Field of Science

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.

9 comments:

  1. How confident can we be that Chancelloriids lacked differentiated organ systems under their spiny coats? To my knowledge we've only found complete scleritomes in life position but no actual soft-tissue underneath it all. I strongly suspect that if we had a living Chancelloriid to dissect we'd find a U-shaped gut, muscles, etc. etc.

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  2. The preserved complete scleritomes also preserve soft tissue; chancelloriids appear to have had a quick thick, leathery integument. However, as yet no-one's noticed any sign of anything inside the integument; it just seems to be pretty much a hollow sack.

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  3. These fossils might make good candidates for some hi-res micro CT scanning,kind of like what was done with those weird little blobby things from the Paleoproterozoic of Gabon. Maybe some thin sheets of compressed internal structure could be teased out from between the upper and lower integumentary layers.

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  4. This may be completely wrong, but is it possible there is symbiosis involved somehow? Could it be that the sclerotomes are produced by symbiotic microorganisms that associate with several different life forms? I expect there is good reason that will not work as an explanation though

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  5. Here are two questions:

    1 - Can we be certain that a Chancelloriid is a single animal? Could it be a colony of individuals like a coral or salp?

    2 - Has any one looked backward in time to see if a Chancelloriid has affinities to pre-cambrian life? Could they be the most evolved species of something that didn't survive past the cambrian but had a long history before it? (e.g. in the small shelly fauna)

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  6. Could it be that the sclerotomes are produced by symbiotic microorganisms that associate with several different life forms?

    Why? And why would it only be with two small ecologically widely-separated groups? And why do we see no comparable situation among other taxa?

    Can we be certain that a Chancelloriid is a single animal? Could it be a colony of individuals like a coral or salp?

    No reason to think so. The known anatomy of a chancelloriid is more consistent with a single terminal mouth or osculum (which hasn't ever been actually observed, but the arrangement of sclerites becomes denser at the terminal end as would be expected if they were guarding a terminal mouth or osculum).

    Has any one looked backward in time to see if a Chancelloriid has affinities to pre-cambrian life? Could they be the most evolved species of something that didn't survive past the cambrian but had a long history before it? (e.g. in the small shelly fauna)

    The small shelly fauna is Cambrian, albeit earlier than the more familiar Cambrian faunas such as the Burgess. No connection has been identified between chancelloriids and earlier taxa; still, in the absence of the distinctive sclerites, it would be difficult to recognise such a connection if it did exist, so failure to identify one doesn't necessarily mean anything.

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  7. Very interesting! I just thought of another question...Are we certain this *is* an animal?

    My question comes from stumbling over the Receptaculites (which are currently algae or plants). It has a spiraling pattern with smaller, denser area near the "top".

    Do any known multi-cellular plants/algae use calcite or aragonite?

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  8. one is that chancelloriids are sponge-grade animals...well it's a remote hypothesis

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  9. Are we certain this *is* an animal?

    (Sorry about the slow reply) I'm not aware of any other type of organism that would show the chancelloriids' combination of a flexible, possibly leathery cuticle with isolated sclerites. Calcified algae, in contrast, have more complete calcification.

    Offhand, I wouldn't treat the interpretation of receptaculitids as algae as secure (though it is the interpretation I personally find more believable). Also, different reconstructions of receptaculitids differ as to whether the point of concentration represents the top or the bottom.

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