Roller Coaster Behavior in the Cruziana Rugosa Group from Penha Garcia (Portugal): Implications for the Feeding Program of Trilobites

Abstract

Trilobite burrows of the Cruziana rugosa group are common and well preserved in the Armorican Quartzite Formation (Lower to Middle Ordovician) of the Ponsul River gorge in Penha Garcia. Based on morphological and behavioral peculiarities, Cruziana beirensis is reinstated herein and included in the rugosa group. Apart from the broad range in morphology and size, Cruziana from Penha Garcia show higher behavioral diversity using food sources than has ever been documented in a single section. This behavioral diversity, mainly in circling behavior, has been analyzed using the Capacity Fractal Dimension by implementation of the box-counting theorem applied to the bedding plane. Fractal Dimension also suggests that Cruziana rouaulti can be included in the rugosa group, despite the obscurity of the scratch patterns, as products of juveniles. Circling, sinusoidal or teichichnoid behavior modifications reflect a generalist mode of sediment feeding, claimed mudtrophobacterivory, while most of the interactions with worm burrows originated previously or (mostly) later, by interpreted trilobite necrophagy and/or worm commensalism. Patchy exploitation of biomat grazing fields is inferred from Cruziana preservation styles, physical interactions with biomat-related sedimentary structures and area-limited high bioturbational indices in the explored tier.

Keywords:

• Cruziana
• ethotype diversity
• feeding management
• biotic interactions
• Lower to Middle Ordovician
• Portugal