EARLY MAMMALIAN FAUNAL RECOVERY FOLLOWING THE CRETACEOUS-PALEOGENE MASS EXTINCTION EVENT IN MCGUIRE CREEK, MONTANA, USA

The changes in mammalian faunal composition and structure following the Cretaceous-Paleogene (K-Pg) mass extinction are central to understanding not only how terrestrial communities recovered from this ecological perturbation, but also the early evolution of archaic groups leading to extant mammalian clades. Previous studies of mammals following the K-Pg extinction have investigated recovery on continent-wide scales over millions of years, or focused on faunas right after the extinction and ca. 300 Ka later, potentially missing processes occurring during the earliest recovery. Here, we analyzed changes in mammalian local faunas during the earliest recovery on a small spatiotemporal scale. We compiled a sample of 229 mammalian specimens from four localities spanning 45 m of the Tullock Formation in the McGuire Creek area of McCone County, Montana, and placed these localities into a high-precision chronostratigraphic framework using ⁴⁰Ar/³⁹Ar tephra ages and magnetostratigraphic data. The sample consists of three local faunas: Z-Line, which comprises two localities from between 66.04 Ma (K-Pg boundary) and 66.01 Ma, and Luck O Hutch and Coke’s Clemmys, each of which comprises one locality from between 66.01 and 65.75 Ma. We used faunal composition, heterogeneity, and richness measures to assess ecological structure in each local fauna and quantitatively compare faunas through time.

Between the earliest post-K-Pg Z-Line local fauna and the two younger local faunas, at least four new genera appeared, including the multituberculate Cimexomys and a member of the plesiadapiform genus Purgatorius; raw species richness nearly doubled, from 6 species in Z-Line to 11 species in Luck O Hutch and 10 species in Coke’s Clemmys; and metatherians fell from 10% to 0% of specimens. At least three metatherian specimens from Z-Line may represent taxa usually only found in the Cretaceous. These results show that in the first 250 Ka of the recovery, mammalian local faunas were undergoing rapid changes towards pre-extinction levels of taxonomic richness and heterogeneity, but there also may have been some low level of holdover of latest Cretaceous taxa into earliest Paleogene faunas. Our results further underline the merits of examining biological processes in the fossil record at ecologically relevant scales whenever possible.