A fossil record of developmental events: variation and evolution in epidermal cell divisions in ostracodes.

The carapaces of some ostracode taxa bear reticulate skeletal ridges that outline underlying epidermal cells. This anatomy allows one to identify homologous cells across individuals, to infer the modal sequence of cell divisions that occurs over ontogeny, and to identify individuals with variant cell patterns (e.g., additional or missing cell divisions), even in fossils. Here we explore the variational properties and evolutionary history of this developmental system in the deep-sea ostracode genus Poseidonamicus. Using a sample of over 2000 specimens to capture variation in cell division sequence, we show that phenotypic variation in this system is highly structured: some variants, regions of the carapace, and lineages are much more variable than others. Much of the differences in variation among cells can be attributed to the molt stage in which cells take their final form-cell divisions occurring later in ontogeny are more variable than those earlier. Despite ample variation, only two evolutionary changes in the sequence of cell divisions occur over the 40 Myr history of this clade. The evolutionary changes that do occur parallel the two most common intraspecific variants, suggesting that developmental structuring of variation can have long-term evolutionary consequences. Analysis of the most common variant over the last two molt stages suggests that it suffers a fitness disadvantage relative to the modal form. Such normalizing selection may contribute to the evolutionary conservativeness of this developmental system in the Ostracoda.