New data from Cerdas, Bolivia, a low latitude site that chronicles the end of the Middle Miocene Climatic Optimum (MMCO).

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

Colonia

Congreso; V Congreso Latinoamericano de Paleontologia de Vertebrados,; 2015

Fac. de Ciencias, Uy

Many South American mammal groups apparently underwent northward range contractionsfollowing the MMCO including primates, porcupines (Erethizontidae), palaeothentid marsupials(Paucituberculata), and astrapotheres (a native ungulate group). Determining the precise timing ofthese shifts has been hampered by a scarcity of: (1) early middle Miocene (Langhian) sites fromtropical latitudes; and (2) late middle Miocene (Serravallian) sites from the Southern Cone. Cerdas,Bolivia (ca. 21° S) dates to the Langhian age (ca. 16.5-15.3 Ma based on 40Ar/39Ar dating andpaleomagnetic correlation) and is one of only three sites that documents Neotropical mammalspecies of the MMCO. Our team?s recent fieldwork at Cerdas recovered specimens from low insection that represent three previously undocumented groups: a meat-eating metatherian(Sparassodonta), a proboscis-bearing ungulate (Astrapotheria), and a megatheriid sloth. Paleosols ofthe lower Cerdas Beds are weakly to moderately developed, have compound and compositeprofiles, and preserve several types of ichnofossils including lined and unlined burrows,rhizohaloes, and rhizotubules. The sparassodont remains include the basicranium and most of themandible of a species comparable in size to the hathliacynid Cladosictis patagonica from the lateearly Miocene of Santa Cruz, Argentina. However, several features suggest borhyaenoid rather thanhathliacynid affinities including a jugular fossa, a non-pneumatized squamosal, and the lack of ahypoconulid on m4. The astrapothere remains consist of many tooth fragments with an unusualcombination of features not typical of late early Miocene Astrapotherium magnum nor late middleMiocene members of the Uruguaytheriinae; these include relatively smooth premolar ectolophs andvery large upper molar cingulae. A partial megatheriid sloth dentary preserving the last molariformlikely pertains to a Megatheriinae, suggesting that this subfamily could have originated in lowerlatitudes 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 asa new species. The osteoderms of this specimen are characterized by a surface texture of smalltubercles 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 aunique, radial pattern. The large proportion of regionally endemic species at Cerdas providesevidence that the MMCO was a key factor in the differentiation of Miocene mammal assemblages.Ongoing studies at Cerdas aim to clarify the evolutionary relationships of these species and to placethem in a refined paleoenvironmental context.