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?

REFERENCES

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.