Earth and Planetary Sciences ETDs

Publication Date

Spring 3-2025

Abstract

The Nicoya region in northwestern Costa Rica contains a rich record of Upper Cretaceous fossiliferous sediments, which have received variable levels of study over the past several decades. In particular, rudists from these deposits have been well-characterized, however, the non-rudist ammonite, gastropod, and bivalve fauna have received much less attention. However, this area is particularly important because the Nicoya region was attached to the southernmost tip of North America, making it a unique crossroad between the western Tethys (Caribbean) and Pacific Oceans. Here I present a detailed investigation of these rocks and their fauna between the Late Campanian to Early Maastrichtian (~ 75 – 70 Ma). Fifty-one localities were examined, 42 of which yielded fossil fauna and/or stratigraphic data useful for paleoenvironmental interpretation. Results indicate that after an early Late Campanian tectonic uplift (around 75 Ma), carbonate and mixed platforms allowed the growth of rudist reefs accompanied by a diverse community of other mollusks. During the following intervals through the latest Campanian and into the Early Maastrichtian, sea level oscillated which contributed to changes in molluscan community structure from low diversity, rudist reef-dominated assemblages to deeper water cephalopod-dominated assemblages, and finally shallower water coral reef-dominated assemblages. From a more global view of the Late Cretaceous, each assemblage described here shows a mixed paleobiogeographic affinity: rudists, nerineids, trigoniids, and other bivalves and gastropods show a strong Caribbean affinity, while most ammonites show affinity with the Central (Mediterranean) Tethys; some bivalves are most similar to Californian faunas, and the presence of belemnites and scaphitids (along with scaphitid jaws, the first identified from the Late Cretaceous of the Caribbean region) shows paleobiogeographic affinity with the Gulf and Atlantic Coastal Plains and the Western Interior Seaway. These results provide a baseline to further investigate patterns of diversity and paleobiogeography in molluscan fauna of the tropical Caribbean Tethys, which will help elucidate the effects of the Cretaceous-Paleogene extinction event in the region and subsequent evolution and structure of molluscan communities.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Ph.D. Corinne E. Myers

Second Committee Member

Ph.D. Tyler Mackey

Third Committee Member

Ph.D. Warren D. Allmon

Language

English

Keywords

Campanian, Maastrichtian, paleobiogeography, Nicoya, inoceramids, nerineids

Document Type

Thesis

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