Late Cretaceous infructescences of Operculifructus (Cornales) from Mexico
Project
ID: 00000C564
Public
PREMISE OF THE STUDY: 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 southern 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 from the Late Cretaceous (Campanian) El Gallo Formation, Baja California, México bear previously undescribed characters that suggest a relationship to Grubbiaceae. METHODS: Microstructures of the fossils were examined through light microscopy and X-ray micro-computed 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. Analyses applied several 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) 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 the 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 within North America by the Campanian.