Morphology, function and taphonomy of Lower Paleozoic trilobites and their close relatives: insights from North American Konservat-Lagerstätten

Abstract

Eurthropods are extremely abundant and diverse in the word today, living in a broad range of environments and occupying many different ecological niches. Not only are euarthropods significant in the environment today, but they have played important roles since the earliest species appeared in the Cambrian Period. Understanding how this group evolved to such an impressive range of diversity requires utilizing fossils with preserved non-biomineralized tissues such as limbs, gills and digestive systems to fully understand the adaptations and selective pressures facing them leading to their abundance and diversity today. Artiopoda is a group of early crown group arthropods containing the well-known trilobites and there close non-biomineralized relatives. In the Introduction, I review 16 formations where the 40 species of trilobites with preserved appendages are found and the morphology of the limbs. The biramous appendages of trilobites were very variable depending on lifestyle and environment. In Chapter 1, I formally describe Helmetia expansa a concilitergan artiopod known from the Burgess Shale (Cambrian; Stage 4; Wuliuan; British Columbia, Canada) since 1917 but which has never been comprehensively studied. This work demonstrates the presence of biramous appendages and a complex digestive system, as well as resolving the relationships within Conciliterga. In Chapter 2, I show that a well-known trilobite from the Burgess Shale had specialized appendages in the middle of the body, likely used by adult males to grasp onto females prior to and during mating. The following three chapters focus on the trilobites from the Walcott-Rust Quarry (Upper Ordovician; Katian – Hirnantian; New York, USA), where appendages are preserved in three- dimensions by calcite infills. Chapter 3 examines the steps leading to this unusual mode of preservation using Scanning Electron Microscopy and Raman Spectroscopy to study the elemental and mineralogical composition. I show the crucial role of the sediment supported external mold in preserved non-biomineralized tissues. In Chapter 4, I demonstrate the convergent evolution of enrollment mechanisms in trilobites, millipedes, and isopods. This defensive strategy protects the more vulnerable ventral tissues but requires musculature to move the ventral plates (i.e., sternites) and limb morphology that allows complete encapsulation. Finally, in Chapter 5, I use over 260 thin sections of trilobite specimens with preserved non-biomineralized tissues to resolve the controversial appendicular morphology. Using complimentary methods to understand how these fossils formed, the morphology preserved, and compare with modern arthropods, I show the broad of complex behaviors and morphology present early crown group arthropods.