Megalodon is an extinct species of shark that lived approximately 28 to 1.5 million years ago, during the Cenozoic Era (late Oligocene to early Pleistocene).The taxonomic assignment of C. megalodon has been debated for nearly a century, and is still under dispute. The two major interpretations are Carcharodon megalodon (under family Lamnidae) or Carcharocles megalodon (under family Otodontidae). Consequently, the scientific name of this species is commonly abbreviated C. megalodon in the literature.
C. megalodon is regarded as one of the largest and most powerful predators in vertebrate history, and likely had a profound impact on the structure of marine communities.Fossil remains suggest that this giant shark reached a maximum length of 15.9–20.3 metres (52–67 ft), and also affirm that it had a cosmopolitan distribution.Scientists suggest that C. megalodon looked like a stockier version of the great white shark, Carcharodon carcharias.
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.
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
Megalodon fossils have been excavated from many parts of the world, including Europe, Africa and both North and South America, as well as Puerto Rico, Cuba, Jamaica, 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 earliest megalodon remains were reported from late Oligocene strata, circa 28 million years old.Although fossils are mostly absent in strata extending beyond the Tertiary boundary, they have been reported from subsequent Pleistocene strata. It is believed that megalodon became extinct in the Pleistocene, probably about 1.5 million years ago.
Length
Reconstruction by Bashford Dean in 1909, with fossil teeth assembled from various localities.
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 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 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.
Enamel height
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.[26][27] Randall extrapolated this method to estimate 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.Largest anterior tooth height
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 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.
Root width
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.
Crown height
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 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 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).
Dentition and jaw mechanics
Reconstruction showing the position of the replacement teeth.
A team of Japanese scientists, T. Uyeno, O. Sakamoto, and H. Sekine, discovered and excavated partial remains of a megalodon, with a nearly complete associated set of its teeth, from Saitama, Japan in 1989.Another nearly complete associated megalodon dentition was excavated from Yorktown Formations of Lee Creek, North Carolina in the United States and served as the basis of a jaw reconstruction of megalodon at the American Museum of Natural History in New York City.These associated tooth sets solved the mystery of how many teeth would be in the jaws of the megalodon in each row. As a result, highly accurate jaw reconstructions became possible. More associated megalodon dentitions were found in later years. Based on these discoveries, scientists S. Applegate and L. Espinosa published an artificial dental formula (representation of dentition of an animal with respect to types of teeth and their arrangement within the animal's jaw) for megalodon in 1996. Most accurate modern megalodon jaw reconstructions are based on this dental formula.
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