PALEOGENE RODENTS OF THE HUAYABAMBA FORMATION, OF THE ALTO RÍO MADRE DE DIOS AND ITS TRIBUTARIES, AMAZONIAN PERU

KAY, RICHARD F.; ARNAL MICHELLE; SALENBIEN, WOUT; VALDIVIA A. L.; MORSE, PAUL E.; BORTHS, MATTHEW R.; FLEMING, PERRY R.; MARTÍNEZ, JEAN- NOËL; GONZALES, LAUREN A.

Congreso; SVP Annual Meeting; 2021

The fossil record of South America includes caviomorphrodents and anthropoid primates of proposed PaleogeneAfrican origin. Interpreting their origins and earlyevolution is obscure due to their best fossil record beingin high latitudes with a scarcity of fossils from the tropicswhere major radiations or oldest representatives might befound. Recent excavations at two sites in the HuayabambaFormation (HF) along the Río Alto Madre de Dios in thePeruvian Amazon (13 °S) have yielded new faunas ofsmall rodents, expanding the geographical and temporalranges of Paleogene tropical rodents. The HF is the oldestCenozoic formation within the Madre de Dios basin,based on dated detrital zircon samples from twocaviomorph-bearing localities. Distinctive features of thezircons include: (1) Over half the zircons are sourcedfrom the west, meaning the Andes were a major sedimentsupply to a foreland basin by at least ~58 Ma. (2) Zirconswith Cenozoic ages are rare, accounting for only ~1% ofthe total, indicating no major volcanic sources wereproximate to the Madre de Dios basin during deposition.Site RA-1 is located on the Río Aguaroa, with a minimumdepositional age of 30.28 ± 0.74 Ma. RA-1 documents afluvial sedimentary environment with abundant paleosolscontaining rodents, metatherians, and other terrestrialvertebrates. Two rodent genera, Eobranisomys andCachiyacuy, are represented, having been previouslydocumented at Contamana, Peru (7 °S, late middleEocene Pozo Formation) and Santa Rosa, Peru (9 °S,lower Oligocene Yahuarango Formation). Eobranisomysis also known from La Cantera, Chubut ProvinceArgentina (46°S; lower Oligocene Sarmiento Formation).Site RC-1 on the Río Carbón is assigned to the HF on thebasis of overall similarity to the zircon provenance profileof RA-1 and a distinctive ~235 Ma zircon age peak absentfrom younger formations, though it has yet to yield anyCenozoic zircon grains. RC-1 has produced Cachiyacuykummeli and Pozomys, hitherto recorded at Contamanaand Santa Rosa. Several other new rodent taxa are alsorecorded, including the first subtropical record ofAcaremyidae, a family previously restricted to the lateOligocene?middle Miocene of Argentina.These new records from the HF reveal considerabletaxonomic diversity, but not a correspondingmorphological disparity. Notably, all rodents from thesePaleogene localities are brachydont, unlike the wide rangeof hypsodonty characterizing rodent faunas in Neogenelocalities.