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

Abstract

Sharks have a long and rich fossil record that consists predominantly of isolated teeth due to the poorly mineralized cartilaginous skeleton. Tiger sharks (Galeocerdo), which represent apex predators in modern oceans, have a known fossil record extending back into the early Eocene (ca. 56 Ma) and comprise 22 recognized extinct and one extant species to date. However, many of the fossil species remain dubious, resulting in a still unresolved evolutionary history of the tiger shark genus. Here, we present a revision of the fossil record of Galeocerdo by examining the morphological diversity and disparity of teeth in deep time. We use landmark-based geometric morphometrics to quantify tooth shapes and qualitative morphological characters for species discrimination. Employing this combined approach on fossil and extant tiger shark teeth, our results only support six species to represent valid taxa. Furthermore, the disparity analysis revealed that diversity and disparity are not implicitly correlated and that Galeocerdo retained a relatively high dental disparity since the Miocene despite its decrease from four to one species. With this study, we demonstrate that the combined approach of quantitative geometric morphometric techniques and qualitative morphological comparisons on isolated shark teeth provides a useful tool to distinguish between species with highly similar tooth morphologies.

The first Miocene fossils from coastal woodlands in the southern East African Rift

“…Sharks of the genus Galeocerdo (Figures 8, 9) were top predators in these estuaries and nearshore environments. Specimens of Galeocerdo are known from the Eocene to the present (Türtscher et al, 2021), whilst the species G. aduncus , present in the Gorongosa sample, has a temporal range from the Oligocene to the late Miocene (Soto Ovalle, 2016; Türtscher et al, 2021). The genus was widely distributed in the tropical and temperate seas of the Miocene, with specimens found in Madagascar (Andrianavalona et al, 2015), North Africa (Argyriou et al, 2015; Cook, Murray, Simons, Attia, & Chatrath, 2010), Oceania (Fitzgerald, 2004), Eurasia (Marsili, Carnevale, Danese, Bianucci, & Landini, 2007; Villafaña et al, 2020), and the Americas (Carrillo-Briceño, Maxwell, Aguilera, Sánchez, & Sánchez-Villagra, 2015; Landini et al, 2017).…”

“…Four of these are fragmentary teeth from GPL-1 (PPG2017-P-121, PPG2018-P-224) and GPL-12 (PPG2019-P-126, PPG2019-P-127), and two are complete crowns and roots from GPL-12 (PPG2019-P-127, PPG2019-P-129) (Figure 8). For shark teeth we use the terminology of Türtscher et al 2021. The following descriptions and analyses are based on the two complete teeth.…”

“…When classifying the fossils using the species categories, Gorongosa A was classified within Galeocerdo cuvier , whilst Gorongosa B was strongly categorized within Galeocerdo aduncus . The species G. aduncus has a temporal span from the Oligocene to the end of the Miocene, whilst the earliest record of the extant species G. cuvier is from the Middle Miocene (Soto Ovalle, 2016; Türtscher et al, 2021).…”

“…Galeocerdo upper and lower teeth are very similar, but they increase in breadth relative to height posteriorly. The teeth of juvenile tiger sharks have fewer serrations than those of adults (Türtscher et al, 2021). The Gorongosa fossil teeth are functionally similar to those of the extant tiger shark, and we may infer similar function in piercing large prey.…”

Stratigraphy, chronology and palaeontology of the Tertiary rocks of the Cheringoma Plateau, Mozambique

“…comm.). This genus ranges in age from Lutetian to Extant (Türtscher et al 2021) but the form of the fossil tooth illustrated by Habermann et al (2019) suggests that it does not belong to any of the species of this genus that are older than middle Lutetian. It is likely to be Priabonian or younger.…”