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 Ichthyostega. In comparison to fishes and crown tetrapods, our results show that Ichthyostega 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 Ichthyostega and highlight a hitherto underappreciated level of morphological experimentation during the tetrapod transition to land.