The Cephalodiscids

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.

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 analysis reveals that Rhabdopleura is an extant graptolite. Lethaia 46: 34–56.

Lilies of Blood

The flora of southern Africa is renowned for being remarkably diverse and, in many cases, remarkably eye-catching. The region is home to more than its fair share of ornamental plants, many of which have become popular garden subjects. Among the remarkable members of the southern African flora are the blood lilies of the genus Haemanthus.

Haemanthus coccineus, copyright Peter Coxhead.

Haemanthus is a genus of 22 known species found in the very southern part of the continent, in the countries of South Africa and Namibia (species from further north that have historically been included in Haemanthus are now treated as a separate genus Scadoxus). It is a member of the belladonna family Amaryllidaceae and, like many other members of that family, grows as a herb from a fleshy bulb that is partially or entirely concealed underground. The plant above ground may be annual or persistent, depending on species. Each individual Haemanthus plant produces very few leaves at a time: two is the most common number (Van Jaarsveld 2020). The leaves are more or less fleshy, often hairy, and may be directed upwards or spread outwards.

In those species that shed their leaves, flower stalks are produced before the next season's leaves appear, in a similar matter to the related naked ladies Amaryllis belladonna. Flowers are produced in dense umbels, subtended by bracts that are often brightly coloured, so at a glance the inflorescence of some species might be taken for a single large flower up to ten centimetres in diameter. Depending on the species, the supporting stalk may vary from over a foot in height to only a few centimetres. The first species to be described bear flowers of a bright red colour, explaining both the genus and vernacular names, but flowers may also be pale pink or white. Species that lack the red colour may be referred to as 'paintbrush lilies' rather than 'blood lilies'. Fruits are soft fleshy berries.

Haemanthus albiflos, copyright Krzysztof Ziarnek, Kenraiz.

Phylogenetic analyses of the genus have identified two major clades, a mostly eastern clade found in regions with summer rainfall and a mostly western clade associated with winter rainfall. A notable outlier is the eastern summer-rainfall species H. montanus which is the sister taxon to the winter rainfall clade. Members of the summer-rainfall clade have white or pale pink flowers; members of the winter-rainfall clade have pale pink to dark red flowers. Members of both clades have been grown as pot plants for their unusual appearance though the scent of the flowers is not regarded as pleasant. Perhaps the most widely grown species is H. albiflos, a species native to both the western and eastern parts of South Africa that bears flowers in umbels up to seven centimetres wide. This species is evergreen, carrying its leaves year-round.

REFERENCE

Van Jaarsveld, E. 2020. Haemanthus. In: Eggli, U., & R. Nyffeler (eds) Illustrated Handbook of Succulent Plants: Monocotyledons 2^nd ed. pp. 441–443. Springer.

The Oligorhynchiidae

Dorsal view of Oligorhynchia subplana gibbosa, from Cooper (1935).

From Oligochiton, we move onto Oligorhynchia. The Oligorhynchiidae are a family of very small brachiopods known from the Middle and Late Ordovician. They were among the earliest representatives of the Rhynchonellida, a major group of brachiopods that survives to the present day. Rhynchonellidan shells are usually characterised by a strong beak associated in life with a well-developed pedicel. In oligorhynchiids, this beak is suberect and the shell as a whole is an elongate subtriangular shape. The valves of the shell are folded into coarse plicae (ridges). At least towards the base of the shells, the major folds are in what is called an inverted arrangement, with a ridge in the dorsal valve matched by a valley in the ventral valve (Schmidt & McLaren 1965). Other structural features defining the group include small plates projecting into the pedicel opening, distinct vertical dental plates and divided hinge plates in the valve articulation, and the usual absence of a median septum or cardinal process inside the shell (Savage 1996).

The oligorhynchiids first arose in the east of what was then the continent of Laurentia (corresponding to modern North America). They subsequently spread across the Iapetus Ocean to the continents of Baltica and Kazakhstan (Jin 1996). The end of the Ordovician saw their replacement by other rhynchonellid families. Nevertheless, their genetic lineage would continue for some time yet as they have been identified as ancestors of later families: the Trigonirhynchiidae and Camarotoechiidae (Jin 1989). The brief oligorhynchiid spark would blossom into later rhynchonellid success.

REFERENCES


Jin, J. 1989. Late Ordovician–Early Silurian rhynchonellid brachiopods from Anticosti Island, Quebec. Biostratigraphie du Paléozoïque 10: 1–127, 130 pls.

Jin, J. 1996. Ordovician (Llanvirn–Ashgill) rhynchonellid brachiopod biogeography. In: Copper, P., & J. Jin (eds) Brachiopods pp. 123–132. CRC Press.

Savage, N. M. 1996. Classification of Paleozoic rhynchonellid brachiopods. In: P. Copper, & J. Jin (eds) Brachiopods pp. 249–260. CRC Press.