Taxonomic revision, stratigraphic significance and phylogeny of the Bithynian ammonoid genus Kocaelia Fantini Sestini, 1990 (Anisian, Middle Triassic)

The potential of the integrated study of historical type specimens and newly collected (bed-by-bed) topotypes towards a much improved definition of modern, population-based and phylogeny-oriented taxonomy is here demonstrated by our case study of Kocaelia FANTINI SESTINI, 1990 and Semibeyrichites KRYSTYN & TAZTREITER, 1991.The systematic relationship of these two Middle Anisian Beyrichitidae has been a matter of considerable debate for the past three decades. Our study of the type specimens of Aspidites toulai ARTHABER, 1914, type species of Kocaelia, collected from Gebze (NW Turkey), reveals that this species was incorrectly figured by Arthaber in 1914. The ventral side of this species is subtabulate instead of rounded as reported in the original figuration. As a result, the original definition of Kocaelia by Fantini Sestini, based mainly on its subtabulate venter, is indeed correct. Thus, Semibeyrichites KRYSTYN & TATZREITER, 1991, also based on a subtabulate venter and typified by S. ruttneri KRYSTYN & TATZREITER, 1991 from Aghdarband (NE Iran), becomes a junior synonym of Kocaelia. The study of the type specimens of Semibeyrichites ruttneri allows us to demonstrate that this species is congeneric with K. toulai, differing by its thicker whorl section. Most of the type specimens of K. toulai share a subtabulate venter, while their ammonitic suture lines differ only in the depth of the external lobe E from specimen to specimen. Unfortunately, the type specimens of K. toulai and K. ruttneri were collected without information as to their stratigraphic position in the section; consequently the specimens cannot be studied utilizing a populationistic and stratigraphic (stratophenetic) approach in order to test the significance of the slight difference in suture lines. However, such a study is possible with the Assereto collection, which was formerly studied by FANTINI SESTINI in the 1980s. Assereto amassed his collection from the exact same locality (Gebze, Turkey) where the ammonoids studied by Arthaber were found. Our study of this collection is focused on the suture line, which is well preserved in some tens of specimens. A comparison of specimens collected from the same beds with those from stratigraphically higher beds, reveals the existence of three main components in the variation of the suture line. The most interesting one is stratigraphic and consists of a gradual deepening of the external lobe E as exhibited by specimens from the Nicomedites osmani chronozone as compared to those of the overlying Aghdarbandites ismidicus chronozone. Both chronozones, originally defined at Gebze, are therefore revised as to their boundaries and scope. Stratophenetic comparison of bed-by-bed collected topotypes of Kocaelia toulai, with bed-by-bed topotypes of Nicomedites osmani TOULA and Aghdarbandites ismidicus (ARTHABER), species both originally described from Gebze, has enabled us to establish the first reconstruction of the phylogeny of the early Tethyan Beyrichitidae, mainly based on their suture lines. In the Tethys Realm this family developed in the Middle Anisian a first lineage Nicomedites-Kocaelia, whereas Aghdarbandites, in part coeval with Kocaelia, bears no relationship with the latter. Evidence for the ancestor/descendant relationship of Nicomedites and Kocaelia is found in the ammonitic suture line with its shallow external lobe, and in the shell ornamentation. Aghdarbandites and Nicomedites share a subammonitic suture line with a shallow external lobe, but the former differs by its stronger ribbing and the presence of three rows of nodes on the flank. Further investigation is necessary to clarify the possible relationships between Aghdarbandites and Nicomedites.