PALEOECOLOGY AND FUNCTIONAL MORPHOLOGY OF AN UNUSUAL BRACHIOPOD FROM THE MIDDLE JURASSIC OF JORDAN

The brachiopod faunas of the Hamam and Mughanniyya formations (Callovian) in northwest Jordan consist of articulated shells and are dominated by taxa of the rhynchonellid genus Daghanirhynchia. Among these specimens we have identified a new rhynchonellid genus and species which morphologically appears to have characters that are shared with Daghanirhynchia and Septirhynchia; however specimens are much smaller than Septirhynchia and are closer in size to Daghanirhynchia. High resolution digitized 3D images show that our new genus has a short and low dorsal median septum but lacks a ventral median septum that characterizes Septirhynchia. Its crura are shorter than those of Daghanirhynchia and the dorsal valve is more strongly convex. The position of the lateral commissure would have been perpendicular to the sediment-water interface in life position due to the weight of secondary shell material in the apical regions of the ventral valve that acted as a counterweight to the far larger dorsal valve. Thus, as ontogeny progressed with accompanying atrophy of the pedicle, the brachiopod would have been in a stable upright position on the sea floor with the posterior portion of the shell partially buried in the sediment. The ventral planareas overlap slightly thus preventing some sediment from infiltrating the interior of the shell. Soft tissue probably served as a backup and prevented additional clogging as is the case in Recent terebratulids. The recognition of this new genus is another example of how modern 3D reconstruction techniques help solve the problem of homeomorphy in Mesozoic brachiopods.