El Gallo Formation; Baja California; Cretaceous angiosperm
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Over the past eight months, I have collaborated with paleontologists from the Universidad Nactional Autónoma de México to formally describe plant fossils they uncovered in Baja California between their 2013 and 2016 field seasons. We report the first floral assemblage of the dinosaur-aged El Gallo Formation, Baja California, Mexico, a small (82-specimen) collection of plant fossils from fossil soils contained within the 75.84 +/- 0.05 to 74.55 +/- 0.09 million-year-old El Disecado Member of the unit. A compound fruit belonging to the genus Operculifructus, previously reported from the 3.40 to 1.08 million year younger Cerro del Pueblo Formation, Coahuila, Mexico, dominates the flora, and displays an unusual suite of characters.
We identify a fruit tissue seed cap that lies within a crown defined by distal extensions of the seed coat as a derived character unique to the taxon. Because this character has not yet been demonstrated in a grossly similar fossil reproductive structure from the younger El Cien Formation, Baja California, Mexico, we maintain that the unequivocal record of Operculifructus in Baja California is presently limited to the Mesozoic, or the age of dinosaurs. We also note a lack of characters diagnostic of family- and order-level groups within the fossils, and so consider their taxonomic position uncertain within flowering plants. Likewise, resulting from sparse character data, species-level referral is not yet possible. Temporal constraints on our analysis of relationships in the taxon suggest that character states represented by the El Gallo Operculifructus are the most primitive known within it.