New social insect nests from the Upper Jurassic Morrison Formation of Utah

Abstract

This paper reports a new assemblage of social insect ichnofossils from the Brushy Basin Member of the Upper Jurassic Morrison Formation near Green River, Utah. At least seven distinct nests are visible in the locality horizon, identifiable at the outcrop scale by loci of anastomosing, and orthogonally connected hor-izontal burrows and vertical shafts. A boulder-sized block from the in situ horizon has eroded and rolled downhill, revealing the ventral aspect of the nest, showing a view of the overall nest architecture. Burrow and shaft clusters are organized into mega-galleries which have branching arms and ovate, bulbous cham-bers. The organization of distinct trace morphologies is consistent with ethological complexity of the social insects. A small sample was collected and analyzed by serial sectioning and petrographic thin sectioning to observe small-scale morphological features. Centimeter-scale analysis shows chamber, gallery, and burrow walls have complex topography. Pebble-sized, hollow, ellipsoid features are distributed throughout the up-permost facies of the nest and have undergone complete silicification of their outer surfaces. The ellipsoids share similarity with pellet structures made of mud or carton produced by modern termites. This trace fossil assemblage suggests it is possible that termites had acquired subterranean nesting behavior, and mud or carton utilization in nest construction in seasonally arid habitats by the Late Jurassic.