Variations in population structure of Diplocraterion parallelum: Hydrodynamic influence, food availability, or nursery settlement?

Abstract

In the Triassic of the Betic Cordillera, the U-shaped trace fossil Diplocraterion parallelum exhibits pronounced spatial variations in its abundance, size, and orientation. Higher density populations are found in small patches on bedding planes that were topographic lows on the palaeo-seafloor. These patches consist mainly of small specimens, separated into two size-groups: group A with larger specimens, and a similar preferred orientation, and group B of smaller individuals, densely distributed, and with a more variable orientation.Between the patches, isolated large individuals occur sparsely. Comparison with populations of crustaceans (which are among the potential trace makers of Diplocraterion parallelum) allows possible population dynamics of D. parallelum trace makers in this setting to be considered. Important factors determining changes in the abundance and distribution of crustacean populations are protection of small individuals against predators and/or comparatively high-energy conditions, nursery settlement, and/or temporal, typically seasonal, variations in environmental parameters. Depressions of the seafloor appear to have represented hospitable habitats in which juveniles of the producer of D. parallelum developed, reflecting either a deliberate reproductive strategy or the consequence of accumulation of eggs deposited elsewhere. In these depressions energy conditions may have been lower and food availability higher. During early ontogeny, defenseless young individuals used these areas as an ecological refuge against predators and high energy conditions. When the juveniles of the D. parallelum trace makers grew, they presumably spread from the nursery to settle in the surrounding area.