Geology Reference
In-Depth Information
Udoteacean green algae
filamentous (nodular), delicately branched, and phyl-
loid (bladed) growth forms. The basic pattern of the
internal organization is differentiated into a central re-
gion ( medulla ) and a peripheral zone ( cortex ). The med-
ullary filaments are arranged parallel to the axis of the
thallus, interwoven or branched, and give rise to lat-
eral cortical filaments. The medulla is poorly calcified.
Hence, in thin sections the central region often exhib-
its no structures and is strongly recrystallized. The thin-
ner cortical filaments differ greatly in shape and are
straight, swollen or constricted. Calcification of living
udoteaceans varies in intensity and takes place by pre-
cipitation of needle-shaped aragonite crystals on the
surface of filament membranes within the thallus. In
fossil udoteaceans, calcification is mainly recorded in
the cortex. Many udoteacean fossils appear as calcare-
ous 'tubes' or blades, exhibiting a peripheral zone per-
forated by pores and an often strongly recrystallized or
spar-filled central region with faint or no filaments.
Fossil reproductive structures are commonly absent,
reported occurrences are questionable.
Bladed udoteaceans are described in the context of
the discussion of phylloid algae (Sect. 10.2.1.6).
This algal group is well-known to sedimentologists be-
cause of the fundamental importance of Halimeda in
the production of carbonate muds and sands in modern
shallow seas.
Which name is the right one? Taxonomists like to
change systematic categories. Both, erect and filamen-
tous forms were formerly called Codiaceae, a group of
the Bryopsidales (formerly Siphonales). Since modern
Codiaceae include biologically different types, the
group has recently been split up into Udoteaceae, mor-
phologically represented by the genus Halimeda , and
into the non-calcified Codiaceae, represented by the
genus Codium . Most erect fossil forms are now included
within the Udoteaceae. Some authors use the name Hali-
medaceae which has priority over Udoteaceae (Hillis
1991). Filamentous fossil taxa are attributed to the po-
rostromate Cyanobacteria or are sometimes regarded
as members of separate groups of siphonate green al-
gae.
Definition and morphology: Udoteacean algae are
plants consisting of cm-sized segmented thalli attached
to the substrate by rhizoids. The algae develop erect,
Ecological constraints: Modern codiacean green al-
gae occur in normal saline warm waters of tropical shelf,
Fig. 10.10. Udoteacean algae. Species richness and sedimentological role. A : The figure shows the number of species of
three closely related genera characterized by similar patterns in the differentiation of cortical filaments (Pl. 57). In recent
papers some more new species have been described, but the general trend in species richness is the same as shown in the
figure. Halimeda was also reported from Permian deposits. B : The contribution of erect udoteaceans to the sand fractions of
platform and reef carbonates increased notably during the Tertiary and reached in Miocene lagoons values of more than 75%
similar to those of today. PL: Pleistocene. Modified from Flügel (1991). Pictures of the morphotypes after Elliott (1982).
Search WWH ::




Custom Search