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Megapiranha is an extinct Serrasalmidae characin fish from the Late Miocene (8–10 million years ago) Ituzaingó Formation of Argentina, described in 2009. Live Science: Toothy 3-foot Piranha Fossil Found The type species is M. paranensis. Megapiranha at Fossilworks.org It is thought to have been about in length and in weight. The holotype consists only of and a zigzag tooth row; the rest of its body is unknown. ( Summary of the paper). This dentition is reminiscent of both the double-row seen in , and the single row seen in the teeth of modern , suggesting that M. paranensis is a transitional form. Its bite force is estimated between .
The name Megapiranha is a combination of the word "mega" in reference to the animal's large size and piranha, a common name for typically carnivorous members of Serrasalmidae. The word piranha itself is a Portuguese merging of words originating in the Tupi language and may have several meanings including "tooth fish", "cutting fish", "devil fish"Britton, A. Scott. Guaraní: Guaraní-English, English-Guaraní; concise dictionary. New York: Hippocrene Books, 2005. Print. or "biting fish". The species name was chosen to reflect Megapiranha's place of origin near the city of Paraná.
Each teeth only shows a single tooth crown the shape of an almost equilateral triangle, which sits atop a constriction of the tooth. Towards the apex of the crown the teeth take on a more sloping edge which is finely serrated in addition to slight labio-lingual compression. Much like in the arrangement of the teeth, Megapiranha differs significantly from its modern relatives. Pacus generally have more complex and broad teeth while true piranhas have teeth with multiple cusps, well developed serration and strong compression, making them thin and well suited for cutting. Between the three preserved teeth the size varies greatly, with the third being the largest and the fourth the smallest. The attachment scars likewise differ in size, showing a similar size distribution.
The preserved premaxilla is almost straight and the teeth are all positioned on the same horizontal plane. The dorsal surface of the bone is slightly concave and slopes upwards towards the front as it transitions to the ascending process, which is barely tapering. Here too Megapiranha provides a unique combination of features amongst its family, with serrasalmids that share the straight axis of the premaxilla typically having a straight dorsal margin and two different planes on which the teeth are placed, while those with a single horizontal plane and concave dorsal surface lack a straight axis. The entire premaxilla is long with a rugose outer surface that most likely housed nerves and blood vessels. The symphyseal joint is interlocking.
Based on the size of the holotype, Megapiranha has originally been estimated to have reached a length of and a weight of , larger than any other member of the family, living or extinct. Later research using Serrasalmus rhombeus as a basis arrived at a more conservative size estimate of long and heavy.
This unique combination of characters supports the idea that within serrasalmids an evolutionary trend led to the shift from double-rowed dentition with broad teeth to the single row of flattened teeth observed in piranhas. Megapiranha represents an intermediate form between the two, with triangular, slightly compressed teeth but maintaining two rows of teeth that are still relatively broad.
In 2012 Justin R. Grubich and colleagues suggest that the dentition of Megapiranha may have been a transitional form between feeding on hard prey and specialising in slicing flesh. To arrive at this conclusion, they conducted extensive measurements of the bite force of the extant Serrasalmus rhombeus as a standin for its Miocene relative. With this method they calculated a bite force of 1240 Newton for the smaller estimates and 4749 Newton for the older, larger size estimates. Even the more conservative estimates would put the bite-force of Megapiranha four times higher than that of the largest extant piranha species. The authors additionally note that the fact that the measurements were taken on live animals may lead to underestimates caused by fatigue and stress. Furthermore, the measurements were restricted to the anterior bite-force, not including the potential of doubled bite-force along the lower jaw. This may result in forces between 2480 and 9498 Newton.
Tests using a bronze-alloy replica of Megapiranha'
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