Abstract Twelve conodont communities and assemblages in the Cambrian-Ordovician interval of western Newfoundland are recognized by four cluster analyses, which include 18 468 identifiable conodont specimens recovered from 230 conodont-bearing samples from four stratigraphic sections. Each section represents a different facies: platform, upper proximal slope, lower proximal slope, and distal slope. The 12 conodont communities and assemblages were differentiated into pelagic and nektobenthic communities as well as both the assemblages with a mixture of pelagic and nektobenthic communities and transported assemblage. The distribution of conodont communities along the platform-to-slope environmental gradient shows some gradational relationships. Relative sea-level curves derived in part from the distribution pattern of conodont communities reveal a major sea-level drop in the latest Cordylodus proavus Zone time, and two major sea-level rises in the early Iapetognathus fluctivagus Zone time and early Cordylodus angulatus Zone time, respectively.

Abstract Conodont diversity was severely reduced during the terminal Ordovician mass extinction. Few species, mainly coniform taxa, survived across the Ordovician-Silurian boundary. The nature and timing of the post-extinction recovery and diversification have previously been difficult to assess, due to limited knowledge and preservation of the earliest Silurian faunas. Recent documentation of early Llandovery conodont faunas from a complete stratigraphic succession on Anticosti Island, Québec, along with the data re-examined from earlier detailed studies, provides the basis for a new analysis. Five evolutionary cycles are recognized through the Llandovery, together with a distinct set of bio-events that are interpreted as immigration, emigration, origination, and extinction events through consideration of global occurrence data for Llandovery conodonts. These events are supported by the detailed sampling and stratigraphic range data, as well as the first cladistic analysis of the four key genera: Oulodus, Ozarkodina, Pterospathodus , and Rexroadus . The Anticosti Basin may have been an important centre of evolutionary radiation, given that several genera and species have first appearances in this region and that initial evolutionary lineages can be established for many taxa. Many faunal variations appear to be correlated to eustatic events as well as the changing ocean-climate state through this boundary interval.

Abstract Conodonts, the tiny, phosphatic, toothlike remains of an extinct group of early vertebrates, are the most important fossil group for biostratigraphy throughout their stratigraphic range from Late Cambrian to Late Triassic. The monograph presents the results of a significant project in remote regions of northeastern British Columbia. It extends the knowledge of the stratigraphic framework and conodont faunas into a region where information of this kind is largely unknown. Complete stratigraphic sections exposed in the high alpine of the Northern Rocky Mountains allowed examination of strata across a platform-shelfbreak-basin transect. The conodont faunas from the Kechika Formation, Skoki Formation, and Road River Group are described from an extensive collection of nine stratigraphic sections (over 9000 m measured) that yielded abundant conodont elements (38 600 total). This monograph represents a benchmark study of these important zonal fossils. The detailed paleontological work not only provides a taxonomic basis for future studies on early Paleozoic conodonts but also focuses on the evolution of conodonts in the early Ordovician, a time of extraordinary adaptive radiation. The taxonomic work provides detailed descriptions and illustrations of 185 species representing 69 genera. Seven new genera and 39 new species are described. The high diversity of taxa across the platform-to-basin transect shows the biogeographic differentiation and spatial ecological partitioning of conodonts through time. The taxonomy permits the refinement to the biostratigraphic zonation within two faunal realms for British Columbia that can be correlated with schemes elsewhere in North America and also internationally.

Abstract The northwestern margin of the northern Appalachian Orogen and adjacent craton is a Lower Paleozoic, low-latitude carbonate platform that originally lay along the northern margin of the Iapetus Ocean. Parts of the platform interior are now exposed in Quebec, but much lies beneath the Gulf of St. Lawrence. Outer-shelf and deep-water deposits crop out in western Newfoundland. The shelf break and upper slope are nowhere exposed, but their nature has been determined from numerous clasts in sediment gravity flows redeposited on the lower slope and now stacked in allochthonous thrust complexes. The relatively thin part of the platform (approximately 2 km) preserved in western Newfoundland lay well inboard of the shelf edge and accumulated on rigid crust that underwent relatively little thermal subsidence. Four separate phases of platform evolution can be differentiated, reflecting the interplay between tectonics, eustasy, and the evolving Lower Paleozoic biota. Phase 1: preplatform shelf, reflects initial siliciclastic deposition and volcanism on rifted crystalline basement, followed by a short period of carbonate sedimentation with archaeocyathan buildups and ooid-sand shoals that was terminated by offlap of thick quartz arenites. Phase 2: narrow, high-energy platform, is characterized by mixed siliciclastic-carbonate, non-bioclastic, peritidal sedimentation throughout and is locally manifest as three Grand Cycles. Contemporaneous deep-water sediment composes basal welded conglomerate overlain by quartzose carbonate turbidites. Phase 3: wide, low-energy platform, is an onlap package of muddy, bioclastic, subtidal and peritidal carbonates arranged in the form of two unconformity-bounded sequences. The adjacent deep-water slope was a narrow belt of debris flows and a wide apron of carbonate and shale, deposited in the lower part of an oxygen-minimum zone. Phase 4: foundered platform, documents the initial uplift, faulting, subsidence, and fragmentation of the platform in a sequence of peritidal to subtidal to deep-water carbonates, reflecting the initial stages of the Taconic Orogeny. The entire sequence is buried by synorgenic flysch and overlain by thrust sheets containing deep-water sediments of phases 1, 2, and 3, as well as oceanic lithosphere and other exotic assemblages. The main control governing platform initiation, thickness, and demise was tectonics. Internal stratigraphy, arrangement of shallow-water facies, and style of deep-water sedimentation were governed by eustasy with a local tectonic overprint. Climate was temperate tropical. The sediments themselves, dominated by periods of bioclastic versus non-bioclastic (ooids, peloids) sedimentation, reflects temporal changes in the early Paleozoic global biosphere.

The ultrastructure of 24 form-species of Ordovician conodonts has been examined with the scanning electron microscope. The conodonts represent two provincial and three subprovincial faunas and include hyaline forms, neurodonts (a subgroup of the hyalines), and cancellate forms (conodonts with white matter). The robust neurodont elements are constructed of cone-in-cone lamellae separated by distinct interlamellar spaces. There is limited fusion between adjacent lamellae and between the long, needle-like crystallites within each lamella. Crystallites become granular in form near the base in some neurodonts. A sheet-like septum bisects the elements longitudinally, but does not pass through the central growth canal. The latter is surrounded by fused lamellae producing a strengthened wall. Minute spheres occur along crystallites in some neurodonts. In the non-neurodont hyaline conodonts, greater fusion occurs between lamellae and between crystallites, and white matter may develop along the central growth canal; no septum or spheres have been noted within this group. Increased fusion is considered to have produced stronger elements capable of acquiring lateral compression, costae, and keels. Cancellate conodonts develop white matter primarily in their cusps and denticles. White matter is finely crystalline with no lamellar structure, but with abundant circular and linear voids. There is no pattern to the occurrence of the holes, but most linear voids are oriented transversely. White matter is formed by secondary transformation from lamellar material; this change is reflected in a transitional zone of incipient white matter, where a reorientation of the hard tissue is evident. Minute spheres occur in white matter; the central growth canal is destroyed and does not penetrate beyond the transition zone. White matter is believed to provide extra strength to the element allowing marked lateral compression and sharp margins. Although separated on structural criteria, these three conodont groups appear to occupy distinct and evolving ecosystems during the Ordovician. Three theories are proposed which, individually or in combination, may indicate the functional advantages conveyed by the development of white matter: (1) that the factors of element weight and phosphate availability were important, (2) that greater strength to cope with variable stresses was achieved, and (3) that white matter was induced by a disruption in vascular supply, possibly resulting from a partial eruption through the secreting tissues.

Distribution of Middle and Upper Ordovician conodonts in North America is marked by provincialism at many levels. Thus, although zonal sequences can be established in several regions, none is completely continental in scope. Conodonts in the eastern Appalachians are closely related to those of northern Europe and both are discussed by Bergström in another report in this volume. Synthesis of regional sequences west of the Appalachians permits recognition of 12 distinct faunas, composed largely of species known best or exclusively from North America. The lower four, in the Whiterock Stage, have been reported only from Cordilleran, Marathon, and Arbuckle sections. Faunas 5 and 6 are represented in Cordilleran, Midcontinent, and southern Appalachian sections, and six ranges to the top of the Chazy in Quebec; 7 through 9, in the Black River-Trenton, are best known in the eastern United States and the Midcontinent, but are also represented in the Cordilleran region. The distribution of faunas 10 through 12, in the Cincinnatian Series, has been most thoroughly established in the eastern Midcontinent, but representatives of these faunas are known from Hudson Bay to Texas and Nevada. As yet, this sequence of 12 faunas cannot be correlated very closely with that of the Anglo-Scandinavian-Appalachian Province.