Paleobiology of complex trace fossils

Complex trace fossils include structurally elaborate biogenic structures such as the large, spiral forms of Zoophycos and the intricately organized graphoglyptids; compound structures consisting of connected parts that would receive different ichnotaxonomic labels if preserved in isolation; and recurrent spatial associations of different trace fossils that probably represent strong ecologic interaction. These complicated animal artefacts should not be viewed so much as preservational rarities or taxonomic puzzles, but rather as neglected sources of information on the ethologic, ecologic and evolutionary characteristics of trace producers. Trying to fit these structures into the traditional behavioral classification scheme of Seilacher is bound to fail. Many complex trace fossils appear to have been occupied for long intervals and to record re-engineering or active control of proximal habitats by the trace-producing organisms. Attempting to understand their actual biologic significance will require new descriptive and interpretive approaches. One possible descriptive approach involves making comprehensive inventories of structural elements that record particular kinds of adaptive behavior or interaction (behavioral tokens). Portraying the spatial and temporal order of tokens could be done using a kind of systems diagram (paleoethologic ‘blueprint’) that summarizes construction, operation and maintenance of complex biogenic structures. New approaches to interpretation involve viewing the structures as evidence of physiologic projection of the trace producers, or as the extended phenotypes of these organisms. A reorganized approach to the study of complex biogenic structures emphasizing biologic properties would not only lead to some very old puzzles finally being solved, but also would revitalize ichnology and forge new and productive connections to ecology and evolutionary biology.