Presentations

Digitizing Fossil Morphology in 3-D: A First Step Toward Range-of-Motion Analysis

Calvin J. Frye

Poster presented at the Annual Meeting of the Geological Society of America, October 26, 1992, in Cincinnati, Ohio.

The complex motions of arthropod limbs constrain the ecological limits of the organism. The activities of food gathering and manipulation, walking, burrowing, and swimming require different styles and degrees of motion. The complexity of the joint system in arthropods previously has allowed only a qualitative description of their motion.

A system has been developed of digitizing, in three dimensions, the positions of the limiting ranges of motion of each joint axis of an arthropod limb. These, in turn, can be integrated with other joint axes of an appendage to define its total range of motion. The technique involves only simple equipment, such as vernier calipers or precision scales. No digitizing pad or three-dimensional digitizer is necessary.

Measurements of the specimen are taken, recording the distances between morphological landmarks defining ranges of joint motion. A software routine has been developed that uses these measurements in a triangulation network to construct a wire-frame model of the specimen, giving the three-dimensional coordinates of each feature as a vertex of the framework.

The calculated results of this routine have been checked against the theoretical positions of the vertices of regular polyhedra, as well as with biological specimens digitized using a conventional three-dimensional digitizer. Reasonable accuracy and consistency have been obtained with this approach; but, care must be taken to avoid reliance on points which are too close together or nearly coplanar. This technique should be applicable to a wide range of biomechanical studies.

'Tintenflecken' from the Michigan Basin: Preservation, Ontogeny, and Variation of Aptychi Found in Upper Devonian Rocks at Paxton, Michigan

Joseph T. Hannibal, Robert Carr, and Calvin J. Frye

Talk presented (by Hannibal) at the Annual Meeting of the Geological Society of America, October 28, 1992, in Cincinnati, Ohio.

While beds containing concentrations of 'Tintenflecken' (fossil aptychi resembling ink spots) have long been known, most species of aptychi have been named on the basis of only a very few specimens. More than sixty specimens of a species of aptychus, Sidetes ('Spathiocaris') cf.S. emersoni (Clarke), have been recovered during three years of field work in the Upper Devonian Antrim Shale exposed in the LaFarge quarry in Paxton, Michigan.

Most specimens occur singly, but others occur in groups. One specimen, preserved in three dimensions in limestone matrix, retains the presumed original, scooplike shape of the aptychus. Many specimens found in shale matrix are unfolded, flattened, and have well-preserved outer margins, and thus can be readily measured. The length-to-width ratio of these specimens changes during ontogeny and also varies among specimens of similar size. This indicates that species determinations based primarily on length-to-width ratios, a commonly used criterion, should be treated with caution.

Analysis of composition using X-ray spectroscopy indicates that these specimens, like those found in the Ohio Shale in the Cleveland region, are carbon films. The Paxton locality, however, is less diverse in species of Sidetes than the Ohio Shale. Ammonites of sizes compatible with the Michigan aptychi are found at the Paxton site.