Ala. Johnson, Evidence and cause of small size in Bathonian (Middle Jurassic) marine bivalves of north-western Europe, PALAEONTOL, 42, 1999, pp. 605-624
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.