ARTHROPOD EVOLUTION DURING THE PALEOZOIC: INSIGHTS FROM THE FEZOUATA BIOTA

The earliest stages of arthropod evolution are recorded by an exceptional fossil record preserving whole body specimens and soft tissues in exquisite detail, found typically in Burgess Shale-type lagerstätten (BSTs). Such fossils have been used, for example, to reconstruct the stem lineage leading to the establishment of crown group Euarthropoda. This evolution spans from the earliest Cambrian through to the earliest Ordovician, where one of the youngest known BSTs, the Fezouata Shale, records a mix of Cambrian Explosion animals and later Paleozoic taxa. Our recent work has been clarifying the diversity of arthropods at the Fezouata Shale by examining their anatomy using diverse imaging techniques and through the careful examination of their taphonomic context. Incident and polarized light photography is combined with multispectral macro-imaging, and synchrotron radiation approaches such as X-ray fluorescence major-to-trace elemental mapping and microtomography. An analytical approach to deciphering taphonomic biases has also been developed, which applies probability statistics to a database of the presence/absence of different tissue types within the assemblage of fossil taxa found at a given locality, to estimate which types of animals might be under- or over-represented. Combining these approaches with detailed fossil descriptions is clarifying the assemblage of arthropods typifying the Fezouata Shale. A diverse assemblage of suspension feeding radiodonts has been identified, in contrast to the abundant raptorial predation of Cambrian radiodonts. The establishment of several major chelicerate lineages, for example the oldest known horseshoe crabs, has also been revealed at this locality. Soft tissue preservation in trilobites shows the dynamics of biramous limb evolution within the group, and a new species of nektaspid arthropod suggests a biogeographic migration compared to Cambrian taxa. The arthropods of the Fezouata Shale are truly transitional in nature.