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MIDDLE AND LATE TURONIAN OYSTERS OF 

 THE LOPHA LUGUBRIS GROUP 



By ERLE G. KAUFFMAN 



U. S. National Museum 

 Smithsonian Institution 



(With Eight Plates) 



INTRODUCTION 



Fossil oysters are among the most common and well preserved 

 faunal elements in Cretaceous sediments. With few exceptions, how- 

 ever, they have been ignored in evolutionary studies and biostra- 

 tigraphy. Although countless species have been described, the tax- 

 onomy of the group is confusing and inconsistent. It seems ironic, 

 therefore, that they are one of the groups most suited to modern 

 population systematics. The present investigation attempts to demon- 

 strate the feasibility of detailed systematic and evolutionary study, 

 faunal zonation, and regional correlation based on oysters. It employs 

 simple biometric analysis of large collections, from numerous locali- 

 ties, distinct stratigraphic levels, and diverse sediment types. I have 

 chosen for this purpose a typical lophid species group centered around 

 Lopha lugubfis (Conrad), and including L. bellaplicata hellaplicata 

 (Shumard), L. hellaplicata novamexicana n. subsp., and varieties of 

 these forms. 



The Lopha lugubris lineage is the predominant ostreid species 

 group in Middle and Upper Turonian (Upper Cretaceous) sediments 

 of the Western Interior and western Gulf Coast. These plicate 

 oysters are abundant and well preserved at various stratigraphic 

 levels in the upper Eagle Ford Shale of Texas, the Mancos Shale of 

 New Mexico, and the Carlile Shale of Colorado, New Mexico, and, 

 rarely, Kansas. They are ideally suited for a modern systematic 

 study because the structural complexity of lophid oysters is greater 

 than in many other ostreid types. Detailed analysis of this group 

 provides a good test for the systematic, taxonomic, and evolutionary 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 148; NO. 6 



