Article: The feeding mechanisms and affinities of the Triassic brachiopods Thecospira Zugmayer and Bactrynium Emmrich
The morphology of Thecospira and Bactrynium is analysed functionally. It is inferred that the spiral brachidium of Thecospira, with its unique grooved lamellae, bore a simple spirolophe, and that the 'lobate apparatus' of ridges and grooves in Bactrynium bore a complex ptycholophe. The morphology of both genera is consistent with ciliary feeding mechanisms similar to those of living brachiopods: it is inferred that Thecospira operated a spirolophous current system of 'exhalant' or 'spirifer' type, analogous to the living Discinisca; and that Bactrynium operated a ptycholophous system analogous to the living Megathiris. Both brachiopods were cemented in early growth stages but later became free-lying; both had a strophic hinge, and normal articulation and musculature. Bactrynium is pseudopunctate, Thecospira includes punctate and impunctate (or obscurely pseudopunctate) species. Affinity with Permian Davidsoniacea is accepted for Thecospira', the nature of its derivation is discussed in functional terms. Bactrynium is assigned to the Thecideacea; its resemblance to Permian Lyttoniacea is interpreted as due to functional convergence. It is concluded that the Thecideacea were derived from Davidsoniacea, probably during Permian time and probably by neoteny; affinity to Spiriferida or Tere-bratulida is thus rejected, and the older view reaffirmed, that Thecideacea are surviving Strophomenida. The phylogenetic history of the feeding mechanisms of Thecideacea and Thecospira is interpreted in terms of a concept of 'functional zones', and is regarded as an example of 'size-required allometry'.