Catalogue of Organisms: The Cephalodiscids

Among the many extra obscure inhabitants of the world’s oceans are the Cephalodiscidae, a household of small (just a few millimetres in size), largely sessile animals that largely reside in colonies inside a shared domicile. Although hardly ever noticed, cephalodiscids have obtained their fair proportion of consideration because of being among the many closest dwelling kin of the graptolites that after dominated the world’s oceans in the course of the early Palaeozoic period.

Preserved Cephalodiscus colony, copyright E. A. Lazo-Wasem.

Cephalodiscids are one of many two dwelling branches of the pterobranchs (the opposite being the Rhabdopleuridae), which along with the acorn worms make up the phylum Hemichordata. Hemichordates are in flip one of many three dwelling phyla of the deuterostomes, along with the echinoderms and chordates (to which, in fact, we ourselves belong). Pterobranchs are filter feeders, utilizing an association of tentaculated arms arising simply behind the pinnacle to gather particles from the water. In cephalodiscids, every particular person often possesses a number of pairs of arms in distinction to the one pair in rhabdopleurids (although no less than one species of Cephalodiscus has small males with a single pair). The pinnacle carries a big glandular disc (therefore the identify of the household) that’s used to secrete the attractive tissue making up the exterior dwelling (known as the tubarium) through which a colony of Cephalodiscus lives. Each cephalodiscids and rhabdopleurids have a contractile stalk on the finish of the physique from which new people (zooids) are budded. Nonetheless, whereas the zooids of rhabdopleurids (and presumably their extinct graptolite kin) stay hooked up to one another all through their life, cephalodiscid zooids cut up away from their dad or mum by the point they mature. The vast majority of cephalodiscid species have distinct men and women although a small quantity could also be hermaphrodites. Some species exhibit sexual dimorphism; males could also be significantly smaller than females.

Particular person zooid of Cephalodiscus dodecalophus, from Sedgwick et al. (1898).

About twenty species of dwelling cephalodiscids are at the moment recognised. The vast majority of these have been included in a single genus Cephalodiscus, albeit divided between various subgenera. The one outlier, Atubaria heterolopha, was described in 1936 from a single dredge haul close to Japan (Mitchell et al. 2013). No dwelling materials was discovered within the haul so it was presumed this species doesn’t assemble a tubarium like different cephalodiscids. Nonetheless, its zooids have been in any other case little totally different from these of Cephalodiscus. The subgenera of Cephalodiscus are largely distinguished by tubarium construction. In some species, every particular person within the colony can have its personal separate tube closed off on the base. In different species, tubes will open right into a central chamber shared between a number of zooids (Maletz 2014). Openings of the tubarium could also be surrounded by spines and the like, secreted by the zooids as they creep out from their domicile.

Current research have indicated that cephalodiscids symbolize the sister group to all different pterobranchs/graptolites, implying an historical past which will lengthen again to the Cambrian. Nonetheless, the fossil report of cephalodiscids themselves is minimal. That is largely because of sensible difficulties: as a result of the soft-bodied zooids usually are not preserved, fossils can solely be recognized from the exterior tubarium construction alone. Except the origin level of the tubarium is preserved and identifiable, there may be little to tell apart a cephalodiscid tubarium from a benthic graptolite (graptolite colonies start with a differentiated larval chamber referred to as a sicula, cephalodiscids produce no such construction). A handful of fossil cephalodiscids have been recognized, notably the early Devonian Eocephalodiscus, however as but they inform us little in regards to the evolution of this historic lineage.

REFERENCES

Maletz, J. 2014. The classification of the Pterobranchia (Cephalodiscida and Graptolithina). Bulletin of Geosciences 89 (3): 477–540.

Mitchell, C. E., M. J. Melchin, C. B. Cameron & J. Maletz. 2013. Phylogenetic evaluation reveals that Rhabdopleura is an extant graptolite. Lethaia 46: 34–56.