PROTOCISTA, EUBACTERIA, & PORIFERA
PORIFERA & ARCHAEOCYATHA

Sponges, stromatoporoids, and archaeocyathids are included in this part because individual members within each group share, at least partially, similar skeletal features and symmetry leading some workers consider them all as being members of the same phylum. All are benthic, sessile, suspension-feeders which inhabited a wide variety of exclusively marine environments throughout much of the Phanerozoic.

CLASSIFICATION & GEOLOGIC RANGES

Phylum Porifera

Class Demospongea (Cambrian - Recent

Class Hexactinellida (Cambrian - Recent)

Class Calcarea (Cambrian - Recent)

"Class Stromatoporoida" (Ordovician - ?Recent)

Phylum Archaeocyatha (Cambrian)

 

Phylum PORIFERA

Figure 2 - Basic Sponge Morphology

From Boardman et al (1987)

The poriferans (sponges) are characterized by cell groups that are independent of each other and have the ability to change their function during their life cycle. The skeletons of sponges can be composed of an organic substance called spongin (the stuff of an ordinary bath sponge), or they may have calcareous or siliceous skeletons composed of chambers, or more commonly rod-like branched elements called spicules. After death, spicules are scattered across the sea floor and may be found as disarticulated microfossils.

Large spicules (visible to the unaided eye) are termed megalascleres, whereas small ones are called microscleres. Spicules have four basic symmetries: (i) monaxon, (ii) triaxon, (iii) tetraxon, and (iv) polyaxon. Examine Figure 2 below. Together the spicular symmetry and mineral composition serve as the primary basis in poriferan classification.
Figure 2 - Nomenclature of Common Megascleres & Microscleres in Fossil and Modern Sponges

Modified from Boardman et al (1987)

Class Demospongea

Sponges with skeletons of spongin, spongin and siliceous spicules, or a skeleton of fused opaline silica. When present, spicules are commonly monaxon, tetraxon, or polyaxon, but never triaxon. Here is an example of a modern demosponge with spongin. Here is a good example of a fossil demosponge. Note in this specimen the canals in the siliceous walls.

Class Hexactinellida (previously Hyalospongae)

Sponges with siliceous spicules that are usually triaxons and commonly fused to form a net or box-like pattern. They are often called glass sponges. For a recent example of a glass sponge, view this image. Compare it with Hydnoceras from the Devonian of New York.

Class Calcarea

Sponges that have calcareous spicules as in Astaeospongia (the disc shaped fossil in the following image) or more commonly, non-spicular porous chambers (the other three fossils in the image). When spicules are present, they are not fused and are typically monaxons and/or tetraxons.

"Class Stromatoporoida"

Although some texts treats this group as a member of the demospongea, some paleontologists consider stromatoporoids not as true sponges, but belonging to their own phylum. The middle road is taken in this course, treating the stromatoperoids as a separate class within the Porifera. The sheet-like or hemispherical skeletons of stromatoporoids are of two types. The first type have small mounds called mamelons from which canals called astorhizae radiate. This group has horizontal partitions called laminae and vertical partitions called pillars. The space between the laminae and pillars is called the gallery. Compare the image with the accompanying figure below.

Figure 3 - Stromatoporoid Morphology

From Boardman et al (1987)

The second type of stromatoporoid is similar to the first in having laminae and possibly pillars, yet it lacks the astorhizae and mamelons. This form is quite similar to algal stromatolites, but differ in possessing a true calcareous skeleton. Stromatoporoids such as these are quite common from the Silurian and Devonian shallow-water carbonates of central New York and also the Canadian Rockies where they often built reefs.

Phylum ARCHAEOCYATHA

The Archaeocyathids are predominantly an Early Cambrian phylum with no living representatives. They generally have skeletons that formed a porous calcareous cup or cone that resembles later Paleozoic corals. In fact, the archaeocyathids where the reef builders of the Early-Middle Cambrian. The cone-shaped skeletons are commonly constructed of two perforate walls separated by radially arranged vertical blades called septa. As shown in the accompanying figure, the skeletons of archaeocyathids come in two varieties: (i) regulars that have both septa and tabulae but lack dissepiments (small curved plates), and (ii) irregulars that lack septa, but have dissepiments and rod-like bars similar to sponge spicules. This similarity has led some to believe that archaeocyathids belong with the Phylum Porifera. See the diagram below for a representation of an archaeocyathid.

Figure 4 - Archaeocyathid Morphology

From Boardman et al (1987)

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