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In the 1980s, megalodon was assigned to "Carcharocles".
Before this, in 1960, the genus "Procarcharodon" was erected by French ichthyologist Edgard Casier, which included those four sharks and was considered separate from the great white shark.
It is now considered a junior synonym of "Carcharocles".
The genus "Palaeocarcharodon" was erected alongside "Procarcharodon" to represent the beginning of the lineage, and, in the model wherein megalodon and the great white shark are closely related, their last common ancestor.
It is believed to be an evolutionary dead-end and unrelated to the "Carcharocles" sharks by authors who reject that model.
Another model of the evolution of this genus, also proposed by Casier in 1960, is that the direct ancestor of the "Carcharocles" is the shark "Otodus obliquus", which lived from the Paleocene through the Miocene epochs, 60 mya to 13 mya.
The genus "Otodus" is ultimately derived from "Cretolamna", a shark from the Cretaceous period.
In this model, "O. obliquus" evolved into "O. aksuaticus", which evolved into "C. auriculatus", and then into "C. angustidens", and then into "C. chubutensis", and then finally into "C. megalodon".
Another model of the evolution of "Carcharocles", proposed in 2001 by paleontologist Michael Benton, is that the three other species are actually a single species of shark that gradually changed over time between the Paleocene and the Pliocene, making it a chronospecies.
Some authors suggest that "C. auriculatus", "C. angustidens", and "C. chubutensis" should be classified as a single species in the genus "Otodus", leaving "C. megalodon" the sole member of "Carcharocles".
The genus "Carcharocles" may be invalid, and the shark may actually belong in the genus "Otodus", making it "Otodus megalodon".
A 1974 study on Paleogene sharks by Henri Cappetta erected the subgenus "Megaselachus", classifying the shark as "Otodus" ("Megaselachus") "megalodon", along with "O.
(M.) chubutensis".
A 2006 review of Chondrichthyes elevated "Megaselachus" to genus, and classified the sharks as "Megaselachus megalodon" and "M. chubutensis".
The discovery of fossils assigned to the genus "Megalolamna" in 2016 led to a re-evaluation of "Otodus", which concluded that it is paraphyletic, that is, it consists of a last common ancestor but it does not include all of its descendants.
The inclusion of the "Carcharocles" sharks in "Otodus" would make it monophyletic, with the sister clade being "Megalolamna".
One interpretation on how megalodon appeared was that it was a robust-looking shark, and may have had a similar build to the great white shark.
The jaws may have been blunter and wider than the great white, and the fins would have also been similar in shape, though thicker due to its size.
It may have had a pig-eyed appearance, in that it had small, deep-set eyes.
Another interpretation is that megalodon bore a similarity to the whale shark ("Rhincodon typus") or the basking shark ("Cetorhinus maximus").
The tail fin would have been crescent-shaped, the anal fin and second dorsal fin would have been small, and there would have been a caudal keel present on either side of the tail fin (on the caudal peduncle).
This build is common in other large aquatic animals, such as whales, tuna, and other sharks, in order to reduce drag while swimming.
The head shape can vary between species as most of the drag-reducing adaptations are toward the tail-end of the animal.
Since "Carcharocles" is derived from "Otodus", and the two had teeth that bear a close similarity to those of the sand tiger shark ("Carcharias taurus"), megalodon may have had a build more similar to the sand tiger shark than to other sharks.
This is unlikely since the sand tiger shark is a carangiform swimmer which requires faster movement of the tail for propulsion through the water than the great white shark, a thunniform swimmer.
Due to fragmentary remains, there have been many contradictory size estimates for megalodon, as they can only be drawn from fossil teeth and vertebrae.
Also because of this, the great white shark is the basis of its reconstruction and size estimation, as it is regarded as the best analogue to megalodon.
Using length estimates extrapolated from 544 teeth found throughout geological time and geography, including adults and juveniles, a 2015 study estimated an average length of .
In comparison, the maximum recorded size of the great white shark is , and the whale shark (the largest living fish) can reach .
It is possible that different populations of megalodon around the globe had different body sizes and behaviors due to different ecological pressures.
If it did attain a size of over , it would have been the largest known fish that has ever lived, surpassing the Jurassic fish "Leedsichthys".
Mature male megalodon may have had a body mass of , and mature females may have been , given that males could range in length from and females .
A 2015 study linking shark size and typical swimming speed estimated that megalodon would have typically swum at –given that its body mass was typically –which is consistent with other aquatic creatures of its size, such as the fin whale ("Balaenoptera physalus") which typically cruises at speeds of .
Its large size may have been due to climatic factors and the abundance of large prey items, and it may have also been influenced by the evolution of regional endothermy (mesothermy) which would have increased its metabolic rate and swimming speed.
Since the otodontid sharks are considered to have been ectotherms, and megalodon was a close relative to them, megalodon may have also been ectothermic.
Contrary to this, the largest contemporary ectothermic sharks, such as the whale shark, are filter feeders, implying some metabolic constraints with a predatory lifestyle.
That is to say, it is unlikely that megalodon was ectothermic.
Gordon Hubbell from Gainesville, Florida, possesses an upper anterior megalodon tooth whose maximum height is , one of the largest known tooth specimens from the shark.
In addition, a megalodon jaw reconstruction developed by fossil hunter Vito Bertucci contains a tooth whose maximum height is reportedly over .
The first attempt to reconstruct the jaw of megalodon was made by Bashford Dean in 1909, displayed at the American Museum of Natural History.
From the dimensions of this jaw reconstruction, it was hypothesized that megalodon could have approached in length.
Dean had overestimated the size of the cartilage on both jaws, causing it to be too tall.
In 1973, John E. Randall, an ichthyologist, used the enamel height (the vertical distance of the blade from the base of the enamel portion of the tooth to its tip) to measure the length of the shark, yielding a maximum length of about .
However, tooth enamel height does not necessarily increase in proportion to the animal's total length.
In 1994, marine biologists Patrick J. Schembri and Stephen Papson opined that "C. megalodon" may have approached a maximum of around in total length.
In 1996, shark researchers Michael D. Gottfried, Leonard Compagno, and S. Curtis Bowman proposed a linear relationship between a shark's total length and the height of the largest upper anterior tooth.
The proposed relationship is: total length in meters = − (0.096) × [UA maximum height (mm)]-(0.22).
They asserted that "C. megalodon" could have reached a maximum of in total length.
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 of root width, there are approximately of shark length.
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 tool for estimating jaw perimeter.
The largest tooth in Jeremiah's possession had a root width of about , which yielded in total length.
In 2002, paleontologist Kenshu Shimada of DePaul University proposed a linear relationship between tooth crown height and total length after conducting anatomical analysis of several specimens, allowing any sized tooth to be used.
Shimada stated that the previously proposed methods were based on a less-reliable evaluation of the dental homology between megalodon and the great white shark, and that the growth rate between the crown and root is not isometric, which he considered in his model.
Using this model, the upper anterior tooth possessed by Gottfried and colleagues corresponded to a total length of .
Among several specimens found in the Gatún Formation of Panama, one upper lateral tooth was used by other researchers to obtain a total length estimate of using this method.
In 2019, Shimada revisited the size of megalodon and discouraged using non-anterior teeth for estimations, noting that the exact position of isolated non-anterior teeth is difficult to identify.
Shimada stated that the maximum total length estimates, based on upper anterior teeth that are available in museums, are , depending on the estimation method used.
The most common fossils of megalodon are its teeth.
Diagnostic characteristics include a triangular shape, robust structure, large size, fine serrations, a lack of lateral denticles, and a visible V-shaped neck (where the root meets the crown).
The tooth met the jaw at a steep angle, similar to the great white shark.
The tooth was anchored by connective tissue fibers, and the roughness of the base may have added to mechanical strength.
The lingual side of the tooth, the part facing the tongue, was convex; and the labial side, the other side of the tooth, was slightly convex or flat.
The anterior teeth were almost perpendicular to the jaw and symmetrical, whereas the posterior teeth were slanted and asymmetrical.
Megalodon teeth can measure over in slant height (diagonal length) and are the largest of any known shark species.
In 1989, a nearly complete set of megalodon teeth was discovered in Saitama, Japan.
Another nearly complete associated megalodon dentition was excavated from the Yorktown Formations in the United States, and served as the basis of a jaw reconstruction of megalodon at the National Museum of Natural History (USNM).
Based on these discoveries, an artificial dental formula was put together for megalodon in 1996.
The dental formula of megalodon is: .
As evident from the formula, megalodon had four kinds of teeth in its jaws: anterior, intermediate, lateral, and posterior.
Megalodon's intermediate tooth technically appears to be an upper anterior and is termed as "A3" because it is fairly symmetrical and does not point mesially (side of the tooth toward the midline of the jaws where the left and right jaws meet).
Megalodon had a very robust dentition, and had over 250 teeth in its jaws, spanning 5 rows.
It is possible that large megalodon individuals had jaws spanning roughly across.
The teeth were also serrated, which would have improved efficiency in cutting through flesh or bone.
The shark may have been able to open its mouth to a 75° angle, though a reconstruction at the USNM approximates a 100° angle.
In 2008, a team of scientists led by S. Wroe conducted an experiment to determine the bite force of the great white shark, using a long specimen, and then isometrically scaled the results for its maximum size and the conservative minimum and maximum body mass of megalodon.
They placed the bite force of the latter between in a posterior bite, compared to the bite force for the largest confirmed great white shark, and for the placoderm fish "Dunkleosteus".
In addition, Wroe and colleagues pointed out that sharks shake sideways while feeding, amplifying the force generated, which would probably have caused the total force experienced by prey to be higher than the estimate.
Megalodon is represented in the fossil record by teeth, vertebral centra, and coprolites.
As with all sharks, the skeleton of megalodon was formed of cartilage rather than bone; consequently most fossil specimens are poorly preserved.
To support its large dentition, the jaws of megalodon would have been more massive, stouter, and more strongly developed than those of the great white, which possesses a comparatively gracile dentition.
Its chondrocranium, the cartilaginous skull, would have had a blockier and more robust appearance than that of the great white.
Its fins were proportional to its larger size.
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, in 1926.
It comprises 150 vertebral centra, with the centra ranging from to in diameter.
The shark's vertebrae may have gotten much bigger, and scrutiny of the specimen revealed that it had a higher vertebral count than specimens of any known shark, possibly over 200 centra; only the great white approached it.
Another partially preserved vertebral column of a megalodon was excavated from the Gram Formation in Denmark in 1983, which comprises 20 vertebral centra, with the centra ranging from to in diameter.
The coprolite remains of megalodon are spiral-shaped, indicating that the shark may have had a spiral valve, a corkscrew-shaped portion of the lower intestines, similar to extant lamniform sharks.
Miocene coprolite remains were discovered in Beaufort County, South Carolina, with one measuring .
Gottfried and colleagues reconstructed the entire skeleton of megalodon, which was later put on display at the Calvert Marine Museum in the United States and the Iziko South African Museum.
This reconstruction is long and represents a mature male, based on the ontogenetic changes a great white shark experiences over the course of its life.
Megalodon had a cosmopolitan distribution; its fossils have been excavated from many parts of the world, including Europe, Africa, the Americas, and Australia.
It most commonly occurred in subtropical to temperate latitudes.
It has been found at latitudes up to 55° N; its inferred tolerated temperature range was .
It arguably had the capacity to endure such low temperatures due to mesothermy, the physiological capability of large sharks to conserve metabolic heat by maintaining a higher body temperature than the surrounding water.
Megalodon inhabited a wide range of marine environments (i.e., shallow coastal waters, areas of coastal upwelling, swampy coastal lagoons, sandy littorals, and offshore deep water environments), and exhibited a transient lifestyle.
Adult megalodon were not abundant in shallow water environments, and mostly inhabited offshore areas.
Megalodon may have moved between coastal and oceanic waters, particularly in different stages of its life cycle.
Fossil remains show a trend for specimens to be larger on average in the southern hemisphere than in the northern, with mean lengths of , respectively; and also larger in the Pacific than the Atlantic, with mean lengths of respectively.