- "If we can demonstrate the strong link between Carcharocles and Otodus from such skeletal remains, we may be able to settle the evolutionary and taxonomic debates."
- — Kenshu Shimada, referring to the Sacaco dentition.
There is a major disagreement among scientists as to how C. megalodon should be classified. The controversy is that whether C. megalodon is a close relative of the extant great white shark or whether the two species are distant relatives. The trend among shark researchers is to dismiss the statement that C. megalodon is a close relative of the great white shark, in favor of citing convergent evolution as the reason for the dental similarity.
One school of thought insists that both megalodon and the White Shark are derived from Cretalamna via 'Carcharodon' orientalis, and are thus both members of the same genus (Carcharodon) and family (Lamnidae). Proponents of this theory include paleontologists Shelton Applegate, John Maisey, Robert Purdy and shark systematist Leonard Compagno.
The opposing school of thought insists just as strongly that Megalodon is derived from Cretalamna via Otodus obliquus, 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:
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 serrations 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.
Although the supporters of Carcharodon raise 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.