neseuretus avus, trilobites fossil from middle ordovician

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