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• Ichthyology is the branch of zoology devoted to the study of fish, including bony fish (Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha).
• Although their exact relationship to the only other living group of jawless fish, the lampreys, was long the subject of controversy, genetic evidence suggests that hagfish and lampreys are more closely related to each other than to jawed vertebrates, thus forming the clade Cyclostomi.
• The autochthonous ichthyofauna consists of at least 50 species of two classes, Cephalaspidomorphi (jawless fishes) and Actinopterygii (ray-finned fishes), and five orders, Petromyzontiformes (lampreys), Siluriformes (catfish), Gadiformes, Esociformes (pikes and allies) and Perciformes.
• These armored jawless fish superficially resemble their relatives, the cyathaspids, though, researchers place the tolypelepids as a sister group to the cyathaspids and the pteraspidids (and both groups' daughter taxa).
• Lepidaspis serrata ("serrated scaly shield") is an extinct heterostracan jawless fish from Early Devonian Canada.
• Anaspida ("shieldless ones") is an extinct group of jawless fish that existed from the early Silurian period to the late Devonian period.
• The class Osteostraci (meaning "bony shells") is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian.
• Pteraspidomorphs have been first regarded as related to bony fishes, then to sharks, then ancestral to hagfishes, and finally as the closest jawless relatives of the gnathostomes.
• Cyclostomi, often referred to as Cyclostomata , is a group of vertebrates that comprises the living jawless fishes: the lampreys and hagfishes.
• The head of a fish includes the snout, from the eye to the forward most point of the upper jaw, the operculum or gill cover (absent in sharks and jawless fish), and the cheek, which extends from eye to preopercle.
• for the taxon Cyclostomi, which comprises the extant jawless fishes: the hagfish (Myxini) and the lampreys (Petromyzontidae).
• Fish can be grouped into the more basal jawless fish and the more common jawed fish, the latter including all living cartilaginous and bony fish, as well as the extinct placoderms and acanthodians.
• Jamoytius kerwoodi is an extinct species of primitive, eel-like jawless fish known from the Patrick Burn Formation in Scotland, dating to the Llandovery epoch of the Early Silurian period.
• In the middle Devonian the armored jawless fish known as ostracoderms were declining in diversity and instead the jawed fish were thriving and increasing in diversity in both the oceans and freshwater.
• He made fundamental advances in the knowledge of graptolites and of the extinct, primitive jawless fishes referred to as ostracoderm that lived some 400 million years ago.
• stensioi may have evolutionary traits which connect its morphology to modern or crown-grouped jawed vertebrates, despite its resemblance to ancient jawless fish, showing an instability in the prevailing hypotheses of placoderm evolution.
• However, in sea lampreys, an early jawless fish species that originated over 500 million years ago, 11-deoxycortisol plays a crucial role as the primary and ultimate glucocorticoid hormone with mineralocorticoid properties; 11-deoxycortisol also takes part, by binding to specific corticosteroid receptors, in intestinal osmoregulation in sea lamprey at metamorphosis, during which they develop seawater tolerance before downstream migration.
• Lasianus has a pair of eyes, and a notochord, and while it has rows of bony scutes running along its back, it lacks the armour typical of other Paleozoic jawless fish like ostracoderms.
• Bony fish (osteichthyans) have four pairs of arches, cartilaginous fish (chondrichthyans) have five to seven pairs, and the more basal jawless fish ("agnathans") have up to seven.
• The Gnathobdellida were jawed and carnivorous or parasitic while the Pharyngobdellida were jawless and carnivorous.

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