Preservation Of Trace Fossils Research Articles

Alluvial ichnofacies from Upper Triassic red beds in India: Implications for palaeoenvironment and palaeoclimate

Trace fossils are highly sensitive to the hydrodynamic conditions of depositional systems, substrate, and climate and hence yield information about spatio-temporal changes in palaeoenvironment and palaeoclimate. In this paper, we report alluvial trace fossil assemblages from the Upper Triassic Tiki Formation of the Rewa Basin, central India. Based on detailed facies analysis and architecture, two depositional environments are identified in this formation: (1) perennial braided channel belts and (2) extensive and complex floodplains containing ephemeral small fluvial channels, lakes, and crevasse splay deposits. Upper flow regime primary structures in laterally extensive sandstones encased in the floodplain deposits indicate short-lived high magnitude flood discharge, suggestive of seasonal climates. Trace fossils are sporadically distributed through these facies, generally in suites of low abundance, although in places the bioturbation index is higher. Three major ichnofacies, namely Skolithos, Scoyenia and Mermia are recognized, based on the association of different trace fossils preserved in the channel, crevasse splay of proximal floodplain and pond/lake of distal floodplain deposits, respectively. Vertical Beaconites in palaeosols and an abrupt change from Skolithos to Scoyenia ichnofacies indicate a highly variable water table, controlled by seasonal precipitation. The transitions between the Skolithos, Scoyenia and Mermia ichnofacies reveal that trace fossil preservation was variously controlled by channel abandonment, floodplain dewatering, substrate desiccation or periodic waterlogging of overbank areas. Specifically, increased water discharge over the floodplain during high precipitation, followed by post-flood recovery of hot, dry, low-water table conditions may have been a key controlling factor in shaping the ichnological characteristics of these Upper Triassic red beds.

TRACE FOSSILS OF PROBLEMATIC ORIGIN: ASSESSING SILICIFIED TRACE FOSSILS FROM THE PERMIAN OF UTAH, U.S.A.

ABSTRACTSilicification is a diagenetic process that can affect the fidelity of trace fossil preservation. The combination of compaction and oversilicification associated with chert precipitation can alter the original fabric created by the trace producers. Sedimentary structures and trace fossils in mixed chert-carbonate systems are especially prone to these processes, leading to the preservation of a limited or biased ichno-assemblage and the omission of important paleoecologic detail. We summarize useful criteria for identifying chert-associated trace fossils and present a formal naming scheme. Using this nomenclature, we identified silicified trace fossils in the chert-rich Permian strata from northwestern Utah. An assemblage of burrows is present in outcrops of the Trapper Creek Formation, consisting of nodule-like growths of microcrystalline quartz in close association with bioturbated horizons. Thin section micro-textures of burrow fill lend additional support to the bioturbated origin of chert nodules. Silicified Thalassinoides fossils are present, as are chert nodules with a Rhizocorallium-like morphology.

Firm evidence for a post-extinction ichnofauna: earliest CarboniferousCruziana reticulataassemblage from the Anti-Atlas of Morocco

Cruziana reticulata is an arthropod-related ichnospecies that is characterized by a conspicuous net-like scratch pattern whose initial formation and later preservation require the presence of consolidated substrates in shallow marine fine-grained bottoms. There are two scenarios in which epifaunal to shallow infaunal benthic organisms may access firm siliciclastic substrates: first, by exposure of the compacted bottom after erosion of the upper, water-saturated and usually soft portion of sediment column; and, second, by primary fast substrate stabilization in the absence of biogenic sediment mixing. Whereas the first mode occurs throughout the Phanerozoic, the latter is only prevalent in marine bottoms during the Early Palaeozoic mainly as a consequence of poorly developed infaunal bioturbation. However, by eradicating burrowing organisms, mass extinctions are known (i.e. the end-Permian extinction) to ‘reset’ intensity of ecospace utilization, which entails the return to this anachronistic style of trace fossil preservation in younger times. An earliest Carboniferous ichno-assemblage dominated by Cruziana reticulata from Morocco attests the spread of firm substrates in the aftermath of the Hangenberg Event – a major extinction at the end of the Devonian period. Since evidence for erosion is lacking in accompanying sedimentary rocks and the ichno-assemblage shows characteristics of opportunistic exploitation of the nutritious muddy seabed, we favour the interpretation of this assemblage to represent a post-extinction ichnofauna. It shows that other taxonomically less severe mass extinctions may also exhibit transient but severe ecological effects in open marine ecosystems such as the collapse of vital sediment mixing. Such post-extinction effects may distort the perception of the stratigraphical record as a firmground ichno-assemblage often taken as evidence for submarine erosion and are crucial in highlighting sequence boundaries. We, thus, encourage appreciating macroevolutionary framework of respective ichnofaunas.

Trace fossils associated with Burgess Shale non-biomineralized carapaces: bringing taphonomic and ecological controls into focus.

The association of trace fossils and non-biomineralized carapaces has been reported from Cambrian Lagerstätten worldwide, but the abundance, ichnodiversity, taphonomy and ecological significance of such associations have yet to be fully investigated. Two main end-member hypotheses are explored based on the study of a relatively wide variety of trace fossils preserved associated to Tuzoia carapaces from the middle Cambrian Burgess Shale in British Columbia. In the ecological Tuzoia garden hypothesis, the bacterially enriched surface of carapaces provides opportunities for intricate ecologic interactions among trophic levels. In the taphonomic shielding hypothesis, the trace fossil–carapace association results from preferential preservation of traces as controlled by compaction independent of any association in life. In an attempt to better understand the role of the carapace as a medium for preservation of trace fossils and to evaluate the effects of mechanical stress related to burial, a numerical model was developed. Results indicate that the carapace can shield underlying sediment from mechanical stress for a finite time, differentially protecting trace fossils during the initial phase of burial and compaction. However, this taphonomic model alone fails to fully explain relatively high-density assemblages displaying a diversity of structures spatially confined within the perimeter of carapaces or branching patterns recording re-visitation.

Microbial mediation in invertebrate trace fossil preservation in Sousa Basin (Early Cretaceous), Brazil

Well-preserved invertebrate and vertebrate trace fossils commonly occur in reddish, fine to very fine-grained sandstone and mudstone bedsets of the Antenor Navarro and Sousa formations in the Sousa Basin, referring to an Early Cretaceous pull-apart basin in northeastern Brazil, related to the South Atlantic opening. Palaeoenvironments are understood as oxidizing terrestrial (alluvial fans and meandering rivers with extensive floodplains or perennial flooded areas), under hot and humid palaeoclimates punctuated by drought episodes. The ichnodiversity is low but trace fossils are abundant in the invertebrate ichnoassemblages analyzed in this paper. The ichnoassemblages are composed mainly of shallow endostratal traces. Repichnia and Fodinichnia behaviours are represented by Aulichnites isp., Palaeophycus isp., ?Arenicolites isp., Planolites isp., Taenidium isp. and Phycodes isp. and are a taphonomic challenge in terms of their production and preservation, particularly considering its palaeoenvironment. Field observations and collected samples reveal common presence of microbially induced sedimentary structures associated to these traces. Biofilms and thin microbial mats seem to produce the adequate conditions of nutrient supply, moisture and substrate biostabilization to allow trace makers' survival and taphonomically enhance the preservation of the invertebrate trace fossils in an unstable and even sometimes inhospitable environment.

Bioirrigation in Alph n. igen., arthropod cubichnia from the Mississippian Hartselle Sandstone of Alabama (USA)

Cubichnia from the Hartselle Sandstone, described as Rusophycus hartselleanus and herein renamed Alph hartselleanus n. igen., includes specimens with well-defined, terminal cylindrical structures. We interpret these oblique shaft-like structures as having a respiratory function based on: (1) exact placement of cylindrical structures on the cubichnial axis; (2) unique sculpture with both transverse and longitudinal striation, absent elsewhere in the assemblage despite excellent preservation of trace fossils; (3) associated features in the trace such as bubbly structure that also could be caused by bioirrigation. Alph hartselleanus is much larger than known Carboniferous trilobites. While similar to Rusophycus, the Hartselle cubichnia has a complex morphology suggesting a crustacean tracemaker. The trace is quadrilobate, with distinct anterior and posterior regions. The posterior region has 5 or 6 pairs of coarse, obliquely transverse striae corresponding to digging appendages, crosscut by fine, obliquely longitudinal striae that may record bioirrigation. The anterior region has a bubbly structure suggestive of thixotropic movement of sand, again, possible evidence for bioirrigation. The axial shafts do not match known trilobite appendages, but could have been made by crustaceans. Were the cubichnia merely resting traces, or did they have additional functions such as nesting or protection during moulting? In a broader context, the physical evidence for bioirrigation represents a nearly untapped opportunity for understanding the Cambrian Substrate Revolution as well as Phanerozoic ecosystems. Where preserved, the apertures of burrows should be investigated for evidence of bioirrigation.

Xiphosurid trackways in a Lower Cretaceous tidal flat in Patagonia: Palaeoecological implications and the involvement of microbial mats in trace-fossil preservation

Lower Cretaceous trackways assignable to xiphosurids were recently found in tidally influenced marginal-marine deposits of the Agrio Formation (Patagonia, Argentina). The aim of this paper is to describe these trace fossils in detail, discuss their conditions of preservation, make palaeoecological and taphonomic inferences from them, and analyse the importance of their palaeogeographic location. These trace fossils are assigned to Kouphichnium, and five track morphotypes are established. Microbially induced sedimentary structures (MISS) are associated with the trackways. Scanning electron microscope (SEM) studies document the involvement of microbial mats in the preservation of the trackways by the presence of cyanobacteria-like filament sheaths. The microbial mats enabled preservation of the tracks by binding and biostabilisation of the sediment surface. The mostly likely producers of the trackways are from the subfamily Limulinae. The studied surface could represent a high-tide mating ground associated with a very shallow water deposit on a warm Cretaceous tidal flat. These records are the first convincingly documented trackways produced by xiphosurids in the Early Cretaceous worldwide and the second fossil record of this group from the Cretaceous in the Southern Hemisphere.

ICHNOLOGICAL ANALYSIS OF LATERAL ENVIRONMENTAL HETEROGENEITY WITHIN THE BONARELLI LEVEL (UPPERMOST CENOMANIAN) IN THE CLASSICAL LOCALITIES NEAR GUBBIO, CENTRAL APENNINES, ITALY

Dark, pelagic sediments of the uppermost Cenomanian Bonarelli Level (OAE2 event) interval, in two classical sections Contessa and Bottaccione in the Central Apennines, contain unbioturbated and bioturbated beds suggesting fluctuations in pore water oxygenation from anoxic to oxic or dysoxic conditions. The oxic and dysoxic improvement events prior to, during, and after the event are marked by biogenic structures showing an increase in the diversity of the trace fossils (Chondrites, Planolites, Thalassinoides, Trichichnus, and Zoophycos) from none to five ichnotaxa in individual beds. The number of anoxic events differs in the Contessa and Bottaccione sections, even though they are only 2.5 km apart. Comparison with sections from the Carpathians (Poland) and the Betic Cordillera (Spain), reveals that minor anoxic events below and above the Bonarelli Level are absent in the studied Apennine sections. Moreover, the diversity and density of trace fossils in the Apennine sections are lower than those from other studied sections in the Tethys, most likely indicating a comparatively lower availability of food in the Gubbio area as a result of its paleogeographic location. The preservation of trace fossils, controlled by the consistency of sediments and diagenetic processes, can mask diversity and density in some beds.

The evolutionary significance of a Lower Cambrian trace-fossil assemblage from the Meguma terrane, Nova Scotia

We report a diverse assemblage of trace fossils from the Lower(?) Cambrian High Head member of the Church Point Formation near Yarmouth, Nova Scotia. Based on the presence of sharp-based beds, load casts, and flute casts, along with other sedimentological criteria, the strata are interpreted to have a turbidite origin. Other characteristics, including wrinkle structures and high fidelity of trace-fossil preservation, suggest that the inter-episode biotope was influenced by the presence of biomats. The trace-fossil assemblage includes (in approximate order of abundance) Planolites , Helminthopsis , Oldhamia , Chondrites , Gordia , Cladichnus , Psammichnites gigas , Treptichnus , Phycodes , Lorenzinia , Palaeophycus , and Teichichnus . Trace fossils are increasingly abundant upwards in the section, but they are ultimately mitigated by an increase in (inferred) sedimentation rates. The trace-fossil assemblage, which bears many similarities to other Lower Cambrian locales (e.g., Puncoviscana Formation of northwest Argentina, Chapel Island Formation of southeastern Newfoundland), and the distribution of ichnofossils strongly support an earliest Cambrian age for the studied strata. More importantly, evolutionary trends can be interpreted from the trace-fossil assemblage, namely (1) an increase in size and diversity of animals in deep water; (2) an expansion of motile strategies used to coexist with biomats; and, (3) increasingly complex mining strategies below the sediment-biomat interface.

Trace fossils of a cyclic chalk–marl succession; the upper Maastrichtian Rørdal Member, Denmark

Trace fossils from an upper Maastrichtian cyclic chalk–marl succession, the Rørdal Member, exposed in the Rørdal quarry, Denmark, are analysed in order to test whether the changes in substrate lithology exerted any control over the ichnodiversity, tiering complexity, and density of the infauna. The cyclicity is interpreted as caused by orbital changes within the Milankovitch frequency band. The carbonate content varies between 71 and 82 weight% in the marl and 82–92 weight% in the chalk beds. The material is based on 19 samples collected from six chalk and marl beds. The investigated bedding-normal sample surfaces vary in area between 29 and 155cm2. Eight ichnogenera and two undetermined ichnogenera are recognised. The member is characterised by three ichnofabrics (A, B and C). The ichnofabric analysis is based on texture and internal structure of the sediments resulting from bioturbation. Ichnofabric A is found only in chalk samples and shows a poor preservation of trace fossils, whereas ichnofabric C is found in a few chalk and all marl samples and comprises a very dense, diverse and well preserved ichnofauna representing a high tiering complexity. Ichnofabric B represents an intermediate situation between ichnofabrics A and C and occurs in chalk samples immediately adjacent to marl beds. The observed changes in ichnofabrics between chalk and marl are related to the amount of clay in the samples and the differences in the occurrence of trace fossils are interpreted as due to differences in the visibility of traces between chalk and marl and not due to differences in ecological stress upon the endobenthic community of the two lithologies. The study thus provides an excellent example of how the effect of taphonomic factors may give a misleading and biased impression of apparent differences in the endobenthic community between chalk and marl.

Trace fossil preservation and the early evolution of animals

The trace fossil record is an important element in discussions of the timing of appearance of bilaterian animals. A conservative approach does not extend this record beyond about 560–555 Ma. Crucial to the utility of trace fossils in detecting early benthic activity is the preservational potential of traces made close to the sediment–water interface. Our studies on the earliest Cambrian sediments suggest that shallow tiers were preserved to a greater extent than typical for most of the Phanerozoic. This can be attributed both directly and indirectly to the low levels of sediment mixing. The low levels of sediment mixing meant that thin event beds were preserved. The shallow depth of sediment mixing also meant that muddy sediments were firm close to the sediment–water interface, increasing the likelihood of recording shallow tier trace fossils in muddy sediments. Preservation of surficial trace fossils in this type of setting remains problematic but the above factors suggest that also these can be expected to have left a reasonable record. Overall, the trace fossil record can be expected to provide a sound record of the onset of bilaterian benthic activity. The lack of convincing trace fossils significantly before the Cambrian supports models of late appearance of macroscopic benthic bilaterians.

The Ichnogenus Gastrochaenolites and Its Tracemakers from Firmgrounds of the Bohemian Cretaceous Basin (Czech Republic)

Gastrochaenolites ornatus Kelly and Bromley 1984 occurs on the sub-Cretaceous unconformity surface and intra-Cretaceous omission surfaces in the area of Brandys nad Labem (central Bohemia). A variety of substrates (Proterozoic shales, Ordovician claystones and siltstones, and poorly lithified Cenomanian sandstones) were affected by the same biogenic activity. Considering the mode of construction and preservation of trace fossils, all can be considered as firmgrounds. Some specimens contain tracemakers preserved in situ, represented by thick-ribbed molluscs (cf. Myopholas sp.).

Bioturbation in an Active Deep-Sea Area: Implications for Models of Trace Fossil Tiering

The Nova Scotian continental rise below 4750 m water depth is subject to intermittently strong currents (>30 cm/s) a few times a year in deep-sea storms. These erode and rapidly deposit sediment on the bed, with deep erosion (>5 cm) occurring in an annual to decadal time scale. X-radiographs of subsamples from box cores taken from this area exhibit a repetitive organization of primary and biogenic structures. Four tiers are recognized: 1, 2, and 3a and b. Tiers 1 and 2 are produced by ichnoguilds dominated by Mycellia B and Protopalaeodictyon, and resemble the archetypical Nereites ichnofacies. Tier 3 is dominated by guilds comprising members of the Scolicia group (especially Bichordites) with an overprint of Planolites and Chondrites. However, Tier 3 exhibits a vertical gradient of potential trace fossil preservation from 3a to 3b. At depth (3b), the tier becomes thoroughly bioturbated and finally forms the historical of other authors. At the base of this layer some split box cores revealed Thalassinoides 20-25 cm down. However, no vertical shafts were seen linking these occurrences to the modern sea floor, and they are thought to be part of a fossil ichnocoenosis belonging to an early post-glacial colonization of the sea bed. The extreme short-term rates of erosion and deposition in this area, the latter being up to 2.7 cm per year, result in a very low probability of preservation of Tiers 1 and 2. The material providing the long-term accumulation rate of 5 cm/ky is dominated by Tier 3. Thus only one tier would be preserved in lithified deposits. The record of intense biological and sedimentary activity seen in x-radiographs of Tiers 1 and 2 is not preserved. The accumulating sediment is ichnologically biased. This slowly accumulating sediment column is stratigraphically incomplete at the 1-year level required to preserve a record of benthic storm events, but is probably complete at the 100-year scale.

Deep marine trace fossil assemblages from the Lower Carboniferous of Menorca, Balearic Islands, western Mediterranean

Sediments of Lower Carboniferous age in eastern Menorca, Balearic Islands contain a diverse and exceptionally well preserved ichnofauna, including Neonereites biserialis, Nereites isp., Arthrophycus isp., Dictyodora liebeana, two ichnospecies of Chondrites, several ichnospecies of Lophoctenium, two ichnospecies of Phycosiphon, Syncoprulus pharmaceus, annulated burrows and a vertical burrow. The host lithologies are conglomerates, sandstones, siltstones and mudstones; most are the product of debris flows, and of high concentration [Ta(bc) intervals] to relatively dilute (Tcd/Tde intervals) turbidity currents. The rocks are interpreted as deposits of an inner- to mid-fan palaeoenvironment. Channelized deposits, sequences of overbank deposition and interchannel deposits interspersed with the deposits of unconfined debris flows and high concentration turbidity currents are present. The ichnofauna is most frequently, and best, preserved within the Td interval of turbidities, which are interpreted as interchannel deposits, produced by low concentration turbidity flows. The distribution of the ichnofauna is partly controlled by the lithologies in which they are preserved. The rarity and poor preservation of trace fossils in the coarser grained facies contrasts with the detailed preservation of very delicate traces in the finer grained lithologies. However, the ichnofauna is also partitioned between different subenvironments of the mid-fan to produce a series of palaeoichnocoenoses. Thin intervals of interchannel deposits, separated by deposits of high concentration turbidity currents, repeatedly contain only Phycosiphon incertum and small (?juvenile)Dictyodora liebeana. These traces are interpreted as the products of opportunistic colonization of near-channel environments during episodes of quiescent deposition. Thicker intervals of interchannel deposits contain diverse assemblages of trace fossils characteristic of more stable environments, in which widespread colonization occurred. Overbank deposits at Cabo de Favaritz are medium- to thick-bedded, fine-grained beds. In these, the ichnofauna occurs in a simple, two-tier profile. The upper tier is dominated by Nereites isp.; this is underlain by a partially bioturbated layer characterized by large Dictyodora liebeana and Arthrophycus isp.

Dinosaur tracks in intrusive igneous rock

Well preserved brontosaur footprints are replicated as impressions at the top of a dacitic igneous sill that intruded the track‐rich Cretaceous Jindong Formation, Kyeongsang basin South Korea. Although an unusual occurrence, the example shows the potential of such intrusions for enhancing preservation of trace fossils in some cases

A diverse ichnofauna from silurian flysch of the aberystwyth grits formation, Wales

AbstractThe most extensive ichnofauna yet recorded from a deep water Lower Palaeozoic sequence occurs within the distal turbidites of the Lower Silurian Aberystwyth Grits Formation of Central Wales.The strata contain an abundant assemblage comprising 25 ichnogenera: Asteriacites, Bergaueria, Chondrites, Cochlichnus, Cosmorhaphe, Glockerichnus, Gordia, Helicolithus, Helminthopsis, Helminthoida, Hormosiroidea, Lorenzinia, Megagrapton, Monomorphichnus, Neonereites, Nereites, Palaeophycus, Paleodictyon, Planolites, Protopaleodictyon, Spirorhaphe, Spirophycus, Squamodictyon, Subphyllochorda, Taphrhelminthopsis; 36 ichnospecies are described, three of which (Asteriacites aberensis, Helminthopsis regularis, Cosmorhaphe elongata) are new.The inorganic sedimentary structures and trace fossils of some 418 sandstone beds were examined in detail; 16 per cent of the beds commence with Divisions A or B and 84 per cent with Division C of the turbidite sequence. This indicates a relatively distal environment, mainly receiving low velocity turbidity currents, and favouring trace fossil preservation. The most common traces were Helminthopsis, Paleodictyon, and Squamodictyon which were found on 46 per cent, 34 per cent, and 19 per cent of the beds examined.Data from this, and other recently described sequences, confirms that there was a gradual increase in trace fossil diversity in the deep oceans throughout the Lower Palaeozoic, in contrast to the situation in shallow water shelf seas where a peak was reached as early as the Lower Cambrian.

Burial Preservation of Trace Fossils as Indicator of Storm Deposition: ABSTRACT

Burial Preservation of Trace Fossils as Indicator of Storm Deposition: ABSTRACT

Trace fossils in the Mesozoic sediments of Kachchh Western India

The Mesozoic succession of Kachchh, western India, is a classic example of the value of trace fossils in the interpretaion of depositional sequences. Because this sequence has traditionally been referred to as Gondwana and because the previous emphasis has been on body fossils, a misconception has been established concerning this fascinating nearshore sequence. Preliminary investigations of generally well-preserved physical and biogenic structures in the Mesozoic rocks of Kachchh reveal that the entire succession represents deposition in a shallow marine coastal setting, i.e., lagoons, embayments, tidal flats, estuarine channels, shoals and shallow shelf. The Patcham Formation shows poor preservation of trace fossils due to lack of appropriate lithologies. The Chari Formation contains a rich ichnofauna. Of special interest are horizons of “bioconglomerates” showing reworked fillings of Thalassinoides burrows and pholad borings. The Katrol Formation exhibits a diversified and well-preserved assemblage of burrows, including an unusual feeding burrow here recorded for the first time. The Umia Formation though devoid of body fossils contains a rich and varied assemblage of ichnofossils. The uppermost part of this unit, the Bhuj Member, contains several thick horizons of densely Rhizocorallium-bioturbated sand, which supports a marine setting contrary to the general belief of its fluvial origin.

Trace fossils in upper cretaceous argillaceous marine facies of the U.S. Western interior

Argillaceous strata of the Greenhorn and Niobrara depositional cycles contain an extensive, though spottily developed, record of tracemaking organisms. These beds are assignable to five facies, four of which represent the gradation from nearshore sandy shales to far offshore calcareous shales, the fifth representing anoxic or nearly anoxic bottom conditions. In strata studied by us the best potential for trace fossil preservation was in deposits characterized by pronounced textural variation, such as shales with sandstone interbeds, shales with thin lenses of siltstone or sandstone, or sandy and silty shales (= mudstones). In pure clayey shales and calcareous shales, trace fossils are preserved best in concretionary structures. Our study suggests that except for shales rich in organics, trace-making organisms were common and widespread in argillaceous muds of the Western Interior. Initial fluidity, high degree of compaction, and textural homogeneity are principal reasons for the poor record of trace makers in pure, clayey shales.

Preservation of Trace Fossils in Flint in European Chalk: ABSTRACT

It has long been known that many flint concretions in the European Upper Cretaceous chalk represent burrows of organisms. In particular, the ichnogenus Thalassinoides is readily recognizable in branched networks of flint developed more or less parallel to bedding, while other ichnogenera such as Zoophycos and Chondrites are commonly preserved on the surfaces of such nodules. However, until recently, the value of this qualitative information has been restricted because the trace fossils are more or less invisible beyond the boundaries of the flint. It has not been clear, therefore, to what extent trace fossils preserved in flint zones are present at flintless zones. For example, in flint-bearing sequences, was the formation of beds of flints restricted in an degree to horizons especially bioturbated with Thalassinoides suevicus? Conversely, is heavy Thalassinoides suevicus bioturbation restricted to zones bearing thalassinoid flints? The development of a technique involving painting oil onto polished surfaces of chalk is now greatly refining our knowledge of the distribution of trace fossils in pure white chalk, where they are otherwise barely visible, and we are now in a better position to evaluate the ichnological evidence provided by flint preservation. Patterns of distribution are emerging. It is becoming clear, for example, that the many different morphologies of thalassinoid flints are characteristically associated with different sedimentological settings. Thus, fully developed, sizable thalassinoid flint boxworks occur in autochthonous chalks containing correspondingly large, idiomorphic T. suevicus boxworks; straggly, malformed thalassinoid or finger flints have been found in slightly allochthonous chalk where the burrows have been dislocated or streaked out; and in unbioturbated allochthonous chalk, thalassinoid concretions are absent and flint may occur as rounded or irregular forms incorporating siliceous sponge body fossils. Such flint fabrics are easily recognizable and visible in the field and are showing themselves to be a useful aid in detailed sedimentological interpretation of chalk sequences. End_of_Article - Last_Page 430------------