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Geology; August 2006; v. 34; no. 8; p. 657-660; DOI: 10.1130/G22560.1
© 2006 Geological Society of America
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Tunneling trilobites: Habitual infaunalism in an Ordovician carbonate seafloor

Lesley Cherns1, James R. Wheeley1 and Lars Karis2

1 School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff CF10 3YE, UK
2 Geological Survey of Sweden, Box 670, SE-751 28 Uppsala, Sweden

Asaphus trilobites preserved in tunnel systems of the trace fossil Thalassinoides from the mid-Ordovician (ca. 465 Ma) Holen Limestone, Sweden, are interpreted as the trace makers, enabled by shallow carbonate firm grounds to construct open tunnel networks and develop habitual infaunal behavior. Their in situ preservation confirms an infaunal ethology inferred for some trilobite taxa from functional morphology. We suggest that predation pressure from large omnivorous nautiloid cephalopods ("Orthoceras" Limestone facies) may have triggered ecologic opportunism. In trilobites well adapted for predatory-scavenging behavior as well as excavation, the tunnel networks functioned primarily for protection, possibly assisting in feeding, breathing, and breeding strategies. Previously, "trilobite burrows" have referred to seafloor traces of locomotion, feeding, and resting (Cruziana, Rusophycus). Infaunal, tunneling trilobites provide new evidence of mid-Ordovician partitioning of the skeletal benthos, adding to an ecologic and trophic tier hitherto interpreted as occupied by soft-bodied organisms. Such trilobites also provide an identity for Thalassinoides tracemakers prior to Devonian evolution of decapod crustaceans.

Key Words: trilobites • Thalassinoides • Ordovician • carbonates • paleoecology • early diagenesis • Sweden




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