Evolution, diversity, and disparity of the tiger shark lineage Galeocerdo in deep time
Türtscher, Julia et al. (2021), Evolution, diversity, and disparity of the tiger shark lineage Galeocerdo in deep time, Dryad, Dataset, https://doi.org/10.5061/dryad.rv15dv47c
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.
Material – Isolated teeth of extinct and extant shark species as well as dried jaws of the extant tiger shark Galeocerdo cuvier 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 †G. acutus, †G. aduncus, †G. aegyptiacus, †G. bigelowi, †G. capellini, †G. casei, †G. clarkensis, G. cuvier, †G. davisi, †G. eaglesomei, †G. gajensis, †G. latidens, †G. mayumbensis, †G. paulinoi, †G. rosaliaensis and †G. triqueter. However, teeth of morphologically similar species of Hemipristis (†H. curvatus and †H. serra) and †Physogaleus (†P. alabamensis and †P. contortus) were also included. Five species were represented with their holotype (†G. casei, †G. davisi, †G. gajensis, †G. paulinoi, †P. alabamensis) and three with the whole type series (†G. clarkensis, †G. eaglesomei, †G. rosaliaensis).
Geometric Morphometrics – The tooth shape of fossil and extant Galeocerdo species as well as Hemipristis and †Physogaleus 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 Galeocerdo 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 geomorph (v. 3.1; Adams et al. 2016) and RRPP (Collyer and Adams 2018).
Disparity Through Time – To evaluate how the dental morphological disparity through geologic time changed among Galeocerdo 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 geomorph (v. 3.1; Adams et al. 2016) and performed post-hoc pairwise comparisons considering the variance between the epochs to estimate differences between them.
Austrian Science Fund, Award: P 33820