Graham A. Young

Exceptionally preserved Late Ordovician biotas from Manitoba, Canada

There are few body fossil biotas known from early Paleozoic accretionary shorelines, and very few examples of Ordovician soft-bodied assemblages. This study documents two recently discovered biotas from separate sedimentary basins in Manitoba, Canada, that provide unique information about tropical shoreline communities shortly before the Late Ordovician extinction event. Each site represents a distinct depositional environment, but they share biotic elements, including eurypterids, xiphosurids, and large problematic tubes. The William Lake biota, representing more restricted conditions, includes jellyfish that are among the best hydromedusan body fossils known. Rocks at the Airport Cove site, deposited under more open circulation, contain scolecodonts and noncalcified algae. These biotas have some parallels with the recently described Middle Ordovician Winneshiek Lagerstatte, but are also similar to some Late Silurian assemblages. Considered together, early Paleozoic marginal marine deposits are a rich but as yet poorly known source of paleobiodiversity data.

THE WORLD'S BIGGEST TRILOBITE—ISOTELUS REX NEW SPECIES FROM THE UPPER ORDOVICIAN OF NORTHERN MANITOBA, CANADA

Abstract The largest known trilobite fossil, a virtually complete articulated dorsal shield of the asaphid Isotelus rex new species, has been recovered from Upper Ordovician (Cincinnatian, Richmondian) nearshore carbonates of the Churchill River Group in northern Manitoba. At over 700 mm in length, it is almost 70 percent longer than the largest previously documented complete trilobite, and provides the first unequivocal evidence of maximum trilobite length in excess of one-half metre. Comparisons with other fossil and extant members of the phylum suggest that in terms of maximum linear dimensions it was among the biggest arthropods ever to have lived. Sediments of the Churchill River Group were deposited in an equatorial epeiric setting and the extremely large size of I. rex n. sp. thus marks a striking example of low-latitude gigantism, in sharp contrast to the widespread phenomenon of “polar gigantism” in many modern marine benthic arthropods. Lack of extensive epibiontic colonization of the exoskeletal surface and the presence of large distinctive trace fossils in the same unit suggest that I. rex n. sp. may have been a semi-infaunal predator and scavenger that employed a shallow furrowing and probing mode of benthic feeding. The extinction of the isotelines (and virtually the entire asaphide lineage) at the end of the Ordovician cannot be related to the near contemporaneous achievement of exceptionally large adult size in some representatives. Failure to survive the terminal Ordovician extinction event was most likely a consequence of a pelagic larval life-style that proved ill-adapted to the rapid onset of global climatic cooling and loss of tropical shelf habitats.

Silurian Proporidae and Plasmoporidae (Anthozoa, Tabulata) from the Chaleurs Bay region, Canada

Six species belonging to the families Proporidae and Plasmoporidae occur in the Lower and Upper Silurian rocks of the Limestone Point and La Vieille Formations of northern New Brunswick and of the Anse a Pierre-Loiselle, La Vieille, and Gascons Formations of the Gaspe Peninsula of Quebec. The three species of Propora are widely distributed but show varying faunal affinities, while both species of Plasmopora, Plasmopora logani and Plasmopora corrugata , are new and are almost endemic. Revised concepts of Propora and Plasmopora are proposed. The holotype specimen for the type species of Camptolithus was examined and confirms the genus as distinct from Propora , rather than a synonym as has been previously suggested. The facies distribution of these corals is variable. In general, the proporids, which mostly occur in facies indicating shallow carbonate banks and patch reefs, are more restricted in distribution than the plasmoporids, which occur in these facies and also in others representing a variety of open-shelf environments.

Silurian Heliolitidae (Anthozoa, Tabulata) from the Chaleurs Bay region, Canada

The Early to Late Silurian sedimentary rocks of the Limestone Point and La Vieille Formations of northern New Brunswick and the Anse a Pierre-Loiselle, La Vieille, and Gascons Formations of the Gaspe Peninsula possess diverse and abundant tabulate coral faunas that include six species of Heliolitidae distributed among Heliolites and Stelliporella. Two species, Heliolites laxus and Heliolites distinctus , are new. A revised concept of the genus Heliolites is proposed. The Heliolitidae from this region show a substantial degree of endemism and are most similar to northern European faunas. The distribution of individual heliolitid species was under a high degree of facies control. The heliolitids are more restricted in distribution than are other groups of Tabulata in this region and are most abundant in open shelf and carbonate bank facies.

Relationships between internal and external morphology in Paleofavosites (Tabulata); the utility of growth and growth form

During growth of colonial corals, the basic organization of skeletal elements was determined by inherent factors, but arrangement of corallites within a colony could be affected if environmental change induced a modified growth form. Comparisons of internal and external characters during colony development indicate how environmental and genetic factors determined growth form. The results of these comparisons have implications for understanding of colony integration, functional morphology, and systematics. This study is based on serially sectioned coralla of the cerioid tabulate Paleofavosites subelongus, from the uppermost Ordovician to lowermost Silurian of the east-central United States. Colony growth from resulted from changes in maximum growth angle of marginal corallites, and in the shape of the growth surface. These features were coordinated with corallite characters and were dependent on variation in corallite growth. At the same time that a colony became broader by expanding its maximum growth angle and developing a taller growth surface, its corallites became larger, more new corallites were initiated, and recently initiated corallites expanded more rapidly. When a colony9s maximum growth angle was reduced and the growth surface became flatter, corallites also became smaller, fewer corallites were initiated, and those corallites that were recently initiated expanded slowly. Genetic constraint of growth is illustrated by consistent patterns of initial colony growth, and by relationships among characters of internal and external morphology. Frequent small-scale variations in growth angle and growth surface height: width during astogeny indicate fluctuating environmental factors. Sedimentation and subsidence of the colony were probably the major environmental controls on form.

Chapter 9 Coral biogeography in the Late Ordovician (Cincinnatian) of Laurentia

Abstract During the Late Ordovician, Cincinnatian, the epicontinental seas and continental margin of Laurentia provided habitats that were suitable for corals. Biogeographical differentiation occurred within this equatorially placed continent, when corals were introduced to areas that had fundamentally different environments. There were four biogeographical divisions, characterized by distinctive faunas that included some endemic taxa: the Red River–Stony Mountain Province, Richmond Province, Edgewood Province and the less well understood, informal ‘Continental Margin’ Area. In each division, the potential for diversification and the capacity for diversity were determined by factors such as the duration and size of the division, the amount of immigration, the extent of evolution and biogeographical differentiation, faunal responses to changes in sea-level and climate, and the complexity of the ecological structure. The development of multiple biogeographical divisions, each contributing to overall diversity, enhanced the ‘Great Ordovician Biodiversification Event’. During the latest Ordovician mass extinction, there was a reduction of diversity and loss of biogeographical divisions within Laurentia. The divisions were terminated when their characteristic taxa disappeared, in response to major environmental changes associated with glaciation in Gondwana and subsequent global warming.

ENIGMATIC FOSSIL ENCRUSTING AN UPPER ORDOVICIAN ROCKY SHORE ON HUDSON BAY, CANADA, IS A CORAL

An enigmatic encruster from the Upper Ordovician rocky shore exposed near Churchill, Manitoba, was recently described by Johnson et al. (1998). The specimen was found attached to a quartzite boulder enclosed in carbonate matrix. The matlike, calcareous fossil consists of densely packed, vertical, cylindrical columns with upward-radiating structures issuing from their centers. It was identified as a new genus and species, Storeacolumnella hudsonensis, of uncertain taxonomic affinity but with possible characteristics of sponges and calcareous green algae. In particular, the radiating structures within columns were compared with spicules of a sclerosponge and with the siphon system of siphonous green algae. We are engaged in a comprehensive study of corals from the locality at which the enigmatic encruster was obtained. Based on material prepared to date, and following an examination of the two thin sections illustrated by Johnson et al. (1998, fig. 3), we can demonstrate that their specimen is a heliolitid tabulate coral. It represents the genus Ellisites Dixon, Bolton, and Copper, 1986. Ellisites is peculiar in possessing features that resemble some heliolitid corals and some stromatoporoid sponges (Dixon et al., 1986). Its species have calcitic skeletons that combine, in various proportions, zones of contiguous vertical trabeculae and zones of horizontal vesicular plates. Trabeculae are pinnately fibrous, with fibers diverging …

Halysitid and auloporid tabulate corals from the Gascons and West Point Formations (Silurian), Gaspe, Quebec, Canada

The uppermost Lower Silurian and Upper Silurian Gascons and West Point Formations of the southern Gaspe Peninsula were deposited under a broad range of environmental conditions from deep offshore-shelf to reef facies. Halysitid and auloporid tabulate corals occur in a number of facies and show a high degree of endemism. Two species of Halysitidae and three species of Auloporida are found in these formations. Both halysitids have been previously described from this region. The auloporids include the new species Syringopora minuta and another species that may represent the first known Silurian occurrence of the fletcheriellid genus Pseudofletcheria . A neotype for the halysitid Cystihalysites amplitubulatus (Lambe) is proposed.

Late Ordovician Conulariids from Manitoba, Canada

Abstract Six species of conulariids, assigned to four genera, were recovered from the type locality of the Cat Head Member of the Red River Formation in southern Manitoba, Canada. These are middle Katian (Late Ordovician) in age. The most abundant conulariid species from this locality, Conularia porcella, is new, and is represented by 21 specimens. Additionally, 28 three-dimensionally preserved micromorphic conulariids, assigned to Eoconularia aff. loculata, were recovered using acetic acid preparation from limestone samples of late Katian (Late Ordovician) age. These samples had been collected from Churchill, northern Manitoba, by the Geological Survey of Canadas J. B. Tyrrell in 1894. These taxa are unusually abundant for conulariids, which are normally represented by only a few specimens from any given locality, and this abundance may be a reflection of the exceptional preservation at these two localities.