Page 276 - Introduction to Paleobiology and The Fossil Record

THE BASAL METAZOANS: SPONGES AND CORALS 263

DEMOSPONGEA

Siphonia
(Cretaceous-Tertiary)
Archaeoscyphia
(Ordovician)
HEXACTINELLIDA

Protospongia Hydnoceras Prismodictya
(Cambrian-Ordovician) (Silurian-Carboniferous)
(Devonian-Carboniferous)

CALCAREA

Rhaphidonema Corynella
(Triassic-Cretaceous) (Triassic-Cretaceous)
Astraeospongium
(Silurian-Devonian)
Figure 11.3 Some examples of the main groups of sponges: Archaeoscyphia (×0.25), Siphonia
(×0.4 and 0.8), Protospongia (×0.4), Hydnoceras (×0.25), Prismodictya (×0.6), Rhaphidonema
(×0.8), Corynella (×0.8) and Astraeospongium (×0.4).

The relationships among the three major groups of Porifera are obscure. Analyses of poriferan

morphology and structure, cytology and molecular biology suggest that, first, sponges are a para-
phyletic grouping (Sperling et al. 2007) and, second, that the Calcarea and Hexactinellida form
monophyletic groups that are close to the base of the Eumetazoa. The Demospongea is more basal
(Fig. 11.4).

Continued