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End-Cretaceous Ecosystems

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The Cretaceous-Paleogene (K-Pg) mass extinction resulted in the extinction of all non-avian dinosaurs, consequently allowing for the rise of mammals. The prevailing hypothesis for the cause of this event is global climatic change resulting from an asteroid impact that struck the Yucatan peninsula 66 million years ago. More recently, however, studies have presented increasing evidence that ecosystems may have already been declining leading up to the impact event. This study aims to assess the stability of local, late-Cretaceous ecosystems preserved in the rocks of the Lance Formation. We use food web network analyses to evaluate whether these ecosystems were already in decline, resulting in an amplified event subsequent to asteroid impact. Microvertebrate fossil assemblages from the Lance Creek Formation of southeastern Wyoming capture a time-series of ecosystem diversity from a nearshore drainage of the Mesozoic intercontinental seaway, making it an ideal study system to test for ecosystem stability leading up to the K-Pg. Using over 300 identified vertebrate microfossils, we established a network highlighting an ecosystem through the latest Cretaceous. From this network, we tested metrics including connectedness, degree distribution, cliques, and diversity. Here, we present a single food web from one our ecosystems sites that will provide as the basis for upcoming research, where multiple networks will be created, analyzed, and compared against each other to evaluate Lance Creek's stability. This network exhibits a connectance of 0.118, an evenness of 0.455, a Shannon's diversity index of 1.59, and a Simpson's diversity index of 0.563, with nodes presenting varying levels of betweenness and degrees. Continued assessment of Lance Creek ecosystem stability will provide a localized perspective into ecosystem stability during this monumental chapter in vertebrate evolution. Furthermore, this research may provide insights into how modern biodiversity will react at the local scale to rapid global climatic change.

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Loyola University Chicago

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End-Cretaceous Ecosystems