Foster, W.; Gensbigler, P.; Bletcher, M.; Forjaz, A.; Li, B.; Matos Romero, V.; Kwon, C.; Lyson, T.; Kiemen, A. L.; Bever, G. S.

Modern biodiversity is built on a series of disparate body plans whose origins are obscured by deep time. Our most direct source of clarifying data is the fossilized skeleton, where morphology reflects an evolving functionality realized through the development of associated tissues. We apply this dualistic perspective to the shelled body plan of turtles whose Paleozoic initiation is marked by a derived relationship between ribs and dermis (Lyson & Bever, 2020). Current developmental models remain in conflict with an increasingly informative fossil record, suggesting critical steps remain unrecognized. Here we explore the hypothesis that the breakdown of rib-spanning muscles, an evolutionary transformation mirrored in embryogenesis, is one such step. Multi-modal imaging of turtle embryos, including a novel application of histology-based deep learning (Kiemen et al., 2022; Matos-Romero et al., 2025; Forjaz et al., 2026), establishes intercostal muscle degradation as preceding turtle-specific rib development and highlights the heuristic power of 3D, whole-embryo analysis (Forjaz et al., 2026). Quantified divergence from mouse pinpoints the timing and tempo of this organized, apoptotic breakdown. Initial evidence suggests an associated non-pathological inflammatory response, which has been shown capable of driving evolutionarily stable hyperossification (Rashid et al., 2023). These patterns support trunk muscles as a critical signalling centre whose ontogenetic loss set the phylogenetic stage for a morphogenetic transformation remarkable in a non-metamorphic species.