Giant trilobite burrows and their paleobiological significance (Lower-to-Middle Ordovician from Penha Garcia, Portugal)

The largest ever described Cruziana, more than 200 mm wide, are not uncommon in the Lower-to-Middle Ordovician Armorican Quartzite Formation, at the Ichnological Park of Penha Garcia, central Portugal. They are the most visible trace fossils of ichnocoenoses dominated by classic arthropod intrastratal burrows, tunnels, and trackways, and Skolithos piperocks in a shallow marine, transgressive siliciclastic succession. The typical “Armorican Quartzite” of the Skolithos Ichnofacies is overlaid by a sequence of alternating storm-related quartzites and pelites of the Cruziana Ichnofacies, where the Cruziana rugosa Group is far abundant and shows a unique diversity of behaviors. A quantitative comparison of the Cruziana ichnospecies from Penha Garcia with the trilobite communities from the Middle Ordovician of Canelas, where large body clusters of asaphids are common, was preliminarily performed with the aim of establishing a genetic relationship and eventual trophic migratory paths across trilobite ontogenesis. The gregarious behavior commonly found in discrete beds totally covered by Cruziana is explained under the trilobite clustering paleobiology, as detritus feeding congregations where the opportunity for scavenging cannot be ruled out. The large body size inferred for some of the Cruziana rugosa trilobite producers may be correlated with the high latitude position of Penha Garcia in the shores of Gondwana during the Ordovician times, the diversification of food resources and abundance, and the capacity to explore them and thrive in oxygen-depleted substrates, that may have resulted in “polar gigantism” among them.