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.
Xenarthrans possess a suite of characteristics that make them among the most unusual of mammals. Understanding the functional significance of these traits is one prominent reason why xenarthrans matter. In addition, Xenarthra is currently considered one of the basal clades of placental mammals, and the only one to originate in South America. Consequently, studies of xenarthrans can provide important insights into the evolution of early placentals. The fossil record contains hundreds of recognized species of xenarthrans but this rich evolutionary history is currently distilled into just 31 extant species. Preserving this heritage through various conservation initiatives is yet another reason why xenarthrans matter. This Special Feature on xenarthrans provides an overview of current work and identifies many areas requiring further study. It is our hope that this Special Feature will raise the profile of xenarthrans among mammalogists and perhaps entice some to consider addressing one or more of the many lingering questions that remain about this enigmatic group.
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.
Armadillos (Cingulata: Dasypodidae and Chlamyphoridae) are the only mammals bearing an osseous carapace. Most mammals have rapid growth rates and require high mineral levels. However, armadillos feed mainly on insects, a poor source of calcium and phosphate. This raises the question how lactating females obtain the needed minerals to provide their offspring sufficient quantities for the development and hardening of their carapace. The objective of this study was to evaluate whether there are differences in the mineral density of the carapace and endoskeleton of reproductive (especially lactating) and non-reproductive female armadillos, and those of males. We performed computed tomography of 32 female and 13 male dead pichis (Zaedyus pichiy) of different age classes and reproductive status and compared bone mineral density (BMD) of their marginal scutes, compact and spongy bone of the carapace, and femoral bone. Juveniles of both sexes had significantly lower BMD than adults. Average BMD values of the marginal scutes, compact and spongy bone of the carapace, femoral head, spongy bone, and femoral cortical bone of lactating females were similar to those of adult males and significantly higher than values of adult females without signs of gestation or lactation. We conclude that lactating females do not seem to lose bone density, at least from the carapace and femur, during lactation. However, our results suggest that a high BMD may be necessary to support reproduction in female pichis.
Armadillo pups rapidly grow a bony carapace, suggesting a large transfer of calcium and phosphorus from mother to pups via milk. Because Ca and P in milk are bound in casein protein micelles, we predicted armadillo milk to be high in protein content. Milk samples (n = 25) from 10 lactating nine-banded armadillos (Dasypus novemcinctus) collected at days 1-6, 14-15, 33-38, and 49-51 after birth were assayed for macronutrient composition (water, fat, protein, sugar, ash, Ca, and P). Gross energy (GE) was calculated from protein, sugar, and fat. Protein concentration (8-11%) was the highest of the milk solids at all time points, and ash (total mineral) values increased from 1.6% to 3.6% and were higher than sugar values (2.4%) after 1 month. Calcium concentration increased from 0.4% to over 1.0% by 1 month of lactation. Calcium and phosphorus were strongly correlated with milk protein (r = 0.796 and 0.815, respectively; P < 0.001). Protein contributed the majority of milk GE over the first 2 weeks of lactation (51%) and was equal to fat after 1 month (both about 47% of GE). The proportion of GE from sugar declined from 14.5% to 6.6%. We suggest that a milk high in protein was an ancestral trait for armadillos that allowed the evolution of a bony carapace by enabling large amounts of Ca and P to be transferred via casein micelles. Based on data from giant anteater (Myrmecophaga tridactyla) milk, a high-protein milk may be ancestral to all Xenarthrans, but this suggestion awaits confirmation from data on the milk of sloths. We suggest that armadillo pups likely catabolize a significant amount of milk protein for metabolic energy.
We present a survey of the morphological diversity of the bony labyrinth of the inner ear in Xenarthra, including the fossil ground sloth Megatherium. Using a combination of traditional and geometric morphometrics, correlation analyses, and qualitative observations, we attempt to extract independent and informative phylogenetic characters of the bony labyrinth for the superorder. Geometric morphometric analyses demonstrate a strong imprint of phylogenetic history on the shape of the bony labyrinth of xenarthrans and a weak influence of allometry. Discrete characters mapped on a consensus cladogram for xenarthrans show support for many traditional nodes within the superorder and may also provide critical information for problematic nodes within Cingulata. A relatively large lateral semicircular canal may, for instance, represent a synapomorphy for the molecular clade allying fairy armadillos (Chlamyphorinae) to the Tolypeutinae. Striking convergences were detected when comparing Megatherium, the giant ground sloth, with extant armadillos and Chlamyphorus, the pink fairy armadillo, with the extant three-and two-toed sloths. These findings have the potential to help understand the phylogenetic relationships of fossil xenarthrans.
A detailed analysis of 1,039 scientific studies of extant armadillos (Xenarthra: Cingulata, Dasypodidae) published in the last 25 years (1989-2013) revealed substantial biases in coverage, including taxonomically, the locales where field studies were conducted, and in the topics investigated. Examination of the number of other publications that cited each paper revealed that 470 (45%) papers had been cited no more than 10 times, 249 (24%) had never been cited, and 112 (11%) were not even found in the Google Scholar database. The most heavily cited papers were molecular phylogenetic analyses that often used tissues from one or more species of armadillo but were not about the animals per se. Thus, it appears that research on armadillos is plagued by numerous gaps in coverage and is not reaching a wide audience. These data indicate obvious opportunities for future research. In addition, recent findings suggest that even relatively well-studied phenomena may require reexamination. Here, we review recent advances in the study of armadillos and highlight promising areas for future work. One critical need is for a thorough systematic revision of Dasypodidae to be completed. This will make it possible to prioritize those species and populations most in need of study. Additionally, more long-term field studies of populations of marked individuals are required. Although there are many important and interesting questions waiting to be answered, the small number of researchers currently conducting studies of armadillos, particularly in the wild, means that progress will be slow.
Hairy armadillos constitute an ecologically homogeneous and morphologically similar group with currently 5 species classified in the subfamily Euphractinae. Among them, the Andean hairy armadillo Chaetophractus nationi (Xenarthra, Cingulata, Dasypodidae) is a small, endangered armadillo that has long been suspected to represent a high-altitude variant of Chaetophractus vellerosus. Here, we report the 1st phylogenetic systematics assessment of hairy armadillos using morphological and molecular analyses of all described species with focus on the status of the Andean hairy armadillo. Multivariate analyses of shape variation based on 3-dimensional landmark coordinates of skulls allowed a clear differentiation of each species with the exception of C. vellerosus and C. nationi, within which only a latitudinal and/or altitudinal gradient in size was apparent. Moreover, analyses of mitochondrial DNA control region (D-loop) revealed a single C. nationi haplotype that appeared to be identical with a C. vellerosus haplotype from Argentina. Identical sequences in C. vellerosus and C. nationi were also observed for 3 of the 5 non-coding nuclear markers investigated. Based on these data, we propose that C. nationi should be considered as a synonym of C. vellerosus. However, this taxonomic change should not preclude the protection of the high-altitude Bolivian populations that are steadily declining because of their overexploitation for traditional purposes. Finally, phylogenetic analyses of euphractine armadillos based on a combination of 6 non-coding nuclear markers and 2 nuclear exons suggest the paraphyly of the genus Chaetophractus, with C. vellerosus being more closely related to Zaedyus pichiy than to C. villosus.