Archaeocyatha (, "ancient cups") is a taxon of extinct, sessile, reef-building that lived in warm tropical and subtropical waters during the Cambrian Period. It is thought that the centre of the Archaeocyatha origin is now located in East Siberia, where they are first known from the beginning of the Tommotian Age of the Cambrian, 525 million years ago (mya).In other regions of the world, they appeared much later, during the Atdabanian, and quickly diversified into over a hundred families. They became the planet's first reef-building animals and are an index fossil for the Lower Cambrian worldwide.
Preservation
The remains of Archaeocyatha are mostly preserved as
carbonate structures in a
limestone matrix. This means that the fossils cannot be chemically or mechanically isolated, save for some specimens that have already eroded out of their matrices, and their morphology has to be determined from thin cuts of the stone in which they were preserved.
Geological history
Today, the archaeocyathan families are recognizable by small but consistent differences in their
structures: Some archaeocyathans were built like nested bowls, while others were as long as 300mm. Some archaeocyaths were solitary organisms, while others formed colonies. In the beginning of the
Toyonian Age around 516 mya, the archaeocyaths went into a sharp decline. Almost all species became
extinct by the Middle Cambrian, with the final-known species,
Antarcticocyathus webberi, disappearing just prior to the end of the Cambrian period.
[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 the
Demosponges. As for the earliest archaeocyathan, the
Ediacaran sponge
Arimasia from the
Nama Group may be within the clade and specifically allied with Monocyathea; however, this is unclear.
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The archaeocyathans became the planet's first reef-building animals and are an index fossil for the Lower Cambrian worldwide.
Antarcticocyathus was considered the only late Cambrian archaeocyath, but its reinterpretation as a lithisid sponge
Morphology
[[Image:Archaeocyatha.png|thumb|300px| 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 – Rhizoid|left]]The typical archaeocyathid resembled a hollow horn coral. Each had a conical or vase-shaped porous skeleton of calcite similar to that of a sea 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 ice cream cup). At the base, these pleosponges were held to the substrate by a holdfast. The body presumably occupied the space between the inner and outer shells (the intervallum).
Ecology
Flow tank experiments suggest that archaeocyathan morphology allowed them to exploit flow gradients, either by passively pumping water through the skeleton, or, as in present-day, Extant taxon sponges, by drawing water through the , removing nutrients, and expelling spent water and wastes through the pores into the central space.
The size of the pores places a limit on the size of plankton that archaeocyaths could have consumed; different species had different sized pores, the largest large enough to conceivably consume mesozooplankton, possibly giving rise to different ecological niches within a single reef.
Although archaeocyaths have commonly been thought of as stenobionts narrowly adapted to carbonate-dominated marine settings, they were also present in siliciclastic-dominated environments as well.
Distribution
A 2010 study showed that the centre of the Archaeocyatha origin is located in East Siberia, where they are first known from the beginning of the Tommotian Age of the Cambrian, 525 million years ago (mya).
The archaeocyathans inhabited coastal areas of shallow seas. Their widespread distribution over almost the entire Cambrian world, as well as the taxonomic diversity of the species, might be explained by surmising that, like true sponges, they had a planktonic stage that enabled their wide spread.
Taxonomy
phylogenetic 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 without spicules, thus morphologically similar to the archaeocyaths. ] thus sometimes called a pleosponge. But some invertebrate 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, one cladistic 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 a clade nested within the phylum Porifera (better known as the true sponges).
True archaeocyathans coexisted with other enigmatic sponge-like animals. Radiocyatha and Cribricyatha were two diverse Cambrian classes comparable to Archaeocyatha, alongside genera such as Boyarinovicyathus, Proarchaeocyathus, Acanthinocyathus, and Osadchiites.[ Treatise on Invertebrate Paleontology Part E, Revised. Porifera, Volumes 4 & 5: Hypercalcified Porifera, Paleozoic Stromatoporoidea & Archaeocyatha, liii + 1223 p., 665 figs., 2015, available here. .]
The clade Archaeocyatha have traditionally been divided into Regulares and Irregulares (Rowland, 2001):
However, Okulitch (1955), who at the time regarded the archaeocyathans as outside of Porifera, divided the phylum in three classes:
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Phylum Archaeocyatha Vologdin, 1937
Notable fossil sites
The Ajax Mine Fossil Reef in the Flinders Ranges of South Australia contains a large number of Lower Cambrian Archaeocyath fossils, exposed in limestone at ground level.[ Text may have been copied from this source, which is available under a Attribution 4.0 International (CC BY 4.0) licence. (See here. ][
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Footnotes
Further reading
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Cesare Emiliani. (1992). Planet Earth : Cosmology, Geology, & the Evolution of Life & the Environment. Cambridge University Press. (Paperback Edition ), p 451
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Okulitch, V. J., 1955: "Part E – Archaeocyatha and Porifera. Archaeocyatha, E1-E20", in Moore, R. C., (ed.) 1955: Treatise on Invertebrate Paleontology. Geological Society of America & University of Kansas Press, Lawrence, Kansas, 1955, xviii-E122.
External links
