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Comments on the Extinction of the Inoceramid Bivalves: A Case Study from the Saint Paul Area, Eastern Desert, Egypt | OMICS International
ISSN: 2157-7617
Journal of Earth Science & Climatic Change

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Comments on the Extinction of the Inoceramid Bivalves: A Case Study from the Saint Paul Area, Eastern Desert, Egypt

Manal S. Mekawy*
Geology Department, Faculty of Science, Suez Canal University, Ismailia, Egypt
Corresponding Author : Geology Department
Faculty of Science
Suez Canal University, Ismailia, Egypt
E-mail: [email protected]tmail.com
Received May 27, 2013; Accepted June 23, 2013; Published June 28, 2013
Citation: Mekawy MS (2013) Comments on the Extinction of the Inoceramid Bivalves: A Case Study from the Saint Paul Area, Eastern Desert, Egypt. J Earth Sci Clim Change 4:140. doi: 10.4172/2157-7617.1000140
Copyright: © 2013 Mekawy MS. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

Inoceramid bivalves first existed in the Permian and became dominant during the Jurassic and Cretaceous. Previous studies are in agreement that these bivalves experienced a rapid decline and became extinct at the end of the Mesozoic. The primary and actual cause for their extinction remains unclear and doubtful. In the present study, the systematic paleontology of four Inoceramus species from the Saint Paul area, Eastern Desert, Egypt, is reported. These species include Inoceramus cf. atlanticus, Inoceramus (Inoceramus) dunveganensis, Inoceramus (Mytiloides) labiatus, and Inoceramus species. This study presents the first record of Inoceramus (I.) dunveganensis in Egypt. Additionally, an attempt had been made to answer an important question: Are the inoceramid bivalves really extinct?

Keywords
Inoceramid bivalves; Extinction; Mesozoic; Saint Paul area; Egypt
Introduction
Inoceramids are a valuable biostratigraphic group of pterioid bivalves that disappeared at the end of the Cretaceous period [1]. Inoceramids became dominant components of many level-bottom communities and achieved global dispersion during the Jurassic and Cretaceous [2]. Inoceramids are found in a wide range of facies and environments, suggesting that they had a relatively wide ecological tolerance at the genus and species level [3].
In Egypt, inoceramid shells commonly found in argillaceous layers of marine Upper Cretaceous sequences. Inoceramids are rarely found complete, although external and internal molds are commonly preserved. Few studies have recorded of inoceramid species from the Egyptian Upper Cretaceous rocks (Table 1).
This paper focuses on the systematic paleontology of four inoceramid species from the Saint Paul area, Eastern Desert, Egypt. In addition, an attempt had been made to answer an important question: Are the inoceramid bivalves really extinct?
Materials and Methods
Six specimens of inoceramid bivalves were collected from the Galala Formation, Wadi El Deir, Saint Paul area, Eastern Desert, Egypt (Figure 1A). Three specimens were collected from the middle part of Galala Formation (bed no. 4) corresponding to an ammonite zone (Acanthoceras amphibolum) which is considered to be of late Middle Cenomanian. The remaining samples were collected from the uppermost part of the Galala Formation (bed number 10), in an ammonite zone (Choffaticeras segne) belonging to the Early Turonian. In the laboratory, the collected specimens are carefully cleaned by brushing and washing with water and then left to dray in air. The specimens were investigated and identified using a hand lens. The specimens were classified into four species as follows: Inoceramus cf. atlanticus (late Middle Cenomanian), Inoceramus (Inoceramus) dunveganensis (late Middle Cenomanian), Inoceramus (Mytiloides) labiatus (Early Turonian), and Inoceramus species (Early Turonian). The samples photographed using digital camera, are shown on Plate 1.
Geological Setting
The Wadi El Deir section represents a nearly complete Upper Cretaceous succession ranging in age from the Cenomanian to the Maastrichtian (Figure 1B). The Wadi El Deir section can be subdivided into five rock units as follows: Galala Formation (Cenomanian- Early Turonian), Umm Omeiyid Formation (early Middle Turonian), Wata Formation (late Middle-Late Turonian), Matulla Formation (Coniacian-? Early Campanian) and Sudr Chalk (Campanian- Maastrictian). In the present study, the Inoceramus specimens were collected from the Galala Formation, which measures 67 m in thickness and is composed mainly of shale, fossiliferous marl and dolomitic limestone with minor sandstone, claystone and siltstone interbeds in the lower part of the formation.
Systematic Paleontology
Phylum Mollusca Cuvier, 1795
Class Bivalvia Linné, 1758
The terminology follows that of the glossary presented by [4] in the Treatise on Invertebrate paleontology, part N.
Subclass Pteriomorphia Beurlen, 1944
Order Pterioida Newell, 1965
Suborder Pteriina Newell, 1965
Superfamily Ambonychiacea Miller, 1877
Family Inoceramidae Giebel, 1852
Genus Inoceramus J. Sowerby, 1814
Inoceramus cf. atlanticus (Heinz 1936)
Material: One internal mold of an incomplete specimen and attached to rock was recorded from the middle part of Galala Formation (bed no. 4).
Remakes: The studied specimen is well preserved with a small size. The outline of the specimen, as well as the form of a fine commarginal growth costae with close interspaces, is very similar to that of Inoceramus atlanticus, which was identified and described in detail by [5-8]. The present species was recorded for the first time in Egypt by [8] (the present author).
Age of the present study: late Middle Cenomanian.
Inoceramus (Inoceramus) dunveganensis Mclearn, 1926
1926 Inoceramus dunveganensis Mclearn.
1960 Inoceramus (Inoceramus) dunveganensis Mclearn.
Material: Two incomplete specimens of an internal mold were collected from the middle part of the Galala Formation (bed number 4)
Remakes: The present species closely resemble Inoceramus (Inoceramus) dunveganensis, which was described and illustrated by [9]. This species was recorded from an Acanthoceras amphibolum ammonite zone (late Middle Cenomanian), and according to [9], this species ranges in age from the Late Albian to the Middle or Late Cenomanian.
Age of the present study: Late Middle Cenomanian.
Stratigraphic range: Late Albian to Late Cenomanian.
Subgenus Mytiloides J. Sowerby, 1814
Inoceramus (Mytiloides) labiatus (Schlotheim, 1813)
Material: Two internal mold specimens (one complete and another incomplete) were recorded from the upper-most part of the Galala Formation (bed number 10). [11-15]
Remakes: The present species closely resemble Inoceramus (Inoceramus) dunveganensis, which was described and illustrated by [9]. This species was recorded from an Acanthoceras amphibolum ammonite zone (late Middle Cenomanian), and according to [10], this species ranges in age from the Late Albian to the Middle or Late Cenomanian.
Age of the present study: Early Turonian.
Stratigraphic range: Turonian.
Inoceramus species
Material: One incomplete specimen, internal mold, was recorded from the upper most part of the Galala Formation (bed number 10).
Remakes: The specimen is a poorly preserved internal mold, and owing to the incompleteness, the author cannot identify it. It is reported from the Choffaticeras segne ammonite zone.
Age of the present study: Early Turonian.
Is the Extinction of Inoceramids Bivalves Real?
Although many studies have been conducted on inoceramid bivalves, there remains a wide range of problems and questions to be investigated. The extinction of inoceramid bivalves is doubtful for many reasons, the most important of which are the following:
1- The incompleteness of recorded shells, with most specimens being fragments.
2- The specimens commonly occur as molds (internal or external), which is not sufficient for accurate identification.
3- Despite the wide geographic distribution of inoceramid bivalves, they are rare and limited in occurrence.
4- The cause of rapid evolutionary rates of Cenomanian-Turonian inoceramids remains unclear.
5- The high similarly in external shell morphology between Inoceramus and Mytiloides.
6- Problems related to inoceramid taxonomy at species-level owing to the high similarity between the specimens.
7- The wide ecological tolerances at the genus and species level and the survival of many inoceramid bivalves after the Cenomanian-Turonian mass extinction make us wonder why the inoceramid bivalves did not survive after the Cretaceous- Tertiary catastrophe.
The issue of the extinction of inoceramid bivalves is very complicated and requires further study, revision and cooperation between paleontologists. We can make a difference in understanding our environment and save time through cooperation.
Acknowledgements
Many thanks to everyone in Journal of Earth Science & Climatic Change who has contributed toward the progress of this work especially Gracia S. Oliver, Assistant Managing Editor to respond to any request I asked her. I am deeply grateful to Ted Eckmann Editor, Department of Geography, Bowling Green State University, USA for critical review of the manuscript. Deep and grateful thanks to the reviewers for their critical review of the manuscript and useful comments.
References

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