Page 275 - Introduction to Paleobiology and The Fossil Record

262 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

osculum

ostium
bud

spongocel
canal
Ascon Sycon Leucon
root tuft
Figure 11.2 Main grades of sponges.
Figure 11.1 Basic sponge morphology.

Box 11.1 Classifi cation and spicule morphology of the sponges

CLASSIFICATION OF THE SPONGES
The phylum Porifera was traditionally subdivided into four classes, the Demospongea, Calcarea,
Sclerospongea and Hexactinellida, based mainly on the composition of the skeleton and type of
spicules. Higher-level taxonomy is based exclusively on soft-tissue morphology. Some workers have
suggested the exclusion of the glass sponges from the Porifera but this is poorly supported; rather
they are closely related to the demosponges. However, the sclerosponges, with some additional cal-
careous skeletons, are now placed within the Demospongea. Thus three classes now comprise the
phylum (Fig. 11.3).

Class CALCAREA (calcareous sponges)
• Sponges with calcitic spicules, usually simple, and/or porous calcareous walls. Marine
environments
• Cambrian to Recent

Class DESMOSPONGEA (common sponges)
• Sponges with skeletons of spongin, a mix of spongin and siliceous spicules or only siliceous
spicules. The spicules may be of two different sizes and the larger are represented by monaxons
and tetraxons. Marine, brackish and freshwater environments. Living sponges previously assigned
to the Sclerospongiae (coralline sponges) – sponges with a compound skeleton of siliceous spic-

ules, spongin and an additional basal layer of laminated fibrous aragonite or calcite – are now
also included here
• Cambrian to Recent

Class HEXACTINELLIDA (siliceous sponges)

• These are the glass sponges with complex siliceous spicules having six rays directed along three
mutually perpendicular axes. Deep-water marine environments
• Precambrian (?) and Cambrian to Recent

However, two form-groups of sponge, the sphinctozoans (with a segmented chambered skeleton)
and the chaetetids (with microscopic tubules) have representatives within the Calcarea and Demo-
spongea; both were important reef builders.