Page 282 - Introduction to Paleobiology and The Fossil Record

THE BASAL METAZOANS: SPONGES AND CORALS 269

Ajacicyathida
Monocyathida

branching

intervallum

septum aporous septum solitary pseudo-
cerioid
inner wall catenulate
outer wall
Archaeocyathida Kazachsetanicyathida
Tabulacyathida

porous septum branching
?
massive
solitary

sole
branching
(a)
Direction of water flow:
Inhalent
encrusting
Exhalent
Inferred soft tissue distribution

branching
(b)

Figure 11.9 The Archaeocyatha: (a) morphology and (b) classification, function and growth modes of
the main groups. (Based on Wood et al. 1992.)
representatives of the group exist there has Unfortunately many Cambrian carbonates
been, in the past, considerable speculation have been recrystalized, often destroying the
about the taxonomic affinities of the archaeo- details of skeletal morphology. The exoskele-

cyaths: they have been classifi ed with algae, ton of the archaeocyathan animal is aspiculate
calcified protozoans, poriferan-grade metazo- and usually composed of a very porous,

ans, animals with a grade of organization inverted cone composed of two nested concen-
intermediate between protozoans and meta- tric walls separated from each other by radi-
zoans, and cnidarians – none of which now ally arranged, vertical septa (Fig. 11.9). Both
seems likely. the inner and outer walls are densely perfo-
rated and together defi ne the intervallum, or
Morphology and classifi cation: central cavity, partitioned into a number of
archaeocyath individuals and modules segments (loculi) by the radial septa, which
are often less porous than the walls or some-
Archaeocyaths are most commonly found in times aporous. The inner wall circumscribes
carbonates, and details of their morphology the central cavity, open at the top and closed
are usually reconstructed from thin sections. at its base to form a tip. The apex of the