A GIANT PTERYGOTID EURYPTERID FROM THE EARLY ORDOVICIAN FEZOUATA BIOTA OF MOROCCO

The Early Ordovician (latest Tremadocian) Fezouata biota of Morocco has yielded a diverse exceptionally preserved fauna including giant filter-feeding anomalocaridids, trilobites with preserved limbs, and the oldest horseshoe crabs. Here, we report the occurrence of eurypterids from the fauna; these predate the previous oldest record by some 20 million years. The Moroccan eurypterid is known from several specimens including a swimming paddle, telson, and two grasping claws (chelicerae). The morphology clearly places the new taxon within the pterygotids, a highly derived group previously known only from the Silurian to mid-Devonian. Morphometric analysis of chelicerae places the new taxon among predatory taxa such as Jaekelopterus and Pterygotus, suggesting that the Moroccan pterygotid was an active hunter. The new taxon achieved gigantic proportions, with a maximum estimated body size (excluding the enlarged chelicerae) of 195–215 cm, suggesting that it was an apex predator.

The assignment of the Moroccan eurypterid to Pterygotidae infers a large ghost range for all eurypterid clades into the Lower Ordovician. This suggests either an explosive radiation of eurypterids during the earliest Ordovician, or an origin sometime during the Cambrian; the latter notion may be supported by the presence of chasmataspidid fossils in the late Cambrian of Texas. The new taxon also reveals that large body size evolved in pterygotids long before the first appearance of large armored placoderms. The co-occurrence of traditional Cambrian faunal elements such as anomalocaridids with Paleozoic faunal elements like eurypterids in the Lower Ordovician further underscores that the turnover between the Cambrian an Paleozoic Evolutionary Faunas was a considerably more protracted and complex affair than previously considered; as a consequence, the distinction between these evolutionary faunas may not be as clear as thought. The Fezouata fauna comprises a complex, multi-tiered ecosystem with multiple large predatory species, showing that such ecosystems evolved early in the Great Ordovician Biodiversification Event.