A rare psammosteid heterostracan (Agnatha) occurrence in the Devonian of the Prague Basin is indicative of a dry land influence in the regional palaeogeography. This argument is strongly supported by the appearance of vascular plants, but the studied vertebrates allow us to presume a local brackish water environment. These conditions are considered to be related to a supposed land in the vicinity of the preserved relic of Devonian rocks in the Teplá-Barrandian unit. Such unique conditions were not repeated in the area despite the increasing extent of continental environment related to the rising Variscan Orogeny.