REEVALUATION OF AGNOSTOID ARTHROPOD MORPHOLOGY AND REASSESSMENT OF TRILOBITE AFFINITY USING SILICIFIED SPECIMENS FROM THE GREAT BASIN

Agnostoids are an extinct, cosmopolitan group of blind, benthic arthropods. With known occurrences from the early Cambrian through the Late Ordovician, agnostoids are some of the most common Cambrian fossils and are important biostratigraphic indices. Despite their widespread biostratigraphic use, the group continues to be enveloped in disagreement over their phylogenetic affinities, namely their potential relationship to trilobites and position on the arthropod tree. While traditionally considered trilobites, agnostoids lack a number of significant trilobite features (such as facial sutures, a rostral plate, and a calcified protaspid larval stage) and also substantially differ from trilobites in their appendage structure and placement, lack of a transitory pygidium, differences in thoracic articulation, and differences in the relationship between pairs of appendages and thoracic tergites due to each of their two thoracic tergites not representing individual true metameric body segments. These significant morphological differences that set agnostoids apart from trilobites are well supported, with some having been continually described in agnostoid literature since the mid-19^th century. Such differences have often been overlooked, overridden, or mischaracterized in phylogenetic analyses, resulting in the classification of agnostoids within Trilobita being maintained.

Secondarily silicified agnostoid specimens representing at least five genera (Kormagnostus, Proagnostus, Acmarhachis, Lejopyge, and Oedorhachis) have been collected from the Marjuman (Miaolingian; Guzhangian) Lincoln Peak Formation at a locality near Cleve Creek, northwest Schell Creek Range, eastern Nevada, and freed from their host limestone using hydrochloric acid digestion. These well preserved silicified agnostoids provide abundant new supporting data, including ample ontogenetic material, which demonstrates important agnostoid-specific morphological characters, such as their lack of a trilobite-like transitory pygidium and their unique style of thoracic articulation. Reconsideration of agnostoid hard parts does not support recent codings of putative dorsal exoskeleton synapomorphies shared with trilobites, but instead suggest that agnostoids are not ingroup Trilobita.