THE SEILACHERIAN APPROACH TO INTERPRETING ENIGMATIC TRACE FOSSILS: AN EXAMPLE FROM THE LATE ORDOVICIAN OF EASTERN NORTH AMERICA

Professor Seilacher's legacy to sedimentary geology and paleontology includes his methodology for interpreting enigmatic specimens. This paper applies a Seilacherian approach (steps I-III, below) to the interpretation of two unusual trace fossils from the Late Ordovician of Eastern North America. The two Rusophycus assemblages (one from Ontario, Canada, the other from Indiana, USA) are unique in their circular arrangement of multiple Rusophycus burrows.

I. Constraining sedimentological reality; the conditions under which the structure formed and was preserved: Both specimens are preserved in convex hyporelief. The excavation was presumably made in mud and later filled with the casting medium (silt). Rusophycus are impressed along the circumference of the circle, but scratch marks do not cover the entire depression, indicating that the tracemakers impressed themselves very closely into a pre-existing cavity but did not excavate the depression themselves.

II. Identifying the trace –maker; likely suspects, motive (behavior), and means to create the trace: The size and shape of the Rusophycus are consistent with traces attributed to the trilobite Flexicalymene. This genus is present and relatively abundant in both localities. Different sizes of individual Rusophycus indicate that multiple individuals were present, although the size range is relatively narrow.

III. Determining the behavior represented by the trace fossils; building on what is known, and entertaining novel possibilities for novel specimens: Rusophycus is generally regarded as a feeding trace, and Rusophycus assemblages likely reflect aggregations of trilobites in response to a local concentration of food (in contrast to mass assemblages of trilobite body fossils which may be related to molting/mating). The Ontario specimen is donut-shaped, with a central depression that would have been a topographically positive feature when the excavation was made. This feature may have been constructed by a tube-dwelling organism that mined the surrounding sediment radially from its domichnia, creating the depression that the trilobites exploited. Alternatively, the depression could have been formed by the burial and decay of a radially symmetric organism, creating a circular, organic-rich depression that attracted the trilobites.