El aparato locomotor de los perezosos (Xenarthra, Folivora) santacrucenses (Mioceno temprano). Implicancias paleoecológicas

• Autores: Néstor Toledo
• Directores de la Tesis: Sergio Fabián Vizcaíno (dir. tes.), María Susana Bargo (codir. tes.)
• Lectura: En la Universidad Nacional de La Plata ( Argentina ) en 2012
• Idioma: español
• Enlaces
  • Tesis en acceso abierto en:  sedici.unlp.edu.ar  doi.org 
• Resumen
  • español

    El objetivo general de esta tesis es contribuir al conocimiento paleoecológico integral de las faunas de mamíferos de Edad Santacrucense, basándose en estudios morfofuncionales de detalle, tanto cualitativos como cuantitativos, y con controles filogenéticos, tafonómicos y estratigráficos adecuados. Los objetivos específicos son realizar un estudio morfofuncional del aparato locomotor de los perezosos santacrucenses con el fin de detectar diferencias morfológicas que permitan generar hipótesis sobre preferencia y uso del sustrato (terrestre, semiarborícola, arborícola, cavador, etc.), características posturales y de locomoción (caminadores cuadrúpedos o facultativamente bípedos, trepadores lentos, braquiadores, etc.) y generar hipótesis acerca de la inserción de los perezosos en el contexto paleoecológico de la fauna de la Formación Santa Cruz. Las hipótesis de trabajo son las siguientes: 1) Los perezosos santacrucenses de menor tamaño corporal serían formas arborícolas y los de mayor tamaño semiarborícolas; la morfología y función de su esqueleto apendicular y musculatura asociada serían más semejantes a las de los osos hormigueros que a los de los perezosos actuales, indicando un modo de vida similar (trepadores cuadrúpedos con capacidad de escarbar el sustrato). 2) La presencia de esta formas arborícolas y semiarborícolas de perezosos, sumada a la diversidad de otros mamíferos arborícolas (como primates, osos hormigueros, roedores y marsupiales), indicarían ambientes arbolados en la Formación Santa Cruz y estrategias de partición del uso de recursos tróficos y espaciales.

  • English

    Xenarthrans are one of the most conspicuous and ancient groups of placental mammals of South America. They include two main clades: Cingulata (armadillos and glyptodonts) and Pilosa; the latter comprises Vermilingua (anteaters) and Folivora (sloths).

    Living sloths are represented by the genera Bradypus (three-toed sloths) and Choloepus (twotoed sloths) and six species. They all inhabit the rainforests of Central and South America, are folivores and strictly arboreal at less than 10 kg of body mass. By contrast, the fossil record of Folivora is extremely abundant and diverse, with about 100 named genera. The first remains, scarce and fragmentary, come from the Eocene and Oligocene of Patagonia, but after the Miocene (about 20 Millions of years) they become one of the most well-represented groups of the Cenozoic faunas of South America. Miocene sloths are particularly well represented in the Santa Cruz Formation (Santacrucian Age; late Early Miocene). They comprise taxa of larger body size than the living sloths but much smaller than their giant relatives of the Pleistocene. Although the taxonomy at the species level is still unresolved, which has restricted systematic studies to higher ranks, eleven genera are recognized in the Santa Cruz Formation, belonging to three of the four sloth clades: Eucholoeops and Megalonychotherium (Megalonychidae), Planops and Prepotherium (Megatheriidae), Hapalops, Hyperleptus, Analcimorphus, Schismotherium and Pelecyodon (Megatherioidea basales) and Nematherium and Analcitherium (Mylodontidae).

    The fauna of the Santa Cruz Formation represents a phase in the history of the Cenozoic of South America, in which the communities consisted of a complex mixture of lineages of "old South American ancestry" (Marsupialia, Xenarthra, Litopterna, Notoungulata and Astrapotheria), along with forms from other continents (Rodentia and Primates).

    Exposures of the Santa Cruz Formation along the Atlantic coast of the Santa Cruz province, between the Coyle and Gallegos rivers, are exceptional for the quality of preservation of the fossil vertebrates (mostly mammals), often fairly complete and articulated. This exceptional preservation makes them suitable for paleoautoecological studies, characterizing each taxon according to three parameters: body size, diet and substrate preference (including locomotion).

    The main goal of this thesis is to contribute to the comprehensive paleoecological knowledge of the Santacrucian mammal faunas, based on detailed morphofunctional studies, both qualitative and quantitative, with phylogenetic, taphonomic and stratigraphic controls.

    The specific goals are to perform a morphofunctional study of the locomotory apparatus of the Santacrucian sloth, in order to identify morphological differences that permit generating hypotheses about the substrate preference and use (terrestrial, semiarboreal, arboreal, digger, etc.), posture and locomotion characteristics (ambulatory quadrupeds or facultatively bipeds, slow climbers, scansorial, etc.), and generate hypotheses about the place of these sloths in the paleoecological context of the fauna of the Santa Cruz Formation.

    The working hypotheses are: 1) the smaller Santacrucian sloths were arboreal and the larger semiarboreal; the morphology and function of the appendicular skeleton and associated musculature would be more similar to those of anteaters than those of the living sloths, indicating a similar lifestyle (climbing quadrupeds capable of scratching the substrate). 2) The presence of these arboreal and semiarboreal sloths, coupled with the diversity of other arboreal mammals (e.g., primates, anteaters, rodents and marsupials), would indicate the presence of forested environments in the Santa Cruz Formation, and very probably strategies for partitioning of trophic and space resources.

    In order to test the proposed hypotheses, the following activities were performed:

    1. Description of the bony elements of the appendicular skeleton, with emphasis on features of functional interest.

    2. Linear measurements describing functional traits, proportions and lengths were taken.

    3. Construction of a morphometric database of living mammals from various systematic groups, including diversity of body sizes, substrate preferences and locomotor styles.

    4. Estimation of body mass using equations specially generated by applying estimated error indices.

    5. Reconstruction of the musculature of the locomotory apparatus.

    6.Qualitative morphofunctional analysis based on the evidence provided by the descriptions and muscles reconstruction.

    7. Ecomorphological analysis, in order to explore the morphometric resemblance to living mammals, generating functional hypotheses to explain this similarity.

    8. Application of several biomechanical and functional indices.

    9. Integration of the information obtained by these approaches to characterize the Santacrucian sloths functional and ecologically.

    10. Evaluation and discussion of the inferred ecological role of the Santacrucian sloths within the vertebrate assemblages of the Santa Cruz Formation.

    The analysis of the results produced the following conclusions:

    • The morphology of the appendicular skeleton of the Santacrucian sloth was markedly different from their living relatives.

    • The proportions of the appendicular bony elements and the large development of feaures for the origin and insertion of muscles and ligaments are more similar to those of the anteaters and other living mammals capable of digging.

    • The musculature of the locomotory apparatus of the Santacrucian sloths was greatly developed, allowing great forces during adduction of the limbs, extension and flexion of the forearm and calf, flexion and prehension of the hand, and extension and prehension of the foot.

    • The appendicular skeleton was capable of wide abduction at the hip and shoulder, and flexed postures of the elbow and knee. Supination ability was limited and the ankle was restricted to flexion-extension. The foot was efficient in prehension.

    • The analysis of these functional features suggests that among Santacrucian sloths were arboreal forms (Hapalops, Eucholoeops, Pelecyodon, Schismotherium and Analcimorphus), semiarboreals (Analcitherium and Nematherium), facultatively arboreal and terrestrial (Prepotherium).

    • Regarding the use of substrate, the Santacrucian sloths were diggers or at least they had good digging capabilities, especially the mylodontids Analcitherium and Nematherium and the megatherioid Hapalops.

    • Santacrucian sloths were large mammals when compared with the living sloths, with body masses between 40 and 80 kg (Hapalops, Eucholoeops, Pelecyodon, Schismotherium and Analcimorphus) and between 80 and nearly 200 kg (Analcitherium, Nematherium and Prepotherium).

    • The integration of these lines of evidence leads us to propose Hapalops, Eucholoeops, Pelecyodon, Analcimorphus and Schismotherium as conforming to a guild of arboreal and folivorous mammals that did not compete for resources with other arboreal herbivorous Santacrucian mammals. Analcitherium and Nematherium were semiarboreal forms of closed habitats, leaf eaters but also feeding on tubers, fruits or roots, due to their digging abilities. Prepotherium, the most terrestrial and folivorous, probably competed with closed-habitat and large-sized litopterns.

    • Due to their body size, muscle strength and large claws, sloths were potential prey only for predators of large size.

    • Qualitative and detailed anatomical studies are essential for the understanding and assessment of functional traits of interest and to generate consistent paleobiological hypotheses. They allow highly detailed analysis with a significant methodological simplicity. Exploratory ecomorphological analysis are also very useful, despite their the lower resolution. Biomechanical and functional indices constitute a simple methodology that allows quantitative and fast comparison of functional features. The combination of these different methodological approaches made it possible to characterize the sloths of the Santa Cruz Formation based on their body size and substrate preference and use.