Silurian phytoplankton of the type Ludlow area
by Gary Mullins, Richard Aldridge and David Siveter
Holotype
Deunff 1955, p. 146, pl. 4, fig. 8; H–77, RU; matrix of a loose coral (Favosites turbinata), Onondaga Limestone, Devonian, Ontario, Canada.
Diagnosis (Playford 1977, p. 19–20)
"Vesicle originally spherical or nearly so; outline variable–circular, subcircular, square, triangular, rectangular, or polygonal. Wall of vesicle single–layered, up to 1.5 µm thick (average 0.7 µm); almost smooth to faintly granulate or scabrate under light microscope; microgranulate, granulate, microgranulate to micropunctate, microrugulate, or microreticulate under scanning electron microscope. Long, slender, ± flexible processes distributed fairly regularly on vesicle; number variable, usually 4–12, average 6–7, rarely as many as 18–20. Processes heteromorphic or near heteromorphic, with essentially hollow interiors, and constituting drawn–out, basically subcylindrical extensions of vesicle wall, freely communicating with vesicle cavity. At proximal ends, processes have curved to angular contacts with wall of vesicle; distal extremties simple or, more typically, branched (bifurcate to first, second or rarely third order; or digitate). Branching often somewhat asymmetrical. Length of processes variable, from about 0.8 to 2.0 times vesicle diameter. Processes sculptured sparsely to moderately densely echinate to granulate elements, and in some specimens, with discontinuous, fine, subparallel (longitudinal) ridges that are usually developed proximally as a type of substriate subsidiary sculpture. Spines on processes up to 2 µm long, usually about 0.5–1 µm. Excystment, rarely observed, is by simple rupture of vesicle wall."
Description of specimens from the Ludlow type area (after Mullins 2001)
"Vesicle thin-walled, laevigate to faintly scabrate under transmitted light, with a tetrahedral, rectangular, square, polygonal or spherical outline. With a SEM, the vesicle surface appears laevigate, granulate, microfoveolate, micropunctate or granoreticulate. The processes are hollow, slender, straight or slightly flexuous and taper to simple or digitate tips, although they may rarely branch from half way along their length. The processes appear slightly echinate or echinate under transmitted light; with a SEM the processes appear echinate and the majority of their bases are striate (e.g. Pl. 17, figs 3–4), with the striae just extending onto the vesicle surface. Excystment occurs via a linear rupture, which can be ornamented with large granae along its length."
Dimensions (type material)
Vesicle width = 35 µm, Process length = 55 µm, Process number = 4–20.
Dimensions (in the type Ludlow area)
Vesicle width = 13–40 µm, Process length = 18–53 µm, Process diameter at base = 2.5–8.5 µm, Process number = 4–13.
Occurrence in the type Ludlow area
Much Wenlock Limestone–Lower Bringewood formations, Pitch Coppice Quarry and Goggin Road.
Reported occurrence elsewhere in the UK
Llandovery–Prídolí series, Welsh Borderland (Downie 1963, Lister 1970, Lister & Downie 1974, Hill 1974, Dorning 1983, Hill & Dorning 1984, Mabillard & Aldridge 1985, Turner et al. 1995); Wenlock Series, Ayrshire (Dorning 1982); Cheviot Hills, NE England (Barron 1989).
Reported stratigraphical range (worldwide)
Silurian–Devonian.
References
BARRON, H. F. 1989. Mid–Wenlock acritarchs from a Silurian inlier in the Cheviot Hills, NE England. Scottish Journal of Geology, 25, (1), 81–98, 4 pls.
DEUNFF, J. 1955. Un microplancton fossile Dévonien a Hystrichosphères du Continent Nord–Américain. Bulletin de la microscopie applquée, sér. 2, 5, (11–12). 138–149, pls 1–4.
DORNING, K. J. 1982. Early Wenlock acritarchs from the Knockgardner and Straiton Grit formations of Knockgardner, Ayrshire. Scottish Journal of Geology, 18, (4), 267–273, pls 1–2.
DORNING, K. J. 1983. Palynology and stratigraphy of the Much Wenlock Limestone Formation of Dudley, central England. Mercian Geologist, 9, (1), 31–40, pls 5–7.
DOWNIE, C. 1963. 'Hystrichospheres' (acritarchs) and spores of the Wenlock Shales (Silurian) of Wenlock, England. Palaeontology, 6, (4), 625–52, pls 91–92.
HILL, P. J. 1974. Stratigraphic palynology of acritarchs from the type area of the Llandovery and the Welsh Borderland. Review of Palaeobotany and Palynology, 18, (1–2), 11–23.
HILL, P. J. & DORNING, K, J. 1984. The Llandovery Series of the type area. Appendix I. Acritarchs. In: COCKS, L. R. M., WOODCOCK, N. H., RICKARDS, R. B., TEMPLE, J. T. & LANE, P. D. The Llandovery Series of the type area. Bulletin of the British Museum of Natural History (Geology), 38, (3), 174–176.
LISTER, T. R. 1970. A monograph of the acritarchs and Chitinozoa from the Wenlock and Ludlow Series of the Ludlow and Millichope areas, Shropshire. Palaeontographical Society [Monograph], 124, (1), 1–100, pls 1–13.
LISTER, T. R. & DOWNIE, C. 1967. New evidence for the age of a primitive echinoid Myriastiches gigas. Palaeontology, 10, (2), 171–174, pl. 23.
MABILLARD, J. E & ALDRIDGE, R. J. 1985. Microfossil distribution across the base of the Wenlock Series in the type area. Palaeontology, 28, (1), 89–100.
MULLINS, G. L. 2001. Acritarchs and prasinophyte algae of the Elton Group, Ludlow Series, of the type area. Monograph of the Palaeontographical Society, no. 616 for vol. 155, 1–154, pls 1–18.
PLAYFORD, G. 1977. Lower to Middle Devonian acritarchs from the Moose River Basin, Ontario. Geological Survey of Canada, Bulletin, 279, 1–87, pls 1–20.
TURNER, N., SPINNER, E. & DORNING, K. J. 1995. A palynological study of the Lower Carboniferous Lydebrook Sandstone and adjacent late Wenlock and Langsettian strata, Shropshire, England. Review of Palaeobotany and Palynology, 84, 305–329, pls 1–5.
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