Page 155 - Carbonate Facies in Geologic History

142 The Advent of Framebuilders in the Middle Paleozoic

mon in well-bedded interreef limestones. Colonial Rugosa have more complex
adaptations. Their Siluro-Devonian growth forms include nodular to massive
lumps, e.g., Hexagonaria. Tsien has demonstrated how in the Belgium Devonian
this genus varies from platy to globular proceeding upslope on buildups and how
the calyx of individual corallites becomes deeper in lagoonal forms. The largest
colonial tetracorals are the spectacular dendroid-fasciculate Devonian forms such
as Disphyllum, whose colonies were as much as 2 m in diamter. Presumably the
Rugosa, like the Tabulata, were more capable than stromatoporoids of withstand-
ing a turbid environment. During dominance of the Tabulata in the Silurian and
earlier Devonian and where conditions of marine salinity were normal, and in
water with moderate turbidity, there formed a general vertical sequence in build-
ups consisting upwards of Rugosa through Tabulata to a stromatoporoid cap.
There is considerable overlap offorms in this sequence.
Colonial Rugosa also occur as isolated clumps in muddy marly beds in bio-
stromes and downslope on edges of banks or reefs in somewhat deeper muddier
water. The dendroid-fasciculate form is quite characteristic of the latter environ-
ment and has a morphological equivalent in the Jurassic-Triassic "Thecosmilia."
Tsien's analysis (1971) of growth forms for these follows. His characterizations
have been somewhat modified based on experience in other areas and by compar-
isons with modern forms of Hexacorals.
a) Fasciculate forms: It is easy to over-generalize on these forms, many of which dwelt in
especialIy protected niches.
(1) Small and delicate: far downslope in argilIaceous substrate.
(2) Large and abundant: higher up slope, shalIow water of moderate agitation, like mod-
ern Acropora cervicornis.
(3) Bushy-knobby: agitated and muddy water (growth form like some modern Porites).
b) Massive growth forms:
(1) Large and globular: Shallow water with frequent sporadic agitation alternating with
quieter water, muddy conditions. "Knollenriffe" of German authors. Common in bio-
stromes, not in real buildups, and equivalent to the biostromes of globular (cabbage-
head) stromatoporoids.
(2) Large and irregular: Wave base zone, clear-water reefy boundstone. "Blockriffe" of
German authors. Comparable to ledge-flat areas of modern coral growth on Bermuda.
(3) Large dish-shaped: Buildups, but somewhat downslope.
(4) Small, thin, dish-shaped: Far downslope of buildups.
(N ote similarity of these shapes to that of modern genus M ontastrea at depths of 30-
50m.)
3. The stromatoporoids: These probable Hydrozoans or sponges are charac-
terized by very tiny pores and a low-lying, encrusting growth form (excepting
some of the later dendroid types). Such morphology prevented successful compe-
tition with corals where water was muddy. On the other hand, in the Middle
Paleozoic, stromatoporoids are more common than corals across vast platforms
or banks of pure carbonate. Probably they were more tolerant than corals of
warm, nutrient-depleted water of variable salinity, but could also exist with corals
where water was generally clear. There is good documentation of stromatopo-
roids replacing both Tabulata and Rugosa toward the top of many reefs which
show other independent evidence of having grown into wave base. Several basic
growth forms of stromatoporoids are ecologically significant and their positions
relative to slopes and tops of carbonate buildups have been tested by studies in
Canada, Belgium, and Sweden.