Evidence and cause of small size in Bathonian (Middle Jurassic) marine bivalves of north-western Europe

Authors
Citation
Ala. Johnson, Evidence and cause of small size in Bathonian (Middle Jurassic) marine bivalves of north-western Europe, PALAEONTOL, 42, 1999, pp. 605-624
Citations number
116
Categorie Soggetti
Earth Sciences
Journal title
PALAEONTOLOGY
ISSN journal
00310239 → ACNP
Volume
42
Year of publication
1999
Part
4
Pages
605 - 624
Database
ISI
SICI code
0031-0239(199908)42:<605:EACOSS>2.0.ZU;2-Z
Abstract
Data from the monographic literature indicate a general size reduction amon gst Bathonian marine bivalves in southern England: mean size is reduced in all subclasses and major mode-of-life categories relative to the immediatel y preceding Bajocian stage (27 per cent. reduction) and the later Oxfordian stage (41 per cent, reduction). Smaller size cannot be explained in terms of sedimentary facies, nor as a 'pure' evolutionary phenomenon (involving n o environmental change); it therefore probably reflects some quality of the ambient water. A comparable reduction in average size (19 per cent. relati ve to Bajocian, 46 per cent. relative to Oxfordian) is evident amongst oyst er and scallop lineages studied throughout Europe and a sedimentary-facies (and pure evolutionary) control can be ruled out at least for an area exten ding from southern England to the east side of the Paris Basin. A lowering of salinity to the mid-twenties per mil is the likeliest cause of size redu ction in this area. Isolation of the region from the oceans, combined with high fresh water runoff, was probably the determining factor in such widesp read lowering of salinity. High runoff may have been caused by increased ra infall. Tests are proposed for the ubiquity of the pattern identified, and the suggested proximate and ultimate causes. It is noted that recognition o f widespread reduced salinity has implications for the salinity tolerances of certain 'stenohaline-marine' taxa and the salinity ranges ascribed to ce rtain nearshore faunal associations. In addition, recognition that environm entally determined size reduction may occur over a broad area has implicati ons for the general issue of interpreting phyletic size change.