neseuretus avus, trilobites fossil from middle ordovician

animals arthropods avus biostratigraphy bulging calymenoidea colpocoryphe edunculated elongated eyes fauna fossil geological glabella heritage isolated marine mediumsized middle neseuretus nesuretus order ordovician outline overhead ovus phacopida prehistorical pygidium records salterocoyphe sedimentary sediments segmented shape striations stylized thorax triangular trilobite trilobites view well