SENSITIVITY OF VARIOUS METHODS OF MEASURING THECA VOLUME IN BLASTOIDS

Volume is a basic parameter that paleobiologists measure as a proxy for biomass for a variety of purposes. However, precisely measuring the volume of irregularly-shaped specimens can be problematic. The this difficulty increases in minute specimens because of the imprecision of the tools currently used, as well as confusion as to which volumetric measuring methods are most suitable in a variety of situations. This study evaluates the performance of various methods used to measure volume through a study of the blastoid Pentremites tulipiformis, an extinct echinoderm. Blastoid theca were measured using four different techniques that were evaluated in the context of measurement time, cost, precision, repeatability, and effectiveness over a range of sizes. The thirty-one specimens covered theca heights from 3.27 to 17.3 mm and were measured in triplicate in random order for each method. Statistical tests were used to compare the results of the methods.

Simple displacement of water by the specimen in a graduated cylinder was fast and inexpensive, but had low precision and repeatability. This method was particularly poor at measuring small volumes. Calculating volume by dividing the mass of specimens by the density of calcite was fast, extremely precise, very repeatable, but has moderate up front costs of an analytical balance. This method was particularly effective when measuring extremely small specimens but concerns arise where potentially incomplete infilling of the theca by calcite, or other minerals of different densities, may become problematic. Estimating volume by measuring the mass of water displaced by immersed specimens was somewhat time consuming, reasonably precise, very repeatable, but has moderate up front costs of an analytical balance. This method works well on large specimens even if they are incompletely in filled or of mixed mineralogy, but becomes less repeatable at small sizes because of the nature of the apparatus and the potential of surface bubbles on all specimens. Measuring volume with a 3-D laser scanner was very time consuming, but had high precision and repeatability at large sizes, but was poor at measuring small volumes because of the limitations of the scanner resolution. This method has very high up front costs because of price of the scanner and associated desktop computer.