A preliminary analysis of the first 150.000 years of the Paleocene molluscan rebound interval in the Brazos River area, Texas reveals a rapid diversity increase comprised of a roughly equal blend of local and immigrant Cretaceous survivors and new Paleocene species. This increase plateaus around 30.000–40.000 years after the K-T boundary at a relatively low level compared to former Cretaceous or later Paleocene diversity levels, suggesting a prolonged environmental stress after the K-T event. Species of uncertain evolutionary origin constitute a major portion of the assemblage, and until these taxa are identified, we won't know the precise proportions of new and survivor taxa, or the earliest occurrence of new Paleocene species. The presence of a very low diversity, numerically dominated assemblage just above the K-T boundary tsunamite and the appearance of the first definite new Paleocene species some distance above this, suggests the nature of the Paleocene rebound is more consistent with a rapid, catastrophic extinction than with a gradual environmental deterioration.