Developmental and Evolutionary Perspectives on the Origin and Diversification of Arthropod Appendages

Jointed, segmented appendages are a key innovation of arthropods. The subsequent diversification of these appendages, both along the body axis and across taxa, has contributed to the evolutionary success of arthropods. Both developmental and fossil data are informative for understanding how these transitions occurred. Comparative analyses help to pinpoint the developmental novelties that distinguish arthropod appendages from the lobopodous appendages of other panarthropods, and that distinguish different appendage types. The fossil record of stem group arthropods is diverse and preserves intermediate steps in these evolutionary transitions, including some that cannot be directly inferred based on extant taxa. These lead to hypotheses that can be tested with comparative developmental data, as well as to reinterpretations of developmental results. One developmental novelty of arthropods is the reiterated deployment of the joint formation network, which divides the appendages into segments. The fossil record raises questions about how this joint formation network was first deployed, given the contrasting morphologies of appendages in stem group versus extant arthropods. The fossil record supports a character tree for appendage diversification showing progressive individuation of appendages in an anterior-to-posterior sequence. However, to date, developmental evidence provides at best limited support for this character tree. Recent interpretations of the fossil record suggest that the labrum of extant arthropods is a greatly reduced protocerebral appendage pair; this hypothesis is consistent with the extensive shared developmental patterning of the labrum and jointed appendages. Reciprocal illumination from fossils and developmental patterning in a phylogenetic context both makes sense of some results and helps motivates questions for future research.