Differential Cretaceous/Tertiary extinctions of nonmarine vertebrates; Evidence from northeastern Montana

Abstract

This chapter documents survivorship across the Cretaceous/Tertiary (K/T) (and Lancian/Puercan) boundary for all 111 species of nonmarine vertebrates from Garfield and McCone counties, Montana. Species-level survivorship appears to be between about 53 and 64 percent after the artifacts caused by paleobiogeography, rarity of some species, and differing evolutionary rates are taken into account. Without correcting for these artifacts, survival is an artificially low 32 percent. If specimens from the Bug Creek interval are early Paleocene (Puercan) in age, survivorship is an almost certainly incorrectly high value of 82 percent. This is because an undetermined number of latest Cretaceous (Lancian) species in the Bug Creek interval probably were reworked from Cretaceous sediments and were extinct by Bug Creek time. Comparison of the Lancian/Puercan transition with the older Judithian/Lancian and younger Puercan/Torrejonian transitions suggests percentage survival is similar among the three (55, 48, and 58 percent, respectively) after the Lazarus effect is considered. These results are not easily explained by a catastrophic mass-extinction scenario for the K/T transition, at least for nonmarine vertebrates. Rather, a geologically rapid but noncatastrophic change, such as the loss of range and habitat diversity during the Late Cretaceous marine regression, is commensurate with the analysis.