Cornalean affinities, phylogenetic significance, and biogeographic implications of Operculifructus infructescences from the Late Cretaceous (Campanian) of Mexico

IN ENGLISH: 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 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 () 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 () 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. RESUMEN EN ESPANOL Hipotesis de la Investigacion Cornales cretacicos representan un registro esencial en la historia de los asteridos. Casi todos los linajes del orden estan bien representados en el registro fosil, pero las familias africanas surenas Curtisiaceae y Grubbiaceae permanecen pobremente entendidas. Setenta y tres ejemplares de una infrutescencia fosil perteneciente al genero Operculifructus Estrada-Ruiz & Cevallos-Ferriz emend. Hayes & Smith de la formacion campaniana (Cretacico Tardio) El Gallo, Baja California, Mexico, poseen caracteres no descritos previamente y sugieren una relacion con Grubbiaceae. Metodologia Microestructuras de los fosiles fueron examinadas con microscopio de luz y microtomografia computarizada (micro-CT) de rayos X. Frutos actuales de Grubbia tomentosa (Thunb.) Harms fueron escaneados para su comparacion con el material fosil. Se realizaron analisis filogeneticos usando los 77 caracteres de frutos de Atkinson () para probar las relaciones de los fosiles con los linajes principales del orden. En algunos analisis se aplicaron restricciones topologicas a los taxa actuales basandose en varias hipotesis, soportadas geneticamente, de las relaciones dentro de Cornales. Resultados Centrales (Cruciales) Las estructuras novedosas de Operculifructus, recientemente observadas aqui, incluyen (1) ovulos anatropos; (2) frutos drupaceos; (3) disco epigino; y (4) un canal estilar en el centro del disco alineado con la protuberancia micropilar de la semilla. Los analisis filogeneticos consistentemente dan como resultado que Operculifructus es hermano de Grubbiaceae. Conclusiones Operculifructus proporciona evidencia directa de la presencia de Grubbiaceae en el Cretacico Tardio, mucho antes que la previa evidencia en el Eoceno. La filogenia de Atkinson () indica que la nueva posicion filogenetica recuperada para Operculifructus tambien resalta la presencia de frutos drupaceos cornaleanos mas basales en Norteamerica durante el Campaniano.