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.
Highlights
Despite their individual cognitive simplicity, eusocial insects are responsible for building some of the most complex structures in the animal kingdom (Theraulaz and others, 1998)
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
Individual ellipsoids appear as circles in cross section. These findings demonstrate a new occurrence of social insect nest ichnofossils from the Morrison Formation, morphologically distinct from reported social insect ichnofossils both within and outside of the Morrison
Summary
Despite their individual cognitive simplicity, eusocial insects (termites, ants, bees, and wasps) are responsible for building some of the most complex structures in the animal kingdom (Theraulaz and others, 1998). Eusociality, the highest degree of animal sociality, is a remarkable biological phenomenon where members of a species are differentiated into reproductive castes and provide alloparental care (Crespi and Yanega, 1995). Eusocial insects that create the most complex structures are ants and termites, which are some of the most ecologically successful groups of organisms on Earth. Social insects can create structures that are spectacularly more complex than other solitary and subsocial arthropods because of stigmergy and self-organization (Theraulaz and others, 1998). Armour Smith, E., Loewen, M.A., and Kirkland, J.I., 2020, New social insect nests from the Upper Jurassic Morrison Formation of Utah: Geology of the Intermountain West, v.
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