NORTH AMERICAN AND EUROPEAN STROPHEODONTIDS: THEIR MORPHOLOGY AND SYSTEMATICS

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Abstract
Morphological studies of North American and European stropheodontid brachiopods appear to indicate that they were a genetically related group which first became distinct from the strophomenids in Upper Ordovician times. By Lower Silurian times this ancestral group had diverged into the 6 fundamental lines of descent: subsequent modification, including especially the complementary growth and degeneration of the pseudodeltidium and chilidium, the spread of denticles along the hinge line, the loss or specialization of the socket ridges and the posterior growth of the cardinal process, proceded in a similar manner in all stocks but with an increasing discrepancy in the time which various lines of descent took to attain the more advanced phases of development (heterochronous parallelism). Applying these observations to a systematic treatment of the stropheodontide it seems advisable to allocate the described 27 supra-specific groups to 19 genera and regard the remaining 8 as chronological subgenera.

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