Ontogeny, Systematics and Soft-Part Anatomy of Early Cambrian Trilobites from South Australia

Abstract

The Cambrian, explosion, is one of the most important events in the history of life on Earth. Much of the higher level taxonomic diversity recognised in metazoan animals appeared during this period, and it is at this time that we see the rapid development of modern-style marine ecosystems and associated ecological interactions such as predation. Rare fossil deposits known as Konservat-Lagerstätte that preserve the soft parts of organisms offer us, windows, into life in the immediate aftermath of the Cambrian explosion. The Emu Bay Shale from Kangaroo Island, South Australia, contains the only known Cambrian Konservat- Lagerstätte in the Southern Hemisphere, and offers important insights into life in the epicontinental seas of East Gondwana approximately, million years ago. Unlike the majority of other similar deposits, the Emu Bay Shale Lagerstätte is dominated by trilobites, an extinct class of biomineralising arthropods. Trilobites are one of the first important, diverse animal groups to appear in the fossil record, and as such are useful for answering important questions about early animal evolution. The conditions that allowed the preservation of soft parts in the Emu Bay Shale were also conducive to preserving complete, articulated trilobite specimens that are required to conduct detailed morphological investigations, including those related to growth, in these early arthropods. This thesis reviews the systematics and taxonomic descriptions of three species of trilobite from the Emu Bay Shale, Estaingia bilobata, Redlichia takooensis and Redlichia rex sp. nov., as well as Redlichia cf. versabunda from the Ramsay Limestone, Yorke Peninsula, South Australia, with particular emphasis on ontogenetic development. Instances of extremely rare soft-part preservation of appendages and digestive structures in the two Redlichia species from the Emu Bay Shale are described. A complete post-embryonic ontogenetic series of articulated specimens is described for E. bilobata, and the segmental growth dynamics of this trilobite, s trunk region are investigated using morphometrics. An ontogenetic series is also described for R. cf. versabunda based largely on cranidia, and includes exceptionally well-preserved protaspides, the earliest trilobite larval stage, . Finally, results from these studies are incorporated into a phylogenetic analysis aimed at exploring the importance of ontogenetic characters in trilobite evolutionary relationships. Results reaffirm the importance of studying developmental processes in extinct organisms such as trilobites, particularly in, primitive, examples such as those from the early Cambrian Period, to understanding the evolution of complex life on Earth.