The Complex Fungal Microboring Trace Saccomorpha stereodiktyon isp. nov. Reveals Growth Strategy of its Maker

A complex microboring trace of fungal affinity is described in shells as a new ichnotaxon Saccomorpha stereodiktyon isp. nov. and compared with the earlier established ichnotaxon Saccomorpha terminalis Radtke, 1991. The new trace is characterized by a three-dimensional network of tunnels composed of a bifurcate horizontal (parallel to substrate surface) network with an upright (perpendicular to surface) system of tunnels and by the formation of cylindrical to multilobate terminal sporangial swellings. The new trace shares with Saccomorpha terminalis Radtke, 1991 the terminal position of sporangial swellings but differs from this ichnotaxon by its complexity in spatial arrangement, segmented construction, and ramification of tunnels. The horizontal parts of the network in the new taxon adhere to the substrate surface and regularly produce thinner tunnels that explore the interior of the substrate, allowing the producer to participate in digestion of organic lamellae incorporated in the shell. Microborings similar to the new trace fossil have been observed in modern bivalve shells of the Atlantic Ocean, North Sea, Adriatic Sea and Red Sea at depths ranging from the intertidal down to 1,550 m. The fossil record of the trace reaches back to the Jurassic and the type material stems from a Lower Oligocene oyster shell. The study shows that complex microboring traces reflect both behaviour and developmental strategy of their makers.