Geology Reference
In-Depth Information
Plate 53 Calcified Cyanobacteria: Porostromate Calcimicrobes
Cyanobacteria (also known as cyanophytes or blue-green algae) are procaryotes. They contribute to sediment
production (microbial mats), sediment construction (e.g. travertines and tufa; reefs), and sediment destruction
(boring and micritization). Cyanobacterial microboring is the oldest known form of bioerosion. The first calci-
microbes appeared near the Precambrian-Cambrian boundary. Calcified cyanobacteria were abundant in Paleo-
zoic and Mesozoic shallow-marine carbonates. Late Mesozoic and Early Tertiary calcimicrobes are commonly
found in lacustrine or marginal-marine limestones.
Calcified cyanobacteria (
calcimicrobes
) form millimeter- to centimeter-sized bushy masses, radial fans or
tangled masses, often composed of very small 'tubes' (designed by calcification within organic sheaths), or
irregularly shaped bodies. Calcimicrobes comprise microfossils consisting of tubes organized in erect radial
clusters of branched tubes (e.g.
Cayeuxia:
-> 1, 2), prostrate felts composed of non-oriented intermingled tubes
(e.g.
Girvanella:
-> 7), dendritic micritic filaments, apparently chambered (e.g.
Epiphyton;
Fig. 10.5)), or ag-
gregates of hollow reniform bodies (e.g.
Renalcis;
Fig. 10.5). Most recent Cyanobacteria use Low-Mg calcite,
but a few also High-Mg calcite or aragonite. Fossil calcimicrobes consist of Low-Mg calcite. At present, intense
cyanobacteria calcification appears to be essentially a freshwater phenomenon and is rare in modern subtidal
environments, in contrast to ancient cyanobacteria which occupied tidal and subtidal environments and were
common in reef and platform settings.
Common criteria used in the differentiation of groups or taxa of tubiform porostromate forms are the arrange-
ment of tubes (e.g. erect, pendant, prostrate; -> 1, 7, 8), branching patterns (e.g. simply dichotomous,
polydichotomous, V-shaped or right-angle branching; -> 1, 3, 6), and the shape and dimensions of the tubes
(inner diameter, wall thickness and wall structure). Growth forms (hemispherical, -> 1; tufted, -> 4; nodular, -
> 6; laterally expanded crusts and sheets, -> 7, 8) as well as growth zonation patterns (-> 2, 4) reflect environ-
mental and biological constraints.
Do not confuse tubular porostromate calcimicrobes with other thin-section fossils!
Solenoporacean filaments
shown in longitudinal sections have smaller dimensions and often exhibit horizontal partitions (Pl. 55). Bryo-
zoan tubes are conspicuously larger and morphologically differentiated (Pl. 85).
1
Cayeuxia
Frollo. Longitudinal section of a nodular thallus. The small nodules exhibiting a hemispherical outline are
composed of outward radiating tubes that originate from a point located peripherally on the thallus. The tubes diverge
upward at a narrow angle.
Cayeuxia
is very similar to the recent alga
Rivularia
; some authors consider the genera as being
identical (Dragastan 1985). Late Jurassic (Tithonian): Kapfelberg, Southern Franconian Alb, Germany.
2
Cayeuxia
Frollo. SEM sample. Note growth zonation (GZ) and branching pattern (arrows). Same locality and age as -> 1.
3
Cayeuxia
. Note nodular growth form, long branched tubes (arrow) and the difference in tube diameter as compared with
-> 1. Species of
Cayeuxia
(and
Rivularia
) are differentiated according to divergence angle, tube diameter and branching
pattern. Late Jurassic (Tithonian): Sulzfluh Mountains, Graubünden, Switzerland.
4
Porostromate cyanobacteria (
Pycnoporidium lobatum
Yabe and Toyama) are characterized by erect growth and differen-
tiation into marginal tufts (arrow). Late Jurassic: Lower Saxonia, Germany.
5
Ortonella
Garwood
.
Cross-section (CS) and longitudinal section (LS) of a globular growth form. High-energy grain-
stones of an inner carbonate ramp. Early Carboniferous (Kohlenkalk, Viséan): Aachen, Germany.
6
Garwoodia
Wood. Note difference in thickness of the tubes (as compared with -> 1 and 5) and right-angle branching
pattern (arrow). Some authors place this genus into the green algae.
Garwoodia
and morphologically similar taxa are
known from Paleozoic and Mesozoic platform carbonates. Jurassic and Cretaceous species are abundant in back-reef and
inner shelf environments. Early Cretaceous: Subsurface, Hohenlinden, Bavaria, Germany.
7
Girvanella ducii
Wethered, characterized by thin-walled tangled, coiled or aligned tubes and prostrate growth. The genus
is common in Paleozoic carbonates. Filaments of micrite-walled calcimicrobes as
Girvanella
might have been a major
source of lime mud in the Early Paleozoic when the tubes fell apart (Pratt 2001). Middle Devonian (Givetian reef):
Sauerland, Germany.
8
Rothpletzella
Wood, characterized by flat, curved or encrusting sheets of tubes branching dichotomously in one plane and
appearing as 'strings of pearls' in oblique sections.
Rothpletzella
is a common Devonian calcimicrobe occurring both in
shallow and deep-water settings, and characteristically found at reef-margins and on fore-reef slopes. The genus acted as
constructors, but also as crypts within cavities. Early Devonian: Near Canberra, Australia.
