Peter Pervesler

SURFACE LEBENSSPUREN PRODUCED BY AMPHIPODS AND ISOPODS (CRUSTACEANS) FROM THE ISONZO DELTA TIDAL FLAT, ITALY

Abstract Substrate properties, such as grain size, water content, shear strength, and content of organic mucus, influence the life activity of benthic organisms and their trace-fossil record. This study deals with actualistic experiments using small crustaceans (amphipods and isopods) moving mainly over plaster of Paris surfaces in various stages of hardening. Several consistencies, such as semi-fluid, very soft, soft, soft-stiff, stiff, and very stiff are distinguished. The morphology of surface lebensspuren shows a broad variety that directly depends on stiffness of the substrate and the capability of the organisms to cope with it. Semi-fluid substrates hinder the organisms in their motility— they move by plowing, whereby sediment flows back behind the animal and refills the furrow, leaving an indistinct line on the surface. With increasing stiffness, traces acquire additional morphological details such as levees and median furrows. On stiff and very stiff substrates, the crustaceans do not penetrate into the sediment, but move by jumping, with the consequence of producing jumping traces instead of furrows. Some lebensspuren obtained in the experiments, especially furrows, are similar to some trace fossils attributed to several non-arthropod animal groups such as annelids, bivalves, and gastropods. This study helps clarify the interpretation and taxonomy of trace fossils and in reconstruct of substrate properties during their formation.

Fossil evidence for chemoautotrophic bacterial symbiosis in the thyasirid bivalve thyasira michelottii from the middle Miocene (Badenium) of Austria

Many modern thyasirid bivalves are known to engage with chemoautotrophic bacteria in symbiosis, which can be inferred from the specialized feeding and digestive systems of these bivalves, as well as from stable carbon isotope ratios of their soft tissues. Fossil evidence for this life habit consists largely of facies criteria and bio‐geochemical markers. Further evidence for chemosymbiosis is found in the characteristic burrow‐systems of these bivalves, by which sulfide is obtained to sustain the symbiotic bacteria. These burrows were previously known only from observations made in aquaria. Such a burrow is described here for the first time in a fossil thyasirid bivalve. Thyasira michelottii (R. Hörnes, 1875) burrowed in sandy sediment in an anterior‐up position, approximately 5–10 cm below the surface, to which it was connected by an inhalant tube. It also produced a prominent posterio‐ventral tunnel, extending up to 300 mm into the sediment. Like modern thyasirids, this animal lacked an exhalant tube. By comparison with the ecology of modern thyasirid bivalves, we suppose that oxygen was obtained by active ventilation through the inhalant tube and that the prominent posterio‐ventral tunnel reflects search by the probing, vermiform foot for short‐lived pockets of sulfidic material in an otherwise low‐sulfide environment. The monospecific occurrence of Thyasira and the paleogeographic setting indicate dysaerobic, warm water conditions. A short distance to the coast and the occurrence of proximal tempestites suggest an inner shelf setting, prone to occasional disturbance by storms.

A New Y-Shaped Trace Fossil Attributed to Upogebiid Crustaceans from Early Pleistocene of Italy

Y-shaped trace fossil (U-shaped upper part with a basal shaft), Parmaichnus stironensis igen. nov. et isp. nov. penetrates from a discontinuity surface cut in Early Quaternary mudstones in the Stirone Valley, Northern Italy. It is attributed to upogebiid decapod crustaceans. Parmaichnus differs from Psilonichnus by the presence of turning chambers in the upper part of the burrow. The turning chambers are considered to be an important taxonomic feature of upogebiid burrows. P. stironensis occurs together with Thalassinoides cf. paradoxicus (produced probably by callianassid crustaceans) and wide U-shaped pyritised cylinders (supposedly produced by balanoglossid hemichordates).

Secondary hardground-communities in the Northern Gulf of Trieste, Adriatic Sea

Secondary hardground-communities in the Bay of Panzano were investigated with respect to their taxonomic composition, distribution and to the taphonomic processes, which may influence their preservation potential. Every hardground category (mainly serpulid aggregates, various mollusc shells andChaetopterus tubes) at the investigated soft-bottom is attractive for potential epigrowth.All epigrowth organisms are suspension feeders; serpulids are quantitatively by far the most important. “Constructors” (barnacles, bryozoans, oysters, serpulids) enlarge the hardground, have a considerable preservation potential and show limited taphonomic disintegration. In contrast, soft-bodied epigrowth (anemones, ascidians, sponges) and thin-walledChaetopterus tubes have a poor fossilisation potential. Vagile colonizers (ophiurids, echinoids, decapods, holothurians) show also strong taphonomic disintegration, but their species composition is partly reconstructable from fragments embedded in the sediment and provides indirect evidence for ascidians and sponges.The density of overgrown secondary hardgrounds decreases distinctly with decreasing distance from the river mouth due to changing types of available hardgrounds, a markedly change of sedimentary facies and bottom trawling.

Orientation of crustacean burrows in the Bay of Panzano (Gulf of Trieste, Northern Adriatic Sea)

Three burrow systems produced by Jaxea nocturna and one cluster of burrows produced by Upogebia pusilla where investigated in the Bay of Panzano, Northern Adriatic Sea, to determine preferred orientations. The distributions of dip directions differ between both producers. Steep shafts into the consolidated mud followed by large, shallowly inclined tunnels oriented in a manner similar to a spiral are characteristic for Jaxea nocturna burrows. In contrast, the Y-shaped burrows of Upogebia pusilla have entrance shafts that are less steep which are connected by a near-horizontal section, where a blind ending tunnel branches. Orientations of the dominating, shallowly inclined burrow parts are not randomly distributed in either Jaxea or in Upogebia burrows. Three preferred axial orientations with almost equal proportions in each direction are characteristic for the Jaxea burrow from the centre of the bay. This system transforms to a squared structure near the coast, where one direction parallels the shoreline and the other is oriented at right angles to the coast. The latter orientation demonstrates significant constancy in all investigated burrows. Upogebia burrow clusters coincide with the three preferred orientations of the Jaxea burrow from the bay centre, but the proportions of the directions are unequal. Burrow segments connecting the steep entrance shafts in Upogebia are oriented almost parallel to the shoreline, whereas at right angles to the coast the flat blind-ending tunnels incline towards the open sea. The study shows a strong coincidence between empirical and theoretical distributions of the dominating orientations in both species.

ICHNOLOGICAL RECORD OF ENVIRONMENTAL CHANGES IN EARLY QUATERNARY (GELASIAN–CALABRIAN) MARINE DEPOSITS OF THE STIRONE SECTION, NORTHERN ITALY

ABSTRACT Nineteen ichnotaxa occur in a 94-m-thick section of Gelasian–Calabrian siliciclastic deposits along the Stirone River. The most common trace fossils are grouped in 17 clusters based on a K-Means Cluster Analysis. This grouping is related to distal, archetypal and proximal Cruziana ichnofacies. Successions of ichnotaxa and clusters as represented in Detrended Correspondence Analyses determine environmental trends that helped establish a sequence stratigraphic scheme, which is not always obvious in the sedimentary record. The depositional sequences are separated by three sequence boundaries (A–C), which are well expressed in the ichnological record. Primary fabrics are less frequent in the archetypal and proximal Cruziana ichnofacies than in their idealized models. This is probably due to less intense storms in the small and protected paleo-Adriatic Sea and to intense bioturbation. Small-scale intervals with low ichnodiversity and ichnofabrics commonly dominated by one trace fossil are present. This suggests stressed conditions and opportunistic colonization related to small, but probably frequent, seafloor disturbances. The record of these disturbances— possibly caused by storm and bottom currents, deposition, or possibly erosion—has been obliterated by subsequent bioturbation. The disappearance of Schaubcylindrichnus, the smaller size of Scolicia and Ophiomorpha, and a slight decrease in trace fossil diversity in the Calabrian part of the section is interpreted to record climate cooling. This is also supported by the general decrease in ichnodiversity in Pleistocene versus Pliocene shallow-marine to slope siliciclastic facies.