Page 279 - Introduction to Paleobiology and The Fossil Record

266 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Cnidarians Times of major
Parazoans
reef-building
Cenozoic
100
Cretaceous
Jurassic
200
Triassic
Permian
Age (Ma) Carboniferous Scleractinia
300
Devonian
400
Silurian Archaeocyathans Inozoans
Ordovician Sphinctozoans aclonal, solitary
500 clonal, modular
Cambrian Chaetetids degree of integration

600 Stromatoporoids

Figure 11.6 Stratigraphic distribution of reef-building sponges and related parazoans, together with the
scleractinian corals.

Late Carboniferous the chaetetid calcifi ed often correlated with the rise of the scleractin-
sponges were important reef builders. In the ian corals, equipped with a superior nutrition-
Permian and mid-Triassic, structures involv- gathering system, associated with symbiotic
ing sphinctozoans were common and the Mid zooxanthellae (see p. 285).
to Late Jurassic was marked by bioherms of
lithistid demosponges, while the hexactinel-
lids migrated into deeper-water environments.
Jurassic sponge reefs dominated by hexacti- Stromatoporoidea
nellids and lithistids have been documented The stromatoporoids were mound and sheet-
throughout the Alpine region. Cup-shaped like marine, modular organisms that appeared
and discoidal morphotypes dominated hard in the Mid Ordovician. These animals were
and soft substrates, respectively, and these common components of Late Ordovician,
developed a substantial topography above the Silurian and Early to Mid Devonian shallow-
seafloor, and modern analogs of these hexac- water marine communities, forming irregular

tinellid reefs are now known from off the mounds on the seabed, associated with calcar-
coast of Canada. eous algae and corals. They have a superfi cial
As noted earlier, the acquisition of a calcar- resemblance to some tabulate corals. The

eous skeleton was not confined to any one group reached an acme during the Mid Devo-
class; the calcareous skeleton was developed nian but declined during the later Paleozoic
a number of times, convergently, across the and Mesozoic. Although stromatoporoids
phylum, with a few basic plans superimposed have been understandably classified with the

on pre-existing sponge morphology. Conse- cnidarians, their similarity to the modern cal-
quently, various groups have been recognized cified sponges and the discovery of spicules

on the basis of the calcareous skeleton, but within the skeleton suggest that these, too, are
components of each group arose indepen- poriferans and may well be a grade of orga-
dently in different clades. In broad terms, the nization within the Demospongea. In common
chaetetids and sphinctozoans, together with with a number of other poriferans, the group
the archaeocyaths and stromatoporoids, were is polyphyletic, with stromatoporoid taxa
the most important calcareous reef builders. showing gross morphological convergence
However, the decline of the calcareous sponges towards a common body plan or grade of
in reef ecosystems during the Mesozoic is organization. Because most stromatoporoids