Criteria for recognizing alien (historically introduced) plants in continental floras are largely inferential, involving range disjunctions, absence in the first botanical records for a region, pioneer status in secondary successions ('weedy habit'), and, to a lesser extent, the fossil record. Decisions on whether a species is native or introduced commonly appear in local and regional floras. In a few cases, the plant's occurrence in the fossil record has reversed its presumed alien status1. Ironically, the appearance of assumed introduced species in prehistoric contexts could also be taken as an index of modern contamination due to stratigraphical mixing. We now present conclusive evidence for the prehistoric occurrence of Corispermum L. (Chenopodiaceae), a herbaceous annual growing on sandy soils throughout western North America, where it was previously reported as adventive from Eurasia. The use of tandem accelerator mass spectrometry (TAMS) for direct radiocarbon measurement of seeds eliminates problems with stratigraphical mixing and using this method we verify a pre-Columbian age for Corispermum.
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