Body dimensions of the extinct giant shark Otodus megalodon: a 2D reconstruction

Abstract

Inferring the size 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 megalodon is known almost exclusively from fossilised teeth. Estimates of †O. megalodon body size have been made from its teeth, using the great white shark (Carcharodon carcharias) as the only modern analogue. This can be problematic as the two species likely belong to different families, and the position of the †Otodus lineage within Lamniformes is unclear. Here, we infer †O. megalodon body dimensions based on anatomical measurements of five ecologically and physiologically similar extant lamniforms: Carcharodon carcharias, Isurus oxyrinchus, Isurus paucus, Lamna ditropis and Lamna nasus. We first assessed for allometry in all analogues using linear regressions and geometric morphometric analyses. Finding no evidence of allometry, we made morphological extrapolations to infer body dimensions of †O. megalodon at different sizes. Our results suggest that a 16 m †O. megalodon likely had a head ~ 4.65 m long, a dorsal fin ~ 1.62 m tall and a tail ~ 3.85 m high. Morphometric analyses further suggest that its dorsal and caudal fins were adapted for swift predatory locomotion and long-swimming periods. Estimating the body size of exceptionally large extinct taxa is a difficult task because the fossil record is inherently incomplete and because allometry, if present, can make extrapolations hard to model. Palaeontologists therefore have to rely on the relationships between often isolated and fragmented body-part remains and length in extant relatives to estimate the body size of extinct giants 1, 2. The extinct †Otodus megalodon has been estimated to be the largest macropredatory shark known to have existed 3. Based on its fossil teeth and using the modern great white shark (Carcharodon carcharias) as an analogue, it has been calculated that it reached a maximum total length (herein, TL) of ~ 15 to 18 m 3-5. †Otodus megalodon was originally classified in the family Lamnidae (order Lamniformes) with C. carcharias considered its closest living relative 3, 6-8. This classification was based on similar tooth morphologies 3, 7, 8 , which also implied that the two species shared an ecological function as apex macropredators. Carcharodon carcharias has therefore been widely used as the main modern analogue of †O. megalodon 3, 4, 9, 10. Accordingly, linear relationships between tooth crown height and TL recorded in C. carcharias 5, 11 have been used extensively to infer the size and skeletal anatomy of †O. megalodon 3-5, 9, 12-14. A detailed examination of tooth morphology challenged the relationship between C. carcharias and †O. megalodon, revealing that C. carcharias descended from a lineage that includes the mako sharks (Isurus spp.) and other closely related taxa (i.e. †Cosmopolitodus) rather than †O. megalodon 15. This hypothesis has further been supported by the fossil record of Carcharodon 16-...

Body length estimation of Neogene macrophagous lamniform sharks (Carcharodon and Otodus) derived from associated fossil dentitions

“…This is particularly important when attempting to estimate the size of taxa that are even more distantly related (e.g., Otodus obliquus). Cooper et al (2020) stated that "Lamna nasus is considered the best dental analogue for both Cretalamna and Megalolamna, mako sharks (Isurus spp.) have similar dental morphology to Otodus, and C. carcharias has similar dentition to Otodus (Carcharocles) and Otodus (Megaselachus)."…”

“…(C) Body length of fossil taxa is calculated under the assumption that the ratio of summed crown width to total body length (TL) is proportional in ecologically and taxonomically related species. Silhouette proportions for O. megalodon are based on Cooper et al (2020). A/a = anterior, I = intermediate, and L/l = lateral.…”

Ontogenetic growth pattern of the extinct megatooth shark Otodus megalodon—implications for its reproductive biology, development, and life expectancy

“…Its most commonly cited estimated maximum size is about 18-20 m in total length (TL) (e.g. Pimiento and Balk 2015;Nelson et al 2016;Razak and Kocsis 2018;Pimiento et al 2019;Cooper et al 2020), but individuals exceeding 14.1-15.3 m TL have not yet been convincingly substantiated in the scientific literature (Shimada 2019;Shimada et al 2020). The conservative estimates are nonetheless still gigantic especially given that nonplanktivorous sharks have a general size limit of 7 m TL (Pimiento et al 2019;Shimada et al 2020).…”

Biomechanical insights into the dentition of megatooth sharks (Lamniformes: Otodontidae)

“…This group experienced a trend towards gigantism throughout the Cenozoic that culminated with Otodus megalodon , the largest macropredatory shark ever to exist 4 . This species overpassed 15 m in total length and likely weighed more than 50 tons 4 – 7 . Historically, the evolution of such titanic body sizes in otodontids has been related to the emergence of various marine mammal lineages during the Paleogene (i.e., pinnipeds, sirenians, and cetaceans) 5 , 8 , 9 .…”