A fossil from the Early Eocene London Clay flora of southern England provides the earliest confirmed seed record of Myristicaceae (Magnoliales). The specimen, which was fractured transversely to show internal structure, reveals prominent longitudinal ruminations of the kind found today only in the Myristicaceae. We describe this fossil as Myristicacarpum chandlerae sp. nov. and discuss its phylogenetic and biogeographic implications. Its Early Eocene age might seem to contradict molecular evidence that Myristicaceae diversified in the Miocene, but this depends on whether it belongs in the crown group of the family or on the stem lineage leading to it. To address this question, we review the distribution of ruminations, aril type, and seed size and shape on a molecular and morphological phylogeny of extant Myristicaceae. Myristicacarpum chandlerae resembles some extant genera and not others in presence of ruminations, small size, and elongate shape, but these characters are highly homoplastic in living Myristicaceae and are equally consistent with a position in the crown group or on the stem lineage. However, biogeographic arguments favor a pre-Miocene age for the crown group. Myristicaceae join a growing list of taxa in which modern-appearing fossils predate ages inferred from molecular divergences.