AGNOSTID TEST VOLUME INCREASES IN THE LATE CAMBRIAN PRELUDE TO THE GOBE

During the Ordovician and thereafter, polymerid trilobites included many taxa whose convexity yielded higher body volumes than most of their Cambrian predecessors. Among agnostids, a pronounced increase in test inflation appears in the Late Cambrian and is especially evident within the diversifying Pseudagnostinae. Increased volume due to inflatedness, especially within the agnostid pygidium, would have accommodated proliferating unique agnostid limbs whose club-shaped structures are thought to have functioned in oxygen absorption. Among the Pseudagnostinae, the morphological innovation of the deuterolobate axis produced a smoothly convex pygidial cross-section, facilitating rapid expansion of internal volume. Preliminary morphometrics reported here from Great Basin USA field collections, U.S.G.S. collections, and estimates from monographs and museum materials, demonstrate pygidial volume increases in mature holaspids of common Late Cambrian species when compared to mature holaspids of representative Middle Cambrian species and narrowly axiolobate taxa.

In agnostids, test inflation thus may reflect an important phase in the prelude to the GOBE (Great Ordovician Biodiversification Event) and the consequent establishment of the Paleozoic Fauna. As phytoplankton diversity increased through the Cambrian, evidenced by the acritarch record, new niches based on particle size opened for zooplankton and small particle suspension feeders like agnostids, who likely partitioned their niches based on particle size. Brisk suspension feeding in small modern arthropods increases oxygen absorption as a side effect, and the same was likely the case with agnostids. Clades of agnostids capable of increasing oxygen absorption due to limb morphology and test volume may have had advantages in the fluctuating oxygen conditions of the Late Cambrian well documented from isotope curves. Thus the roots of the GOBE and the initial morphologic traces of its dramatic oxygen increases are discernable in volumetrics of Late Cambrian fossil arthropods.