Coelenterata

Archiasterella hirundo sclerite, from Porter (2008).

Belongs within: Eumetazoa.
Contains: Trilobozoa, Ctenophora, Cnidaria.

The cactus animals
Published 24 January 2008
Chancelloria, from Bengtson (2004).

Chancelloriids are arguably the most frustrating group of fossil Problematica. 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, 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.

Chancelloriid sclerite with close-up of basal foramina, from Janussen et al. (2002).

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 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, 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.

Wiwaxia, from Palaeos.

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. 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?

Systematics of Coelenterata
<==Coelenterata [Acalephae, Radiata]
    |  i. s.: Cosmetirella davisiPO99
    |         Rhabdoon singularePO99
    |         Heterotiara anonymaPO99
    |         Eudoxoides spiralisPO99
    |--+--TrilobozoaBJ17
    |  `--+--Eoandromeda octobrachiataBJ17, EL11
    |     `--CtenophoraOH17
    `--+--CnidariaOH17
       |--Haootia quadriformisBJ17
       `--Chancelloriidae [Chancelloriida]MP10
            |--Ginospina araniformis Missarzhevsky 1989D94
            |--Elkanospina trispinata Missarzhevsky 1989D94
            |--Chancelloria Walcott 1920CA06
            |    |--*C. eros Walcott 1920CA06
            |    |--C. iranica Mostler & Mosleh-Yadzi 1976WS93
            |    |--C. lenaica Zhuravleva & Korde 1955WS93
            |    `--C. racemifundis Bengtson in Bengtson et al. 1990MP10
            |--Allonnia Doré & Reid 1965C12, MP10 [incl. Dimidia Jiang in Luo et al. 1982MP10]
            |    |--*A. tripodophora Doré & Reid 1965CA06
            |    |--A. erromenosa Jiang in Luo et al. 1982CA06
            |    |--A. phrixothrix [incl. A. junyuan Janussen et al. 2002]C12
            |    |--A. simplex (Jiang in Luo et al. 1982)CA06
            |    `--A. tetrathallisL04
            `--Archiasterella Sdzuy 1969MP10
                 |--*A. pentactina Sdzuy 1969CA06
                 |--A. antiqua Sdzuy 1969CA06
                 |--A. fletchergryllus Randell et al. 2005P08
                 |--A. hirundo Bengtson in Bengtson et al. 1990P08
                 `--A. robusta Vasilieva 1986CA06

*Type species of generic name indicated

References

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

[BJ17] Budd, G. E., & S. Jensen. 2017. The origin of the animals and a ‘savannah’ hypothesis for early bilaterian evolution. Biological Reviews 92 (1): 446–473.

[C12] Chen, J.-Y. 2012. Evolutionary scenario of the early history of the animal kingdom: evidence from Precambrian (Ediacaran) Weng’an and Early Cambrian Maotianshan biotas, China. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 239–379. Springer.

[CA06] Clausen, S., & J. J. Álvaro. 2006. Skeletonized microfossils from the Lower–Middle Cambrian transition of the Cantabrian Mountains, northern Spain. Acta Palaeontologica Polonica 51 (2): 223–238.

[D94] Dzik, J. 1994. Evolution of ‘small shelly fossils’ assemblages of the early Paleozoic. Acta Palaeontologica Polonica 39 (3): 247–313.

[EL11] Erwin, D. H., M. Laflamme, S. M. Tweedt, E. A. Sperling, D. Pisani & K. J. Peterson. 2011. The Cambrian conundrum: early divergence and later ecological success in the early history of animals. Science 334: 1091–1097.

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.

[L04] Li G.-X. 2004. Early Cambrian hyolithelminths—Torellella bisulcata sp. nov. from Zhenba, southern Shaanxi. Acta Palaeontologica Sinica 43 (4): 571–578.

[MP10] Moore, J. L., S. M. Porter, M. Steiner & G. Li. 2010. Cambrothyra ampulliformis, an unusual coeloscleritophoran from the Lower Cambrian of Shaanxi Province, China. Journal of Paleontology 84 (6): 1040–1060.

[OH17] Ou, Q., J. Han, Z. Zhang, D. Shu, G. Sun & G. Mayer. 2017. Three Cambrian fossils assembled into an extinct body plan of cnidarian affinity. Proceedings of the National Academy of Sciences of the USA 114 (33): 8835–8840.

[PO99] Pagès, F., & C. Orejas. 1999. Medusae, siphonophores and ctenophores of the Magellan region. Scientia Marina 63 (Suppl. 1): 51–57.

[P08] Porter, S. M. 2008. Skeletal microstructure indicates chancelloriids and halkieriids are closely related. Palaeontology 51 (4): 865–879.

[WS93] Wills, M. A., & J. J. Sepkoski Jr. 1993. Problematica. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 543–554. Chapman & Hall: London.

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