Hostname: page-component-586b7cd67f-gb8f7 Total loading time: 0 Render date: 2024-11-28T18:04:27.719Z Has data issue: false hasContentIssue false
  • English
  • Français

Two new graptolites from the early Silurian (Llandovery: Aeronian) of central Wales: an origin for monoclimacid thecal morphology

Published online by Cambridge University Press:  01 May 2009

J. A. Zalasiewicz
J. A. Zalasiewicz
Affiliation:
British Geological Survey, Keyworth, Notts, NG12 5GG, U.K.
Get access Rights & Permissions [Opens in a new window]

Abstract

Monograptus chrysalis sp. nov. from the magnus Zone (Aeronian) of central Wales shows a previously unrecognized combination of thecal types. Distally, the thecae possess marked lappets and short lateral horns, and approximate to those of the revolutus group of monograptids and some described examples of Pribylograptus. Proximally, the thecae are elongate and slender, and possess slit-like apertures below a distinct geniculum. They resemble the proximal thecae of Monograptus imago sp. nov., one of the earliest recognized British monoclimacid monograptids, from the succeeding leptotheca Zone of central Wales and the Lake District. It is suggested that M. chrysalis arose from a graptolite of the revolutus group by retraction or suppression of the hooked proximal thecae, while the distal thecae remained constant, and that M. imago then evolved from M. chrysalis by a spreading of the proximal end characteristics distally. The proximal morphology and overall record of the monoclimacids, though, suggests that they may be polyphyletic; in this paper, the generic name Monoclimacis is reserved for the late Telychian graptolites of the vomerina group.

Type
Articles
Information
Geological Magazine , Volume 129 , Issue 6 , November 1992 , pp. 779 - 785
Copyright
Copyright © Cambridge University Press 1992

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Bjerreskov, M. 1975. Llandoverian and Wenlockian graptolites from Bornholm. Fossils and Strata 8, 1–93, pls. 1–13, Oslo.CrossRefGoogle Scholar
Chen, X. 1984. Silurian graptolites from southern Shaanxi and northern Sichuan with special reference to classification of the Monograptidae. Palaeontologica Sinica, New Series B 166, 1–102.Google Scholar
Chen, X. & Lin, Y. 1978. Lower Silurian graptolites from Tongzi, northern Guizhou. Memoirs of the Nanjing Institute of Geology and Palaeontology, Academia Sinica 12, 1–76.Google Scholar
Elles, G. L. & Wood, E. M. R. 1911. A monograph of British Graptolites. Palaeontographical Society (Monograph), part 8, 359–414.Google Scholar
Gould, S. J. 1977. Ontogeny and Phylogeny. Cambridge, Massachusetts: Harvard University Press. 501 pp.Google Scholar
Hutt, J. E. 1974. A new group of Llandovery biform monograptids. In Graptolite Studies in Honour of O.M.B. Bulman (eds Rickards, R. B., Jackson, D. E. and Hughes, C. P.), pp. 189–203. Special Papers in Palaeontology, London no. 13.Google Scholar
Hutt, J. E. 1975. The Llandovery graptolites of the English Lake District. Part 2. Palaeontographical Society (Monograph), 57–137.Google Scholar
Lapworth, C. 1876. On Scottish Monograptidae. Geological Magazine 3, 308–21.CrossRefGoogle Scholar
Loydell, D. K. In press. Worldwide correlation of Telychian (Upper Llandovery) strata using graptolites. Geological Society of London Special Publication.Google Scholar
Melchin, M. J. 1989. Llandovery graptolite biostratigraphy and palaeobiogeography, Cape Phillips Formation, Canadian Arctic Islands. Canadian Journal of Earth Sciences 26, 1726–46.CrossRefGoogle Scholar
Mitchell, C. E. 1987. Evolution and phylogenetic classification of the Diplograptacea. Palaeontology 30, 353–405.Google Scholar
Obut, A. M., Sobolevskaya, R. F. & Merkuryeva, A. P. 1968. Graptolite llandoveri v kernakh burovykh skvezhin noryl'skogo rayona. Akademia Nauk SSSR. Sibirskoe otdelenie Institut Geologii i Geofiziki, 1–136 (in Russian).Google Scholar
Přibyl, A. 1943. O nékolika nových graptolitech z českého a némeckeho siluru. II. (Einige neue Graptolithen aus dem bohmischen und deutschen Silur). Vestník Kràlovské Česke Spoleénosti Nauk. Tñida. Matematicka Přiřodovědecká, no. 6, 1–16.Google Scholar
Rickards, R. B. 1968. The thecal structure of Monoclimacis? galaensis. Lethaia 1, 303–9.CrossRefGoogle Scholar
Rickards, R. B. 1974. A new monograptid genus and the origins of the main monograptid genera. In Graptolite Studies in Honour of O. M. B. Bulman (eds Rickards, R. B., Jackson, D. E. and Hughes, C. P.), pp. 141–7. Special Papers in Palaeontology no. 13.Google Scholar
Rickards, R. B., Hutt, J. E. & Berry, W. B. N. 1977. Evolution of the Silurian and Devonian graptoloids. Bulletin of the British Museum (Natural History), Geology 28, 1–120.CrossRefGoogle Scholar
Rickards, R. B. & Rushton, R. B. 1968. The thecal form of some slender Llandovery Monograptus. Geological Magazine 105, 264–74.CrossRefGoogle Scholar
Štorch, P. 1988. Earliest Monograptidae (Graptolithina) in the Lower Llandovery sequence of the Prague Basin (Bohemia). Sbornik Geologickych Ved: Palaeontologie 29, 9–48.Google Scholar
Sudbury, M. 1958. Triangulate monograptids from the Monograptus gregarius Zone (Lower Llandovery) of the Rheidol Gorge (Cardiganshire). Philosophical Transactions of the Royal Society, Series B, 241, 485–555.Google Scholar
Zalasiewicz, J. A. 1990. Silurian graptolite biostratigraphy in the Welsh Basin. Journal of the Geological Society, London 147, 619–22.CrossRefGoogle Scholar
Zhao, Y. 1984. On the evolution of the Monograptids based on the contraction of the apertural thecal wall. Acta Geologica Sinica 2, 97–105.Google Scholar