"Porostromata" is an antiquated

form taxon Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncertainty; the goal of sc ...

that refers to

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

cyanobacteria. The term porostromate is also used as a descriptor of textures and microstructure of

stromatolites Stromatolites () or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). ...

and oncolites which contain tubules or other cellular structures. The group was defined by Pia (1927) as containing calcareous algae bearing calcified tubules; these may run parallel to the growth surface as in '' Girvanella'', '' Sphaerocodium'' or perpendicular, as in '' Hedstroemia'', '' Ortonella'', and '' Mitcheldaenia''; however, in 1937 Pia restricted the group to only ''Girvanella'' and ''Sphaerocodium'', placing the other genera in the

Codiaceae Codiaceae is a family of green algae in the order Bryopsidales. Genera * † '' Abacella'' Maslov * '' Appeninocodium'' O.Dragastan * '' Arabicodium'' G.F.Elliott * '' Bevocastria'' E.J.Garwood * '' Botryella'' V.P.Shuysky * '' Boueina'' F ...

. In reality it seems that most or all of the species included in "Porostromata" represent fossils of cyanobacteria. Monty (1981) advocated continued use of porostramata in an informal sense to describe a habit and structure of bacterial colonies, rather than in a taxonomic sense. He defined a "porostromate" microstructure as follows:

"Porostromate microstructures are defined by the growth of loose or tangled, vertical, flabellate or flat-lying, straight or sinuous calcified filaments or threads, or even of calcified unicells."

Porostromate stromatolites and oncolites are mostly absent throughout the

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

, with calcified filaments apparently first appearing in the uppermost Precambrian. They then persist throughout the entire

Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anima ...

, though from the

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

and beyond they are only known from freshwater environments. Stromatolites and oncolites not bearing clear cellular structures are referred to as "

spongiostromate "Spongiostromata" is an antiquated form taxon that refers primarily to fossil cyanobacteria. "Spongiostromate" is also used to describe stromatolites and oncolites that do not preserve clear tubules or other cellular microstructure. Pia (1927 ...

"; throughout geologic history they seem to have always vastly outnumbered porostromate forms.

References

Paleobotany Cyanobacteria Stromatolites Trace fossils {{trace-fossil-stub