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The Permian-Triassic Boundary: A Crisis for Bivalves?
A conventional plot of the number of bivalve genera at different times throughout the Permian and Triassic results in a spindle diagram with a narrow waist in the earliest Triassic. Only 19 marine bivalve genera (9 new) are known from this period of time, in contrast to about 70 genera known from the early Late Permian and approximately 140 known from the Late Triassic. Part of this decline in diversity is due to an imperfect fossil record; the remainder results mainly from the disappearance of many Permian epifaunal genera. Few families or superfamilies are extinguished at the end of the Paleozoic and a number of characteristically Mesozoic families first appear in mid-Permian times.
Earliest Triassic bivalve assemblages are widespread and distinctive. Unusual features of these faunas are: 1) sudden abundance of new genera like Claraia; 2) absence of many Permian pteriomorphs (particularly scallops); 3) temporary absence of virtually all bivalves with eulamellibranch gills; and 4) sudden worldwide homogeneity. One explanation of these observations is that the preserved assemblages are composed of unusually hardy forms able to flourish in a rigorous environment. Less tolerant genera may have been forced into deep water by adverse conditions on the narrow Early Triassic continental shelves. Present-day deep water faunas are dominated by the types of bivalves which temporarily disappear in the Early Triassic, and contain few of the epifaunal groups which are extinguished at or near the end of the Permian. Progressive changes in the gross aspect of Late Permian and Early Triassic molluscan assemblages from geosynclinal sequences in Japan suggest that changes in seawater salinity could have been responsible for the exodus, although other factors such as niche crowding may also have been operating.
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