Cephalodiscidae

Cephalodiscus nigrescens, copyright Smithsonian National Museum of Natural History.

Belongs within: Hemichordata.

The cephalodiscids
Published 25 September 2021

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.

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

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.

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

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.

Systematics of Cephalodiscidae
<==Cephalodiscidae [Cephalodiscida, Cephalodiscoidea, Eocephalodiscidae]
|--Melanostrophus Öpik 1930 [Melanostrophidae]M14
| `--*M. fokini Öpik 1930MU04
|--Atubaria Sato 1936B70
| `--*A. heterolopha Sato 1936B70
|--Pterobranchites Kozłowski 1967B70
| `--P. antiquus Kozłowski 1967B70
|--Eocephalodiscus Kozłowski 1949B70
| `--*E. polonicus Kozłowski 1949B70, MM13
|--Aellograptus Obut 1964M14
| `--*A. savitskyi Obut 1964B70
`--Cephalodiscus McIntosh 1882MSF05
| i. s.: C. calciformisM14
| C. fumosusM14
| C. lutetianus Abrard, Dollfus & Soyer 1950MSF05
| C. (Acoelothecia John 1931)B70
| `--C. (*A.) kempi John 1931B70
|--C. (Demiothecia Ridewood 1906)MM13, M14
| `--C. (D.) hodgsoni Ridewood 1918MM13, MU04 [=C. (Idiothecia) hodgsoniK48]
`--+--C. (Cephalodiscus)MM13
| |--*C. (C.) dodecalophus M’Intosh 1887B70, M14 [=C. (Demiothecia) dodecalophusK48]
| |--C. (C.) gracilis Harmer 1905MM13 [=C. (Demiothecia) gracilisK48]
| |--C. (C.) inaequatus [=C. (Demiothecia) inaequatus]K48
| `--C. (C.) sibogae [=C. (Demiothecia) sibogae]K48
`--+--C. (Idiothecia Lankester in Ridewood 1906)MM13, B70
| |--C. (*I.) nigrescens Lankester 1905B70
| `--C. (I.) levinseniK48
`--C. (Orthoecus Andersson 1907)MM13, MU04
|--C. (*O.) solidus Andersson 1907B70
|--C. (O.) densus Andersson 1907MU04
|--C. (O.) graptolitoides Dilly 1993MM13
`--C. (O.) rarusK48

*Type species of generic name indicated

References

[B70] Bulman, O. M. B. 1970. Graptolithina with sections on Enteropneusta and Pterobranchia. In: Teichert, C. (ed.) Treatise on Invertebrate Paleontology pt V 2nd ed. pp. V1–V149. The Geological Society of America, Inc.: Boulder (Colorado), and the University of Kansas: Lawrence (Kansas).

[K48] Kozłowski, R. 1948. Les graptolithes et quelques nouveaux groupes d’animaux du Tremadoc de la Pologne. Palaeontologica Polonica 3: i–xii, 1–235.

[M14] Maletz, J. 2014. The classification of the Pterobranchia (Cephalodiscida and Graptolithina). Bulletin of Geosciences 89 (3): 477–540.

[MSF05] Maletz, J., M. Steiner & O. Fatka. 2005. Middle Cambrian pterobranchs and the Question: what is a graptolite? Lethaia 38: 73–85.

[MU04] Mierzejewski, P., & A. Urbanek. 2004. The morphology and fine structure of the Ordovician Cephalodiscus-like genus Melanostrophus. Acta Palaeontologica Polonica 49 (4): 519–528.

[MM13] 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.

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