THE BIOPRINT OF BURROWING PALEOZOIC ARTHROPODS

Forty years ago, ichnologists were aware that arthropods were common Paleozoic animals, but were hard put to identify their burrows. Since then, a great deal of progress has been made, and now it is time to take stock.

Why were arthropod burrows overlooked for so long? One reason is that many arthropod burrows superficially resemble “worms” or worm burrows. For instance, controversy continues on whether Arthrophycus was made by arthropods or by polychaetes, although historically both ideas were broached more than a hundred years ago. Another reason is that some arthropod burrows were misidentified as trails; e.g., Cruziana is still called a trail, despite the realization that most Cruziana was produced beneath the sediment-water interface. A third reason is that the greatest diversification of decapod crustaceans did not occur until the Mesozoic, and Paleozoic arthropods such as trilobites were not regarded as strong burrowers. Finally, until taxonomists began to pay attention to the bioprint – that is, the morphologic characteristics that allow recognition of the tracemaker – caution prevented assignment of Paleozoic burrows to any particular group of animals.

The bioprint of arthropod burrows includes such features as: (1) continuity with or morphologic similarity to bilaterally symmetrical resting traces shaped like arthropods; (2) sets of striae (scratchmarks) made probably by limbs with an exoskeleton; or less commonly (3) continuity with trackways made by multiple limbs. Based on such information, at least some members of the following Paleozoic ichnogenera can be assigned to arthropods: Arthrophycus, Beaconites, Cruziana, Nereites, Rusophycus, Taenidium, Teichichnus, Thalassinoides, Trichophycus. Hence, interpretations of arthropod burrows in Paleozoic deposits should lend to more realistic assessments of tracemaker diversity in ichnoassemblages.