AMMONOID ZONATION OF THE TYPE GUADALUPIAN (MIDDLE PERMIAN SERIES)—ANALYSES OF ZONAL BOUNDARIES VERSUS CHRONOSTRATIGRAPHIC BOUNDARIES AND THEIR HISTORICAL CONTEXTS

Recent papers have demonstrated confusion on the part of non-ammonoid specialists regarding current ammonoid systematics and ranges in Middle Permian strata. While classic monographs from the middle of the past century (e.g., Miller and Furnish 1940) provide the primary framework for ammonoid zonation, considerable refinement has occurred since then. Most of these refinements reside in more narrowly focused papers published in various venues around the world. A summary of these changes is both timely and appropriate. Formal ammonoid zonation of the Middle and Upper Permian is based on phylogenesis of the Cyclolobinae. Miller and Furnish 1940 recognized three cyclolobid zones: the Zone of Waagenoceras represented what we now recognize as the Roadian and Wordian Stages, the Zone of Timorites represented the Capitanian Stage, and the Zone of Cyclolobus represented post-Guadalupian strata. By way of example, Waagenoceras has undergone considerable taxonomic revision: Demarezites was erected by Ruzhentsev 1955 for a group of earlier, simpler morphologies that had previously been assigned to Waagenoceras . The generic concept of Demarezites remained unsettled until stabilized by Glenister and Furnish (1987). Classic Waagenoceras is now clearly differentiated from coeval Newellites Furnish and Glenister 1969 (an advanced offshoot) and Mexicoceras Ruzhentsev 1955 (a variable, generally simpler form). Mexicoceras is very abundant in the type Guadalupian, but has misled several workers because phylogenetically it belongs to the morphologically similar Kufengoceratinae. These taxonomic revisions have altered generic ranges, as should be expected. Also, the GSSP definitions are based on conodont speciation events, so even though the ammonoid zones characterize their respective stages, their zonal boundaries do not coincide precisely. An example is given by the Demarezites/Waagenoceras transition (Lambert et al., 2000). Ammonoids with characters transitional between the two genera occur at the traditional Roadian/Wordian lithostratigraphic boundary (top of Road Canyon Fm, Glass Mtns; base of Brushy Canyon Fm, Gaudalupe Mtns), but precede the conodont-defined basal Wordian GSSP horizon (just above top of Willis Ranch Mbr, Word Fm, Glass Mtns; just up into Getaway Mbr, Cherry Canyon Fm, Guadalupe Mtns).