Celyphus rallus, probable early cretaceous rivulariacean blue-green alga

The morphological resemblance between the sheaths of modern Gloeotrichia and Early Cretaceous Celyphus rallus Batten strongly suggests that the latter is a Mesozoic relative of extant blue-green algal species belonging to the family Rivulariaceae. The fossils are typically encountered in argillaceous sediments that were deposited in non-marine environments. When living, they could have been planktonic, but because all the colonies recovered to date are compressed, the possibility that they formed hemispherical rather than spherical masses and grew attached to a substrate cannot be entirely excluded. In common with many blue-greens, C. rallus may have been a pioneering species, perhaps at the transition from brackish to fresh water deposition. Local abundances might well reflect “blooms” in neutral or slightly alkaline conditions at this juncture, but could also be connected with seasonal changes and fixing nitrogen from the atmosphere in nitrogen-poor environments.