by Becca Shelton, RJD Intern
There are few things I love more than sharks and a good debate. The white shark, or great white, (Carcharodon carcharias) is my favorite species of extant sharks and the megalodon shark (Carcharodon megalodon) is my favorite extinct species. It just so happens that both species are in the center of an interesting dispute. Who is the ancestor to the white shark? For a long time, I personally had no doubt it was the megalodon shark because of similar looking teeth and jaws. In reality, this is not an easy question to answer. One of the reasons is that the species of sharks that are theorized to be the “closest” ancestor are extinct. Since sharks possess a cartilaginous skeleton, there is almost never a fully preserved skeleton since cartilage does not preserve well, unlike animals with boney skeletons. However, shark teeth are covered in enamel which helps in preservation and fossilized shark teeth can be found all over the world. Most of the debates surrounding this white shark ancestry involve teeth, especially morphology and serration. The two major theories I will be discussing are the megalodon hypothesis (Carcharodon megalodon) and the hastalis hypothesis (Isurus hastalis).
Before I get into the hypotheses, I think it is important to start at the beginning. All three species of shark belong to the Family Lamnidae, making them related. They split off at the Genus level with C. megalodon and C. carcharias in the Genus Carcharodon and I. hastalis in the Genus Isurus. Megalodons are considered to be the largest predatory fish to ever have existed, growing up to 67 feet long and existed until 1.5-2 million years ago (Gottfried et al., 1996). Information on I. hastalis is very limited, with most of the focus on their fossilized teeth in relation to C. carcharias. So which shark gave rise to the largest carnivorous fish alive today?
The megalodon hypothesis is the older theory that proposes that white sharks (Carcharodon carcharias) share a more common ancestor with extinct megalodons (C. megalodon) than with the extinct “broad-toothed” mako (Isurus hastalis) (Nyberg et al., 2006). This theory was proposed in 1835 by Louis Agassiz and has been supported by numerous scientists including Purdy et al. (2001), Gottfried et al. (1996) and Applegate and Espinosa-Arubarrena (1996). Figure 1-A, is a phylogenic tree representing the megalodon hypothesis.
On the opposite side, the more “recent” hypothesis is the hastalis hypothesis, which basically states that C. carcharias shares a more recent ancestor with I. hastalis than C. megalodon (Nyberg et al. 2006). This hypothesis is supported by Cappetta (1987), Muizon and De Vries (1985) and Casier (1960). Figure 1-B (above), is a phylogenic tree representing the hastalis hypothesis.
In the paper, tracing the ancestry of the great white shark, Carcharodon carcharias, using morphometric analyses of fossil teeth, Nyberg et al distinguish between the two hypotheses by various morphometric analyses. When comparing tooth shapes, they found no significant difference between C. carcharias and I. hastalis. The next test correlated tooth size with age with upper and lower anterior positions and discovered that the growth rates were once again more similar between C. carcharias and I. hastalis. For their final examination, they used scanning electron microscopy to compare serrations between the three species. They found that the tooth serrations on C. carcharias are more similar to I. hastalis and are distinct from C. megalodon. Nyberg et al conclude that “these results indicate that C. carcharias originated from an extinct group of mako sharks” (I. hastalis) “and not from the megatoothed sharks” (C. megalodon).
Does this mean that the hastalis hypothesis is the right answer to our original question; who is the ancestor to the white shark? It could be. If the answer to ancestry can be traced by tooth morphology similarities, then Nyberg et al. makes a solid case. But then again, there are a lot of scientists who believe in the megalodon hypothesis. This debate has been going on for well over a century and it will most likely continue for some time. I have briefly presented both hypotheses and one paper to get you, the reader, excited about learning more about this topic.
I grew up with the megalodon hypothesis and I still have trouble believing that white sharks are not “mini-me” versions of the megalodon. I also assumed that I. hastalis’ teeth were similar to the extant mako shark. This is not the case whatsoever, but I will let you be the judge. Figure 2 is a C. megalodon tooth compared to white shark teeth and figure 3 is an I. hastalis tooth.
Now you can probably see why this debate has gone on for so long. These teeth look relatively similar without the morphometric analyses. Without genetic material, these teeth and skeletal similarities may be our only clues to who really is the ancestor of the white shark.
Applegate, S. P., and L. Espinosa-Arrubarrena. 1996. The fossil history of Carcharodon and its possible ancestor, Cretolamnaa: study in tooth identification; pp. 19-36 in A. P.
Klimley and D. G. Ainley (eds.), Great White Sharks: The Biology of Carcharodon carcharias. Academic Press, San Diego, California.
Casier, E. 1960. Note sur la collection des poissons Paleocenes et Eocenes de L’Enclaver de Cabinda (Congo). Annales du Musee. Royal du Congo Belge (A.3) 1, 2: 1-48.
Cappetta, H. 1987. Handbook of Paleoichthyology.C hondrichthys II: Mesozoic and Cenozoic Elasmobranchii. Gustav Fischer Verlag, Stuttgart and New York.
Gottfried, Michael D., Leonard JV Compagno, and S. Curtis Bowman. “Size and skeletal anatomy of the giant “megatooth” shark Carcharodon megalodon.” Great White Sharks: the biology of Carcharodon carcharias (1996): 55-66.
Muizon, C., and T. J. DeVries. 1985. Geology and paleontology of late Cenozoic marine deposits in the Sacaco area (Peru). Geologische Rundschau 74:547-563.
Nyberg, Kevn G., Ciampaglio, Charles N., and Wray, Gregory A. Tracing the Ancestry of the Great White Shark , Carcharodon carcharias , Using Morphometric Analyses of Fossil Teeth Author ( s ): Kevin G . Nyberg , Charles N . Ciampaglio and Gregory A . Wray Reviewed work ( s ): Source : Journal of Vertebrate Paleontology , Vol . 26 , No . 4 ( Dec . 11 , 2006 ), pp . 806-814
Purdy, R. W., V. P. Schneider, S. P. Applegate, J. H. McLellan, R. L. Meyer, and B. H. Slaughter. 2001. The Neogene sharks, rays, and bony fishes from Lee Creek Mine, Aurora, North Carolina. Smithsonian Contributionsto Paleobiology 90:71-202.