What is the significance of the coelacanth fish latimeria

The coelacanth pronounced SEEL-uh-kanth is an enormous, bottom-dwelling fish that is unlike other living fishes in a number of ways. They belong to an ancient lineage that has been around for more than million years.

Coelacanths can reach more than six feet long and weigh about pounds, and they're covered in thick, scaly armor. It's estimated they can live up to 60 years or more. There are two living species of coelacanth, and both are rare. The West Indian Ocean coelacanth Latimeria chalumnae lives off the east coast of Africa, while the Indonesian coelacanth Latimeria menadoensis is found in the waters off Sulawesi, Indonesia. They are the sole remaining representatives of a once widespread family of lobe-finned fishes; more than species are known from the fossil record.

Coelacanths were thought to be extinct until a live one was caught in Coelacanths were known only from fossils until a live Latimeria chalumnae was discovered off the coast of South Africa in Until then, they were presumed to have gone extinct in the late Cretaceous period, over 65 million years ago. The second living species of coelacanth, Latimeria menadoensis , was discovered in an Indonesian market in , and a live specimen was caught one year later.

Coelacanths might be important for understanding the transition from water to land. Coelacanths were thought to be the ancestors of tetrapods four-legged, land-living animals , but a recent analysis of the coelacanth genome suggests that lungfish are actually more closely related to tetrapods. The divergence of coelacanths, lungfish, and tetrapods is thought to have occurred about million years ago. Coelacanths might occupy a side branch of the vertebrate lineage, closely related to, yet distinct from, the ancestor of tetrapods.

Coelacanths have a unique form of locomotion. One striking feature of the coelacanth is its four fleshy fins, which extend away from its body like limbs and move in an alternating pattern.

The movement of alternate paired fins resembles the movement of the forelegs and hindlegs of a tetrapod walking on land. Their jaws are hinged to open wide. Unique to any other living animal, the coelacanth has an intracranial joint, a hinge in its skull that allows it to open its mouth extremely wide to consume large prey.

Instead of a backbone, they have a notochord. Coelacanths retain an oil-filled notochord, a hollow, pressurized tube that serves as a backbone. Several morphological studies have shown that the coelacanth lineage has not displayed critical morphological transformation during its evolutionary history following an early diversification episode in the Devonian Schaeffer, ; Cloutier, ; Forey, ; Schultze, ; Friedman and Coates, and several molecular studies revealed a low genomic substitution rate for Latimeria for some categories of genes at least Amemiya et al.

They challenged the low substitution rate in Latimeria genome by scanning publications dealing with nuclear gene analyses.

A majority of studies 8 among 12 papers indicates no conclusive evidence for a slow evolution of the coelacanth genome, and those pointing out a slowly evolving genome are mostly based on studies of the HOX gene clusters 2 among At the same time, the full genome sequencing of Latimeria chalumnae was made available Amemiya et al.

Casane and Laurenti also questioned the morphological stability of Actinistia by arguing that no fossils of the genus Latimeria have been found so far, suggesting that morphological differences between extant and extinct coelacanths are important enough to be grouped into distinct genera. They also stated that the morphological stability of coelacanths is not supported by paleontological evidence, and illustrated the actinistian morphological disparity by comparing the body morphology of nine extinct genera ranging in age from the Lower Devonian to the Cretaceous along with the Recent Latimeria.

Among anatomical structures taken as examples of characters varying through time, they listed the number of vertebral elements, the ratio between the abdominal and the caudal regions, and pointed out differences in the general body morphology. They briefly compared the anatomy of Latimeria with the Cretaceous Macropoma and noticed differences concerning the orientation of the gape and the shape and relative proportions of some skull bones.

Here we address the issue of coelacanth morphological transformations through deep time. We do not question the statement that morphological differences are observed between extinct actinistian taxa and Latimeria but rather test whether morphological changes occurred in actinistians at the same pace as in other comparable major vertebrate lineages by comparing rate of anatomical novelties acquisition along corresponding evolutionary trajectories.

Although computations of morphological acquisitions along the actinistian lineage were performed by several authors Schaeffer, ; Forey, , ; Cloutier, ; Schultze, , their purpose was to assess fluctuations of morphological changes and not to compare transformations rates with other clades, as proposed here. In order to address the issue of morphological transformations through time, we selected three clades of crown-group vertebrates: the Actinistia coelacanths , the tetrapods here encompassing Tetrapoda and their closest relatives , and the Actinopterygii ray-finned fishes Figure 1 and propose phylogenetic reconstructions of corresponding extant genera, the coelacanth Latimeria , a hummingbird Trochilus and the perch Perca Figure 2.

The Chondrichthyes chondrichthyans and the Dipnoi lungfishes could not be included in this study because the post-Paleozoic fossil record of these clades lacks sufficient taxa represented by complete and articulated specimens, which prevents homogeneous comparison with the other extant clades considered. The phylogenetic reconstructions presented here are based on a similar model as in the Figure 1 of Casane and Laurenti , but with the notable difference that they are plotted against a time frame Figure 2A.

For each actinistian terminal taxon retained in the original figure of Casane and Laurenti , we looked for approximately coeval tetrapod and ray-finned fish taxa, which have been proposed as close relatives of hummingbird and perch, respectively.

Because of the incompleteness of the fossil record, variations in the stratigraphic ranges occur for some of the corresponding genera between the three lineages, and the selected taxa do not represent the closest phylogenetic relatives in all instances, but a genus close to it with a reasonably well-known anatomy similarly, the actinistian cladogram displays a selection of genera. In this figure, the genera corresponding to the coelacanth Macropoma are Ichthyornis and Hoplopteryx for tetrapods and actinopterygians, respectively, and the genera corresponding to Mawsonia are Gansus and Enchodus , respectively, and so on through deep time.

In addition, note that because Miguashaia —the sister group to all other actinistians in Casane and Laurenti's phylogeny—is not the oldest known representative of this clade, the patterns of the three compared cladograms slightly differ at their bases. Moreover, a few choices were made between concurrent phylogenies, such as, for instance, the placement of Dialipina as sister group of all other ray-finned fishes Zhu et al.

Figure 1. Interrelationships among the five crown-vertebrate clades. The lineages with stippled lines are not included in this study, because their fossil record is mostly composed of fragmentary remains teeth and tooth plates for the second half of their evolutionary histories. Figure 2. A Cladograms of the three crown-vertebrate clades included within a time frame. Numbers indicate uniquely derived apomorphies supporting each node [for actinistians, numbers separated by a slash bar are counted based on data from Dutel et al.

B Scale indicating the acquisition rate of uniquely derived apomorphies per million years for the three clades. Red arrows indicate rates calculated with the total number of unique apomorphies, blue arrows indicate rates without the basalmost node for each clade gray frame in the phylogenies above. For actinistians, D and Z indicate calculation with data from Dutel et al.

The three calibrated cladograms represent the evolutionary histories of the coelacanth, hummingbird and perch during a similar time interval of almost million years. In a second step, we searched in literature for uniquely derived morphological character states that occurred along the corresponding internodes of the three cladograms.

Uniquely derived characters, referred to as unique apomorphies below, may or may not include unambiguous character transformations. The rate of unique apomorphies acquisition per million years is calculated for the three lineages by dividing the total number of unique apomorphies by Figure 2B.

Because of the different patterns between the basal part of the three phylogenies, and because of the taxonomic uncertainties mentioned above, the number of unique apomorphies in the three lineages were also computed by excluding the basalmost nodes, i. In this case, we consider an average total time interval of Ma for calculating unique apomorphy acquisition per million years.

For actinistians, we used the set of characters from Dutel et al. Because no phylogenies including all the genera used in the tetrapod and the ray-finned lineages are available, we mapped the characters using several published sources. We selected from each study unique apomorphies that support the six nodes.

Unique apomorphies correspond to character changes with a consistency index of 1, although we cannot exclude that for some of them, reversions are possible if the total set of terminal taxa was considered in the phylogenetic analyses. Moreover, we made the assumption that adding taxa would not add unique apomorphies. Because some of the studies used here do not provide lists of character transformations associated with their consistency indexes, we ran the original datamatrices and searched directly for unique apomorphies supporting the nodes Supplementary Table S1.

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