The Trouble with Coelosclerites

A couple of years ago, I posted a brief review of the chancelloriids, mysterious sessile animals from the Cambrian period. As explained in that post (which I'd recommend reading before this one), chancelloriids are remarkable for how much we know about them while still being unable to place them anywhere in the animal family tree. However, there are two main options that are currently supported: one is that chancelloriids are sponge-grade animals, probably in the stem-group of modern Epitheliozoa (the clade including coelenterates and bilaterians, which differ from sponges in having a differentiated external skin around their bodies); the other is that chancelloriids are related to other Cambrian animals such as halkieriids, which have themselves been shown to be closely related to molluscs (Vinther & Nielsen, 2005).


Diagram of coelosclerite microstructure from Porter (2008).

The sponge interpretation of chancelloriids has some strong points marshalling in its favour: chancelloriids lack any sign of bilateral symmetry and no sign has been recognised in them of differentiated organ systems. The main feature associating them with halkieriids is the microstructure of their sclerites. Chancelloriids and halkieriids (and a couple of other Cambrian families) possessed sclerites with a microstructure unknown for any other animal group. Known as coelosclerites, these structures were hollow and would have been secreted as a single unit without any subsequent growth*. The greater part of the sclerite was formed of aragonite fibres, arranged parallel to the axis of the sclerite. External protrusions on the sclerite were formed by aragonite bundles sitting at an angle to the main body. A thin layer, probably originally organic, covered the outer surface of the sclerite (Porter, 2008).

*But see Jakob Vinther's comment on the earlier post.

Porter (2008) felt that the similarity between chancelloriid and halkieriid sclerites was so great that it was unlikely that they had evolved independently. The coelosclerite was far from being the only way to develop such a structure: the Cambrian and subsequent periods have seen the evolution of many other sclerite-possessing animals, all of which exhibited different sclerite microstructures. Nor could any convergence be explained by selective pressures: the sessile chancelloriids and slug- or chiton-like halkieriids would have ecologically very different animals. If the coelosclerite structure arose independently in the two groups, the similarities would have to be accepted as pure coincidence.

However, if we accept that coelosclerites had a single origin, we have to explain the complete absence of apparent bilaterian traits in chancelloriids. Many groups of bilaterians have lost their ancestral bilateral symmetry: tunicates, entoprocts, echinoderms, for instance. None of them, however, have lost all trace of their ancestry to quite the same degree that chancelloriids would have had to. Porter (2008) proposed two options: (1) chancelloriids were indeed highly derived bilaterians forming a clade with halkieriids, or (2) chancelloriids were sponge-grade stem-epitheliozoans; coelosclerites arose in the common ancestor of chancelloriids and bilaterians but were subsequently lost by bilaterians other than halkieriids.

Option 2 might appear tempting if halkieriids were close to the base of bilaterians, but it is well-established that they are not. If halkieriids are interpreted as stem-trochozoans (a fairly conservative interpretation) then coelosclerites would have had to have been lost at least six times, in the ancestors of ctenophores, cnidarians, deuterostomes, ecdysozoans, bryozoans and platyzoans (and that is ignoring more phylogenetically contentious groups such as acoelomorphs and chaetognaths that could potentially increase the number even further). If, as seems more likely, halkieriids are stem-molluscs, we have to factor in another two losses for brachiozoans and annelids (and, again, I'm ignoring phylogenetic renegades such as entoprocts). And in the case of brachiozoans, the greater part of the brachiozoan stem group appear to have themselves possessed sclerites; Porter's hypothesis 2 would require the stem-brachiozoans to have lost coelosclerites, only to re-evolve a distinct new sclerite form shortly afterwards.

So, in my opinion, the only really viable options are coelosclerites evolved convergently in two entirely separate lineages, or coelosclerite-possessing animals formed a single monophyletic clade. My personal inclination would be to favour the latter; the examples of ascidians and others demonstrate that significant re-organisations of the bilaterian body plan are not a priori impossible. Of course, the supporting evidence either way remains shaky, and the whole structure could still come tumbling down tomorrow.

REFERENCES

Porter, S. M. 2008. Skeletal microstructure indicates chancelloriids and halkieriids are closely related. Palaeontology 51 (4): 865-879.

Vinther, J., & C. Nielsen. 2005. The Early Cambrian Halkieria is a mollusc. Zoologica Scripta 34: 81-89.