Evolution and paleontology ( fish )
Although a great many fossil fishes have been found and described, they represent a tiny portion of the long and complex evolution of fishes and knowledge of fish evolution remains relatively fragmentary. In the classification presented in this article fishlike vertebrates are divided into seven classes, the members of each having a different basic structural organization and different physical and physiological adaptations for the problems presented by the environment. The broad basic pattern has been one of successive replacement of older groups by newer, better adapted groups. One or a few members of a group evolved a basically more efficient means of feeding, breathing, swimming, or several better ways of living. These better adapted groups then forced the extinction of members of the older group with which they competed for available food, breeding places, or other necessities of life. As the new fishes became well established, some of them evolved further and adapted to other habitats, where they continued to replace members of the old group already there. The process was repeated until all or almost all members of the old group in a variety of habitats had been replaced by members of the newer evolutionary line.
Agnatha: early jawless fishes
The earliest vertebrate fossils of certain relationships are fragments of dermal armour of jawless fishes (class Agnatha, order Heterostraci) from the Middle Ordovician Period in North America, about 450,000,000 years in age. Early Ordovician toothlike fragments from the U.S.S.R. are less certainly remains of the class Agnatha. It is uncertain whether the North American jawless fishes inhabited shallow coastal marine waters, where their remains became fossilized, or were freshwater vertebrates washed into coastal deposits by stream action.
Jawless fishes probably arose from ancient small, softbodied filter-feeding organisms much like and probably also ancestral to the modern sand-dwelling filter feeders, the Cephalochordata (Amphioxus and its relatives). The body in the ancestral animals was probably stiffened by a notochord. Although a vertebrate origin in fresh water is much debated by paleontologists, it is possible that mobility of the body and protection provided by dermal armour arose in response to streamflow in the freshwater environment and to the need to escape from and resist the clawed invertebrate eurypterids that lived in the same waters. Because of the marine distribution of the surviving primitive chordates, many paleontologists doubt that the vertebrates arose in fresh water.
Heterostracan remains are next found in what appear to be delta deposits in two North American localities of Silurian age. By the close of the Silurian, about 400,000,000 years ago, European heterostracan remains are found in what appear to be delta or coastal deposits. In the Late Silurian of the Baltic area, lagoon or freshwater deposits yield jawless fishes of the order Osteostraci. Somewhat later in the Silurian from the same region, layers contain fragments of jawed acanthodians, the earliest group of jawed vertebrates, and of jawless fishes. These layers lie between marine beds but appear to be washed out from fresh waters of a coastal region.
It is evident, therefore, that by the end of the Silurian both jawed and jawless vertebrates were well established and already must have had a long history of development. Yet paleontologists have remains only of specialized forms that cannot have been the ancestors of the placoderms and bony fishes that appear in the next period, the Devonian. No fossils are known of the more primitive ancestors of the agnaths and acanthodians. The extensive marine beds of the Silurian and those of the Ordovician are essentially void of vertebrate history. It is believed that the ancestors of fishlike vertebrates evolved in upland fresh waters, where whatever few and relatively small fossil beds were made probably have been long since eroded away. Remains of the earliest vertebrates may never be found.
By the close of the Silurian, all five known orders of jawless vertebrates had evolved, except perhaps the modern cyclostomes, which are without the hard parts that ordinarily are preserved as fossils. Cyclostomes were unknown as fossils until 1968, when a lamprey of modern body structure was reported from the Middle Pennsylvanian of Illinois, in deposits almost 300,000,000 years old. Fossil evidence of the four orders of armoured jawless vertebrates is absent from deposits later than the Devonian. Presumably they became extinct at that time, being replaced by the more efficient and probably more aggressive placoderms, acanthodians, selachians (sharks and relatives), and by early bony fishes. Cyclostomes survived probably because they early evolved from anaspid agnaths and developed a rasping tonguelike structure and a sucking mouth, enabling them to prey on other fishes. With this way of life they apparently had no competition from other fish groups.
Early jawless vertebrates probably fed on tiny organisms by filter feeding, as do the larvae of their descendants, the modern lampreys. The gill cavity of the early agnaths was large. It is thought that small organisms taken from the bottom by a nibbling action of the mouth, or more certainly by a sucking action through the mouth, were passed into the gill cavity along with water for breathing. Small organisms then were strained out by the gill apparatus and directed to the food canal. The gill apparatus thus evolved as a feeding, as well as a breathing, structure. The head and gills in the agnaths were protected by a heavy dermal armour; the tail region was free, allowing motion for swimming.
Most important for the evolution of fishes and vertebrates in general was the early appearance of bone, cartilage, and enamel-like substance. These materials became modified in later fishes, enabling them to adapt to many aquatic environments and finally even to land. Other basic organs and tissues of the vertebrates such as the central nervous system, heart, liver, digestive tract, kidney, and circulatory system undoubtedly were present in the ancestors of the Agnatha. In many ways, bone, both external and internal, was the key to vertebrate evolution.
Acanthodii: early jawed fishes
The next class of fishes to appear was the Acanthodii, containing the earliest known jawed vertebrates, which arose in the Upper Silurian, over 400,000,000 years ago. The acanthodians declined after the Devonian but lasted into the Lower Permian, a little less than 280,000,000 years ago. The first complete specimens appear in Lower Devonian freshwater deposits, but later in the Devonian and Permian some members appear to have been marine. Most were small fishes, not over 75 centimetres (approximately 30 inches) in length.
We know nothing of the ancestors of the acanthodians. They must have arisen from some jawless vertebrate, probably in fresh water. They appear to have been active swimmers with almost no head armour but with large eyes, indicating that they depended heavily on vision. Perhaps they preyed on invertebrates. The rows of spines and spinelike fins between the pectoral and pelvic fins give some credence to the idea that paired fins arose from “fin folds†along the body sides.
The relationships of the acanthodians to other jawed vertebrates are obscure. They possess features found in both sharks and bony fishes. They are like early bony fishes in possessing ganoid-like scales and a partially ossified internal skeleton. Certain aspects of the jaw appear to be more like those of bony fishes than sharks, but the bony fin spines and certain aspects of the gill apparatus would seem to favour relationships with early sharks. Acanthodians do not seem particularly close to the Placodermi although, like the placoderms, they apparently possessed less efficient tooth replacement and tooth structure than the sharks and the bony fishes, possibly one reason for their subsequent extinction.
Placodermi: plate-skin fishes
The first record of the jawed Placodermi is from the Early Devonian, about 390,000,000 years ago. The placoderms flourished for about 60,000,000 years and were almost gone at the end of the Devonian. Nothing is known of their ancestors, who must have existed in the Silurian. The evolution of several other, better adapted, fish groups soon followed the appearance of the placoderms and this apparently led to their early extinction. Their greatest period of success was approximately during the middle of the Devonian, when some of them became marine. As their name indicates (placoderm means “plate skinâ€), most of these fishes had heavy coats of bony armour, especially about the head and anterior part of the body. The tail remained free and heterocercal (i.e., the upper lobe long, the lower one small or lacking). Most placoderms remained small, 30 centimetres or less in length, but one group, the arthrodires, had a few marine members that reached 10 metres in length. Important evolutionary advances of the placoderms were in the jaws (which usually were amphistylic—i.e., involving the hyoid and quadrate bones) and development of fins, especially the paired fins with well-formed basal or radial elements. The jaws tended to be of single elements with strongly attached toothlike structures. These were too specialized to be considered ancestral to the more adaptable jaws of subsequent bony fish groups. It has been proposed that sharks arose from some group of placoderms near the Stensioelliformes and that the chimaera line (class Holocephali) arose from certain arthrodires; this suggestion, however, is uncertain.
A peculiar, five-centimetre fish, Palaeospondylus, from Middle Devonian rocks in Scotland, is probably not a placoderm, although classed with them here. Various suggestions that its relationships are with the agnaths, placoderms, acanthodians, sharks, and even lungfishes and amphibians are unconvincing and its relationships remain completely unknown.
Selachii: sharks and rays
Sharks (class Selachii) first appear in the Middle Devonian about 375,000,000 years ago, became quite prominent by the end of the Devonian, and are still successful today. Two Early Devonian orders of primitive sharklike fishes, the Cladoselachiformes and the Cladodontiformes, became extinct by the end of the Permian, about 230,000,000 years ago, while the freshwater order Xenacanthiformes lasted until the Middle Triassic, about 200,000,000 years ago. The final Devonian order, Heterodontiformes, still has surviving members.
Modern sharks and rays arose during the Jurassic Period, about 135,000,000 to 190,000,000 years ago, probably from an older group, the hybodont sharks. Presumably marine cladoselachians gave rise to the hybodont Heterodontiformes during the close of the Devonian. These had the placoderm amphystylic jaws but had paired fins of a more efficient type. In turn the hybodonts are thought to have given rise to the living but archaic mollusk-eating Port Jackson sharks (heterodonts). The relationships of the surviving (but archaic) hexanchiform sharks are unknown. The two main orders of modern Selachii, the Lamniformes (sharks) and Rajiformes (skates and rays), appeared between 140,000,000 and 180,000,000 years ago during the Jurassic Period. They are characterized by a hyostylic jaw (in which articulation involves only the hyoid bone), an improvement allowing greater mobility of the jaws and an important feature in the methods of predation used by modern selachians.
Skates and rays evolved from some bottom-living sharklike ancestor during the Jurassic. The primary evolution and diversification of modern sharks, skates, and rays took place in the Cretaceous Period and Cenozoic Era. Thus, along with the teleost fishes (discussed below) most surviving sharks, skates, and rays are essentially of relatively recent origin, their main evolutionary radiation having taken place within the last 140,000,000 years.
Holocephali
The class Holocephali, the chimaeras or ratfishes as their modern survivors are called, first appeared in the Upper Devonian but were most common and diversified during the Mesozoic Era. Only one of the seven known orders survived beyond the close of the Cretaceous Period about 65,000,000 years ago. Although not many modern species of chimaeras are known, they are sometimes relatively abundant in their deep-sea habitat. The relationships of these fishes are in question. It has been proposed that they are related to the Devonian ptyctodont arthrodires, which had a chimaera-like shape and pelvic claspers. It has also been suggested that they are closely related to the Selachii because both selachians and holocephalians have many characters in common, such as placoid scales, pelvic claspers, and absence of true bone. It has been suggested recently that both holocephalians and selachians are related to the acanthodians on the basis of the gill arch structures. Further evidence is needed to solve the problem of their classification and relationships.
Sarcopterygii: fleshy-finned fishes
The class Sarcopterygii are extremely ancient in origin, their first remains appearing in Lower Devonian strata of Germany, about 390,000,000 years old. The most important group, the rhipidistians, which gave rise to the amphibians by the end of the Devonian, became extinct about 120,000,000 years later, near the beginning of the Permian. Two lesser groups, the coelacanths and the dipnoans (lungfishes), have barely survived. Recognition of the class Sarcopterygii is controversial in that some ichthyologists believe that the two major groups, the Crossopterygii (including the rhipidistians and coelacanths) and the Dipnoi, have arisen from independent origins, the present structures of the two groups being widely divergent. The primitive members, however, show several similarities, supporting the view that they had a common ancestor. The nature of the ancestor or ancestors remains a mystery. The Sarcopterygii probably evolved from unknown Silurian jawed freshwater fishes that may also have been ancestral to the actinopterygians.
The rhipidistian crossopterygians apparently flourished in the fresh waters of the Middle Devonian where, in adapting to a habitat subject to seasonal droughts, some evolved pectoral and pelvic appendages strong enough and flexible enough to enable them to leave drying pools to seek out those ponds that retained water. Paradoxically, terrestrial amphibians first rose through the need to survive in water.
The early coelacanths of the Upper Devonian were small freshwater and inshore fishes, and it was not until the Late Permian and Triassic that they became marine and grew larger and more diverse. They are not known as fossils later than the Cretaceous, and it was therefore a great surprise when in 1938 a live, 160-centimetre (63-inch) specimen was taken at 120 metres (approximately 390 feet) off the coast of eastern South Africa.
The dipnoans first appeared in the Lower Devonian and were fully differentiated at that time. They flourished until the close of the Triassic, when their numbers became greatly reduced. The modern Australian lungfish differs little from one of the Triassic forms. The living South American and especially African lungfishes are elongated, specialized fishes adapted to live and survive in more or less annual ponds.
Actynopterygii: ray-finned fishes
The Actinopterygii, or “ray-finned†fishes, is the largest class of fishes. In existence for about 390,000,000 years, since the Lower Devonian, it consists of some 52 orders containing more than 480 families, at least 80 of which are known only from fossils. The class contains the great majority of known living and fossil fishes, with about 20,000 living species. The history of actinopterygians can be divided into three basic stages or evolutionary radiations, each representing a different level of structural organization and efficiency.
The Chondrostei have a 300,000,000-year history. They arose first in the Lower Devonian, increased in numbers and complexity until about the Permian, and thereafter declined, becoming almost extinct by the middle of the Cretaceous, 100,000,000 years ago. The chondrostean order Palaeonisciformes is the basal actinopterygian stock from which all other chondrosteans and the holosteans evolved. They were the most common fishes of their time, relatively small and typically like later fishes in appearance. In comparison with today’s fishes they had peculiar looking jaws and tails. Their tails were heterocercal. On their bodies were thick ganoid scales that abutted each other, rather than overlapping as in most modern fishes. Palaeonisciformes often had large eyes placed far forward, long mouths with the upper jaw firmly bound to the fully armoured cheek, and a relatively weak lower jaw muscle. They gave rise to a great variety of types, with elongate bodies and jaws, bottom-living types that fed on microorganisms, deep-bodied marine reef fishes and coral-eating reef fishes. Almost all of these were replaced by modern teleosts. Surviving Chondrostei are the bottom-feeding marine and freshwater sturgeons, the strange plankton-feeding paddlefishes of the Mississippi of North America and the Yangtze River of China, and the freshwater bichirs and reed fishes (family Polypteridae) of Africa. The relationship of the polypterids is in some doubt, and the group is sometimes placed in the Sarcopterygii.
Several of the chondrostean orders developed characteristics that approached the holostean level of anatomic organization and are sometimes called subholosteans. One of these orders, the Parasemionotiformes, evolved from the Palaeonisciformes in the Lower Triassic and may have given rise to at least some of the holosteans. This evolutionary line leads to the Pholidophoriformes, which gave rise to modern bony fishes, or teleosts.
The holosteans are thought to be of mixed origin and to represent a stage in the evolution of a group of chondrostean orders. If so, the infraclass Holostei does not represent a single lineage. Important holostean characteristics are the approach of the tail toward the homocercal condition and the equal number of fin rays and basal elements of the fin rays. Both of these conditions make the holostean a more efficient swimmer than the chondrostean, as does thinning of the holostean body scales. Another important advance of holosteans was the freeing of the upper jaw from the preopercular bone of the cheek, allowing greater movement of the gill chamber and jaws, with more powerful development of the lower jaw muscle.
Five orders of holosteans are known, with their greatest evolutionary radiation occurring during the Triassic, Jurassic, and Cretaceous periods, when the chondrosteans were declining and the teleosts just beginning to expand. Two holostean groups survive today: the bowfin, Amia calva, and the several species of gars, Lepisosteus, all found in North America.
The modern bony fishes, infraclass Teleostei, include the great majority of living fishes. They first appear in the fossil record about 190,000,000 years ago (as the family Leptolepididae) with their homocercal caudal fin and caudal skeleton already fully developed. They arose from an order of holosteans now extinct, the Pholidophoriformes. This group was intermediate in character between the chondrosteans and the teleosts. Teleosts have reached their fullest extent within the last 50,000,000 years and represent a distinct functional advance over their holostean ancestors. They have greater swimming ability, owing to the improvement in the tail structure, and have a still more efficient feeding and gill ventilating apparatus.
The bony fishes represent the culmination of a long evolution toward a body plan with maximum swimming efficiency. Particularly important in this evolution have been changes in fins and in the tail. Some authorities believe that the paired fins arose from a single continuous tail and anal fin that was divided at the vent and extended forward along each side to the head. Later the sections between the pectoral, pelvic, anal, and caudal fins were lost. The fin rays of sharks and rays are of a horny material, but those of many primitive fossil fishes are of bone. The bony fin rays of sarcopterygians and actinopterygians probably arose from scales lying in the fin folds. Modern teleost fishes have flexible fin rays (called soft rays) of jointed segments of bone, or spiny rays, each of solid continuous bone. The first dorsal fin of acanthopterygian fishes is of the spiny type. The original tail fin of primitive fishes was not an effective swimming organ, because of its asymmetry. The steady improvement in tail shape over 400,000,000 years is one of the prominent features of fish evolution. In primitive fishes the tail (vertebral) axis turned upward (heterocercal) or downward (hypocercal) and a lobe of flesh projected from it. This form of tail cannot provide a powerful driving mechanism, because the driving force is unevenly distributed relative to the body axis. With an asymmetrical tail, the fish swims by an undulating motion of the body and tail. In some fishes with a diphycercal tail (with the axis of the vertebrae extending down the middle of the fin lobe), developed in both modern and ancient fishes, the tail remains relatively ineffective because it has remained too rigid for proper propulsive action. The development of a true homocercal tail fin, in which powerful muscles move strong fin rays with a very flexible basal joint and in which the upper and lower lobes are about equal, is a development exclusive to teleost fishes.
As suggested by the existence of more than 400 families, teleosts are extremely varied in anatomical form and in the habitat occupied. They can be divided into about nine superorders, each with distinct evolutionary significance. The Leptolepidimorpha, an extinct, relatively primitive group, has uncertain relationships with other teleosts and is as yet poorly understood. The second group, the Elopomorpha, retains some relatively primitive living members, such as the tarpons, but is mostly represented by the large variety of specialized true eels. The Clupeomorpha includes the herrings and anchovies, relatively primitive fishes, mostly specialized for existence near the surface of the open ocean. A few species are anadromous, breeding in fresh water but spending most of their lives in the sea. The fourth superorder, the Osteoglossomorpha, consists of a group of relatively primitive teleosts, most of which are now extinct. The few surviving members are tropical and worldwide in distribution but adapted for restricted habitats. The Protacanthopterygii is a varied collection of relatively primitive orders, marine, deep-sea, and freshwater in distribution; trouts, smelts, and argentines are examples. The sixth superorder, the Ostariophysi, is an important group of primarily freshwater fishes, including the characins, carps, minnows, loaches, suckers, and catfishes.
The remaining three superorders have a complex fossil history and are not yet fully understood, but all seem to possess similar evolutionary trends. Each group shows a tendency to develop spiny fin rays in the dorsal and anal fins (reduced in some) and a shelf of bone under the eye. There is a tendency for the pelvic fins to move forward on the body, with a reorganization of swimming methods and a slight gain in manoeuvrability. All three groups probably are related and presumably arose from some early protacanthopterygian ancestor. The Scopelomorpha includes a wide variety of deep-sea open-ocean plankton feeders and predators, some of which bear light organs. The Paracanthopterygii is a rather miscellaneous collection of fishes, the most important to man being the cods. The final superorder, the Acanthopterygii, is the result of the great radiation of modern spiny-rayed fishes and contains the dominant fishes in marine shore habitats, tropical, temperate, and Arctic. They also have penetrated the freshwater environment, especially lakes, slow-moving streams, and ponds. The superorder has some important open-ocean members, such as tunas. The key to the successful acanthopterygian radiation probably has been their mobile, protractile mouth.
[tags]evolution, paleontology, fish, fossil[/tags]
