New Mexico Geological Society Annual Spring Meeting — Abstracts

A comprehensive biostratigraphy using selachians can be constructed for the Cretaceous Interior Seaway. There are 7 selachian faunachrons defined for the Cretaceous Interior seaway: middle Cenomanian-middle Turonian Ptychodus anonymous-Odontaspis faunachron, middle Turonian P. anonymous-P. whipplei faunachron, middle Turonian-Middle Coniacian, P.whipplei,Squalicorax falcatus faunachron, middle P. mortoni-S. falcatus faunachron, middle Coniacian- Late Santonian P. mortoni-S.kaupi faunachron, early Campanian-middle Campanian S.kaupi-S. pristodus faunachron, and the middle-late Campanian S. pristodus-Cretodus faunachron. The primary species that define the faunachrons belong to the genera Ptychodus and Squalicorax. These species that turn over nearly in step throughout the Late Cretaceous. So, they define both long time intervals from the middle Cenomanian to the middle Turonian, where both Squalicorax falcatus and Ptychodus anonymous are present. They are common members of the P. anonymous-Odontaspis faunachron. There are also very short time intervals, such as the anonymous-whipplei faunachron, where there are overlapping Ptychodus species in the middle Turonian. This biochronological unit is a very short amount of time, spanning just about the length of the deposition of the Juana Lopez Member of the Mancos Shale. A slightly longer, yet relatively short selachian faunachron is in the middle Coniacian; this is a short span of time during the deposition of the El Vado Member of the Mancos Shale and of the Gallup Sandstone. This biochronological unit is determined by the presence of S. falcatus and P. mortoni. Thus, nearly all of the selachian faunachrons are defined by the presence of the species of Squalicorax and Ptychodus.