Articulated remains of the extinct shark Ptychodus (Elasmobranchii, Ptychodontidae) from the Upper Cretaceous of Spain provide insights into gigantism, growth rate and life history of ptychodontid sharks

Abstract

Due to their cartilaginous endoskeleton and the continuous tooth replacement, the chondrichthyan fossil record predominantly consists of isolated teeth, which offer diagnostic features for taxonomic identifications, but only provide very limited information of an organism's life history. In contrast, the calcified vertebral centra of elasmobranchs (sharks, skates and rays) yield important information about ecological and biological traits that can be utilized for constructing age-structured population dynamic models of extant species and palaeoecological reconstructions of such aspects in extinct groups. Here, we describe two large shark vertebrae from the Santonian (Upper Cretaceous) of Spain, which show a unique combination of characters (asterospondylic calcification pattern, with concentric lamellae and numerous parallel bands that are oriented perpendicular) that is only known from ptychodontid sharks, a distinct, extinct group of giant durophagous sharks of the Cretaceous era. Based on linear regression models for large extant sharks a total length between 430 and 707cm was estimated for the examined specimen. Our results indicate that ptychodontid sharks were large viviparous animals, with slow growth rates, matured very late and, therefore, show typical traits for K-selected species. These traits combined with a highly specialized feeding ecology might have played a crucial role for the success but also, eventually, extinction of this group.

Evolution, diversity, and disparity of the tiger shark lineageGaleocerdoin deep time

“…The second species, † G. priscus , is based solely upon some isolated scales from the late Eocene (Heckel 1853). Although isolated scales may bear paleoecological signals, they only provide very limited taxonomic information (Ferrón et al 2014), because different morphotypes occur across the body (e.g., Ankhelyi et al 2018; Jambura and Kriwet 2020). Due to a lack of an illustration, it is not possible to unambiguously assign the described scales to a species, hence we consider † G. priscus a nomen dubium .…”

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

“…up to 6.4 m TL for the Greenland shark, Somniosus microcephalus; and up to 6.1 m TL for the great hammerhead shark, Sphyrna mokarran (Compagno 1984). It has been suggested that at least some members of the Late Cretaceous durophagous shark genus, Ptychodus, possibly reached at least 10 m TL (Shimada et al 2010;Jambura and Kriwet 2020), and if so, durophagy may be another specialisation pathway that has the potential for elasmobranchs to attain gigantism (Pimiento et al 2019). In addition, some isolated, over-sized lamniform vertebrae are known from Albian (Lower Cretaceous) deposits, and whereas the largest specimen is estimated to have come from an individual that possibly measured 8.3-9.8 m TL, their taxonomic identity remains uncertain (Shimada 1997e;Frederickson et al 2015).…”

The Italian record of the Cretaceous shark,Ptychodus latissimusAgassiz, 1835 (Chondrichthyes; Elasmobranchii)

“…Fossa-Mancini (1921) also noted that the ratio between "useful weight" (soft tissues) and "harmful weight" (skeleton) could be greater in ammonites than in other mollusks. This author thus concluded that these shelled cephalopods could ensure sufficient food even for individuals of Ptychodus reaching considerable body sizes (see also Shimada et al, 2010;Jambura & Kriwet, 2020). Like their extinct relatives (e.g., ammonites), living cephalopods have important and diversified roles in marine ecosystems, ranging from occasional scavengers (e.g., nautiloids) at lower trophic levels to carnivorous predators (e.g., squids; Olóriz & Villaseñor, 2010;Lukeneder, 2015).…”

“…Ptychodus Agassiz, 1834 is a genus of Cretaceous elasmobranchs ranging from the Albian to the Campanian with worldwide distribution (Woodward, 1912;Cappetta, 2012). The fossil record of this enigmatic predator mainly includes isolated teeth, very rare associated, but also articulated dentitions and mineralized cartilaginous elements (Nicolis, 1889;Woodward, 1889;Tan, 1949;MacLeod, 1982;Everhart & Caggiano, 2004;Shimada et al, 2010;Cappetta, 2012;Shimada, 2012;Diedrich, 2013;Hamm, 2017;Amadori et al, 2019aAmadori et al, , 2019bAmadori et al, , 2020Jambura & Kriwet, 2020). Ptychodus possessed polygonal, molariform teeth arranged in dental plates suitable for crushing shelled preys (Williston, 1900a(Williston, , 1900bWoodward, 1912;Cappetta, 2012;Shimada, 2012;Diedrich, 2013;Amadori et al, 2019bAmadori et al, , 2020.…”