Opisoma menchikoffi Dubar, 1948: New occurrences from the Lower Jurassic of the High Atlas, Morocco and Tethyan Himalaya, southern Xizang (Tibet), China

Abstract

The alatoform bivalve Opisoma is characterized by highly antero-posteriorly compressed shells with broad lateral wings formed by pronounced shell wall folds, and a robust, ventrally elongate hinge plate. As a key component of the Lithiotis-fauna, Early Jurassic species of Opisoma thrived in tropical shallow-marine environments along the Tethyan and Panthalassan coasts during the Sinemurian to early Toarcian interval. This study documents a new occurrence of well-preserved Opisoma menchikoffi specimens from the Pliensbachian Bou Dahar limestones of the eastern High Atlas Mountains, Morocco. Numerous natural transverse sections through articulated shells at various growth stages, along with natural longitudinal sections of disarticulated shells provide comprehensive morphological insights, enabling a taxonomic revision of this previously poorly understood species. The dentition of O. menchikoffi is redefined, with the right valve possesses three cardinal teeth and the left valve bears two. Furthermore, a single occurrence of the Lithiotis-fauna from the upper Pliensbachian–lower Toarcian Pupuga Formation in Zanda County (Tethyan Himalaya), originally attributed to Lithiotis by Li et al. (2013), is reappraised and assigned to O. menchikoffi, Lithioperna scutata, and megalodontids. O. menchikoffi is now documented from the Pliensbachian to lower Toarcian of Morocco and Algeria in the western Tethys, as well as from the upper Pliensbachian to lower Toarcian of southern Xizang (Tibet) in the eastern Tethys. Its broad trans-Tethyan distribution constitutes a critical biostratigraphic marker that facilitates precise correlation of Early Jurassic sequences between the eastern and western Tethyan realms.