Введение
Остракодермы – панцирные бесчелюстные рыбы палеозойской эры. В современных классификациях этот термин используется нечасто, поскольку он является парафилетическим (исключая челюстных рыб) (может быть также полифилетическим, если анаспиды ближе к круглоротым) и, следовательно, не соответствует единой эволюционной линии. Тем не менее, термин до сих пор применяется как неофициальный способ объединения панцирных бесчелюстных рыб. Инновацией остракодерм стало использование жабр не для питания, а исключительно для дыхания. Ранние хордовые с жаберными предшественниками использовали их как для дыхания, так и для питания. Остракодермы имели отдельные жаберные мешки по бокам головы, которые постоянно были открыты и не имели защитного оперкуля. В отличие от беспозвоночных, перемещающих пищу с помощью движения ресничек, остракодермы использовали свою мускулистую глотку для создания всасывания, затягивающего мелкую и медленно движущуюся добычу в рот. Швейцарский анатом Луи Агассиз в 1830-х годах получил из Шотландии окаменелости костных панцирных рыб. Ему было сложно их классифицировать, так как они не напоминали ни одно из известных живых существ. Сначала он сравнивал их с современными панцирными рыбами, такими как сомы и осетровые, но позже понял, что у них отсутствуют подвижные челюсти. Поэтому в 1844 году он отнес их к новой группе, назвав "остракодермами", что означает "покрытые раковиной" (от греческого ὄστρακον + δέρμα). Остракодермы имели головы, покрытые костным щитом. Они одни из первых существ с костными головами. Микроскопические слои этого щита, по мнению эволюционных биологов, "состоят из крошечных зубоподобных структур". Нил Шубин пишет: "Разрежьте кость черепа [остракодермы], откройте ее и рассмотрите под микроскопом – вы обнаружите практически ту же структуру, что и в наших зубах. Там есть слой эмали и даже слой пульпы. Весь щит состоит из тысяч маленьких зубов, сращенных вместе. Этот костный череп, один из самых ранних в летописи окаменелостей, полностью состоит из маленьких зубов. Зубы изначально возникли для захвата добычи (см. Конодонты); позже их версия была использована по-новому – для защиты". После появления челюстных рыб (плакодерм, акантодий, акул и т.д.) около 420 миллионов лет назад большинство видов остракодерм пришло в упадок, а последние остракодермы вымерли в конце девонского периода. Более поздние исследования показывают, что рыбы с челюстями сыграли меньшую роль в вымирании остракодерм, чем считалось ранее, поскольку они сосуществовали без заметного упадка около 30 миллионов лет. Подкласс Ostracodermi был отнесён к разделу Agnatha вместе с существующим подклассом Cyclostomata, включающим миног и миксин.
Armored jawless fish of the Paleozoic
Ostracoderms are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes) (may also be polyphyletic if anaspids are closer to cyclostomes) 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 Louis Agassiz received some fossils of bony armored fish from Scotland in the 1830s. He had difficulty classifying them, as they did not resemble any living creature. He compared them at first with extant armored fish such as catfish and sturgeon, but later realized that they lacked movable jaws. Hence, he classified them in 1844 as a new group, named "ostracoderms" to mean 'shell skinned' (from Greek ὄστρακον + δέρμα ). Ostracoderms have heads covered with a bony shield. They are among the earliest creatures with bony heads. The microscopic layers of that shield appear to evolutionary biologists, "like they are composed of little tooth like structures." Neil Shubin writes: "Cut the bone of the [ostracoderm] skull open pop it under a microscope and you find virtually the same structure as in our teeth. There is a layer of enamel and even a layer of pulp. The whole shield is made up of thousands of small teeth fused together. This bony skull one of the earliest in the fossil record is made entirely of little teeth. Teeth originally arose to bite creatures (see Conodonts); later a version of teeth was used in a new way to protect them." After the appearance of jawed fish (placoderms, acanthodians, sharks, etc.) about 420 million years ago, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period. More recent research indicates that fish with jaws had far less to do with the extinction of the ostracoderms than previously assumed, as they coexisted without noticeable decline for about 30 million years. The Subclass Ostracodermi has been placed in the division Agnatha along with the extant Subclass Cyclostomata, which includes lampreys and hagfishes.
Ostracoderms are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes) (may also be polyphyletic if anaspids are closer to cyclostomes) 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 Louis Agassiz received some fossils of bony armored fish from Scotland in the 1830s. He had difficulty classifying them, as they did not resemble any living creature. He compared them at first with extant armored fish such as catfish and sturgeon, but later realized that they lacked movable jaws. Hence, he classified them in 1844 as a new group, named "ostracoderms" to mean 'shell skinned' (from Greek ὄστρακον + δέρμα ). Ostracoderms have heads covered with a bony shield. They are among the earliest creatures with bony heads. The microscopic layers of that shield appear to evolutionary biologists, "like they are composed of little tooth like structures." Neil Shubin writes: "Cut the bone of the [ostracoderm] skull open pop it under a microscope and you find virtually the same structure as in our teeth. There is a layer of enamel and even a layer of pulp. The whole shield is made up of thousands of small teeth fused together. This bony skull one of the earliest in the fossil record is made entirely of little teeth. Teeth originally arose to bite creatures (see Conodonts); later a version of teeth was used in a new way to protect them." After the appearance of jawed fish (placoderms, acanthodians, sharks, etc.) about 420 million years ago, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period. More recent research indicates that fish with jaws had far less to do with the extinction of the ostracoderms than previously assumed, as they coexisted without noticeable decline for about 30 million years. The Subclass Ostracodermi has been placed in the division Agnatha along with the extant Subclass Cyclostomata, which includes lampreys and hagfishes.
Основные группы
Основные группы остракодермов
Major groups of ostracoderms Group Class Image Description Cephalaspido morphi Cephalaspidomorphi or cephalaspids ('head shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. †Galeaspida(extinct) Galeaspida ('helmet shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, and one species, Tujiaaspis vividus, had paired fins. Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase. †Pituriaspida(extinct) Pituriaspida ('pituri shields') are a small group of extinct armoured jawless fishes with tremendous nose like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). They are known only by two species, Pituriaspis doylei and Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia. †Osteostraci(extinct) Osteostraci ('bony shells') lived in what is now North America, Europe and Russia from the 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 of pituriaspids. Together, these two taxa of jawless vertebrates are the sister group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone. Othergroups Other groups †Pteraspido morphi(extinct) †Pteraspidomorphi ('wing shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida. †Thelodonti(extinct) Thelodonti ('feeble teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of 'thelodont scales'. This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was. Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free swimming organisms. †Anaspida(extinct) Anaspida ('no shields') is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
Группа Класс Описание изображения
Major groups of ostracoderms Group Class Image Description Cephalaspido morphi Cephalaspidomorphi or cephalaspids ('head shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. †Galeaspida(extinct) Galeaspida ('helmet shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, and one species, Tujiaaspis vividus, had paired fins. Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase. †Pituriaspida(extinct) Pituriaspida ('pituri shields') are a small group of extinct armoured jawless fishes with tremendous nose like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). They are known only by two species, Pituriaspis doylei and Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia. †Osteostraci(extinct) Osteostraci ('bony shells') lived in what is now North America, Europe and Russia from the 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 of pituriaspids. Together, these two taxa of jawless vertebrates are the sister group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone. Othergroups Other groups †Pteraspido morphi(extinct) †Pteraspidomorphi ('wing shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida. †Thelodonti(extinct) Thelodonti ('feeble teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of 'thelodont scales'. This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was. Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free swimming organisms. †Anaspida(extinct) Anaspida ('no shields') is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
Cephalaspido morphi Cephalaspidomorphi или cephalaspids («головные щиты»), как и большинство современных рыб, были очень хорошо бронированы. Особенно хорошо развит был щит головы, защищающий голову, жабры и переднюю часть внутренних органов. Тело в большинстве форм также было хорошо бронировано. На головном щите по всей поверхности был ряд борозд, образующих обширный боковой орган. Глаза были довольно маленькими и располагались на вершине головы. Челюсти как таковой не было. Отверстие рта было окружено маленькими пластинами, делающими губы гибкими, но лишенными способности кусаться. Большинство биологов считают этот таксон вымершим, но название иногда используется в классификации миног, поскольку ранее считалось, что миноги связаны с цефаласпидами. Если включить миног, это расширит известный ареал группы от силурийского и девонского периодов до наших дней.
Major groups of ostracoderms Group Class Image Description Cephalaspido morphi Cephalaspidomorphi or cephalaspids ('head shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. †Galeaspida(extinct) Galeaspida ('helmet shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, and one species, Tujiaaspis vividus, had paired fins. Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase. †Pituriaspida(extinct) Pituriaspida ('pituri shields') are a small group of extinct armoured jawless fishes with tremendous nose like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). They are known only by two species, Pituriaspis doylei and Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia. †Osteostraci(extinct) Osteostraci ('bony shells') lived in what is now North America, Europe and Russia from the 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 of pituriaspids. Together, these two taxa of jawless vertebrates are the sister group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone. Othergroups Other groups †Pteraspido morphi(extinct) †Pteraspidomorphi ('wing shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida. †Thelodonti(extinct) Thelodonti ('feeble teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of 'thelodont scales'. This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was. Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free swimming organisms. †Anaspida(extinct) Anaspida ('no shields') is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
†Galeaspida (вымершие) Галеаспиды («шлемные щиты») имели массивный костный щит на голове. Галеаспиды обитали в мелководных пресных и морских средах в течение силурийского и девонского периодов (430–370 миллионов лет назад) на территории современной Южной части Китая, Тибета и Вьетнама. Поверхностно их морфология больше напоминает Heterostraci, чем Osteostraci, а у одного вида, Tujiaaspis vividus, были парные плавники. Galeaspida считаются более близкими родственниками Osteostraci, основываясь на большем сходстве морфологии черепа.
Major groups of ostracoderms Group Class Image Description Cephalaspido morphi Cephalaspidomorphi or cephalaspids ('head shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. †Galeaspida(extinct) Galeaspida ('helmet shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, and one species, Tujiaaspis vividus, had paired fins. Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase. †Pituriaspida(extinct) Pituriaspida ('pituri shields') are a small group of extinct armoured jawless fishes with tremendous nose like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). They are known only by two species, Pituriaspis doylei and Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia. †Osteostraci(extinct) Osteostraci ('bony shells') lived in what is now North America, Europe and Russia from the 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 of pituriaspids. Together, these two taxa of jawless vertebrates are the sister group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone. Othergroups Other groups †Pteraspido morphi(extinct) †Pteraspidomorphi ('wing shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida. †Thelodonti(extinct) Thelodonti ('feeble teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of 'thelodont scales'. This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was. Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free swimming organisms. †Anaspida(extinct) Anaspida ('no shields') is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
†Pituriaspida (вымершие) Питуриаспиды («щиты питури») — небольшая группа вымерших бронированных бесчелюстных рыб с огромным носом, похожим на клюв, обитавших в морских, дельтовых средах Среднего девона Австралии (около 390 млн лет назад). Известны только два вида — Pituriaspis doylei и Neeyambaspis enigmatica, обнаруженные в песчанике бассейна Джорджина в Западном Квинсленде, Австралия.
Major groups of ostracoderms Group Class Image Description Cephalaspido morphi Cephalaspidomorphi or cephalaspids ('head shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. †Galeaspida(extinct) Galeaspida ('helmet shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, and one species, Tujiaaspis vividus, had paired fins. Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase. †Pituriaspida(extinct) Pituriaspida ('pituri shields') are a small group of extinct armoured jawless fishes with tremendous nose like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). They are known only by two species, Pituriaspis doylei and Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia. †Osteostraci(extinct) Osteostraci ('bony shells') lived in what is now North America, Europe and Russia from the 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 of pituriaspids. Together, these two taxa of jawless vertebrates are the sister group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone. Othergroups Other groups †Pteraspido morphi(extinct) †Pteraspidomorphi ('wing shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida. †Thelodonti(extinct) Thelodonti ('feeble teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of 'thelodont scales'. This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was. Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free swimming organisms. †Anaspida(extinct) Anaspida ('no shields') is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
†Osteostraci (вымершие) Osteostraci («костные раковины») обитали на территории современной Северной Америки, Европы и России с Среднего силура до Позднего девона. Анатомически остеостраканы, особенно девонские виды, были одними из самых продвинутых из всех известных бесчелюстных. Это связано с развитием парных плавников и их сложной анатомией черепа. Остеостраканы были более похожи на миног, чем на челюстных позвоночных, поскольку имели две пары полукружных каналов во внутреннем ухе, в отличие от трех пар, обнаруженных во внутренних ушах челюстных позвоночных. Считается, что они являются сестринской группой питуриаспид. Вместе эти два таксона бесчелюстных позвоночных являются сестринской группой гнатостомов. Несколько синапоморфий подтверждают эту гипотезу, такие как наличие: склеротических косточек, парных грудных плавников, дермального скелета с тремя слоями (базальный слой изопедина, средний слой губчатой кости и поверхностный слой дентина) и перихондральной кости.
Major groups of ostracoderms Group Class Image Description Cephalaspido morphi Cephalaspidomorphi or cephalaspids ('head shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. †Galeaspida(extinct) Galeaspida ('helmet shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, and one species, Tujiaaspis vividus, had paired fins. Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase. †Pituriaspida(extinct) Pituriaspida ('pituri shields') are a small group of extinct armoured jawless fishes with tremendous nose like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). They are known only by two species, Pituriaspis doylei and Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia. †Osteostraci(extinct) Osteostraci ('bony shells') lived in what is now North America, Europe and Russia from the 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 of pituriaspids. Together, these two taxa of jawless vertebrates are the sister group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone. Othergroups Other groups †Pteraspido morphi(extinct) †Pteraspidomorphi ('wing shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida. †Thelodonti(extinct) Thelodonti ('feeble teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of 'thelodont scales'. This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was. Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free swimming organisms. †Anaspida(extinct) Anaspida ('no shields') is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
Другие группы
Major groups of ostracoderms Group Class Image Description Cephalaspido morphi Cephalaspidomorphi or cephalaspids ('head shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. †Galeaspida(extinct) Galeaspida ('helmet shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, and one species, Tujiaaspis vividus, had paired fins. Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase. †Pituriaspida(extinct) Pituriaspida ('pituri shields') are a small group of extinct armoured jawless fishes with tremendous nose like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). They are known only by two species, Pituriaspis doylei and Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia. †Osteostraci(extinct) Osteostraci ('bony shells') lived in what is now North America, Europe and Russia from the 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 of pituriaspids. Together, these two taxa of jawless vertebrates are the sister group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone. Othergroups Other groups †Pteraspido morphi(extinct) †Pteraspidomorphi ('wing shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida. †Thelodonti(extinct) Thelodonti ('feeble teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of 'thelodont scales'. This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was. Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free swimming organisms. †Anaspida(extinct) Anaspida ('no shields') is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
†Pteraspido morphi (вымершие) †Pteraspidomorphi («формы крыловидных щитов») имеют обширную защиту головы. Многие имели гипоцеркальные хвосты для создания подъемной силы, облегчающей передвижение по воде для их бронированных тел, покрытых костной кожей. У них также были присосательные ротовые придатки, и некоторые виды могли обитать в пресной воде. Таксон включает подгруппы Heterostraci, Astraspida, Arandaspida.
Major groups of ostracoderms Group Class Image Description Cephalaspido morphi Cephalaspidomorphi or cephalaspids ('head shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. †Galeaspida(extinct) Galeaspida ('helmet shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, and one species, Tujiaaspis vividus, had paired fins. Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase. †Pituriaspida(extinct) Pituriaspida ('pituri shields') are a small group of extinct armoured jawless fishes with tremendous nose like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). They are known only by two species, Pituriaspis doylei and Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia. †Osteostraci(extinct) Osteostraci ('bony shells') lived in what is now North America, Europe and Russia from the 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 of pituriaspids. Together, these two taxa of jawless vertebrates are the sister group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone. Othergroups Other groups †Pteraspido morphi(extinct) †Pteraspidomorphi ('wing shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida. †Thelodonti(extinct) Thelodonti ('feeble teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of 'thelodont scales'. This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was. Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free swimming organisms. †Anaspida(extinct) Anaspida ('no shields') is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
†Thelodonti (вымершие) Thelodonti («слабые зубы») — группа мелких вымерших бесчелюстных рыб с отличительными чешуями вместо крупных пластин брони. Существует много споров о том, представляет ли группа палеозойских рыб, известная как Thelodonti (ранее coelolepids), монофилетическую группировку или разрозненные стволовые группы к основным линиям рыб без челюстей и с челюстями. Телодонтов объединяет наличие «телодонтовых чешуек». Этот определяющий признак не обязательно является результатом общего происхождения, поскольку он мог развиться независимо в разных группах. Таким образом, телодонты обычно рассматриваются как полифилетическая группа, хотя по этому вопросу нет твердого согласия; если они монофилетичны, то нет твердых доказательств их предкового состояния. Телодонты были морфологически очень похожи и, вероятно, тесно связаны с рыбами классов Heterostraci и Anaspida, отличаясь главным образом покрытием отличительными, маленькими, шиповатыми чешуями. Эти чешуи легко рассеивались после смерти; их небольшой размер и прочность делают их наиболее распространенными окаменелостями позвоночных того времени. Рыбы обитали как в пресноводных, так и в морских средах, впервые появившись в ордовикском периоде и вымершие во время вымирания фрасна-фаменна в Позднем девоне. Они были преимущественно донными фильтраторами, хотя есть свидетельства того, что некоторые виды поднимались в толщу воды, чтобы стать свободноплавающими организмами.
Major groups of ostracoderms Group Class Image Description Cephalaspido morphi Cephalaspidomorphi or cephalaspids ('head shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. †Galeaspida(extinct) Galeaspida ('helmet shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, and one species, Tujiaaspis vividus, had paired fins. Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase. †Pituriaspida(extinct) Pituriaspida ('pituri shields') are a small group of extinct armoured jawless fishes with tremendous nose like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). They are known only by two species, Pituriaspis doylei and Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia. †Osteostraci(extinct) Osteostraci ('bony shells') lived in what is now North America, Europe and Russia from the 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 of pituriaspids. Together, these two taxa of jawless vertebrates are the sister group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone. Othergroups Other groups †Pteraspido morphi(extinct) †Pteraspidomorphi ('wing shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida. †Thelodonti(extinct) Thelodonti ('feeble teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of 'thelodont scales'. This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was. Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free swimming organisms. †Anaspida(extinct) Anaspida ('no shields') is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
†Anaspida (вымершие) Anaspida («без щитов») — вымершая группа примитивных бесчелюстных позвоночных, обитавших в силурийском и девонском периодах. Анаспиды были мелкими морскими агнатами, лишенными тяжелого костного щита и парных плавников, но обладали сильно гипоцеркальным хвостом. Они впервые появились в раннем силуре и процветали до вымирания в Позднем девоне, где большинство видов, за исключением миног, вымерли из-за экологических потрясений в то время.
Major groups of ostracoderms Group Class Image Description Cephalaspido morphi Cephalaspidomorphi or cephalaspids ('head shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. †Galeaspida(extinct) Galeaspida ('helmet shields') have massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, and one species, Tujiaaspis vividus, had paired fins. Galeaspida are regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase. †Pituriaspida(extinct) Pituriaspida ('pituri shields') are a small group of extinct armoured jawless fishes with tremendous nose like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). They are known only by two species, Pituriaspis doylei and Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia. †Osteostraci(extinct) Osteostraci ('bony shells') lived in what is now North America, Europe and Russia from the 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 of pituriaspids. Together, these two taxa of jawless vertebrates are the sister group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone. Othergroups Other groups †Pteraspido morphi(extinct) †Pteraspidomorphi ('wing shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida. †Thelodonti(extinct) Thelodonti ('feeble teeth') are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of 'thelodont scales'. This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was. Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free swimming organisms. †Anaspida(extinct) Anaspida ('no shields') is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.