Quantifying Evolutionary Dynamics of Arthropodization and Arthrodization.

Euarthropods are characterized by segmented, sclerotized, and articulated bodies and appendages, expressed through arthrodization and arthropodization. Both are regarded as defining features of Euarthropoda and have been proposed to evolve in a specific sequence, during which arthropodization preceded arthrodization, and initiated in frontal appendages and was subsequently co-opted into other appendages. Here, we investigate their evolution across Ecdysozoa using morphological comparison, ancestral-state reconstruction (ASR), and semi-quantitative methods. Morphological comparisons and ASR reveal repeated gains, reductions, losses, and regional decoupling of segmentation, sclerotization, and articulation in both body and appendicular systems. Semi-quantitative indices indicate variable degrees of arthrodization and arthropodization across ecdysozoan lineages and do not show a progressive increase from the basal panarthropods and the stem-group euarthropods to crown-group euarthropods. Together, these results support a modular, convergent evolution of arthrodization and arthropodization in different lineages, probably through repeated modification, reduction, or redeployment of the ancestral developmental toolkit.