New Mammal Faunal Data from Cerdas, Bolivia, a Low Latitude Neotropical Site that Chronicles the end of the Middle Miocene Climatic Optimum in South America

CROFT, D. A.; ANAYA, F.; BRANDONI, D.; CARLINI, A. A.; CATENA, A. M.; CIANCIO, M. R.; ENGELMAN, R. K.

Dallas

Congreso; SVP 75th annual meeting; 2015

Many groups of South American mammals apparently underwent northward range contractions following the Middle Miocene Climatic Optimum (MMCO) including primates, porcupines (Erethizontidae), palaeothentid marsupials (Paucituberculata), and astrapotheres (a native ungulate group). Determining the precise timing of these shifts has been hampered by a scarcity of (1) early middle Miocene (Langhian) sites from tropical latitudes, and (2) late middle Miocene (Serravallian) sites from the Southern Cone. Cerdas, Bolivia (ca. 21° S) is one of only three sites of Langhian age that documents Neotropical mammal distributions near the end of MMCO. Our team?s fieldwork at the site recovered specimens from low in section that represent three groups previously undocumented at the site: a meat-eating metatherian (Sparassodonta), a proboscis-bearing ungulate (Astrapotheria), and a megatheriid sloth. Paleosols from this portion of the section are weakly to moderately developed, have compound and composite profiles, and preserve several types of ichnofossils including lined and unlined burrows, rhizohaloes, and rhizotubules. The sparassodont remains include the basicranium and most of the mandible of a species comparable in size to the hathliacynid Cladosictis patagonica from the late early Miocene of Santa Cruz, Argentina. However,several features suggest borhyaenoid rather than hathliacynid affinities including a jugular fossa, a non-pneumatized squamosal, and the lack of a hypoconulid on m4. The astrapothere remains consist of many tooth fragments with an unusual combination of features not typical of late early Miocene Astrapotherium magnum nor late middle Miocene members of the Uruguaytheriinae; these include relatively smooth premolar ectolophs and very large upper molar cingulae. A partial megatheriid sloth dentary preserving the last molariform likely pertains to a Megatheriinae, which suggests that this subfamily could have originated in lower latitudes and later spread into Patagonia. A newly discovered specimen of a horned armadillo (Peltephilidae) from Cerdas includes a partial articulated carapace that supports its identification as a new species. The osteoderms of this specimen are characterized by a surface texture of small tubercles and pits, a central longitudinal elevation (acute in cross section) surrounded by a deep, wide groove extending over most of the osteoderm, and depressions along the border arranged in a unique, radial pattern. Ongoing studies at Cerdas aim to place these mammals in a refined paleoenvironmental context