No matter how you define the term, fish make up the overwhelming majority of vertebrate diversity (as has been the case as long as there have been vertebrates), but they have been woefully underrepresented as Fossil Vertebrates of the Month. To begin remedying that imbalance, I am bringing the feature back from its hiatus by featuring the fish to end all fish. There have been innumerable strange sharks through time, but none as spectacular as Megalodon, the "mega-toothed" shark. As I always caution my students, referring to a species solely by its specific epithet without appending it to a genus is a cardinal sin in biology, but I am doing so here for a reason. Despite being unquestionably the most famous of all fossil fish, Megalodon is a surprisingly enigmatic animal. The great paleoichthyologist Louis Agassiz interpreted it as a member of the same genus as modern great white sharks, Carcharodon, naming the species Carcharodon megalodon. However, other studies have since suggested that it was in fact a member of the extinct family Otodontidae, possibly making it a closer relative of mako sharks and a member of the genus Carcharocles. The debate continues to this day. One of the major reasons for this confusion is the fact that, like all chondrichthyans, C. megalodon had a cartilaginous rather than a bony skeleton, and as such its fossil record is composed predominately of teeth. These teeth are, however, fairly abundant and among the more spectacular vertebrate fossils out there. They have been found worldwide in rocks dating to the Late Miocene and Pliocene, but they are perhaps best represented in the Southeast US. One of the first questions asked when the earliest C. megalodon teeth were identified was just how big the full animal would have been. Reconstructing body size based solely on isolated teeth can be difficult to begin with and this is even more true when uncertainty exists as to exactly what type of shark C. megalodon was. The most conservative estimates place the largest specimens at roughly 12 meters (40 feet) in length, though some researchers have suggested that much larger individuals existed. The logical assumption is that a marine predator of such enormous size would prey on other large animals, particularly cetaceans and this hypothesis is borne out by what appears to be C. megalodon tooth marks on whale bones. As illustrated by a handful of presentations at last month's North American Paleontology Convention, material being uncovered in Panama (Which may represent a C.megalodonnursery!) is helping shed light on other aspects of the species' biology, such as body size trends through time and the factors that led to the extinction of the largest shark ever to have lived.