Improved methodology of ‘conchostracan’ (Crustacea: Branchiopoda) classification for biostratigraphy
Scholze, Frank; Schneider, Jörg W.
Newsletters on Stratigraphy Volume 48 Number 3 (2015), p. 287 - 298
published: Aug 1, 2015
ArtNo. ESP026004803004, Price: 29.00 €
Conchostracans (Crustacea: Branchiopoda) are one of the best prospective invertebrate groups for the biostratigraphy of continental deposits, especially in the otherwise fossil-poor sediments of extreme environments. As pioneer organisms, they can have rapid population growth in a temporary environment and often form fossil mass occurrences. Additionally, their diminutive, desiccation-resistant eggs could easily be transported and regionally distributed by storms. This makes them ideal tools for biostratigraphic correlations. Unfortunately, their classification is hampered by the widely differing terminology of carapace morphology. In order to promote the use of well-defined descriptive terms and to avoid further use of subjective terms, a normalized carapace terminology based on a set of quantitative and semi-quantitative measurements is presented here. This terminology provides definitions for diagnostic features such as growth bands, growth lines, larval valve, umbo, and the outer margins of the valve as well as defining distinct sculptural elements such as concentric ribs, radial ribs and ornamentations of the shell. The terminological inventory includes objective determinations and classifications for the total valve length, and the coefficients of the valve shape, the length of the dorsal margin, the position of the umbo, and the size of the larval valve. The standardized carapace terminology includes measurements in order to describe the positions of the points of maximal curvature at the anterior, posterior, ventral and dorsal margins as well as the intensities of the curvatures. Both the procedure for measurement of valve parameters and carapace terminology are explained in detail and applied to fossil examples. As a result, an objective and reproducible terminology for future studies of conchostracans is proposed. If successfully applied, the proposed standardized terminology would provide a method resulting in easy and exactly reproducible taxonomic diagnoses, which would improve future variability studies and faunal comparisons as essential prerequisites of conchostracan use in biostratigraphy.