STOP MOTION MOVIES OF MICRO-CT CORE SCANS AS TOOLS FOR EXAMINING ETHOLOGY IN TRACE FOSSIL ASSEMBLAGES FROM THE LOWER MISSISSIPPIAN NORTHVIEW FORMATION, SOUTHWESTERN MISSOURI, USA

Burrows and other trace fossils result from behaviors of active biological agents that excavate, move, and modify sedimentary successions; the study of these behaviors is known as ethology. Stop motion movies provide a means for exploring ethology. Directional motion as well as apparent intensity of bioturbation convey a notion of timing even though the relative durations of behaviors remain unknown. Movies compiled from collections of μCT scans of short cores from the lower Mississippian (Kinderhookian Series) Northview Formation in southwestern Missouri illustrate the phenomenon.

The Northview Formation is composed of a cyclic succession of light gray-green shale interbedded with very fine sandstone and siltstone. Ichnospecies include Scalarituba missouriensis Weller 1899, Nereites sp., and Zoophycos sp.; other fauna include foraminifera and rare brachiopods. The aesthetically distinctive pattern of burrowing and convenience of collection as field stone facilitated use of the burrowed sandstone for vernacular architectural applications in and around Springfield, Missouri. It locally is known as "worm rock," and it commonly is used as an exterior facing stone for houses and as a decorative element in public structures. Seven cores (4.5 cm and 7.0 cm dia.) of burrowed fine sandstone with siltstone and mudstone fills were collected from near the Noble Hill stratigraphic section approximately 15 km north of Springfield. Core scans were performed at the University of Missouri with a Zeiss Xradia 510 Versa 3D x-ray microscope.

The movies show several scales of burrowing: (1) the largest burrows, up to 2.5 cm across, are of unknown origin and are reflected in abrupt contrast changes with depth in the core; (2) lined burrows up to 3.0 mm indicated by high contrast in the walls; (3) Scalarituba, ~2.5 mm dia., is characterized by a series of curved transverse ridges that indicate movement in the direction of concavity; (4) the smallest burrows, sometimes preserved as open porosity, appear to be quasi-linear and disposed to narrower horizontal intervals; and (5) intermediate burrows, <1.0–1.5 mm dia., show the greatest intensity of bioturbation and a propensity for vertical movement in the cores. The most compelling aspect of these movies is the vestige of motion attributable to 340-million-year-old actors.