Lower Eocene Crustacean Burrows (Israel) Reflect a Change from K- to r-Type Mode of Breeding Across the K-T Boundary Clarifying the Process of the End-Cretaceous Biological Crisis - Advances in Data, Methods, Models and Their Applications in Geoscience - page 328

narrowing of these neritic habitats during the Late Cretaceous reduced rudistid abundance
(e.g., central-eastern Mediterranean and Middle East region; Steuber and Löser, 2000) and
the associated 'reefal' communities. Late Cretaceous rudist buildups prevailed in restricted
regions of the Tethys Sea, such as in the Caribbean province (e.g., Jamaica; Mitchell et al.,
2004) where a few genera reached the K-T boundary and became suddenly extinct probably
as the result of the asteroid impact at the Yukatan Peninsula in the same region (Steuber et
al., 2002). The following initiated tsunami waves might have broken and killed the rudists or
covered them by sediment (e.g., Scasso et al., 2005; Bralower et al., 2010; with references).
However, the dating of these turbulence-induced deposits relative to the age of the asteroid
impact and the K-T boundary are still controversial (Keller et al., 2007).
Most oysters are attached to substrates in shallow marine environments tending to
concentrate and form oyster banks. A few genera recline on soft bottoms in low-energy
environments (e.g., Gryphaea, Pycnodonte ). The 'tribe' Exogyrini (Stenzel, 1971) was highly
abundant in the Cretaceous neritic zone and their calcitic shells are well preserved in
carbonate platform sediments beside the long-ranging Ostrea . Their attached (left) valve first
grew in a spiral pattern which opened and straightened into an elongated or rounded cup-
shape in which the posterior margin stretched over the substrate and the anterior margin
was raised, whereby the flattened upper valve was inclined to the substrate. This mode of
growth subjected these oysters to penetration of sedimentary particles in between the valves
as well as total cover by sediment. The disadvantageous growth orientation in shallow
marine environments added to possible exposure at low tide or predation, all of which
resulted in the extinction of this group at the end of the Cretaceous. Thereby they differ
from the subfamilies Gryphaeinae and Pycnodonteinae in which the lower valve grew in a
nearly planispiral curvature into a cup-shape, whereby the valve commissure (margins) was
elevated above the substrate and the flat upper valve was in horizontal orientation (Stenzel,
1971). The larvae of these oysters had to attach before undergoing metamorphism. The
shallow marine habitats of the Upper Triassic-Jurassic Gryphaeinae consisted mainly of
friable sediment such as sand and marl. Because these sediments lacked large firm
substrates, any grain or small fragment served as attachment site as evidenced by the small
attachment scar at the oyster beak. With further growth the small substrate lost its
anchorage function and the oyster was tilted, raising the substrate above the ground
whereby the ventral margins of the oyster nearly sunk into the friable sediment. To avoid
the penetration of sedimentary particles, the oyster increased the upward growth of the
lower valve whereby the oyster balance changed and required further tilt and upward
growth. The resulting planispiral curvature increased the living space in between the two
valves beyond the size of the mollusk which was compensated by secondary deposition of
shell material on the inner surface of the lower shell (Lewy, 1976). However, the precipitated
calcitic foliated shell structure increased the oyster total weight and enhanced its sinking
into the sediment. Thereby the curvature and thickness of the Gryphaeinae lower valve
reflect the plasticity of the sediment on which it reclined. The crucial effect of this secondary
deposit in the lower valve was partly solved in Cretaceous times by changing the compact
structure into a vesicular one which characterized the similarly looking Pycnodonteinae.
These oysters thickened their valves by layers of light vesicular structures in between layers
of foliated structures minimizing the weight of the secondary fill. Thereby the
Pycnodonteinae could inhabit very soft bottoms and thrive on marl and planktonic
foraminiferal ooze forming chalk in the Upper Cretaceous. Thanks to their adaptation to
rather deep marine environments, the Pycnodonteinae survived the end-Cretaceous