The interchange between the previously disconnected faunas of North and South America was a massive experiment in biological invasion. A major gap in our understanding of this invasion is why there was a drastic increase in the proportion of mammals of North American origin found in South America. Four nonmutually exclusive mechanisms may explain this asymmetry: 1) Higher dispersal rate of North American mammals toward the south, 2) higher origination of North American immigrants in South America, 3) higher extinction of mammals with South American origin, and 4) similar dispersal rate but a larger pool of native taxa in North versus South America. We test among these mechanisms by analyzing ∼20,000 fossil occurrences with Bayesian methods to infer dispersal and diversification rates and taxonomic selectivity of immigrants. We find no differences in the dispersal and origination rates of immigrants. In contrast, native South American mammals show higher extinction. We also find that two clades with North American origin (Carnivora and Artiodactyla) had significantly more immigrants in South America than other clades. Altogether, the asymmetry of the interchange was not due to higher origination of immigrants in South America as previously suggested, but resulted from higher extinction of native taxa in southern South America. These results from one of the greatest biological invasions highlight how biogeographic processes and biotic interactions can shape continental diversity.

北美和南美先前断开的动物区系之间的互换是一项大规模的生物入侵实验。我们对这种入侵的理解的一个主要空白是为什么在南美发现的北美起源的哺乳动物的比例急剧增加的原因。四种非互斥机制可以解释这种不对称性：1）北美哺乳动物向南方的扩散速度更快，2）南美移民在南美的血统较高，3）南美血统的哺乳动物灭绝更高，以及4）相似的扩散率较高，但北美和南美的原生类群数量更多。我们通过使用贝叶斯方法分析约20,000个化石的发生来推断移民的分散和多样化速率以及分类学选择性，从而在这些机制中进行测试。我们发现移民的散布和原籍率没有差异。相比之下，南美本土哺乳动物则表现出更高的灭绝率。我们还发现，两个起源于北美的进化支（食肉目和Artiodactyla）在南美的移民明显多于其他进化支。总体而言，交换的不对称性并不是由于先前提出的南美洲移民的更高起源，而是由于南美洲南部自然分类群的灭绝所致。这些最大的生物入侵之一的这些结果突出了生物地理过程和生物相互作用如何塑造大陆多样性。我们还发现，两个起源于北美的进化支（食肉目和Artiodactyla）在南美的移民明显多于其他进化支。总体而言，交换的不对称性并不是由于先前提出的南美洲移民的更高起源，而是由于南美洲南部自然分类群的灭绝所致。这些最大的生物入侵之一的这些结果突出了生物地理过程和生物相互作用如何塑造大陆多样性。我们还发现，两个起源于北美的进化支（食肉目和Artiodactyla）在南美的移民明显多于其他进化支。总体而言，交换的不对称性并不是由于先前提出的南美洲移民的更高起源，而是由于南美洲南部自然分类群的灭绝所致。这些最大的生物入侵之一的这些结果突出了生物地理过程和生物相互作用如何塑造大陆多样性。