Austin Crawford

Understanding the Fossil Record of Tiger Sharks

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

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

Summarized by Austin Crawford, a senior obtaining a Bachelor of Science in geology at the University of South Florida. Austin’s current career interests involve several fields in hydrology/hydrogeology, engineering geology, geologic/environmental consulting, geophysics, mine geology, oil and gas (OG), and geomatics. Aside from education, Austin enjoys spending time outdoors, riding his motorcycle, watching sports, listening to music, and spending time with family and friends. 

What data were used? 569 isolated teeth of both extinct and the extant (still living) tiger shark were examined for the basis of determining the geometric morphometrics, which is a method used to quantify shape of the teeth and disparity, which indicates differences between teeth of Galeocerdo. The ages of the teeth were also recorded. 

Methods: The 569 teeth samples (Figure 1) were each photographed with a labial aspect representation, which is the surface towards the lips. In addition, 18 published illustrations were included to compensate for more complete representation. The specimens were analyzed using a two-dimensional geometric morphometric system using landmarks. This means that the same 64 locations on each tooth were marked in a computer program (tpsDIG2). These landmarks were analyzed using a Generalized Procrustes analysis (GPA), which compared the differences in shapes across all of the teeth and summarized how different each of the shapes were. These data were analyzed using a Principal Components Analysis (PCA), which plots all of the variables’ differences into a 2D plane for easier analysis. Qualitative shape analysis by the researchers was also considered in the later portion of the study to describe features that weren’t included in the prior study, such as serrations in the teeth that are key identifiers for species within the genus Galeocerdo. 

Results: The conclusion of the procedure yielded three identified genera: Galeocerdo, Hemipristis, and Physogaleu and two unclear species, G. acutus and G. triqueter . The three genera and two species groupings were identified from the PCA, which showed the three distinct genera plotted on the graph. From the original 23 identified species of tiger shark, researchers here determined only 16 were legitimately considered because seven lacked illustrative characteristics. The distribution of these 16 species of Galeocerdo was made using the same processes used for the broader classification of all 569 shark teeth. The disparity through time for the tiger sharks falls into geologic time spans of Paleogene and Neogene-Quaternary. The PCA showed groupings of sharks by time spans, too. Paleogene sharks (G. clarkensis, G. eaglesomei, and G. latidens) occupy a distinct area of the principal components chart, indicating they are similar in shape. Tiger sharks within the Neogene-Quaternary are notably different in shape. 

The descriptions for all Galeocerdo species teeth are in lingual view (side of the tongue) with the distal side on the right and medial side on the left (away from the center of the mouth and toward it, respectively). Galeocerdo aduncus (A) is tan in color, smooth and is the smallest specimen of the six, coming close in size only to Galeocerdo clarkensis holotype (C). A contains rounded root lobes, strong serrations along distal side, strong notched distal edge, and very fine serrations along one side. Galeocerdo capellini (B) has a darker tan combined with some orangish and red tone, considerably rough texture, and is the largest of the six samples in size. Specimen B has the most rounded root lobes, conjoined rounded serrations, weakly notched distal edge, and medium sized with rounded serrations along the mesial side. Galeocerdo clarkensis holotype (C) is the roughest textured tooth of all six species, relatively small compared to the others, and has a combination of colors in green, grey, and brown. The morphology of C is the most abnormal compared to that of the remaining five shark tooth samples. The specimen has a poor notch at the root, rounded root lobes, a small number of wide serrations, strong distal edge, and curved side with poor serrations. Galeocerdo cuvier (D) is most noticeable by its cleft. The boundary marks the change between dark color and extremely smooth textured surface to a light, rough region of the root lobe. Galeocerdo cuvier (D) is large in size compared to the holotype of Galeocerdo clarkensis (C). Sample D has a square-like root lobe, fairly notched distal edge and prominent serrations on both sides. Galeocerdo eaglesomei (E) holotype is the easiest to recognize shark tooth of the six specimens. E is black in color, smooth texture, and medium sized. The resemblance of Galeocerdo eaglesomei (E) is close to that of the general shark tooth one might think of. It has three strong points in a triangle form with the two-edged root lobes and fine point in the apical region, no distal notch, and contains well-formed serrations along both the mesial and distal sides. Galeocerdo mayumbensis (F) is medium in size, contains some texture, and is mainly tan with some darker areas near the root lobe. Sample F is highly convex and has square root lobes, very weak distal notch, and rolled serrations along both sides.
Different morphology (like serrations, the sharp projections), color, texture, and size of the six significant tiger shark species teeth samples. Scale bars= 10 mm.
A: Galeocerdo aduncus. B: Galeocerdo capellini. C: Galeocerdo clarkensis holotype. D: Galeocerdo cuvier. E: Galeocerdo eaglesomei holotype.F: Galeocerdo mayumbensis

Why is this study important? Evolution, diversity, and disparity of the tiger shark lineage Galeocerdo in deep time serves an important part within the paleontology of Galeocerdo as a whole. The simple acknowledgement is beneficial to the genus. The authors state that this shark has been neglected compared to the other apex shark genera. The article is important to both the diversity and disparity between those diverse species of Galeocerdo throughout the Cenozoic. In doing so, the paleobiology of Galeocerdo can help with knowing phenotypes (the physical expressions of the genetic makeup) of the extant tiger shark today as well as its trend in the future.

The big picture: Apex sharks such as Galeocerdo serve an important purpose in the Earth’s oceans as they maintain the population of other prey. This results in an ecosystem balance for the plenty of other organisms that they feed off. Evidence shows that oceanic sharks and rays have been in decline globally since 1970 meaning a deteriorating diversity of higher order ocean species. Consideration of scientific studies on shark evolution is a way we as humans can protect the future of shark ecology.  

Citation: Türtscher, J., F. A. López-Romero, P. L. Jambura, R. Kindlimann, D. J. Ward, and J. Kriwet. 2021. Evolution, diversity, and disparity of the tiger shark lineage Galeocerdo in deep time. Paleobiology, 47:574–590.