Producing Processes of the Trace Fossil Asteriacites lumbricalis Revisited: Different Ophiuroid Behaviors Produce Different Trace Forms

Abstract

Abstract. The star-shaped trace fossil Asteriacites lumbricalis is produced by ophiuroids and has two different forms. One form has four distinct arms with transverse fine and parallel striations and one indistinct arm without striations, and the producing process has been studied already. In this study, we clarified the producing process of another form with five distinct radiating arms and fine striations by aquarium observations of extant ophiuroids. The form was produced when ophiuroids were covered with a thin sand layer (11–39% of the arm length in thickness). After covered by thin sand, ophiuroids raised their five arms with one to five tips protruded vertically above the sand cover, raised and slanted the disc in the sand, and then obliquely emerged onto the sand cover. Finally, they returned to horizontal postures creeping on the sand. The trace left on the substratum was very similar in shape to the trace fossil from Hettangian (Lower Jurassic) in Blumenrod, Germany. The producing process of the repetitive multiple traces of A. lumbricalis was revealed by aquarium observations. When an ophiuroid was alternately covered with a thin sand layer and then with a thin abrasive layer as mud substratum, the ophiuroid left a vertically stacked series of traces which gradually shifted the horizontal position. This unique form of A. lumbricalis was produced by escaping of ophiuroids from successive deposition events.