Arthur J. Boucot

Glacio-Eustatic Control of Late Ordovician–Early Silurian Platform Sedimentation and Faunal Changes

Late Ordovician–Early Silurian stratigraphic sequences across the worlds platforms bear evidence for onlapping relations in the early part of the Silurian. In general, Late Ordovician marine benthic faunal communities and faunas found in platform rocks differ from those characteristic of the early part of the Silurian. Planktonic graptolites are also conspicuously different in Early Silurian strata from those in Late Ordovician strata. The impressive evidence for Late Ordovician–earliest Silurian continental glaciation in Africa and significant portions of South America and evidence for only restricted orogenic activity in the Late Ordovician-Early Silurian interval suggests that glaciation was the principal agent behind the changes seen among faunas as well as in the stratigraphic record. Shallowing of marine waters across the Late Ordovician platforms was probably related to “locking up” oceanic waters in the glaciers during glaciation, and onlapping relations took place as the glaciers melted and sea level rose. Restriction of platform marine environments and shallowing as well as cooling of the oceans were primary environmental factors behind the changes in marine faunas. Llandovery deglaciation coincides with a time interval during which animal communities were widely spread areally.

Caradocian land plant microfossils from Libya

The oldest paleontologically well-dated spore tetrads and cuti-clelike sheets of cells have been recovered from beds of Caradoc (mid-Ordovician) age from boreholes in the Murzuk (Murzuq) and Rhadames (Ghadāmis) Basins of western Libya. These microfossils together with late Llandovery–early Wenlock vascular plant megafossils (lycophytes and Psilophyton ) known from Libya lead us to conclude that land plants, including vascular plants, probably had a long pre-Silurian record, extending at least into the basal Caradocian. Additonal evidence suggests that this record does not extend as far back as the earlier Late Cambrian (Dresbachian).

Facies patterns and geography of the Yangtze region, South China, through the Ordovician and Silurian transition

Abstract Lithofacies and biofacies changes through the Ordovician and Silurian transition on the Yangtze platform are coincident with stepwise mass extinctions, recovery events, global sea-level changes, and regional palaeogeographic configurations. Facies patterns through three intervals, the late-mid Ashgill, Hirnantian, and early Rhuddanian, indicate that black shale occupied most of the Yangtze platform region during the late–mid Ashgill and early Rhuddanian, while the Hirnantian possesses more diverse facies types. The black shale was replaced by carbonate facies during the Hirnantian coincident with a global sea-level low stand. Biofacies changes from the late–mid Ashgill graptolitic to the Hirnantian shelly facies were apparently signalled by gradual and diachronous existence of a mixed facies, the Manosia facies, in the Yangtze region. However, the biofacies change was abrupt from the Hirnantian to the early Rhuddanian when graptolitic facies occupied most areas on the Yangtze platform leaving only a very narrow, limited belt of shelly facies along the southern margin of the platform in northern Guizhou. The early Rhuddanian unified biofacies pattern is a result of a global sea-level rise, which may be synchronized with a recovery bioevent of graptolites after Hirnantian mass extinction.

A critique of Phanerozoic climatic models involving changes in the CO2 content of the atmosphere

Abstract Critical consideration of varied Phanerozoic climatic models, and comparison of them against Phanerozoic global climatic gradients revealed by a compilation of Cambrian through Miocene climatically sensitive sediments (evaporites, coals, tillites, lateritic soils, bauxites, calcretes, etc.) suggests that the previously postulated climatic models do not satisfactorily account for the geological information. Nor do many climatic conclusions based on botanical data stand up very well when examined critically. Although this account does not deal directly with global biogeographic information, another powerful source of climatic information, we have tried to incorporate such data into our thinking wherever possible, particularly in the earlier Paleozoic. In view of the excellent correlation between CO 2 present in Antarctic ice cores, going back some hundreds of thousands of years, and global climatic gradient, one wonders whether or not the commonly postulated Phanerozoic connection between atmospheric CO 2 and global climatic gradient is more coincidence than cause and effect. Many models have been proposed that attempt to determine atmospheric composition and global temperature through geological time, particularly for the Phanerozoic or significant portions of it. Many models assume a positive correlation between atmospheric CO 2 and surface temperature, thus viewing changes in atmospheric CO 2 as playing the critical role in regulating climate/temperature, but none agree on the levels of atmospheric CO 2 through time. Prior to the relatively recent interval of time in which atmospheric CO 2 is directly measurable, a variety of biological and geological proxies have been proposed to correlate with atmospheric CO 2 level or with p CO 2 /temperature. Atmospheric models may be constructed for the Pre-Cenozoic but the difficulties of assessing variables in their construction are many and complex. None of the modelers have gathered enough biological and geological data to impart reliability to the model constructs. Most modelers focus almost exclusively on one or a few variables as proxy to measure atmospheric CO 2 , nor consider the many other variables involved, nor agree on what these variables are or how to measure them. In this paper, it is the reliability of the present data bases used in these atmospheric models that we wish to consider. We focus most attention on the Berner models, such as GEOCARB I, II and BLAG, because of the basic role they attribute to tracheophytes in regulating atmospheric CO 2 and our own interest in pre-tracheophytic land plants and the atmospheric composition of the pre-tracheophytic Paleozoic. We survey the presence of symbiotic mycorrhizae and question the assumption that all tracheophytes are obligately associated with them. Although pre-tracheophytic embryophytes, cyanobacteria, and possibly other organisms preceded tracheophytes on land by millions of years, Berners models do not consider a significant role for them in affecting pre-Devonian climate/temperature and atmospheric composition. In effect, Berner assumes that pre-tracheophytic life inhabited a world governed largely by abiotic physical and chemical reactions. We consider uncertainties raised by minimizing possible roles for pre-tracheophytic oxygenic and heterotrophic microorganisms analogous to those speculated to be unique to tracheophytes both with regard to an active role in biodeterioration of rock and soil mineral substrates and in the sequestration of organic carbon. Additionally, Berner does not consider marine productivity, which might have been high in the Precambrian and Early Paleozoic and possibly consequent organic carbon sequestration, even in the possible absence of terrestrial organisms, or even in the absence of a significant preserved biomass of terrestrial and marine organisms. The important roles played by cyanobacteria, for example, are briefly reviewed by Giller and Malmqvist in lakes and rivers as regards both planktonic and benthic taxa, and it is not safe to assume that these organisms were absent or of no potential significance in the pre-embryophytic, i.e. earlier Ordovician and well back into the Precambrian. Berners models have met with a large measure of consensus about CO 2 balance during the Phanerozoic, about the role played by tracheophytes, and have been used to test or evaluate other data. After reviewing the biological and geological assumptions and estimates on which these Models are based, we conclude that they do not provide reliable information about atmospheric CO 2 composition through Phanerozoic time, particularly in the Early Phanerozoic. We compare many atmospheric CO 2 models, while considering the numerous proxies on which they are based and conclude that the competing models are inadequate for atmospheric CO 2 estimation. Many possibilities not considered in present models must either be included or eliminated based on reliable evidence. We suggest that assessing Phanerozoic climate/temperature based on the available geological/climatic proxies would appear to provide a more reliable method of estimating variations in CO 2 , and hence atmospheric CO 2 :O 2 balance, than most proxy constructs on which atmospheric models are presently based, because of the critical role postulated for atmospheric CO 2 in regulating Earths surface temperature. We present our own Phanerozoic climate estimate, based on readily available geological/climatic data, for comparison with postulated coeval atmospheric CO 2 levels as a test of the postulated correlation.

Early silurian spore tetrads from new york: earliest new world evidence for vascular plants?

Several taxa of abundant cutinized trilete spores from earliest Silurian shale in New York predate by almost an entire period vascular land plant megafossils. Paleoecological evidence suggests that these spores may represent vascular land or semiaquatic plants but a bryophytic origin cannot be precluded on the basis of spore characters. An algal origin is considered unlikely.

Silurian Trilete Spores and Spore Tetrads from Gotland: Their Implications for Land Plant Evolution

Land-plant type spores occur in argillaceous limestones and platform graptolitic mudstones of Wenlock-Ludlow age in Sweden. Analysis of the stratal sequence demonstrates that occurrence of abundant spores is a function of depositional ecology, particularly water depth and shoreline proximity. This ecological analysis and the seeming absence of any correlation between the first appearances of abundant spores and megafossils of land-plant type raise the possibility that land plants evolved far earlier than the megafossil record suggests.

Silurian Bio-Events

In the history of Silurian biota and ecosystem as a whole no “big” catastrophes occurred like the one at the Ordovician-Silurian boundary. Yet it was not a quiet period either. There were established 15 more or less remarkable bio-events, among others the most severe extinction of conodonts and acritarchs in the very beginning of the Wenlock (Ireviken Event), the Great Crisis or lundgreni Event among graptolites in the Homerian and the middle Ludfordian Event comprising many lineages of vertebrates, graptolites, conodonts and corals. Most remarkable diversity rises of the Silurian biota were in the late Rhuddanian, in the Telychian and in the early Gorstian. Both extinctions and originations were in good correlation with the global sea-level curve, but the effect has to be interpreted as an integrated process partly triggered by development of early Silurian glaciation and climate in general.

Palynological Evidence Bearing on the Ordovician-Silurian Paraconformity in Ohio

Organic microfossils (spore tetrads, acrit-archs, scolecodonts, chitinozoans) from six Late Ordovician through Early Silurian samples (Elkhorn Formation of the Richmond Group; lower and upper beds of Belfast Member of the Brassfield Formation) straddling the Ordovician-Silurian paraconformity provide evidence about water depth. The spore tetrads appear to have been derived from plants growing in environments no deeper than an intertidal, semiterrestrial or very shallow-marine if not a nonmarine position. Their concentration on both sides of the paraconformity indicates at least shallower depth conditions than are present in the overlying upper part of the Brassfield Formation with its Eocoelia and succeeding deeper water Pentamerus benthic marine life zone faunas. A progressive deepening of water in the Early Silurian away from the paraconformity is also indicated by the inverse relationship existing between abundant spores in the basal bed of the Belfast Member and abundant Acanthomorphitae acritarchs in the upper bed. The presence of Sphaeromorphitae acritarchs in all sixsamples indicates that those microplankton are restricted to very shallow water depths. Spores from the Elkhorn Formation may have been derived from plants living in Ordovician time or they have “leaked” into the Elkhorn from the Silurian. In the former event, these spores may represent the oldest vascular plant remains recognized in North America.

Pre-Hirnantian Ashgill climatically warm event in the Mediterranean region

Compilation of the marine, benthic megafossils from approximately the mid-Ashgill of the Mediterranean region, including much of Central and Southern Europe plus North Africa, and elsewhere indicates a warm interval featuring bioclastic limestone and a warm climate marine fauna. These mid-Ashgill faunas immediately precede the latest Ashgillian, Hirnantian, cool interval that featured widespread glaciation, and are underlain by typical, cold water, Mediterranean Realm, older Ordovician rocks and faunas. The cause or causes responsible for the brief warm interval are uncertain, but may have involved a warm water gateway that is geographically still not located. There is a possibility that South Africa was similarly affected by this roughly mid-Ashgillian marine situation. Early Paleozoic bauxite minerals and kaolins in northwestern Sudan and kaolins elsewhere in North Africa may represent the same time interval, which would suggest that there was a non-marine amelioration of the local climate as well as the marine effects.

The classification of athyridid brachiopods

Phylogenetic relationships among higher taxa of articulate brachiopods that developed a spiralium as mineralized support for the lophophore, and the resulting classifications, are not universally agreed upon due to the complex pattern of character evolution within the phylum. Our results, using phylogenetic methodology in conjunction with traditional methods, showed a more complete and easily testable picture of this evolution than either method used alone. A detailed working hypothesis of athyridide phylogeny and the supporting evidence on character distribution is given, and a new classification of the impunctate and punctate athyrids sensu lato is presented with diagnoses to subfamily. Agreement between the stratigraphical first appearance of athyridid subfamilies and their cladistic rank was quite good (except for the koninckinoids), suggesting that both outgroup and traditional paleontological methods indicate a similar direction of character polarity in the evolution of the group. The classification is proposed based on a mix of external and internal characters which persisted during unbroken lineages. New taxa are the families Parazygidae and Triathyrididae; the subfamilies Whitfieldellinae, Triathyridinae, Septathyridinae, Cleiothyridininae, Ochotathyridinae and Plectospirinae, and the genera Brimethyris, Carteridina, Tetraloba and Cooperispira. We studied the great morphological diversity among the athyridids. Their complete stratigraphical ranges help in building phyletic lineages.