UNRAVELING HOMEOMORPHY IN THE GENUS DAGHANIRHYNCHIA USING 3D DIGITIZED SERIAL SECTIONS

The brachiopod faunas of the Hamam and Mughanniyya formations (Callovian) in northwest Jordan consist of articulated specimens and are dominated by taxa of the rhynchonellid brachiopod genus Daghanirhynchia. At first appearance these brachiopods are externally very similar; however, there are variations in the number of costae. For reliable identification of articulated brachiopods, particularly post-Paleozoic taxa that are often homeomorphic, serial sections are an essential tool. We used the modern technique of creating digitized 3D models of the specimens sectioned showing the internal shell morphology and were able to identify five species within this genus. In this work we present these taxa and also the type species of Daghanirhynchia, D. macfadyeni, from the slightly older Dhruma Formation (Bathonian) in Saudi Arabia. The 3D images clearly demonstrate the differences among these species in the development of secondary shell material in the apical region, the presence or absence of median septa in both valves, and the orientation and outline of the cardinalia including the crura. As a result, taxa of Daghanirhynchia allow a detailed biostratigraphy of Middle Jurassic strata from the Middle East. Our technique demonstrates the stratigraphic importance of Mesozoic brachiopod genera even though they are characterized by strong homoplasy and intraspecific variability.