holostean
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What is a holostean fish?
Holostean is a term for a group of primitive bony fishes within the superclass Actinopterygii, also known as ray-finned fishes. Represented today by the bowfins (order Amiiformes) of North America and the gars (order Semionotiformes) of North and Central America and Cuba, they were abundant in the Mesozoic Era.
Holostean characteristics include having the dermal bone of the upper jaw freed from the cheek elements, attachment to the skull near the nasal chambers, and the separation of the palate from the cheek elements. The scales are typically rhomboidal, or diamond-shaped, and covered with an enamel-like substance.
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Which living fish are considered holosteans?
Holosteans are primitive bony fishes within the superclass Actinopterygii (ray-finned fishes). Today, the only living holosteans are bowfins (order Amiiformes) and gars (order Semionotiformes). The bowfin is represented by a single species (genus Amia), while the gars include seven species across the genera Lepisosteus and Atractosteus.
Holosteans were most abundant during the Mesozoic Era (251–65.5 million years ago) and are thought to be of mixed origin, representing an evolutionary stage of several chondrostean orders.
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How do holosteans differ from other types of fish?
Holosteans are primitive bony fishes within the superclass Actinopterygii, which includes ray-finned fishes. They differ from other fish groups through key skeletal features such as a maxilla freed from cheek elements, attachment to the skull near the nasal chambers, and a separation of the palate from cheek elements. The caudal fin is typically hemiheterocercal, and the braincase comprises separate ossifications similar to teleosts.
Compared to Chondrostei, Holosteans have a tail closer to the homocercal condition, an equal number of fin rays and basal elements, thinner body scales, and a freed upper jaw, allowing for greater movement and more powerful jaw muscles.
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What are some key features of holostean skeletons?
Holostean skeletons have several distinguishing features. The upper jaw's dermal bone (maxilla) is detached from the cheekbones, connecting to the skull near the nasal chambers. The palate is separate from the cheekbones.
The adductor mandibulae muscle, responsible for closing the jaws, is larger and more subdivided compared to bichirs, sturgeons, and paddlefish. Primitively, the centrum, the vertebra's central part surrounding the notochord, is absent, though it developed independently in most holostean orders.
Scales are typically rhomboidal (diamond-shaped) and covered in an enamel-like substance, but can be thin and cycloidal (rounded and overlapping) in some groups. The number of fin rays in unpaired fins equals the number of basal supports. The tail fin is usually hemiheterocercal, with the body lobe turning up slightly, and is externally symmetrical. The braincase comprises separate ossifications similar in number and placement to those in teleosts.
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Where do holostean fish live and what do they eat?
Holostean fish are primitive bony fishes within the superclass Actinopterygii ray-finned fishes. Today, they are represented by bowfins (order Amiiformes) of North America and gars (order Semionotiformes) of North and Central America and Cuba.
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Why are holosteans important for understanding fish evolution?
Holosteans, a group of primitive bony fishes, are important in understanding fish evolution because they represent a crucial stage between the earlier Chondrostei and the later Teleostei.
Holosteans show an approach to the homocercal tail condition, have an equal number of fin rays and basal elements, and possess thinner body scales, making them more efficient swimmers than chondrosteans. They also have a freed upper jaw, allowing greater movement of the gill chamber and more powerful development of the lower jaw muscle.
The evolutionary radiation of holosteans occurred during the Triassic, Jurassic, and Cretaceous periods, a time when chondrosteans were declining and teleosts were beginning to expand. The Amiiformes are recognized as the closest living relatives of the teleosts.
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holostean, (infraclass Holostei), any member of a group of primitive bony fishes that make up one of the three major subdivisions of the superclass Actinopterygii (ray-finned fishes). Holosteans are represented today by the bowfins (order Amiiformes) of North America and the gars (order Semionotiformes) of North and Central America and Cuba. Holosteans diverged from their chondrostean ancestors in the order Palaeonisciformes during the Permian Period and were particularly abundant in the Mesozoic Era (251–65.5 million years ago); however, only three living genera remain. The genus Amia contains the single remaining species of bowfin, and the genera Lepisosteus and Atractosteus contain the seven living species of gars. The extinct order Pycnodontiformes is often associated with the holosteans. There is disagreement as to whether infraclass Holosei should be recognized as a natural taxonomic entity, since many authorities believe that bowfins, gars, and their fossil relatives did not descend from a common ancestor.
General features
Bowfins (also known as grindles, mudfishes, and dogfishes) are found in slow-moving waters from the Great Lakes to the Gulf of Mexico. Female bowfins reach a length of about 75 cm (30 inches) and weigh up to 3.5 kg (about 8 pounds); males are smaller. Bowfins eat all kinds of fish and invertebrates and are sometimes destructive to game fish populations. Bowfins are seldom caught as food fish. Relatives once common in Europe and Asia are extinct.
Gars occur only in North America, Central America, and Cuba—from southeastern Canada to Panama—but are not found west of the Rocky Mountains. The longnose gar (Lepisosteus osseus) is the most widely distributed species. Gars are primarily freshwater fish that sometimes venture into salt water or brackish water. The alligator gar (Atractosteus spatula), one of the largest freshwater fishes, is particularly abundant in the Everglades region of southern Florida, where it is caught locally as a food fish. It sometimes grows to a length of nearly 3 metres (10 feet) and may attain a weight of 136 kg (300 pounds).
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The names gar, garfish, and garpike are sometimes also applied, especially in Europe, to the needlefishes (Belonidae). Needlefishes are coastal fishes of warm seas and have very long and slender jaws; however, they are not closely related to the Holostei.
Natural history
Reproduction
Bowfins spawn in weedy areas along the edges of streams and lakes. The male constructs the nest and guards the eggs as well as the newly hatched young. The young bowfin has an adhesive organ at the tip of its snout that enables it to cling to weeds. The fish grows rapidly, and it may be as long as 23 cm (9 inches) by the end of its first year.
In the spring, female gars lay large yolk-filled eggs in shallow water. The gar hatchlings grow rapidly, feeding on minnows. The long rows of needle-sharp teeth are effective in capturing fast-swimming prey.
Ecology
Gars and bowfins are voracious predators that feed on invertebrates and other fishes. All amiiform fishes (that is, the bowfin’s extinct relatives) were probably predaceous. For the most part, the amiiforms were a marine group. The modern bowfin, however, is confined to fresh water. In addition, the bowfin is not limited to its gills for respiration. Its highly developed swim bladder can also function as a lung in times when the water temperature is high and oxygen concentration is low. In contrast, gars are known to occasionally venture into salt water, but apparently they do not attempt to feed there. Gars, which also possess a versatile swim bladder, often float quietly at the surface of slow-moving waters, breathing air.
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Form and function
Skeletal features
In the Holostei the dermal bone of the upper jaw (maxilla) is freed from the cheek elements, and its attachment to the skull only occurs in the ethmoid region or near the nasal chambers. The palate is separated from the cheek elements, and the adductor mandibulae muscle, which closes the jaws, is larger and more subdivided than it is in the Polypteriformes (bichirs) and Acipenseriformes (sturgeons and paddlefish). Primitively, the centrum (that is, the central and circular part of a vertebra) surrounding the notochord (a flexible rod that passes through the vertebral column) is absent, but this structure apparently developed independently in most of the holostean orders. The scales are primitively rhomboidal, or diamond shaped, and covered with an enamel-like substance; however, they have become thin and cycloidal (that is, rounded and overlapping) in several groups. The fin rays of the unpaired fins are always equal in number to their basal supports, and the fins themselves may or may not be bordered at the anterior end by fulcra, which are modified scales or spines. The caudal, or tail, fin is typically hemiheterocercal (that is, the body lobe turns up slightly) and externally symmetrical. The braincase is always composed of separate ossifications (centres of bone formation) that resemble, in number and placement, those found in the teleosts.
The infraclass Holostei includes the orders Semionotiformes, Pycnodontiformes, and Amiiformes. In these orders the preoperculum (an L-shaped bone anterior to the operculum, or gill cover) is tied to the palatal elements and provides part of the originating area for the adductor mandibulae muscle.