Showing papers in "Lethaia in 2003"

Tracing the tropics across land and sea: Permian to present

Abstract: The continuity through the past 300 million years of key tropical sediment types, namely coals, evaporites, reefs and carbonates, is examined Physical controls for their geographical distributions are related to the Hadley cell circulation, and its effects on rainfall and ocean circulation Climate modelling studies are reviewed in this context, as are biogeographical studies of key fossil groups Low-latitude peats and coals represent everwet climates related to the Intertropical Convergence Zone near the Equator, as well as coastal diurnal rainfall systems elsewhere in the tropics and subtropics The incidence of tropical coals and rainforests through time is variable, being least common during the interval of Pangean monsoonal climates Evaporites represent the descending limbs of the Hadley cells and are centred at 10° to 40° north and south in latitudes that today show an excess of evaporation over precipitation These deposits coincide with the deserts as well as seasonally rainy climates, and their latitudinal ranges seem to have been relatively constant through time Reefs also can be related to the Hadley circulation They thrive within the regions of clear water associated with broad areas of downwelling which are displaced toward the western portions of tropical oceans These dynamic features are ultimately driven by the subtropical high-pressure cells which are the surface signature of the subsiding branches of the Hadley circulation Carbonates occupy the same areas, but extend into higher latitudes in regions where terrestrial surface gradients are low and clastic runoff from the land is minimal We argue that the palaeo-latitudinal record of all these climate-sensitive sediment types is broadly similar to their environments and latitudes of formation today, implying that dynamic effects of atmospheric and oceanic circulation control their distribution, rather than temperature gradients that would expand or contract through time

Patterns of Phanerozoic carbonate platform sedimentation

Abstract: Carbonate platforms changed substantially in spatial extent, geometry, composition and palaeogeographical distribution through the Phanerozoic. Although reef construction and carbonate platform development are intimately linked today, this was not the case for most of the Phanerozoic. Carbonate production by non-enzymatic precipitation and non-reefal organisms is mostly responsible for this decoupling. Non-reefal carbonate production was especially prolific during times of depressed reef growth, balancing losses in reef carbonate production. Palaeogeographical distribution and spatial extent of Phanerozoic carbonate platforms exhibit trends related to continental drift, evolutionary patterns within carbonate platform biotas, climatic change and, possibly, variations in ocean chemistry. Continental drift moved large Palaeozoic tropical shelf areas into higher latitudes, thereby reducing the potential size of tropical platforms. However, the combined global size of carbonate platforms shows no significant decline through the Phanerozoic, suggesting that availability of tropical shelf areas was not a major control of platform area. This is explained by the limited platform coverage of low-latitude shelves (42% maximum) and occasional high-latitude excursions of platform carbonates. We speculate that reduced tropical shelf area in the icehouse tropics forced the migration of the many carbonate-secreting organisms into higher latitudes and, where terrigenous input was sufficiently low, extensive carbonate platform could develop.

Ordovician conodont biogeography – reconsidered

Abstract: Review of the traditional separation of global Ordovician conodont distribution into the North American Midcontinent Province (NAMP) and North Atlantic Province (NAP) reveals a confusing variety of concepts and definitions that hinder biogeographic analysis. Use of this twofold scheme and its subsequent variants should be­discontinued in favour of the more detailed divisions proposed here. Major biogeographical entities of the Shallow-Sea and Open-Sea Realms, separated by the shelf-slope break, are both further subdivisible into Tropical, Temperate and Cold Domains. In the Cold domains, faunal differences between the two Realms and their subdivisions are not easily discernible, since biofacies zones and different habitats were highly condensed. Faunal differences are amplified in the tropical regions, where the North American Midcontinent Province and North Atlantic Province were originally defined. Recognition of endemic taxa is essential for finer classification within domains of the Shallow-Sea Realm (SSR). Our preliminary analysis of Early Ordovician conodont distribution identifies the Laurentian Province (in the Tropical Domain), Australian­(Tropical Domain), North China (Tropical Domain), South China (Temperate Domain), Argentine Precordillera (Temperate Domain) and Balto-Scandian Province (in the Cold Domain). The Open-Sea Realm (OSR) is dominated by cosmopolitan and widespread taxa, and formal subdivision at provincial level is yet to be achieved. The North Atlantic Province encompasses both the Open-Sea Realm and the Temperate and Cold Domains of the Shallow-Sea Realm. The North American Midcontinent Province sensu stricto is more or less equivalent to the Laurentian Province, representing shallow-water regions fringing Laurentia; in a broader sense the North American Midcontinent Province includes all provinces of the Tropical Domain within the Shallow-Sea Realm.

Pre‐Hirnantian Ashgill climatically warm event in the Mediterranean region

Abstract: Compilation of the marine, benthic megafossils from approximately the mid-Ashgill of the Mediterranean region, including much of Central and Southern Europe plus North Africa, and elsewhere indicates a warm interval featuring bioclastic limestone and a warm climate marine fauna. These mid-Ashgill faunas immediately precede the latest Ashgillian, Hirnantian, cool interval that featured widespread glaciation, and are underlain by typical, cold water, Mediterranean Realm, older Ordovician rocks and faunas. The cause or causes responsible for the brief warm interval are uncertain, but may have involved a warm water gateway that is geographically still not located. There is a possibility that South Africa was similarly affected by this roughly mid-Ashgillian marine situation. Early Paleozoic bauxite minerals and kaolins in northwestern Sudan and kaolins elsewhere in North Africa may represent the same time interval, which would suggest that there was a non-marine amelioration of the local climate as well as the marine effects.

The fossil record of biodiversity: nutrients, productivity, habitat area and differential preservation

Abstract: It is hypothesized that the Phanerozoic record of fossil diversity is a function of a secular increase in nutrient availability and productivity (food, energy), and cyclic changes in sea level and habitat area due to supercontinent assembly and rifting. Both variables may have affected biodiversity through the combined variable of {productivity × area}. {Productivity × area} remained relatively constant after the Cambro-Ordovician until the end of the Permian, as did the traditional curve for biodiversity. During assembly of Pangea, decreasing sea level and habitat area were counteracted by increasing nutrient inputs due to uplift and the spread of vascular plants and enhanced continental weathering. As Pangea underwent its final assembly, interior drainage increased, so that by the end of the Permian both habitat area and nutrient runoff decreased. Following the end-Permian extinctions, the traditional curve of diversity began to increase, habitat area, nutrient levels and productivity all increased. Despite the confounding factors of differential preservation and sampling bias toward the present, the fossil record reflects a real response by the marine biosphere to tectonism, sea level, paleoceanographic regime and climate, and the spread of terrestrial floras, and their influence on habitat area, nutrient inputs, and productivity through time.

Biodiversity curves for the Ordovician of Baltoscandia

Abstract: Biodiversity curves for the Ordovician of Baltoscandia show a substantial increase in taxonomical diversity through the period, as seen also in global data sets. A database of 10,340 records of first and last appearances of species at different localities in the region has been analysed using simple species counts, and partly validated with resampling methods. While the biodiversity curve for all fossil groups combined is probably reasonably accurate except for an unknown scaling constant, taxonomical or geographical subsets may not be sufficiently well sampled to allow precise measurement of their species counts through time. The analysis shows a major diversification event commencing in the middle Arenig (Ibex-Whiterock boundary), and more limited diversification events in the Llanvirn, Caradoc and Ashgill. The Scandinavian (Norwegian and Swedish) biodiversity curves are broadly correlated with major changes in sea level, with low biodiversity at highstands and high biodiversity at lowstands, although this pattern is not clear for all fossil groups. In the Arenig, graptolites and trilobites appear to have higher diversities at high sea levels, while the brachiopods and ostracodes show higher diversities at low sea level. As a consequence, the Arenig diversification is delayed for the latter two groups until the upper end of the interval.

Fidelity of molluscan life and death assemblages on sublittoral hard substrata around granitic islands of the Seychelles

Abstract: This study quantified the degree of coincidence between living and dead molluscan faunas in a shallow-water coral reef environment in the Indian Ocean. The results were compared with those from a similar life:death study in the northern Red Sea, and with those published for reef corals and soft substrata molluscs. The proportions of quantitatively important taxa are robust to sampling intensity, but fidelity indices and rank-order correlations are strongly influenced by quantitatively unimportant taxa. Distinct differences between life and death assemblages were recognized, which are due to distinct biases in the death assemblage. Bivalves that lived in close contact with living corals are preferentially overgrown after death and should provide considerable temporal and ecological information in a potential fossil record, as they will be preserved within a rapidly growing reef framework. Some gastropod taxa are preferentially transported into surrounding soft substrata postmortem. Here they will be affected by time-averaging and taphonomic disintegration typically occurring in sediments resulting in the associated loss of much temporal information. Most gastropod shells, however, are inhabited by hermit crabs postmortem, which may strongly alter the fossil gastropod community structure. The results are similar to a comparable study in the northern Red Sea, with a major exception being the strong dominance of hermit crab-inhabited gastropod shells in the death assemblage of the Seychelles. Comparison of life:death assemblages between hard and soft substrata, in keeping with the northern Red Sea study, showed the strong dominance of dead shells in the soft substrata with the converse on the hard substrata. This results from different accumulation conditions for dead shells in soft substrata. Fidelity indices are well suited to demonstrate that sedimentary death assemblages are typically remarkably robust reflections of local community composition but they do not record the strong biases in death assemblages of coral reef associated hard substrata molluscs and are therefore unsuitable for comparisons of life and death assemblages in reef environments.

The earliest myodocopes: ostracodes from the late Ordovician Soom Shale Lagerstätte of South Africa

Abstract: The late Ordovician Soom Shale Lagerstatte of South Africa has yielded Myodoprimigenia fistuca n. gen. and n. sp., the earliest and only known Ordovician occurrence of myodocopes, one of the major groups of ostracodes. M. fistuca is a likely sister group of the Upper Silurian ‘cypridinid’ myodocopes and allied forms. It had a thin, lightly mineralized and flexible shell with microstructures resulting from in vivo calcification processes. It probably fed on cephalopod carrion, thus extending evidence for a carnivorous scavenging lifestyle in ostracodes back by 200 Ma. The species was probably nektobenthic and thus consistent with the notion that the origin of the late Silurian pelagic myodocopes - and therefore of pelagic ostracodes - is to be charted in a benthic to pelagic ecological shift in the group.

Infestation of Middle Devonian (Givetian) camerate crinoids by platyceratid gastropods and its implications for the nature of their biotic interaction

Abstract: One of the classic examples of biotic interactions preserved in the fossil record is that between crinoids and infesting platyceratid gastropods. This relationship, spanning an interval from the Middle Ordovician to the end of the Permian, is recognized by the firm attachment and positioning of platyceratids over the anal vent of their hosts. Several hypotheses have been proposed to explain this interaction; the most widely accepted is that the gastropods were coprophagous commensals, feeding on crinoid excrement without any significant detriment to their hosts. The purpose of this investigation was to test this hypothesis. Two species of Middle Devonian camerate (Monobathrida, Compsocrinina) crinoids, Gennaeocrinus variabilis Kesling & Smith 1962 and Corocrinus calypso (Hall 1862), were used in this investigation. The data consisted of 426 individuals of G. variabilis collected near Rockport, Michigan, 30 of which were infested, and 188 individuals of C. calypso collected near Arkona, Ontario, Canada, of which 25 were infested. Length and volume were measured for each crinoid to determine whether a significant difference existed in the size of infested versus uninfested individuals. The results indicated that for both species of crinoids individuals infested by snails were significantly smaller than uninfested individuals (p < 0.05). We explored a variety of scenarios to explain this pattern and conclude that they falsify the null hypothesis that the crinoid-gastropod relationship was strictly commensal. The smaller size of the infested crinoids is interpreted as a consequence of nutrient-stealing by the parasitic gastropods, a strategy that finds analogs in modern seas. Moreover, the absence of platyceratids on the largest crinoids suggests that large size may have inferred immunity from lasting infestation.

The Miocene palaeobathymetry and palaeoenvironments of Carriacou, the Grenadines, Lesser Antilles

Abstract: Carriacou, a small island in the Grenadines, Lesser Antilles, has a Cenozoic rock record that has been important in interpreting the geologic history of the Southern Lesser Antilles Arc Platform. The Lower-Middle Miocene sedimentary succession of the southeast and east coasts, consisting of the Belmont, Kendeace, Carriacou and Grand Bay formations, has been interpreted as a shallowing-upward sequence from turbidite basin to nearshore?/beach? palaeoenvironments. An earlier interpretation of the Belmont Formation as having been deposited in shallow water is at variance with the turbiditic nature of the succession; the included fossils are considered allochthonous. However, an interpretation of the Grand Bay Formation as deep water is supported by multiple lines of evidence, including sedimentology (turbidites), ichnology (autochthonous association of burrows typical of deep-water environments) and palaeontology (terrestrial, planktic, and shallow and deep water benthic species mixed together). The minimum depth of deposition of the Grand Bay Formation was 150–200 m. This suggests that the (unseen) contact between the Carriacou and Grand Bay formations is either an unconformity, formed following rapid deepening of the basin, or a fault, the Grand Bay Formation being deposited in a separate basin from the shallowing-upwards Belmont-Kendeace-Carriacou formations, against which it is now juxtaposed

A giant Upper Jurassic turtle revealed by its trackways

Abstract: Exceptional fossilization of large tetrapod swimming traces occurs in the Cerin Lagerstatte (Jura Mountains, France). These trackways are imprinted in Jurassic (Late Kimmeridgian) lagoonal fine-grained limestones and are attributed to giant turtles, which swam with a simultaneous movement of their forelimbs like the modern ones. These turtles swam in very shallow waters close to land, perhaps near a nesting area. As a major consequence, these new ichnologic data place the origin of true large marine turtles during the Jurassic period and not during the Cretaceous period as previously considered on the basis of skeletal remains.

Intense drilling in the Carboniferous brachiopod Cardiarina cordata Cooper, 1956

Abstract: The brachiopod Cardiarina cordata, collected from a Late Pennsylvanian (Virgilian) limestone unit in Grapevine Canyon (Sacramento Mts., New Mexico), reveals frequent drillings: 32.7% (n = 400) of these small, invariably articulated specimens (<2 mm size) display small (<0.2 mm), round often beveled holes that are typically single and penetrate one valve of an articulated shell. The observed drilling frequency is comparable with frequencies observed in the Late Mesozoic and Cenozoic. The drilling organism displayed high valve and site selectivity, although the exact nature of the biotic interaction recorded by drill holes (parasitism vs. predation) cannot be established. In addition, prey/host size may have been an important factor in the selection of prey/host taxa by the predator/parasite. These results suggest that drilling interactions occasionally occurred at high (Cenozoic-like) frequencies in the Paleozoic. However, such anomalously high frequencies may have been restricted to small prey/host with small drill holes. Small drillings in C. cordata, and other Paleozoic brachiopods, may record a different guild of predators/parasites than the larger, but less common, drill holes previously documented for Paleozoic brachiopods, echinoderms, and mollusks.

Preservation of in situ reef framework in regions of low hurricane frequency: Pleistocene of Curaçao and Bonaire, southern Caribbean

Abstract: Luxuriant fringing reefs along the southwestern shores of the Caribbean islands of Curacao and Bonaire (12°N), located outside the most frequent hurricane tracks, are rarely affected by major storms. Consequently, reef growth and long-term preservation are potentially optimal and distinct from reefs experiencing greater hurricane frequency. Hurricane Lenny (November 1999) took an unusual west-to-east track, bisecting the Caribbean Basin north of these islands, but generated heavy waves (3-6 m) that severely damaged reefs along the normally leeward shores. Massive coral colonies >100 years old were toppled, but even at the most severely damaged sites, 82–85% of colonies remained in growth position. Late Pleistocene (125 ka) elevated reefs in the Lower Terrace of Curacao record even higher proportions of corals in growth position (93%), possibly reflecting a low hurricane frequency during the Pleistocene highstand. In comparison, coeval Pleistocene reefs in regions that today experience a high hurricane frequency (Great Inagua Island and San Salvador, Bahamas) have lower proportions of corals preserved in growth position (79% and 38%, respectively). These results are consistent with the hypothesis that reefs in regions experiencing very low hurricane frequency, like the southern Caribbean, are more likely to be preserved with corals in primary growth position in comparison to regions with higher hurricane frequency.

Ecology and evolution of Jurassic trigoniid bivalves in Europe

Abstract: A modern palaeobiological approach to the taxonomy, making full allowance for a wide range of variation, has allowed the distinction of 22 Jurassic trigoniid species in Europe, grouped into 7 genera. Most species were confined to shallow marine habitats ranging up to a few tens of metres in depth but were adapted to low energy environments as well as environments of moderate to high energy. The presence of external ornament on the shell flanks was probably an adaptation to facilitate burrowing, but in the absence of useful external ornament other adaptations to increase burrowing efficiency may have been realized by the development of an elongate shape. Because their occurrence in Jurassic strata is only intermittent, confident inferences on evolutionary patterns are limited to the two commonest genera, Trigonia and Myophorella. Phyletic size increase has been recognized in the latter but not the former genus. The calculated species longevities correspond fairly closely with those established for Jurassic bivalve species in general. The mode of speciation is dominantly punctuational, but only one likely example of species splitting is recognized.

Completeness of a fossil record: the Pleistocene echinoids of the Antilles

Abstract: Diversity of Pleistocene marine invertebrates with single component, robust, easily identifiable skeletons shows a good correspondence to that of the Recent when compared between similar environments in the same region. Such comparisons are less easily made for multi-element skeletons. In the Antilles there are 24 extant species of echinoids in 0–30 m water depth. Considering all available fossils - complete tests, disarticulated spines, and test fragments and ossicles - the Pleistocene echinoids of the Antillean region have a high similarity with the extant shallow water fauna (c. 63% specific and 72% generic similarity). Many of the extant, shallow water, regular echinoid taxa are known from the Pleistocene, although irregular echinoids are less well represented. These results are comparable with those determined for coeval benthic invertebrates from this and other geographic regions, and indicate a high degree of similarity from a survey which is so far limited to only a few of the islands. Not all Pleistocene units considered herein were deposited in shallow water, but they include autochthonous specimens of taxa whose depth range extends beyond shallow water at the present day or presumed allochthonous specimens derived by downslope transport from shallower water, or both.

Lancetfish teeth (Neoteleostei, Alepisauroidei) from the Early Cretaceous of Alcaine, NE Spain

Abstract: Isolated teeth of fossil lancetfishes (Neoteleostei, Alepisauroidei) are reported from Early Cretaceous strata of the Oliete subbasin (Iberian basin) in NE Spain. These are the oldest remains attributable to alepisauriform teleosts. The fossil-bearing strata are dated as lower Barremian. The teeth were recovered from shallow marine to lagoonal sediments. They closely resemble teeth of the fossil alepisauriforms Enchodus and Cimolichthys in overall morphology. The combination of fang-like morphology, sculpture consisting of apico-basal striations, postapical barb, absence of distal cutting edge, and wide pulp cavity surrounded by a rather thin layer of dentine is interpreted as the most basal tooth pattern in alepisauriforms. These remains may be isolated, but this study emphasizes the true value of such remains for palaeobiological studies, predicting the fossils provide characters for phylogenetic analyses.

Early Cambrian coelobiontic communities in tectonically unstable crevices developed in Neoproterozoic andesites, Ossa‐Morena, southern Spain

Abstract: A well-preserved biota of Lower Cambrian cavity-dwelling organisms is recorded within fissures in Neoproterozoic andesites in Ossa-Morena (southern Spain). The cavities are unique among described Lower Cambrian coelobiontic communities due to the igneous character of the host rock. Coelobiontic habitat was episodically enlarged by synsedimentary tectonic fracturing reflecting polyphase infill of recurrent facies. The pioneer coelobiontic biota was diverse, and consisted of encrusting stromatolites and thromboids (dominated by Epiphyton and Renalcis), attached to walls and ceilings of the cavities, associated with archaeocyaths. Sponge spicules and chancelloriid sclerites occur as dense clusters indicating in situ growth, death and decay of spiculate sponges and coeloscleritophorans. Other organisms, such as echinoderms, trilobites and brachiopods, are also found within the cavities as reworked skeletons, indicating that they were washed in from the overlying, open seafloor. The main feature of the coelobiontic biota is the dominance of a sessile, chemosynthetic and filter-feeding epibenthos, composed of microbial communities, archaeocyaths, spiculate sponges (demosponges and rarer hexactinellides) and coeloscleritophorans.

Growth patterns of Lower Palaeozoic sponges

Abstract: Detailed studies of the growth patterns of modern siliceous sponges are restricted to demosponges and theoretical models. It is generally assumed that sponge growth is essentially incremental, with completion of one arbitrary unit being followed by external addition. All recent species are thick-walled, but Lower Palaeozoic sponges are dominated by thin-walled hexactinellids, with most Cambrian taxa consisting of a single spicule layer. Large populations of a primitive dictyospongiid have allowed the reconstruction of the growth patterns of their spicules and body morphology. The results indicate that growth occurred through continuous expansion of the globose body, accompanied by continuous enlargement of existing spicules, with a spicule size limit being reached only during the lifetime of a few individuals. It is noted that this skeletal growth pattern is otherwise restricted to deuterostomes. Consecutive appearance of successive spicule size orders appears to have maintained a maximum inhalant pore area. Comparisons with more limited data from two acanthose hexactinellids and a hazeliid demosponge indicate that an identical growth pattern operated in these species. The subsequent evolution of growth patterns is discussed, with various mechanisms producing the later thick-walled morphologies of hexactinellids and demosponges. The implications of these observations are discussed with reference to identification and systematics, since spicule size and arrangement are shown to vary during growth.

Cyathophycus and the origin of demosponges

Abstract: The relationships between the poriferan classes are currently obscure. Molecular phylogenies appear to be reaching a consensus that the hexactinellids and demosponges are closely related, despite previous attempts to separate the Hexactinellida from other sponges on cytological grounds, but the details of the transition are unknown. Similarities of spicule morphology and structure are used to infer that the transition probably occurred after the onset of silicification, and should therefore be seen in fossils. The similarity between protosponges and early demosponges has been noted previously, based largely on a thin, reticulated wall of simple spicules (monaxons in Leptomitus, stauracts in Protospongia). A close relationship is, however, unlikely, since the protospongiids possessed a precise geometric arrangement of multiple spicule size orders that is lacking in demosponges. A close morphological similarity exists between the skeletons of transitional protosponge-dictyosponge reticulosids, such as Cyathophycus, and the early hazeliid demosponges. The inner spicule layer of primitive dictyosponges consists of a cross-hatched array of fine monaxons, as seen in the wall of some simple hazeliids. Although most described species of hazeliids were morphologically complex, a simple globose species is here recorded from the Caradoc of Wales. The evolutionary link between the classes is suggested to lie within the dictyospongioid and hazeliid lineages. Although the direction of evolution cannot be certainly fixed, it is conceptually much easier to derive the demosponges from the hexactinellids, rather than vice versa.

Early Cambrian Salterella and Volborthella (Phylum Agmata) re‐evaluated

Abstract: The genus Volborthella is removed from synonymy under Salterella and placed in the extinct phylum Agmata. Volborthella is more restricted geographically and occurs in older Lower Cambrian strata than Salterella, but distinguishing younger specimens of that genus from poorly preserved Salterella is difficult. Vologinella is not related to Volborthella or other Agmata. Other genera assigned to the phylum by various workers are discussed, but their assignment to this phylum is either enigmatic or rejected. The published concept that specimens of Volborthella and Salterella are agglutinated sclerites or spines on a larger soft-bodied organism is discussed. The material on which this notion is based is re-illustrated and the notion rejected in favor of the view that these conical forms are the hard parts of discrete organisms.

Form and environment of Gryphaea arcuata

Abstract: Gryphaea arcuata is one of the most studied fossils, but its detailed palaeoecology has been largely neglected. Specimens were collected within a short stratigraphic range (three ammonite zones) in the ‘Calcaire a gryphees’ of Xeuilley (Lorraine, France) dated Hettangian to Lower Sinemurian. As far as possible, they were sampled from each marly bed of the section. A biometric study and an isotopic analysis are compared in regard to organic matter measurements and palynological data, the results demonstrating a clear relationship between the shape of G. arcuata and environmental parameters. Factors responsible for the various shapes are temperature, oxygen levels on the sea floor and nutrient levels. Two main morphotypes can be related to two kinds of environment. In the first, controlled by a relatively hot and humid climate and tending towards eutrophication, the growth rate of Gryphaea was low, and the shells small, wide and thin. In the second environment, cooler than the first one and closer to the optimal living conditions of G. arcuata, the shell was large, thick and narrow, and exhibited a high growth rate.

Ecospace utilization in early Phanerozoic deep-marine environments: deep bioturbation in the Blakely Sandstone (Middle Ordovician), Arkansas, USA

Abstract: Ichnofabric analysis of alternating light and dark-coloured mudstone layers in the Blakely Sandstone (Middle Ordovician) at Crystal Springs Landing, Lake Ouachita (west of Hot Springs, western Arkansas, USA) reveals two equilibrium palaeoichnocoenoses. The first was emplaced under variable, but low, oxygen levels during deposition of the dark-coloured layers; small diameter transition layer burrows overprint a mixed layer ichnofabric. The transition layer infauna was tiered with abundant Chondrites representing the deeper of two shallow tiers. Light-coloured layers accumulated during prolonged intervals in which the sediments were oxygenated to a greater extent and depth. Preservation of a mixed layer ichnofabric within them is the result of limited, but deep (up to at least 400 mm), reworking subsequently in the transition layer by an equilibrium community. These transition layer trace fossils are not tiered. If representative of oxygenated sediment columns in Ordovician deep-marine environments, an extensive volume of infaunal ecospace was colonized (in this case by deposit feeders); its more efficient use subsequently, including vertical partitioning of the infaunal community into specific environmental niches (tiering), could have accommodated increases in diversity and community complexity. Changes over time in the maximum depth to which sediments were bioturbated, alone, would therefore be a poor measure of the extent of ecospace utilization.

Morphogenetic significance of the conchal furrow in nautiloids: evidence from early embryonic shell development of Jurassic Nautilida

Abstract: As reported by many workers over the past two centuries, the inner part of the shell of various straight and coiled Palaeozoic to tertiary nautiloid taxa bears a continuous mid-ventral furrow that extends into the phragmocone and the body chamber nearly to the aperture. Study of the early embryonic shell development of Jurassic Nautilida shows that the most apical part of this so-called conchal furrow originates from the inner part of the initial, calcified shell apex, in line with the inner ventral termination of the central linear depression of the cicatrix, the initial site of shell deposition. The conchal furrow corresponds to a morphological feature arising as a developmental by-product. Rare specimens of scattered ammonoid species (and possibly of bactritoids) display a similar feature, whereas their protoconch lacks a cicatrix. However, the protoconch of recent cuttlefish, Sepia officinalis, often displays a longitudinal fold of the primary shell epithelium. A longitudinal groove or a pair of grooves appears connected with this cicatrix-like structure. Although the mid-ventral ridge in ammonoids must probably be viewed as an incidental ‘fabricational noise’, whether or not it originates from a so far undocumented optional ridge on the protoconch or from some other structure related to shell development remains an open question.

Sexual reproductive structures in the green alga Ivanovia triassica

Abstract: Ivanovia triassica is a new species of calcified Codiaceae from Triassic rocks of the Yukon Stikine Terrane in Canada. The genus was known previously only from the late Paleozoic. Stalked outgrowths of the thalli are interpreted as oogonia and dome-shaped outpocketings as male gametangia. These represent the first sexual reproductive structures observed in Ivanovia. Sectional views of the thalli indicate the species was cyathiform and that the inner and outer cortices of the membranes comprising the thallus were dimorphic.

Biogeography of Eocene bryozoans from the St Vincent Basin, South Australia

Abstract: The first extensive and stratigraphically detailed taxonomic study of the Middle to Late Eocene Bryozoa of the St Vincent Basin has identified more than 200 species of Cheilostomata and 50 species of Cyclostomata. There are three biogeographic groups: basin endemic, Australian and global. Two-thirds (116) of the cheilostome species and seven genera are currently considered endemic to this basin. Most species are endemic to Australia and similar to those found in the Oligo-Miocene of Victoria. The Cellariidae are a common component of most Australian Cainozoic deposits, but the species are highly dissimilar, with 13 of the 17 species here being new. The global component indicates that biogeographic links with regions outside Australia still existed in the Eocene. The cyclostome genus Reticrescis is only known from the Australian and Antarctic Eocene. Ten genera have their first occurrence in the Eocene St Vincent Basin. The Phidoloporidae and Smittinidae represent the most diverse and ubiquitous groups at a geological time close to their time of origination. Contemporaneous sediments in Antarctica, eastern Europe and North America also have a diverse fauna of this family, pointing to a strong Tethyan link. Rhamphosmittina lateralis (MacGillivray) is still extant in New Zealand, having an exceptionally long time range of 40 million years. Overall, the fauna has a distinct Late Cretaceous character. A new genus of Onychocellidae appears similar to genera that were common in Cretaceous Tethyan faunas but rare during the Cainozoic. This similarity ends in the Oligocene, after which the Australian bryozoan became endemic

Unusual preservation of crustaceans and microbial colonies in a vadose zone, northwest Morocco

Abstract: Exceptional fossil preservation is observed in self-sealing microcavities in limestones where lichens, cyanobacteria and fungi together entombed themselves and organic walled microfossils, crustaceans and their eggs. Preservation has been enabled by calcite coating, lining and impregnation of the exoskeleton of the crustaceans, which had a high original calcium content and acted as a nucleus for precipitation. The good preservation was facilitated by the microcavities, the surrounding limestone, the seasonality and rapidity of precipitation, the microbial colonies living on the specimens, and the fluxing of vadose waters through the karst. The microbes and the crustaceans probably became trapped by the very high rate of calcite precipitation promoted within the cavities. Entrapment in mucous-secreting mats of cyanobacteria and fungi preceded the destruction of some of the soft parts of the crustaceans. By the time the mats and the incorporated biota were sealed into the cavities in the limestone, the mucosic mats and their bacterial communities had mediated production of a range of calcite cements promoting preservation of the refractory tissues. This process has important implications for cementation studies in arid zones (and especially in the Martian subsurface), since a range of microbes are involved in progressive biomineralization leading to fossilization within a perched, vadose karst.