Multiple-Rusophycus (Arthropod Ichnofossil) Assemblages and Their Significance

Abstract

Rusophycus is an ichnogenus comprising shallow burrows generally attributed to trilobites and other arthropods. The paleoethological interpretation of these structures is not conclusively known; workers variously have attributed the ichnogenus to feeding, resting, hiding/escape, hunting or nesting behaviors. Rusophycus morphology varies from unornamented, bilobed forms to forms that preserve details of ventral anatomy of the trace maker. Rusophycus occurs as single or multiple impressions. Some examples of multiple-Rusophycus assemblages are clearly the result of the activity of a single animal (e.g., serially arranged impressions of the same size), but others represent the activity of several individuals. Associated traces representing multiple individuals are especially interesting, for they may give evidence of complex behavior, for example, interactions between the trace makers. Some of the multiple-trace assemblages show alignment (congruent anterior-posterior orientation of the individuals) suggesting rheotactic behavior; other assemblages comprise randomly oriented traces. The difference between the aligned and randomly oriented assemblages may reflect differences in the current energy and amount of available food, and may also suggest the following feeding modes for these benthic-feeding arthropods: (1) alignment with head into the current in resource-poor environments, (2) orientation with head at an oblique angle to the current in high-energy, resource-poor environments, and (3) random orientation in low-energy, resource-rich environments.

Keywords:

• Rusophycus
• arthropods
• trilobites
• behavior

ACKNOWLEDGEMENTS

I thank the late Wayne Pryor for his excellent introduction to ichnology, and the type-Cincinnatian Series for yielding so many excellent arthropod trace fossils. Ron Pickerill kindly provided prints of Figure 5, and the paper was improved by the careful editing and comments of John Pollard and Mike Romano. I thank the 2007 Trace Fossils Seminar for valuable editorial assistance.

Notes

*Osgood and Drennen (1975) refigured and reinterpreted the specimen originally illustrated by Hall (1852)

*These papers describe the same fauna

*These papers describe the same fauna

*These papers descibe the same Late Silurian fauna

^#These papers describe the same Australian fauna.