Element morphology and taxonomic relationships of the Ordovician conodonts Phragmodus primus Branson and Mehl, 1933, the type species of Phragmodus Branson and Mehl, 1933, and Phragmodus undatus Branson and Mehl, 1933

Rediscovery of Branson and Mehl's (1933b) classical outcrop of the Middle Ordovician Joachim Dolomite near Holstein in eastern Missouri, from which they described 32 new species and four new genera of conodonts, makes it possible to obtain the large topotype collections needed to interpret their taxa in terms of modern multielement taxonomy. Of special interest in their Holstein fauna is Phragmodus primus Branson and Mehl, 1933, by original designation the type species of Phragmodus Branson and Mehl, 1933, whose morphology and relationships have been very poorly understood. Study of topotype collections, as well as of Branson and Mehl's syntypes, has failed to reveal any notable morphological differences between the elements of P. primus and those of P. undatus Branson and Mehl, 1933, a very well known and widely distributed species in the Middle and Upper Ordovician Midcontinent Realm faunas. These species are herein considered to be synonymous. Because the original descriptions of P. primus and P. undatus were published simultaneously, neither name has priority over the other. However, P. undatus has figured far more prominently in the taxonomic and biostratigraphic conodont literature than P. primus and therefore we favor use of the former designation for this species. The recognition that P. primus and P. undatus are the same species extends the stratigraphic range of P. undatus downward to considerably below the Deicke K-bentonite, that is, well into the Blackriveran.