Here are the errors identified in your text:
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"People have reported seeing big sharks, being nearly as big as a megalodon."
- The phrase "being nearly as big as a megalodon" is awkward and could be clearer.
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"This led people to believe it was a megalodon, but there are other big sharks apart from the megalodon."
- "apart from the megalodon" could be better phrased as "besides the megalodon."
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"The basking shark is a huge shark, growing up to 29 – 40 feet."
- The dash should be a hyphen: "29–40 feet."
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"These lengths are close to the size of the megalodon, which was estimated to be around 33 – 60 feet."
- Same issue with the dash; it should read "33–60 feet."
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"However, basking sharks are completely harmless and only eat plankton."
- This sentence is grammatically correct but might be clearer if it were rephrased for flow.
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"If the megalodons were truly alive, we would be seeing them all the time."
- "we would be seeing" can be simplified to "we would see" for clarity.
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"Fossils of the megalodon were found warm water oceans and seas, and nowhere near the north and south pole."
- This phrase should include "in" before "warm water oceans" to read: "found in warm water oceans."
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"So, if the megalodons were alive, we’d be seeming them often."
- "seeming" should be corrected to "seeing."
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"Fossils of the megalodon were common, showing that they were not rare in the ocean."
- This sentence is correct but could be revised for clarity regarding the pronoun "they." You may specify "they" as "megalodons."
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"They would also leave big bite marks on animals they fed on."
- The wording could be simplified to "animals on which they fed," but it is not grammatically incorrect.
Overall, most errors are minor and involve clarity, punctuation, or word choice rather than strictly grammatical mistakes.