- Archaeocyatha
Taxobox
name = Archaeocyatha
fossil_range = Early - LateCambrian
domain =Eukaryota
unranked_regnum =Opisthokonta
regnum =Animalia
subregnum =Parazoa
phylum =Porifera ? (sponges)
unranked_classis = Archaeocyatha
thumb|300px|right|*">1 – Gap ("intervallum")
*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 – RizoidThe Archaeocyatha or archaeocyathids ("ancient cups") were
sessile ,reef -building [Archaeocyathid reef structures ("bioherms"), although not as massive as later coral reefs, might have been as deep as ten meters (Emiliani 1992:451).] marine organisms of warm tropical and subtropical waters that lived during the early (lower)Cambrian period. They are first known from the beginning of theTommotian Age of the Cambrian, about 530 million years ago (mya), and quickly diversified into over a hundred families. They became the planet's very firstreef building animals.Today, the archaeocyathan families are recognizable by small but consistent differences in their
fossil ized structures: Some archaeocyathans were built like nested bowls, while others were as long as 30 cm. Some archaeocyaths were solitary organisms, while others formed colonies. Then, around 520 mya, the archaeocyaths went into a sharp decline. Almost all species becameextinct by the Middle Cambrian, with the final-known species disappearing just prior to the end of the Cambrian period, "Antarcticocyathus webberi". [The last-recorded archaeocyathan is a single species from the late (upper) Cambrian of Antarctica.] Their rapid decline and disappearance coincided with a rapid diversification of theDemosponges .The typical archaeocyathd resembled a hollow
horn coral . Each had a conical or vase-shaped porousskeleton ofcalcite similar to that of a sponge. The structure appeared like a pair of perforated, nested ice cream cones. Their skeletons consisted 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, these pleosponges were held tosubstrate withholdfast . The body presumably occupied the space between the inner and outer shells (the intervallum). Flow tank experiments suggest that archaeocyathan morphology allowed them to exploit flow gradients, either by passively pumping water through theskeleton , or, as in present-day, extant sponges, by drawing water through thepores , removing nutrients, and expelling spent water and wastes through the pores into the central space.The archaeocyathans inhabited coastal areas of shallow seas. Their widespread distribution over almost the entire Cambrian world, as well as the
taxonomic diversity of thespecies , might be explained by surmising that that they wereplanktonic during theirlarva l stage. Theirphylogenetic affiliation has been subject to changing interpretations: Yet the consensus is growing that the archaeocyath was indeed a kind of sponge, [ Scuba divers have discovered living calcareous sponges, including one species that -- like the archaeocyathans -- is withoutspicule s, thus morphologically similar to the archaeocyaths. (Rowland 2001).] thus sometimes called a pleosponge. But someinvertebrate paleontologist s have placed them in an extinct, separate phylum, known appropriately as the Archaeocyatha. [Debrenne, F. and J. Vacelet. 1984. "Archaeocyatha: Is the sponge model consistent with their structural organization?" in "Palaeontographica Americana", 54:pp358-369.] However, onecladistic analysis [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.] suggests that Archaeocyatha is aclade nested within thephylum Porifera (better known as the true sponges).Notes
References
*Emiliani, Cesare. (1992). "Planet Earth : Cosmology, Geology, & the Evolution of Life & the Environment". Cambridge University Press. (Paperback Edition ISBN 0-521-40949-7), p 451
External links
* [http://www.ucmp.berkeley.edu/porifera/archaeo.html (UCMP Berkeley) Archaeocyathans]
* [http://www.palaeos.com/Invertebrates/Porifera/Archaeocyatha.html (Palaeos Invertebrates) Archaeocyatha]
Wikimedia Foundation. 2010.