Carcharodon versus Carcharocles:

What's in a Name?

The giant-toothed shark, Megalodon, is among the most celebrated and contentious of fossil fishes. How Megalodon is related to the extant White Shark (Carcharodon carcharias) and - by extension - which is the correct genus to use for the extinct species are both hotly contested. There are two basic schools of thought on these issues.

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One school insists that both Megalodon and the White Shark are derived from Cretolamna via Carcharodon orientalis, and are thus both members of the same genus (Carcharodon) and family (Lamnidae).

Proponents of this school include paleontologists Shelton Applegate, John Maisey, Robert Purdy and shark systematist Leonard Compagno.

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The opposing school insists just as strongly that Megalodon is derived from Cretolamna via Otodus, and that Carcharodon derived from a separate lineage. Further, 'Carcharodon' orientalis belongs in its own genus, Paleocarcharodon, and was an evolutionary dead end that did not give rise to any extant shark. Thus megalodon belongs in a separate genus (Carcharocles) and family (Otodontidae) from the modern white shark:

Proponents of this school include paleontologists Henri Cappetta, John Long, Mikael Siverson, and David Ward.

This table provides a brief point-counterpoint synopsis of some of the arguments on both sides of the Carcharodon-Carcharocles debate.

Megalodon is a Carcharodon

Megalodon is a Carcharocles

As C. carcharias teeth grow, they become increasingly similar to those of Megalodon in morphology, with increasingly finer and more numerous serrations and more robust proportions. A Megalodon-type tooth would result from extrapolating these ontogenetic changes seen in C. carcharias.

The serrae of C. carcharias teeth do NOT change frequency or size with age and the robustness of its teeth does not at all approach that of Megalodon. Serrae of C. carcharias are larger and highly irregular, those of Megalodon are smaller and much more regular.

Teeth of presumed subadult Megalodon are very similar in form to those of C. carcharias, except for defining characters such as more massive, deeper roots and smaller, more numerous serrations.

Similarities between the teeth of Megalodon and C. carcharias are convergent. If one ignores the root shape and serration size, all that is left is a large triangular blade. Teeth of C. carcharias have a small, narrow scar at the base of the enameloid (where the root meets the blade) on the lingual (inner) surface; those of megalodon have instead a large chevron-shaped scar (the bourlette).

Bendix-Almgreen (1983) observed that the fine structure of the teeth of Megalodon and C. carcharias are similar and indicate a close relationship. Unfortunately, Bendix-Almgreen did not examine the teeth of other lamnids, but his observations support placing megalodon in the genus Carcharodon.

Bendix-Almgreen's histology proves nothing. Of course the teeth of Megalodon and C. carcharias are similar - so are those of most lamnoids. The teeth of C. carcharias have cloacal pores (where nerve and blood vessels enter the root) that are large and grouped, those of Megalodon are small and scattered.

Several natural Megalodon tooth sets (including Uyeno 1989) have relatively large intermediate teeth showing a pattern of tooth reversal (in which the cusp points toward the center of the jaw rather than the jaw corner) similar to that seen in Carcharodon. Reversal of intermediate teeth is interpreted as a derived state for Carcharodon, as it does not occur in C. hastalis or modern Lamna.

The apparent reversal of the intermediate teeth in Megalodon and C. hastalis are artifacts of reconstruction. In the most complete natural Megalodon set (70+ teeth) David Ward has seen, tooth reversal is not apparent; Pliocene C. hastalis show the same tooth pattern - including reversed intermediates - as C. carcharias.

So what does all this mean? Although the Carcharodon cartel raises some compelling points, their evidence shows enough weaknesses to warrant caution in lumping Megalodon in the same genus as the modern White Shark. Extrapolation from one species to another is risky in practice, being fraught with inherent problems and perils. Several shark lineages have independently evolved teeth with a triangular, serrated blade, probably due to convergence borne of similar dietary and feeding habits. Interpreting fossil shark teeth is a highly subjective combination of training and experience; since no two researchers have exactly the same backgrounds, differences of opinion are bound to appear. And lastly, taphonomy (the science of puzzling out by what happened to a fossil or artifact after it was deposited - including how it was moved from its original resting place to where it was eventually found) can be quite convoluted and tricky. Fossil shark teeth that happen to be found together may not have come from the same individual, any more than leaves clogging a storm drain necessarily all came from the same tree.

Given the specimens and information available to me as I write this (and with all due respect to Drs. Applegate, Compagno, Maisey, and Purdy), I am inclined to side with the Carcharocles cartel, pending further evidence.

 

ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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