EXCEPTIONALLY WELL-PRESERVED ANT NEST FOSSILS IN CALCIC PALEOSOLS OF THE OGALLALA FORMATION (MIOCENE), SCOTT COUNTY, KANSAS, USA

The Ogallala Formation is composed of pedogenically modified fluvial silts, sands, and gravels. Calcic paleosols occur with high stratigraphic frequency and are characterized by abundant carbonate nodules and root traces, and thick paleophreatic calcretes. Honeycomb-like structures common to some calcretes have been attributed to algal structures, rhizoliths, or as recent dissolution features. These structures are preserved within and below calcrete beds as CaCO₃-filled casts weathering in full relief, or are composed of powdery CaCO₃ showing no relief. The honeycombs are composed of two basic elements: 1) vertically tiered, horizontally oriented chambers, wider than they are tall; and 2) vertical to sub-vertical, small-diameter shafts intersecting the chambers. Three distinct types are present based on differences in plan-view chamber-shape, shaft orientation, and point of intersection between shaft and chamber. Type A is composed of flat-bottomed, lobed chambers ranging in plan view from small teardrop-shapes to larger heart-shaped or multi-lobed chambers. Lobed chambers radiate from and are connected at their sides to a central, helical shaft. Type B chambers are circular to oblate in plan view and intersected by vertical shafts towards the center of the chamber or entering from above and exiting from below at opposite chamber edges. Type C is composed of spherical to ovoid chambers with vertically-oriented long axes connected from above to one or more vertical shafts. Many honeycomb structures are nearly identical to nest of extant ants in their size, basic structural elements, and architectural morphology. Type A are nearly identical to the nests of extant harvester ants; specifically the genus Pogonomyrmex. Such ants are highly specialized granivores and prefer arid to semi-arid regions and sandy soils. Type B are most similar to the nests of extant funnel ants which are fairly cosmopolitan with a wide range of habitats. Type C are most similar to fungus-gardening chambers of attine ants. Thick calcretes imply extended periods of depositional hiatus with high evapotranspiration of relatively shallow groundwater. The nests most likely predate calcrete formation and indicate well-drained vadose conditions during their construction and likely episodic sedimentation.