Stable for 15 million years: scanning electron microscope investigation ofMiocene euglyphid thecamoebians from Germany, with description of the new genus Scutiglypha

Material of the Middle Miocene kieselgur deposit of Beuern (Germany, Vogels berg), about 15 million years old limnic sediment of a volcanic crater lake was investigated with the scanning electron microscope without any previou s preparations. The samples mainly consisted of diatom shells and sponge sp iculae, but also contained well-preserved tests of euglyphid thecamoebians where, for the first time, individual test scales could be clearly seen. Tw o species, each represented by several specimens, were investigated in deta il. Both had highly characteristic body and aperture scales, which were ind istinguishable from those of extant Euglypha crenulata Wailes,1912 and E. s cutigera Penard, 1911. Thus, the fossil specimens were assigned to these ex tant species which, however, were referred to a new genus, Scutiglypha, cha racterised by scutiform, crenate body scales. Clearly, details of the scale s remained stable over millions of generations, although testate amoebae ar e largely asexual. The mechanism responsible for this stability is unknown, but intrinsic ("internal") selection might override environmental ("extern al") selection, thus promoting evolutionary stability of main parts of the system.