THE CALCAREOUS SHELL OF THE BRACHIOPODA and ITS IMPORTANCE TO THEIR CLASSIFICATION

• DOI: 10.1111/j.1469-185x.1956.tb01591.x
• Publication Date: 2008-01-21T23:02:42Z
• Authors: ALWYN WILLIAMS
• Keywords: Polyphyly; Outer core

SUMMARY The most important ordinal classifications of the articulate brachiopods are founded on radical changes skeleton. They all suffer from two serious defects: first a reliance morphogeny only one or features to segregate past and present representatives into ‘natura orders, secondly lack data growth nature living A study shell structure shows that have two‐layered shell, differentiation which is controlled by outer epithelium mantle. Three distinct conditions known: endopunctate with caecal outgrowths mantle penetrating almost external surface layer, pseudopunctate rods (taleolae) granular calcite embedded in inner layer an impunctate free caeca taleolae. were probably monophyletic, polyphyletic ones certainly were. delthyrium accommodating pedicle more commonly constricted confined valve deltidial plates pseudodeltidium. disposition latter suggests possessing it pediculate‐outer epithelial junction was limited valve, while other shared both valves. pseudodeltidium characteristic three stocks, unrelated. Deltidial developed number independent groups. Skeletal supports for lophophore always consisted secondary substance secreted epithelium. simplest types, crura, arose polyphyletically, as comparatively late modifications ridges concerned primarily strengthening sockets affording attachment dorsal adjustor muscles. also associated rather than suspended diaphragm result supporting ridges, platforms, etc. brachial valves unrelated stocks. Calcareous spires homologized side arms terebratuloids not helicoid brachia rhynchonelloid lophophore. Spires loops pre‐eminently spiriferoids terebratuloids, but neither exclusive their respective pallial sinus patterns factor classification been largely neglected. them modification time indicative considerable anatomical readjustment. circulation mantles earliest performed pair sinuses pairs valve. This arrangement reminiscent some inarticulates, addition gonads, usually coelomic visceral cavity projected contained sacs. subsequent development complex gonadal sacs concomitant reduction circulatory sinuses. Certain stages this change‐over attained independently several stocks during existence. Ordinal any these basic ultimately satisfy no seems best build up generic level process morphological comparison. In manner well‐tested will eventually emerge. superfamily, used to‐day, represent taxonomic limit close Twenty‐two superfamilies recognized suggested they tend themselves six groups, each conveniently typified well‐known brachiopods: Orthis, Strophomena, Pentamems, Rhynchonella, Spirifer Terebratula.