Geology Reference
In-Depth Information
Angulocellularia Group: Precambrian to JurassIc
Criteria: Micritic, solid, bushlike form, erect or pen
dant growth; similar to recent Oscillatoriaceae
Examples: Angulocellularia: Cambrian, Early Ordovi
cian; Frutexites: Paleozoic, Jurassic
Epiphyton Group: Cambrian and Early Ordovician;
Late Devonian
Criteria: Micritic narrow dendritic filaments, which
may be rodlike, apparently chambered or tubular;
erect or pendant growth
Examples: Epiphyton: Cambrian to Devonian; Thar
ama: Silurian to Devonian
Girvanella Group: Abundant in the Paleozoic
Criteria: Thinwalled tubes, tangled, coiled or
aligned; prostrate growth; similar to recent Oscilla
toriaceae
Examples: Girvanella: Cambrian to Cretaceous
Hedstroemia Group: Cambrian to Cretaceous
Criteria: Branching clusters of tubes; clusters with
fanlike longitudinal sections, radial erect growth. Ad
jacent tubes may share common walls or may be
separate; taxa separated according to patterns of
branching. Similar to recent rivulariacean cyanobac
teria
Examples: Hedstroemia: Silurian; Ortonella: Car
boniferous; Cayeuxia: Triassic to Cretaceous
Renalcis Group: Abundant in Cambrian and Early
Ordovician; Late Devonian; common in reefs
Criteria: Clusters of hollow reniform bodies, usually
with thick walls which may be micritic, fibrous or clot
ted; irregular or erect growth
Examples: Renalcis: Cambrian to Early Carbonif
erous; Shuguria: Cambrian to Carboniferous
Garwoodia/Mitcheldeania Group: Late Paleozoic
and Mesozoic
Criteria: Coarse, thinwalled, radiating clusters of
tubes which are commonly juxtaposed (systematic
position uncertain, may be green algae)
Examples: Garwoodia: Devonian to Cretaceous;
Mitcheldeania: Carboniferous to Cretaceous
Rothpletzella Group: Silurian and Devonian
Criteria: Flat, curved and encrusting sheets of jux
taposed tubes which branch dichotomously in one
plane, systematic position uncertain
Examples: Rothpletzella: Devonian
Fig. 10.5. Practical classification of calcified cyanobacteria based on growth forms and filament patterns appearing as
variously oriented tubiform structures or irregular bodies (modified from Riding 1991). The tubes occur in clusters within
commonly millimeter-sized nodules, bushes or sheets. Preferred growth direction is erect, pendant or prostrate (horizontal,
parallel to the substrate). The Hedstroemia group and the Garwoodia/Mitcheldeania group comprise microfossils, often
described as porostromate algae or microbes. The systematic position of these fossils (cyanobacteria or green algae) as well
as the generic differentiation is still a matter of discussion.
Thin-section pictures: Frutexites occurring in a deep-water stromatolite. Note the bush-like growth form and the micro-
structure consisting of radially arranged fibres of Fe-Mn oxides and calcite. Frutexites was probably non-phototrophic and
seems to have preferred oxygen deficient, low-energy environments (Böhm and Brachert 1993). Early Jurassic: Near Lunz,
Austria. - Epiphyton: The genus appears close to the Precambrian-Cambrian boundary and is widespread in Cambrian reef
limestones. Early Cambrian: Southern Sardinia. - Girvanella forms small nodules, oncoids or sheets composed of loosely or
densely tangled thin-walled tubes. Early Ordovician: Western Newfoundland. - Cayeuxia and morphologically similar taxa
are abundant fossils in Mesozoic platform carbonates. The genus was placed in the synonymy of Rivularia , a modern
cyanobacterium occurring in freshwater and marine environments. Late Jurassic (Tithonian): Sulzfluh, Switzerland. - Renal-
cis often occurs together with Epiphyton (bottom of the picture). Both genera are principal constituents of Cambrian archaeo-
cyath reefs. Early Cambrian: Southern Sardinia. - Garwoodia and other taxa of this group are characterized by coarser tubes
than of Hedstroemia and similar genera. The size of the tubes has been used as an argument for assigning these microfossils
to udoteacean green algae. Late Jurassic (Tithonian): Sulzfluh, Switzerland. - Rothpletzella occurs in platform and reef
carbonates. Early Devonian (Currajong limestone): Canberra, Australia.
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