From Fin to Limb: Orientational Shift and Evolution of Diagonal-Couplet Gait in Tetrapods

The transition from fins to limbs represents a pivotal evolutionary shift that enabled vertebrates to engage terrestrial environments through coordinated transformations in appendicular structure and neuromuscular control. Here, we propose that this transition can be understood as a biomechanical reorganization of force transmission across multi-joint musculoskeletal linkages. Rather than a simple rotational modification transforming fins directly into forelimbs, this process involved progressive reorganization of sarcopterygian appendages, including axial rotation, directional force production, and coordinated stance-swing dynamics that likely emerged prior to fully terrestrial locomotion. This review applies the two-joint link model, originally developed for human limb biomechanics, to extant sarcopterygians such as the coelacanth (Latimeria) and to early-diverging tetrapods, including archosaurs such as the American alligator (Alligator mississippiensis). The model demonstrates how sequential activation of biarticular and monoarticular muscles generates predictable, axis-aligned endpoint force outputs, providing a mechanistic explanation for how ancestral fins and early limbs could support weight bearing, propulsion, and postural stabilization during the water-to-land transition. The framework further clarifies the evolution and persistence of diagonal-couplet lateral sequence gait, a locomotor pattern widespread among tetrapods and consistent with Paleozoic trackway evidence. Integration of electromyographic data with conserved spinal circuitry including central pattern generators, interneurons, and the topographic organization of lateral motor column motor pools reveals how intrinsic spinal architecture governs muscle activation sequences and limb-level force redirection. Although supraspinal pathways modulate locomotion, core coordination of diagonal-couplet gait emerges from spinal mechanisms. A central property of the model is musculoskeletal redundancy: multiple muscle combinations can generate equivalent endpoint forces within linked multi-joint systems, thereby preserving functional capacity across postural variation. This robustness arises from the four-bar linkage organization of biarticular muscles, a design principle conserved across vertebrate musculoskeletal systems. Together, these findings position the two-joint link model as a unifying neuromechanical framework for understanding fin-to-limb evolution, the emergence of tetrapod gait, and the conserved spinal organization underlying vertebrate locomotion.