A reciprocal and apparently symmetrical interchange of land mammals between North and South America began about 3 million years ago, after the appearance of the Panamanian land bridge. The number of families of land mammals in South America rose from 32 before the interchange to 39 after it began, and then back to 35 at present. An equivalent number of families experienced a comparable rise and decline in North America during the same interval. These changes in diversity are predicted by the MacArthur-Wilson species equilibrium theory. The greater number of North American genera (24) initially entering South America than the reverse (12) is predicted by the proportions of reservoir genera on the two continents. However, a later imbalance caused by secondary immigrants (those which evolved from initial immigrants) is not expected from equilibrium theory.

Integration of sequence stratigraphy, magnetostratigraphy, Ar/Ar dating, and paleontology considerably advances knowledge of the Late Cretaceous-early Paleogene chronostratigraphy and tectonic evolution of Bolivia and adjacent areas. The partly restricted marine EI Molino Formation spans the Maastrichtian and Danian (-73-60.0 Ma). Deposition of the alluvial to lacustrine Santa Lucía Formation occurred between 60.0 and 58.2 Ma. The widespread erosional unconformity a t the base of the Cayara Formation is 58.2 Ma. This unconformity separates the Upper Puca and Corocoro supersequences in Bolivia, and is thus coeval with the Zuni-Tejas sequence boundary of North America. The thick overlying Potoco and Camargo formations represent a late Paleocene-Oligocene foreland fill. The onset of shortening along the Pacific margin at-89 Ma initially produced rifting in the distal foreland. Santonian-Campanian eastward-onlapping deposits indicate subsequent waning of tectonic activity along the margin. Significant tectonism and magmatism resumed along the margin a t-73 Ma and produced an abrupt increase in subsidence rate and other related phenomena in the basin. Subsidence was maximum between-71 and-66 Ma. Due to the early Maastrichtian global sea-level high, marine waters ingressed from the northwest into this underfilled basin. Subsidence decreased during the Late Maastrichtian and was low during the Danian. It increased again in the latest Danian, for which a slight transgression is recorded, and peaked in the early Selandian. Tectonism between 59.5 and 58.2 Ma produced a variety of deformational and sedimentary effects in the basin and correlates with the end of emplacement of the Coastal batholith. The subsequent 58.2 Ma major unconformity marks the onset of continental foreland basin development, which extended into Andean Bolivia during the late Paleocene-Oligocene interval. This basin underwent internal deformation as early as Eocene time in the Altiplano and Cordillera Oriental. These early structures, previously assigned to the late Oligocene-early Miocene orogeny, probably accommodated observed tectonic rotations in the Eocene-Oligocene.

Caenolestinae of the classification most commonly used for the last 70 years would have to be replaced by Abderitidae and Garzoninae. This usage would upset continuity and stability in nomenclature of this diverse and important group of South American mammals.In view of this, Marshall & Tedford (1978, p. 58) submitted an application to the Commission to safeguard the family-group names Caenolestidae Trouessart, 1898, and Palaeothentidae (Sinclair, 1906) Osgood, 1921 , from the threat to their stability represented by the prior names Abderitidae Ameghino, 1889, Epanorthidae Ameghino, 1889, Garzonidae Ameghino, 1891b, and Decastidae Ameghino, 1893b. They asked the Commission:(1) to use its plenary powers to rule that the family-group names Abderitidae Ameghino, 1889, Garzonidae Ameghino, 1891b, and Decastidae Ameghino, 1893b, not be given nomenclatural precedence over the family-group names Caenolestidae Trouessart, 1898, and Palaeothentinae Sinclair, 1906;(2) to place the following family-group names on the Official List of Family-Group Names in Zoology with the endorsements that:(a) Caenolestidae Trouessart, 1898, be given nomenclatural precedence over Abderitidae Ameghino, 1889, Garzonidae Ameghino, 1891b, and Decastidae Ameghino, 1893b;

From radioisotopic (potassium-argon) age determinations of tuffs and magnetostratigraphy of Late Tertiary mammal-bearing beds in Catamarca Province, northwest Argentina, refined estimates have been obtained for the durations and boundaries of beds of Chasicoan (Middle Miocene) through Chapadmalalan (Pliocene) age. An age of 9.0 million years is tentatively accepted for the Chasicoan-Huayquerian boundary, 5.0 million years for the Huayquerian-Montehermosan boundary, and 3.0 million years for the Montehermosan-Chapadmalalan boundary. Procyonids (raccoons and their allies), a group of North American origin, are first recorded in South America in a level immediately below a unit dated at 6.0 million years. Cricetine rodents of the tribe Sigmodontini are first recorded in South America in beds of Montehermosan age in Argentina. Ground sloths, a group of South American origin, first appear in North America in Early Hemphillian time in beds dated between 9.5 and 9.0 million years. The Panamanian land bridge was established by 3.0 million years ago, and an interchange of the terrestrial faunas was well under way by Late Blancan time (around 2.5 million years before present) in North America and by Chapadmalalan time in South America.

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