Cosmopolitodus, originally coined by Glikman(1964), is used to describe Miocene and early Pliocene members of the great white lineage. The Cosmopolitodus dentition design was quite similar to that of the modern Great White.
The narrow-form Miocene teeth of C. hastalis had a circumglobal distribution. The broad-form (a.k.a. C. xiphodon) appears to pop-up in the Western Atlantic and the serrate species, C. escheri (with its narrow lower teeth), in the Eastern Atlantic. Whether the Pacific species (represented by the Sacaco dentition), is a variation of a currently described species (serrate teeth have been previously ascribed to hastalis) requires in-depth research.
History of the GenusEdit
Paleoichthyologist Henri Cappetta, one of the most distinguished researchers on fossil sharks, noticed that fossil teeth of 'Isurus' hastalis are very similar to those of the modern White Shark. In fact, Cappetta has remarked that the two are so similar that fossil Carcharodon carcharias teeth in which the serrations have been abraded away by geological activity are virtually impossible to differentiate from specimens of hastalis. In 1995, paleoichthyologist Mikael Siverson began to question the assignment of hastalis to the genus Isurus. Based on striking similarities between the root shape and overall structure of the tooth blade, Siverson now believes that hastalis and escheri are not makos at all, but direct ancestors of the modern White Shark. Siverson has therefore suggested that they should be re-assigned to the genus Cosmopolitodus. This view has also been adopted by paleontologist David Ward and seems to be gaining acceptance in at least some paleontological and fossil collecting circles.
The assumption that saw-toothed Carcharodon evolved from smooth-toothed Isurus is based on the idea that the appearance of serrations coincides with the origin of the genus Carcharodon. But it's relatively easy to serrate a tooth, as shown by many clearly separate shark lineages which have independently evolved serrated teeth. A newer interpretation of the lamnoid fossil record holds that the Carcharodon lineage was originally smooth-toothed and is actually older than that of Isurus. According to this scenario, the Carcharodon lineage can be traced back to the smooth-toothed Isurolamna inflata, which lived about 65 to 55 million years ago. I. inflata gave rise to Macrorhizodus praecursor, which lived about 55 million years ago and had smooth edged but broader teeth than its ancestor. Praecursor gave rise to Cosmopolitodus hastalis, which lived about 35 million years ago and developed even braoder teeth. Hastalis, in turn, gave rise to Cosmopolitodus escheri, which lived about 25 to 20 million years ago and had weak serrations on its teeth. And finally, escheri gave rise to the modern White Shark, Carcharodon carcharias, which appeared some 11 million years ago and had the coarsely serrated teeth for which the genus is renowned today. Therefore, Carcharodon and Isurus both descended from Isurolamna inflata and many smooth-edged fossil teeth originally named Isurus are in fact part of the Carcharodon lineage.