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GEOREF RECORD

Silurian (late Llandovery-Ludlow) atrypid brachiopods from Gotland, Sweden, and the Welsh Borderlands, Great Britain

Paul Cooper
Silurian (late Llandovery-Ludlow) atrypid brachiopods from Gotland, Sweden, and the Welsh Borderlands, Great Britain
National Research Coucil of Canada, Research Press, Ottawa, ON, Canada (2004) 215 pp.

Abstract

A rich and well-preserved atrypoid brachiopod fauna from the Silurian carbonate platform of Gotland (Sweden) in the Baltic Basin, and the siliciclastic-dominated ramp setting of the Welsh Borderlands in the Anglo-Welsh Basin, Britain, is assigned to three suborders (Anazygidina, Lissatrypidina, Atrypidina), four families (Atrypidae, Anazygidae, Lissatrypidae, Glassiidae), 13 genera, and 48 species. Sixteen new species are described. These span ca. 20 million years of evolution in the Atrypida at a critical stage in their early diversification, ranging from the late Llandovery (late Telychian, crenulata Zone) to late Ludlow (Ludfordian, Whitcliffian, bohemicus Zone). For more than two centuries, the Baltic and Anglo-Welsh basins have provided the source for many of the type species of well-established, key genera, including the founding genus and type species of the order Atrypida, Atrypa reticularis (Linnaeus, 1785). For the first time, the interior structure of the complete lophophore support, the brachidium (crura, spiralia, and jugal processes), is reconstructed for the genera Atrypina, Oglupes, Plectatrypa, Spirigerina, Septatrypa, and Reticulatrypa, as well as for other well-known as well as new species. Synonymies are clarified for several species-groups, and evolutionary trends in the succession of species are demonstrated for several genera, assisted by comparisons of infraspecific and interspecific variation within and between species. None of these trends demonstrate evolutionary stasis of long-lived species, not even the grab-bag species Atrypa reticularis, but instead record a constant flux, and commonly random, evolutionary spectrum of atrypid species and variation through the time analysed for this region. Appearances of new genera were initiated either via immigration from the eastern tropics (e.g., Atrypoidea from the eastern tropics), or evolution within the west European domain of Baltica. Transitional species occur in the Gotatrypa-Atrypa-Oghupes-Endrea complex, and the Plectatrypa-Xanthea-Eospinatrypa lineage, marking the arrival of spinose atrypines. Evaluation of the modes of life, and biogeographic distribution, shows that the atrypids occupied a range of benthic ecological niches, from deeper water, slope, and distal shelf or ramp to shallow, onshore oncoid, ooid, and carbonate sand to mud shoals, under more restricted conditions: none occupied the shallowest subtidal, or intertidal zones. A number of atrypids became reef dwellers for the first time, especially Spirigerina, Xanthea, and Endrea, although most taxa were off-reef, and a few were almost exclusively deeper water taxa. None of the distinctive Late Silurian atrypid fauna from the Urals appears to have been present, suggesting a partial block to migration during the Middle and Late Silurian, or unique Uralian reef niches absent in the Baltic. The upper Llandovery-upper Ludlow succession of NW Europe was marked by six benthic "events" in the history of the atrypids: these are documented as radiations, declines, extinctions, and migrations of genera across the shelf area of the paleocontinent Baltica. These events coincided with reef expansions and declines, but did not coincide with those recorded for conodont and graptolite pelagic biota, suggesting that the tropical benthos was less perturbed by regional sea-surface changes, and was probably decoupled from the pelagic biota.


Serial Volume: 46318
Title: Silurian (late Llandovery-Ludlow) atrypid brachiopods from Gotland, Sweden, and the Welsh Borderlands, Great Britain
Author(s): Cooper, Paul
Affiliation: Laurentian University, Department of Earth Sciences, Sudbury, ON, Canada
Published: 2004
Text Language: English
Summary Language: French
Publisher: National Research Coucil of Canada, Research Press, Ottawa, ON, Canada
ISBN: 0-660-19011-7
ISBN: 978-0-66019-011-2
ISBN: 978-0-66019-266-6
Number of pages: 215
References: 615
Accession Number: 2006-015241
Categories: Invertebrate paleontology
Document Type: Book
Bibliographic Level: Monographic
Annotation: Includes appendix
Illustration Description: illus. incl. strat. cols., 32 plates, sketch map
N57°00'00" - N58°00'00", E18°00'00" - E19°30'00"
N51°30'00" - N52°45'00", W03°15'00" - W02°10'00"
Source Note: NRC
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute.
Update Code: 200605
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