Sutural form and shell morphology of Placenticeras and systematic descriptions of Late Cretaceous ammonites from the Big Bend region, Texas
Waggoner, Karen J.
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Two related investigations document the sutural form, shell morphology, and taxonomy of several Late Cretaceous ammonites. A morphometric study utilizes geographical information systems (GIS) and multivariate statistical tools to examine the taxonomy, phylogeny, and biostratigraphic utility of ten Placenticeras species common in North American Cretaceous strata. A second biostratigraphic study provides systematic descriptions of ammonites collected from Upper Cretaceous (Campanian) strata in the Big Bend and Sierra Vieja areas of West Texas, using the GIS and statistical methods employed in the first study to assist with identification of the Placenticeras specimens. A high degree of intraspecific variation and strong interspecific similarity among the ten Placenticeras species results in gradational shell morphology and sutural form to such an extent that confident discrimination is only possible for the extreme end members of the gradational series of species. Two distinct sutural lineages are recognized when GIS and morphometric analyses are based on sutural similarity, disregarding previous species assignment. The two separate sutural lineages persisted through most of Late Cretaceous time while shell morphology varied widely. This suggests that sutural form may be a more conservative character, whereas most aspects of shell morphology are more plastic. The Pen and Aguja formations in the Big Bend area and the Ojinaga and San Carlos formations in the Sierra Vieja area preserve ammonites representative of the Scaphites hippocrepis III zone, the Submortoniceras tequesquitense zone, the Menabites (Delawarella) delawarensis zone, and the Baculites maclearni zone. These Campanian biostratigraphic zones allow for correlation of Big Bend and Sierra Vieja strata with Gulf Coast and Western Interior strata. The two sutural form lineages documented in the morphometric study are also recognized in four species of Placenticeras represented in the Big Bend and Sierra Vieja collections. The importance of sutural form for examining the phylogeny of North American Placenticeras species suggests that future application of the methods utilized in this study to specimens representing various ontogenetic stages collected from well constrained stratigraphic successions may result in a complete revision of the genus Placenticeras.