AGNOSTIDS: REVOLUTION AND INNOVATION IN THE LATE CAMBRIAN

Agnostids were more ecologically varied – and likely more active – than is generally assumed. I use three-dimensional imagery, volumetrics, and landmarking to explore a model of agnostid functional morphology that highlights unique adaptations to conditions during a time of serial environmental disruptions.

Occurrence data and morphology indicate that agnostids were not exclusively epifaunal. Arguably-pelagic taxa of these small arthropods increased from the Middle to the Late Cambrian; they were among the few genera surviving the oxygen-related "biomere" crises of the later Cambrian. In some Late Cambrian forms, a novel adaptation, the deuterolobe, created more intra-test volume for drastically increased respiration surfaces on bulbous appendage extensions. More oxygen absorption supported higher metabolism; higher metabolism in turn supported more active feeding and locomotion. Analogies with small modern arthropods, along with hydrodynamic considerations, show that agnostids were capable of meaningful propulsion. Bristled appendages reached the posterior of a large, natant, flexibly-positioned hypostome for small-particle feeding. Coincident with these innovations, the incipient stages of the Plankton Revolution increased available food supply.

Capacious Late Cambrian agnostids grew more allometrically than their dorso-ventrally compressed predecessors and contemporaries. Ontogenetic series representing each of these types, compared to newly originating taxa, support the probability that multiple species arose by heterochrony during the Late Cambrian. The enhanced respiratory capacity of such taxa may have conferred a selective advantage under pulses of hypoxia. For agnostids, a time interval purportedly marked by extinctions proved remarkably generative – a sort of “diversity pump”. Agnostid diversity and morphologic disparity peaked in the Cambrian, predating the disparity peak of polymerid trilobites in the Ordovician. Agnostid diversity dropped in the Ordovician, but multiple genera achieved cosmopolitan distribution across a latitudinal gradient marked by size clines.