EARLY ORDOVICIAN RADIODONT (ARTHROPODA) DIVERSITY FROM THE FEZOUATA BIOTA (MOROCCO): EVOLUTIONARY AND ECOLOGICAL IMPLICATIONS

The Fezouata Shale Formation (Early Ordovician, Tremadocian) of Morocco yields exceptionally preserved fossils of marine organisms that illustrate the transition between the Cambrian Explosion and the Great Ordovician Biodiversification Event (GOBE). Arthropods are abundant in this Lagerstätte, especially radiodonts (informally known as anomalocaridids). Radiodonta is an order of stem-lineage arthropods characterised by a segmented body with wide swimming flaps, and a head bearing a pair of appendages, a pair of eyes on stalks, and a mouth surrounded by spiney plates.

We examined 121 radiodont frontal appendages and applied imaging techniques including photography, computed tomography (CT scan), synchrotron luminescence, and Raman spectroscopy to identify new characteristics. We revise Aegirocassis, the only named radiodont yet described from the Fezouata Shale, and identify several new Hurdiidae taxa. The enigmatic Pseudoangustidontus, previously described as a spinose fragment of an unknown raptorial appendage, is identified as a hurdiid radiodont. Fezouata Shale Radiodonts were highly diverse and employed a variety of different feeding strategies. Suspension feeding was dominant (3 species, 112 specimens), whereas only 9 specimens were sediment sifters. This prevalence of suspension feeding may be linked to the “Ordovician Plankton Revolution”, which saw a huge radiation in plankton diversity during the GOBE. The study also points to the decline of active raptorial predation in radiodonts, as suggested by their absence so far from the Fezouata Shale.

Other radiodont material from the Fezouata Shale includes disarticulated head carapaces, which are often associated with brachiopods interpreted as ectosymbionts of living nektonic radiodonts, or preserve imprints of traces. Additionally,, 2 oral cones (mouthparts) are found in the Fezouata Shale, which show unique characters and allow us to better understand the evolutionary implications of this arthropod group.