Richard G. Bromley

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.

Trace Fossils at Omission Surfaces

Trace fossils at discontinuity surfaces may be classified as three distinct ’ assemblages, according to their time relationship with the depositional hiatus. These assemblages may be called the “preomission,” “omission “and “postomission” suites. Discrimination of these suites is essential for the stratinomic interpretation of the discontinuity surface. The three suites may represent the same ichnocoenose in different modes of preservation, or the appearance of a new ichnocoenose may suggest the environmental conditions responsible for the omission of sediments and the resumption of deposition.

Composite ichnofabrics and tiering of burrows

Infaunal communities in marine environments typically are tiered; that is, different taxa live at different depths within the sediment. Tiered suites of biogenic structures yield complex biogenic sedimentary fabrics (ichnofabrics), with the traces of deep-burrowing organisms overprinted on those of shallow-burrowing organisms. Careful analysis of crosscutting relationships of burrows in such composite ichnofabrics allows reconstruction of the tiered nature of fossil endobenthic communities. It is important to recognize that the best preserved and most prominently displayed trace fossils in most assemblages usually represent the deepest tier. Thus, they were farther removed from the sea floor and therefore less indicative of actual sea floor conditions than the more poorly preserved traces of the shallower tiers, on which the deeper traces are juxtaposed.

Bioerosion of Bermuda reefs

Abstract In order to test the usefulness of borings as indicators of palaeoenvironment a number of transects were sampled over a variety of reefs in Bermuda and an analysis was made of the borings and their inhabitants. The borings are described of 25 endolithic species, including sponges, a sea anemone, sipunculids, annelids, bivalves, a gastropod, a barnacle and an echinoid. Several embedding species are also described. The distribution data indicate that bathymetric control is largely overprinted by that of hydraulic energy. Other controls are discussed such as living versus dead coral as substrate, morphology and structure of substrate, and location within special microenvironment. These relationships within the complex reef milieu would be difficult to disentangle in the fossil record.

Tracks and trackways of Myotragus balearicus Bate (Artiodactyla, Caprinae) in Pleistocene aeolianites from Mallorca (Balearic Islands, Western Mediterranean)

Tracks and trackways of the ruminant goat, Myotragus balearicus Bate, 1909 are abundant in the Late Pleistocene aeolianites of the south coast of Mallorca, Spain. The carbonate aeolianites form impressive dunes and related sand ramps that were initiated as echo dunes in front of a 20–40 m high paleocliff of Miocene reefal limestone. Where the dunes built up to the cliff top, they covered it as an aeolian ramp. The dunes and sand ramps were active during the dry summer months but were passive during the wet winter period. The small, goat-like animals were abundant on the island, and for unknown reasons crossed the aeolian accumulations in large numbers. This study deals with the tracks formed in the dune deposits. All preserved trackways indicate impression into moist sand. Special features of the tracks include the structure produced by the withdrawal of the foot, here called the ‘axis’, and a disturbance zone of plastic deformation around this. On dune crests, the disturbance zone surrounds the axis more or less symmetrically. However, in addition, a ‘pressure pad’ of dislocated, slightly rotated sediment bound by curved microfaults is commonly produced posterior to the axis by propulsive pressure of the foot. On steep stoss and lee slopes, the pressure pad becomes oriented in a down-slope position as a result of gravitational slip of the walking animal. Combination of disturbance of the sediment in this way by manus followed by overprinting of similar disturbance by pes produces highly complicated track structure. This structure may be characteristic of artiodactylous mammals in soft sand, particularly aeolian deposits. The structure is designated as Bifidipes aeolis isp. nov.

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...

Structure and function of large, lobed Zoophycos, Pliocene of Rhodes, Greece

Abstract In the Kolymbia limestone facies (upper Pliocene) of the Rhodes Formation of Rhodes, Greece, a study was made of a large, spiral form of Zoophycos, named herein Zoophycos rhodensis. In the small bay south of Cape Vagia, two individuals were excavated and dissected while several more were serially sectioned in a vertical plane. The upper parts of the Zoophycos were missing through modern beach erosion, but in one individual the two lowest whorls were almost completely exposed. These comprised a skirt-like zone of spreite surrounded by a zone of 63 marginal lobes. The diameter of the trace fossils was about 1 m. Vertical sections demonstrated that the spreiten were composed of material advected downward from a higher horizon, indicating that the spreite was produced nearly 1 m below the sea floor. The skirt-like zone of the spreite was constructed of sigmoidal minor lamellae in bundles separated by major lamellae. The radiating major lamellae are not replaced by ridges such as is the case in many other lobed, spiral Zoophycos. The marginal tube and, to a lesser extent, the spreite are mineralised with an oxidised iron mineral, probably once pyrite. The marginal tube is completely filled with sediment and shows no signs of collapse or compaction. This is taken to indicate active backfilling, and that the operative burrow was thus J-shaped and not U-shaped The two zones of the spreite are considered to represent two different modes of behaviour. The skirt-like zone probably represents deposit feeding. The mineralised marginal tube and lobes possibly represent sulphide wells connected with chemosymbiosis between the trace-making animal and sulphide-oxidising bacteria.

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.

PHYCOSIPHON INCERTUM REVISITED: ANCONICHNUS HORIZONTALIS IS ITS JUNIOR SUBJECTIVE SYNONYM

Comparisons between Phycosiphon incertum Fischer-Ooster (1858) and Anconichnus horizontalis Kern (1978) show that these two monotypic ichnogenera do not differ significantly in geometry, size, sediment fill, and environmental occurrence. Therefore, taxonomic priority has to be given to Phycosiphon and Anconichnus becomes a subjective junior synonym. The trace fossil is a small, highly lobed spreite structure produced by a zoned backfill consisting of a pale mantle and dark core. In a few cases, a meniscate structure is visible in the core. The spreite sediment is an extension of the mantle material; it commonly is poorly visible. The spreite-producing behavior may be locally interrupted, leaving a single tube of backfill. The spatial arrangement of the trace fossils seems to depend on the host sediment; in muddy and homogeneous material the lobes tend to be oriented randomly, including vertically, whereas in laminated sand and silty sediments they tend to lie parallel to bedding. This latter orientation is exaggerated by compaction. Textural differences between host sediment and spreite or mantle may be subtle and not always evident. Observations in modern sediments are in agreement with the type material of both ichnogenera.

Bioerosional innovation for living in carbonate hardgrounds in the Early Ordovician of Sweden

Some of the worlds oldest macroborings occur in hardgrounds in lower Ordovician (Arenig) limestones exposed on the island of Oland, southern Sweden. The trace fossils, which are described here as Gastrochaenolites oelandicus isp. nov., appear to be dwelling structures excavated in the indurated substrate by invertebrates of unknown taxonomic affinity. They are the oldest examples of this ichnogenus. The appearance of a macroboring life habit at this early time represents a revolutionary new adaptive strategy for inhabiting carbonate hardgrounds. However, this innovative strategy apparently was not successful for the long term, because this particular macroboring taxon seems to have disappeared shortly after its early Ordovician appearance.