PLOWING THROUGH THE PALEOZOIC: TRILOBITE ICHNOFOSSILS AS STRATIGRAPHIC INDICES

The presence of Cruziana and Rusophycus in rock sequences that lack body fossils has proven to be an effective stratigraphic marker for the paleocontinent Gondwana. The Gondwanan “Cruziana stratigraphy” provides an ichnostratigraphic paradigm that can be tested on the Laurentian paleocontinent. We examine Cruziana and Rusophycus ichnospecies from Laurentia and discuss their application to a Laurentian and to a global “Cruziana stratigraphy.” Distinction between different species of Cruziana and Rusophycus is based on unique appendage impressions and scratch patterns. Comparative analysis of North American Cruziana and Rusophycus ichnospecies indicates that stratigraphic correlation is possible on the Laurentian paleocontinent. However, Laurentia has a trilobite ichnofossil fauna distinct from that of Gondwana, suggesting that a global “Cruziana stratigraphy” is not possible.

Cruziana and Rusophycus ichnofossils also provide information about the ventral morphological features of their trace-maker. Such features include impressions of biramous limbs, coxae, and cephalic doublure, as well as drag marks from genal and pleural spines. The study of these ichnofossils lends itself to improving the understanding of trilobite limb evolution. Except for the extremely rare occurrences of soft-part preservation, there is little evidence available to study trilobite limbs. The breadth of appendage diversity seen in many arthropod lineages was never attained by trilobites. It has been suggested that this lack of variation may be the product of a common substrate relationship shared among all trilobites; alternatively, it may be the effect of having a constrained basal-arthropod prototype. Systematic examination of the temporal patterns of limb markings on Cruziana and Rusophycus can help explain the basis for their homogenous limb morphology.