Science. 1979 Apr 20;204(4390):272-9. doi: 10.1126/science.204.4390.272.
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.
从阿根廷西北部卡塔马卡省的凝灰岩放射性同位素（钾-氩）年龄测定和晚第三纪含哺乳动物的地层的磁力地层学，对查西科安（中中新世）到查帕德马兰（上新世）时期地层的持续时间和边界进行了更精确的估计。查西科安-瓦伊奎尔边界的年龄暂定为 900 万年，瓦伊奎尔-蒙特赫尔莫斯边界的年龄为 500 万年，蒙特赫尔莫斯-查帕德马兰边界的年龄为 300 万年。浣熊及其同类的原鼬科动物最初在北美的一个水平记录在一个 600 万年的地层之下。在阿根廷的蒙特赫尔莫斯地层中首次记录到南美洲的西格莫丁尼族的仓鼠科啮齿动物。地懒是一个起源于南美洲的群体，最早出现在北美洲的早海芬阶，年代在 950 万到 900 万年之间。巴拿马地峡在 300 万年前就已经建立，到了晚布兰坎期（距今约 250 万年），北美洲和南美洲的陆地动物群已经开始交流，到了查帕德马兰期，这种交流已经非常活跃。
