Biological, ecological and taphonomic insights into problematic animal and macroalgae fossils: Case studies from the Cambrian of North America and Mongolia and the Ordovician of France

Abstract

The Ediacaran-Cambrian transition (~540 Ma) was one of the most significant times in the evolution of animals. It was during this time that most of the major modern animal groups first evolved and diversified. The impetus for the timing of this radiation remains enigmatic, and various drivers have been suggested, including biological, environmental, and ecological processes. Each of these factors likely played a role, and it was the combination of each of these things together that prompted such a rapid explosion of diversity unparalleled in the history of animals. Here I present a series of case studies on early Cambrian and Ordovician deposits across the globe that investigate these factors influencing the evolution of early animals at scales ranging from the individual organism to entire communities. In my first chapter, I investigated the phylogenetic placement of the enigmatic early Cambrian fossil Salterella. This organism was unique in that it built a shell incorporating both a biomineralized outer layer and internal agglutinated layers. The combination of petrographic, mineralogical, and shell microstructure data, suggest that Salterella was most likely a Cnidarian. Thus, cnidarians have likely had the capacity to biomineralize and agglutinate sediments (both behaviors they exhibit in the modern) since the early Cambrian. My second chapter involved the description of algae fossils from the early Ordovician Cabrières Biota. This deposit records a polar marine community. The morphologically simple algae from this deposit more closely resemble Precambrian forms than later Paleozoic forms, supporting a delayed diversification of macroalgae compared to animals at the start of the Phanerozoic. My third chapter focused on early Cambrian stratigraphy from southwestern Mongolia, recording the fossil assemblage. In the Bayangol Formation, I documented the occurrence of abundant iron oxide filaments that represent convergent taphonomies of algae, tubular animals, and trace fossils. The diversity of this assemblage is cryptic due to the tendency for each of these fossils to lose their distinctive characteristics and converge on a single form through taphonomic, diagenetic, and weathering processes. Together, these projects provide new insights into the evolution of early animals as well as the ecosystems they lived in and how the fossil record is preserved. Each is a critical component to reconstructing the history of life on Earth.