Ontogeny and phylogeny of Early Cambrian olenelloid trilobites, with emphasis on the late Dyeran Biceratopsidae

Lack of detailed paleobiological information has resulted in disagreement among workers regarding the evolutionary history of Early Cambrian olenelloid trilobites, in terms of the broad relationships among olenelloid taxa and the modes by which particular taxa evolved. This dissertation details the ontogenetic development, intraspecific variability, and evolutionary history of a diverse, monophyletic clade of olenelloids (the Biceratopsidae), and lays the foundations upon which a more complete understanding of olenelloid phylogeny can be built.; The cephalon of all olenelloid species passed through four phases of development during ontogeny, defined by changes in glabellar growth patterns and by the appearance of genal spines. These evolutionarily conserved phases provide a framework within which interspecific comparison of rates and modes of development can be meaningfully conducted.; Detailed comparison of Nephrolenellus geniculatus and its ancestral sister-species N. multinodus throughout their development reveals that the modes by which the ancestral ontogeny was evolutionarily modified were more complex than more traditional investigations would suggest. While less detailed studies would interpret all interspecific differences in terms of an increased rate of development in the descendant (heterochrony), geometric morphometric analyses find evidence of ontogenetic rate accelerations, rate decelerations, modified growth allometries, and deletion of morphological structures. A comprehensive classification of modes by which ontogeny can be evolutionarily modified is presented, recognizing rate modifications, timing modifications, change in location of structures (heterotopy), change in type of structures (heterotypy), change in number of structures (heterometry), and change in growth patterns of structures (allometric repatterning) as discrete classes.; Previous cladistic analyses of the Olenelloidea are shown to have underestimated the degree of intraspecific variability and taphonomic overprint on observed morphology, to have employed inappropriate outgroup taxa, and to have used non-independent characters pertaining to adult morphology. A comprehensive cladistic analysis of the Biceratopsidae (plus many outgroup species), utilizing adult and ontogenetic characters of defendable independence, differs from previous evolutionary hypotheses in terms of clade membership, generic relationships, species relationships, and generic monophyly. A revised classification of the Biceratopsidae, including comprehensive descriptions of all nephrolenelline species, is presented. Four new species are described as Bolbolenellus klondikensis, B. besti, B. inflatus, and Arcuolenellus mccollumi.