Insect trace fossil associations in paleosols: The Coprinisphaera ichnofacies

Analysis of fifty-eight paleosol trace fossil assemblages, ranging from the Triassic to the Recent, allows refinement of continental ichnofacies model s and the proposal of a Coprinisphaera ichnofacies. The Coprinisphaera ichn ofacies consists of trace fossils of bees, wasps, ants, beetles, termites, and other unassigned insects. Meniscate burrows, mammal caves, and rhizolit hs also may be present. This ichnofacies is named after the dung beetle nes t Coprinisphaera, the most common component of this archetypal assemblage. In mature paleosols, the Coprinisphaera ichnofacies has moderate to relativ ely high trace fossil diversity and high abundance. Ethologically, this ass emblage is dominated by nesting traces (calichnia) and exhibits a relativel y complex tiering pattern, reflecting variable depths of emplacement of hym enopterous, termite, and dung beetle nests. Common components include the b ee cells Celliforma, Uruguay, Ellipsoideichnus, Palmiraichnus, and Rosellic hnus; the wasp nest Chubutolithes; the ant traces Attaichnus and Parowanich nus, and other beetle traces, such as Monesichnus, Fontanai, and Teisseirei . Termite nests may occur, but are less common components of the Coprinisph aera ichnofacies. The Coprinisphaera ichnofacies fulfills all the requirements to qualify as a Seilacherian or archetypal ichnofacies, namely recurrence in time anal sp ace, and distinct paleoenvironmental implications. Proposal of the Coprinis phaera ichnofacies is based on the analysis of twenty-eight cases, ranging from the? Late Cretaceous to the Recent. The Coprinisphaera ichnofacies cha racterizes paleosols developed in paboecosystems of herbaceous communities. These herbaceous communities range from dry-and-cold to hum id-and-warm cl imates. More detailed paleoclimatological inferences can be obtained by eva luating the relative abundance of the various traces within the assemblage. A dominance of hymenopterous traces would indicate drier conditions, where as the presence of termite nests would indicate more humid. The Coprinispha era ichnofacies occurs in paleosols developed in various depositional syste ms subject to subaerial exposure, such as alluvial plains, desiccated flood plains, crevasse splays, levees, abandoned point bars, and vegetated eolian environments. This and other potential terrestrial ichnofacies are control led by ecological parameters (e.g., vegetation, climate, and soil) rather t han by depositional processes. The association of fossil insect nests indic ates the extent of soil development and consequently, such ichnofossils are one of the best indicators of paleosols. The previously proposed Termitichnus ichnofacies was defined to include all paleosol trace fossil assemblages. However, the available information indi cates that terrestrial environments are far more complex. Therefore, it is suggested that the Termitichnus ichnofacies as presently defined be abandon ed because it does not reflect the diversity of paleosol settings and fails to provide significant paleoecologic information. Formal definition of a T ermitichnus ichnofacies in a more restricted sense, to include assemblages dominated by termite nests in paleosols of closed forest ecosystems, should await documentation of additional studies to prove recurrence. Other fossi l insect-nest associations in paleosols (e.g, halictid nests in calcareous soils) do not have enough recurrence in time and space to be considered Sei lacherian ichnofacies, but do represent potential ichnofacies. The model pr oposed in this paper includes the paleoecologically defined Coprinisphaera ichnofacies plus a definite number of associations, each one possessing its own paleoenvironmental implications, which do not show the necessary recur rence to be considered ichnofacies, at present. Climate and vegetation are considered hey factors in the shaping of terrestrial ecosystems and should be taken into account for the definition of additional terrestrial ichnofac ies.