Mammals are among the most dynamic constituents of modern terrestrial ecosystems, and their rise to ecological prominence has long been attributed to the demise of non-avian dinosaurs during the Cretaceous-Paleogene (K-Pg) mass extinction event. Over the last 20+ years, however, it has become apparent that mammals began diversifying much earlier, in the Mesozoic, and the major lineages that characterize mammalian faunas in the aftermath of the K-Pg mass extinction began their ecological ascendency beginning in the Cretaceous. Nonetheless, because placental mammals are the most abundant and diverse extant mammals, there is a still an assumption that they must have evolved key behavioral and life-history traits that allowed them to diversify up to the present day (the ‘placental-mammal paradigm’). Further, the earlier adaptive radiation of mammals in the Cretaceous has been nearly universally ascribed to the rise of angiosperms (flowering plants), with mammals simply tracking the emerging ecospaces new plant groups facilitated (the ‘Cretaceous-angiosperm paradigm’). In this talk, I challenge the ‘placental-mammal’ and ‘Cretaceous-angiosperm’ paradigms.

My first vignette will highlight my research on multituberculate mammals, demonstrating how this oft-considered ‘archaic’ lineage shows evidence of being as behaviorally and reproductively ‘advanced’ as extant small-bodied placental mammals. My second vignette will highlight recent work exploring the role that tectonism may have played in stimulating the Cretaceous diversification of mammals and other terrestrial lineages. I will then outline ongoing field-based projects and new results that aim to test, and lend support to, that hypothesis. Collectively, these projects underscore a need for challenging expectations in the realm of early mammalian paleontology, building upon first principles, and re-focusing our attention on field- and specimen-based science.