Stephen R. Westrop

The Ecology of the Cambrian Radiation

A. Yu. Zhuravlev and R. Riding (Editors), 2000, Columbia University Press, New York, 576 p. (Hardcover

New uppermost Cambrian U–Pb date from Avalonian Wales and age of the Cambrian–Ordovician boundary

85.00) ISBN: 0-231-10612-2; (Softcover

TRILOBITE ALPHA DIVERSITY AND THE REORGANIZATION OF ORDOVICIAN BENTHIC MARINE COMMUNITIES

40.00) ISBN: 0-231-10612-0. The title of this book certainly grabs ones attention. The Cambrian Radiation was one of the pivotal points in the history of life, and the roles of intrinsic biological factors (e.g., Stanley, 1973) and extrinsic physical factors (e.g., Rhodes and Morse, 1971) have been debated for more than thirty years. Despite this long interest in the triggering mechanisms, surprisingly little has been published on the paleoecology of the radiation. This book is an important step forward. In three sections, twenty-one chapters by thirty-three authors work their way through the physical setting of the Cambrian world, community paleoecology, and the paleoecology of the major radiating groups of animals. Most of the chapters are review articles, and some have been published elsewhere. However, this is actually a strength of the book because it brings a great deal of information together under a single cover. One-third of the chapters are written by Russian paleontologists, and these add breadth to the book through their incorporation of research that is poorly known and seldom cited by western workers. The section on the physical environment starts with two chapters on Vendian-Ordovician global geography. …

Silurian trilobite alpha diversity and the end-Ordovician mass extinction

A crystal-rich volcaniclastic sandstone in the lower Peltura scarabaeoidesZone at Ogof-ddu near Criccieth, North Wales, yields a U-Pb zircon age of 491 ± 1 Ma. This late Late Cambrian date indicates a remarkably young age for the Cambrian-Ordovician boundary whose age must be less than 491 Ma. Hence the revised duration of the post-Placentian (trilobite-bearing) Cambrian indicates that local trilobite zonations allow a biostratigraphic resolution comparable to that provided by Ordovician graptolites and Mesozoic ammonites.

Terminal Cambrian and lowest Ordovician succession of Mexican West Gondwana: biotas and sequence stratigraphy of the Tiñu Formation

The Ordovician saw the transformation of marine benthic communities from the trilo- bite-based Cambrian Fauna to the brachiopod-dominated Paleozoic Fauna. An evaluation of the changing importance of trilobites during the Ordovician can be made from accurate assessments of taxonomic richness in various habitats. Here we present a new compilation of trilobite alpha diversity based on field collections and survey of the literature. The data indicate that trilobite spe- cies richness within nearshore, shallow subtidal, carbonate buildup and deep subtidal shelf envi- ronments was essentially constant between the Late Cambrian and the Late Ordovician. The alpha diversity patterns do not support the notion that trilobites became displaced from inner shelf en- vironments during the Ordovician. Rather, the data are consistent with a decline in relative im- portance of the group through dilution as newly radiating invertebrate groups entered Ordovician paleocommunities. They also imply that direct interactions between elements of the Cambrian and Paleozoic faunas were not involved in the Ordovician reorganization of paleocommunities. Like many other major faunal transitions during the Phanerozoic, the Ordovician radiations appear to have been essentially non-competitive in nature.

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

Abstract Following the end-Ordovician extinction, global clade diversity of Silurian trilobites dropped to about half of Ordovician levels. Although clade diversity failed to recover, this extinction had surprisingly little long-term impact on the number of trilobite species that occupied local habitats (alpha diversity). A new compilation of data from Laurentia and other continents indicates that Silurian trilobite alpha diversities in all major environments were comparable to those of the Late Cambrian and Ordovician; shallow subtidal diversity reached an all-time high during the Late Ordovician. The profound differences in patterns at local and global levels demonstrate the necessity for a hierarchical approach to analyses of diversity. Factors governing global clade diversity are lodged at hierarchical levels beyond those controlling local species richness and must be sought in studies of between-habitat (beta) or geographic (gamma) diversity.

UPPERMOST CAMBRIAN–LOWER ORDOVICIAN FAUNAS AND LAURENTIAN PLATFORM SEQUENCE STRATIGRAPHY, EASTERN NEW YORK AND VERMONT

The Tinu Formation of Oaxaca State is the only fossiliferous lower Palaeozoic unit between the Laurentian platform in northwest Mexico and Gondwanan successions in Andean South America. The Tinu traditionally has been referred to the Lower Ordovician (Tremadoc) and regarded as having a provincially mixed fauna with Laurentian, Avalonian, and Gondwanan elements. Bio- and lithostratigraphic re-evaluation demonstrates that the Tinu is a Gondwanan, passive margin succession. It includes a lower, thin (to 16 m), condensed, uppermost Cambrian Yudachica Member (new). The Yudachica nonconformably overlies middle Proterozoic basement as a result of very high late Late Cambrian eustatic levels. The Yudachica changes from storm-dominated, but slightly dysoxic, shelf facies (fossil hash limestone and shale) in the south to an upper slope facies with debris flows 50 km to the north. Three biostratigraphically distinct depositional sequences comprise the Yudachica. The Yudachica has Gondwanan-aspect trilobites with low-diversity conodonts characteristic of unrestricted marine/temperate facies. The upper Tinu, or Rio Salinas Member (new), is a Lower Ordovician (Tremadoc) depositional sequence that records strong early, but not earliest, Tremadoc eustatic rise marked by graptolite- and olenid-bearing dysoxic mudstones. Higher strata shoal upward into shell-hash limestones and proximal tempestite sandstones with upper lower Tremadocian unrestricted marine/temperate conodonts. New taxa include Orminskia rexroadae Landing gen. et sp. nov. from the Cordylodus andresi Zone; this euconodont is related to hyaline coniform genera best known from Ordovician tropical platform successions. Cornuodus ? clarkei Landing sp. nov. resembles the coeval, upper lower Tremadoc tropical species Scalpellodus longipinnatus (Ji & Barnes).

Declining importance of trilobites in Ordovician nearshore paleocommunities; dilution or displacement?

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.

Marjuman (Cambrian) agnostoid trilobites of the Cow Head Group, western Newfoundland

Abstract The Cambrian–Ordovician boundary is a type 1 depositional sequence boundary with dramatic local erosional incision in restricted marine facies on the easternmost New York Promontory. The systemic boundary is bracketed below by Late Cambrian, upper Cordylodus proavus Zone (s.s). conodonts from carbonates of the upper Little Falls Formation (=Whitehall Formation, abandoned). Presumed Lower Ordovician ellesmeraceratoid cephalopods from the upper Little Falls are uppermost Cambrian and among the oldest known in North America. The overlying deepening–shoaling cycle of the Tribes Hill Formation (=Cutting and Great Meadows Formations, abandoned) is the local expression of a lowermost Ordovician (Rossodus manitouensis Zone) depositional sequence recognizable across Laurentia. Complete replacement of conodonts takes place in the late Tremadocian or Tremadocian–Arenigian boundary interval with onlap of the “Fort Ann Formation” across the paleokarst cap of the Tribes Hill. The trilobites Hystricurus sp. and Symphysurina myopia Westrop new species occur in less restricted, thrombolitic facies of the middle Tribes Hill that have the highest conodont diversity. Ulrichodina Furnish, 1938, emend. is regarded as the senior synonym of the conodont Colaptoconus Kennedy, 1994 (=Glyptoconus Kennedy, 1980).

Taphonomy and Paleoecology of Ordovician Trilobite Clusters, Bromide Formation, South-central Oklahoma

Declining importance of trilobites was a key feature of Ordovician community evolution. Previous work has shown that replacement of trilobitedominated paleocommunities by molluscand brachiopod-rich paleocommunities was diachronous, and began in nearshore environments. However, the processes responsible for these changes remain unclear. New data from northern North America indicate that trilobite species diversity in nearshore settings maintained a constant, low level between the Late Cambrian (Marjuman) and Middle Ordovician (Whiterockian). As the total number of species of other groups, especially molluscs, increased in nearshore environments, the relative importance of trilobites in these paleocommunities declined through a process of dilution, rather than actual displacement. The apparent offshore retreat of trilobite-dominated paleocommunities is also at least partly a reflection of this dilution process: through OFFSHOREI the Cambrian and Ordovician, trilobite assemblages tended to be most speciose in offshore environments, so that the apparent rate of dilution proceeded more slowly than in the nearshore. Thus, trilobite-dominated paleocommunities maintained their integrity longer in the offshore, and this produced an overall pattern of diachronous replacement. Similarly, progressive offshore replacements of other community types in younger strata may also be dilution phenomena related to species diversity gradients.