NEW NON-TRILOBITE ARTHROPODS FROM THE EARLY ORDOVICIAN FEZOUATA BIOTA OF MOROCCO

The late Tremadocian to late Floian Fezouata Biota of south-eastern Morocco represents the first exceptionally preserved Ordovician fauna from a normal open marine environment. Significantly, the biota combines a classical Ordovician shelly fauna with both a large number of typical Early–Middle Cambrian Burgess Shale-type organisms and later, more derived unmineralized forms. Such co-occurrence indicates that replacement of Cambrian biotas during the Ordovician radiation was more protracted than hitherto recognized, and also suggests that significant diversification had already started well before the onset of the Tremadocian. These findings are further supported by a number of recent arthropod discoveries. One new arthropod of problematic affinity shows a thinly mineralized, strongly vaulted ovoid shield with a delicate dimpled ornament and a short, sharply delimited, annulated posterior axis. Another specimen belongs to Nektaspidida and exhibits a spatulate cephalic shield overlapping the first of three trunk tergites and a tail shield of comparable shape but smaller size. It most likely resolves within the family Liwiidae, reaffirming a Gondwana/Baltica affinity for this clade. An additional fossil may represent a badly preserved naraoiid. The discovery in the Fezouata Biota of the rare and poorly known arthropod genus Mollisonia marks the first occurrence of this taxon in the Ordovician, being previously known only from the Middle Cambrian of North America and China. Although overall very similar to Cambrian representatives of the genus, the detailed morphology of the Moroccan specimen differs from that of earlier forms. Importantly, the Fezouata specimen for the first time provides some information on the appendages of Mollisonia, with a pair of lamellate exopods and thin, probably cylindrical endopods being partially exposed to the left of the tail shield. Combined with its general morphology, this finding strongly suggests a position for Mollisonia within Lamellipedia. These recent arthropod discoveries reinforce the status of the Fezouata Biota as an intermediate in the evolution from Cambrian to later Paleozoic ecosystems.