Lipid droplets

OK, they are cute, but why should a developmental biologist care about them? Onset of calcification early in ontogeny coupled with gradual, progressive morphological development permits the recognition of growth sequences for over a hundred trilobite species. Striking variation in the number of trunk segments at maturity across ontogeny and phylogeny, coupled with the curious pattern of segment exchange between the pygidium and thorax during early development, provides opportunities to explore the co-evolution of different aspects of segmentation and body patterning. Recent results suggest an evolutionary trade-off between the ability to vary the overall number of thoracic segments and the advantages of regional specialization along the body axis. The striking, general decrease over time in the degree of intraspecific variation in the aftermath of the Cambrian radiation offers an opportunity to assess the roles of regulatory evolution and palaeoenvironmental change in shaping the evolutionary trajectory of the clade as a whole.