Field of Science

Scleritome Week: The Cactus Animals

They may be a day late, but I promised chancelloriids and here they are!

Chancelloriids are arguably the most frustrating group of animals I'm going to be covering for Scleritome Week. in the cases of machaeridians, Microdictyon and palaeoscolecidans, the identification of articulated specimens revolutionised our understanding of the sclerite-bearing animal. In the case of chancelloriids, despite the availability of a number of well-preserved articulated specimens, we remain very much in the dark. We know what the animals looked like, we have a reasonably good idea of how they were put together, we can infer a lot about their probable life-style. And after all that, we're left with something that just doesn't make a lot of sense.

As you can see in the figure above from Bengtson (2004), chancelloriids were sessile animals, probably filter feeders, with an external covering of star-shaped sclerites. Bengtson (2004) compares their appearance to a cactus, which sounds like a pretty good description to me. Different species varied somewhat in overall shape, from the cylindrical Chancelloria to the more globular Allonnia. Chancelloriids were restricted to the Cambrian, and became extinct by the end of that period (Janussen et al., 2002). Some specimens show a root thickening at the base of the animal that probably served to anchor it in soft sediment.

When first described, chancelloriids were regarded as sponges, with the sclerites compared to sponge spicules. It is true that their overall appearance would have been very sponge-like, but the finer details don't stack up. Sponge spicules are internal structures, secreted by an enveloping layer of sclerocyte cells. In contrast, the chancelloriid sclerites appear to have been at least partially external (the base may have been embedded in the animal's body, with only the spines protruding) and possessed a hollow central cavity that in life probably contained soft tissue (the figure above from Janussen et al., 2002, shows the basal foramina in each individual spine that would have connected the spicule tissue with the rest of the body). Well-preserved specimens from Chengjiang show evidence of a thick epidermis, completely different from the thin and undifferentiated pinacoderm of sponges. On this basis, Janussen et al. (2002) decided that chancelloriids must at least belong to the Epitheliozoa, the clade of all animals except for sponges (Trichoplax plus Eumetazoa). However, as I've recently learnt from Palaeos, one group of sponges, the Homoscleromorpha, does possess a true eumetazoan-like epithelium, though spicules are still of typical sponge construction and nothing like the chancelloriid sclerites.

In 1981, Bengtson and Missarzhevsky suggested an alternative position for chancelloriids as a member of their Coeloscleritophora, along with two order groups of sclerite taxa, the siphonoguchitids and wiwaxiids. The three groups were united by the possession of a hollow sclerite with no evidence of accretionary growth (that is, the sclerite was probably secreted as a unit rather than being added to over the course of the animal's life). Later, Bengtson was to suggest a molluscan affinity for coeloscleritophorans due to similarities in shell secretion.

As far as I know, siphonoguchitids have not yet been found as articulated fossils, but finds from the Burgess Shale mean that the living appearance of wiwaxiids is well-known (picture above from Palaeos). In stark contrast to the sessile chancelloriids, wiwaxiids were mobile animals, a bit like an armoured slug. Authors have differed over whether wiwaxiids were more closely related to annelids or molluscs, but their position somewhere within the trochozoans seems secure. It is a lot more debatable whether the Coeloscleritophora is a monophyletic group, or if the coelosclerite has arisen polyphyletically in unrelated groups. Even before the identification of the wiwaxiid body form, doubts had been cast based on the bilateral nature of individual wiwaxiid and siphonoguchitid sclerites compared to the radial arrangement of chancelloriid sclerites. I can't help asking myself, though, if sea squirts were only known from adult fossils, without any understanding of their development, would any connection be made to other chordates?


Bengtson, S. 2004. Early skeletal fossils. In Neoproterozoic-Cambrian Biological Revolutions (J. H. Lipps & B. M. Waggoner, eds.) The Paleontological Society Papers 10: 67-77.

Janussen, D., M. Steiner & Zhu M. 2002. New well-preserved scleritomes of Chancelloriidae from the Early Cambrian Yuanshan Formation (Chengjiang, China) and the Middle Cambrian Wheeler Shale (Utah, USA) and paleobiological implications. Journal of Paleontology 76 (4): 596-606.


  1. Hey Chris.
    Hip huray for scleritomes. Excellent review of all those bisarre critters, resolved, unresolved, highly debated, almost neglected.....

    Chancelloriids have always been one of my worst enemies. As I have worked with coelosclerites (halkieriids) I know chancelloriids quite well and have studied endless articulated and SSF specimens. Once in a while I believe the coelosclerite model. Other times I am more in favour of the polyphyletic model- that halkieriids and chancelloriids have evolved coelosclerites convergently. At least their body plan are so distinctly different one is undoubtedly a bilaterian (halkieria), it has preserved a gut and looks like a mollusc (or a brachiopod if you have a good imagination ;-) The Chancelloriids are sac shaped, sessile animals with no evidence for internal organs in spite of hundred of fossil in localities that is known for that kind of preservation.
    If accepting the coelosclerite model, then they must be lophotrochozoans. Then it that case an agile metazoan became sessile. As we know halkieriids and wiwaxiids are mollusc relatives then it might be worthwhile having a look at Andy Wanninger et al. recent paper on the entoprocts and their possible molluscan affinities.
    Susannah Porter is supposed to have a paper in press in Palaeontology on additional evidence for a link between chancelloriids and halkieriids. you should post on that Chris when it comes.
    On the other side (the one that I am leaning towards now). is that the sclerites have evolved convergently. I have studied the microstructure of phosphatized halkieriid sclerites and compared with chancelloriids and see some distinct differences that could suggest a different ontogenetic mode of formation. For example, there is good evidence for incremental growth in halkieriid sclerites. They grew to a finite size, however, and were continually shed. But the growth of the sclerite was from an external (basal) epithelium and not from the internal cavity (the tissue inside the sclerite served a different purpose). The chancelloriids grew in different manner that I am not able to explain right now.

    In favour of the polyphyly-of-coelosclerites-model is Sperling and Peterson, based on molecular work of sponges. They believe that sponges easily could have given rise to chancelloriids as they think spicules in these forms are polyphyletic and why not making some extracellular ones in at least one case. This relates to the Homoscleromorph story you also mentioned. Heres a link..
    The paper is published in Geological society of London, special papers. You might be able to find it on their webpage.

    So, where does this put us? I really don´t know. I would like to stay agnostic and await some further evidence. There is not enough evidence to convince me in either direction. Frustratingly, chancelloriids are preserved in all possible ways, phosphatized, articulated and even with soft parts. What more can we ask for and yet we are not able to solve this one!

  2. The chancelloriids grew in different manner that I am not able to explain right now.

    "Not able to explain right now" as in you don't know, or "not able to explain right now" as in wait for the paper? ;-)

  3. Ax suggested a mollusc-entoproct sister relationship in 1999, but unfortunately I can't recall what his grounds for doing so were.

    Ax, P. 1999. Das System der Metazoa II. Ein Lehrbuch der phylogenetischen Systematik. Gustav Fisher Verlag: Stuttgart.

  4. For some reason, Jakob's URL for the Sperling & Peterson paper isn't showing properly on my screen - if anyone else wants a look at it, you can find it here. It's interesting - I may have to post on it soon.

  5. Hey Chris
    As if I dont really know yet. I have some ideas that can easily be refuted. I need to see some more material and think a lot more on them before I will come out and send something for peer review. So a blog is great to get some steam out.
    Have a look at Wanninger et al 2007 and haszprunar and Wanninger 2007 to get an idea of why some people think entoprocts and molluscs could be sistergroups....

  6. Christopher Taylor said:
    "I may have to post on [Sperling & Peterson] soon."

    I hope you do. I originally based the whole sponge revision series on it. Then I did a 180 and came to disagree with almost everything they argued except sponge paraphyly -- had to rewrite everything. I'd be very interested in your opinion.

    The reason you won't find the S&P paper in its original form may be that it has already been published as: Sperling EA, D Pisani & Peterson (2006), Poriferan paraphyly and its implications for Precambrian palaeobiology. In P Vickers-Rich & P Komarower (eds.) The Rise and Fall of the Ediacaran Biota. Geol. Soc. Lond. Spec. Publ. 286: 355–368.

    I don't know why there are two versions, but it seems to be the same paper.

    Toby White


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