Uncovering the early diversification of core eudicots in western North America: structurally preserved fruits from Late Cretaceous deposits of Sucia Island

Abstract

Core eudicots are the most diverse lineage of flowering plants with a fossil record that dates back to the Late Cretaceous. Making up much of the diversity in flowering plants today, it is important to investigate the core eudicot fossil record to better understand the early diversification of angiosperms. Recent fossil discoveries from Cretaceous deposits within the western coast of North America may help shed new light on the early diversity of core eudicots and angiosperms as a whole. Late Cretaceous deposits along the western coast of North America remain relatively undersampled compared to the eastern coast or western interior. However, ongoing efforts to recover and characterize fossil flora of western North America within a phylogenetic framework will result in a better understanding of the turnover of lineages through time. Here I report two new species of core eudicots based on fossil fruits recovered from Campanian (Late Cretaceous) deposits of Sucia Island, Washington state, USA. The fossil fruits were studied using sectioning, light microscopy, X-ray micro-computed tomography, and phylogenetic analyses. Based on morphological comparisons and support from phylogenetic analyses, one fossil fruit was assigned to the extant genus Ceratopetalum (Cunoniaceae), as a new species Ceratopetalum suciaensis Tang & Atkinson sp. nov., and the second fruit to the extinct genus Esgueiria (Esgueiriaceae), as a new species Esgueiria aligera Tang & Atkinson sp. nov. These fossils are the first unequivocal evidence of both genera within North America which greatly extend their distribution. Continuing to recover and describe both extant and extinct lineages will undoubtedly help us to better understand the early evolutionary patterns of flowering plants.