Irregulares - 1
Cambrian
We don't have a lot to say about the Irregulares at this point. The Archaeocyatha were traditionally divided into Regulares and Irregulares. Over the 1990's the archaeocyath people, and ultimately the sponge community in general, trashed most of their own classification schemes in a fit of melancholy. The Irregulares were thrown out along with everything else, although they may yet be monophyletic, as previously discussed. This emotional crisis was brought on by a growing suspicion that sponges are infected with chronic homoplasy. That is, sponges seemed to evolve in circles, with different groups continually re-inventing the same basic variations on the sponge body plan. In some areas of phylospace, paleontologists moved on to untangle these knots using cladistic methods. In others, there has been a sense of fear and trembling, existential dispair, and a belief that phylogeny is no longer their responsibility because, "the next major development in sponge phylogeny will be molecular studies that will refine the evolutionary relationships between clades." Rowland (2001).
Actually, no. But that's another story, and one we have told too often elsewhere. In any case, the DNA folks are of no use here because the archaeocyaths are all dead ... probably.So many possible lazarus taxa have shown up in the sponge world recently that it isn't safe to bet on that sort of thing. In any case, we're not going to repeat the usual rant about over-reliance on sequence data. Instead, we're essentially going to repeat a few words about morphology. Then, once we've driven off the riff-raff through sheer boredom, we will launch into one of the most outrageous, baseless, and far-fetched speculations in our long history of baseless and far-fetched speculata.
Special Credits: special thanks to Prof. Carlos M. da Silva of the Universidade de Lisboa, for posting really good material on archaeocyaths; to Lângia Colli Montresor, Universidade Federal de Minas Gerais, for help with the translation; and to Adam P. White of Trinity University for a really useful suggestion about sponge evolution. Finally, our thanks to Lin Wei-Hong for translating portions of Feng et al. (2002).
Irregulares Develomental Synapomorphies. The characteristics which are really supposed to hold the Irregulares together are, oddly enough, a suite of essentially developmental features. Otherwise, little distinguishes the typical archaeocyathid (for example) from an oddball ajacicyathid. The developmental model goes like this. All archaeocyaths start out as little cups without pores. In irregulars, the aporous stage is prolonged, and the dissepiments ("framing") start up right away before the organism starts builing the radial septa that make ajacithyacids in particular (see image of the ajacicyathid Rasetticyathus) look so much like a coral in cross-section. Only after this does the animal start to build more outer wall, punch out some pores, put in the septa and tabulae "drywall"), construct the inner wall, and what have you. Archaeocyaths seem to be able to remodel and reinforce all parts throughout life. However, the Irregulares always start with a basal pore-less cup and a network of dissepiments. Benton & Harper (1997); Perejón & Moreno-Eiris (2006).
Outer Wall: The pores in the outer wall tend to be larger than in ajacicyathids. However, the irregulars have a tendency to block off some pores, later in life, with other structures which may have microporosities of their own. Perejón & Moreno-Eiris (2006).
Intervallum: Since the dissepiments are built early, everything else has to work around them. Perhaps for this reason, the partitions of the intervallum tend to be less ... regular. What one is usually told is that Irregulares are structurally more "complex." Rowland (2001). At least in some cases, this seems to mean "chaotic." Compare the neatly arranged septa of Rasetticyathus with the complex Archaeocyathus, or the simply untidy Protopharetra. Irregulars also tend to have other messy habits. For example, the septa get wavy and fuse into one another.
Another frequent characteristic is that the tabulae are continuous with the outer wall. Incidently, you may get tired of our use of "in most cases," "usually," etc. Recall that archaeocyath taxonomy has always been run by stratigraphers and paleoecologists. They want rapid identification tools apomorphies) and ecomorphs, respectively. They don't care much about synapomorphies, phylogeny, or even complete circumscription. Consequently, many archaeocyaths are probably misclassified from a phylogenetic standpoint, and nothing is completely consistent.
In addition, structures of the intervallum in some Irregulares seem to have a greater tendency to grow centripitally, as projections of the outer wall, and thus from the outer wall inwards. This makes good biological sense. Irregulares have to build their drywall around pre-existing framing. There are two ways to approach that problem. The first is the Archaeocyathus approach: start from the outside in an organized fashion and build straight inwards, in a great many places, until one hits a dissepiment. Alternatively, one may apply the Protopharetra technique: start wherever the hell one happens to feel like it and run the septae at any old angle around the beams. (We once lived in a house which was build in the latter fashion, during a week-long beer party. The owner fell apart after that, but the house held together for many years.) The Archaeocyathus approach is referred to as centripetal growth, while the Protopharetra technique is referred to as thromboid growth. The former is typical of Archaeocyathida, and the latter of Kazachstanicyathida. Benton & Harper 1997); Perejón & Moreno-Eiris (2006).
Inner Wall and Central Cavity: It is notable that the inner cavity may become partially, and sometimes completely blocked by all this construction activity.
Ecology: "This is Highly Irregular!"
If you have read the preceding notes with some care, you will sense that something is amiss. Let us restate a few characteristics of Irregularia: 1) prolonged developmental stage with aporous outer wall, (2) secondary blocking of pores, 3) obstruction of the central cavity, and (4) septal divisions which make no hydrodynamic sense. How can this be reconciled with the sponge model of archaeocyath physiology? Truthfully, we don't know. However, sphinctozoan sponges and stromatoporoid sponges had much the same problem and manged well enough. In fact, many archaeocyaths (not all of them Irregulares) are referred to as having sphinctozoan or stromatoporoid growth patterns. This isn't a very satisfactory answer, but it's all we have at the moment.
Hopefully, by this point, we have no readers, and we can do a bit of unconstrained speculation without anyone asking embarassing questions. Our thesis proceeds in four steps.
1) Most of the morphological features of archaeocyaths in general, and Irregulares in particular, are also found in microbialite structures that do not contain any archaeocyaths, or any sponges, or even any metazoans.
2) There are good reasons for (1) that have nothing to do with phylogeny.
3) It is possible to see, with existing data, the beginnings of a phylogenetic progression from microbialite "nest parasites" to archaeocyaths.
4) Irregulares in particular, and metazoans in general, didn't evolve a body plan so much as adapt to pre-existing microbial structures.
The Morphology of Mats
Today, when we think of "microbial mats" or microbialites -- if we think about them at all, that is -- we generally visualize stromatolites or flat, massively layered structures. But these are just the most common structures. Another important form is the thrombolite. In their (likewise massively layered) discussion of marine hard substrates, Taylor & Wilson (2003) discuss thrombolites in the following terms:
Stromatolites are microbial structures (microbialites) with internal
laminations; thrombolites are microbial masses with clotted internal textures. ... Thrombolites range from the
Neoproterozoic to the Recent. They were common in the Cambrian and
Ordovician and the Devonian, but rarer though present in every other
Phanerozoic system. Thrombolites have a much more varied environmental distribution than stromatolites, being found in cryptic spaces such as cavity walls as well as on exposed surfaces.
Thrombolites come in many forms. The only way to do this properly is a walkthrough of some real thrombolites, illustrating the features we have in mind:
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