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Session Schedule & Abstracts

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Sunday 3rd July, 2016

Symposium: Morphology & evolution of the Xenarthra 1

Room: Salon H   9:30 am–11:00 am

Moderator(s): S. Bargo & J. Nyakatura
XEN1-1  9:30 am  Recent progress and future prospects in fossil xenarthran studies. De Iuliis G*, University of Toronto
Abstract: The ICVM meetings have become near-regular gatherings for us to present our latest findings and share our ideas and thoughts. It is appropriate that we occasionally reflect on our recent efforts, certainly to recognize and celebrate our successes but also to consider where we might do better and plot out a path for future endeavors. One highlight since our first Symposium (ICVM 6, Jena) is that there are more of us. The need for producing "intellectual descendants" is keenly felt and strong efforts have been made toward this end, but some of us, including fairly active members, have produced few, if any, students (mainly due to institutional restrictions). In such cases, it is important to promote more active participation of such members in student committees. Our field efforts to recover fossil xenarthrans have also increased, leading to further systematic work. Such activity continues in more traditionally worked areas, but efforts have increased in regions traditionally less well known, the northwestern and generally older deposits of South America. Certainly this has yielded much new information. However, perhaps the most fruitful efforts over the past 15 years are those focusing on functional morphology and paleobiology, which have produced a wealth of new understanding. Among these, the La Plata Museum-Duke University expeditions stand out for their recovery of numerous remains that have promoted paleobiological studies while also allowing reconsideration of long-standing systematic issues, thus combining the main thrusts of our research. One pressing issue is the continued need to recognize the importance of variation among fossil remains. We have learned much from recent studies of extant xenarthrans and the recovery of large fossil collections in this regard, but have yet to apply it more consistently to our systematic work. Closer integration of studies on fossil and living xenarthrans should prove fruitful in this regard and for functional morphology.

XEN1-2  10:00 am  Potential distribution of fossil xenarthrans during the late Pleistocene. Varela L., Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay; Tambusso P.S., Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay; Di Giacomo M., Department of Art Conservation, University of Delaware, Delaware, USA; Patiño S.J., Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay; Fariña R.A.*, Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay
Abstract: Species distribution models (SDMs) are helpful for understanding actual and potential biogeographical traits of organisms. SDMs for the last interglacial (LIG), the last glacial maximum (LGM) and the Holocene climatic optimum (HCO) were generated for 11 South American late Pleistocene xenarthrans; five Cingulata, Glyptodon clavipes, Doedicurus clavicaudatus, Panochthus tuberculatus, Neosclerocalyptus paskoensis, Pampatherium humboldtii, and six Pilosa, Glossotherium robustum, Lestodon armatus, Mylodon darwinii, Catonyx cuvieri, Megatherium americanum and Eremotherium laurillardi. Co-occurrence records were studied based on the overlap of their generated areas of potential distributions and compared with the available biome reconstructions of South America during the LGM to analyze species distribution patterns, ecological requirements and possible interactions. Our results suggest that sloths could have had a large co-occurrence area mainly in the Chaco-Paraná and the plains in the Río de la Plata regions, and a smaller area mainly in northeastern Brazil. At least two cases of possible competitive exclusion might have occurred in tropical South America, between Megatherium and Eremotherium , and possibly between Glyptodon and Glyptotherium. In both cases, potential distributions overlap but no co-occurrence is recorded. Areas of high suitability were observed for submerged parts of the continental shelf exposed during the LGM, showing an overall increase in potential habitat compared to the LIG and HCO. This, coupled with warmer conditions, suggests a drastic reduction in total available areas of preferred habitat at the end of the Pleistocene for most taxa. When all potential distributions are considered together, a northern latitudinal shift could be observed between the LGM and HCO both in Cingulata and Pilosa.

XEN1-3  10:15 am  Phylogeny and historical biology of sloths. Tambusso PS*, Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay; Varela L, Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay; Patiño SJ, Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay; McDonald HG, Facultad de Ciencias, UniversBureau of Land Management, Utah State Office, Salt Lake City, Utah, USAidad de la República, Montevideo, Uruguay; Fariña RA, Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay
Abstract: After 34 Ma of evolutionary history in South America, sloths reached North America during the GABI, were part of the impressive Pleistocene megafauna and only Choloepus and the basal Bradypus survived the Quaternary extinction. Here several recently described taxa were added to a previous morphological data matrix. Bayesian phylogenetic inference and Tip-Dating were applied to 57 genera to investigate their diversity, morphological disparity and biogeographical history. The obtained phylogeny supports the commonly recognized clades but some taxa, both traditional and recently described, are placed in different families. Our results give divergence time estimates for the major clades, which cannot be dated with molecular methods. Diversity increased early until Middle Miocene and then remained steady until the Quaternary extinction. Two diversity drops are observed, at the end of the Santacrucian and at the end of the Huayquerian. Phenotypic rates were initially high, with the diversification of superfamilies Mylodontoidea and Megatherioidea, but declined ca. 24 My ago, followed by a slow increase in rates as families diversified. Similarly, morphological disparity showed an early increase and a stable phase during much of the Miocene followed later by an increase associated with familial diversification. Southern South America was the most probable area of origin and of early diversification. Both megatheriid and nothrotheriid basal nodes were strongly correlated with Andean uplift events, while the complex, somewhat unresolved mylodontid early history showed an early occupation of the northern regions of South America. Quaternary South American megalonychids have North American forerunners while Choloepus have Antillean ancestors. After a complex ecomorphological and biogeographical history, the Quaternary extinction left only two living small genera, phylogenetically distant though very similar in general morphology and tree-dwelling habits.

XEN1-4  10:30 am  Osteoderms in ground sloths: plesiomorphic or apomorphic? McDonald HG*, Bureau of Land Management
Abstract: Osteoderms occur in many tetrapods. In mammals osteoderms are only present in xenarthrans and for the Cingulata, armadillos, pampatheres and glyptodonts, form a distinctive feature, the carapace. The number of carapace osteoderms may exceed the number of bones of the skeleton and the earliest xenarthran records are based primarily on osteoderms, leading to the assumption that osteoderms are plesiomorphic in xenarthrans rather than a bias reflecting a common, easily preserved bone. If osteoderms are not considered but only the rest of the skeleton, the earliest records of both cingulates and pilosans occur at essentially the same time. In pilosans, osteoderms are only documented for a small number of mylodontine sloths and one species of megathere and have never been reported from megalonychids and nothrotheres. If osteoderms are pleisomorphic for xenarthrans they should be present in all the earliest sloth clades, and their absence should occur later in time with perhaps retention in only a few clades. This is not the case and all known osteoderm records in sloths occur later in time. It is proposed that osteoderms in a few closely related mylodontine sloths and a single megathere are instead neomorphs, and not derived from an ancestral cingulate.

XEN1-5  10:45 am  When development and paleontology meet: novel developmental data shed new light on the evolutionary history of the Xenarthra. Hautier L*, Institut des Sciences de l`Evolution de Montpellier, Université Montpellier, CNRS; Billet G, Sorbonne Universités, CR2P, UMR CNRS 7207, Univ Paris 06, Muséum national d’Histoire naturelle; Gomes Rodrigues H, Sorbonne Universités, CR2P, UMR CNRS 7207, Univ Paris 06, Muséum national d’Histoire naturelle; Oliver J, Harvard University, Museum of Comparative Zoology; Pierce SE, Harvard University, Museum of Comparative Zoology
Abstract: Some mammalian clades are quite novel to morphologists (e.g., tenrec-golden mole-paenungulate; hippo-whale) and would not have been recognized without the recent analyses of molecular data. In contrast, the mammalian clade Xenarthra has a long history in comparative anatomy. The superorder includes the armadillos, anteaters, and tree sloths and is one of the four major lineages of placental mammals recently defined. Since the 19th century, biologists have been fascinated by their morphological peculiarities and have made some intriguing observations on their development. However, previous analyses rarely provided a comparative basis upon which to analyse possible homologies with the morphology of other mammals. Our past developmental studies have focused on the sequence of ossification events during growth in the skull and skeleton, and have shown that xenarthrans display heterochronic shifts as compared to other placental mammals. Here we extend our work by focusing on the development of specific anatomical features: xenarthrous vertebral articulations in armadillos and caniniform teeth in sloths. While the condition of xenarthrous vertebrae has been known to researchers since the first descriptions of xenarthrans, there is still uncertainty regarding the function and development of the involved articulations, and intermediate vertebral conditions remain unknown in the fossil record. Our study provides a novel developmental perspective on the evolution of xenarthrous morphology. In terms of tooth development, our developmental data directly supports the claim that the sloth lower caniniform teeth are not homologous to canines of other mammals and that upper caniniforms are not homologous between the two-toed and the three-toed sloths. Applied to the tooth row of all extinct sloths, these data support the interpretation that the dental morphology of the three-toed sloth is unusual and illuminate a potential ancestral dental formula for sloths.

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