THE CALCAREOUS SHELL OF THE BRACHIOPODA and ITS IMPORTANCE TO THEIR CLASSIFICATION
Authors: WILLIAMS ALWYN 
Published: January 2008

The most important ordinal classifications of the articulate brachiopods are founded on radical changes of the skeleton. They all suffer from two serious defects: first a reliance on the morphogeny of only one or two features to segregate past and present representatives into ‘natura orders, secondly a lack of data on the growth and nature of the living skeleton.

A study of the shell structure shows that all articulate brachiopods have a two‐layered shell, the growth and differentiation of which is controlled by the outer epithelium of the mantle. Three distinct conditions are known: the endopunctate shell with caecal outgrowths of the mantle penetrating almost to the external surface of the outer layer, the pseudopunctate shell with rods (taleolae) of granular calcite embedded in the inner layer and an impunctate shell which is free of caeca and taleolae. The pseudopunctate brachiopods were probably monophyletic, the endopunctate brachiopods were probably polyphyletic and the impunctate ones certainly were.

The delthyrium accommodating the pedicle is more commonly constricted and confined to the pedicle valve by the growth of deltidial plates or a pseudodeltidium. The disposition of the latter suggests that in brachiopods possessing it the pediculate‐outer epithelial junction was limited to the pedicle valve, while in all other brachiopods the junction was shared by both valves. A pseudodeltidium is characteristic of three stocks, all probably unrelated. Deltidial plates were developed in a number of independent groups.

Skeletal supports for the lophophore have always consisted of outgrowths of secondary shell substance secreted by outer epithelium. The simplest types, the crura, arose polyphyletically, probably as comparatively late modifications of ridges which were concerned primarily with strengthening the sockets and affording attachment for the dorsal adjustor muscles. The lophophore was also commonly associated with the dorsal mantle rather than suspended from the diaphragm and as a result supporting ridges, platforms, etc. were developed in the brachial valves in a number of unrelated stocks. Calcareous spires are homologized with the side arms of the terebratuloids and not with the helicoid brachia of the rhynchonelloid lophophore. Spires and loops are pre‐eminently characteristic of the spiriferoids and terebratuloids, but neither are exclusive features of their respective groups.

The pallial sinus patterns as a factor in classification have been largely neglected. A study of them suggests that their modification in time is indicative of considerable anatomical readjustment. The circulation in the mantles of the earliest articulate brachiopods was performed by one pair of sinuses in the pedicle valve and two pairs in the brachial valve. This arrangement is reminiscent of that in some inarticulates, but in addition the gonads, which are usually limited to the coelomic visceral cavity of living inarticulates, projected into the mantle and were contained in a pair of sacs. The subsequent development of complex sinuses from the gonadal sacs was concomitant with the reduction of the circulatory sinuses. Certain stages of development in this change‐over were attained independently by several stocks during their existence.

Ordinal classifications founded on the morphogeny of any one of these basic features of the brachiopods ultimately satisfy no one and it seems best to build up a classification from generic level by a process of morphological comparison. In this manner a well‐tested ordinal classification will eventually emerge. The superfamily, as used to‐day, seems to represent the taxonomic limit to classification by close morphological comparison. Twenty‐two superfamilies are recognized and it is suggested that they tend to segregate themselves into six groups, each conveniently typified by one of six well‐known brachiopods: Orthis, Strophomena, Pentamems, Rhynchonella, Spirifer and Terebratula.