Studies of the Mowry shale (Cretaceous) and contemporary formations in the United States and Canada

Cretaceous ammonites from the lower part of the Matanuska Formation, southern Alaska

“…Lumping together of possibly intergrading forms also demands precise stratigraphic control and the demonstration that the fossils under consideration may have coexisted as a single population. This latter requirement can best be met by finding many specimens together in a single bed or in a single concretion, as in the thoroughly convincing study of ammonite variation published by Reeside and Cobban (1960).…”

The Lower Cretaceous (Albian) ammonite genera Leconteites and Brewericeras

“…In recent years, studies of large populatiOns of ammonites have shown that some species exhibit a 1 University of California, Riverside, Calif. 2 Stanford University, Stanford, Calif. surprisingly Wide range of morphologic variation. Studies of the Upper Cretaceous Collignoniceratids by Haas (1946), the Triassic genus Tropites by Silber ling (1959), and the mid-Cretaceous genus Neogastroplites by Reeside and Cobban (1960) conclusively demonstrate that intraspecific variation from finely ribbed compressed individuals to robust coarsely ornamented forms is normal for some ammonites and that finely drawn taxonomic distinctions based on differences in whorl proportions or strength of ornamentation do not hold up when a sufficiently large sample is available. On the other hand, some groups of ammonites are remarkably constant morphologically and show little range in variation.…”

Support for a Vraconnian Stage between the Albian sensu stricto and the Cenomanian (Cretaceous System)

“…In addition, the faunas are completely discrete and mainly endemic with, in the Gulf of Mexico, populations dominated by Engonoceras, Mortoniceras (Angolaites) and Mortoniceras (Subschloenbachia), see KENNEDY et alii (1998). In the Western Interior the associations are almost exclusively Neogastroplites (REESIDE & COBBAN, 1960). It is for this reason that the idea of a connection, either transient or permanent, which was not taken into account by WILLIAMS & STELCK (1975) but considered by OWEN (1996a), is completely speculative.…”