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Ostracodermi
Ostracodermi () or ostracoderms is an informal group of vertebrate animals that include all armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes and possibly the cyclostomes if anaspids are closer to them) and thus does not correspond to one evolutionary lineage. However, the term is still used as an informal way of loosely grouping together the armored jawless fishes. An innovation of ostracoderms was the use of gills not for feeding, but exclusively for respiration. Earlier chordates with gill precursors used them for both respiration and feeding. Ostracoderms had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. Unlike invertebrates that use ciliated motion to move food, ostracoderms used their muscular pharynx to create a suction that pulled small and slow-moving prey into their mouths. Swiss anatomist Lou ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Agnatha
Agnatha (; ) or jawless fish is a paraphyletic infraphylum of animals in the subphylum Vertebrata of the phylum Chordata, characterized by the lack of jaws. The group consists of both extant taxon, living (Cyclostomi, cyclostomes such as hagfishes and lampreys) and Extinction, extinct clades (e.g. conodonts and Cephalaspidomorphi, cephalaspidomorphs, among others). They are sister taxon, sister to vertebrates with jaws known as gnathostomes, who evolution, evolved from jawless ancestors during the early Silurian by developing folding joint, articulations in the first pairs of gill arches. Sequencing, Molecular data, both from rRNA and from mtDNA as well as Embryology, embryological data, strongly supports the hypothesis that both groups of living agnathans, hagfishes and lampreys, are more closely related to each other than to Gnathostomata, jawed fish, forming the Class (biology), superclass Cyclostomi. The oldest fossil agnathans appeared in the Cambrian. Living jawless fish c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Osteostraci Janvier
The class (biology), 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 Wenlock epoch, Middle Silurian to Late Devonian. Anatomically speaking, the osteostracans, especially the Devonian species, were among the most advanced of all known agnathans. This is due to the development of paired fins, and their complicated cranial anatomy. The osteostracans were more similar to lampreys than to jawed vertebrates in possessing two pairs of semicircular canals in the inner ear, as opposed to the three pairs found in the inner ears of jawed vertebrates. They are thought to be the sister-group to Pituriaspida, pituriaspids, and together, these two taxa of jawless vertebrates are the sister-group of Gnathostomata, gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cilium
The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike projection that extends from the surface of the much larger cell body. Eukaryotic flagella found on sperm cells and many protozoans have a similar structure to motile cilia that enables swimming through liquids; they are longer than cilia and have a different undulating motion. There are two major classes of cilia: ''motile'' and ''non-motile'' cilia, each with two subtypes, giving four types in all. A cell will typically have one primary cilium or many motile cilia. The structure of the cilium core, called the axoneme, determines the cilium class. Most motile cilia have a central pair of single microtubules surrounded by nine pairs of double microtubules called a 9+2 axoneme. Most non-motile cilia have a 9+0 axoneme that lacks the centr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lateral Line
The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial cells, known as hair cells, which respond to displacement caused by motion and transduce these signals into electrical impulses via excitatory synapses. Lateral lines play an important role in schooling behavior, predation, and orientation. Early in the evolution of fish, some of the sensory organs of the lateral line were modified to function as the electroreceptors called ampullae of Lorenzini. The lateral line system is ancient and basal to the vertebrate clade, as it is found in fishes that diverged over 400 million years ago. Function The lateral line system allows the detection of movement, vibration, and pressure gradients in the water surrounding an animal. It plays an essential role in orientation, predation, and fish ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cephalaspidomorphi
Cephalaspidomorphi (alternatively called Monirhina, or simply cephalaspids) is a Class (biology), class of agnatha, jawless fishes that is presently regarded as uniting the Osteostraci, osteostracans, Galeaspida, galeaspids and Pituriaspida, pituriaspids. Most biologists regard this taxon as extinct, but the name is still sometimes used in the classification of lampreys because they were once thought to be descended. If lampreys are included they would extend the known range of the group from the Silurian and Devonian periods, when they are traditionally assumed to have lived, to the Holocene, present day. Modern works typically assume the cephalaspidimorphs to be the closest relatives of jawed fishes, who may have emerged from within them; if this is true, they would survive if the Gnathostomata, jawed fish are included. The cephalaspidomorphs possessed armored head-shields, a heterocercal tail fin, and in some groups paired pectoral fins. Biology and morphology Cephalaspidomorphs ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hagfish
Hagfish, of the Class (biology), class Myxini (also known as Hyperotreti) and Order (biology), order Myxiniformes , are eel-shaped Agnatha, jawless fish (occasionally called slime eels). Hagfish are the only known living Animal, animals that have a skull but no vertebral column, although they do have rudimentary vertebrae. Hagfish are marine animal, marine predators and scavengers that can defend themselves against other larger predators by releasing copious amounts of slime coat, slime from mucous glands in their skin. Although their exact relationship to the only other extant taxon, living group of Agnatha, 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 superclass Cyclostomi. The oldest-known stem group hagfish are known from the Pennsylvanian (geology), Late Carboniferous, around Moscovian (Carboniferous), 310 million years ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lamprey
Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterized by a toothed, funnel-like sucking mouth. The common name "lamprey" is probably derived from Latin , which may mean "stone licker" ( "to lick" + "stone"), though the etymology is uncertain. "Lamprey" is sometimes seen for the plural form. About 38 extant species of lampreys are known, with around seven known extinct species. They are classified in three families—two small families in the Southern Hemisphere (Geotriidae, Mordaciidae) and one large family in the Northern Hemisphere (Petromyzontidae). Genetic evidence suggests that lampreys are more closely related to hagfish, the only other living group of jawless fish, than they are to jawed vertebrates, forming the superclass Cyclostomi. The oldest fossils of stem-group lampreys are ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cyclostomata
Cyclostomi, often referred to as Cyclostomata , from Ancient Greek κύκλος (kúklos), meaning "circle", and στόμα (stóma), meaning "mouth", is a superclass of vertebrates that comprises the living jawless fish classes: the lampreys and hagfishes. Both groups have jawless mouths with horny epidermal structures that function as teeth called ceratodontes, and branchial arches that are internally positioned instead of external as in the related jawed fishes. The superclass was named by Joan Crockford-Beattie. Possible external relationships This taxon is often included in the paraphyletic infraphylum Agnatha, which also includes several groups of extinct armored fishes called ostracoderms. Most fossil agnathans, such as galeaspids, thelodonts, and osteostracans, are more closely related to vertebrates with jaws (called gnathostomes) than to cyclostomes. Biologists historically disagreed on whether cyclostomes are a clade. The "vertebrate hypothesis" ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Devonian Period
The Devonian ( ) is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at million years ago ( Ma), to the beginning of the succeeding Carboniferous period at Ma. It is the fourth period of both the Paleozoic and the Phanerozoic. It is named after Devon, South West England, where rocks from this period were first studied. The first significant evolutionary radiation of life on land occurred during the Devonian, as free- sporing land plants (pteridophytes) began to spread across dry land, forming extensive coal forests which covered the continents. By the middle of the Devonian, several groups of vascular plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants ( pteridospermatophytes) appeared. This rapid evolution and colonization process, which had begun during the Silurian, is known as the Silurian-Devonian Terrestrial Revo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cephalaspidomorphi
Cephalaspidomorphi (alternatively called Monirhina, or simply cephalaspids) is a Class (biology), class of agnatha, jawless fishes that is presently regarded as uniting the Osteostraci, osteostracans, Galeaspida, galeaspids and Pituriaspida, pituriaspids. Most biologists regard this taxon as extinct, but the name is still sometimes used in the classification of lampreys because they were once thought to be descended. If lampreys are included they would extend the known range of the group from the Silurian and Devonian periods, when they are traditionally assumed to have lived, to the Holocene, present day. Modern works typically assume the cephalaspidimorphs to be the closest relatives of jawed fishes, who may have emerged from within them; if this is true, they would survive if the Gnathostomata, jawed fish are included. The cephalaspidomorphs possessed armored head-shields, a heterocercal tail fin, and in some groups paired pectoral fins. Biology and morphology Cephalaspidomorphs ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Heterostraci
Heterostraci (Ancient Greek, ἕτερος+ὄστρακον "those [with] a different shell" [-i is pl. of -us]) is an extinct subclass (biology), subclass of Pteraspidomorphi, pteraspidomorph, ostracoderm, jawless vertebrate that lived primarily in Marine (ocean), marine and estuary environments. Heterostraci existed from the mid-Ordovician to the conclusion of the Devonian. Description and anatomy The heterostracans differed from other Paleozoic agnathan taxa both in the arrangement and histology of their scales. Most heterostracans had two plates which form a large dorsal shield and a large ventral shield, and had series of scales arranged in various patterns on the sides of their bodies, the exact pattern differing from one group to another. In a few primitive forms, such as ''Lepidaspis'', the dorsal and ventral shields are composed of a mosaic of tiny scales. In most other known forms, though, these tiny scales have fused together to form the shield-plates. The scale ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Conodonts
Conodonts, are an extinct group of marine jawless vertebrates belonging to the Class (biology), class Conodonta (from Ancient Greek κῶνος (''kōnos''), meaning "cone", and ὀδούς (''odoús''), meaning "tooth"). They are primarily known from their hard, mineralised tooth-like structures called "conodont elements" that in life were present in the oral cavity and used to process food. Rare soft tissue remains suggest that they had elongate eel-like bodies with large eyes. Conodonts were a long-lasting group with over 300 million years of existence from the Cambrian (over 500 million years ago) to the beginning of the Jurassic (around 200 million years ago). Conodont elements are highly distinctive to particular species and are widely used in biostratigraphy as indicative of particular periods of geological time. Discovery and understanding of conodonts The teeth-like fossils of the conodont were first discovered by Heinz Christian Pander and the results published in Sain ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
