The long-term utilization of conodonts as one of the primary elements of Paleozoic and Triassic biostratigraphy has resulted in a database of stratigraphic information that far exceeds that available for any other Paleozoic or early Mesozoic metazoan taxon (Sweet 1988). The geographically and sedimentologically widespread distributions of many conodont taxa, the group's long stratigraphic history, and the stratigraphic density of the conodont fossil record make these organisms ideal targets for evolutionary paleobiological questions. The present paper applies these properties of the conodont record to long-standing questions of microevolutionary dynamics, an exercise not unique to this paper (for example, Murphy et al. 1981), but one that has not been disseminated broadly into the mainstream paleobiological literature. In turn, current ideas and techniques of morphometric analysis and the analysis of fossil time-series (Roopnarine et al. 1999; Roopnarine 2001) are applied here to the study of the Lower Devonian genus Wurmiella from the Great Basin of Nevada.

Patterns of microevolutionary change are of central importance to our understanding of organismal diversification over geological time. While the time-scale of microevolution challenges the finest temporal resolutions available from the fossil record, the major theory of diversification (that is, the origin of species by natural selection) claims that microevolution is fundamental to the production of new diversity. On the other hand, more recent macroevolutionary hypotheses such as species selection (Stanley 1975) and the possible random nature of mass extinction events (Gould 2002) have called into question the degree to which microevolutionary processes might exert any control over long-term patterns of diversity. The fossil record remains one of the primary sources of information capable of resolving the conflict between a strict Darwinian view versus hierarchical approaches to selection as a structuring agent in diversification. Accessing this information, however, requires work at the finest stratigraphic resolutions available, detailed quantitative morphological analyses, and objective assessments of microevolutionary series.

Evolutionary change and character variation can be expected to be largely continuous and quantitative as temporal intervals approach generational timescales. Two questions of great importance to understanding the role of microevolution in macroevolutionary phenomena are 1) the extent to which character variability within ancestral populations determines variation among descendant species, and 2) whether within-species evolution is at all relevant to the differences among species. The first question addresses variation at the organismal level, such as ontogenetic change and the processes of ontogenetic change, for example allometry, heterochrony and heterotopy. The second returns to one of the central arguments of punctuated equilibrium theory, and that is the restriction of major morphological change to times of speciation (Gould 2001). Whether most of the geological span of a species is characterized by constrained stasis (Roopnarine 2001) or oscillatory `no net change' evolution, the implication is that speciation, as identified by a significant morphological discontinuity, is essentially independent of intraspecific evolution. These issues should be addressed by high resolution, quantitative studies of intraspecific morphological change, coupled with methods appropriate for the classification of microevolutionary modes. This paper addresses both these issues, examining within-sample variation, as well as stratophenetic change in Wurmiella.

The highly variable P1 element forms the basic means of taxonomic differentiation in ozarkodinide conodonts (Sweet 1988) (Figure 1). The shape and outline of the lower profile of the element is a key feature supporting taxonomic and phylogenetic hypotheses (Murphy et al. 1981) and is the focus of analysis in this paper. The lower profile, however, is a curved or sinuous structure with very few discrete landmarks and therefore does not lend itself to a typical geometric morphometric approach (Bookstein 1991). Here we present a new method combining a landmark approach and a spline-based description of the P1 element's basal platform in lateral profile. The technique relies on the presence of two geometrically homologous landmarks on the element's platform to convert spline coordinates to Bookstein-style shape coordinates for subsequent analysis of within-sample dispersion and allometry, and among-sample discrimination. Finally, the evolutionary mode of the stratigraphically ordinated samples is classified using the Hurst-estimation method outlined in Roopnarine (2001).