Suborder TEREBRATELLIDINA

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Introduction

The suborder Terebratellidina comprises eight families which are differentiated on the form and development of the skeleton (brachidium) supporting the lophophore. It is generally called 'the loop' although it is modified in four of the families (Thaumatosiidae, Platidiidae, Megathyrididae and Kraussinidae).

A long reflected loop is the final stage of development in the Laqueidae, Dallinidae, Macandraviidae and Terebratellidae. It develops from two sources, posteriorly from the crura and anteriorly from the median septum, and the pattern of development differs to some extent in each family. These differences are identifiable only in intermediate stages of development from the position of connecting bands between different parts of the loop (Richardson 1975). Resorption of the connecting bands gives a long loop, similar in form in each of the four families.

The lophophore of the loopless families is supported by some of the components which contribute to the loop in the looped families i.e. the median septum, and ascending and descending lamellae, and they occur in a variety of dispositions, for example, the brachidium of species of Argyrotheca consist of descending branches only. The members of these families are small in size and are specialised for a bonded sedentary life style. These characters, together with the undeveloped nature of the brachidium, suggest that they are neotenous derivatives of forms similar to those of members of looped familles (Foster 1989). Spicules occur commonly in loopless forms but are rare in looped genera.

Most Australian genera are in the Terebratellidae. The families Laqueidae, Dallinidae, Megathyrididae and Kraussinidae each contain one Australian genus, but the Macandreviidae, Platiidae and Thaumatosiidae are unrepresented in Australian waters.

Occurrence: Triassic to Recent.

General References

Foster, M.W. 1989. Brachiopodas from the extreme South Pacific and adjacent waters. Journal of Paleontology 63: 33

Richardson, J.R. 1975. Loop development and the classification of terebratellacean brachiopods. Palaeontology 18: 29

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