Evolution, diversity, and disparity of the tiger shark lineage Galeocerdo in deep time.

Julia Türtscher,Patrick L. Jambura,David J. Ward,Faviel A. López-Romero,René Kindlimann,Jürgen Kriwet

doi:10.1017/pab.2021.6

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.

Highlights

Sharks, rays, and skates belong to the cartilaginous fish (Chondrichthyes), forming a monophyletic group, the Neoselachii sensu Compagno (1977) or Elasmobranchii sensu Maisey (2012)
Our results indicate that †G. triqueter is a junior synonym of †P. contortus based on similar tooth morphologies and the same morphospace occupation
We demonstrate here that the presumed high taxonomic diversity of extinct tiger sharks, compared with only one extant species, was much lower than generally assumed

Summary

Introduction

Rays, and skates belong to the cartilaginous fish (Chondrichthyes), forming a monophyletic group, the Neoselachii sensu Compagno (1977) or Elasmobranchii sensu Maisey (2012). More than 500 species of sharks have been described, which amounts to more than 40% of all known extant chondrichthyans (Ebert et al 2013; Weigmann 2016) They are low in number compared with bony fishes, sharks represent a diverse group, occupying many different ecological niches and trophic levels—from zooplanktivorous species at low trophic levels (e.g., whale shark Rhincodon typus), to secondary consumers (e.g., nurse shark Ginglymostoma cirratum), and third-order consumers (e.g., white shark Carcharodon carcharias and tiger shark Galeocerdo cuvier), which is similar to marine mammals (Cortès 1999).

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