How Archaeopteryx could run over water
ABSTRACT Archaeopteryx may have exploited the running over water technique used by Basilisk lizards in combination with lift generated by its wings.* Direct measurements of the dimensions of the hind limbs of the Berlin specimen of Archaeopteryx with an estimated mass of 0.25 kg, are compared with predictions of the leg sizes of a Basilisk lizard with the same body mass using GLASHEEN and McMAHON'S (1996b) allometric equations. GLASHEEN and McMAHON'S (1996a,b) model of the Basilisk technique is applied to estimate the impulses produced by the hind legs in interaction with the water during the slap and stroke phases of the running cycles of Archaeopteryx and of the lizard. Lift and drag forces generated by Archaeopteryx running at 2 m· S-l with its wings and tail in fixed gliding position are estimated using an approximate mass flux model (SUNADA & ELLINGTON 2000) for a range of downwash angles (£). Extra lift due to ground effect is included in the predictions. Results show that the legs of Archaeopteryx are longer than those of Basilisk lizards of the same body mass; the feet are of similar size. The stride frequencies achieved by Archaeopteryx are the crucial parameter determining the impulse needed. According to the model, a 0.25 kg Archaeopteryx using a stride frequency of 10Hz generates 29 % more impulse from slap plus stroke than needed while running over seawater. However, if it used a more realistic stride frequency of 5 Hz, a lift force of 0.9 N, generated by the wings and tail, would be required to make surface running possible. The lift and induced drag force estimates for the Berlin Archaeopteryx (wingspan 0.6 m) gradually increase with increasing downwash angles. The lift force of 0.9 N required at a stride frequency of 5 Hz was generated at £=26°. At that downwash angle Archaeopteryx would need an increase of the backward stroke angle of only 7.5° to produce the thrust impulse required to overcome the drag of 0.16 N mainly induced by the generated lift. Archaeopteryx might have used the running over water technique using wings and tail in gliding stance to generate lift to increase the foraging area and to have an extra possibility to escape from terrestrial predators.