Archaeocyatha
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*2 – Central cavity
*3 – Internal wall
*4 – Pore (all the walls and septa have pores, not all are represented)
*5 – Septum
*6 – External wall
*7 – Rizoid
The Archaeocyatha or Archaeocyathids were sessile, reef-building[1] marine organisms of warm tropical and subtropical waters that lived during the Early Cambrian period; they appeared in the Tommotian period about 530 Ma and quickly diversifying into over a hundred families, which are recognizable by small but consistent differences in their fossilized structures, some built like nested bowls, others as long as 30 cm, some solitary, others colonial. Virtually all species became extinct by the mid-Cambrian,[2] coincidental with a rapid diversification of sponges.
Archaeocyatha resemble hollow horn corals. Each has a conical or vase-shaped porous skeleton of calcite similar to that of a sponge. The structure is something like a pair of perforated, nested ice cream cones. Their skeletons consist of either a single porous wall (Monocyathida), or more commonly as two concentric porous walls, an inner and outer wall separated by a space. Inside the inner wall was a cavity (like the inside of an empty ice cream cone). At the base, they were held to substrate with holdfast. The body presumably occupied the space between the inner and outer shells; flow tank experiments suggest that their morphology allowed them to exploit flow gradients, either by passively pumping water through the skeleton, or, as in modern sponges, by drawing water through the pores, removing nutrients and expelling spent water and wastes through the pores into the central space.
Archaeocyatha inhabited areas of shallow seas that were near the shoreline. Their widespread distribution over almost the entire Cambrian world, as well as the diversity of the species, can be explained by surmising that that they were planktonic during their larval stage.
Their phylogenetic affiliation has been subject to changing interpretations: consensus now has it that they were indeed a kind of sponge,[3] but some authorities have placed them in an extinct separate phylum, the Archaeocyatha.[4] Archaeocyathids were the planet's first reef builders. Cladistic analysis[5] suggests that Archaeocyatha is a clade nested within the Porifera (sponges).
[edit] Notes
- ^ Archaeocyathid reef structures ("bioherms"), though not as massive as later reefs, may be as deep as ten meters (Emiliani 1992:451).
- ^ The last recorded archaeocyathan is a single species from the Upper Cambrian of Antarctica.
- ^ Scuba divers have discovered living calcareous sponges, including one species that like Archaeocyathans is without spicules, morphologically similar to archaeocyaths. (Rowland 2001).
- ^ Debrenne, F. and J. Vacelet. 1984. "Archaeocyatha: Is the sponge model consistent with their structural organization?" in Palaeontographica Americana, 54:pp358-369.
- ^ J. Reitner. 1990. "Polyphyletic origin of the 'Sphinctozoans'", in Rutzler, K. (ed.), New Perspectives in Sponge Biology: Proceedings of the Third International Conference on the Biology of Sponges (Woods Hole) pp. 33-42. Smithsonian Institution Press, Washington, DC.
[edit] References
- Emiliani, Cesare, Planet Earth 1992, p 451
[edit] External links
- (UCMP Berkeley) Archaeocyathans
- (Palaeos Invertibrates) Archaeocyatha, including an abstract of Stephen M. Rowland, 2001. "Archaeocyatha: a history of phylogenetic interpretation" in Journal of Paleontology, 75.6 pp. 1065-1078.
