Gary L. Mullins

GLOBAL PATTERNS OF ORGANIC-WALLED PHYTOPLANKTON BIODIVERSITY DURING THE LATE SILURIAN TO EARLIEST DEVONIAN

Abstract Numerous environmental factors as well as oceanic circulation patterns and geographic constraints all contribute to the abundance, distribution, and diversity of present-day marine phytoplankton assemblages. These same factors presumably affected the Paleozoic marine phytoplankton, which was dominated by organic-walled acritarchs and prasinophytes. During the Late Silurian (Gorstian, Ludfordian, and Přídolí) and earliest Devonian (Lochkovian), important paleogeographic, paleooceanographic, and geochemical changes were occurring as well as major compositional changes and diversity fluctuations in the marine organic-walled phytoplankton. Innovative morphologies appeared during the Late Silurian, in both low and high latitude assemblages, but with significant quantitative differences. This was followed by a turnover in assemblage composition during the Silurian/Devonian transition, and an initial radiation of new acritarch and prasinophyte taxa in the Early Devonian. Observed changes in total phytoplankton diversity during the Gorstian through earliest Lochkovian are based on organic-walled microphytoplankton data derived from published and unpublished key stratigraphic sections where independent age control has been firmly established. These key sections are from: Missouri and Oklahoma, U.S.A. and western Newfoundland, Canada (Laurentia); Gotland, Sweden, and Podolia, Ukraine (Baltica); the Welsh Basin and Borderland (Avalonia); northern France and northern Spain (Armorica); and Libya in northern Africa, and Argentina and Bolivia, South America (Gondwana). Regional biodiversity changes for the organic-walled microphytoplankton were determined for the warm low latitude areas (Baltica, Laurentia, and Avalonia) and temperate to cool higher latitude areas (northern and southern Gondwana). The Late Silurian—earliest Devonian organic-walled phytoplankton was divided into three major categories to facilitate comparison of compositional fluctuations, both within stratigraphic sections as well as between geographic areas. The three categories, based on overall morphology, are marine chlorophytes and prasinophytes, marine acritarchs, and nonmarine types, including coenobial forms. This triparate grouping is both broad and detailed enough to mark critical changes in both the phytoplankton assemblages, as well as the paleoenvironment. In general, high phytoplankton diversity peaks occurred during the Early and Late Gorstian in the warm low latitude areas, followed by varying fluctuations during the Ludfordian and Přídolí for both the warm low latitude and cool high latitude areas. An initial radiation of new phytoplankton taxa and the appearance of more cosmopolitan assemblages mark the beginning of the Lochkovian.

Integrated Silurian Chitinozoan and Graptolite Biostratigraphy of the Banwy River Section, Wales

The succession of 38 upper Llandovery–lower Wenlock chitinozoan taxa from graptolitic horizons in the Banwy River section (Powys, Wales) is described. Five new species are named: Bursachitina nestoraeConochitina leviscapulaeConochitina mathrafalensisBelonechitina caveiBelonechitina meifodensis. A further ten taxa are described under open nomenclature. Seven chitinozoan biozones are recognized in the Banwy River section, three of which (Cingulochitina bouniensisConochitina acuminataSalopochitina bella) are new. The base of each biozone is correlated with the graptolite biostratigraphical scheme as follows: Angochitina longicollis Biozone — upper spiralis Biozone; Conochitina acuminata Biozone — lowermost lapworthi Biozone; Margachitina banwyensis Biozone — upper lapworthi Biozone; Margachitina margaritana Biozone — lowermost insectus Biozone; Cingulochitina bouniensis Biozone — upper murchisoni Biozone; Salopochitina bella Biozone — upper firmus Biozone. The succession of chitinozoan biozones in the Banwy River section is compared with that in other sections which have graptolite biostratigraphical control. This has highlighted the following: (1) the correlation of the base of the dolioliformis Biozone with the graptolite biozonation is imprecise; (2) E. dolioliformis is recorded only from levels after the first A. longicollis in Sweden (although this may reflect previously unrecognized synonymies); (3) the longicollis Biozone may be diachronous, its base correlating with levels low in the Telychian in Sweden, Norway and Estonia and with the upper Telychian spiralis Biozone in Wales and the Prague Basin; (4) data herein and from the Prague Basin indicate that the base of the margaritana Biozone correlates with a level low in the insectus Biozone.

Upper Silurian microplankton of the Leintwardine Group, Ludlow Series, in the type Ludlow area and adjacent regions

The acritarchs and prasinophyte algae from the Upper Bringewood, Lower Leintwardine, Upper Leintwardine and Lower Whitcliffe formations of the Ludlow type area and surrounding regions are described. The following new taxa are proposed: Cymatiosphaera pumila sp. nov., Melikeriopalla pustula sp. nov., Cheleutochroa beechenbankensis sp. nov., Cymbosphaeridium molyneuxii sp. nov., Flammulasphaera bella gen. et sp. nov., Percultisphaera incompta sp. nov., Salopidium aldridgei sp. nov. and Umbellasphaeridium? wicanderi sp. nov. A holotype is defined for Lophosphaeridium galeatum Hill and a further 34 new taxa are described under open nomenclature. Two biozones, identified by the first appearance of the zone taxon, are defined in the Sunnyhill section, Ludlow [Global Stratotype Section and Point (GSSP) for the base of the Ludfordian Stage, Ludlow Series]. The base of the Leoniella vilis Biozone is identified at 4.52m below the top of the Upper Bringewood Formation at Beechenbank, Aymestrey. The base of the Triangulina sanpetrensis Biozone is identified at 15.49m above the base of the Lower Leintwardine Formation and Ludfordian Stage at the Sunnyhill section. These biozones allow correlation with sections in north–west Spain, Podolia and Gotland. The possibility of using Visbysphaera whitcliffense and U.? wicanderi as biozonal indicators is suggested.

M ICROPLANKTON BIOSTRATIGRAPHY OF THE B RINGEWOOD G ROUP , L UDLOW S ERIES , S ILURIAN , OF THE TYPE AREA

Synopsis The distribution of the acritarchs and prasinophyte algae in the Lower Bringewood Formation and Upper Bringewood Formation, Ludlow Series, Silurian, of the type Ludlow area and nearby Downton area is reported. Ten new taxa, Melikeriopalla fissura, Buedingiisphaeridium crassum, Dateriocradus ludloviensis, Diexallophasis downtongorgensis, Dorsennidium europaeum quintum, Dorsennidium gogginense, Dorsennidium pyramidale, Onondagella deunffii downtonensis, Visbysphaera lamina and Visbysphaera bringewoodense are described. The new combination Eisenackidium gordonense (Crame, 1964b) is proposed. In the type Ludlow area there is an increase in the abundance per gram of sample, but a decline in diversity, of the acritarchs and prasinophyte algae across the Lower Bringewood Formation‐Upper Bringewood Formation boundary. However, the microplankton show a general trend of decreasing abundance per gram of sample, but increasing diversity, through the Bringewood Group in the Downton area. The fluctuations in the abundance and diversity of the microplankton cannot be used to correlate the Ludlow and Downton areas and this probably reflects differences in their local depositional environments, which affected the rate of sedimentation and/or the living microplankton populations. However, the first occurrences of M. fissura, Leoniella vilis, O. deunffii downtonensis, V. bringewoodense and D. pyramidale and last occurrences of O. deunffii downtonensis and V. lamina do allow correlation of the Ludlow and Downton areas. Furthermore, the base of the Leoniella vilis Biozone is now recognised as correlating approximately with the base of the Upper Bringewood Formation. In the Ludlow area, the L. vilis Biozone first occurs with certainty 3.03 m above the base of the Upper Bringewood Formation and this compares well with the Downton area, where it was recorded 4.5 m above the base of that unit.

Revision of acritarchs and prasinophyte algae from the lower Silurian of Belgium

The type acritarch and prasinophyte algal material of Stockmans & Willière (1963), from the Silurian of the Courtrai region, Belgium, has been re-examined. Revised descriptions for the holotypes of several type species are given, which detail the greater morphological diversity recognized. Four new combinations are proposed (Buedingiisphaeridium parveroquesi, Dictyotidium deflandrei, Helosphaeridium echinodermum and Multiplicisphaeridium pentagonale). Micrhystridium? radians is provisionally retained, although it is probable that this species should be referred to a new, unrecognized genus (it possesses a pylome and spinose, simple and branched processes). The synonymy of Diexallophasis denticulata and Diexallophasis remota (proposed by Playford, 1977) is rejected, as the latter has broader-based and more robust processes. The generic assignment of Diexallophasis mucronata is confirmed. Several of the taxa proposed by Stockmans & Willière have been re-illustrated to show clearly the features of taxonomic importance. These descriptions and illustrations will enable the reliable recognition of these taxa in contiguous sections elsewhere, and will help to form the basis of future population studies.

Aggregates of the acritarch Dilatisphaera laevigata: faecal pelletization, phytoplankton bloom or defence against phagotrophy?

Monospecific aggregates of 2–7 individuals of the Silurian acritarch Dilatisphaera laevigata Lister are described. Each generally consists of a central collection of vesicles surrounded by elongate, radiating processes. Acritarchs could aggregate by chance during sample processing, or they may have formed within a sporangia-like structure, although such structures are considered unlikely for D. laevigata. Analogies with modern algae suggest that the aggregates of D. laevigata could have formed by faecal pelletization in the surface waters, or by coagulation of individuals during phytoplankton blooms. In this latter instance the baculate/spinose vesicle ornament and digitate-like branching of the processes may have increased the chances of adhesion. It is also plausible that the aggregates may represent a morphological defence against predation or parasitic infection.