Utente:Marrabbio2/Pesci

I primi pesci fossili: ostracodermi (sopra) e Placodermi

L'evoluzione dei pesci non è studiata come un singolo evento da quanto la riorganizzazione sistematica dei pesci ha portato al disconocimento di questi come gruppo monofileticoTemplate:Sfn. I primi pesci apparvero durante la cosidetta esplosione cambriana, circa 530 milioni di anni fa: questi ritrovamenti fossili appartengono ad un gruppo di piccoli pesci corazzati senza mascelle, noti come ostracodermi, oggi estinti. Un clade imparentato ed esistente, comprendente le lamprede. Le prime mascelle appaiono invece nei Placodermi. La diversità di questi vertebrati indica il vantaggio evolutivo di una bocca provvista di mascelle, anche se ancora non è chiaro chiaro se il vantaggio comprenda un morso con maggior resistenza, una respirazione migliorata o una combinazione di entrambi i fattori.

OverviewModifica

Template:Multiple image

 Lo stesso argomento in dettaglio: Vertebrate paleontology e List of transitional fossils § Invertebrates to Fish.

Fish may have evolved from an animal similar to a coral-like sea squirt, whose larvae resemble primitive fish in important ways. The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today), although perhaps the reverse is the case. Vertebrates, among them the first fishes, originated about 530 million years ago during the Cambrian explosion, which saw the rise in organism diversity.Template:Sfn

 
Outdated evolutionary view of continual gradation (click to animate)
 
The early vertebrate Haikouichthys, from about Template:Ma in China, may be the "ancestor to all vertebrates" and is one of the earliest known fish.Template:R

The first ancestors of fish, or animals that were probably closely related to fish, were Pikaia, Haikouichthys and Myllokunmingia.Template:RTemplate:Sfn These three genera all appeared around 530 Ma. Pikaia had a primitive notochord, a structure that could have developed into a vertebral column later. Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail.Template:R All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.Template:Sfn

These were followed by indisputable fossil vertebrates in the form of heavily armoured fishes discovered in rocks from the Ordovician Period 500–430 Ma. The colonisation of new niches resulted in massive body sizes. In this way, fishes with increasing sizes evolved during the early Paleozoic, such as the titanic placoderm Dunkleosteus, which could grow 7 meters long. The Devonian Period (395 to 345 Ma) brought in the changes that allowed primitive air-breathing fish to remain on land as long as they wished, thus becoming the first terrestrial vertebrates, the amphibians.

The first jawed vertebrates appeared in the late Ordovician and became common in the Devonian, often known as the "Age of Fishes".Template:Sfn The two groups of bony fishes, the actinopterygii and sarcopterygii, evolved and became common.Template:Sfn The Devonian also saw the demise of virtually all jawless fishes, save for lampreys and hagfish, as well as the Placodermi, a group of armoured fish that dominated much of the late Silurian. The Devonian also saw the rise of the first labyrinthodonts, which was a transitional between fishes and amphibians.

The reptiles appeared from labyrinthodonts in the subsequent Carboniferous period. The anapsid and synapsid reptiles were common during the late Paleozoic, while the diapsids became dominant during the Mesozoic. In the sea, the bony fishes became dominant. The dinosaurs gave rise to the birds in the Jurassic.Template:Sfn The demise of the dinosaurs at the end of the Cretaceous promoted expansion of the mammals, which had evolved from the therapsids, a group of synapsid reptiles, during the late Triassic Period.

The later radiations, such as those of fish in the Silurian and Devonian periods, involved fewer taxa, mainly with very similar body plans. The first animals to venture onto dry land were arthropods. Some fish had lungs and strong, bony fins and could crawl onto the land also.

Pesci senza mascelleModifica

 
A modern jawless fish, the lamprey, attached to a modern jawed fish
 Lo stesso argomento in dettaglio: Agnatha.

Jawless fishes belong to the superclass Agnatha in the phylum Chordata, subphylum Vertebrata. Agnatha comes from the Greek, and means "no jaws".[1] It excludes all vertebrates with jaws, known as gnathostomes. Although a minor element of modern marine fauna, jawless fish were prominent among the early fish in the early Paleozoic. Two types of Early Cambrian animal apparently having fins, vertebrate musculature, and gills are known from the early Cambrian Maotianshan shales of China: Haikouichthys and Myllokunmingia. They have been tentatively assigned to Agnatha by Janvier. A third possible agnathid from the same region is Haikouella. A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian Burgess Shale of British Columbia.

Many Ordovician, Silurian, and Devonian agnathians were armoured with heavy bony-spiky plates. The first armoured agnathans—the Ostracoderms, precursors to the bony fish and hence to the tetrapods (including humans)—are known from the middle Ordovician, and by the Late Silurian the agnathans had reached the high point of their evolution. Most of the ostracoderms, such as thelodonts, osteostracans, and galeaspids, were more closely related to the gnathostomes than to the surviving agnathans, known as cyclostomes. Cyclostomes apparently split from other agnathans before the evolution of dentine and bone, which are present in many fossil agnathans, including conodonts.[2] Agnathans declined in the Devonian and never recovered.

The agnathans as a whole are paraphyletic,[3] because most extinct agnathans belong to the stem group of gnathostomes.[4][5] Recent molecular data, both from rRNA[6] and from mtDNA[7] strongly supports the theory that living agnathans, known as cyclostomes, are monophyletic.[8] In phylogenetic taxonomy, the relationships between animals are not typically divided into ranks, but illustrated as a nested "family tree" known as a cladogram. Phylogenetic groups are given definitions based on their relationship to one another, rather than purely on physical traits such as the presence of a backbone. This nesting pattern is often combined with traditional taxonomy, in a practice known as evolutionary taxonomy.

 
Evolution of jawless fishes. The diagram is based on Michael Benton, 2005.[9]

Il cladogramma sottostante dei pesci senza mandibole è basato sugli studi effettuati Philippe Janvier e altri per il progetto Tree of Life Web Project.[10] († = indica che il gruppo è estinto)


Pesci senza mascelle

Hyperoartia (lamprede)

?†Euconodonta (Animali simili ad anguille)

unnamed

Pteraspidomorphi (Pesci senza mascelle)

?†Thelodonti (Pesci senza mascelle con scaglie)

unnamed

?†Anaspida (antenati senza mascelle delle lamprede)

unnamed

Galeaspida (Pesci senza mascelle con scudo osseo cranico)

unnamed

?†Pituriaspida (Pesci senza mandibola corazzati con rostro)

Osteostraci (Pesci senza mandibola corazzati con scudo osseo cranico)

Vertebrati con mandibola (vedi la prossima sezione)

ConodontiModifica

Conodonts resembled primitive jawless eels. They appeared 495 Ma and were wiped out 200 Ma.Template:R Initially they were known only from tooth-like microfossils called conodont elements. These "teeth" have been variously interpreted as filter-feeding apparatuses or as a "grasping and crushing array".Template:R Conodonts ranged in length from a centimeter to the 40 cm Promissum.Template:R Their large eyes had a lateral position of which makes a predatory role unlikely. The preserved musculature hints that some conodonts (Promissum at least) were efficient cruisers but incapable of bursts of speed.Template:R In 2012 researchers classify the conodonts in the phylum Chordata on the basis of their fins with fin rays, chevron-shaped muscles and notochord.Template:R Some researchers see them as vertebrates similar in appearance to modern hagfish and lampreys,Template:R though phylogenetic analysis suggests that they are more derived than either of these groups.Template:R

OstracodermiModifica

Ostracoderms (shell-skinned) are armoured jawless fishes of the Paleozoic. The term does not often appear in classifications today because it is paraphyletic or polyphyletic, and has no phylogenetic meaning.Template:Sfn However, the term is still used informally to group together the armoured jawless fishes.

The ostracoderm armour consisted of 3–5 mm polygonal plates which shielded the head and gills, and then overlapped further down the body like scales. The eyes were particularly shielded. Earlier chordates used their gills for both respiration and feeding, whereas ostracoderms used their gills for respiration only. They had up to eight 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.

The first fossil fishes that were discovered were ostracoderms. The Swiss anatomist Louis Agassiz received some fossils of bony armored fish from Scotland in the 1830s. He had a hard time classifying them as they did not resemble any living creature. He compared them at first with extant armored fish such as catfish and sturgeons but later realizing that they had no movable jaws, classified them in 1844 into a new group "ostracoderms".[11]

Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids. Later, about 420 million years ago, the jawed fish evolved from one of the ostracoderms. After the appearance of jawed fish, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period.[12]

Pesci con mascellaModifica

Template:Further2

The vertebrate jaw probably originally evolved in the Silurian period and appeared in the Placoderm fish which further diversified in the Devonian. The two most anterior pharyngeal arches are thought to have become the jaw itself and the hyoid arch, respectively. The hyoid system suspends the jaw from the braincase of the skull, permitting great mobility of the jaws. While there is no fossil evidence directly to support this theory, it makes sense in light of the numbers of pharyngeal arches that are visible in extant jawed vertebrates (the Gnathostomes), which have seven arches, and primitive jawless vertebrates (the Agnatha), which have nine.

As in most vertebrates, fish jaws are bony or cartilaginous and oppose vertically, comprising an upper jaw and a lower jaw. The jaw is derived from the most anterior two pharyngeal arches supporting the gills, and usually bears numerous teeth.

It is thought that the original selective advantage garnered by the jaw was not related to feeding, but to increased respiration efficiency. The jaws were used in the buccal pump (observable in modern fish and amphibians) that pumps water across the gills of fish or air into the lungs in the case of amphibians. Over evolutionary time the more familiar use of jaws (to humans), in feeding, was selected for and became a very important function in vertebrates. Many teleost fish have substantially modified their jaws for suction feeding and jaw protrusion, resulting in highly complex jaws with dozens of bones involved.

Jawed vertebrates and jawed fish evolved from jawless fish, and the cladogram below for jawed vertebrates is a continuation of the cladogram in the section above. († = group is extinct)


Jawed vertebrates

Placodermi (armoured fishes)

unnamed

Chondrichthyes (cartilaginous fishes)

Teleostomi

Acanthodii (spiny sharks)

Bony fishes

Actinopterygii (ray-finned fishes) <dominant class of fish today

Lobe-finned fishes

?†Onychodontiformes (lobe-finned)

Actinistia (coelacanths)

unnamed

Porolepiformes (lobe-finned)

Dipnoi (lungfishes)

unnamed

Rhizodontimorpha (predatory lobe-finned)

Tristichopteridae (tetrapodomorphs)

Tetrapods (four legged animals)

PlacodermiModifica

 
Evolution of the (now extinct) placoderms. The diagram is based on Michael Benton, 2005.[9]
 
Placoderms (extinct) were armoured jawed fishes (compare with the ostracoderms above)

Template:Further2

Placoderms, class Placodermi (plate skinned), are extinct armoured prehistoric fish, which appeared about 430 Ma in the late Silurian. They were mostly wiped out at the end of the Devonian 360 Ma, though a few survived another 5 million years. Their head and thorax were covered with massive and often ornamented armoured plates. The rest of the body was scaled or naked, depending on the species. The armour shield was articulated, with the head armour hinged to the thoratic armour. This allowed placoderms to lift their heads, unlike ostracoderms. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches. The chart on the right shows the rise and demise of the separate placoderm lineages: Acanthothoraci, Rhenanida, Antiarchi, Petalichthyidae, Ptyctodontida and Arthrodira.

Spiny sharksModifica

 
Spiny sharks (extinct) were the earliest known jawed fishes. They resembled sharks but were an independent branch.

Template:Further2

Spiny sharks, class Acanthodii, are extinct fishes which share features with both bony fishes and cartilaginous fishes. Despite being called "spiny sharks," acanthodians predate sharks. They evolved in the sea at the beginning of the Silurian Period, some 50 million years before the first sharks appeared. Eventually competition from bony fishes proved too much, and the spiny sharks died out in Permian times about 250 Ma. In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans (gars, bowfins). They may have been an independent phylogenetic branch of fishes, which had evolved from little-specialized forms close to recent cartilaginous fishes.

Pesci cartilagineiModifica

 
Radiation of cartilaginous fishes, derived from work by Michael Benton, 2005.[13]

Template:Further2

Cartilaginous fishes, class Chondrichthyes, consisting of sharks, rays and chimaeras, appeared by about 395 million years ago, in the middle Devonian. The class contains the sub classes Holocephali (chimera) and Elasmobranchii (sharks and rays). The radiation of elasmobranches in the chart on the right is divided into the taxa: Cladoselache, Eugeneodontiformes, Symmoriida, Xenacanthiformes, Ctenacanthiformes, Hybodontiformes, Galeomorphi, Squaliformes and Batoidea.

Pesci osseiModifica

Template:Further2

Bony fishes, class Osteichthyes, are characterised by bony skeleton rather than cartilage. They appeared in the late Silurian or early Devonian, about 416 million years ago. Both bony and cartilaginous fishes may have arisen from either the spiny sharks or the placoderms. A subclass of the Osteichthyes, the ray-finned fishes (Actinopterygii), have become the dominant group of fishes in the post-Paleozoic and modern world, with some 30,000 living species.

The bony (and cartilaginous) fish groups that emerged after the Devonian, were characterised by steady improvements in foraging and locomotion.Template:Sfn

Lobe-finned fishesModifica

Template:Further2

 
The Queensland lungfish is a lobe-finned fish which is a living fossil. Lungfish evolved the first proto-lungs and proto-limbs. They developed the ability to live outside a water environment in the middle Devonian (397-385 Ma), and have remained virtually the same for over 100 million years.[14]

Lobe-finned fishes, fish belonging to the class Sarcopterygii, are mostly extinct bony fishes, basally characterised by robust and stubby lobe fins containing a robust internal skeleton, cosmoid scales and internal nostrils. Their fins are fleshy, lobed, paired fins, joined to the body by a single bone.[15] The fins of lobe-finned fish differ from those of all other fish in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. The pectoral and pelvic fins are articulated in ways resembling the tetrapod limbs they were the precursors to. The fins evolved into the legs of the first tetrapod land vertebrates, amphibians. They also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of ray-finned fish. The braincase of lobe-finned fishes primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early lobe-finned fishes have a symmetrical tail. All lobe-finned fishes possess teeth covered with true enamel.

Lobe-finned fishes, such as coelacanths and lungfish, were the most diverse group of bony fishes in the Devonian. Taxonomists who subscribe to the cladistic approach include the grouping Tetrapoda within the Sarcopterygii, and the tetrapods in turn include all species of four-limbed vertebrates.Template:Sfn The fin-limbs of lobe-finned fishes such as the coelacanths show a strong similarity to the expected ancestral form of tetrapod limbs. The lobe-finned fish apparently followed two different lines of development and are accordingly separated into two subclasses, the Rhipidistia (including the lungfish, and the Tetrapodomorpha which include the Tetrapoda) and the Actinistia (coelacanths). The first lobe-finned fishes, found in the uppermost Silurian (ca 418 Ma), closely resembled spiny sharks, which went extinct at the end of the Paleozoic. In the early–middle Devonian (416 - 385 Ma), while the predatory placoderms dominated the seas, some lobe-finned fishes came into freshwater habitats.

 
The coelacanth is another lobe finned fish which is a living fossil. It is thought to have evolved into roughly its current form about 408 million years ago, during the early Devonian,Template:R and has not essentially evolved further from its ancient form.Template:Sfn

In the Early Devonian (416-397 Ma), the lobe-finned fishes split into two main lineages — the coelacanths and the rhipidistians. The former never left the oceans and their heyday was the Late Devonian and Carboniferous, from 385 to 299 Ma, as they were more common during those periods than in any other period in the Phanerozoic; coelacanths still live today in the oceans (genus Latimeria). The Rhipidistians, whose ancestors probably lived in estuaries, migrated into freshwater habitats. They in turn split into two major groups: the lungfish and the tetrapodomorphs. The lungfish's greatest diversity was in the Triassic period; today there are fewer than a dozen genera left. The lungfish evolved the first proto-lungs and proto-limbs; developing the ability to live outside a water environment in the middle Devonian (397-385 Ma). The first tetrapodomorphs, which included the gigantic rhizodonts, had the same general anatomy as the lungfish, who were their closest kin, but they appear not to have left their water habitat until the late Devonian epoch (385 - 359 Ma), with the appearance of tetrapods (four-legged vertebrates). Tetrapods are the only tetrapodomorphs which survived after the Devonian. Lobe-finned fishes continued until towards the end of Paleozoic era, suffering heavy losses during the Permian-Triassic extinction event (251 Ma).

Ray-finned fishesModifica

Ray-finned fishes, class Actinopterygii, differ from lobe-finned fishes in that their fins consist of webs of skin supported by spines ("rays") made of bone or horn. There are other differences in respiratory and circulatory structures. Ray-finned fishes normally have skeletons made from true bone, though this is not true of sturgeons and paddlefishes.[16]

Ray-finned fishes are the dominant vertebrate group, containing half of all known vertebrate species. They inhabit abyssal depths in the sea, coastal inlets and freshwater rivers and lakes, and are a major source of food for humans.[16]

TimelineModifica

Template:Phanerozoic eon

Pre-Devoniano: origine dei pesciModifica

Cambriano Cambriano (542–488 milioni di anni fa): l'inizio del Cambriano è caratterizzato dalla cosiddetta "Esplosione cambriana", un'improvvisa apparizione di quasi tutti i phyla animali invertebrati (molluschi, meduse, vermi e artropodi, come i crostacei) in grande abbondanza. I primi vertebrati apparvero sotto forma di pesci primitivi, che si sono evoluti poi in modo molto diversificato nel Siluriano e Devoniano.
  Pikaia

Pikaia, along with Myllokunmingia and Haikouichthys ercaicunensis immediately below, are all candidates in the fossil record for the titles of "first vertebrate" and "first fish". Pikaia is a genus that appeared about 530 Ma during the Cambrian explosion of multicellular life. Pikaia gracilens (pictured) is a transitional fossil between invertebrates and vertebrates,Template:Sfn and may be the earliest known chordate.Template:RTemplate:R In this sense it may have been the original ancestor of fishes. It was a primitive creature with no evidence of eyes, without a well defined head, and less than 2 inches (5 centimetres) long. Pikaia was a sideways-flattened, leaf-shaped animal which swam by throwing its body into a series of S-shaped, zig-zag curves, similar to movement of snakes. Fish inherited the same swimming movement, but they generally have stiffer backbones. It had a pair of large head tentacles and a series of short appendages, which may be linked to gill slits, on either side of its head. Pikaia shows the essential prerequisites for vertebrates. The flattened body is divided into pairs of segmented muscle blocks, seen as faint vertical lines. The muscles lie on either side of a flexible structure resembling a rod that runs from the tip of the head to the tip of the tail.Template:Sfn

  Haikouichthys Haikouichthys (il cui nome scientifico significa pesce di Haikou) è un altro genere che appare nei ritrovamenti fossili di circa 530 milioni di anni fa, e segnala la transizione da invertebrati a vetrebratiTemplate:Sfn Gli Haikouichthys appartengono ai craniati (animali con teschio e testa distinta dal corpo). Diversamente dai Pikaia, vi è la presenza di occhi. Inoltre hanno un teschio definito e altre caratteristiche che hanno convinto i paleontologi a classificarli come craniati e come il primo vero pesce. Analisi cladistiche successive hanno suggerito che questi animali fossero cordati o craniati; Template:R senza dare risultati indiscutibili e incontrovertibili, pertanto si fanno rientrare in entrambi i gruppi.Template:RTemplate:Sfn
  Myllokunmingia Myllokunmingia è un genere apparso circa 530 milioni di anni fa. Apparteneva ai cordati, aveva dimensioni minute (28 mm di lunghezza e 6 di altezza) ed è tra le più antiche forme di vita vertebrate.
  Conodonti Conodonts (cone-teeth) resembled primitive eels. They appeared 495 Ma and were wiped out 200 Ma.Template:R Initially they were known only from tooth-like microfossils called conodont elements. These "teeth" have been variously interpreted as filter-feeding apparatuses or as a "grasping and crushing array".Template:R Conodonts ranged in length from a centimeter to the 40 cm Promissum.Template:R Their large eyes had a lateral position of which makes a predatory role unlikely. The preserved musculature hints that some conodonts (Promissum at least) were efficient cruisers but incapable of bursts of speed.Template:R In 2012 researchers classify the conodonts in the phylum Chordata on the basis of their fins with fin rays, chevron-shaped muscles and notochord.Template:R Some researchers see them as vertebrates similar in appearance to modern hagfish and lampreys,Template:R though phylogenetic analysis suggests that they are more derived than either of these groups.Template:R
  Ostracodermi Ostracoderms (shell-skinned) are any of several groups of extinct, primitive, jawless fishes that were covered in an armour of bony plates. They appeared in the Cambrian, about 510 million years ago, and became extinct towards the end of the Devonian, about 377 million years ago. Initially Ostracoderms had poorly formed fins, and paired fins, or limbs, first evolved within this group. They were covered with a bony armour or scales and were often less than 30 cm (1 ft) long.
Ordoviciano Ordoviciano (488–443 milioni di anni fa): i pesci, la prima forma di vita vertebrata, continuano ad evolvere: compaiono i primi pesci con mandibole (Gnathostomata).
La vita sulla terraferma non si è ancora differenziata.
  Heterostraci Heterostraci è una classe estinta di agnati vertebrati che viveva principalmente in ambienti marini e di estuario. Sono apparsi all'inizio dell'Ordoviciano e si estinse alla fine del Devoniano. Differivano dagli altri pesci senza mandibola del periodo nella disposizione e nell'istologia delle loro scaglie.
Astraspis Astraspis (star shield) is an extinct genus of primitive jawless fish related to other Ordovician fishes, such as Sacabambaspis and Arandaspis. Fossils show clear evidence of a sensory structure (lateral line system). The arrangement of these organs in regular lines allows the fish to detect the direction and distance from which a disturbance in the water is coming. Arandaspis are thought to have had a mobile tail covered with small protective plates and a head region covered with larger plates. A specimen described by Sansom et al. had relatively large, lateral eyes and a series of eight gill openings on each side.Template:R
  Pteraspidomorphi Pteraspidomorphi is an extinct class of early jawless fish. The fossils show 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.
Thelodonti Thelodonts (nipple teeth) are a class of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is debate over whether these represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish.Template:R Thelodonts are united by their characteristic "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.Template:Sfn If they are monophyletic, there is no firm evidence on what their ancestral state was.Template:Sfn These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time.Template:SfnTemplate:R 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 some species may have been pelagic.
The Ordovician ended with the Ordovician–Silurian extinction event (450–440 Ma). Two events occurred that killed off 27% of all families, 57% of all genera and 60% to 70% of all species.Template:R Together they are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct.
Siluriano Silurian (443–419 Ma): Many evolutionary milestones occurred during this period, including the appearance of armoured jawless fish, jawed fish, spiny sharks and ray-finned fish.
  While it is traditional to refer to the Devonian as the age of fishes, recent findings have shown the Silurian was also a period of considerable diversification. Jawed fish developed movable jaws, adapted from the supports of the front two or three gill arches
Anaspida Anaspida (without shield) is an extinct class of primitive jawless vertebrates that lived during the Silurian and Devonian periods.Template:Sfn They are classically regarded as the ancestors of lampreys.Template:Sfn Anaspids were small, primarily marine agnathans that lacked heavy bony shield and paired fins, but have highly exaggerated hypocercal tails. They first appeared in the early Silurian, and flourished until the Late Devonian extinction,Template:Sfn where most species, save for lampreys, went extinct. Unusually for an agnathan, anaspids did not possess a bony shield or armour. The head is instead covered in an array of smaller, weakly mineralised scales.Template:Sfn
  Spiny sharks Spiny sharks, more formally called "Acanthodians" (having spines), comprise of the class Acanthodii. They first appeared by the late Silurian ~420 Ma, and were among the first fishes to evolve jaws. They share features with both cartilaginous fish and bony fish, but they are not true sharks. They became extinct before the end of the Permian ~250 Ma. However, scales and teeth attributed to this group, as well as more derived jawed fish, such as cartilaginous and bony fish, date from the Ordovician ~460 Ma. Acanthodians were generally small shark-like fishes varying from toothless filter-feeders to toothed predators. They were once often classified as an order of the class Placodermi, but recent authorities tend to place the acanthodians nearer to or within the living jawed fishes. They are distinguished in two respects: they were the earliest known jawed vertebrates, and they had stout spines supporting all their fins, fixed in place and non-movable (like a shark's dorsal fin), an important defensive adaptation. Their fossils are extremely rare.
  Placodermas Placodermis, (plate-like skin), are a group of armoured jawed fishes, of the class Placodermi. The oldest fossils appeared during the late Silurian, and became extinct at the end of the Devonian. Recent studies suggest that the placoderms are possibly a paraphyletic group of basal jawed fishes, and the closest relatives of all living jawed vertebrates. Some placoderms were small, flattened bottom-dwellers, such as antiarchs. However many, particularly the arthrodires, were active midwater predators. Dunkleosteus, which appeared later in the Devonian below, was the largest and most famous of these. The upper jaw was firmly fused to the skull, but there was a hinge joint between the skull and the bony plating of the trunk region. This allowed the upper part of the head to be thrown back and, in arthrodires, allowed them to take larger bites.
  Guiyu oneiros Guiyu oneiros è il più antico pesce osseo oggi conosciuto. Presenta una combinazione di caratteri tipici degli Actinopterygii e dei Sarcopterygii, anche se l'analisi scientifica lo pone più vicino a questi ultimi.
Andreolepis Il genere Andreolepis include il più antico attinopterigio oggi conosciuto, Andreolepis hedei, che comparve nel tardo Siluriano, circa 420 milioni di anni fa.[17][18]

Devoniano: l'era dei pesciModifica

Template:Include timeline

The Devonian Period is broken into the Early, Middle and Late Devonian. By the start of the Early Devonian 419 mya, jawed fishes had divided into four distinct clades: the placoderms and spiny sharks, both of which are now extinct, and the cartilaginous and bony fishes, both of which are still extant. The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. Both the cartilaginous and bony fishes may have arisen from either the placoderms or the spiny sharks. A subclass of bony fishes, the ray-finned fishes (Actinopterygii), have become the dominant group in the post-Paleozoic and modern world, with some 30,000 living species.

Sea levels in the Devonian were generally high. Marine faunas were dominated by bryozoa, diverse and abundant brachiopods, the enigmatic hederelloids, microconchids and corals. Lily-like crinoids were abundant, and trilobites were still fairly common. Among vertebrates, jawless armoured fish (ostracoderms) declined in diversity, while the jawed fish (gnathostomes) simultaneously increased in both the sea and fresh water. Armoured placoderms were numerous during the lower stages of the Devonian Period and became extinct in the Late Devonian, perhaps because of competition for food against the other fish species. Early cartilaginous (Chondrichthyes) and bony fishes (Osteichthyes) also become diverse and played a large role within the Devonian seas. The first abundant genus of shark, Cladoselache, appeared in the oceans during the Devonian Period. The great diversity of fish around at the time, have led to the Devonian being given the name "The Age of Fish" in popular culture. every known aquatic environment.

The first ray-finned and lobe-finned bony fish appeared in the Devonian, while the placoderms began dominating almost every known aquatic environment. However, another subclass of Osteichthyes, the Sarcopterygii, including lobe-finned fishes including coelacanths and lungfish) and tetrapods, was the most diverse group of bony fishes in the Devonian. Sarcopterygians are basally characterized by internal nostrils, lobe fins containing a robust internal skeleton, and cosmoid scales.

During the Middle Devonian 393–383 Ma, the armoured jawless ostracoderm fishes were declining in diversity; the jawed fish were thriving and increasing in diversity in both the oceans and freshwater. The shallow, warm, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop essential characteristics such as well developed lungs, and the ability to crawl out of the water and onto the land for short periods of time. Cartilaginous fishes, class Chondrichthyes, consisting of sharks, rays and chimaeras, appeared by about 395 million years ago, in the middle Devonian

During the Late Devonian the first forests were taking shape on land. The first tetrapods appear in the fossil record over a period, the beginning and end of which are marked with extinction events. This lasted until the end of the Devonian 359 mya. The ancestors of all tetrapods began adapting to walking on land, their strong pectoral and pelvic fins gradually evolved into legs (see Tiktaalik).Template:R In the oceans, primitive sharks became more numerous than in the Silurian and the late Ordovician. The first ammonite mollusks appeared. Trilobites, the mollusk-like brachiopods and the great coral reefs, were still common.

The Late Devonian extinction severely affected marine life, killing off all placoderms, and all trilobites, save for a few species of the order Proetida. This major extinction occurred at the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about Template:Geologic Ages Inline Ma, when all the fossil agnathan fishes, save for the psammosteid heterostracans, suddenly disappeared. A second strong pulse closed the Devonian period. The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota, more drastic than the familiar extinction event that closed the Cretaceous. The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems.

Devoniano (419–359 mya): The start of Devonian saw the first appearance of lobe-finned fish, precursors to the tetrapods (animals with four limbs). Major groups of fish evolved during this period, often referred to as the age of fishes.Template:R See Category:Devonian fish.
Devoniano inferiore (419–393 Ma):
  Psarolepis Psarolepis (speckled scale) is a genus of extinct lobe-finned fish which lived around 397 to 418 Ma. Fossils of Psarolepis have been found mainly in South China and described by paleontologist Xiaobo Yu in 1998. It is not known for certain which group Psarolepis belongs, but paleontologists agree that it probably is a basal genus and seems to be close to the common ancestor of lobe-finned and ray-finned fishes.Template:Sfn
  Holoptychius Holoptychius is an extinct genus from the order of porolepiform lobe-finned fish, extant from 416 to 359 Ma. It was a streamlined predator about 50 centimetri (20 in) long (though it could grow up to 2.5 m), which fed on other bony fish. Its rounded scales and body form indicate that it could have swum quickly through the water to catch prey.Template:SfnTemplate:R Similar to other rhipidistians, it had fang-like teeth on its palate in addition to smaller teeth on the jaws. Its asymmetrical tail sported a caudal fin on its lower end. To compensate for the downward push caused by this fin placement, Holoptychius's pectoral fins were placed high on the body.
Ptyctodontida The ptyctodontids (beak-teeth) are an extinct monotypic order of unarmored placoderms, containing only one family. They were extant from the start to the end of the Devonian. With their big heads, big eyes, and long bodies, the ptyctodontids bore a strong resemblance to modern day chimaeras (Holocephali). Their armor was reduced to a pattern of small plates around the head and neck. Like the extinct and related acanthothoracids, and the living and unrelated holocephalians, most of the ptyctodontids are thought to have lived near the sea bottom and preyed on shellfish.
Petalichthyida The Petalichthyida was an order of small, flattened placoderms which were extant from the beginning of the Devonian to the Late Devonian. They were typified by splayed fins and numerous tubercles that decorated all of the plates and scales of their armour. They reached a peak in diversity during the Early Devonian and were found throughout the world. Because they had compressed body forms, it is supposed they were bottom-dwellers that chased after or ambushed smaller fish. Their diet is not clear, as none of the fossil specimens found have preserved mouth parts.
Laccognathus Laccognathus (pitted jaw) was a genus of amphibious lobe-finned fish that existed 398–360 Ma.[19] They were characterized by the three large pits (fossae) on the external surface of the lower jaw which may have had sensory functions.[20] Laccognathus grew to 1–2 metri (3–7 ft) in length. They had very short dorsoventrally flattened heads, less than one-fifth the length of the body.[21] The skeleton was structured so large areas of skin were stretched over solid plates of bone. This bone was composed of particularly dense fibers – so dense that exchange of oxygen through the skin was unlikely. Rather, the dense ossifications served to retain water inside the body as Laccognathus traveled on land between bodies of water.[22]
Devoniano medio (397–385 Ma): Cartilaginous fishes, consisting of sharks, rays and chimaeras, appeared about 395 Ma.
  Dipterus Dipterus (two wings) is an extinct genus of lungfish from 376–361 Ma. It was about 35 centimetri (14 in) long, mostly ate invertebrates, and had lungs, not an air bladder. Like its ancestor Dipnorhynchus it had tooth-like plates on its palate instead of real teeth. However, unlike its modern relatives, in which the dorsal, caudal, and anal fin are fused into one, its fins were still separated. Otherwise Dipterus closely resembled modern lungfish.Template:Sfn
  Cladoselache Cladoselache was the first abundant genus of primitive shark, appearing about 370 Ma.Template:Sfn It grew to 6 piedi (1,8 m) long, with anatomical features similar to modern mackerel sharks. It had a streamlined body almost entirely devoid of scales, with five to seven gill slits and a short, rounded snout that had a terminal mouth opening at the front of the skull.Template:Sfn It had a very weak jaw joint compared with modern-day sharks, but it compensated for that with very strong jaw-closing muscles. Its teeth were multi-cusped and smooth-edged, making them suitable for grasping, but not tearing or chewing. Cladoselache therefore probably seized prey by the tail and swallowed it whole.Template:Sfn It had powerful keels that extended onto the side of the tail stalk and a semi-lunate tail fin, with the superior lobe about the same size as the inferior. This combination helped with its speed and agility which was useful when trying to outswim its probable predator, the heavily armoured 10 metri (33 ft) long placoderm fish Dunkleosteus.Template:Sfn
  Coccosteus Coccosteus (seed bone) is an extinct genus of arthrodire placoderm. The majority of fossils have been found in freshwater sediments, though they may have been able to enter saltwater. They grew up to 40 centimetri (16 in) long. Like all other arthrodires, Coccosteus had a joint between the armour of the body and skull. In addition, it also had an internal joint between its neck vertebrae and the back of the skull, allowing for the mouth to be opened even wider. Along with the longer jaws, this allowed Coccosteus to feed on fairly large prey. As with all other arthrodires, Coccosteus had bony dental plates embedded in its jaws, forming a beak. The beak was kept sharp by having the edges of the dental plates grind away at each other.Template:Sfn
Bothriolepis

Bothriolepis è stato il genere di placodermi più diffuso nei ritrovamenti fossili, con oltre 100 specie scoperte negli strati rocciosi del Devoniano medio in ogni continente.

Pituriaspida Pituriaspida (hallucinogenic shield) is a class containing two bizarre species of armoured jawless fishes with tremendous nose-like rostrums. They lived in estuaries around 390 Ma. The paleontologist Gavin Young, named the class after the hallucinogenic drug pituri, since he thought he might be hallucinating upon viewing the bizarre forms.Template:Sfn The better studied species looked like a throwing-dart-like, with an elongate headshield and spear-like rostrum. The other species looked like a guitar pick with a tail, with a smaller and shorter rostrum and a more triangular headshield.
Late Devonian extinction: 375–360 Ma. A prolonged series of extinctions eliminated about 19% of all families, 50% of all generaTemplate:R and 70% of all species. This extinction event lasted perhaps as long as 20 Ma, and there is evidence for a series of extinction pulses within this period.
Devoniano superiore (383–359 Ma):
  Dunkleosteus

Dunkleosteus is a genus of arthrodire placoderms which was extant from 380 to 360 Ma. It grew up to 10 metri (33 ft) longTemplate:RTemplate:Sfn and weighed up to 3.6 tonnes.Template:R It was a hypercarnivorous apex predator. Apart from its contemporary Titanichthys (below), no other placoderm rivalled it in size. Instead of teeth, Dunkleosteus had two pairs of sharp bony plates which formed a beak-like structure. Apart from megalodon, it had the most powerful bite of any fish,Template:R generating bite forces in the same league as Tyrannosaurus rex and the modern crocodile.Template:R

  Titanichthys Titanichthys is a genus of giant, aberrant marine placoderm which lived in shallow seas. Many of the species approached Dunkleosteus in size and build. Unlike its relative, however, the various species of Titanichys had small, ineffective-looking mouth-plates that lacked a sharp cutting edge. It is assumed that Titanichthys was a filter feeder that used its capacious mouth to swallow or inhale schools of small, anchovy-like fish, or possibly krill-like zooplankton, and that the mouth-plates retained the prey while allowing the water to escape as it closed its mouth.
  Materpiscis

Materpiscis (mother fish) is a genus of ptyctodontid placoderm from about 380 Ma. Known from only one specimen, it is unique in having an unborn embryo present inside, and with remarkable preservation of a mineralised placental feeding structure (umbilical cord). This makes Materpiscis the first known vertebrate to show viviparity, or giving birth to live young.Template:R The specimen was named Materpiscis attenboroughi in honour of David Attenborough.Template:R

Rhizodonts Rhizodonts were an order of lobe-finned fish which survived to the end of the Carboniferous, 377–310 Ma. They reached huge sizes. The largest known species, Rhizodus hibberti grew up to 7 metres in length, making it the largest freshwater fish known.

Dai pesci ai tetrapodiModifica

 Lo stesso argomento in dettaglio: Tetrapod § Evolution e List of transitional fossils § Fish to Tetrapods.
 
A cladogram of the evolution of tetrapods showing some of the best-known transitional fossils. It starts with Eusthenopteron at the bottom, indisputably still a fish, through Panderichthys, Tiktaalik, Acanthostega and Ichthyostega to Pederpes at the top, indisputably a tetrapod

The first tetrapods are four-legged, air-breathing, terrestrial animals from which the land vertebrates descended, including humans. They evolved from lobe-finned fish, appearing in coastal water in the middle Devonian, and giving rise to the first amphibians.Template:R

The group of lobe-finned fishes that were the ancestors of the tetrapod are grouped together as the Rhipidistia,Template:R and the first tetrapods evolved from these fish over the relatively short timespan 385–360 Ma. The early tetrapod groups themselves are grouped as Labyrinthodontia. They retained aquatic, fry-like tadpoles, a system still seen in modern amphibians. From the 1950s to the early 1980s it was thought that tetrapods evolved from fish that had already acquired the ability to crawl on land, possibly in order to go from a pool that was drying out to one that was deeper. However, in 1987, nearly complete fossils of Acanthostega from about Template:Ma showed that this Late Devonian transitional animal had legs and both lungs and gills, but could never have survived on land: its limbs and its wrist and ankle joints were too weak to bear its weight; its ribs were too short to prevent its lungs from being squeezed flat by its weight; its fish-like tail fin would have been damaged by dragging on the ground. The current hypothesis is that Acanthostega, which was about 1 metro (3,3 ft) long, was a wholly aquatic predator that hunted in shallow water. Its skeleton differed from that of most fish, in ways that enabled it to raise its head to breathe air while its body remained submerged, including: its jaws show modifications that would have enabled it to gulp air; the bones at the back of its skull are locked together, providing strong attachment points for muscles that raised its head; the head is not joined to the shoulder girdle and it has a distinct neck.Template:R

 
Until the 1980s early transitional lobe-finned fishes, such as the Eusthenopteron shown here, were depicted as emerging onto land. Paleontologists now widely agree this didn't happen, and they were strictly aquatic.Template:R

The Devonian proliferation of land plants may help to explain why air-breathing would have been an advantage: leaves falling into streams and rivers would have encouraged the growth of aquatic vegetation; this would have attracted grazing invertebrates and small fish that preyed on them; they would have been attractive prey but the environment was unsuitable for the big marine predatory fish; air-breathing would have been necessary because these waters would have been short of oxygen, since warm water holds less dissolved oxygen than cooler marine water and since the decomposition of vegetation would have used some of the oxygen.Template:R

There are three major hypotheses as to how tetrapods evolved their stubby fins (proto-limbs). The traditional explanation is the "shrinking waterhole hypothesis" or "desert hypothesis" posited by the American paleontologist Alfred Romer. He believed limbs and lungs may have evolved from the necessity of having to find new bodies of water as old waterholes dried up.Template:R

The second hypothesis is the "inter-tidal hypothesis" put forward in 2010 by a team of Polish paleontologists lead by Grzegorz Niedźwiedzki. They argued that sarcopterygians may have first emerged unto land from intertidal zones rather than inland bodies of water. Their hypothesis is based on the discovery of the 395 million-year-old Zachełmie tracks in Zachełmie, Poland, the oldest ever discovered fossil evidence of tetrapods.Template:RTemplate:R

The third hypothesis, the "woodland hypothesis", was proposed by the American paleontologist Gregory J. Retallack in 2011. He argues that limbs may have developed in shallow bodies of water in woodlands as a means of navigating in environments filled with roots and vegetation. He based his conclusions on the evidence that transitional tetrapod fossils are consistently found in habitats that were formerly humid and wooded floodplains.Template:R

Research by Jennifer A. Clack and her colleagues showed that the very earliest tetrapods, animals similar to Acanthostega, were wholly aquatic and quite unsuited to life on land. This is in contrast to the earlier view that fish had first invaded the land — either in search of prey (like modern mudskippers) or to find water when the pond they lived in dried out — and later evolved legs, lungs, etc.

Two ideas about the homology of arms, hands and digits have existed in the past 130 years. First that digits are unique to tetrapods[23][24] and second that antecedents were present in the fins of early sarcopterygian fish.[25] Until recently it was believed that "genetic and fossil data support the hypothesis that digits are evolutionary novelties".[26]p. 640. However new research that created a three-dimensional reconstruction of Panderichthys, a coastal fish from the Devonian period 385 million years ago, shows that these animals already had many of the homologous bones present in the forelimbs of limbed vertebrates.[27] For example, they had radial bones similar to rudimentary fingers but positioned in the arm-like base of their fins.[27] Thus there was in the evolution of tetrapods a shift such that the outermost part of the fins were lost and came to be replaced by early digits. This change is consistent with additional evidence from the study of actinopterygians, sharks and lungfish that the digits of tetrapods arose from pre-existing distal radials present in more primitive fish.[27][28] Controversy still exists since Tiktaalik, a vertebrate often considered to be the missing link between fishes and land-living animals, had stubby leg-like limbs that lacked the finger-like radial bones found in the Panderichthys. The researchers of the paper commented that it "is difficult to say whether this character distribution implies that Tiktaalik is autapomorphic, that Panderichthys and tetrapods are convergent, or that Panderichthys is closer to tetrapods than Tiktaalik. At any rate, it demonstrates that the fish–tetrapod transition was accompanied by significant character incongruence in functionally important structures.".[27]p. 638.

From the end of the Devonian to the Mid Carboniferous a 30 million year gap occurs in the fossil record. This gap, called Romer's gap, is marked by the absence of ancestral tetrapod fossils and fossils of other vertebrates that look well-adapted for life on land.Template:R

Transition from lobe-finned fishes to tetrapods
 ~385 milioni di anni fa Eusthenopteron

Genus of extinct lobe-finned fishes which has attained an iconic status from its close relationships to tetrapods. Early depictions of this animal show it emerging onto land, however paleontologists now widely agree that it was a strictly aquatic animal.Template:R The genus Eusthenopteron is known from several species that lived during the Late Devonian period, about 385 Ma. It was the object of intense study from the 1940s to the 1990s by the paleoichthyologist Erik Jarvik.Template:R

  Gogonasus Gogonasus (snout from Gogo) was a lobe-finned fish known from 3-dimensionally preserved 380 million-year-old fossils found in the Gogo Formation. It was a small fish reaching 30–40 cm (1 ft) in length.[29] Its skeleton shows several tetrapod-like features. They included the structure of its middle ear, and its fins show the precursors of the forearm bones, the radius and ulna. Researchers believe it used its forearm-like fins to dart out of the reef to catch prey. Gogonasus was first described in 1985 by John A. Long. For almost 100 years Eusthenopteron has been the role model for demonstrating stages in the evolution of lobe-finned fishes to tetrapods. Gogonasus now replaces Eusthenopteron in being a better preserved representative without any ambiguity in interpreting its anatomy.
 
~385 milioni di anni fa
Panderichthys Adapted to muddy shallows, and capable of some kind of shallow water or terrestrial body flexion locomotion. Had the ability to prop itself up.[30]
They had large tetrapod-like heads, and are thought to be the most crownward stem fish-tetrapod with paired fins.
 
~375 milioni di anni fa
Tiktaalik A fish with limb-like fins that could take it onto land.[31] It is an example from several lines of ancient sarcopterygian fish developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the evolution of tetrapods.[32] Paleontologists suggest that it is representative of the transition between non-tetrapod vertebrates (fish) such as Panderichthys, known from fossils 380 million years old, and early tetrapods such as Acanthostega and Ichthyostega, known from fossils about 365 million years old. Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as a "fishapod".[33][34]
 
365 milioni di anni fa
Acanthostega A fish-like early labyrinthodont which occupied weed-filled swamps and changed views about the early evolution of tetrapods.Template:R It had eight digits on each hand (the number of digits on the feet is unclear) linked by webbing, it lacked wrists, and was generally poorly adapted to come onto land.[35] Sunsequent discoveries revealed earlier transitional forms between Acanthostega and completely fish-like animals.Template:R
 
374–359 milioni di anni fa
Ichthyostega

Until finds of other early tetrapods and closely related fishes in the late 20th century, Ichthyostega stood alone as the transitional fossil between fish and tetrapods, combining a fishlike tail and gills with an amphibian skull and limbs. It possessed lungs and limbs with seven digits that helped it navigate through shallow water in weed-filled swamps.

 
359–345 milioni di anni fa
Pederpes Pederpes is the earliest known fully terrestrial tetrapod. It is included here to complete the transition of lobe-finned fishes to tetrapods, even though Pederpes is no longer a fish.

By the late Devonian, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems to develop, with increasingly complex food webs that afforded new opportunities. Freshwater habitats were not the only places to find water filled with organic matter and choked with plants with dense vegetation near the water's edge. Swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed at this time, and there is much to suggest that this is the kind of environment in which the tetrapods evolved. Early fossil tetrapods have been found in marine sediments, and because fossils of primitive tetrapods in general are found scattered all around the world, they must have spread by following the coastal lines — they could not have lived in freshwater only.

Post-DevonianoModifica

L'Era Mesozoica inizia circa 250 milioni di anni fa sulla scia dell'Estinzione di massa del Permiano-Triassico, la più grande estinzione della storia della Terra, e finisce circa 65 milioni di anni fa, con l'Estinzione di massa del Cretaceo-Paleocene, un'altra estinzione massiva che uccise i dinosauri e moltre specie di piante e animali. É spesso definita come l'età dei rettili, perché i rettili sono stati i vertebrati dominanti in questa era. Durante il Mesozoico inoltre il maxicontinente Pangea si divide gradualmente in continenti più piccoli. Il clima alterna periodi molto caldi a periodi più freddi, ma nel complesso la Terra era più calda di quanto non lo sia oggi.

Carbonifero Carboniferous (359–299 Ma): Sharks underwent a major evolutionary radiation during the Carboniferous.[36] It is believed that this evolutionary radiation occurred because the decline of the placoderms at the end of the Devonian period caused many environmental niches to become unoccupied and allowed new organisms to evolve and fill these niches.[36]
 
Cretoxyrhina and Squalicorax circle a dead Claosaurus
Cretoxyrhina


Squalicorax
The first 15 million years of the Carboniferous has very few terrestrial fossils. This gap in the fossil record, is called Romer's gap after the American palaentologist Alfred Romer. While it has long been debated whether the gap is a result of fossilisation or relates to an actual event, recent work indicates the gap period saw a drop in atmospheric oxygen levels, indicating some sort of ecological collapse.[37] The gap saw the demise of the Devonian fish-like ichthyostegalian labyrinthodonts, and the rise of the more advanced temnospondyl and reptiliomorphan amphibians that so typify the Carboniferous terrestrial vertebrate fauna.

The Carboniferous seas were inhabited by many fish, mainly Elasmobranchs (sharks and their relatives). These included some, like Psammodus, with crushing pavement-like teeth adapted for grinding the shells of brachiopods, crustaceans, and other marine organisms. Other sharks had piercing teeth, such as the Symmoriida; some, the petalodonts, had peculiar cycloid cutting teeth. Most of the sharks were marine, but the Xenacanthida invaded fresh waters of the coal swamps. Among the bony fish, the Palaeonisciformes found in coastal waters also appear to have migrated to rivers. Sarcopterygian fish were also prominent, and one group, the Rhizodonts, reached very large size.

Most species of Carboniferous marine fish have been described largely from teeth, fin spines and dermal ossicles, with smaller freshwater fish preserved whole. Freshwater fish were abundant, and include the genera Ctenodus, Uronemus, Acanthodes, Cheirodus, and Gyracanthus.

  Stethacanthidae

As a result of the evolutionary radiation carboniferous sharks assumed a wide variety of bizarre shapes including sharks belonging to the family Stethacanthidae which possessed a flat brush-like dorsal fin with a patch of denticles on its top.[36] Stethacanthus' unusual fin may have been used in mating rituals.[36] Apart from the fins, Stethacanthidae resembled Falcatus (below).

  Falcatus Falcatus è un genere di piccoli cladodonti dai denti appuntiti e squaliformi, che vissero da 335 a 318 milioni di anni fa. Di piccole dimensioni, raggiungevano i 25-30 cm di lunghezza. [38] Sono caratterizzati dalla particolare forma della pinna dorsale, che presenta i raggi incurvati verso la testa.
  Orodus Orodus è un altro genere di squalo del Carbonifero, appartenenti alla famiglia Orodontidae, vissuto da 303 a 295 milioni di anni fa. Raggiungeva una lunghezza di 2 metri.
Permiano Permiano (298–252 Ma):
  Acanthodes Acanthodes are an extinct genus of spiny shark.[39] It had gills but no teeth,[40] and was presumably a filter feeder.[41] Acanthodes had only two skull bones and were covered in cubical scales. Each paired pectoral and pelvic fins had one spine, as did the single anal and dorsal fins, giving it a total of six spines, less than half that of many other spiny sharks.[41] Acanthodians share qualities of both bony fish (osteichthyes) and cartilaginous fish (chondrichthyes), and it has been suggested that they may have been stem chondrichthyans and stem gnathostomes.[42][43]
The Permian ended with the most extensive extinction event recorded in paleontology: the Permian-Triassic extinction event. 90% to 95% of marine species became extinct, as well as 70% of all land organisms. It is also the only known mass extinction of insects.[44][45] Recovery from the Permian-Triassic extinction event was protracted; land ecosystems took 30M years to recover,[46] and marine ecosystems took even longer.Template:R
Triassico Triassico (252–201 Ma): The fish fauna of the Triassic was remarkably uniform, reflecting the fact that very few families survived the Permian extinction. A considerable radiation of ray-finned fishes]] occurred during the Triassic, laying the foundation for many modern fishes.[47] See Category:Triassic fish.
  Perleidus Perleidus was a ray-finned fish from the Early Triassic. About 15 centimetri (5,9 in) in length, it was a freshwater predatory fish with jaws that hung vertically under the braincase, allowing them to open wide. Perleidus had highly flexible dorsal and anal fins, with a reduced number of fin rays, which would have made the fish more agile in the water.[41]
  Pachycormiformes

Pachycormiformes are an extinct order of ray-finned fish which were extant from the Middle Triassic to the K-Pg extinction (below). They were characterized by serrated pectoral fins, reduced pelvic fins and a bony rostrum. Their relations with other fish are unclear.

  Pholidophorus Pholidophorus was an extinct genus of teleost, around 40 centimetri (16 in) long, from about 240–140 Ma. Although not closely related to the modern herring, it was somewhat like them. It had a single dorsal fin, a symmetrical tail, and an anal fin placed towards the rear of the body. It had large eyes and was probably a fast swimming predator, hunting planktonic crustaceans and smaller fish.Template:Sfn A very early teleost, Pholidophoris had many primitive characteristics such as ganoid scales and a spine that was partially composed of cartilage, rather than bone.Template:Sfn
The Triassic ended with the Triassic–Jurassic extinction event. About 23% of all families, 48% of all genera (20% of marine families and 55% of marine genera) and 70% to 75% of all species went extinct.[48] Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments.[48]
Giurassico Giurassico (199–145 milioni di anni fa):
  Leedsichthys During the Jurassic period, the primary vertebrates living in the seas were fish and marine reptiles. The latter include ichthyosaurs who were at the peak of their diversity, plesiosaurs, pliosaurs, and marine crocodiles of the families Teleosauridae and Metriorhynchidae.[49] Numerous turtles could be found in lakes and rivers.[50][51] See Category:Jurassic fish.
  Ichthyodectidae
 
This fossil Ichthyodectidae from the Lower Jurassic is one of the best conserved fossil fishes worldwide

The family Ichthyodectidae (literally "fish-biters") was a family of marine actinopterygian fish. They first appeared 156 Ma during the Late Jurassic and disappeared during the K-Pg extinction event 65 Ma. They were most diverse throughout the Cretaceous period. Sometimes classified in the primitive bony fish order Pachycormiformes, they are today generally regarded as members of the "bulldog fish" order Ichthyodectiformes in the far more advanced Osteoglossomorpha. Most ichthyodectids ranged between 1 and 5 meters (3–15 ft) in length. All known taxa were predators, feeding on smaller fish; in several cases, larger Ichthyodectidae preyed on smaller members of the family. Some species had remarkably large teeth, though others, such as Gillicus arcuatus, had small ones and sucked in their prey. The largest Xiphactinus was 20 feet long, and appeared in the Late Cretaceous (below).

Cretacico Cretacico (145–65 milioni di anni fa)
  Storioni Gli storioni appaiono nei ritrovamenti fossili nel Cretacico superiore e, da quel momento, la loro morfologia ha subito ben pochi cambiamenti evolutivi, dando loro lo status di fossili viventi[52][53].

La loro lentissima evoluzione è dovuta ad alcuni fattori fondamentali: un lungo intervallo generazionale, un'ampia tolleranza alle diverse temperature e salinità delle acque, la mancanza di predatori a causa delle loro grosse dimensioni e l'abbondanza di prede nel loro ambiente.

  Enchodus
  Xiphactinus

Il più grande dei teleostei conosciuti, questo pesce del Cretacico raggiungeva una lunghezza di 4,5 metri [54].

The end of the Cretaceous was marked by the Cretaceous–Paleogene extinction event (K-Pg extinction). There are substantial fossil records of jawed fishes across the K–T boundary, which provides good evidence of extinction patterns of these classes of marine vertebrates. Within cartilaginous fish, approximately 80% of the sharks, rays, and skates families survived the extinction event,[55] and more than 90% of teleost fish (bony fish) families survived.[56] There is evidence of a mass kill of bony fishes at a fossil site immediately above the K–T boundary layer on Seymour Island near Antarctica, apparently precipitated by the K–Pg extinction event.[57] However, the marine and freshwater environments of fishes mitigated environmental effects of the extinction event.[58]
Cenozoico Cenozoic Era (65 Ma to present): The current era has seen great diversification of bony fishes. Over half of all living vertebrate species (about 32,000 species) are fishes (non-tetrapod craniates), a diverse set of lineages that inhabit all the world's aquatic ecosystems, from snow minnows (Cypriniformes) in Himalayan lakes at elevations over 4,600 metres (15,000 feet) to flatfishes (order Pleuronectiformes) in the Challenger Deep, the deepest ocean trench at about 11,000 metres (36,000 feet). Fishes of myriad varieties are the main predators in most of the world’s water bodies, both freshwater and marine.
  Amphistium Amphistium is a 50-million-year-old fossil fish which has been identified as an early relative of the flatfish, and as a transitional fossil.[59] In a typical modern flatfish, the head is asymmetric with both eyes on one side of the head. In Amphistium, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top of the head.[60]
  Megalodon

Carcharodon megalodon è una specie estinta di squalo che visse tra i 28 e 1,5 milioni di anni fa. Fisicamente simile al Sembrava molto simile al grande squalo bianco, era tuttavia decisamente più grande, con una lunghezza stimata su ritrovamenti fossili che raggiunge i 20,3 metri [61] Found in all oceans[62] it was one of the largest and most powerful predators in vertebrate history,[61], Il Megalodon viveva in tutti gli oceani ed era uno dei più grandi e potenti predatori nella storia dei vertebrati che ha avuto un profondo impatto sulla vita marina[63]

Pesci preistoriciModifica

 Lo stesso argomento in dettaglio: Elenco dei pesci preistorici.

I pesci preistorici sono pesci estinti conosciuti soltanto tramite ritrovamenti fossili. Considerati tra i vertebrati più antichi oggi conosciuti, comprendono il primo pesce estinto, vissuto tra il Cambriano e il Terziario. La branca della paleontologia che studia i pesci preistorici si chiama paleoittiologia. Alcune specie viventi di pesci sono classificati come pesci preistorici, come il celacanto che è anche un fossile vivente, a causa della loro rarità attuale e somiglianza con forme preistoriche estinte.

Fossili viventiModifica

 
The jawless hagfish is a living fossil which has not essentially changed for 300 million years.[64]

The coelacanth was thought to have gone extinct Template:Ma, until a living specimen belonging to the order was discovered in 1938

Siti di fossiliModifica

 
Miguasha National Park: outcrop of Devonian beds rich in fossil fish
 Lo stesso argomento in dettaglio: List of fossil sites.

Some fossil sites which have produced notable fish fossils.

Collezioni di fossiliModifica

Some notable fossil fish collections.

PaleoittiologiModifica

La paleoittiologia è la scienza che studia i pesci preistorici. Nella lista sottostante sono elencati i paleoittiologi che con i loro studi hanno contribuito a svelare i segreti di queste antiche forme di vita.

Voci correlateModifica

ReferencesModifica

CitationsModifica

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BibliographyModifica

Further readingModifica

External linksModifica

Template:Diversity of fish Template:Evolution of fish Template:Evolution Template:Fins, limbs and wings