Complex ichnofossils of solitary and social soil organisms: understanding their evolution and roles in terrestrial paleoecosystems

The range of extant nest architectures for different types of solitary to social insects as well as the key features in their architecture has assisted in the identification of their structures in the geologic record. The recognition and interpretation of complex ichnofossils as the product of insects that indicate varying degrees of sociality represents a major development in the study of continental ichnology. Complex ichnofossils constructed since the Mesozoic by termites (Isoptera), bees (Hymenoptera: Apoidea), wasps (Hymenoptera: Apoidea and Vespoidea), and ants (Hymenoptera: Formicidae) represent unique solutions through degrees of social cooperation to the problems of fossorial life in terrestrial environments. Other such trace-making organisms as various types of beetles and vertebrates also construct a range of simple to complex burrows that indicate solitary, subsocial, and gregarious behaviors. Burrows of vertebrates result from relative degrees of social behavior that are unlike those of social insects. Complex ichnofossils are highly variable in architecture and indicate the type of organism, the number of individuals per nest, the length of time the structure was used, the degree of sociality, and, in some cases, the amount of time the substrate has been exposed to surface processes. A pattern of interconnected structures of varying length, width, height, and number usually distinguishes complex traces. Nests of insect societies have the greatest variability in ichnofossil complexity – being simple to extremely elaborate structures. These traces also preserve major innovations in soil ecosystems that include food hoarding, adaptations to disturbance from flooding and precipitation, enduring unpredictable hypercapnic and hypoxic conditions, and reproductive strategies by employing a subterranean, hemimetabolous or holometabolous life cycle. Polychresichnia is proposed for trace fossils that were involved in many simultaneous, multiple behaviors and uses. Aedificichnia and calichnia could likely be subsets of polychresichnia because many of the ichnofossil nests originally included in those categories were protected by the adults during brood rearing, used as living and sleeping quarters for the adults, and used as shelter from adverse weather. Nest architecture is an important source of information on the evolution of behavior of social insects as well as for other social organisms. Many of the organisms mentioned here have trace-fossil records that extend to the earliest Mesozoic and predate their earliest body-fossil records in the Cretaceous. Most of these trace fossils have changed remarkably little in 225 million years, indicating evolutionary stasis of the basic building blocks in nest construction.