Ulla Asgaard

Triassic freshwater ichnocoenoses from Carlsberg Fjord, East Greenland

This paper describes the occurrence of an unusually diverse and well preserved series of freshwater ichnocoenoses from redbeds of continental origin. The material includes seventeen ichnospecies. In addition, it allows the taxonomic revision of several controversial genera, particularly Cylindricum and Isopodichnus (placed in synonymy with Skolithos and Cruziana, respectively). Two new taxa are named: Fuersichnus communis and Steinichnus carlsbergi. The trace fossils were formed in two distinct continental environments: aquatic and terrestrial. Four ichnocoenoses are recognized: the Fuersichnus and Arenicolites assemblages, which are entirely aquatic, originating in lacustrine conditions; the Scoyenia assemblage, from extremely shallow lacustrine conditions, and the Rusophycus assemblage, representing a regularly desiccated fluviatile environment.

Endolithic community replacement on a Pliocene rocky coast

A limestone karst topography on the island of Rhodes, Greece, was flooded by a tectonically dominated transgression starting in late Pliocene time. Surfaces in contact with seawater were sculptured by communities of boring and rasping organisms. A raised cliff, studied in detail, comprises habitats such as a cliff‐foot platform, steep surfaces along the cliffline, a cave, and overhangs. Each habitat has a distinctive bioerosion sculpture. Four trace fossil suites were recognized, comprising six ichnocoenoses, the work of different endolithic paleocommunities in environments ranging from illuminated, shallow water cliff‐face to aphotic, tranquil inner reaches of the cave. As the transgression progressed, and surfaces were exposed to deepening water, new trace fossils were superimposed on old. Thirty‐one ichnospecies were identified, belonging to Caulostrepsis, Cen‐trichnus, Conchotrema, Entobia, Gastrochaenolites, Gnathichnus, Maeandropolydora, Oichnus, Phrixichnus (new), Radulichnus, Ren‐ichnus, Rogerella...

Two bioerosion ichnofacies produced by early and late burial associated with sea-level change

In bioerosion, as in trace fossils as a whole, deeply emplaced structures have greater survival value than shallow structures. That is to say, tiering (the relative depth to which rasping, etching and boring organisms penetrate their substrate) is of paramount importance for the preservation potential of individual trace fossils. An Entobia ichnofacies is established for trace fossil assemblages dominated by deep tier borings and arising from long-term bioerosion, such as occurs on sediment-free submarine cliffs or hardgrounds. A Gnathichnus ichnofacies comprises assemblages containing all tiers, including superficial sculptures produced by radulation that have very little preservation potential. Such assemblages occur in short-term bioerosion situations as on shell surfaces and hardgrounds buried early by sedimentation.

Experimental study of sediment structures created by a spatangoid echinoid, Echinocardium mediterraneum

Five narrow aquaria were filled with artificially laminated sediment and buried in the shallow sea floor at Kephallinia, Greece. A single individual of Echinocardium mediterraneum was placed in each aquarium and allowed 5 days to bioturbate the sediment. After recovery and drying, the aquaria were X-radiographed, revealing details of the echinoid burrow, including the drain and shaft. Impregnation of the contents of the aquaria using epoxy resin allowed the internal structures of the incipient trace fossils to be studied.

Echinometrid sea urchins, their trophic styles and corresponding bioerosion

The Echinometridae is a diverse, largely tropical family of echinoids. Several species are active borers in shallow water, at and below the low-tide line, especially in coral reef and beachrock substrates, commonly in high-energy situations. In the Caribbean and Atlantic Ocean, Echinometra lucunter erodes two types of boring. Most commonly, the widely reported elongate grooves are produced. In these traps, drifting fragments of algae are caught on the incoming tide. Drifting algae are also caught actively by ‘chopsticks-like’ manoeuvres of the spines. Some simple gardening is undertaken on the boring walls and floor, where short algal turf and endolithic algae are harvested. Juveniles start by making simple cup-shaped borings. In some cases, however, E. lucunter retains this form of bioerosion to adulthood. Cup-shaped borings must indicate emphasis on alga-catching in highenergy environments and less on the gardening, grazing trophic style.