[ES] Los Xenarthra argentinos de Edad Friasense (Mioceno medio) eran deficientemente conocidos hasta las minuciosas recolecciones de 1990-1993 (convenio Duke University, USA-Museo de La Plata, UNLP). A base de estos nuevos restos se reconocen cuatro g?neros y siete especies nuevos de Dasypodidae (Eutatini), Glyptodontidae (Sclerocalyptini y Palaehoplophorini), Megatheriidae (Nothrotheriinae y Prepotheriinae), Mylodontidae (Scelidotheriinae) y Myrmecophagidae (Myrmecophaginae). Estos taxones permiten caracterizar este lapso y diferenciarlo tanto respecto de la Edad Santacrucense (Mioceno temprano) como de la Chasiquense (Mioceno tard?o m?s temprano). Los Dasypodidae de la Fm. Cerro Boleadoras (Santa Cruz) indicar?an una antig?edad algo menor que la del Santacrucense de la costa atl?ntica patag?nica. En la Fm. Coll?n Cura ("Colloncurense") se registran los primeros indudables Megatheriinae y Mylodontinae. En la Fm. El Pedregoso ("Mayoense") aparecen xenartros m?s avanzados a?n. En conclusi?n, durante el Friasense se verifican importantes cambios composicionales entre los Xenarthra, tal vez en relaci?n con diferentes capacidades de adaptaci?n a las variaciones clim?tico-ambientales: entre los Tardigrada, los tipos "araucopampeanos" aparecen en el "Colloncurense"; en cambio, entre los Cingulata, se mantienen muchos tipos "pansantacrucianos" hasta el "Mayoense", y el recambio mayor se produce a lo largo del Chasiquense.

[EN] The Argentine Xenarthrans from the Friasian Age (Middle Miocene) were poorly known until the detailed collections made during 1990-1993 (Duke University, USA-Museo de La Plata, UNLP Agreement). Four new genera and seven new species of Dasypodidae (Eutatini), Glyptodontidae (Sclerocalyptini and Palaehoplophorini), Megatheriidae (Nothrotheriinae and Prepotheriinae), Mylodontidae (Scelidotheriinae) and Myrmecophagidae (Myrmecophaginae), were described. Through these taxa, this lapse may be characterized and differentiated from the Santacrucian Age (Lower Miocene) as well as from the Chasicoan Age (earliest Late Miocene). The Dasypodidae from the Cerro Boleadoras Formation (Santa Cruz) indicate an age somewhat younger than that of the Santacrucian from the Patagonian Atlantic coast. In the Coll?n Cura Formation ("Colloncuran"), the first undoubted Megatheriinae and Mylodontinae, were recorded. In the El Pedregoso Formation ("Mayoan") appeared even more advanced xenarthrans. Finally, significant changes in the composition of the Xenarthra were verified during the Friasian, probably related to different adaptive capability to environmental and climatic changes. Among the Tardigrada, "araucanian-pampean" types appeared in the "Colloncuran"; instead, among the Cingulata, several "pansantacrucian" types lasted until the "Mayoan", and the greatest re-change occurred during the Chasicoan.