The oldest record of North American Lychnothamnus (northeastern Sonora, Mexico): Implications for the evolution, ecology, and paleogeographic distribution of the genus

Abstract

A large population of Late Cretaceous Lychnothamnus tenuis holding small sized gyrogonites are described after systematically sampling the Fronteras section in the Cabullona Group (northeastern Sonora, Mexico). This discovery represents the oldest record of Lychnothamnus in North America and the first record of this genus in Mexico. The sedimentological analyses of the section suggest that Mexican L. tenuis grew in fluvial and floodplain ephemeral ponds such as their South American counterparts, Lychnothamnus barbosai (oldest known world-wide occurrence of Lychnothamnus) and L. tenuis, from the Turonian–Santonian and Santonian–lower Campanian respectively. The small sized gyrogonites (calcified fructifications of charophytes) of L. tenuis from South America was considered to represent an adaptation to stressful growth conditions since most of the fossil and extant Lychnothamnus species are mainly found in freshwater lacustrine deposits. However, this floodplain-type environment has also been described for a small population of Lychnothamnus sp. recently recovered from the lower Eocene Claron Formation (Utah, USA) and for extant Lychnothamnus barbatus found in Australia, both populations having medium to large gyrogonites. This suggests that rather than being a response to ecological parameters, the gyrogonite size of Lychnothamnus is related to the evolutionary trend of the genus, producing smaller gyrogonites when first appeared in the Upper Cretaceous. Finally, the presence of L. tenuis in Sonora (North America) suggests a south to north colonization route for L. tenuis during the Late Cretaceous.