Page 158 - Carbonate Facies in Geologic History

Faunal Evolution in Buildups of the Middle Paleozoic 145

(1964), at the tops of Silurian reefs by Ingels (1963), and in Siluro-Devonian buildups by
Stauffer (1968). Probably nautiloids were predators living on the buildups and their shells
were washed down the sides or caught in tidal pools at the tops.
e) Coral mats such as Amplexus and Tabulates like Alveolites and Thamnopora formed
down the flanks of some buildups.
o Small satellite bioherms are known in flank beds of Silurian mounds in Illinois-Indiana
shelf areas and on Australian reef slopes. These are similar in position and size to some
known in later strata of Pennsylvanian and Triassic age. They are commonly micritic accu-
mulation below wave-base and probably caused by algal growth and lime mud entrapment
by very small and delicate dendroid framework organisms.
Figure IV-28 is a chart of the distribution of the biota in Devonian reefs and
banks of Eifel district in West Germany; it serves as well for a general summary
for the Silurian.

Faunal Evolution in Buildups of the Middle Paleozoic

Givetian and Frasnian strata in the Devonian mark the climax of the coelenterate
reefoid faunas of the Middle Paleozoic. In North America most of these corals
and stromatoporoids appear fairly abruptly in the Middle Ordovician, with asso-
ciated bryozoans and sponges and gradually evolve in size and diversity until
latest Devonian, Famennian, time when they decrease markedly. Carboniferous
buildups are not coralline nor rich in stromatoporoids.
1. Among the fIrst sediment binding, trapping, and framebuilding organisms which ap-
pear with the coelenterates are the sponges. Lithistid sponges in the Middle Ordovician
replace the Archeocyathids and Calathium of the Cambro-Ordovician as reef builders.
Sponges in the Chazyan reefs appear to cover from between 25-50% of the surface areas of
the lowest mounds and are gradually replaced by lime-secreting corals and stromatoporoids.
2. Bryozoans of both ramose and encrusting types are abundant in micritic early stages
of Ordovician and Silurian mounds in the Appalachian miogeosyncline from New York to
Virginia. Beginning in the Middle Ordovician the bryozoans become associated with a sur-
prisingly varied coelenterate fauna which gradually replaces them. From a biological point of
view, the diverse biota of the Crown Point Chazyan buildups appear to constitute the fIrst
fundamentally integrated communities. These possess all the usual reefoid niche-filling forms,
including the predators. In addition, the earliest nonalgal shelf margin buildup (Holston
Formation of Tennessee is a morphologically diverse but solely bryozoan community. The
bryozoan micrite mound assemblages of the Ordovician and Silurian appear to be replaced
by corals in the Devonian but reappear in the Early Carboniferous when fenestrate bryo-
zoans become important and corals decline.
3. Stromatoporoids are larger and more numerous and have more diverse forms in the
Devonian than earlier. They appear to dominate the tops of mounds even in the Ordovician.
Their characteristic growth forms are much more distinctly zoned ecologically in the De-
voman.
4. Tabulate corals, although less important than rugose corals in the Devonian, are the
fIrst carnivorous coelenterates (Lamottia in the Chazyan) and evolve in Late Ordovician and
Silurian times to a variety of large globose and dish-shaped forms which are gradually
replaced by Rugosa in the Devonian. Even though the Tabulata decline somewhat in the
Devonian they may still be quite varied. The genus Alveolites varies from dendroid to thin,
platy encrustations of micrite to massive, irregular forms depending on wave base relations.
5. Rugose tetracorals evolved steadily throughout the Middle Paleozoic, gradually re-
placing the Tabulata in importance. Silurian Rugosa are smaller than those of the Devonian.
The large dendroid-fasciculate forms such as Disphyllum are not known in the Silurian.
6. Mounds formed by stromatolitic mud-trapping algae appear less commonly in De-
vonian than in Silurian and Ordovician strata. Encrusting boundstone of possible algae occur