Body, jaw, and dentition lengths of macrophagous lamniform sharks, and body size evolution in Lamniformes with special reference to ‘off-the-scale’ gigantism of the megatooth shark, Otodus megalodon

Abstract

Extinct lamniform sharks (Elasmobranchii: Lamniformes) are well represented in the late Mesozoic-Cenozoic fossil record, yet their biology is poorly understood because they are mostly represented only by their teeth. Here, we present measurements taken from specimens of all 13 species of extant macrophagous lamniforms to generate functions that would allow estimations of body, jaw, and dentition lengths of extinct macrophagous lamniforms from their teeth. These quantitative functions enable us to examine the body size distribution of all known macrophagous lamniform genera over geologic time. Our study reveals that small body size is plesiomorphic for Lamniformes. There are four genera that included at least one member that reached >6 m during both the Mesozoic and Cenozoic, most of which are endothermic. The largest form of the genus Otodus, O. megalodon ('megatooth shark') that reached at least 14 m, is truly an outlier considering that all other known macrophagous lamniforms have a general size limit of 7 m. Endothermy has previously been proposed to be the evolutionary driver for gigantism in Lamniformes. However, we contend that ovoviviparous reproduction involving intrauterine cannibalism, a possible synapomorphy of Lamniformes, to be another plausible driver for the evolution of endothermy achieved by certain lamniform taxa.

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

“…Ebert et al 2013). Moreover, the development of such a large neonate strongly suggests that these species had an ovoviviparous (or aplacental viviparity) reproductive mode with an intrauterine cannibalism behaviour in the form of oophagy by 'early hatched' embryos (Shimada et al 2020). This reproductive strategy, that must have evolved by the Late Cretaceous (Shimada and Everhart 2019), is universally present in all extant lamniforms that produce relatively large, well-developed newborns offset by low fecundity (Carrier et al 2004;Gilmore et al 2005; Conrath and Musick 2012).…”

“…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).…”

“…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). The gigantism of O. megalodon is attributed to the evolution of regional endothermy (Ferrón 2017), possibly along with the inferred live-bearing reproductive mode involving intrauterine cannibalism in the form of oophagy (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). The gigantism of O. megalodon is attributed to the evolution of regional endothermy (Ferrón 2017), possibly along with the inferred live-bearing reproductive mode involving intrauterine cannibalism in the form of oophagy (Shimada et al 2020). Yet, exactly how O. megalodon developed throughout its lifetime has remained largely in the realm of speculations (Gottfried et al 1996;Kent 2018).…”

Probing the Ecology and Climate of the Eocene Southern Ocean With Sand Tiger Sharks Striatolamia macrota

“…Paleoceanography and Paleoclimatology (Reguero et al, 2012), including elasmobranch (Engelbrecht et al, 2017a(Engelbrecht et al, , 2017bKriwet, 2005;Kriwet et al, 2016) and also teleost fauna (Přikryl & Vodrážka, 2012;Schwarzhans et al, 2017). Larger body size increases the possible prey range of S. macrota, which makes it a keystone predator in the Eocene Antarctic environment, even though the largest individuals from Seymour Island are slightly smaller than the largest calculated specimen recorded by Shimada et al (2020).…”