MICROEVOLUTIONARY DYNAMICS OF THE EARLY DEVONIAN CONODONT WURMIELLA FROM THE GREAT BASIN OF NEVADA


ABSTRACT
The morphologically smooth basal platform margin of one conodont element (P1) in the spathognathodontid ozarkodinidids is a key diagnostic feature for taxon identification. Differences in the shape of this margin have been used as the basis for genealogical delineation and the interpretation of evolutionary dynamics, yet very little is known of the morphological or ontogenetic processes that generate shape variation within the group over geological time. Furthermore, the absence of reliably recognizable geometric landmarks along this margin prevents the application of standard geometric morphometric techniques. This paper addresses these problems by developing a new method for the quantitative description of margin shape, analyzing the ontogeny of shape change within specific stratigraphic intervals, and describing the pattern of evolution of margin shape over geological time.

Specimens of the genus Wurmiella were obtained from the Windmill Limestone (Pragian) of central Nevada. The resulting morphometric data were used to discriminate three species within the samples, Wurmiella wurmi, W. tuma, and a currently undescribed species, W. n. sp. The taxa are differentiated both on the basis of overall shape, as well as differences in margin allometry as described by spline shape and size (recorded separately as centroid size). An examination of evolutionary pattern in the W. wurmi samples, using a random walk-based technique, demonstrates that three independent aspects of margin shape evolved differently. The overall arch and relative position of the basal cavity were highly constrained and static, while the shape of the margin anterior to the basal cavity underwent an episode of significant directional evolution.

Peter D. Roopnarine. Department of Invertebrate Zoology & Geology, California Academy of Sciences, Golden Gate Park, San Francisco CA 94118-4599 , USA. 
Michael A. Murphy. Department of Geology, University of California Davis, Davis CA 95616, USA.
Current address: 680 Gatewood Lane, Sierra Madre, CA
Nancy Buening. Department of Geology, University of California Davis, Davis CA 95616, USA. 
Current address: Department of Geology, 6000 J Street, California State University, Sacramento, CA 95819

KEY WORDS: conodont, Wurmiella, microevolution, morphometrics

PE Article Number: 8.2.31A
Copyright: Paleontological Society. October 2005
Submission: 4 February 2003. Acceptance: 16 February 2004.