S. Henry Williams

The Pattern of Global Bio-Events During the Ordovician Period

The 70 Ma Ordovician Period is characterized by extensive epeiric seas, paleocontinent dispersal, intervals of intense volcanism and black shale deposition, a greenhouse climate state deteriorating to a brief icehouse state, strong faunal provincialism, and profound changes to the biota including the changeover from the Cambrian Fauna to the Paleozoic Fauna. Although many invertebrate phyla diversify during the Ordovician, precise biostratigraphic and global biogeographic data are provided best by conodonts, trilobites and graptolites. These three groups are used in this chapter to recognize five major bio-events four of which correspond closely to Series boundaries: Basal Tremadoc (BTc), Basal Arenig (B’Ag), Basal Llanvirn (B’Ln), Basal Caradoc (B’Cc) and Upper Ashgill (U’Al). Most of these correspond to significant eustatic events and the latter to the terminal Ordovician glaciation. The first four are each characterized by extinctions but these are overshadowed by a rapid innovation event with a radiation of a more diversified fauna; the U’Al is a severe extinction event, second only to the terminal Permian event in the entire Phanerozoic. Compared to many other Phanerozoic systems, the Ordovician is a period of considerable biologic, climatic and oceanographic complexity within which the balance between the forcing processes that produced the major and minor events is still not well understood.

Graptolites from the Lower–Middle Ordovician St. George and Table Head groups, western Newfoundland, and their correlation with trilobite, brachiopod, and conodont zones

Faunal assemblages of the autochthonous, shelf carbonate sequences belonging to the St. George and Table Head groups are dominated by shelly macrofossils and conodonts. Rare, usually monotypic graptolitic horizons enable correlation with the allochthonous Cow Head Group, which was deposited on the middle to lower slope, and the shelly, conodont, and graptolitic zonal schemes elsewhere in North America.The Catoche Formation of the St. George Group is of Ibexian (Canadian) age and yields graptolites indicative of the Tetragraptus approximatus and Tetragraptus akzharensis zones (early Arenig). The basal Aguathuna Formation belongs to the Pendeograptus fruticosus Zone, whereas higher parts span the Ibexian–Whiterock boundary. Graptolites suggest that the Table Head Group entirely postdates the Cow Head Group. This is supported by a Whiterock (early Llanvirn) trilobite, conodont, and brachiopod fauna.

New biostratigraphic information from the western part of the Hamburg klippe, Pennsylvania, and its significance for interpreting the depositional and tectonic history of the klippe

Biostratigraphic investigation of the graptolite and conodont faunas of the western part of the Hamburg klippe of eastern Pennsylvania permits a revised stratigraphic framework and new conclusions regarding its likely tectonic setting. Graptolite and conodont data reveal an almost complete Lower to lower Upper Ordovician zonal section. No Cambrian strata appear to be present, with the possible exception of the uppermost part. During Early to early Middle Ordovician time, medium- to fine-grained siliciclastics and minor carbonate sediments were deposited in a lower slope and rise setting. These sediments were consolidated and incorporated as olistoliths in an olistostrome, possibly as a trench-fill complex, during the Middle Ordovician (Darriwilian 3/4). This olistostrome, which contains large Lower and lower Middle Ordovician fragments within a matrix of shales, siltstones, and sandstones of Da 3/4 age, is herein named the Shellsville Member of the Dauphin Formation. Turbidites, here assigned to the Nyes Road Member of the Dauphin Formation, were also deposited during Da 3/4 time. These rocks interfinger with red beds of the here-named Manada Hill Member of the Dauphin Formation. The red shales, cherts, and associated rocks of the Manada Hill Member are pelagic deposits that range in age from at least early Arenig through middle Llanvirn time. These allochthonous rocks were emplaced as a gravity-generated klippe into the Martinsburg foreland basin during late Climacograptus bicornis or early Dicranograptus clingani time. These three members compose the Dauphin Formation (new) in the western part of the klippe area. Prior to emplacement of the allochthon, syntectonic flysch and scattered wildflysch of the Martinsburg Formation were deposited. Some graptolite faunas from the Martinsburg Formation, where contiguous with the klippe, may be slightly older than those known from areas farther from the klippe. This could indicate an earlier start of deposition in the Martinsburg foreland basin in advance of the allochthon. The klippe occupied a large space in the Martinsburg foreland basin and it diverted deposition in this area until it was finally covered by late Martinsburg age sediment. The Dauphin Formation is now structurally interleaved and folded with the Martinsburg Formation as a result of late Taconian and later Alleghenian tectonism.

Structure and Secretion of the Graptolite Prosicula, and its Application for Biostratigraphical and Evolutionary Studies

Studies on graptolite taxonomy and phylogeny in recent years have placed great emphasis on the proximal development of the rhabdosome, particularly the presence or absence of a virgella and early thecal growth patterns. As the prosicula was the earliest part of the graptolite skeleton to be secreted, it may also reveal fundamental information about evolutionary relationships within the Graptoloidea. The prosiculae from a variety of Ordovician taxa ranging in age from Tremadoc to Caradoc have been examined using a combination of light microscopy, SEM and TEM. Parameters investigated include the overall morphology, transition into the nema, pattern of longitudinal ridges and spiral line. Taxa show a change from early Tremadoc graptoloids which have a low diaphragm, prominent spiral line and lack longitudinal ridges, through late Tremadoc and early Arenig taxa which have longitudinal cortical bandages or spiralled, paired longitudinal ridges, into later Arenig and Llanvirn forms which have simple longitudinal ridges and indistinct spiral line and diaphragm. With additional work at higher stratigraphical levels, graptolite prosiculae may prove to be useful biostratigraphically when more complete material is absent, such as in palynomorph preparations from subsurface studies.

Growth rates and skeletal secretion of siculae in Early Ordovician (Arenig) graptolites from western Newfoundland; implications for development and paleoecology of graptolites

Measurements of fusellar heights along the metasiculae of several Arenig graptolite species demonstrate considerable variation both between and within taxa. In many cases, heights increased rapidly during initial growth; the rate of increase decreased towards the midpoint of the metasicula then remained constant throughout the remainder of the metasicula. There are, however, many exceptions to this general rule. In some specimens, fusellar height continues to increase throughout the whole of the metasicula, while in a few examples heights actually decrease distally. Based on limited study of thecal fusellar heights, it is believed that in the species examined maximum height was achieved early in the secretion of the metatheca. This contrasts with later Ordovician and Silurian taxa in which maximum height generally increases throughout development of the theca. Although increasing fusellar height may have been related to growth of the sicular and thecal zooids, it is concluded that increase in height was, at least in part, independent of such a control. Alternating light and dark bands visible in transmitted light microscopy are apparently related to fusellar development. Bands are regular; each pair of bands typically involves six to nine fuselli and has diffuse boundaries. Detailed electron microscope study suggests that the banding is related to a regular thickening and thinning of the primary (fusellar) layer of the sicular wall. We consider the variation to have been diurnal in origin, each cycle representing a twenty-four hour period. Although this would indicate somewhat more rapid fusellar secretion than in extant hemichordates, both secretion rates and deduced lifespans of rhabdosomes (several years for most rhabdosomes) would compare well both with hemichordates and other modern colonial invertebrates.