Flora and Climate of the Olmos Formation (upper Campanian-lower Maastrichtian), Coahuila, Mexico: A Preliminary Report

In Coahuila, north-eastern Mexico, the Upper Cretaceous Olmos Formation (upper Campanian-lower Maastrichtian) outcrops in the Sabinas Basin, a region important for coal exploitation. Collections of plant macrofossils in the 1970s by Reinhard Weber indicated the presence of a diverse leaf flora that includes aquatic ferns, conifers, and angiosperms. Over the past three years the Laboratory of Palaeobotany of the Institute of Geology and members of P.A.S.A.C. (Paleontólogos Aficionados de Sabinas, A.C.) have made new collections of leaves and woods from the Olmos Formation that expand upon previous data and provide a test of hypotheses regarding the flora and vegetation. Newly collected leaf assemblages contain common palms and at least 35 species of dicotyledons. The dicotyledons are predominantly entire margined and have a high percentage of species with drip tips. Noteworthy taxa include unlobed Laurales with pinnate and palmate venation, pinnately lobed Laurales, Menispermaceae, and primitive eudicots of uncertain familial affinities. Also present is a new genus of leaf with possible affinities to Nelumbonaceae (water lotus family). The wood flora contains abundant angiosperms that include palms, Lauraceae, Fagaceae, Malvaceae, and Cornales. Physiognomic analysis of the leaf flora using simple and multiple regression functions indicates that the Olmos flora represents Paratropical Rainforest, with a Mean Annual Temperature of 20-23 degrees C and Mean Annual Precipitation of 1.5-3 m. Minimal seasonality is indicated by the estimated Mean Annual Temperature, the common occurrence of palms, and the absence of distinct growth rings in dicot woods. The climate inferred for the Olmos leaf flora is significantly wetter than that reported for Campanian-Maastrichtian assemblages from the southeastern US and Western Interior, but is similar to that reported for early Paleogene assemblages. This underscores the potential importance of the Olmos flora for understanding the origin of wet megathermal vegetation in the US during the early Paleogene.