Jonathan M. Adrain

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

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.

TRILOBITE ALPHA DIVERSITY AND THE REORGANIZATION OF ORDOVICIAN BENTHIC MARINE COMMUNITIES

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.

Silurian trilobite alpha diversity and the end-Ordovician mass extinction

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.

The stability of thoracic segmentation in trilobites: a case study in developmental and ecological constraints.

The decline in origination rate of new metazoan body plans following the Cambrian radiation has been suggested to reflect developmental canalization in derived taxa, limiting their ability to evolve forms with radically different morphotypes. Segmentation is a fundamental aspect of arthropod body plan, and here we show that a derived trilobite that secondarily converged on a morphotype characteristic of basal members of the clade also reverted to a pattern of segmental variability common among basal trilobites. Hence a secular trend in loss of variability of the trilobite thorax was not due to the evolution of an inviolable developmental constraint. This result challenges the notion of developmental canalization in phylogenetically derived taxa. Rather, early variability in trilobites may be the result of ecological factors that promoted segment‐rich thoracic morphotypes during Cambrian time.

Persistent oceanic anoxia and elevated extinction rates separate the Cambrian and Ordovician radiations

Recurrent mass extinction events (at “biomere”—a biostratigraphic unit—boundaries) characterize the middle Cambrian to Early Ordovician (Tremadocian) time interval that is between the major Cambrian and Ordovician radiations of animal life. A role for anoxia in maintaining elevated extinction rates in the late Cambrian has been proposed based on coincidence of an extinction with positive excursions in δ13Ccarb and δ34SCAS (CAS—carbonate-associated sulfate). Here we examine an Early Ordovician extinction event at the base of the North American Stairsian Stage (upper Tremadocian), and demonstrate concurrent onset of positive excursions in δ13C and δ34S inferred to reflect enhanced organic matter burial under anoxic waters. Sea-level rise may have brought anoxic waters onto the shelf to initiate extinctions. The evidence for δ13C excursions and elevated extinction rates appears to wane in the Tremadocian, consistent with progressive oxygenation of the oceans reaching a threshold that helped facilitate initial stages of the Great Ordovician Biodiversification Event.

Early Devonian cephalaspids (Vertebrata: Osteostraci: Cornuata) from the southern MacKenzie Mountains, N.W.T., Canada

ABSTRACT A diverse and well-preserved vertebrate fauna occurs in Early Devonian rocks of the Delorme Formation in the southern Mackenzie Mountains, Northwest Territories, Canada. The osteostracans of this fauna include the new cornuate species Waengsjoeaspis nahanniensis and Diademaspis? mackenziensis. Specimens of the latter species had been described in 1976 by Dineley and Loeffler as “Cephalaspis sp. indet.” The cornuate originally described as “?Cephalaspis gabrielsei” Dineley and Loeffler, 1976 is here assigned to the new genus Superciliaspis and more fully described. Other taxa include a possible example of Parameteoraspis sp. and several other indeterminate cornuates. Only dermal bone has been preserved, but preservation of its external and internal surfaces is excellent. Conspecific osteostracan specimens demonstrate a wide variety of overall proportions, due to differing amounts and directions of compaction. Some preferential orientation of corpses on bedding planes has been seen and several exam...

Sampling at the species level: Impact of spatial biases on diversity gradients

Sample bias is a fundamental issue in analyses of diversity. The adequacy of the fossil record continues to be questioned, especially at fine taxonomic and spatial scales. Here we evaluate the impact of variability in sampling intensity on diversity patterns of Late Cambrian trilobite faunas of Laurentian North America across a spectrum of five shelf environments. The data set consists of nearly 2000 samples from the published literature and 55 field collections that provide an independent estimate of diversity in each environment. Collections from the literature are distributed unevenly among environmental groupings; shallow subtidal carbonates account for almost half of the total. However, despite the strong sampling bias, raw counts of species from the literature reproduce the general shape of the diversity gradient established from field collections, including low species richness in nearshore environments and peak diversity in carbonate buildups. Rarefaction of species records confirmed the overall shape of the gradient, although the rank order of some facies was obscured by sampling problems. The results suggest that the adequacy of the published fossil record depends upon the level of analysis. Gross diversity patterns retrieved in this study appear to be robust, but resolution of fine detail is influenced by sampling issues.

Aulacopleurine trilobites from the Llandovery of northwestern Canada

Aulacopleurine trilobites from Llandovery strata of the Whittaker Formation in the central Mackenzie Mountains, Northwest Territories, Canada, include Au/acop/eura andersoni n. sp., Au/acop/eura? ranfordi n. sp., and Songkania smilhi n. sp. The subfamily Aulacopleurinae consists of only the genera Au/acop/eura Hawle and Corda, 1847, and Songkania Chang, 1974, characterized by smooth, broad early growth stages and holaspides having expanded frontal and genal areas, covered with dense caecal pits, but usually very subdued tuberculate sculpture. The relationship of Au/acop/eura? ranfordi and A.? reedi Pnbyl, 1947, to the remainder of species assigned to the genus is ambiguous.

The lichid trilobite Borealarges n. gen., with species from the Silurian of Arctic Canada

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NEW EARLIEST ORDOVICIAN TRILOBITE GENUS MILLARDICURUS: THE OLDEST KNOWN HYSTRICURID

Abstract Hystricurus millardensis Hintze, 1953 has been recorded widely in Laurentia. Revision on the basis of abundant new silicified topotype material indicates that most of these records are of morphologically distinct, though related, taxa and that a plexus of related species is involved. The new genus Millardicurus, with H. millardensis as type, is proposed for this clade, which also includes two well-documented new species, M. housensis and M. paramillardensis, from the House Formation of western Utah, several named species from Greenland, and several unnamed new species from various parts of Laurentia that have in the past been referred to M. millardensis. Millardicurus is not apparently closely related to younger Skullrockian–Stairsian hystricurids from Laurentia, but may be related to poorly known coeval Siberian Platform species classified in the genus Nyaya Rozova, 1963. Landmark-based geometric morphometric methods comprising principal component analysis of partial warp scores and Goodalls F test of pairwise means indicate significant shape differences among cranidia, librigenae, and pygidia of the House Formation species of Millardicurus.