Fossils in Thin Section: It is Not That Difficult - Microfacies of Carbonate Rocks - page 424

lagoonal and reefal environments. They are commonly
concentrated in depths of a few meters, but extend to
about 100 m ( Halimeda thickets). The plants grow on
sand and mud bottoms below intense water agitation.

ceous carbonate platforms. They became abundant dur-
ing the Cenozoic. Many taxa appear to be restricted to
stage or substage intervals. Udoteaceans related to or
identical to Halimeda are common in shallow-marine
carbonates of Cretaceous and Early Cenozoic age,
where they occur in similar environments as today (Fig.
10.10). For a survey of the Phanerozoic species see
Bassoullet et al. (1983).
Some rock-building fossils of problematic system-
atic position that have been tentatively placed in the
udoteacean or codiacean algae will be discussed in

Distribution: Calcified udoteaceans appear in the
Ordovician. The algae are common in the Ordovician,
abundant in the Early and Middle Devonian, and rela-
tively rare in the Late Paleozoic and Triassic. Calcified
udoteacean algae are locally common in Late Triassic
and Jurassic limestones and abundant in Early Creta-

Plate 57 Calcareous Udoteacean Green Algae and Gymnocodiacean Algae

Modern udoteacean green algae are a major source of calcareous mud from postmortem disintegration of their
skeletons. Halimeda contributes to the formation of carbonate muds and sands in shallow environments but also
to Halimeda 'reefs' occurring today on deeper slopes.
Udoteaceae are calcareous green algae with erect (-> 1-6), nodular or bladed growth forms. The internal
organization of erect segmented forms is exemplified by Halimeda (-> 1, 2). The thallus consists of a central
zone (medulla composed of filaments arranged parallel to the axis) and the peripheral cortex zone exhibiting
smaller branched filaments. Calcification (aragonite) takes place between and around the filaments. Ancient
udoteaceans, known since the Ordovician, are predominantly classified according to growth form (branched,
unbranched, segmented, non-segmented, bladed), and the structure of the medulla and cortex (size, shape, num-
ber, arrangement and branching mode of the filaments).
Gymnocodiacean algae (-> 7, 8) are extinct (Devonian to Tertiary) cylindrical or sack-like algae commonly
occurring as mm-sized fragments. They developed erect, branched or unbranched, segmented or non-segmented
thalli composed of central filamentous medulla and a peripheral thin cortex zone with pores. The primary min-
eralogy is considered to have been aragonite. Mass accumulations in Late Permian and Early Cretaceous Tethyan
carbonate platforms are indicative of warm, shallow seas with low to moderate water energy.

1 Halimeda . Longitudinal section of a segment. The internal structure is characterized by loosely packed filaments forming
an outer cortical (C) zone of thin lateral branches that develops from a broad central zone (medulla, M) with thicker
filaments. Dark areas between the filaments are carbonate. Recent reef lagoon: Florida Bay, U.S.A.
2 Halimeda. Note the differences in the size and arrangement of filaments. The medullary filaments are tangled and form
utricles (swellings; arrow) passing into the cortical zone. Recent reef lagoon: French Reef, Florida Bay, U.S.A.
3 Boueina marondei Flügel. Longitudinal (LS) and tangential section (TS). Note the gross similarity of the internal struc-
ture with Halimeda . M: Central medulla, C: Peripheral cortex. Late Triassic (Norian): Western Thailand.
4 Pseudopalaeoporella lummatonensis (Elliott). The cortex (C) consists of densely spaced, branched and subparallel fila-
ments. Middle Devonian (Eifelian): Graz, Austria.
5 Litanaia graecensis Hubmann. Cross section of a cylindrical thallus, differentiated into central medulla (M) with thick
filaments, and peripheral cortex (C). Note bifurcation and thickening of cortical filaments (arrow). Litanaia and similar
taxa (e.g. Lancicula ) are common in Early and Middle Devonian shelf carbonates. Inner ramp (see Pl. 135). Middle
Devonian (Eifelian): Graz, Austria.
6 Cross section of Boueina hochstetteri Toula, a cosmopolitan Late Jurassic and Cretaceous species occurring predomi-
nantly in Early Cretaceous platform carbonates. Cretaceous (Early Aptian): Southern Carpathians, Romania.
7 Gymnocodiacean grainstone, consisting of broken Permocalculus thalli. Arrows point to thin-walled fragments with fine
densely spaced branched cortical filaments. Permocalculus is characterized by tightly bound segments with a wide,
poorly calcified medulla and a narrow cortex intersected by numerous branched pores (see Fig. 10.11). Parallel alignment
of algal fragments and some coated shells indicates current deposition. Back-reef facies. SMF 18-G YMNO . Middle Per-
mian (Wordian): Djebel Tebaga, southern Tunisia.
8 Compacted gymnocodiacean floatstone, consisting of densely packed thalli of Gymnocodium . Arrows point to longitudi-
nal sections exhibiting the characteristic irregular outline of the thalli caused by thick peripheral filaments. SMF 18-
G YMNO . Late Permian: Taurus Mountain Range, Turkey.
-> 4: Courtesy of B. Hubmann (Graz); 5: Hubmann 1990; 6: Courtesy of I. Bucur (Cluj-Napoca)