Digital volumetric modeling reveals unique body plan experimentation in the Devonian tetrapod Ichthyostega

The water-to-land transition in tetrapods promoted a radical shift in locomotor function and kinematic patterns, from axial- to appendicular-dominated propulsion and from buoyancy- to musculoskeletal-driven support. Many of these facets of locomotion are dictated by an animal's whole-body mass properties, which presumably also changed across this transition. We herein use digital volumetric modeling to reconstruct mass properties in the Late Devonian tetrapod <i>Ichthyostega</i>. In comparison to fishes and crown tetrapods, our results show that <i>Ichthyostega</i> possessed a uniquely "robust" body plan, combining traits typical of both "fishes" (anterior center-of-mass) and "tetrapods" (well-developed limbs, especially forelimbs). We also find that variation in body composition has little effect on body attitude at flotational equilibrium, suggesting that whole-body "evolutionary tinkering" had little impact on hydrostatic profile. These results support previous inferences of forelimb-dominated movements in <i>Ichthyostega</i> and highlight a hitherto underappreciated level of morphological experimentation during the tetrapod transition to land.