Life cycle of 70 Ma-old non-marine pennate diatoms

Life cycle variations in the morphological and behavioral traits of modern pennate diatoms are known for a wide variety of genera and it is assumed that patterns of asexual and sexual reproduction (e.g. size reduction and auxosporulation) have remained essentially unchanged since their origin. We show here that many characteristics of modern pennate diatoms and the communities they inhabit can indeed be recognized in remarkably well-preserved, non-marine fossil material from the Cretaceous (∼70 Ma) Huepac Chert in NW Mexico. Besides the incremental size reduction of the siliceous frustules in species including Fragilaria tarahumara, we also demonstrate the spatial arrangement and distribution of these cells in in situ microalgal communities, and present evidence for local production of dense exopolymeric substances, cell division, and possibly auxospores. This is the first time that such life cycle stages are reported for non-marine fossil diatoms.