Life cycle of a Cretaceous non-marine centric diatom

It has been largely assumed that the reproductive strategies of diatoms have remained unchanged over time since their origin (hypothetically in the Mesozoic), based on worldwide observations of a number of representative centric and pennate modern species. Yet, only rare fossil evidence supports this thesis. I here present centric fossil diatoms related to extant Melosirales that are found in the Late Cretaceous Huepac Chert (northern Sonora, Mexico). This deposit contains permineralized frustules of sizes varying from ∼4 to 14 μm in diameter. This 70% range in size is comparable to that seen in populations of extant diatoms that include several generations of successively smaller frustrules that result from asexual reproduction. In addition, specimens with sexually produced auxospore-like morphologies and evidence of localized arrangement and mucilage production, are preserved in the chert. This study provides a more complete view of the sexual and asexual stages in the life cycle of ∼70 Ma-old non-marine diatoms, and supports the suggestion that Melosirales, and perhaps centric diatoms in general, have not changed their main reproductive strategies since at least the Late Cretaceous.