TAXONOMIC AFFINITY OF LATE DEVONIAN ORGANIC-WALLED MICROFOSSILS: INTERPRETATIONS AND IMPLICATIONS

Devonian strata host a diverse assemblage of organic-walled microfossils (OWM). Most of the microfossils found in Devonian shales are characterized as acritarchs: OWM of uncertain taxonomic affinity. Many smooth-walled OWM from the Paleozoic are inferred to be reproductive structures, called phycoma, made by a single genus within the green algal group Prasinophycea. The inferred green algal affinity of these OWM forms is often used as the basis for various proxies, including phytoplankton diversity and levels of primary production. However, these inferences are often made without a complete understanding of the factors that lead to the formation and preservation of resistant organic structures in the fossil record. Organisms may alter the production and/or morphology of recalcitrant structures in response to varying levels of oxygen, and oxygen levels in sediments also affect the likelihood of such structures being preserved. In addition, some acritarchs may not represent photosynthetic taxa, but instead may be the resting stages of metazoan eggs, which in the modern can form in response to deleterious environmental conditions including low oxygen. This induced resting stage phenomenon is especially interesting in light of evidence that acritarch diversity and abundance may correlate with stressful environmental events. For example, the acritarch resurgence documented at the Triassic-Jurassic extinction event is interpreted as a bloom of “disaster taxa”. Thus, the patterns of diversity and abundance in OWM seen across the late Devonian may reflect a combination of a biological response to deleterious environmental conditions, the selective rise of “bloom” taxa, and a taphonomic signal of high preservation in low oxygen settings. Here, we will discuss the possible taxonomic affinities of Devonian OWM found in New York, detail potential controls on their diversity and abundance, and explore the implications of various affinities on reconstructions of late Devonian marine ecosystems.