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common morphological affinities with Early Tertiary sepiids (Coleoidea) suggesting an
evolutionary transition across the K-T boundary rather than belemnoid extinction (Lewy,
2009).
Five Late Cretaceous
nautiloid
genera crossed the K-T boundary (Kummel, 1964). Many
had a spherical shape which was not easy to catch and crush, despite the fact that the
ammonoids and the nautiloids were predated by mosasaurs (Kauffman, 2004). All
cephalopod groups descended from ancestral nautiloids and the extant ones have similar
anatomical features, except for nautilids. The latter have a primitive eye structure, two pairs
of gills and numerous small tentacles in contrast to vertebrates-like eyes, a single pair of gills
and ten or eight tentacles as seen, for example in cuttlefish, squids and octopods. These
anatomical and physiological differences suggest that the extant nautilid adapted to
darkness and oxygen deficiency, such as exists in the deep ocean where they are found
today, restricted to the southwestern Pacific Ocean (Kummel, 1964). Mesozoic fossil
nautiloids occur in shallow and deep marine sediments. It is reasonable to assume that
nautiloids swimming in open marine waters were attacked by sharks, large fish, large
octopods and squids. The slow swimming nautiloids escaped into deeper marine
environments already millions of years ago during which their anatomy and physiology
considerably changed and therefore cannot be applied to Mesozoic and older nautiloids
(Lewy, 2000). This trend explains how nautiloid genera survived the Late Cretaceous
biological crisis which affected their associated cochleate ammonites. This crisis was not
caused by acid rain (Prinn & Fegley, 1987) which would have killed most nektonic
organisms, but reflects an increase in predation pressure (as reflected by other faunal
groups) from which nautiloids escaped into deeper water and less menacing habitats.
4.3 Bivalvia
Most bivalves are burrowers into the sediment and are thus hidden from predators, in
contrast to epifaunal species. Among the few groups which did not survive into the
Cenozoic are the sessile, epifaunal, gregarious
incoceramid
bivalves, which thrived in large
communities on rather deep marine bottoms of calcareous shale and chalk. These sediments
preserved organic matter in some places, suggesting temporary reduced oxygen content and
hence living conditions unfavorable to other organisms (Kauffman et al., 2007). The
decimeter to over a meter long bivalves are found up to the base of the Upper Maastrichtian,
with questionable relics at higher levels. Their small relative
Tenuipteria
survived to the end
of the Maastrichtian (Dhondt, 1983; Marshall & Ward, 1996). This selective extinction can be
explained by increased predation by sharks and mosasaurian reptiles (Kauffman, 1972),
which dived into the deep bottom for the easy prey due to the fragile nature of the prismatic
shell structure of these sessile, rather large bivalves, which until the beginning of the Upper
Maastrichtian coped with the usual predation rate.
Rudists
were individual marine bivalves attached to substrates or reclining on the soft
sediment. Whether or not they hosted photosynthesizing zooxanthellae, they concentrated
in shallow water where food supply and aeration were optimal. They probably had a short
larval stage and could not have drifted far from their ancestral rudists before settling down
and undergoing metamorphism. Therefore the young rudists are found attached beside, or
on top of the previous generation accumulating into wide thickets forming the carbonate-
platform framework, or building elongated or lenticular biogenic buildups (bioherms) with
or without hermatypic corals, stromatoporoids, calcareous algae and other attached faunal
groups. Some rudists reclined on the bottom in the low-energy neritic zone. The general
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