DOI: 10.5061/dryad.rv15dv47c
title: Evolution, diversity, and disparity of the tiger shark lineage Galeocerdo in deep time

description: 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 recognised 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.
description: <i>Material </i>– Isolated teeth of extinct and
 extant shark species as well as dried jaws of the extant tiger shark
 <i>Galeocerdo cuvier</i> were used in this study. A dagger
 preceding the name identifies extinct species in the text.
 The sample consists in total of 569 shark teeth, photographed in
 labial view. Eighteen published illustrations of tiger shark teeth were
 used if insufficient or no other material of the corresponding species was
 available. The majority of the teeth (n = 450) belongs to tiger sharks and
 is represented by the 16 nominal species †<i>G. acutus,
 </i>†<i>G. aduncus</i>, †<i>G.
 aegyptiacus</i>, †<i>G. bigelowi, </i>†<i>G.
 capellini, </i>†<i>G. casei, </i>†<i>G.
 clarkensis</i>, <i>G. cuvier, </i>†<i>G. davisi,
 </i>†<i>G. eaglesomei</i>, †<i>G.
 gajensis,</i> †<i>G. latidens</i>, †<i>G.
 mayumbensis</i>, †<i>G. paulinoi, </i>†<i>G.
 rosaliaensis </i>and †<i>G. triqueter</i>. However,
 teeth of morphologically similar species of <i>Hemipristis</i>
 (†<i>H. curvatus</i> and †<i>H. serra</i>) and
 †<i>Physogaleus </i>(†<i>P. alabamensis </i>and
 †<i>P. contortus</i>) were also included. Five species were
 represented with their holotype (†<i>G. casei,
 </i>†<i>G. davisi, </i>†<i>G. gajensis,
 </i>†<i>G. paulinoi, </i>†<i>P.
 alabamensis</i>) and three with the whole type series (†<i>G.
 clarkensis, </i>†<i>G. eaglesomei, </i>†<i>G.
 rosaliaensis</i>). <i>Geometric
 Morphometrics </i>– The tooth shape of fossil and extant
 <i>Galeocerdo </i>species as well as
 <i>Hemipristis</i> and †<i>Physogaleus </i>was
 studied with 2D landmark-based geometric morphometrics. Three homologous
 landmarks were digitized using the software tpsDIG2 (v. 2.31; Rohlf 2017).
 Additionally, 64 semilandmarks were digitized between the homologous
 landmarks to capture the overall tooth shape. To
 minimize the variance caused by size, orientation, location and rotation,
 a generalized Procrustes analysis (GPA) was performed on the landmark
 coordinates. The sliding semilandmarks were allowed to slide to minimize
 the bending energy (Gunz and Mitteroecker 2013). The aligned coordinates
 were then subjected to a Principal Component Analysis (PCA) to assess
 shape variation of teeth. Tooth shape differences between genera and
 within <i>Galeocerdo</i> species were estimated with a
 permutational analysis of variance (ANOVA), followed by pairwise
 comparisons between the groups, with the functions procD.lm and pairwise
 considering the distances between means in the R packages
 <i>geomorph</i> (v. 3.1; Adams et al. 2016) and <i>RRPP
 </i>(Collyer and Adams 2018). <i>Disparity
 Through Time </i>– To evaluate how the dental morphological
 disparity through geologic time changed among
 <i>Galeocerdo</i> species, we assigned taxa to the time bins
 Eocene, Oligocene, Miocene, Pliocene and Holocene. We used the Procrustes
 variance (Zelditch et al. 2012), applied the function morphol.disparity in
 the R package <i>geomorph</i> (v. 3.1; Adams et al. 2016) and
 performed post-hoc pairwise comparisons considering the variance between
 the epochs to estimate differences between them.