M. Victoria Sánchez

The Aestivation Chamber of the Giant Earthworm Glossoscolex bergi (Glossoscolecidae) in the Subtropical Rainforest of Misiones (Argentina)

Glossoscolex bergi (Glossoscolecidae) is a giant earthworm from the rainforest of Misiones (Argentina). The large size of its aestivation chamber and the meniscate burrows connected to it allow us to describe morphological details and reinterpret some characteristics of the fossil counterpart Castrichnus incolumis. The concavity of menisci, either in the burrow or in those pellets lining the chamber, shows concentric ridges and radiating striae that result from the impression of the last segments of the body. The meniscate burrows associated to the chamber are different from described ichnospecies of Taenidium, because of the surface texture of the meniscus. The aestivation chamber was produced during an atypical four-month drought in a region that lacks a seasonal climate. This suggests that Castrichnus incolumis would be also an indicator of drought periods even in non seasonal climates.

The Phanerozoic Four Revolutions and Evolution of Paleosol Ichnofacies

The analysis of a database composed of 166 cases of invertebrate and root trace fossils in paleosols allowed us to recognize four major evolutionary steps in the Phanerozoic evolution of paleosol ichnofaunas. Each step constitutes a revolution that is reflected in the appearance of a new ichnofacies. The emergence of the first vascular plants during the late Silurian–Early Devonian produced the most significant change in soil evolution, the appearance of rooted Histosols, Spodosols, Alfisols, Ultisols, and forest Oxisols through the Devonian and Carboniferous. The first revolution, then, occurred in the Early Devonian with these first paleosols that exhibit ichnoassemblages composed of rhizoliths. Paleosols bearing only rhizoliths constitute half of the cases in the Paleozoic and are recorded through the whole Phanerozoic. These cases may compose an archetypal ichnofacies termed the Rhizolith Ichnofacies. The Rhizolith Ichnofacies would be indicative of subaerial exposure and depending on needed studies on root morphology, probably would yield more precise and significative data on paleoenvironment and vegetation in the near future. Other cases of Paleozoic ichnoassemblages can be included in the Scoyenia Ichnofacies. The second revolution took place after the end-Permian mass extinction. It is characterized by the appearance of trace fossil assemblages that include or are dominated by earthworm and crayfish trace fossils. These ichnoassemblages are grouped in a new archetypal ichnofacies, the Camborygma Ichnofacies. The second revolution was followed by stasis that ended during the Cretaceous, when the third revolution occurred. By that time, the appearance and diversification of flowering plants triggered the diversification of groups of soil-inhabiting insects, such as ants, termites, bees, wasps, and certain beetles, which were capable of constructing linings and free-standing walls for their chambers and nests that consequently acquired a high potential of preservation. By the Late Cretaceous, the first recognizable insect trace fossils in paleosols appear as isolated examples, integrating the Camborygma Ichnofacies, or composing a new one: the Celliforma Ichnofacies. The fourth and most diverse revolution took place in the Middle Eocene, with the advent of grass-dominated habitats during the long-term cooling-drying period after the EECO, and the full establishment of all groups of modern insects, including the ball-making dung beetles. The spread of grasses produced the last step in soil evolution, that is, the appearance of soils with fine granular peds that derived in Mollisols. Traces of cicadas, dung beetles, bees, sphinx moths, ants, termites, cleptoparasites, and detritivores appear or diversify during this revolution composing the Coprinisphaera Ichnofacies. By the Oligocene, in closed-forest environments, the first assemblage dominated by termite trace fossils is recorded, composing the Termitichnus Ichnofacies. This last revolution is followed during the Neogene by a stasis only interrupted by the occasional appearance of new trace fossils from the same groups of insects.