|Porifera||Archaeocyatha - 2|
Origins: As stated above, the first known archaeocyaths are found at the beginning of the Tommotian. This turns out not to be a coincidence, because the base of the Tommotian on the Siberian Platform is often diagnosed by the first appearance of the archaeocyath, Nochoroicyathus sunnaginicus. Kouchinsky et al., 2005). Archaeocyaths remain important stratigraphic bookmarks for the rest of the Early Cambrian.
The phylogenetic background of the archaeocyaths is unknown. In fact, they almost out-mystify the conodonts, judging by the number of different taxa to which they have been assigned. Rowland (2001) was forced to resort to a sort of phylogram of phylogenetics in order to describe the evolution of thought about archaeocyath origins. Archaeocyaths have been classified as algae, Cnidaria, some sort of behemoth foraminiferan, and even as vascular plants. Some workers still assert that the archaeocyaths ought to have their own Kingdom. If Monaco can be a kingdom, why not Archaeocyatha?
As Rowland points out, the current dominance of the sponge school of thought (i.e. the spongiform encepahlo-party?) is due mainly to the discovery of structurally analogous modern sponges in the 1970's. These sponges have a massive calcareous base, but also spicules. See image of Astrosclera. However, these "Sclerosponges" or "coralline sponges" are believed to be polyphyletic (Chombard et al., 1997), and we have so far turned up nothing that justifies the placement of the archaeocyaths on the stem lineage of the demosponges in particular. The idea seems to be based on a conference abstract describing some cladistic work in the early 1990's .
Throughout their long history of wandering through the phylogenetic waste, the archaeocyaths have frequently been grouped with other taxonomic vagrants, particularly the receptaculitids and stromatoporoids. The current understanding (Rowland, 2001) is that the receptaculatids (whatever they are) were not close relativesof the archaeocyaths, but that the stromatoporoids probably are -- with both archaeocyaths and stromatoporoids being stem demosponges. This makes fairly good intuitive sense, if Astrosclera is a surviving stromatoporoid as suspected. In that case, we might suppose that the demosponge stem group started with both a massive calcareous framework and silicate spicules, with various groups specializing in one or the other skeletal system over time. However, once agin, there is no evidence that archaeocyaths are more closely related to demosponges than calcareous sponges. We take this matter up again later, in connection with the demosponges.
Evolutionary History: Since the archaeocyath communities were already fully established at the base of the Tommotian in Siberia (Wood, 1998), it is likely that they evolved elsewhere -- or at least earlier. However, Siberia is where, and the Early Cambrian is when, they are first found. Benton & Harper (1997). Their distribution became worldwide, and their diversity grew to about 170 families in the Botomian (Cambrian IV), about 25 My later. Rowland (2001). Two waves of extinction in the later Botomian all but eliminated the entire group. Only one genus has been found from the Middle Cambrian, and another from the Furongian, both discovered in Antarctica. Id. No known archaeocyaths survived into the Ordovician. Their decline may be related to global cooling and the replacement of firm microbial substrates by soft, muddy bottoms over the course of the Cambrian. Bottjer et al. 2000); Álvaro et al. 2003); Dornbos et al. 2005).
Classification: Archaeocyatha have traditionally been divided into Regulares and Irregulares. Rowland (2001) describes the difference as follows:
...Irregularia included those genera in which aporous, concave-upward, curved plates called dissepiments always occur, and in which the dissepiments in the tip of the cup can be seen to have developed before the inner wall or any radial longitudinal elements had developed. Regularia, in contrast, included forms that may or not contain dissepiments, but, where present, they develop after the development of the inner wall and septa and/or tabulae.
He goes on to explain that these are no longer regarded as phylogenetic groups, but that much of the literature still refers uses that terminology. So how might we arrange these groups in a somewhat more phylogenetically useful way? We found two moderately recent attempts to reorganize things. They seem more or less consistent, as shown in the figure. Rather than look for trouble, we've decided to quit while we're ahead. Unfortunately, Perejón & Moreno-Eiris (2006) do not mention Benton & Harper's (1997) Erbocyathoidea. Similarly, Benton & Harper do not mention the Capsulocyathida of Perejón & Moreno-Eiris. Worse, Benton & Harper apparantly put the tabulacyaths under Archaeocyathida.
However, by sweeping all those minor matters under the microbial mat, we can write an incomplete phylogeny something like this:
Of those taxa, Archaeocyathida is probably not monophyletic, and we're a bit dubious about the monocyathids. They may not even be archaeocyaths one reason we still haven't ventured a definition of Archaeocyatha), but the rest look plausibly monophyletic. For a first guess, it will do. ATW070908.