Axial muscle function during lizard locomotion

It was recently reported that the epaxial muscles of a lizard, Varanus salvator, function to stabilize the trunk during locomotion, and it was suggested that this stabilizing role may be a shared derived feature of amniotes. This result was unexpected because it had previously been assumed that the epaxial muscles of lizards function to produce lateral bending during locomotion and that only in mammals and birds were the epaxial muscles active in stabilizing the trunk. These results and the inferences made from them lead to two questions. (1) Is the pattern of epaxial muscle activity observed in V. salvator representative of a basal lizard condition or is it a derived condition that evolved within lizards? (2) If the epaxial muscles do not produce lateral bending, which muscles do carry out this function? These questions were addressed by collecting synchronous electromyographic (EMG) and kinematic data from two lizard species during walking and running. EMG data were collected from the epaxial muscles of a lizard species from a basal clade, Iguana iguana, in order to address the first question. EMG data were collected from the hypaxial muscles of both Iguana iguana and Varanus salvator to address the second question. The timing of epaxial muscle activity in Iguana iguana relative to the kinematics of limb support and lateral trunk bending is similar to that observed in Varanus salvator, a finding that supports the hypothesis that the epaxial muscles stabilize the trunk during locomotion in lizards and that this stabilizing role is a basal feature of lizards. Therefore, a stabilizing function of the epaxial muscles is most parsimoniously interpreted as a basal amniote feature. In both Iguana iguana and Varanus salvator, the activity of two of the hypaxial muscles, the external oblique and rectus abdominis, is appropriately timed for the production of lateral bending. This indicates that elements of the hypaxial musculature, not the epaxial musculature, are the primary lateral bending muscles of lizards.