Bivalvia

In the Cenomanian/Turonian strata of north Wadi Qena area, Bivalvia occasionally oysters occur in accumulations, which generally represent shallow-marine environments. With the beginning of Turonian age, oysters become scarce or vanish, this may reelect the fact that, following the Cenomanian/Turonian transgression, conditions became too deep for oysters to flourish, in addition the same criteria assign for the Turonian oysters of North Africa (Dhondt et al. 1999) and Brazil (Seeling and Bengston 1999). The attachment area in most of the investigated species is very small, or even absent, as in Rhynchostreon mermeti indicate partly buried in, soft substrates without being attached. Therefore, there are indicating that at least parts of oysters represent a transported fauna. Size differentiation of the individuals has observed at different sections. Thus, in section 1 specimen are generally smaller than in the other sections. Size differentiation is possibly the result of sorting by sedimentary processes. According to (Mancini 1978), a transported micromorph fauna formed when the smaller individuals of an assemblage are winnowed out and concentrate separately.

Oysters are strongly inequivalve. The lower, left valve is heavy, cup-shaped and more durable than the upper, right valve, which is thinner, lighter and more fragile. An oyster community buried in situ, or transported only a short distance, would consist of approximately equal numbers of left and right valves. Transport would be expected to destroy or remove the lighter, more fragile right valves and deposit the heavier left valves at a shorter distance from the original site.

envets species

Salinity

Oxygen

Turbidity

Substrate

Iiidur-tion

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biogenic

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course sand

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s o a

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Pycnodonte (P.) vesica ¡arts

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Exogyra cónica

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a

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Exogym rib osa

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E. (C.) olisiponensis Amphidonte obliquaium

7

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Cerutosireon flabeUetium

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*

*

*

*

t

Ceratostreon texanum-

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m

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i

*

Hyotissa semipktna

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*

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Ryrruiiagyra arielina

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Ilymatagyra (A.) ajrican

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Rhynchostnson mermeti

r

t

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R.. suborb iculaiu m

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Gyrastrea deletírei

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Oscilopha acanthoantha

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Qsdlopka. dlchotam,

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Agerosirea ungulata.

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Rasiellum (A.} catinatum

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Curvostrea kéinú

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Cureasirea rouviliei

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Fig. 2. Ecological parameters for some oyster species, combined from literatures

Fig. 2. Ecological parameters for some oyster species, combined from literatures

In the studied material, right and left valves are rarely found together. The much smaller number of right valves in the assemblage is attributed to the fragility and breakage, is a reliable means of determining the relative amount of transport an oyster or other bivalve has undergone since death. In particular, smaller specimens of Pycnodonte (Phygraea) vesiculosa, Rhynchostreon mermeti and IIymatogyra (Afrogyra) africana are not found in life position and are mostly disarticulated. They are only partly fragmented. In contrast, nearly all larger specimens, mainly of Exogyra (Costagyra) olisiponensis and I. (A.) africana, are preserved unbroken and articulated. This indicates transport under high-energy conditions, but of short duration, as abrasion and breakage is usually low and lower weight as described above. The degree of disarticulation, along with abrasion and breakage, is a reliable means of determining. Definite ecological parameters can be determined based on index oyster species (Figs. 3, 4).

Different ecological conditions characterize the Neithea. Herein Neithea was found mainly in carbonate rocks representing Circalittoral environments at the early Turonian. All species are recliners that rested on soft substrates or with their convex right valve partly buried, without being attached. Some of the Neithea occurrences in north Wadi Qena appear to represent un-transported faunas or transported only a short distance as they consist of approximately equal numbers of left and right valves. The degree of disarticulation, along with abrasion and breakage, is a reliable indication of the degree of transport a bivalve has undergone since its death.

Fig. 3. Distribution of oyster species within the depositional environments Gastropoda

Gastropod represent well diverse and dense macrofaunal group. Turritillia are dominating at early Cenomanian as internal molds, while Tylostoma are dominate by the beginning of Turonian. The turritella are a good indicator for cold water while the Tylostoma are of high temperature gradients, this reflect the fact that the Cenomanian/Turonian boundary marked by warming conditions. The gastropod fauna represented by irregular densities from the beginning of the base of the measured sections to the top during all intervals from latest Albian to the late Santonian, important ecological interpretations for index gastropod genera shown in Table 2.

Table 2. Ecological parameters for gastropod genera. CP predatory carnivores; CB browsing carnivores; HO herbivores omnivores; HP plant & algae herbivores; SU suspension feeders; D deposited feeders; IS shallow infaunal ; In infuanal; E epifaunal, * analogic, compiled from literatures

Genus

Mode of Life

Trophic style

Environment

Littoral

Sub litt oral

Outer

Infra-

Circa-

Turrit slla

IS

SU

Mesalia

In

SU

Cerithiellct

E*

CD

Cimolitkium

E*

HP

Ceritkium

E

HM/HR

Turriscaia

In

CB*

Nerinea

IS

HP*

Apiyxiella

IS

HP*

Aporrhais

E

D

Coiumbeihna

E*

HO*

Pterodonta

E*

CP

Strom bus

SU

HO

Rkynchocypraea

IS

CP

Gyrodes

E*

CP*

Tylostoma

E

CP*

Fasciolaria

E

CP

Acteonelia

E

CP

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