Page 277 - Introduction to Paleobiology and The Fossil Record

264 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Eumetazoans Demosponges Calcisponges Demosponges Calcisponges Eumetazoans
gut
gut
(macrophagy)
(macrophagy)
Sponge
water-canal system
? (microphagy) Sponge
water-canal system
(a) (b) (microphagy)

Figure 11.4 Sponge paraphyly. (a) The more traditional view presenting both the eumetazoans
and poriferans as monophyletic groups; feeding strategies cannot be polarized since all the
outgroups are non-metazoan. (b) If, however, poriferans are paraphyletic and calcisponges are
more closely related to eumetazoans then the water canal system is a primitive character and the
gut is more derived.

Monaxons

Tetraxons

Triaxons

Desmas

Polyaxons Microscleres

Figure 11.5 Main categories of spicule morphology. Magnifi cation approximately ×75 for all,
except microscleres which are about ×750.

SPICULE MORPHOLOGY

Commonly the spongin skeletons decay and unfused spicular skeletons disintegrate shortly after
death leaving only a selection of hard parts, such as spicules (Fig. 11.5). Spicule morphology is thus

a fundamental means of identification of those spiculate forms. Spicules may be large (megascleres),
acting as part of the skeleton, or small (microscleres), scattered throughout the sponge and rarely
preserved. Five basic types of spicule have been recognized:

1 Monaxons: single axial forms that may grow in one (monactinal) or two (diactinal) directions.
2 Tetraxons (hexactines): four-rayed forms that may have axes of equal length (calthrop).
3 Triaxons: six-rayed forms that form regular networks within the Hexactinellida or glass sponges.
4 Desmas: irregular-shaped forms with ends modified to articulate with one another.

5 Polyaxons: multirayed forms including spherical or star-shaped spicules.