Stephen M. Reilly Ohio University The proposed research will examine the locomotor system in marsupial and placental mammals in which different constraints and innovations appear to have molded the evolution of locomotor design, gaits and the use of mechanical energy-saving systems. Integrated studies of anatomy, gait, limb and axial movements, muscle activity patterns, ground reaction forces and whole-body mechanics will compare locomotor dynamics in two important examples of evolutionary transitions of vertebrate locomotion: from the primitive tetrapod condition to early mammalian forms and between marsupials and placental mammals. The work will follow up on discoveries from previous NSF support showing that the "epipubic" bones lying in the belly wall in front of the pelvis and associated abdominal muscles function in a "cross-couplet system" controlling trunk bending and footfall patterns during locomotion. The research will test the hypotheses that 1) this system, characteristic of primitive mammals, constrains generalized marsupials to trotting gaits and bouncing mechanics, and 2) release from this morphological constraint in some marsupials and placental mammals (via the loss of the epipubic bones) allows the use of an expanded range of gaits and energy-saving mechanisms in locomotion. The specific aim is to quantify abdominal wall function and locomotor dynamics in a range of marsupials and primitive placental mammals to better understand the epipubic bone function, its constraints on locomotor dynamics and the consequences of epipubic bone loss. The advent of the cross-couplet system and its subsequent retention in all basal mammalian taxa reveals a significant and as yet unrecognized critical innovation in the transition from generalized amniote to mammalian patterns of locomotion. Appearing concomitantly with the key mammalian traits of endothermy, mastication and lactation, the cross-couplet system is hypothesized to have been a key locomotor innovation leading to the early radiation of mammals. Although it remains a viable locomotor system in many primitive mammals, the subsequent reduction or loss of the epipubic bones, as well as the loss of the cross-couplet system and its constraints, appears to have freed some marsupials and the extant eutherians from the locomotor constraints on gait and mechanics predicted for cross-couplet system. Understanding the locomotor consequences of the appearance and subsequent loss of the epipubic bones is therefore of great significance in mammalian evolution. Quantifying the functional consequences of changes in early mammalian belly design is critical to understanding the radiation of locomotor abilities in higher mammals. The proposed study is novel in relating morphology and kinematics to gaits and mechanics, and this approach bridges the realms of functional morphology and biomechanics to understand some of the deeper causes of variation in animal behavior and will add greatly to our understanding of quadrupedal locomotion. This work provides opportunities for colleagues, from Appalachian high school students to foreign collaborators, to gain hands-on expertise in state-of-the-art techniques spanning the disciplines of functional morphology, biomechanics, ecology and behavior, and to share in the discovery of new insights in science. Integrating anatomy and mechanics (not well understood by the public) with animal behavior (inherently visible to the public) has proven to provide valuable ways to illustrate to society the relationships between research and discovery, and the critical role of anatomical variation and physics in organismal design, function and evolution.
