Population-level processes are widely studied in shallow-time ecology but not so in macroevolution and paleontology. In FossPop we will investigate empirical and conceptual links between characteristics of ancient populations and species declines and expansions on geological timescales. Population ecologists, evolutionary biologists, and paleontologists will gather at CAS to develop a common framework, based on hands-on data analyses and empirical insights, upon which to understand fossil population characteristics and their consequences for macroevolutionary dynamics. This unusual union of disparate fields will promote genuine breakthroughs in linking microevolutionary and ecological processes to very long-term evolutionary patterns.