Abstract
Abstract The fossil record of planktonic foraminifera is ideally suited to defining stratigraphic age controls and exploring fundamental questions in evolutionary biology due to its excellent preservation potential that yields continuous, high-resolution fossil archives of large numbers of individuals. For full morphometric analyses foraminifera tests are generally mounted, oriented and imaged manually, while data are processed using standard software such as ImageJ or Image Pro. However, manually induced orientation errors are a source of potential bias in trait measurements even when quantified using the same computational subroutine. Here we test the repeatability of four measures of foraminiferal test shape on six morphologically distinct species and present a calibration (power analysis) of the number of individuals needed to determine a given percentage change in these traits. We mounted and measured every individual twice and analysed the difference between the two measurements to determine the effects of small orientation changes on the studied traits. We show that measurements of test area and aspect ratio are statistically indistinguishable between runs for all species studied, and a power law calibration suggests that between 25 and 50 individuals are needed to detect at least a 10% in- or decrease in either trait. However, despite mounting tests on glass slides to clarify perimeter outlines, test perimeter was only repeatable in the spherical species Orbulina universa , and test roundness was not repeatable for three out of six studied species. We recommend the use of aspect ratios constructed from lengths and avoidance of perimeters and their dependent metrics to reduce orientation induced bias.
Highlights
The planktonic foraminifera bequeath one of the most complete fossil records known to science
The accumulation in deep sea sediments of well-preserved shells of vast numbers of individuals make the planktonic foraminiferal fossil record uniquely suited for both biostratigraphic age controls (Blow, 1969; Bolli et al, 1989; Berggren et al, 1995; Wade et al, 2011), and for answering fundamental questions in evolutionary biology (e.g. Wei and Kennett, 1988; Norris, 1991; Alizon et al, 2008; Hull and Norris, 2009; Ezard et al, 2011)
Planktonic foraminifera are most often measured from two-dimensional images taken by a camera attached to a microscope with individual tests mounted in a given orientation, and trait measurements are extracted from the image's 2D test representation
Summary
The planktonic foraminifera bequeath one of the most complete fossil records known to science. Planktonic foraminifera are most often measured from two-dimensional images taken by a camera attached to a microscope with individual tests mounted in a given orientation, and trait measurements are extracted from the image's 2D test representation. Aspect ratio and roundness are two measures of test shape, further enabling species identification as well as quantifying the test area-to-volume ratio, an important measure in terms of metabolic processes. Both metrics are routinely generated by popular software such as ImageJ or Image Pro. Both metrics are routinely generated by popular software such as ImageJ or Image Pro Together, these four traits form an important part of describing planktonic foraminifera morphologies. Quantifying their precision is crucial to the task of interpreting species morphometric records for both biostratigraphic and evolutionary purposes
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