A star-shaped trace fossil here assigned to Asteriacites quinquefolius (Quenstedt) was found in the Miocene Shirahama Formation, Wakayama Prefecture. This is the first report in Japan and stratigraphically youngest record for the ichnospecies. The fossil has five distinct arms and wide striations on both lateral sides of each arm, and the shape is bilaterally symmetrical. To clarify the fossil producing process, we conducted burial experiments of extant asteroids in aquaria and in situ for the first time. Asteroids buried themselves in the substratum using the tube-feet, and when asteroids were covered with thin sand, they escaped slantingly upward onto the sand tilting their body in a bilaterally symmetrical posture. As a result, the remaining trace was very similar to the fossil of A. quinquefolius from the Shirahama Formation. Accordingly, the results suggest the present fossil was formed by the behavior of escaping from thin sand cover by asteroids.
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Vol. 23 • No. 1