Abstract

Cretaceous Cornales provide a crucial record of the early history of asterids. Most lineages of the order are well represented in the fossil record, but South African families of Curtisiaceae and Grubbiaceae remain poorly understood. Seventy‐three specimens of a fossil infructescence belonging to the genus Operculifructus Estrada‐Ruiz & Cevallos‐Ferriz emend. Hayes & Smith from the Late Cretaceous (Campanian) El Gallo Formation, Baja California, Mexico bear previously undescribed characters that suggest a relationship to Grubbiaceae.
Methods

Microstructures of the fossils were examined through light microscopy and x‐ray microcomputed tomography (microCT) scanning. Modern Grubbia tomentosa (Thunb.) Harms fruits were scanned for comparison to the fossil material. Phylogenetic analyses using the 77 fruit characters of Atkinson (2018) were performed to test relationships of the fossil to major lineages of the order. Several analyses applied topological constraints to the extant taxa, based on various genetically supported hypotheses of relationship within Cornales.

Key Results

Novel structures of Operculifructus newly observed here include (1) anatropous ovules, (2) drupaceous fruits, (3) an epigynous disc, (4) and a stylar canal in the center of the disc aligned with the micropylar protrusion of the seed. Phylogenetic analysis consistently resolves Operculifructus as sister to Grubbiaceae.
Conclusions

Operculifructus provides direct evidence for the occurrence of Grubbiaceae in the Late Cretaceous, much older than previous Eocene evidence. The phylogeny of Atkinson (2018) indicates that the new phylogenetic position recovered for Operculifructus also establishes the presence of the most basal drupaceous cornalean fruits in North America by the Campanian.