Among the more obscure inhabitants of the world’s oceans are the Cephalodiscidae, a family of small (only a few millimetres in length), largely sessile animals that mostly live in colonies within a shared domicile. Though rarely observed, cephalodiscids have received their fair share of attention due to being among the closest living relatives of the graptolites that once dominated the world’s oceans during the early Palaeozoic era.
Cephalodiscids are one of the two living branches of the pterobranchs (the other being the Rhabdopleuridae), which together with the acorn worms make up the phylum Hemichordata. Hemichordates are in turn one of the three living phyla of the deuterostomes, together with the echinoderms and chordates (to which, of course, we ourselves belong). Pterobranchs are filter feeders, using an arrangement of tentaculated arms arising just behind the head to collect particles from the water. In cephalodiscids, each individual usually possesses multiple pairs of arms in contrast to the single pair in rhabdopleurids (though at least one species of Cephalodiscus has small males with a single pair). The head carries a large glandular disc (hence the name of the family) that is used to secrete the horny tissue making up the external dwelling (referred to as the tubarium) in which a colony of Cephalodiscus lives. Both cephalodiscids and rhabdopleurids have a contractile stalk at the end of the body from which new individuals (zooids) are budded. However, whereas the zooids of rhabdopleurids (and presumably their extinct graptolite relatives) remain attached to each other throughout their life, cephalodiscid zooids split away from their parent by the time they mature. The majority of cephalodiscid species have distinct males and females though a small number may be hermaphrodites. Some species exhibit sexual dimorphism; males may be considerably smaller than females.
About twenty species of living cephalodiscids are currently recognised. The majority of these have been included in a single genus Cephalodiscus, albeit divided between a number of subgenera. The single outlier, Atubaria heterolopha, was described in 1936 from a single dredge haul near Japan (Mitchell et al. 2013). No dwelling material was found in the haul so it was presumed this species does not construct a tubarium like other cephalodiscids. However, its zooids were otherwise little different from those of Cephalodiscus. The subgenera of Cephalodiscus are mostly distinguished by tubarium structure. In some species, each individual in the colony will have its own separate tube closed off at the base. In other species, tubes will open into a central chamber shared between multiple zooids (Maletz 2014). Openings of the tubarium may be surrounded by spines and the like, secreted by the zooids as they creep out from their domicile.
Recent studies have indicated that cephalodiscids represent the sister group to all other pterobranchs/graptolites, implying an history that may extend back to the Cambrian. However, the fossil record of cephalodiscids themselves is minimal. This is largely due to practical difficulties: because the soft-bodied zooids are not preserved, fossils can only be identified from the external tubarium structure alone. Unless the origin point of the tubarium is preserved and identifiable, there is little to distinguish a cephalodiscid tubarium from a benthic graptolite (graptolite colonies begin with a differentiated larval chamber called a sicula, cephalodiscids produce no such structure). A handful of fossil cephalodiscids have been identified, notably the early Devonian Eocephalodiscus, but as yet they tell us little about the evolution of this ancient lineage.
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 analysis reveals that Rhabdopleura is an extant graptolite. Lethaia 46: 34–56.