22-5 Reconstructing a Macropredator-Rich Miocene Ocean Using Stable Calcium Isotopes

Session: Working Up an Apatite: Teeth as Paleo -Ecological and -Climatological Archives

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Marine mammals have shaped ecosystems as architects of trophic structure for millions of years. Yet knowledge of their ancient ecology remains elusive, and many extant species are difficult to observe. The Miocene marine mammal community was diverse and transitional, including early baleen whales, raptorial sperm whales, large-bodied eared seals, and sirenians, many of which were adapted to warm, productive coastal ecosystems. Pliocene marine mammals were part of a now-extinct food web dominated by increasingly macropredatory marine vertebrates, forming an unprecedentedly long food chain. Some researchers suspect climate-driven shifts in primary productivity during the Pliocene epoch led to the extinction of many megafaunal marine vertebrate species and extreme migratory behavior in many survivors (i.e., large whales) due to resource scarcity. Calcium (δ^44/40Ca) isotope variations in tooth enamel have the potential to reveal the trophic level of fossils millions of years older than what is currently accessible with nitrogen isotope (δ¹⁵N) analysis. This study presents new δ^44/40Ca data from marine mammal, shark, and fish fossils from the Miocene-Pliocene Yorktown (U.S. East Coast) and Miocene Monterey (California) formations. The results support the emerging hypothesis of an exceptionally long Miocene and early Pliocene marine food chain. This is consistent with carbon (δ¹³C) and oxygen (δ¹⁸O) isotope evidence for early niche partitioning among these sympatric taxa, pointing to early marine mammal ecosystem specialization and resource differentiation. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6026

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