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Heterochronic evolution in the Heliolites interstinctus-decipiens lineage of the Chaleur Bay region, eastern Canada
Authors:DONG-JIN LEE  GRAHAM A YOUNG  JAMES P A NOBEL
Institution:Dong-Jin Lee and James P. A. Noble, Department of Geology, University of New Brunswick, P.O. Box 4400, Fredericton. New Brunswick. E3B 5A3, Canada;Department of Geology, The University, Newcastle upon Tyne, NEI 7RU, UK
Abstract:A study of the Heliolites interstinctus-decipiens lineage from the Silurian of the Chalcur Bay region, eastern Canada, indicates that most morphological characters of this lineage, including tabularium diameter, coenenchymal tubule diameter and corallite spacing, remain relatively invariate through a Llandovery to Pridoli interval of about 20 million years. In contrast, septal development and corallite wall erenulation show pronounced evolutionary changes due to heterochrony, as shown by comparison of astogenies throughout the lineage. The reduction of septa and the replacement of crenulate corallites by smooth ones in the 'decipiens' type descendent appear to have been by paedomorphosis from the 'interstinctus' type ancestor. The problems of speciation in this lineage are discussed including the alternative hypotheses of a speciation event in the early Ludlow or a phyletic trend within a single species whose longevity is due in part to the stabilization processes of niche separation.
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