Abstract
Accurate, quantitative characterization of complex shapes is recognized as a key methodological challenge in biology. Recent development of automated three‐dimensional geometric morphometric protocols (auto3dgm) provides a promising set of tools to help address this challenge. While auto3dgm has been shown to be useful in characterizing variation across clades of morphologically very distinct mammals, it has not been adequately tested in more problematic cases where pseudolandmark placement error potentially confounds interpretation of true shape variation. Here, we tested the sensitivity of auto3dgm to the degree of variation and various parameterization settings using a simulation and three microCT datasets that characterize mammal tooth crown morphology as biological examples. The microCT datasets vary in degree of apparent morphological differentiation, with two that include grossly similar morphospecies and one that includes two laboratory strains of a single species. Resulting alignments are highly sensitive to the number of pseudolandmarks used to quantify shapes. The degree to which the surfaces were downsampled and the apparent degree of morphological differentiation across the dataset also influenced alignment repeatability. We show that previous critiques of auto3dgm were based on poorly parameterized alignments and suggest that sample‐specific sensitivity analyses should be added to any research protocol including auto3dgm. Auto3dgm is a useful tool for studying samples when pseudolandmark placement error is small relative to the true differences between specimens. This method therefore represents a promising avenue forward in morphometric studies at a wide range of scales, from samples that differ by a single genetic locus to samples that represent multiple phylogenetically diverse clades.
Highlights
A major challenge for evolutionary biologists, including paleontologists and paleoanthropologists, is to understand how morphological diversity can inform our understanding of micro- and macroevolution
Characterizations of tooth crown morphology are useful for delimiting mammalian species by size in the fossil record in part because teeth show little size variation due to ontogeny and have well-characterized ranges of size variation within species (Gingerich, 2014)
When interspecific linear dimensions are similar and intraspecific variation in shape is high relative to interspecific variation, broad patterns in tooth morphology and their correspondence to species boundaries can be difficult to assess with simple linear measurements alone (Carrasco, 1998)
Summary
A major challenge for evolutionary biologists, including paleontologists and paleoanthropologists, is to understand how morphological diversity can inform our understanding of micro- and macroevolution. Recent interdisciplinary work between comparative anatomists and mathematicians has produced a new class of morphometric protocols (Boyer et al, 2011, 2015a; Boyer, Winchester, & Kay, 2015b; Gao et al in review) that are designed to reduce the time, subjectivity, and idiosyncrasies of user-determined measurements, as well as to provide quantifications that simultaneously include information about shape diversity and shape disparity These new methods include automatic alignment of all study objects and computation of Procrustes distances between them based on a predetermined number of evenly spread pseudolandmarks (Boyer et al, 2015a, 2015b). The transitive alignment using the final, larger set of pseudolandmarks is more accurate than the alignment computed using the initial set of landmarks, and allows for a more accurate alignment of the original input surfaces (Boyer et al, 2011, 2015; Puente, 2013)
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