DOES TRAIT SELECTION BY PALEONTOLOGISTS INFLUENCE PATTERNS OF TEMPO AND MODE IN THE FOSSIL RECORD?

In paleobiology, tempo and mode of morphological evolution have bubbled to the surface once again. Recent work has shown that, for single traits, stasis and random walks are more common than directional change in the fossil record. However, not all traits should be expected to change in the same pattern or at the same rate. Moreover, myriad traits can be measured for a given lineage. Using a real dataset of landmark coordinates and covariance matrices for a Cambrian trilobite, we generated a set of 10 sequential samples that capture temporal trends in morphological change. Among ratios of lengths between pairs of coordinates divided by a standard length (a traditional "shape" trait), 13% showed directional change, 39% showed a random walk, and 48% showed stasis. Traits secondarily generated from multivariate analysis also showed different patterns of change. One expectation is that as the number of traits measured for any given lineage increases, the likelihood decreases that all traits will show the same pattern of change. This expectation is born out across a comparative dataset of both size and shape traits from over 55 real fossil species lineages, representing a diversity of metazoan groups. Our analysis of over 200 metric and meristic traits demonstrates a weak relationship between the pattern of change for a particular trait and its use in species determination. Within species, where particular traits show patterns of stasis, they are typically used in classification. In contrast, where traits show directional change, they are less likely to be used for classification. However, within lineages made up of two or more species, these results are reversed. Lastly, traits that show random walks produced results similar to traits that show directional change. In building this dataset, we have accepted the delimitation of species or chronospecies by the individual authors, which in total may comprise different species concepts. Nonetheless, the fact that different patterns may be produced from single length, trait ratio, or multi-trait analysis of individual species lineages has implications that deserve further consideration.