Shunxin Zhang

Late Ordovician–Early Silurian (Ashgillian–Llandovery) sea level curve derived from conodont community analysis, Anticosti Island, Québec

Abstract Earlier workers have established sea level curves for the Ashgillian–Llandovery sequence on Anticosti Island, Quebec, Canada, based on sedimentology and on brachiopod and reef communities. The complete Anticosti sequence has been collected for conodonts to provide a database of over 77 600 conodonts derived from 272 samples through the 800–1100 m sequence. This paper uses a statistical analysis of the database to establish the pattern of conodont communities and their interpreted water depths. This allows the establishment of a eustatic sea level curve for the sequence. This curve is in general agreement with those of the earlier workers, but is more detailed and better constrained stratigraphically given the large sampling program. The curve reflects more oscillating sea level changes for the whole sequence than those constructed earlier, and different water depths for certain intervals from those established earlier. The curve is valuable given that the Anticosti sequence is one of the most complete in the world for this interval, and its divergence from global eustatic curves for this interval is noted.

A NEW LLANDOVERY (EARLY SILURIAN) CONODONT BIOZONATION AND CONODONTS FROM THE BECSCIE, MERRIMACK, AND GUN RIVER FORMATIONS, ANTICOSTI ISLAND, QUÉBEC

Abstract An abundant conodont fauna was recovered from 202 samples taken from the Becscie, Merrimack, and Gun River formations of early Llandovery (Early Silurian) age on Anticosti Island, Québec. From the collection of 22,409 identifiable specimens, 28 species and four associations representing 12 genera and one group of drepanodontiform elements are documented and their stratigraphic ranges established. One new genus, Rexroadus, is established for two species earlier assigned with question to Oulodus. New species described are Icriodella dicrana, Oulodus sigmoideus, Ozarkodina strena, and Pterospathodus? originalis. The fauna is dominated by species of Ozarkodina and Oulodus; many species have a first appearance that is slightly older than elsewhere suggesting that the Anticosti Basin may have been an important evolutionary radiative center in the early Silurian. There is a slow and progressive diversity increase in conodonts through this interval after the initial appearance of many new taxa in the basal Silurian following the mass extinction in the terminal Ordovician. The Anticosti Llandovery sequence represents one of the few highly fossiliferous, continuous successions in the world and a new conodont biozonation based on conodonts for the Llandovery is proposed with the following interval zones in ascending order: Ozarkodina hassi (new), Ozarkodina strena (new) [subdivided into Oulodus jeannae (new) and Oulodus panuarensis (new) subzones], Ozarkodina clavula (new), Ozarkodina aldridgei (new), Pterospathodus celloni, and Pterospathodus amorphognathoides zones.

LATE ORDOVICIAN TO EARLY SILURIAN CONODONT FAUNAS FROM THE KOLYMA TERRANE, OMULEV MOUNTAINS, NORTHEAST RUSSIA, AND THEIR PALEOBIOGEOGRAPHIC AFFINITY

Abstract The Late Ordovician and Early Silurian conodont faunas of northeast Russia are described, based on 39 conodont-bearing samples collected from Kanyon, Padun, Tirekhtyakh, Maut, and lower Sandugan Formations at Mirny Creek and Ina River sections, Omulev Mountains. The faunas are represented by a total of 776 identifiable conodont specimens assigned to 35 species representing 25 genera, among which a new species, Periodon mirnyensis, is established. Based on graptolite zones at the same locality and other conodont faunas worldwide, the faunas are assigned to the Caradocian, early Ashgillian, late Ashigillian, early Llandovery, and late Llandovery. The Ordovician faunas belong to outer platform to upper slope facies with a North Atlantic Realm affinity; the Silurian faunas are from slope and basin facies. The conodont faunas are the first to be described from the Kolyma Terrane, which has an enigmatic paleogeographic position during the Early Paleozoic.

Paleoecology of Llandovery conodonts, Anticosti Island, Québec

Abstract Anticosti Island, Quebec, has perhaps the most fossiliferous and complete Llandovery (Early Silurian) sequence in the world. The undeformed limestones are well exposed and yield extremely well preserved conodonts whose taxonomy and biostratigraphy has been studied in detail. In addition to documenting the general temporal and spatial distribution of Llandovery conodonts, a set of 107 conodont samples was taken from four cliff sections in the lower and upper Becscie, lower Gun River, and upper Jupiter formations. These samples were used to examine the relationship of conodont elements and communities with microfacies, and explore their change through time. Over 9300 conodonts elements were identified and subjected to cluster analysis. The sections comprise mainly the background sedimentation of lime mudstones, deposited in 30–100 m of water on a gentle ramp. These mudstones are variably affected by interbeds or channel fill deposits interpreted as tempestites. A pattern of evolving conodont communities is documented for the lime mudstone facies through time. Different communities are found in the tempestite deposits that do not suggest a reworking of the mudstones, but rather the importation of deeper offshore faunas during periods of storm surge onto the ramp. The tempestites contain the highest species diversity and show no evidence of reduced diversity through post-mortem selective sorting by the storms.

Late Cambrian and Early Ordovician conodont communities from platform and slope facies, western Newfoundland: a statistical approach

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.

Conodont bio-events, cladistics and response to glacio-eustasy, Ordovician-Silurian boundary through Llandovery, Anticosti Basin, Québec

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.

ANTICOSTIODUS,A NEW MULTIELEMENT CONODONT GENUS FROM THE LOWER SILURIAN, ANTICOSTI ISLAND, QUEBEC

Abstract The multielement conodont genus, Anticostiodus new genus, is described based on discrete elements from the Lower Silurian (early Aeronian) Gun River Formation of Anticosti Island, Quebec. The apparatus includes pectiniform Pa, Pb, Pc and Pd elements and ramiform M, Sa, Sb and Sc elements. Two new species are included under the new genus, Anticostiodus fahraeusi (type species) and Anticostiodus boltoni. Both species occur near the base of Distomodus staurognathoides Zone and in an open subtidal environment.