Intriguing fossils encrusting oncoids and exhumed carbonate concretions are described from the Middle Jurassic (Bathonian) of Poland. The fossils, up to a few millimeters in diameter and preserved in a form of calcium carbonate, are characterized by a tubular, elongate shape with the external surface covered with distinctive semicircular to oval concavities. These tubes do not match any known fossils and do not represent trace fossils sensu stricto. They are similar to serpulid and/or sabellid polychaetes with which they are associated. The latter, however, have calcitic tubes and none are known to possess the concave structures that characterize the tubular fossils. The shape of the concavities and the presence of a carbonate lining within them suggest that the tubular fossils were originally covered with ooids, a few of which are still preserved in the concavities. These Middle Jurassic fossils may represent the fossilized remnants of agglutinated tubes formed by polychaete worms. Their greatest abundances are noted on media bearing thick microbial crusts (oncoidal in nature) on which they often form dense aggregations. This may indicate that the tube-building worms selected such media not only because they provided a suitable hard surface for settlement, but also because they were a rich source of food in the form of both microbes (Cyanophyta) and other organic matter present in the microbial mats. The selectivity of only ooids for tube building makes them dissimilar to any known recent tube-building polychaetes and, thus, the Middle Jurassic representatives are nonactualistic with respect to particle selection for the tube formation.

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