Paleodictyon nodosum: A living fossil on the deep-sea floor

We report new in situ observations and laboratory studies of specimens of a small (diameter 2.4–7.5 cm) strikingly hexagonal form originally described from sedimented steps in a wall of the axial valley of the Mid-Atlantic Ridge (water depth 3430–3575 m) near 26°N, 45°W that appears to be identical to the iconic form Paleodictyon nodosum described as a trace fossil from Eocene flysch deposits at sites in Europe and Wales. Our findings follow: • The form is apparently agglutinated in sea floor sediment (a veneer of calcareous lutite over red metalliferous sediment) and consists of three equidistant rows of tiny holes (diameter ∼1 mm) that intersect at an angle of 120° and continuously connect through vertical shafts (length 2–3 mm) with an underlying horizontal network of tubes or tunnels identical with the fossil form. • The number of rows of holes and spacing of rows increase with overall diameter of the form indicative of organic growth. • The form is shaped like a shield surrounded by a lip and moat with surface relief (∼0.5 cm) that is absent in the fossil form. The surface relief exposes the underlying red sediment and may have been produced either by excavation (constructional origin) or by infaunal growth (body form). • Protoplasm is absent in recovered specimens, as indicated by negative results of staining techniques, explained by either initial absence or loss. • Genetic sequencing of material from the form identified different foraminifera that had settled on the pattern of holes which acts as a baffle to trap organic matter. • Models in flume tanks show that the shield-like form deflects flow of ocean currents into a self-ventilating structure capable of aerating and of circulating organic particles through the tubes or tunnels. • Microbial counting techniques indicated background abundances within and outside the form. We come to two alternative interpretations of the findings resolvable with further studies: • The modern P. nodosum is a burrow consistent with interpretation of the ancient form as a trace fossil. • The modern P. nodosum is a compressed form of a hexactinellid sponge adapted to a sedimentary substrate, which means that the ancient form is a body fossil with possible affinity to the Ediacara fauna.