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Session Schedule & Abstracts

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Sunday 3rd July, 2016

Symposium: Morphology & evolution of the Xenarthra 2

Room: Salon H   11:30 am–1:00 pm

Moderator(s): S. Bargo & J. Nyakatura
XEN2-1  11:30 am  Expanded diagnosis of the ground sloth Mylodon darwinii (Mammalia: Xenarthra: Pilosa) and the functional implications. McAfee R.*, Ohio Northern University
Abstract: Although Mylodon darwinii was initially described in the mid-1800s, present day characterization of the taxon has been restricted to diagnoses of only the skull, tibia, and astragalus. A partial skeleton assignable to M. darwinii provides the opportunity to expand the diagnosis for this Lujanian-Ensenandan (late Pleistocene) ground sloth, primarily for the atlas (C1) and elements of the left forearm (radius, ulna, and some carpals and metacarpals). M. darwinii is generally larger in size than contemporary mylodontids (i.e. Catoynx, Glossotherium, Scelidotherium) with the exception of Lestodon. Atlas vertebra of M. darwinii bears distinct dorsal and ventral arch tuberosities suggesting well-developed rectus capitis ventralis and rectus capitis dorsalis minor muscles, which is in contrast to mylodontids with only one (e.g. dorsal) or no arch tuberosities. The occipital condyle articulations are very broad and triangular shaped, as in Glossotherium, and are not squared like Scelidotherium. These atlas characters together are adaptations for supporting a long and wide cranium. Radius lacks the distinct pronator flange seen in scelidotheres and Lestodon, and the dorsal margin of the ulna is straight and unbowed. The unciform and magnum each possess a third facet for articulation with the each other, and the lateral metacarpals exhibit a mediolateral thickening similar to that seen in other mylodontinae sloths. The combined forelimb features suggest M. darwinii was primarily quadrupedal with the weight of the manus born by the lateral digits (IV and V) like that of other mylodontinae (e.g. Glossotherium, Paramylodon) in a horizontal position, as opposed to a vertical or "knuckle-walking" posture. Continued diagnosis of new elements (e.g. humerus, scapula) for Mylodon will further these functional inferences and relationships.

XEN2-2  11:45 am  Biomechanical study in claws of extinct sloths and extant xenarthrans. Patiño SJ*, Facultad de Ciencias, Universidad de la República; Fariña RA, Facultad de Ciencias, Universidad de la República
Abstract: The morphological variation in claws and ungual phalanges of vertebrates are associated to differences in the locomotor behavior of the species. These structures are the first to get in touch with the substrate and, consequently, they are adapted to the nature of that contact. This, in turn, depends on the characteristics of that substrate and on the way in which the anatomical structures are applied to it. Here we analyze ungual phalanges of three extinct mylodontid sloths (Lestodon armatus, Glossotherium robustum and Scelidotherium leptocephalum) and claws of various groups of extant xenarthrans for actualistic comparison, aiming at testing hypotheses of substrate usage and locomotor behavior. For each species, the third digit was chosen because of its larger size and nearly perfect bilateral symmetry, which increases the possibilities of revealing functional differences between taxa. The inner and external curvature of the ungual phalanges and claws were measured and the mechanical advantage of its flexion in regard to the rest of the hand was calculated. In living species, it was observed that, as expected on mechanical grounds, in the curvature analyses all the digging species clustered together at lower values than those of non-diggers, which clustered separately. On the other hand, they all showed similar mechanical advantage values. The results obtained in this analysis indicates that the three genera of ground sloths were well adapted for strenuous activities, such as digging, in which force rather than velocity is optimized. The other two analyses allowed to identify the very large species Lestodon armatus and Glossotherium robustum as more able diggers than the relatively smaller Scelidotherium leptocephalum, although it cannot be ruled out that the less strong curvatures in the latter might have been due to a difference in the way the sediment was removed.

XEN2-3  12:00 pm  Early dietary, jaw shape and biomechanical performance differentiation in the evolution of armadillos (Xenarthra: Cingulata). De Esteban-Trivigno S*, Institut Català de Paleontologia Miquel Crusafont / Transmitting Science; Cantalapiedra JL, Museum für Naturkunde. Leibniz-Institut für Evolutions und Biodiversitätsforschung; Marce-Nogue J, Universität Hamburg; Fortuny J, Musèum national d'Histoire naturelle
Abstract: Armadillos are the most diverse group of extant xenarthrans, thus they are the most suited to investigate xenarthran evolutionary trends. Our aim was to explore the co-variation between some indicators of biomechanical performance, diet and the shape of the lower jaw in an evolutionary framework. We used Finite Element Analysis (FEA) to analyse the biomechanical performance of the lower jaw under equivalent loads. Mean values of stress were used as quantitative functional indicators. Also, we used geometric morphometrics to obtain shape variables for the lower jaw. 2D landmarks were digitalised on lateral pictures of jaws belonging to different species. Then, the first PC (explaining more than 80 % of variance) of a Principal Components Analysis (PCA) was used in some of the analyses as a single variable representing the main shape variability in our sample. We evaluated the covariation among jaw shape and jaw biomechanics using a Two-Block Partial Least Squares (PLS) analysis of the Phylogenetic Independent Contrasts (PICs). The influence of the diet on shape and biomechanics was evaluated with phylogenetic ANOVAs, and finally we applied maximum-likelihood evolutionary model to evaluate how shape and biomechanical performance evolved through the clade's history. Our results suggest that jaw morphology, diet and performance evolved slowly and differentiated mainly at the base of the tree. This translates into high within-lineage conservatism towards the present. Jaw shape and performance have a high covariance, indicating that they have coevolved. We also found a significant association of insectivory and omnivory with high and low performance values, respectively, in ordinary ANOVAs. However, when we controlled for phylogeny this relationship was not significant. This supports the scenario where the differentiation and this relationship arose early. Within closely related taxa, the relationship does not exist so it should not be considered a general rule.

XEN2-4  12:15 pm  Conceptual and methodological approaches for a paleobiological integration: the Santacrucian sloths (early Miocene of Patagonia) as a study case. Toledo N*, División Paleontología Vertebrados, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, CONICET; Vizcaíno SF, División Paleontología Vertebrados, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, CONICET; Bargo MS, División Paleontología Vertebrados, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, CIC; Cassini GH, División Mastozoología, Museo Argentino de Ciencias Naturales
Abstract: Paleobiological integration begins with the descriptive morphology and proceeds through a series of stages, finally reaching the paleosynecological reconstruction. This integration occurs through modeling of function, inference of faculties, performance and resource use to hypothesize the fundamental niche. Environmental information is then incorporated to speculate about biological roles and realized niche. The paleobiological integration of Santacrucian sloths is analyzed as a study case. These sloths comprise a diversity of eleven genera ranging from 40 to 120 kg in body mass; they are different morphologically from their living relatives. The realized niche of each taxon is reconstructed via integration of three paleoecological attributes: body size, dietary habits and substrate preference. An ecomorphological approach is applied, using Principal Components Analysis to explore the correlation between postcranial linear dimensions and substrate preference, together with functional indices. Integration of these results with previous studies indicates that the mid-sized genera Hapalops, Pelecyodon, Schismotherium and Analcimorphus (stem Megatherioidea) and Eucholoeops (Megalonychidae) were members of the arboreal folivore paleoguild. The larger mylodonts Analcitherium and Nematherium, may have been semiarboreal consumers of leaves, fruits and tubers thanks to their digging capabilities. Prepotherium (Megatheriidae), the largest and most terrestrial Santacrucian sloth, was also a folivore. Only the last three genera competed for trophic resources with other coexistent mammalian herbivores. The large body size and strength of the limbs with large claws made Santacrucian sloths only subject to predation by the largest Santacrucian carnivores, the borhyaenoids marsupials and phorusrhacoid birds. Finally, the absence of modern analogues to this heavy-sized arboreal and semiarboreal herbivore diversity is discussed.

XEN2-5  12:30 pm  Bend it like an armadillo: An investigation into the bending mechanics of the xenarthran vertebral column. Oliver JD*, Museum of Comparative Zoology, Harvard; Hautier L, University of Montpellier II; Pierce SE, Museum of Comparative Zoology, Harvard
Abstract: The xenarthran vertebral column is characterized by unique intervertebral articulations termed xenarthrae. These articulations are found between the posterior thoracic, lumbar, and first sacral vertebrae in all known xenarthrans, with the exception of glyptodonts and modern sloths. It is posited that xenarthrae stiffen the vertebral column, thereby serving as an adaptation to fossoriality in early xenarthrans. Here we investigate the implications of morphological regionalization of thoracolumbar vertebrae on the intervertebral bending mechanics of the nine-banded armadillo ( Dasypus novemcinctus). We ran bending trials on excised "motion segments", consisting of two vertebrae and an intervertebral joint, in sagittal flexion and extension, and lateral flexion, in order to calculate compliance and range of angular motion across the region. A series of measurements were used to establish correlations between vertebral morphology and mechanics. We found a decrease in compliance in the post-diaphragmatic, xenarthrous vertebrae in all three measured directions, which was associated with a less substantial but notable increase in range of angular motion. Based on preliminary results, we predict that this decrease in compliance and increase in range of motion are indeed correlated with the morphological signatures of xenarthrous and post-diaphragmatic vertebrae. We therefore suggest that xenarthrous and post-diaphragmatic zygapophyseal articulations act in concert to passively stabilize the vertebral column, while allowing for greater mobility upon directed muscle activity. Xenarthrae can thus be considered as an extra layer of stability, precluding the need for additional exertion in these ancestrally digging mammals with particularly low metabolic rates.

XEN2-6  12:45 pm  Bone internal microstructure of the forelimb of xenarthrans (Mammalia) – functional implications. Amson E*, AG Morphologie und Formengeschichte, Bild Wissen Gestaltung - ein interdisziplinaeres Labor & Institut fuer Biologie, Humboldt-Universität; Arnold P, University of Jena, Germany; Nyakatura JA, AG Morphologie und Formengeschichte, Bild Wissen Gestaltung - ein interdisziplinaeres Labor & Institut fuer Biologie, Humboldt-Universität
Abstract: The xenarthrans - sloths, anteaters, and armadillos and their extinct relatives - form an important group of placental mammals, today represented by taxa practicing several locomotor styles. While most armadillos are plantigrade, the tamanduas employ a highly peculiar stance, with the weight born on the lateral side of the hand. Additionally, giant anteaters utilise «pseudo knuckle-walking«, the weight being born, for the hand, on the flexed ungual phalanges. Extinct xenarthrans, "ground sloths" in particular, were also reconstructed as practicing peculiar stances, mostly based on the gross morphology of their postcrania. Inner bone microstructure, and in particular the trabecular architecture, adapts to loadings applied to the bone throughout the whole life. The analysis of bone microstructural characteristics is hence a powerful tool that can potentially discriminate between different loading regimes withstood by the bone. Similarly, a previous analysis focused on bone compactness, was able to argue that the highly derived microstructure of one particular genus, was indicative of its aquatic habits. This past analysis also revealed that most xenarthrans, especially pilosans, depart from the mean mammalian condition in displaying rather compact bones. In the present preliminary analysis, we study the microstructural patterns of the forelimb bones of xenarthrans. With this new study, we aim at finding bone microstructural correlates to the various locomotor styles and modes of hand utilisation observed in extant xenarthrans. Ultimately, we endeavour to propose reconstructions of locomotor style in extinct taxa. EA is funded by the Alexander von Humboldt Foundation.

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