Archaeolithothamnium sp.

Division Rhodophyta
(Red Algae)

(Scale bar is 1 cm)

Rhodoliths are discoid to spherical nodules formed by the skeletal growth of colonial red algae. They are occasionally found in most limestones of the southeast, but they reach huge population sizes in only two: the Bridgeboro Limestone (Oligocene) and in some parts of the Clayton Formation (Paleocene). Those from the Bridgeboro have been extensively studied and are reliably identified as Archaeolithothamnium, those from the Clayton are not as well studied and have not been precisely identified. The Bridgeboro Limestone usually has a much greater density of rhodoliths, which rarely exceed 6-7 cm (3 in) in diameter and are more often spherical. The Clayton rhodoliths are generally larger and more flattened.

Algae, because they photosynthesize, need to live in water depths where light can penetrate to the bottom. For red algae the depth may be as great as 100 m (~300 ft) in clear water, and Archaeolithothamnium seems generally to prefer the low light intensities found at that depth. Thus, to a first approximation, there must have been about 100 m of water over the Coastal Plain of southwestern Georgia during the early Oligocene, implying a shoreline somewhere near the present Fall Line!

AGE: LOWER OLIGOCENE (The genus is a guide fossil.)

FORMATION: BRIDGEBORO LIMESTONE



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