Box Plugging the leaks experimental morphology of archaeocyaths

It is often extremely difficult to reconstruct the life modes of long-extinct organisms that apparently lack modern analogs (see p. 150), particularly when the entire phylum is extinct. In an innovative experimental biomechanical study Michael Savarese (then at Indiana University) constructed models of the three main archaeocyathan morphotypes (aseptate, porous septate and aporous septate), and subjected each to currents of colored liquid in a flume (Fig. 11.12). The first morphotype, a theoretical reconstruction, performed badly with fluid escaping through the intervallum while also leaking through the outer wall. The porous septate form, however, suffered some slight leakage through the outer wall but no fluid passed through the intervallum. The aporous septate form was most efficient with no leakage through the outer walls and no flow through the intervallum. Significantly, ontoge-netic series of the fossils show that an initially porous septate morphotype become aporous in later life, perhaps to avoid leakage through the outer wall (Savarese 1992). This was clearly a great advantage to an organism that survived by pumping huge volumes of seawater through its system!

-current flow aseptate porous septate aporous septate condition condition condition

-current flow aseptate porous septate aporous septate condition condition condition

Figure 11.12 Modeling the functional morphology of the archaeocyaths. (From Savarese 1992.)

varied forms, and brilliant colours. . . . In and out among [the rocks and living corals] moved numbers of blue and red and yellow fishes, spotted and banded and striped in the most striking manner, while great orange or rosy transparent medusa [jellyfish] floated along near the surface. It was a sight to gaze at for hours, and no description can do justice to its surpassing beauty and interest. For once, the reality exceeded the most glowing accounts I had ever read of the wonders of a coral sea.

Alfred R. Wallace (1869) The Malay Archipelago

The cnidarians (or "nettle-bearers") include the sea anemones, jellyfish and corals and are the least complex of the true metazoans (eumetazoans), having cells organized into a

jj Mountains | | Land |||| Continental shelf ^ Reefs

Figure 11.13 Paleogeographic range of Early Cambrian archaeocyathid reefs. (Replotted from Debrenne 2007.)

REGULARES IRREGULARES

REGULARES IRREGULARES

Archaeocyaths Reef
Dokidocyathina

Monocyathida

Figure 11.14 Evolutionary trends within the archaeocyaths; modular forms, appearing iteratively, are indicated by M. (Based on Wood et al. 1992.)

Box 11.3 Neoproterozoic colonies

When was the transition complete from the isolated protist way of life to the loosely integrated colonies of cells in the earliest poriferans? The Neoproterozoic rocks of Namibia yield some clues. Rachel Wood and her colleagues (2002) have described a giant, fully-mineralized, complex colonial skeleton, Namapoikea, from the Northern Nama Group, dated at about 550 Ma (Fig. 11.15). This postdates some of the earliest putative cnidarians and sponges in the Ediacara biota, but predates currently known metazoan reef-type ecosystems. Namapoikea is huge (up to 1 m in diameter), robust, with an irregular structure in transverse section but apparently lacking any internal features. It is uncertain whether this is a sponge or a coral but clearly large, modular, skeletal metazoans were already around in the Late Neoproterozoic, providing a hitherto unexpected complexity to terminal Proterozoic reefs and with the potential to provide both open surface and cryptic habitats. Perhaps these encrusting sheets provided shelter for some of the first micromorphic skeletal metazoans?

Archaeocyaths Reef

50 mm

Figure 11.15 Namapoikea: (a) nodular individual perpendicular to a fissure wall, and (b) section showing tubular construction. (Courtesy of Rachel Wood.)

50 mm

Figure 11.15 Namapoikea: (a) nodular individual perpendicular to a fissure wall, and (b) section showing tubular construction. (Courtesy of Rachel Wood.)

tentacle endoderm mesentery tentacle

Endoderm Ectoderm Mesoglea

[L mesoglea endoderm mesentery ectoderm

[L mesoglea ectoderm

endoderm endoderm

ectoderm musculo-epithelial cell sensory cell nematocyst

mesoglea nerve net ectoderm musculo-epithelial cell sensory cell nematocyst mesoglea nerve net

Figure 11.16 Morphology of Hydra: (a) general body plan, and (b) detail of the body wall.

relatively few different tissue types in a radial plan. They are typified by the well-known hydra (Fig. 11.16). Although there are no specialized organs and only a few tissue types, they are more complex than the parazoans. The group was, in the past, referred to as the Coelenterata, but because that phylum also included the sponges and the gelatinous cteno-phores or comb-jellies, the more restricted term Cnidaria is now generally preferred. Two basic life strategies occur (Fig. 11.17): polyps are usually sessile or attached, although some can jump and somersault, while medusae swim, trailing their tentacles like the deadly and vicious snakes that adorned the head of the mythical Medusa. Although medusoids and polyps appear different, they are essentially the same structures but inverted. Many cnidarians exhibit both forms through their life cycles, others only one. The Portuguese man-of-war, for example, is a spectacular and scary colonial form with a medusoid module for floatation and various types of polyps that help feeding, locomotion and reproduction. As a whole the group is carnivorous, attacking crustaceans, fishes, worms and even microscopic diatoms, with their poisonous stinging cells (cnidoblasts) - the reason they are called "nettle-bearers".

Morphology: the basic cnidarian_

The cnidarians are multicellular, having a single body cavity or enteron; the opening at the top (or bottom in most medusae), sur

Figure 11.17 Cnidarian life cycles: generalized view of the life of the hydrozoan Obelia, alternating between the conspicuous polyp and medusa stages.

rounded by tentacles with stinging cells or nematocysts, functions both as a mouth and an anus. There is thus no head or tail, and nutrients and waste pass through the same opening. The body itself, although diploblas-tic, is, in fact, composed of three layers; the inner endoderm and the outer ectoderm both consist of living cells while the intervening mesoglea is a gelatinous, acellular substance containing rare cells. The outer layer of the body wall contains cnidoblast cells that contain the primed stings or nematocysts that are usually confined to the tentacles. A primitive nerve net is embedded in the mesoglea. Fingers of endoderm commonly poke into the enteron, forming radial partitions that increase the area of absorption of nutrients. These mesenteries can, in the case of the corals, secrete calcium carbonate to form solid, calcified partitions or septa. Most species are found in marine environments although hydrozoans can be very abundant in freshwater habitats.

Classification: design and relationships of the main groups_

The phylum Cnidaria is usually split into three classes: hydrozoans, scyphozoans and anthozoans (Box 11.4). The hydrozoans

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