There is a type of bony fish called Sarcopterygii or lobe-finned fishes, that possess lobulated caudal and pectorals fins. For this reason they are called legged–fish. Know through this post all about these peculiar creatures.
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The Sarcopterygi’s History
Around 390 million years ago, fish with lobed fins, (also called fleshed- fins), appeared for the first time in the sea. Of these ancient sarcopterygian fish only 8 survive today. Two species of coelacanths and six species of lungfish.
There are three main hypotheses related to the lobed fins’ evolution. Let’s know each of them:
The traditional explanation is the water well hypothesis which was proposed by the American paleontologist Alfred Romer. He believed that the limbs and lungs could have evolved from the need to find new bodies of water since the interiors were drying up.
The second conjecture is the inter-tidal hypothesis presented in 2010 by a team of paleontologists who argued that The Sarcopterygii, or lobe-finned fishes, may have first emerged from the earth’s intertidal zone and not from internal bodies of water. This hypothesis is based on the discovery of the Zachełmie tracks of 395 million years in Poland, the oldest discovered fossil evidence of tetrapods.
The third assumption is called the forest hypothesis and was proposed by the American paleontologist Greg Retallack in 2011. It argues that such a limbs may have developed in shallow bodies of water in the forests as a means of navigating in environments full of roots and vegetation. He based his conclusions on the evidence that transitional fossils of tetrapods are found consistently in habitats that were once wet and wooded flood plains.
The taxonomy of these fish is divided into three subclasses. In the first classification we find the Actinistia which contains the coelacanths : The western Indian Ocean and the Indonesian coelacanth.
The Dipnoos classification, also known as lungfish, is a subclass of freshwater fish, known for possessing primitive characteristics within the bony fish group, including the skill to breathe air, and primitive structures within lobular fin fish, including the presence of lobed fins with a well-developed internal skeleton.
Finally there are the Tetrapodomorpha, (tetrapods) and their extinct relatives. They are a class of vertebrates consisting of vertebrate tetrapods of four limbs and their closest sarcopterigia relatives that are more closely related to live tetrapods than to live lungfish.
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The Sarcopterygiis’ Main Characteristics
Their fins, both caudal and pectoral, consist of fleshy muscular lobes. Each lobe is supported by a central nucleus of individual bones articulated to each other. Most of these bones can be related to those of the extremities of terrestrial animals.
The rounded tip of each fin is hardened by bony rays, which open in a fan. The muscles of each lobe can move the fin rays independently. Within this group it stands out as the amphibians’ possible predecessor (or at least the first in chronologically terms), those of the order of the Osteolepiformes, (megalichthyiformes), which became extinct 130 years ago.
Most species of fish with fleshy fins are extinct. The largest known finned-fish was the Rhizodus hibberti (Scottish Carboniferous period) that may have exceeded 7 meters in length.
The lungfish (Subclass: Dipnoi), unlike other fish, have a pair of functional lungs that are a derivation of the ventral wall at the end of the pharynx. They have scales at the base of their pairs of lobed fins. When adults, lungfish have nostrils open to the outside that serve for the smelling sense, as well as external gills during their larval stage. At present there are records of 3 genera: Protopterus in Africa, (4 species) ; Lepidosire in South America, (one species); and Neoceratodus in Australia (one species).
Australian Lungfish (Genus Neoceratodus):
They are also known as «Queensland Lungfish», «Barramunda» or «Burnett Salmon».This is a a surviving species of the Neoceratodontidae family and the Ceratodontiformes order.
Its scientific name is Neoceratodus forsteri. This is one of the only six species of lungfish in the world, endemic to Australia. The Neoceratodontidae are an ancient family belonging to the class of Sarcopterigios.
The fossil records of this group date back to 380 million years ago, around the time when the highest vertebrate classes began to evolve. In northern New South Wales fossils of this fish with volatile legs almost identical to this species have been discovered.
Such a fact indicates that Neoceratodus has remained virtually unchanged for more than 100 million years, making it a living fossil and one of the oldest living vertebrate genera on the planet.
Distribution and Habitat
This legged fish is native only to the Mary and Burnett River systems in southeast Queensland. It has been successfully distributed in other more southerly rivers, including the Brisbane, Albert, Stanley and Coomera rivers, also in the Enoggera reservoir in the last century.
The Queensland lung-legged fish has also been introduced into the Pine, Caboolture and Condamine rivers, but the current survival and reproductive success are unknown, in the past this species was widespread in Australia.
At one time at least seven species of Lungfish were in this area. This species lives in slow rivers and calm waters including reservoirs that have some aquatic vegetation present in the banks.
They are commonly found in puddles with depths between 3 and 10 meters and live in small groups under submerged trunks, in dense banks of aquatic macrophytes or in underwater caves formed by the removal of substratum under the roots of trees. The Queensland lung-legged fish is cold tolerant, but prefers waters with temperatures between 15 and 25 ° C.
The Queensland lung-fish, has an olive green to dull brown color on its back, sides, tail and fins, with a pale yellow to orange color on the underside. They also have a reddish color on their sides that is much brighter in the males during the breeding season.
This coloration is the only distinctive sexual characteristic of this species. Their robust and elongated bodies are crowned with a crushed head with small eyes. Its mouth is small and in a sub terminal position. Australian lungfish can grow up to 150 cm in length and their weight is around 43 kilograms.
The average individual is usually around 100 cm and 20 kg. Both male and female follow similar growth patterns, although the female is slightly larger.
The skeleton of this Sarcopterygiii partly bone, and partly cartilage; the vertebrae are pure cartilage, while the ribs are hollow tubes filled with a cartilaginous substance. Their body is covered with large bony scales, (ten rows are observed on each side), while the scales are embedded in their own pockets and overlap widely, so that the body’s vulnerable areas are covered by a thickness of at least four scales.
Two unusually large and thick interconnected scales cover the back of the head where the bony skull is thin. Australian lungfish have a strong tail that is long and shovel-shaped. The pectoral fins are large, fleshy and fin-like, while the pelvic fins are also fleshy well placed back on the body. The dorsal fin begins at the center of the back and is confluent with the caudal and anal fins.
The Australian lungfish has two incisors, restricted in the upper jaw, which are flat, slightly bent and denticulate on the posterior border, followed by dental plates in the upper and lower jaws.
A distinctive feature of these fish is the presence of a single dorsal lung, which they employ to supplement the supply of oxygen through their gills.
This is also the only facultative species of the air vent, breathing only the air when the oxygen in the water is not sufficient to cover its needs.
Juveniles Australian lung fish have different body proportions than mature adults. Their head is rounder and the fins smaller, the trunk is thinner, the mouth is initially terminal, but it moves backwards when the fish grows. The dorsal fin usually reaches the back of the head in young juveniles.
This absolutely striking species is mottled with a golden base color or olive-brown. It has patches of intense dark pigment that remain long after the speckling has disappeared. Young individuals are capable of changing color very fast in response to light, but this ability is gradually lost as the pigment becomes denser.
This is essentially a sedentary species which spends its life within a single area. This fish with legs is credited as slow and inactive, but is able to make quick movements to scape using its strong tail. It is usually quiet and does not respond by day, becoming more active in the afternoon and evening.
Their diet is based on animals ( carnivorous). In captivity, they feeds on frogs, earthworms, pieces of meat and granulated foods. In their natural habitat the Australian Lungfish’ diet includes frogs, tadpoles, fish, a variety of invertebrates and plant material. The males usually mature sexually at 738-790 mm and the females at 814-854 mm. The courtship consists of three distinct phases.
The first one is the search phase, when the fish will spread over a large area, possibly looking for potential spawning sites. The next phase involves a similar behavior to follow the leader, during which a fish, (the male), shows interest in the female and pushes her with his snout. Up to eight individuals may be involved in this behavior; the male may occasionally take a piece of aquatic plant in his mouth and shake it around.
In the third phase, the fish dive together through the aquatic vegetation. The male follows the female and presumably spills her sperm on the eggs.
The status of Queensland’s biggest pride is safe. It is a protected species under the Queensland Fish and Oyster Act since 1914 and capture in the wild is strictly prohibited. It was placed on the CITES list in 1977, is currently protected from fishing and collection for education or research purposes requires a permit in Queensland, under the 1994 fishing act
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Sarcopterygii: Tiktaalik Rosea
In natural mythology, the true story of the tiktaalik fish is as fascinating as that of Sedna and is related with the Inuit’s land, with the origin of the fingers and with the evolution of water and land animals.
Tiktaalik roseae is a fish of late Devonian (about 375 million years ago) with so many characteristics typical of amphibians that it can be considered a transitional form between fish and terrestrial vertebrates.
The fossil remains of this fish were found in the inhospitable frozen lands of Ellesmere Island, in the Canadian territory of Nunavut, so their discoverers decided to baptize the strange creature with the word tiktaalik, which in the Inuktitut dialect is used to refer to a large freshwater fish related to cod.
Tiktaalik, like all fish, had gills, scales and fins. He also had lungs and rib bones similar to those of terrestrial vertebrates, and his head had movement independent of the rest of the body; that is, it had a neck, an element that does not exist in fish.
In addition to all this, the shape of the head, the structure of the bones of the ear and, above all, the disposition of the bones of the fins suggest that Tiktaalik is a representative of the type of fish that millions of years ago gave origin to terrestrial vertebrates, that is, the ancestors of all amphibians, reptiles, birds and mammals.
One of the structures that most clearly shows the transitional character of Tiktaalik is the pectoral fin, beautifully preserved in the fossil material of Ellesmere. To understand the importance of the fin of Tiktaalik, we must begin by remembering the bony structure of our arms and hands: articulated to the shoulder we have a long bone (the humerus) that in turn connects to a pair of bones (the ulna and the radius ) that form the forearm. the hand is a complex system of bones made-up by the wrist (the eight short bones of the carpus), the palm (the five elongated bones of the metacarpus) and the fingers (the 14 thin bones of the phalanges).
This general structure (a long bone, two bones, several short bones and up to five elongated fingers) is fundamentally the same in all groups of terrestrial vertebrates and we can recognize it (with small or large modifications) in the legs of mammals and reptiles, the wings of the birds and the bats, the fins of the whales and the penguins and even in the vestigial extremities like the tiny hands of the tyrannosaurs.
In the vast majority of modern fish (so-called actinopterygians), the fin bones form a kind of elongated spine. Only in the lungfish and the coelacanths we find an elongated bone at the base of the fin. The fin of Tiktaalik is clearly intermediate between that of modern fishes and the extremities of terrestrial vertebrates.
In it we can clearly recognize the structure of a bone-two bones-several short bone in which specialists can recognize bone by bone the arrangement we see in all terrestrial vertebrates. As Neil Shubin has said, one of his discoverers, Tiktaalik was a fish with a doll.
In deposits older than those of Tiktaalik, fish fossils such as Eusthenopteron have been found in which the fin had the structure a bone-two bones. In more recent layers, fossils of amphibians have been found that show the modern disposition already described for terrestrial vertebrates. Tiktaalik not only shows the correct morphology to be considered a form of transition, but its fossils are in addition to the correct antiquity and correspond to the right environment.
375 million years ago what is now the Canadian Arctic cold was located almost on the equator. The environment in which Tiktaalik developed was formed by shallow lagoons, probably associated with the delta of a river. The anatomy of the fin of Tiktaalik suggests that the fish was able to use its limbs to anchor itself firmly to the muddy bottom of the lagoons to remain partially out of the water. In Inuit mythology, Sedna’s fingers could give rise to new marine life.
In the history of life on Earth, the bones of the limbs of animals give us invaluable clues to understand the long way from the primitive Devonian fish to ourselves. Maybe the next time we turn to see our hand we can understand that in the structure of their bones has been recorded our history of more than 375 million years.
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Sarcopterygii: (Latimeria chalumnae)
This is a «living fossil» that was believed to be extinct from the time of the dinosaurs until the first in 1938. This surprising specimen was nicknamed the «most important zoological discovery of the century». The species is a member of an ancient lineage that has existed for more than 360 million years. Unlike any other living animal, the celacanths have an articulated joint in the skull that allows the front of the head to be lifted during lactation.
Latimeria chalumnae has muscular fins on the limbs, lobed and pelvic pectorals and the trilobed tail, an unusual feature. To increase the excitement surrounding the species is the current controversy as to whether the coelacanths or lungfish are the closest living relatives to the first walking creatures on earth.
Their scaly body is dark blue or brown with white spots, pattern that is unique to each individual and provides a good camouflage against the walls of the caves. An additional unique feature is a receptor on the rostral organ of the head that can detect electric fields and can be used to locate a prey or monitor its environment.
Sizes: Length: up to 2 m. Weight: up to 90 kg.
Due to the depth of its oceanic habitat, little is known about the coelacanths’ natural ecology . The young specimens develop inside their mother (ovoviviparity) and are attached to the outside of a species of egg yolk about 100 millimeters in diameter. The developing fish is connected to the sack in the belly’s frontal region and as the volume of the yolk decreases, the embryo develops.
The sack diminishes in size until it disappears completely. Shortly before birth the scar where the yolk was connected closes and disappears completely. The mother gives birth to a maximum of 26 live offspring. Coelacanths are very long-lived and some scientists believe they can live up to 80 years.
They seem to be more active during the night, spending the day wandering through underwater caves and feeding along the coast at night. The individuals observed in the wild seem to swim from time to time with the head down in the posture of a «head stop», but this is possible due to the light or the electromagnetic field produced by the observation submarines. are also opportunists, which feed mainly on fish, including the lantern and cardinal tetra fish , eels, rays among others.
Coelacanthus fossils have been found on all continents except Antarctica, but the distribution of this particular species is not entirely clear. The first living specimen was discovered in 1938 on the coast of South Africa, but the only known population is found in the Comoros Islands, a small archipelago in the Mozambique Channel.
In spite of this, specimens have been observed in the northeast coast of South Africa in Sodwana Bay as well as in Madagascar, Kenya and Tanzania. The specimens trapped in the waters of Indonesia are considered as a distinct species (Latimeria menadoensis) and are brown.
Latimeria chalumnae lives in the ocean waters of 150 to 700 meters deep where there are underwater caverns, deep reefs and volcanic slopes, but they have also been observed at a depth of only 17 meters.
The population figures are particularly difficult to assess considering the coelacanths’ deep habitats, but the population of Comores has had a dramatic decline since the 1990s. These fish are caught accidentally while local fishermen search in deep water other species. Due to the probable slow reproduction rate and the small number of coelacanths, the species is particularly vulnerable to the elimination of pregnant females.
The coelacanth is protected against international trade by its inclusion in Appendix I of the Convention on International Trade in Endangered Species (CITES). The fishermen of the Great Island of Comore have also imposed a voluntary prohibition of fishing in areas where there are coelacanths (or gombessa as they are locally known), in a vital effort to save the exclusive fauna of their country.
The Celacanto Rescue Mission is also distributing deep-release kits to local fishermen to provide them with an effective method of returning accidentally caught fish to their deep habitat.
Recently there have been encouraging signs that the Comores population is now stable, although careful monitoring will be necessary to ensure that this living fossil survives in the next millennium. We invite You to read our article Coelacanths to learn more about this species.
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