External anatomy of the extinct giant shark Otodus megalodon

Inferring the size and shape of extinct animals is fraught with danger, especially when they were much larger than their modern relatives. Such extrapolations are particularly risky when allometry is present. The extinct giant shark Otodus megalodonis known almost exclusively from fossilised teeth, and estimates have been made from these of its total length and body mass using the great white shark (Carcharodon carcharias) as the modern analogue. This is problematic as the two species likely belong to different families within Lamniformes, and the position of theOtoduslineage is unclear. Here, we infer the body dimensions of the giant O. megalodon based on anatomical measurements of five ecologically and physiologically similar lamniform sharks.We first assessed for allometry in all species using linear regressions and geometric morphometric analyses. Because no evidence of allometry was found, we made morphological extrapolations to infer body dimensions of O. megalodonat different sizes.  Our results suggest that a 16 m O. megalodonlikely had a head ~4.65 m long, a dorsal fin ~1.62 m tall and a tail ~3.85 m high. Our morphometric analyses further reveal dorsal and caudal fins adaptedfor fast predatory swimming, but slower, more sustained long-distance swimming.