Chancelloriids Revised

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Many Cambrian fossils are simply spines and sclerites unassociated with any body. Few of the exceptionally-preserved Cambrian freaks come with spines attached, and some of the most prominent of these are the chancelloriids.

Originally described as sponges by Charles Doolittle Walcott back in 1920 (Walcott, 1920), modern researchers have found that the spines are very similar to those of halkieriids, slug-like spined Cambrian animals and probable stem-group molluscs (Porter, 2008).

ResearchBlogging.orgStefan Bengtson, & Desmond Collins (2015). Chancelloriids of the Cambrian Burgess Shale. Palaeontologia Electronica

If you have any interest in chancelloriids, Stefan Bengtson and Desmond Collins just published an extremely thorough monograph based on examination of hundreds of specimens collected by Collins-led expeditions to the Burgess Shale since 1975, as well as Walcott’s original specimens. As always when it comes to such things, this is not a definitive final say on the structure and affinities of the chancelloriids, but it is the most detailed treatment of them written yet, covering their anatomy, their possible phylogenetic affinities, and even the history of their study,

Here, I will just summarise their main conclusions:

Chancelloriids are classified in the order Chancelloriida and family Chancelloriidae, both with Walcott 1920 as author. The diagnosis for Chancelloriidae is as follows:

Sessile marine animals with radially symmetrical, baglike body, broadening upwards from attachment. Integument flexible, sometimes with fine, rectangularly or rhombically arranged platelets or spines, ~0.1 mm long, directed towards the apex. Armour consisting of spiny coelosclerites, which are usually composite, consisting of individual rays joined at base but having separate internal cavities and foramina; cross-section of rays nearly circular, except where bases are joined; foramen restricted. Sclerites rhombically or irregularly arranged; at the apex concentrated into an apical tuft consisting of modified spine-shaped sclerites surrounding a central apical orifice.

3 genera are recognised: ChancelloriaAllonnia, and Archiasterella. Species-level taxonomy is iffy because of the large number of taxa identified only by their sclerites and not the whole-body morphology.

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That’s a reconstruction of Chancelloria eros, and it demonstrates the general morphology of the chancelloriid animal’s morphology. The animal attaches to a substrate (the sea floor, debris on the sea floor, or even on other living organisms), has a narrow “stalk” that becomes inflated upwards, with an apical opening surrounded by specially modified sclerites.

The body seems to have been muscular, and it most likely had a mechanism of pumping water and wastes through the apical opening. Whether it was also used for feeding is not known. They may have brought prey into the body through the opening, but without any evidence of tentacle-like organs for catching prey, this is unlikely. Also unlikely is filter-feeding, since the body has only one opening. The authors conclude that a symbiotic lifestyle is most likely, evidenced by pyrite getting enriched nearer the apical tip. Pyrite is formed from organic matter, and cnidarian polyps with a similar symbiotic lifestyle have a similar concentration of organic material at the apical opening.

The animal was covered in a skin, with the sclerites being hardened parts of it. They are not independent structures that are embedded in the skin. They most likely served a protective function, the authors likening them to cactus spines.

Their phylogenetic affinity is still inconclusive. They are most definitely not sponges. The lack of a bilateral symmetry and a gut excludes them from being part of the Bilateria. The only extant phyla left are the Placozoa, Ctenophora, and Cnidaria, but they don’t share any characteristics. Basically, they are a black box.

What this also means is that the previous studies placing them as sisters to the halkieriids need to be revisited. If halkieriids are indeed stem-group molluscs, then it is very unlikely that chancelloriids are related, unless we assume a secondary loss of bilaterality and gain of radial symmetry. While by itself not impossible, if you consider that it also must have lost all mesodermal organs, it really strains the imagination.

What the authors conclude is that the Coeloscleritophora, the grouping of chancelloriid and halkieriid, is paraphyletic. That then means that sclerites are a plesiomorphic feature for eumetazoans, and that chancelloriids might even represent the ancestral body plan from which halkieriids and the BIlateria are derived.

There is also the possibility that the homologisation of halkieriid and chancelloriid sclerites is false, and that they evolved convergently. Although the authors discount this in the face of the similarities between the two taxa’s sclerites, I personally think it’s worth keeping this option very open.

The paper has much more detail (and is open access!), this was just a summary of the main findings. For more information, please feel free to request!

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