ABERRANT BIVALVES AND TENACIOUS CORALS OF THE EARLY JURASSIC, MOROCCO: SURVIVORS OF EXTINCTION AND OCEANIC ANOXIA

Reef ecosystems are particularly sensitive to environmental changes, and examination of reef turnover in deep time provides context for evaluating the long-term trajectory of modern ecosystems. Therefore, comprehensive inclusion of diverse paleo-reef ecologies is key, especially those that persist in distinct post-extinction environments (i.e. survivors). The Early Jurassic is characterized by pronounced environmental disruptions; in the late Pliensbachian and early Toarcian there is the emplacement of the Karoo-Ferrar large igneous province, rapidly increased atmospheric CO₂ concentrations, global warming, sea-level rise, and associated perturbations to global geochemical cycles. In the oceans, the early Toarcian is marked by 1) the deposition of organic-rich black shales (Toarcian Oceanic Anoxic event [T-OAE]), which record pulses of oceanic anoxia, 2) possible ocean acidification, 3) a multi-phased mass extinction at the Pliensbachian-Toarcian boundary/T-OAE, and 4) a significant metazoan reef crisis. During the Early Jurassic, a group of large, reef-building bivalves (Lithiotids) proliferated. Unlike other regions at this time, in Morocco, Lithiotids are frequently associated with corals and these communities persist across the Pliensbachian-Toarcian boundary.

For this study, detailed lithostratigraphic data were collected for two sections of late Pliensbachian and early Toarcian strata in the Western High Atlas that record extensive Lithiotid buildups and biostromal growth along a mixed carbonate-siliciclastic ramp. All five Lithiotid genera are documented here and display diverse growth forms and colony architectures. Notably, the occurrence of Lithiotids (and co-occurrence with corals) is rarely observed within the early Toarcian, and this study documents Lithiotid-coral associations bracketing the Pliensbachian/Toarcian boundary. The Moroccan sites studied here provide spectacular, expanded stratigraphy for the study of the proliferation and collapse of unique Lithiotid-coral reef ecosystems not possible elsewhere in the world. Continued examination of this interval has the potential to expand our understanding of the ecology of aberrant reef-forming bivalves and the reef dynamics of intervals of characterized by environmental perturbations.