Even though ammonites are not very abundant in Patagonia, a large number of specimens were collected by geologists from the Empresa Nacional del Petroleo (ENAP), Chile, during the course of several years of field work in the Magallanes Basin. This study describes the ammonite family Kossmaticeratidae present in that collection, which is a typical element of most southern hemisphere Late Cretaceous assemblages. The species described include: Kossmaticeras (Kossmaticeras?) fasciculatum n. sp., K. (Karapadites) constrictus n. sp., K. (Natalites) cecioni n. sp., K. (Natalites) hauthali (Paulcke), K. (Natalites) sp., Neograhamites kiliani Spath, N. taylori Spath, Gunnarites elegans n. sp., G. bhavaniformis (Kilian and Reboul), Grossouvrites gemmatus (Huppe), and Maorites densicostatus (Kilian and Reboul).
The stratigraphic information on this fauna indicates that it is distributed in four assemblages. The oldest (assemblage A), possibly late Santonian to early Campanian in age, includes the species Kossmaticeras (Kossmaticeras?) fasciculatum, Neograhamites taylori, and possibly N. kiliani. This is followed by assemblage B (possibly middle to late Campanian), with K. (Karapadites) constrictus, K. (Karapadites) centinelaense (Blasco, Nullo, and Proserpio), K. (Natalites) hauthali, K. (Natalites) cecioni, Neograhamites morenoi Riccardi, N. taylori, N. kiliani, K. (Natalites) sp., Gunnarites elegans, G. bhavaniformis, and possibly Pseudokossmaticeras paulckei Collignon. The third assemblage (C), possibly late Campanian to early Maastrichtian in age, contains G. elegans, G. bhavaniformis, Grossouvrites gemmatus, and K. (Natalites) hauthali. The youngest assemblage (D) is possibly late Maastrichtian in age and is composed of the species Maorites densicostatus and Grossouvrites gemmatus.