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Evolution of the Early Silurian rhynchonellid brachiopod Fenestrirostra in the Anticosti Basin of Quebec

Published online by Cambridge University Press:  14 July 2015

Jisuo Jin ,
W. G. E. Caldwell  and
Paul Copper
Jisuo Jin
Affiliation:
Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 0W0, Canada
W. G. E. Caldwell
Affiliation:
Department of Geology, University of Western Ontario, London, Ontario N6A 5B7, Canada
Paul Copper
Affiliation:
Department of Geology, Laurentian University, Sudbury, Ontario P3E 2C6, Canada
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Abstract

Among the abundant, well-preserved rhynchonellid brachiopods in the Upper Ordovician–Lower Silurian rocks of Anticosti Island, Fenestrirostra Cooper, 1955, one of the earliest camarotoechiid genera to appear in the fossil record, is probably of oligorhynchiid ancestry and seemingly endemic to the Anticosti Basin. The genus differs most obviously from other camarotoechiids in its peculiar costae, which are angular and strong posteriorly and flat and weak anteriorly.

Three successive species of Fenestrirostra, found in micritic mudstones and calcareous shales of the middle and upper Merrimack and Lower Gun River Formations, provide an example of gradualistic evolution. The most primitive species, F. primaeva, occurs only in the lower part of the middle Merrimack Formation; the descendant intermediate species, F. glacialis, occurs mainly in the upper part of the middle and in the upper Merrimack and in the lowermost Gun River Formations; and the descendant terminal species, F. pyrrha, occurs only in the lower Gun River Formation. Four major morphoseries distinguish the lineage: 1) change in the proportions and size of the shells; 2) increase in the number and concomitant loss of the costae from the anterior halves of the shells; 3) change in the orientation and reduction in the size of the hinge plates and hinge fossettes; and 4) change in the height of the median septum. The evolutionary modifications, estimated to have been accomplished in no more than 1 m.y., may have been adaptive responses to persistent low-energy marine conditions.

Biostratigraphically, the Fenestrirostra lineage provides the basis for three interval or range zones that straddle the Rhuddanian–Aeronian interstage boundary in the Anticosti Basin.

Type
Research Article
Information
Journal of Paleontology , Volume 64 , Issue 2 , March 1990 , pp. 214 - 222
Copyright
Copyright © The Paleontological Society 

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