal group. Revisamos los estudios realizados sobre genética de poblaciones, filogeografía y filogenia de los Xenarthra y mostramos como la información generada se ajusta a la discusión actual acerca de los patrones de diversificación en el Neotrópico. Enfocamos nuestro análisis en la influencia de 4 eventos pasados importantes relacionados con la dinámica del paisaje Neotropical y la diversificación de la fauna, sobre la diversidad genética de los Xenarthra; estos son el levantamiento de los Andes, el gran intercambio de fauna del Pleistoceno, la relación entre el bosque Amazónico y el bosque Atlántico y los patrones latitudinales de diferenciación en el bosque Atlántico. Aunque la información disponible aún es escasa, nuestro análisis comparativo indica que los Andes pudieron haber promovido eventos de diversificación intraespecífica, al menos en 2 especies de Xenarthra; que la historia biogeográfica del bosque húmedo Neotropical tuvo influencia sobre la divergencia de linajes de perezosos; y que los patrones genéticos intra e interespecíficos revelan una gran diversidad en Xenarthra, probablemente mayor que la actualmente reconocida con datos morfológicos. Finalmente, resaltamos a los Xenarthra como un modelo apropiado para investigar patrones biogeográficos en el Neotrópico y sugerimos líneas adicionales a considerarse en futuros estudios sobre este grupo único de mamíferos.
ABSTRACT Increased field collecting over the last few years, combined with the examination of historical collections in Uruguay, has resulted in the discovery of a great number of specimens of the Scelidotheriinae, indicating that this subfamily is better represented in Uruguay's Pleistocene fauna than previously thought. Because much of this new material is diagnostic, in this work we provide a fuller description of some specimens (a skull with associated mandible and the manus, another almost-complete skull, and two partial dentaries) from the late Pleistocene of Uruguay for which only preliminary descriptions have been previously made, with tentatively assignments to Catonyx. A discriminant analysis was performed using 48 adult specimens including Scelidotherium, Catonyx, and Proscelidodon and supports the contention that Catonyx is a valid genus and the inclusion of these new specimens within this genus. This analysis also allows us to identify those cranial characters that better differentiate the ge...
Arthritic lesions have been frequently diagnosed in the fossil record, with spondyloarthropathy (a type of erosive and panmammalian arthritis) being one of the most common types described to date for mammals, though not restricted to this group. Here, we identify spondyloarthropathy in fossil bones from the late Pleistocene in Brazil assignable to a large glyptodont individual. Bone erosions in the peripheral joints (viz., the ulna, radius, left femur and tibiae-fibulae) associated with osteosclerosis allow the diagnosis of spondyloarthropathy. The presence of osteophytes in seven bones of the forelimbs (viz., the ulna and radius) and hind limbs (viz., the tibiae-fibulae, left femur and patellae) and a subchondral cyst in one element (viz., the left femur) indicate secondary osteoarthritis. A calcified deposition on the articular surface of the left patella indicates the presence of calcium pyrophosphate deposition disease, which, like the observed osteoarthritic alterations, likely represents a complication of spondyloarthropathy. This is the first report of spondyloarthropathy for xenarthrans.
To investigate the evolution of xenarthran epaxial muscles, fresh specimens of the North American Common long-nosed armadillo Dasypus novemcinctus and of a marsupial, the Virginia opossum Didelphis virginiana, were dissected. Data from one fixed specimen of a two-toed sloth Choloepus didactylus were also used for comparison, because it is a xenarthran exhibiting a highly derived locomotor mode. The opossum was used to represent a more generalized mammalian condition. Each of the three mammalian epaxial muscle groups, the iliocostalis, longissimus dorsi, and transversospinalis, was removed and its mass was determined. All data were corrected for body mass and length. Unpaired, one-tailed t-tests showed the average mass of the iliocostalis and transversospinalis of Dasypus to be significantly larger than the mass of the same muscles in Didelphis, whereas the average mass of the longissimus dorsi was not statistically different between the two species. In agreement with pronounced lateral bending and de-emphasized dorso-ventral flexion and extension, Choloepus also had a relatively large iliocostalis and small longissimus. Our limited data suggest that this condition was inherited from non-arboreal and probably digging early xenarthrans. We believe the relatively larger iliocostalis and transversospinalis muscles in Dasypus can be attributed to the need to provide vertical stabilization of the trunk and resist lateral reaction forces generated by digging. Thus, for Xenarthra it represents a synapomorphy linked to adaptations for fossoriality.
Numerous climatic fluctuations occurred during the Cenozoic (last 66 Ma BP); some of them were drastic (e.g., during the Eocene-Oligocene boundary) while others were more gradual (e.g., late Tertiary cooling), but both have deep effect on the biotas. Armadillos are exclusively from the Americas; they have an old evolutionary history in South America and faunal replacement and/or local extinctions were detected, linked with climatic fluctuations. The global cooling of the late Eocene - early Oligocene coincides with a well-documented faunal turnover of Dasypodinae by Euphractinae in Patagonia. During cold and arid periods of the Quaternary, Euphractinae and Tolypeutinae moved more than once to the eastern Pampean Region, and Dasypodinae moved northward to central Brazil or even further north to the Guyana Region. During interglacial periods some armadillos went extinct locally and/or moved to Patagonia (Zaedyus), central Argentina (Tolypeutes matacus, Chaetophractus vellerosus), or from the north to Mesopotamia and the Pampean Region (Dasypus). Since the end of the Pleistocene/early Holocene, human activity has strongly impacted armadillo populations. Currently, the eastern Pampean Region (Argentina) is characterized by the presence of the couple C. villosus - D. hybridus (probably established since the late Holocene), but during the Pleistocene was Z. pichiy – T. matacus while Z. pichiy - C. villosus characterized early-middle Holocene. This work serves as evidence that paleozoological studies can be used to assess responses of biological systems to large scale perturbations and is the basis for studying future species distributions, in order to identify species in danger of extinction and establish management actions.
The presence of osteoderms in the skin of some extinct sloths and in cingulates (armadillos, pampatheres, and glyptodonts) has often been considered a pleisomorphic character of the Xenarthra. While osteoderms are known from the earliest cingulates, they are absent in most sloths including the two extant taxa and only appear late in their fossil record. Osteoderms are currently only reported from five genera of mylodonts and two megatheres, out of the over 100 currently recognized genera of sloths. Consequently, rather than a plesiomorphic character of the Xenarthra, which has been secondarily lost in sloths, it is more likely that osteoderms in sloths are the result of parallel evolution to the cingulates that independently evolved in one, possibly two different sloth clades.