O’REILLY, J.C.*; SOUZA, C.A.; COSTA, M.C.; DA SILVA, N.J.; Univ. of Miami, Coral Gables, FL; Univ. Cat�lica de Goi�s, Goi�nia, Brazil; Univ. Cat�lica de Goi�s, Goi�nia, Brazil; Univ. Cat�lica de Goi�s, Goi�nia, Brazil: The Mechanism of Burrowing in Leposternon microcephalum (Squamata: Amphisbaenia)

Amphisbaenians with a dorso-ventrally flattened skulls (�spade� or �shovel-snouted�) have been described as utilizing a two-cycle burrowing mechanism that involves temporally separate forward thrust and head-lifting phases. It is hypothesized that the forward thrust involves the dissipation of momentum after the acceleration of the axial skeleton and skull by muscles that originate in the skin and insert on the ribs (costo-cutaneous muscles). The head-lifting phase is powered by a pair of large, pinnate muscles originating on the tops of the vertebrae and inserting on tendons that, in turn insert on the top of the skull. We tested this functional hypothesis in Leposternon microcephalum, a �spade snouted� amphisbaenian from South America, using kinetic data gathered while animals attempted to burrow on a force plate. Experimental manipulations of tunnel length indicate that the costo-cutaneous muscles of the trunk play only a small role in generating forward-directed forces. The duration and size of the forward forces, excludes a prominent role for the storage and dissipation of momentum in this system and no two-cycle burrowing behavior was observed. Instead, Leposternon utilizes an upward rotation of the skull to simultaneously generate both thrusting and lifting forces. The unusual shape of the skull converts the dorsoventral rotation, in part, to anteriorly directed force – functioning like a cam in a cam-follower mechanism. Large Leposternon can generate well in excess of 100 N of force during burrowing. However, approximately one-half of the total force generated is directed dorsally rather than forward.