"Porostromata" is an antiquated

form taxon Form classification is the classification of organisms based on their morphology (biology), morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncer ...

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 preserve ...

cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...

. 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 ''Girvanella'' is a fossil thought to represent the calcified sheath of a filamentous cyanobacterium known from the Burgess Shale and other Cambrian fossil deposits. Specimens are also known from the Early Ordovician San Juan Formation, Argentin ...

'', ''

Sphaerocodium ''Sphaerocodium'' is a fossil that represents the remains of bacteria in the phylum Cyanobacteria, often called ''blue-green algae''. The species of ''Sphaerocodium'' recorded by the author Rothpletz could be symbiotic intergrowths of different ...

'' or perpendicular, as in ''

Hedstroemia The extinct genus ''Hedstroemia'' was once thought to be a rivulariacean cyanobacterium. It forms oval blobs composed of sinuous tubes that have rounded outlines and occurs in conjunction with '' Solenopora''. ''Hedstroemia'' was formerly assi ...

'', '' 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 (biology), 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.Shu ...

. In reality it seems that most or all of the species included in "Porostromata" represent fossils of

. 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 the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

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

Phanerozoic The Phanerozoic is the current and the latest of the four eon (geology), geologic eons in the Earth's geologic time scale, covering the time period from 538.8 million years ago to the present. It is the eon during which abundant animal and ...

, though from the

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

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) e ...

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

References

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