Abstract
The helical, vertical trace fossil Gyrolithes lorcaensis is described from marine to marginal-marine deposits of the Lower Cretaceous of the Sarcheshmeh Formation near the city Kalat-e-Naderi in the Kopet-Dagh Basin. This is the first report of Gyrolithes from Iran and thus expands the geographic range of this trace fossil. G. lorcaensis is characterized by its unique morphometric parameters, a small burrow size and whorl radius, which distinguishes it as the smallest Gyrolithes ichnospecies. The most likely producers of the Iranian Gyrolithes were crustacean decapods, which is supported by the paleoenvironment (shallow and warm water ramp) in which it is found. The function of the spiral structure in the described Gyrolithes of the Sarcheshmeh Formation can be explained as a permanent dwelling structure with the main purpose of farming bacteria via increasing the surface area of the burrow margin that allows more microbial activities. The small burrow dimensions may reflect reduced burrower body size in response to the dysoxic to anoxic conditions suggested by sedimentological evidence.
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