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biological crisis. This is in contrast to the Exogyrini tribe which lived attached to firm
substrates and therefore inhabited mainly shallow marine environments such as gregarious
oysters which were subjected to local exposure by long low-tides, being covered by
sediment from terrestrial runoff, as well as predation. There they were associated with the
oyster-like Chondrodonta which could withstand wave impact and thus thrived in high-
energy environments and disappeared before the Exogyrini during the Upper Campanian
(Stenzel, 1971).Perhaps Chondrodonta attained larger dimension and had a thin fragile
shell in contrast to Exogyra species. The pectinid Neithea Group thrived in Upper
Cretaceous neritic and continental-slope sediments and probably disappeared at the end
of the Mesozoic (MacLeod et al., 1997). After an early byssate stage they reclined on the
sea bottom and occasionally leaped for a short distance, thus being exposed most of the
time to diving predators, subject to over predation and extinction at the end of the
Cretaceous Period.
4.4 Gastropoda
Most gastropod families crossed the Cretaceous-Tertiary boundary nearly unaffected except
for the Nerineidae and Actaeonellidae . These two families comprised rather large
gastropods which thrived on the Tethyan warm-water carbonate platforms and their
marginal mainly low-salinity zones (Sohl & Kollmann, 1985). The elongated nerineids had a
thick external shell reinforced by folds of the inner shell layer. Actaeonellids likewise had a
thick shell and most of them formed ovate conchs which probably slipped through the teeth
of predators, increasing the resistance of both gastropod groups to predation. Though these
gastropods were mobile, they formed layered concentrations and in places lenticular
structures. These accumulations suggest that these gastropods lived close below (shallow
burrowers) or on the bottom and were thus subjected to exhumation and concentration by
turbulent water in the neritic zone. Generally their representative species survived up to the
end of the Cretaceous although some disappeared earlier from many provinces (Sohl &
Kollmann, 1985, fig. 14) probably as the result of the reduction of the neritic zones and
predation of previously untouched organisms.
5. Marine microorganisms with symbiotic zooxanthellae
Marine floral and faunal microorganisms flourished and diversified throughout the Late
Cretaceous. Therefore the disappearance of most of them at the end of the period seemed
catastrophic (MacLeod et al., 1997). Close to the K-T boundary the calcareous tests of
nannoplankton and the planktonic foraminifera reduced their size (dwarfing) and the
assemblage became dominated by low-oxygen-tolerant small heterohelicid foraminifera and
the disaster opportunist nannofossil Micula decussata (Abramovich & Keller, 2002; Keller &
Abramovich, 2009). These latest Maastrichtian affected microfossils occur in Indian Ocean
drilling samples with volcanic sediments attributed to the Deccan volcanism, hinting to a
connection between the intensive volcanism and the deterioration of the marine ecological
systems (Tantawy et al., 2009).
6. The end-Cretaceous biological crisis caused by the Deccan volcanism
The main volcanic phase, comprising ~80% of the total Deccan Trap volume, occurred
around the K-T boundary and is interpreted as being active during a short time interval in
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