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20.01.2016 00:37 - Largest prehistoric animals Vol.1 Vertebrates part11 Sharks ch.4 Megalodon - All time apex predator
Автор: valentint Категория: Други   
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Последна промяна: 03.12.2016 02:50


An extinct megatoothed shark, C. megalodon is by far the biggest shark known.

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This giant shark reached a total length of more than 16 metres (52 ft).C. megalodon may have approached a maximum of 20.3 metres (67 ft) in total length and 103 metric tons (114 short tons) in mass.

According to Renaissance accounts, gigantic, triangular fossil teeth often found embedded in rocky formations were once believed to be the petrified tongues, or glossopetrae, of dragons and snakes. This interpretation was corrected in 1667 by Danish naturalist Nicolaus Steno, who recognized them as shark teeth, and famously produced a depiction of a shark's head bearing such teeth.He described his findings in the book The Head of a Shark Dissected, which also contained an illustration of a C. megalodon tooth.

Swiss naturalist Louis Agassiz gave the shark its initial scientific name, Carcharodon megalodon, in 1835,in his research work Recherches sur les poissons fossiles (Research on fossil fish), which he completed in 1843. Megalodon teeth are morphologically similar to the teeth of the great white shark. On the basis of this observation, Agassiz assigned megalodon to the genus Carcharodon.While the scientific name is C. megalodon, it is often informally dubbed the "megatooth shark","giant white shark" or "monster shark".

C. megalodon is represented in the fossil record primarily by teeth and vertebral centra.As with all sharks, C. megalodon's skeleton was formed of cartilage rather than bone; this results in mostly poorly preserved fossil specimens.While the earliest megalodon remains were reported from late Oligocene strata, circa 28 million years old,a more generally accepted date for the origin of the species is the Middle Miocene, about 15.9 million years ago.Although fossils are mostly absent in strata extending beyond the Tertiary boundary, they have been reported from subsequent Pleistocene strata. It is believed that C. megalodon became extinct around end of the Pliocene, probably about 2.6 million years ago reported post-Pliocene C. megalodon teeth are thought to be reworked fossils. C. megalodon had a cosmopolitan distribution, its fossils have been excavated from many parts of the world, including Europe, Africa and both North and South America, as well as Australia, New Zealand, Japan,Malta, Grenadines and India. Megalodon teeth have been excavated from regions far away from continental lands, such as the Mariana Trench in the Pacific Ocean.

The most common megalodon fossils are its teeth. Diagnostic characteristics include: triangular shape robust structure large size fine serrations and visible v-shaped neck.Megalodon teeth can measure over 180 millimetres (7.1 in) in slant height or diagonal length, and are the largest in size of any known shark species.

Some fossil vertebrae have been found.The most notable example is a partially preserved vertebral column of a single specimen, excavated in the Antwerp basin, Belgium by M. Leriche in 1926. It comprises 150 vertebral centra, with the centra ranging from 55 millimetres (2.2 in) to 155 millimetres (6.1 in) in diameter.However, scientists have claimed that considerably larger vertebral centra can be expected.A partially preserved vertebral column of another megalodon specimen was excavated from Gram clay, Denmark by Bendix-Almgeen in 1983. This specimen comprises 20 vertebral centra, with the centra ranging from 100 millimetres (3.9 in) to 230 millimetres (9.1 in) in diameter.

Even after decades of research and scrutiny, controversy over C. megalodon phylogeny persists.Several shark researchers (e.g. J. E. Randall, A. P. Klimley, D. G. Ainley, M. D. Gottfried, L. J. V. Compagno, S. C. Bowman, and R. W. Purdy) insist that C. megalodon is a close relative of the great white shark. However, others (e.g. D. S. Jordan, H. Hannibal, E. Casier, C. DeMuizon, T. J. DeVries, D. Ward, and H. Cappetta) cite convergent evolution as the reason for the dental similarity. Such Carcharocles advocates have gained noticeable support.However, the original taxonomic assignment still has wide acceptance.

The traditional view is that megalodon should be classified within the genus Carcharodon along with the great white shark. The main reasons cited for this phylogeny are: (1) an ontogenetic gradation, whereby the teeth shift from coarse serrations as a juvenile to fine serrations as an adult, the latter resembling megalodon's; (2) morphological similarity of teeth of young megalodon to those of C. carcharias; (3) a symmetrical second anterior tooth; (4) large intermediate tooth that is inclined mesially; and (5) upper anterior teeth that have a chevron-shaped neck area on the lingual surface. Carcharodon supporters suggest that megalodon and C. carcharias share a common ancestor, Palaeocarcharodon orientalis.

Around 1923, the genus Carcharocles was proposed by D. S. Jordan and H. Hannibal, to classify the shark C. auriculatus. Later on, Carcharocles proponents assigned megalodon to Carcharocles.Carcharocles proponents also suggest that the direct ancestor of the sharks belonging to Carcharocles is an ancient giant shark called Otodus obliquus, which lived during the Paleocene and Eocene epochs.According to Carcharocles supporters, Otodus obliquus evolved into Otodus aksuaticus, which evolved into Carcharocles auriculatus, and then into Carcharocles angustidens, and then into Carcharocles chubutensis, and then into megalodon. Hence, the immediate ancestor of C. megalodon is C. chubutensis, because it serves as the missing link between C. augustidens and C. megalodon and it bridges the loss of the "lateral cusps" that characterize megalodon.

Shark researchers are apparently reconsidering the genus of entire Carcharocles lineage back to Otodus.

Megalodon as a chronospecies

Shark researcher David Ward elaborated on the evolution of Carcharocles by implying that this lineage, stretching from the Paleocene to the Pliocene, is of a single giant shark which gradually changed through time, suggesting a case of chronospecies.This assessment may have credibility.

Mako sharks as closest relatives of great whites

Carcharocles proponents point out that the great white shark is closely related to an ancient shark Isurus hastalis, the "broad tooth mako", rather than to megalodon. One reason cited by paleontologist Chuck Ciampaglio is that the dental morphometrics (variations and changes in the physical form of objects) of I. hastalis and C. carcharias are remarkably similar. Another reason cited is that megalodon teeth have much finer serrations than C. carcharias teeth.Further evidence linking the great white shark more closely to ancient mako sharks, rather than to megalodon, was provided in 2009 — the fossilized remains of a form of the great white shark about 4 million years old were excavated from southwestern Peru in 1988. These remains demonstrate a likely shared ancestor of modern mako and great whites.

Ciampaglio asserted that dental similarities between megalodon and the great white are superficial with noticeable morphometric differences between them, and that these findings are sufficient to warrant a separate genus.However, some Carcharodon proponents (i.e., M. D. Gottfried, and R. E. Fordyce) provided more arguments for a close relationship between the megatooth and the great white.With respect to the recent controversy regarding fossil lamnid shark relationships, overall morphology – particularly the internal calcification patterns – of the great white shark vertebral centra have been compared to well-preserved fossil centra from the megatooth, including megalodon and C. angustidens. The morphological similarity of these comparisons supports a close relationship of the giant fossil megatooth species to extant whites.

Gottfried and Fordyce pointed out that some great white shark fossils are about 16 million years old and predate the transitional Pliocene fossils.In addition the Oligocene megalodon records contradict the suggestion that C. chubutensis is the immediate ancestor of C. megalodon. These records also indicate that megalodon co-existed with C. angustidens.

Some paleontologists argue that the genus Otodus should be used for sharks within the Carcharocles lineage and the genus Carcharocles should be discarded.

Several Carcharocles proponents (i.e. C. Pimiento, D. J. Ehret, B. J. MacFadden, and G. Hubbell) claim that both species belong to the order Lamniformes and in the absence of living members of the family Otodontidae, the great white shark is the species most ecologically analogous to megalodon.

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Among extant species, the great white shark is regarded as the best analogue to megalodon. The lack of well-preserved fossil megalodon skeletons led scientists to rely on the great white shark as the basis of its reconstruction and size estimation.

Due to fragmentary remains, estimating the size of C. megalodon has been challenging.However, the scientific community has concluded that C. megalodon was larger than the whale shark, Rhincodon typus. Scientists focused on two aspects of size: total length and body mass.

The first attempt to reconstruct a megalodon jaw was made by Bashford Dean in 1909. From the dimensions of this jaw reconstruction, it was hypothesized that C. megalodon could have approached 30 metres (98 ft). Better knowledge of dentition and more accurate muscle structures led to a rectified version of Dean's jaw model about 70 percent of its original size and to a size consistent with modern findings.To resolve such errors, scientists, aided by new fossil discoveries of C. megalodon and improved knowledge of its closest living analogue's anatomy, introduced more quantitative methods for estimating its size based on the statistical relationships between the tooth sizes and body lengths.Some methods are mentioned below.

In 1973, Hawaiian ichthyologist John E. Randall used a plotted graph to demonstrate a relationship between the enamel height (the vertical distance of the blade from the base of the enamel portion of the tooth to its tip) of the largest tooth in the upper jaw of the great white shark and its total length.Randall extrapolated this method to estimate C. megalodon's total length. Randall cited two megalodon teeth in his work, specimen number 10356 at the American Museum of Natural History and specimen number 25730 at the United States National Museum, which had enamel heights of 115 millimetres (4.5 in) and 117.5 millimetres (4.63 in) respectively.These teeth yielded a corresponding total length of about 13 metres (43 ft).In 1991, Richard Ellis and John E. McCosker claimed that tooth enamel height does not necessarily increase in proportion to the animal's total length.

In 1996, after scrutiny of 73 great white shark specimens, Michael D. Gottfried, Leonard Compagno and S. Curtis Bowman proposed a linear relationship between the height of the largest upper anterior tooth and total length in the great white shark. The proposed relationship is: total length in metres = − (0.096) × [UA maximum height (mm)]-(0.22).Gottfried and colleagues then extrapolated their technique to C. megalodon. The biggest megalodon tooth in the possession of this team was an upper second anterior specimen, whose maximum height was 168 millimetres (6.6 in). This tooth had been discovered by Compagno in 1993. It yielded an estimated total length of 15.9 metres (52 ft).Rumors of larger megalodon teeth persisted at the time.The maximum tooth height for this method is measured as a vertical line from the tip of the crown to the bottom of the lobes of the root, parallel to the long axis of the tooth.In layman's terms, the maximum height of the tooth is its slant height.

In 2002, shark researcher Clifford Jeremiah proposed that total length was proportional to the root width of an upper anterior tooth. He claimed that for every 1 centimetre (0.39 in) of width, there is approximately 4.5 feet (1.4 m) of the shark. Jeremiah pointed out that the jaw perimeter of a shark is directly proportional to its total length, with the width of the roots of the largest teeth being a proxy for estimating jaw perimeter. The largest tooth in the possession of Jeremiah had a root width of about 12 centimetres (4.7 in), which yielded 16.5 metres (54 ft) total length. Ward asserted that this method is based on a sound principle that works well with most large sharks.

In 2002, paleontologist Kenshu Shimada of DePaul University proposed a linear relationship between tooth crown height and total length in great white sharks after conducting anatomical analysis of several specimens.This relationship is expressed as: total length in centimetres = a + bx, where a is a constant, b is the slope of the line and x is the crown height of tooth in millimetres. This relationship allowed any tooth to be used for the estimate.The crown height was measured as maximum vertical enameloid height on the labial side. Shimada pointed out that previously proposed methods were based on weaker evaluation of dental homology, and that the growth rate between the crown and root is not isometric, which he considered in his model.Furthermore, this relationship could be used to predict the total length of sharks that are morphologically similar to the great white shark, such as C. megalodon.Using this model, the upper anterior tooth (with maximum height of 168 millimetres (6.6 in)) possessed by Gottfried and colleagues corresponded to a total length of 15.1 metres (50 ft).In 2010, shark researchers Catalina Pimiento, Dana J. Ehret, Bruce J. MacFadden and Gordon Hubbell estimated the total length of C. megalodon on the basis of Shimada's method. Among the specimens found in the Gatun Formation of Panama, specimen number 237956 yielded a total length of 16.8 metres (55 ft).Later on, shark researchers (including Pimiento, Ehret and MacFadden) revisited Gatun Formation and recovered additional specimens, the specimen number 257579 yielded a total length of 17.9 metres (59 ft) on the basis of Shimada's method.

In the 1990s, marine biologists such as Patrick J. Schembri and Staphon Papson opined that C. megalodon may have approached a maximum of around 24 to 25 metres (79 to 82 ft) in total length however Gottfried and colleagues proposed that C. megalodon could likely approach a maximum of only 20.3 metres (67 ft) in total length.Currently, most experts acknowledge that C. megalodon reached a total length of more than 16 metres (52 ft).














 



Тагове:   сова,   prehistoric animals,   giant animals,   sharks,


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