Reassessment of the possible size, form, weight, cruising speed, and growth parameters of the extinct megatooth shark, (Lamniformes: Otodontidae), and new evolutionary insights into its gigantism, life history strategies, ecology, and extinction.

(Lamniformes: Otodontidae) is an iconic Neogene shark, but the lack of well-preserved skeletons has hampered our understanding of various aspects of its biology. Here, we reassess some of its biological properties using a new approach, based on known vertebral specimens of and 165 species of extinct and extant neoselachian sharks across ten orders. Using the median neurocranial and caudal fin proportions relative to the trunk proportion among non-mitsukurinid/non-alopiid lamniforms, we show that could have had a slender body and possibly reached about 24.3 m in length. Allometric considerations indicate that a stout body plan like the extant white shark for could have incurred excessive hydrodynamic costs, further supporting the interpretation that likely had a slenderer body than . A 24.3-m-long may have weighed around 94 t, with an estimated cruising speed of 2.1-3.5 km h. A reanalysis of vertebral growth bands suggests a size at birth of 3.6-3.9 m for , supporting the previous interpretations of its ovoviviparity and embryos' intrauterine oophagous behavior, but less likely the need for nursery areas. Additional inferred growth patterns corroborated by the known fossil record support the hypothesis that the emergence of during the Early Pliocene is at least partly responsible for the demise of due to competition for resources. These interpretations are working hypotheses expected to serve as reasonable reference points for future studies on the biology of .