Geology Reference
In-Depth Information
Plate 65 Charophyta
Charophyta are macrophytic green algae. The plant attains a size ranging from several centimeters to as much as
a meter. The erect and branched thallus is divided into a regular succession of nodes with whorls of small
branches, and internodes.
The reproductive organs develop from the nodes during fertile stages. Ancient charophyte fossils are recorded
by the female gametangia (oogonia) and by internode parts (-> 1). The oogonium consists of spiral cells (Fig.
10.20). These cells are partly calcified (Low- or High-Mg calcite). Only the calcified part of the oogonium
(called gyrogonite) is preserved as a fossil. Many gyrogonites are elliptical in shape and 0.5 to 1.0 mm in size.
Additional calcareous plates can form an envelope (utriculus) for the gyrogonites. The taxonomy of fossil
charophytes is based on isolated oogonia. Generic differentiation of gyrogonites in thin sections is rarely pos-
sible and requires longitudinal sections exhibiting an apex, basal pore and basal plate.
Charophytes have been known since the Silurian and became diversified in the Devonian and during the Late
Jurassic, Early Cretaceous and Tertiary. Valuable charophyte-based biozonations have been established for the
Late Mesozoic and the Tertiary.
Fig. 10.20.
Charophyta and their calcified
parts.
A
: Charophyte plant. The erect branched thal-
lus is divided into a regular succession of
nodes, with whorls of small branches, and
internodes.
B
: Calcification takes place at the nodes only
with the elliptical female reproductive organ
(oogonium) and near the stem nodes and in-
ternodes.
C
: Oogonium consisting of the internal egg
cell, surrounded by spiral cells whose inner
part is calcified (black). This part, called the
gyrogonite, is preserved as a fossil.
D
: High-level systematic units are differen-
tiated according to gyrogonite morphology
and sculpture. From left to right: Sycidiales
(Late Silurian to Early Carboniferous),
Trochiliscales (Late Silurian to Permian),
Charales (Devonian to Recent). A, B after
Wray (1977). C, D after Schudack (1993).
1 Charophyte packstone. Cross sections (white arrows) and longitudinal section (black arrow) of internodia. The algae
occur in association with thin-shelled ostracods. Brackish-water facies. LMF 7. Lowermost Cretaceous (Upper Münden
beds, lowermost Berriasian): Lower Saxony, Germany.
2 Charophyte gyrogonites (G) and internodia (IN). Fine skeletal elements in the matrix are charophyte debris. Late Creta-
ceous (Turonian): Divaca, Slovenia.
3 Lacustrine charophyte limestone. Cross sections and oblique sections of internode parts of Characeae. Spinose surface
structure (arrows) is caused by internode rips. Black grains are reworked micrite clasts (MC). White areas are open pores
(P). Carbonate sands near green algal bioherms. Late Tertiary (Miocene): Marktoffingen, Ries, Southern Germany.
4 Gyrogonite. Cross section. The interior was originally occupied by the egg cell. The 'wall' consists of the calcified parts
(calcine) of the spiral cells (SC) surrounding the egg cell. Early Cretaceous: Subsurface, southern Bavaria, Germany.
5 Gyrogonite. Oblique tangential section displaying the orientation of the spiral cells. Lowermost Cretaceous (Purbeck
facies, Berriasian): Subsurface, Kinsau, southern Germany.
6 Gyrogonite. Lowermost Cretaceous (Purbeck facies, Berriasian): Same locality as -> 5.
7 Wackestone with gyrogonites. LMF 7. Same locality as -> 2.
8
Sycidium
. Middle and Upper Devonian lagoonal carbonates can yield microfossils resembling charophyte oogonia and
attributed to the genera
Sycidium, Karpinskya
or
Trochiliscus
. Middle Devonian: Eifel, Germany.
-> 1, 6: Courtesy of M. Schudack (Berlin).
