Trilobite Biofacies Research Articles

Faunal and paleoenvironmental changes at a Cambrian (Jiangshanian; Steptoean–Sunwaptan boundary interval) trilobite extinction event, in contrasting deep- and shallow-subtidal settings, Nevada and Oklahoma

Abstract Successions in Oklahoma and Nevada record trilobite extinction and replacement near the Steptoean–Sunwaptan boundary in inner-shelf and outer-shelf settings, respectively. Prior to the extinctions, different trilobite biofacies occupied these environments, but faunas became similar in composition across the environmental gradient in the overlying I. “major” and Taenicephalus zones. Faunal changes in the outer shelf at the I. “major” Zone begin at a drowning unconformity that brought dark, laminated calcisiltite and silty lime mudstone above a subtidal carbonate succession. In contrast, Oklahoma shows facies continuity in a succession of tidally influenced bioclastic carbonates. Loss of genera and a dramatic abundance “spike” of Irvingella are features of the I. “major” Zone in both regions. Turnover of biofacies occurred in the succeeding Taenicephalus Zone, with both the inner and outer shelf dominated by Orygmaspis (Parabolinoides). Blooms of orthid brachiopods in shallow water settings are underappreciated signals of faunal change in the extinction interval. Although absent from the outer shelf in Nevada, orthids became abundant enough in Oklahoma to form shell beds in the lower Taenicephalus Zone, but became rare in overlying strata. Carbon isotope stratigraphy includes a modest positive δ13C excursion that peaks in the extinction interval at 1.4‰ (Oklahoma) and 2.2‰ (Nevada), which is congruent with previous reports from Utah and Wyoming. Although consistent with regional upwelling of dysoxic waters, the absence of sedimentary evidence for significant environmental change over much of the shelf is problematic. This suggests that physical environmental change acted primarily as a catalyst for cascading ecological and biogeographic effects.

Middle Ordovician trilobites from the Castillejo Formation, Eastern Iberian Range (NE Spain): taxonomic reappraisal, biostratigraphy, and correlation

Middle Ordovician trilobites found in 11 fossil localities from the areas of Alpartir, Aladrén, Luesma and Fombuena (Zaragoza province), and south of Calamocha (Teruel province) are studied. Most records come from the Alpartir Member of the Castillejo Formation, which is dated as Dobrotivian in age (late Darriwilian according to the global scale), with the only exception of two localities that are slightly older, from late Oretanian age (mid−late Darriwilian). In the overlying Sierra Member, three additional localities are recorded, which have estimated age ranges from late early to late Dobrotivian (latest Darriwilian, possibly reaching the Sandbian). A total of 21 different trilobite species were identified including Uralichas hispanicus, Parabarrandia crassa and “Panderia” beaumonti that are recorded for the first time in the Eastern Iberian Range. The main trilobite assemblage from the Alpartir Member belongs to the Placoparia tournemini Biozone, which correlates with numerous areas of southwestern Europe, and here occurs in a biofacies dominated by asaphids. The scarce data derived from the Sierra Member do not allow to distinguish biozones, although the incipient development of a trilobite biofacies adapted to sandy bottoms is recognized, which ends at the top of the Placoparia borni Biozone in the whole of southwestern Europe. The correlation of the trilobite assemblages with those of other formations represented in the Cantabrian, West Asturian-leonese and Central Iberian zones is also discussed, the latter probably related through the enigmatic Central Iberian Arc, a debated orocline of the Iberian Massif.

A Jiangshanian (Cambrian; Furongian) trilobite fauna from the Cow Head Group, western Newfoundland

Three shelf-derived boulders in debris flow conglomerates of the Downes Point Member of the Shallow Bay Formation of western Newfoundland yielded the first record of an Upper Cambrian shelf-margin trilobite fauna from the Elvinia Zone (Steptoean; Jiangshanian) in eastern North America. The fauna is dominated by the “catillicephalid” Buttsia Wilson, 1951, and resembles trilobite biofacies from microbial buildups in the Gatesburg Formation of Pennsylvania. It is correlative with the Cliffia latagenae Subzone, which is the youngest Steptoean biostratigraphic unit in the Gatesburg. New taxa are Triorygma burkhalteri gen. et sp. nov. and Buttsia trema sp. nov.

Trilobite biofacies and sequence stratigraphy: an example from the Upper Ordovician of Oklahoma

There have been surprisingly few empirical investigations of the fundamental principle that the architecture of depositional sequences exerts considerable control on observed patterns of faunal distribution and replacement. In this paper, we examine trilobite associations in two sequences of the Upper Ordovician (Sandbian) Bromide Formation of southern Oklahoma. Cluster analysis and ordination of genus abundance data identified five lithofacies-related biofacies that are also differentiated by diversity patterns. Biofacies of the transgressive system tract (TST) of successive sequences are more similar to each other than they are to biofacies in the highstand systems tract (HST) of the same sequence. This similarity likely records dominance of large, robust convex sclerites in taphonomically degraded samples from condensed, strongly winnowed grainstone and rudstone. Horizons with articulated exoskeletons of isoteline trilobites preserved by obrution deposits occur most commonly in the early HST and record behavioural aggregations. Grainstone and rudstone of the later HST are less winnowed than those of the TST and show less fragmentation and sorting of sclerites. These changes in taphonomic conditions preserve ecological patterns more clearly. In most biofacies, rarefied alpha diversity (samples) and gamma diversity (biofacies) of middle- and outer-ramp HST deposits are greater than in the TSTs, and biofacies replace each other down ramp. Diversity patterns do not agree with model predictions and other data sets that indicate low beta and high alpha diversity in the TST, likely because of taphonomic degradation. Vertical replacement of biofacies is expressed by the appearance of peritidal facies in which trilobites are rare. Biofacies shifts also characterize sequence boundaries and are most profound in the inner-ramp successions characterized by sharp facies offsets. Comparison with bathymetrically similar deposits in the Taconic foreland basin showed similar diversity trends along environmental gradients, with some differences in shallow-water settings attributed to taphonomic differences.

TRILOBITE BIOFACIES ALONG AN ORDOVICIAN (SANDBIAN) CARBONATE BUILDUP TO BASIN GRADIENT, SOUTHWESTERN VIRGINIA

The Upper Ordovician Effna, Botetourt, and Liberty Hall limestones of Virginia record deposition in several paleoenvironments along a carbonate ramp during the early history of the Taconic Foreland Basin. Four distinct lithofacies are present, i.e., biohermal mudstone and wackestone, biostromal grainstone, nodular deep-flank limestone and shale, and basinal mudstones and shale, and each of these is associated with different trilobite biofacies. Cluster analysis and ordination define biofacies, and characterize gradients and patterns within the trilobite relative abundance data. Biofacies are clearly delineated along a depth gradient, with gradational compositional changes between associations. The moderate-diversity bioherm biofacies is dominated by illaenids (Illaenus, Bumastoides) and cheirurids (Sphaerexochus). The high-diversity biostromal biofacies is also an illaenid-cheirurid association, but is proportionally enriched in cheirurids, and includes off-bank elements not common in the bioherm. The high-diversity deep-flank facies is associated with an Ampyx-Isotelus-Nileus assemblage that gradationally replaces the biostromal biofacies. The deep flank gives way to a low-diversity Isotelus-Calyptaulax-Raymondaspis association of cosmopolitan trilobites in the outer slope and basin environment. These biofacies characterize local environments in the Taconic Foreland Basin just prior to, and in the initial phases of tectonically driven oceanographic change that led to regional extirpation of the buildup fauna and the reorganization of deep-water trilobite biofacies. New associations of taxa not found elsewhere in Laurentia are common within the foreland basin, suggesting that the archetypal biofacies described here are reorganized with the onset of regional environmental change.

Trilobite biofacies and palaeogeographic development in the Arenig (Ordovician) of the Yangtze Block, China

Cluster analysis was conducted using Pearson's coefficient based on the relative abundance data of 73 trilobite genera and subgenera in 72 collections, and fourteen Arenig (Floian–early Darriwilian) trilobite biofacies are recognized from the inner shelf to deep outer shelf of the Yangtze Block. The biofacies patterns and faunal dynamics across the eight successive time intervals suggest that the progressive changes in generic composition, diversity, and dominant components may have been affected by bathymetric gradients and lithofacies transition along the palaeoramp of the block, and by temporal constraints on the different trilobite clades. On the whole, the total faunal diversity reached its peak in the shallow outer shelf, from which it decreased both seawards and shorewards. The benthic forms generally exhibited increased endemicity shorewards, but atheloptic taxa were confined largely to the outer shelf, with the generic diversity increased seawards, while the mesopelagic trilobites were associated mainly with the deep-outer-shelf biofacies. The progressive alternations of trilobite biofacies spectra indicate that continual transgressions may have played an important role in a consecutive palaeogeographic development in the Yangtze Block, including mainly the drowning of the huge platform in the early Arenig (early Floian) (TI 2), the shoreward expansion of the deep outer shelf (TI 5) and the ongoing shoreward retreats of the inner shelf (TIs 6–7) in the Yangtze Sea, and the successive deepening of the southern Shaanxi sea especially in the late Arenig (Dapingian–early Darriwilian) (TIs 5–8) since its taking shape in TI 3.

Deep shelf trilobite biofacies from the upper Katian (Upper Ordovician) of the Grangegeeth Terrane, eastern Ireland

AbstractThe trilobite fauna of the upper Katian (mid‐Ashgill in terms of Anglo‐Welsh chronostratigraphy) Oriel Brook Formation in the Grangegeeth Terrane, eastern Ireland comprises 16 species. It is dominated numerically by mesopelagic cyclopygids (Cyclopyge cf. marginata Hawle and Corda and much rarer Symphysops sp.), which comprise 63% of the 156 identifiable sclerites known unequivocally from the formation. Other elements of the trilobite fauna include species of Staurocephalus, Phillipsinella, Nankinolithus and the agnostids Trinodus and Sphaeragnostus. The abundance of cyclopygids in a terrane interpreted as lying within the Iapetus Ocean closer to Laurentia than the Anglo‐Welsh sector of Avalonia is consistent with the Late Katian spread of cyclopygids onto low latitude shelves seen in Quebec and Maine in eastern North America, Girvan in SE Scotland and Co. Clare in the Irish Central Terrane. With the exception of the agnostid trilobites, whose mode of life is strongly debated, associated benthic trilobites lacking eyes are fairly rare and the bottom‐dwelling fauna does not conform to the atheloptic assemblages that typified many mid‐ to high latitude Ordovician outer shelf and upper slope settings. The benthic assemblage can be considered in terms of a bathymetrically arranged spectrum of faunas and is closest to that of the deep shelf Phillipsinella parabola—Staurocephalus clavifrons fauna, known from the Upper Ordovician of Avalonia and Baltica and to some extent marginal Gondwana and South China. This is the first record of such a fauna from Laurentia or its marginal terranes and reflects the Late Ordovician breakdown of provincialism in the Iapetus Ocean. The deep‐water setting is compatible with that of the associated Foliomena brachiopod fauna. Copyright © 2010 John Wiley & Sons, Ltd.

Unusual trilobite biofacies from the Lower Ordovician of the Argentine Cordillera Oriental: new insights into olenid palaeoecology

Balseiro, D., Waisfeld, B.G. & Buatois, L.A. 2010: Unusual trilobite biofacies from the Lower Ordovician of the Argentine Cordillera Oriental: new insights into olenid palaeoecology. Lethaia, Vol. 44, pp. 58–75. The study of biofacies has proven to be relevant in the understanding of trilobite palaeoecology, palaeobiogeography and macroevolution. The widespread Olenid biofacies is one of the best known, and is usually interpreted as occuring in dysoxic environments. Tremadocian successions of the Argentinian Cordillera Oriental bear a diverse and long-studied olenid-dominated fauna. Based on cluster analysis, five distinct biofacies are defined for the middle Tremadocian (Tr2 stage slice), distributed from shelf (below storm wave base) to lower-shoreface settings (above fair-weather wave base). Ordination shows biofacies along two gradients, a bathymetrical one and another related to oxygen content. All of them are dominated both taxonomically and ecologically by olenids. This detailed quantitative palaeoecological study challenges current views suggesting instead that the Olenidae dominated a broad range of environments, from oxygenated shallow-marine to dysoxic deep-marine. Comparisons with largely coeval trilobite records from geodynamically and palaeobiogeographically disparate sites suggest that siliciclastic sedimentation appears as the most influential controlling environmental factor upon olenid distribution and dominance. Further comparisons across different climatic belts show that siliciclastic input controlled trilobite diversity gradients, even more than latitude. From an autoecological viewpoint distribution of traditional olenid morphotypes shows no relation to depth or to oxygen content, and at least some members of the group appear to have had the possibility of coping with low oxygen content, rather than being restricted to oxygen-deficient environments. The analysis performed herein, together with recent research on the group, demonstrate that factors controlling olenid distribution are more complex than previously envisaged. □Biofacies, diversity, Olenidae, palaeoecology, Tremadocian, trilobite.

Trilobite-based biostratigraphic model (biofacies and biozonation) for the Middle Cambrian carbonate platform of the Argentine Precordillera

A preliminary biostratigraphic model that combines trilobite biofacies and biozonation is proposed for the early Marjumian (Middle Cambrian) of the Precordillera of western Argentina. This interval involves a variety of paleogeographic–paleoenvironmental settings across the carbonate platform, ranging from its interior to exterior areas beyond the platform margin and including both open-marine and near-shore environments. This model is based on three stratigraphic sections of the basal part of the Alojamiento Formation, which crops out in the Cordon del Alojamiento (Provincia de Mendoza) and Cordon de Santa Clara (Provincia de San Juan); and three sections of the Soldano Member of La Laja Formation in the Sierra Chica de Zonda (Provincia de San Juan). Three biofacies were recognized on the basis of trilobite associations that show a marked environmental distribution: 1) The Ptychagnostidae Biofacies is represented by an assemblage exclusively dominated by agnostoids, most of them belonging to Ptychagnostus; this assemblage occurs in mudstones and calcareous shales deposited below storm wave-base seaward of the platform margin; 2) the Peronopsidae Biofacies also developed in the external areas but it occurs in limestones deposited between fair-weather and storm wave-base; this assemblage is likewise dominated by agnostoids, but instead belonging to Peronopsis, with the rest of the fauna comprising polymeroids (Dolichometopidae, Oryctocephalidae, Zacanthoididae and Alokistocaridae); 3) the Alokistocaridae Biofacies occurs in grainstones, mudstones and calcareous shales deposited between fair-weather and storm wave-base under changeable conditions located near the platform margin and in the interior. It is dominated by alokistocarids whose relative abundance increases landward from 52% to 58% and to 95%. The remaining components belong to Dorypygidae, Zacanthoididae and Oryctocephalidae; Eodiscidae and Peronopsidae comprise minor proportions in the most seaward location. Due to the contrasting, environmentally mediated associations, these sections belong to three essentially synchronous biozones erected previously for the Middle Cambrian of western Laurentia, the Ptychagnostus gibbus, Oryctocephalus and Ehmaniella Biozones.

SEDIMENTARY FACIES AND TRILOBITE BIOFACIES ALONG AN ORDOVICIAN SHELF TO BASIN GRADIENT, VIOLA GROUP, SOUTH-CENTRAL OKLAHOMA

The Upper Ordovician Viola Group of south-central Oklahoma records deposition on a carbonate ramp that extended from platformal settings to a basin within the Southern Oklahoma aulacogen. Lithofacies analysis allows identification of inner, mid- and outer-ramp environments in the Viola Springs Formation, each of which hosted a distinct trilobite biofacies. There is a sharp break in both composition and species diversity of biofacies between the outer ramp within the aulacogen and shallower environments. The outer-ramp Cryptolithine Biofacies is characterized by low species diversity, whereas both the midramp Thaleops Biofacies and the inner-ramp Bumastoides Biofacies contain up to four times as many trilobite species and rare cryptolithines. Comparisons with Cambrian and older Ordovician shelf-to-basin trilobite distributions suggest that the pattern recorded in the Viola Springs is depth related. The overlying Welling Formation includes two biofacies in inner- to midramp environments, with the faunas of the aulacogen being less diverse than those of the platform. The high trilobite species diversity in shallow water environments of the Viola Group supports a dilution model for community reorganization during the Ordovician.

Relative and absolute abundance of trilobites and rhynchonelliform brachiopods across the Lower/Middle Ordovician boundary, eastern Basin and Range

Relative abundance data are of primary importance in paleoecology, but it is not always obvious how they should be interpreted. Because relative abundance is expressed as a proportion of the total sample, change in the abundance of one group necessarily changes the relative abun- dance of all groups in the sample. There are two possible interpretations for a trend in the relative abundance of a taxon: an ''active'' scenario in which the trend reflects change in the population density of the group itself, or a ''passive'' scenario in which the change is driven by population changes in other taxa. To discriminate between these scenarios it is necessary to collect absolute abundance data (abundance expressed as a function of sample area or volume). We examine both absolute and relative abundance trends through a major paleoecological tran- sition: the shift from trilobite-dominated to brachiopod-dominated paleocommunities in shallow marine carbonates spanning the Lower/Middle Ordovician boundary in western Utah and eastern Nevada. We sampled 61 carbonate mudstone and wackestone beds from the upper Ibex Series (Low- er Ordovician) and lower Whiterock Series (Middle Ordovician) at three sections that span the boundary. All samples come from the shallow subtidal Bathyurid trilobite biofacies. Samples were broken into small pieces, and all skeletal fragments .2 mm were identified to the finest possible taxonomic level. Consistent with previous work on this interval, the relative abundance of trilobites declines sharply across the boundary, while the relative abundance of brachiopods increases. Ab- solute abundance data indicate that the decline in trilobite abundance is genuine and not an artifact of normalization. The trend is not easily explained by sampling bias, facies distribution, tapho- nomic regime, or sedimentation style. The dramatic shift in abundance contrasts with relatively minor changes in relative genus rich- ness across the boundary. This is partly ascribable to differences in the relative abundance structure of trilobite faunas. Though comparable numbers of trilobite and brachiopod genera occur above and below the boundary, the trilobite fauna from the upper Ibex Series has lower evenness then the lower Whiterock Series fauna. Hence sampled trilobite richness is high in the lower Whiterock de- spite the small number of specimens. This highlights the importance of collecting abundance data. Although these data suggest that in at least some cases richness and abundance patterns are not closely coupled, more robust richness data are necessary to confirm this conclusion.

Facies of a Lower Ordovician carbonate shelf (Mungok Formation: Taebaeksan Basin, Korea)

The lithologic associations within the Lower Ordovician Mungok Formation in Korea define four depositional facies that formed across a continental margin fringing the Sino-Korean block: these facies represent lagoonal/restricted marine, shoal, inner shelf, and outer shelf environments. The stacking pattern of these facies reveals two systems tracts composed of five depositional sequences. The lower highstand systems tract consists of the lagoonal/restricted marine and shoal facies, whereas the upper lowstand systems tract comprises, in ascending order, inner shelf, outer shelf, and inner shelf facies. Three trilobite biofacies are recognized in the Mungok Formation: i.e.,Yosimuraspis, Kainella, andShumardia biofacies in ascending order. TheYosimuraspis Biofacies is dominated byYosimuraspis but also containsJujuyaspis andElkanaspis. The predominance of the endemic eponymous taxon suggests a lagoonal/restricted marine environment. The nearly monotaxicKainella Biofacies, which comprises pandemic genera such asKainella and occasionallyLeiostegium, may represent a less restricted environment than theYosimuraspis Biofacies. TheShumardia Biofacies occurs in the marlstone/shale lithofacies through relatively thick stratigraphic interval and is dominated by cosmopolitan trilobite taxa with some endemic species. The lithofacies and cosmopolitan trilobite assemblage of theShumardia Biofacies indicate that it occupied an outer shelf environment. The vertical succession of lithofacies and trilobite biofacies in the Mungok Formation records in general a shift from a restricted, shallow water environment to deeper-water environment.

Llanvirn-early Caradoc trilobite biofacies of western Hubei and Hunan, China

On the basis of representative collections made from seven measured sections in western Hubei and Hunan, four early Llanvirn and five late Llanvirn-early Caradoc trilobite biofacies are distinguished along a bathymetric gradient from shallow to deep outer shelf. The diversity and relative abundance of mesopelagic cyclopygids gradually increased southwards or seawards. According to the facies association exhibited in the region, the upper limit of living depth for different forms is inferred as follows: 1) more than 70 m (M. (Microparia)); 2) less than 100 m (M. (Quadratapyge), Pricyclopyge, Aspidaeglina); 3) at approximately 100 m (Cyclopyge); and 4) more than 100 m (Symphysops, Psilacella). Benthic forms were absent in the deep outer-shelf basin because of stagnant conditions. In shallower sites of the outer shelf, genera of nileids in the carbonate facies and raphiophorids in the clastic facies were relatively abundant, but some endemic forms, such as the asaphid Birmanites and remopleuridid Hexacopyge, became dominant during the late Llanvirn-early Caradoc. Rorringtoniids are mainly associated with deeper outer-shelf facies. Panderia is common in an early Llanvirn nileid fauna at Yichongqiao, Cili area, and may well indicate development of shelf carbonate buildups. A transgressive event is suggested by the sudden appearance of rich Cyclopyge in the early Caradoc at Jiuxi, Changde.

EVOLUTION OF TRILOBITE BIOFACIES IN CAMBRIAN BASINS OF THE SIBERIAN PLATFORM

Cambrian biotic zonation on the Siberian Platform reflects differentiation of the depositional environments (inner shelf, outer shelf and open basin). The combination of the chart of trilobite biofacies replacement and the curve of sea-level fluctuations shows that trilobite biofacies replacement occurs as a rule at times of sign reversal and distinct change in the rates of sea-level rise or fall. The boundaries of major Siberian platform Cambrian chronostratigraphic units, such a stages and series, frequently coincide with the boundaries of biofacies in stratigraphic succession related to sea-level fluctuations. If these fluctuations are gradual and restricted, then the boundaries of the Cambrian stages and series cannot be isochronous levels at a global scale. The known levels for intercontinental correlation on the Siberian Platform include boundaries of the adjacent Triplagnostus gibbus and Tomagnostus fissus Zones from the uppermost Amganian Stage (Middle Cambrian) and the Glyptagnostus stolidotus and Glyptagnostus reticulatus Zones of the lower Upper Cambrian. Both levels correspond to boundaries between highstands and lowstands on the Siberian Platform and appear to serve as boundaries of high rank. Evolution of the trilobite biofacies zonation is illustrated by genera typical for each of the various Cambrian paleogeographic environments on the Siberian Platform.

Evolution of trilobite biofacies in Cambrian basins of the Siberian Platform

Cambrian biotic zonation on the Siberian Platform reflects differentiation of the depositional environments (inner shelf, outer shelf and open basin). The combination of the chart of trilobite biofacies replacement and the curve of sea-level fluctuations shows that trilobite biofacies replacement occurs as a rule at times of sign reversal and distinct change in the rates of sea-level rise or fall. The boundaries of major Siberian platform Cambrian chronostratigraphic units, such a stages and series, frequently coincide with the boundaries of biofacies in stratigraphic succession related to sea-level fluctuations. If these fluctuations are gradual and restricted, then the boundaries of the Cambrian stages and series cannot be isochronous levels at a global scale. The known levels for intercontinental correlation on the Siberian Platform include boundaries of the adjacent Triplagnostus gibbus and Tomagnostus fissus Zones from the uppermost Amganian Stage (Middle Cambrian) and the Glyptagnostus stolidotus and Glyptagnostus reticulatus Zones of the lower Upper Cambrian. Both levels correspond to boundaries between highstands and lowstands on the Siberian Platform and appear to serve as boundaries of high rank. Evolution of the trilobite biofacies zonation is illustrated by genera typical for each of the various Cambrian paleogeographic environments on the Siberian Platform.

Le modèle des biofaciès à Trilobites: un test dans l'Ordovicien inférieur de l'Anti-Atlas, Maroc

The faunal analysis of the Upper Fezouata Shales Formation (Arenig, central Anti-Atlas, Morocco) shows five distinct associations, one of graptolites and four of trilobites, and an environment is proposed for each one. The results obtained from the sedimentary facies confirm this interpretation and allow the environment of the trilobite biofacies and transport phenomena on a storm-dominated marine platform to be defined.

Temporal persistence and stability of Cambrian biofacies: Sunwaptan (Upper Cambrian) trilobite faunas of North America

Abstract During the Upper Cambrian Sunwaptan Stage (= “Ptychaspid Biomere”), trilobite faunas diversified following a mass extinction at the base of the unit. During and immediately after the extinction interval, a small number of low diversity trilobite biofacies were spread widely over the shelf. Turnover at all taxonomic levels was brisk and, in all facies, there was a sequence of rapid biofacies replacements. By the end of the Sunwaptan, a well-developed spectrum of environmentally-circumscribed trilobite biofacies was developed in carbonate depositional systems and tracking of lithofacies by biofacies can be demonstrated. However, rapid turnover still took place at the species level, so persistence and stability of biofacies was expressed at the clade level. A preliminary assessment of other Sunwaptan faunal elements, including articulate and inarticulate brachiopods, suggests that they were characterized by more persistent, stable assemblages, even at the species level. The pattern of stability of Cambrian trilobite biofacies stands in marked contrast to the species level, coordinated stasis documented for the articulate brachiopod-rich biofacies of the Devonian of New York State. The difference in behavior of these biofacies may simply be an expression of differences in the extinction and speciation rates of component taxa. Biofacies assembled from volatile taxa, such as Cambrian trilobites, will inevitably display rapid turnover, whereas coordinated stasis is likely to be characteristic of less volatile groups, such as articulate brachiopods or bivalves. The extent of biofacies stability will therefore be controlled ultimately by factors influencing evolutionary rates, such as behavioral complexity, niche breadth, population size, and dispersal ability.

Conodonts, stratigraphy, and relative sea-level changes of the Tribes Hill Formation (Lower Ordovician, east-central New York)

Tremadocian onlap is recorded by the Tribes Hill Formation. The formation is a lower Lower Ordovician (upper conodont Fauna B Interval(?)-Rossodus manitouensisZone) depositional sequence that unconformably overlies the Upper Cambrian Little Falls Formation.Depositional environments and stratigraphy indicate that the Tribes Hill was deposited on a wave-, not tide-, dominated shelf and that a uniform, “layer-cake” stratigraphy is present. The deepening-shoaling sequence of the Tribes Hill includes the: 1) Sprakers Member (new; peritidal carbonate and overlying tempestite limestone and shale); 2) Van Wie Member (new; subtidal shale and limestone); 3) Wolf Hollow Member (revised; massive carbonates with thrombolitic cap); and 4) Canyon Road Member (new; glauconitic limestone and overlying evaporitic dolostone). The shoaling half-cycle of the Tribes Hill is older than a shoaling event in western Newfoundland, and suggests epeirogenic factors in earliest Ordovician sea-level change in east Laurentia. Conodont and trilobite biofacies track lithofacies, andRossodus manitouensisZone conodonts andBellefontiaBiofacies trilobites appear in the distal, middle Tribes Hill Formation.Twenty-four conodont species are illustrated.Ansella?protoserratanew species,Iapetognathus sprakersinew species,Leukorhinion ambonodesnew genus and species, andLaurentoscandodusnew genus are described.

Mid-Cambrian (Marjuman) trilobites from the Pika Formation, southern Canadian Rocky Mountains, Alberta

The carbonates and fine siliciclastics of the Mid-Cambrian (Marjuman) Pika Formation in the southern Canadian Rocky Mountains provide an important record of inner-shelf trilobite biofacies. The faunas are characterized by low diversity and, in contrast to the more diverse open-shelf faunas of other parts of North America, agnostid trilobites are absent. A new biostratigraphic framework of two zones and two informal faunas is proposed for use in inner-shelf settings. Twenty-seven collections from three sections yielded at least eight genera and thirteen species; Marjumia bagginsi is a new species.

Review of trilobite biofacies distributions at the Cambrian–Ordovician Boundary

AbstractDetailed analysis of trilobite assemblages at the Cambrian–Tremadoc transition reveals the presence of four, possibly five, biogeographic provinces: Baltic, North China, Southeast China, North American, and possibly Siberian. One or more discrete biofacies characterize the transition in each province. When plotted palaeogeographically these appear to be largely temperature controlled. Interdigitation of provincialized trilobite biofacies is recognized by the representation of dominant types of trilobite, olenaceans, ceratopygaceans, remopleuridaceans, dikelocephalaceans and leio-stegiaceans in particular, on key boundary sections. These sections, at Nochixtlán, Mexico; Batyrbay, southern Kazakhstan; Broken Skull River, District of Mackenzie, Canada; and Jiangshan–Chang-shan, western Zhejiang, China, permit the correlation of biogeographic provinces globally. As a result of the application of trilobite biostratigraphy relatively few problems prevent the maintenance of a Cambrian–Ordovician Boundary stratotype section in its traditional Baltic biogeographic setting.