Page 280 - Introduction to Paleobiology and The Fossil Record
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THE BASAL METAZOANS: SPONGES AND CORALS 267
astrorhizal
canal
astrorhiza on
mamelon
latilamina
pillar
lamina
gallery
Figure 11.7 Stromatoporoid morphology.
look like solidified cow pats, and are all super- most morphological evidence places them
ficially very similar, paleontologists must use firmly in the sponges.
thin sections to describe the microstructure
and classify the species.
Autecology and synecology:
stromatoporoid life and times
Morphology and classifi cation
Stromatoporoids were marine organisms
Typical stromatoporoids have a calcareous usually associated with shallow-water car-
skeleton with both horizontal and vertical bonate sediments often deposited in turbulent
structures and often a fi brous microstructure environments. Many genera were important
(Fig. 11.7). The skeleton is constructed from constituents of reefs, particularly during the
undulating layers of calcareous laminae punc- Silurian and Devonian. For example, the spec-
tuated perpendicularly by vertical pillars. The tacular Silurian reefs on the Swedish island of
surfaces of some forms are modifi ed by small Gotland are characterized by a variety of stro-
swellings or mamelons together with astrorhi- matoporoid growth forms (Kershaw 1990),
zae, radiating stellate canals, which are the whereas throughout North America and
traces of the exhalant current canal system. northern Europe Devonian reef complexes
Siliceous spicules have been identifi ed in some and bioherms are dominated by stromatopo-
Carboniferous and Mesozoic taxa, suggesting roids. These animals had complex water
that the primary skeleton was in fact spicu- systems and grew in a variety of different
late; the calcareous casing is secondary with ways: columnar, dendroid, encrusting and
probably low magnesium calcite precipitated hemispherical forms were associated
within a framework of spongin. with specific energy and turbulence levels
Some authors have included the extinct (Fig. 11.8).
stromatoporoids within the sclerosponges, a Stromatoporoids were also associated with
small group of enigmatic sponges with sili- their own diverse microecosystems; those pre-
ceous spicules embedded in aragonite, com- served in the Silurian of Gotland provided
monly found today in cryptic environments in habitats for communities with over 30 epibi-
the tropics. Others have classifi ed them as ont species (see p. 97) that lived attached to
cyanobacteria, foraminiferans or even as a the animals. Boring, encrusting and epifaunal
separate phylum. But these assignments are organisms made good use of the cavities and
probably only of historical interest because substrates available in and on the stromato-
