ENVIRONMENTAL INFLUENCE ON BURROW SYSTEM FEATURES OF A COLONIAL AND FOSSORIAL RODENT: IMPLICATIONS FOR INTERPRETING FOSSIL TETRAPOD BURROWS

Abstract: This is a neoichnologic study of Microcavia australis (Rodentia: Caviidae) burrow systems from two environments of the semiarid region of central Argentina, with the main purpose of contributing to the interpretation of fossil tetrapod burrows. We compared three burrow systems from the Monte and three from the Espinal biogeographic provinces to discern which burrow system features vary with environmental parameters (soil texture, climatic conditions, and vegetation type) and identify the distinctive ichnologic features of M. australis burrow systems. Burrow systems from the Monte occur in nebkhas with sparse xerophytic, psammophilic, and halophilic shrubs in sandy and loose soils. In the Espinal province the burrows appear in Prosopis caldenia forest, with shrubs and herbs in silty and harder soils. The Monte burrow systems comprise an intricate pattern with two levels, closed circuits, and larger tortuosity and fractal dimension. The burrow systems from the Espinal display an L-shaped or linear pattern with a single level and commonly lack closed circuits. The average ratio of total chamber volume to tunnel volume and the tunnel diameter is significantly larger in the Monte systems, which is interpreted as a reflection of larger colony size and individuals having larger body mass. Primary surface ornamentation (sets of claw traces related to producer digging) is better developed in the Espinal burrow systems, and secondary surface ornamentation (herein interpreted as arthropod burrows) dominated in the Monte systems. We propose that the distinctive features of M. australis burrow systems can be used as a model to recognize fossil burrows of colonial and fossorial herbivorous rodents that construct a permanent burrow structure with open entrances in semiarid settings. A set of ichnotaxobases for fossil vertebrate burrows is also suggested.