巨齿鲨：巨齿鲨广泛的食性

At least that's what it did should a whale come long, says Dr. Jeremy McCormack from the Department of Geosciences at Goethe University Frankfurt. It appears, after all, that megalodon partook of a much broader range of prey than previously assumed, as the geoscientist discovered together with scientists from Germany, France, Austria and the US. The researchers examined fossilized megalodon teeth, which are more or less all that has remained of the cartilaginous fish that gave the shark its name, megalodon, meaning "big tooth."

The researchers extracted zinc from the fossil teeth, an element that occurs in atomic variants (isotopes) of different weights. Zinc is ingested with food, whereby less of the heavier isotope zinc-66 than the lighter isotope zinc-64 is stored in muscles and organs. Accordingly, the tissue of fish that eat fish absorbs significantly less zinc-66, and those which, in turn, hunt them for food absorb even less. That is why Otodus megalodon and its close relative Otodus chubutensis had the lowest ratio of zinc-66 to zinc-64 at the top of the food chain.

"Since we don't know how the ratio of the two zinc isotopes at the bottom of the food pyramid was at that time, we compared the teeth of various prehistoric and extant shark species with each other and with other animal species. This enabled us to gain an impression of predator-prey relationships 18 million years ago," explains McCormack. The giant teeth they used for their study mostly came from fossil deposits in Sigmaringen and Passau -- 18 million years ago, a relatively shallow estuary, less than 200 meters deep, flowed along the Alps, teeming with various other shark species alongside megalodon.

McCormack explains: "Sea bream, which fed on mussels, snails and crustaceans, formed the lowest level of the food chain we studied. Smaller shark species such as requiem sharks and ancestors of today's cetaceans, dolphins and whales, were next. Larger sharks such as sand tiger sharks were further up the food pyramid, and at the top were giant sharks like Araloselachus cuspidatus and the Otodus sharks, which include megalodon." McCormack stresses, however, that the Otodus sharks cannot be sharply differentiated from the lower levels of the pyramid: "Megalodon was by all means flexible enough to feed on marine mammals and large fish, from the top of the food pyramid as well as lower levels -- depending on availability."

According to McCormack, this means that the idea of Otodus sharks homing in on marine mammals when it comes to food needs to be revised: "Our study tends rather to draw a picture of megalodon as an ecologically versatile generalist." Comparisons between the fossils from Sigmaringen and Passau, for example, showed that the creatures from Passau fed more on prey from lower levels of the food pyramid, which also points to regional differences in the range of prey or changes in its availability at different times.

Analyzing teeth on the basis of zinc content is a very new method, and McCormack is delighted with the comprehensive and coherent results it produced not only for prehistoric shark and whale species but also for herbivorous prehistoric rhinoceroses and even shark species that exist today. McCormack: "Determining tooth zinc isotope ratios has once again proven to be a valuable instrument for paleoecological reconstructions." "It gives us important insights into how the marine communities have changed over geologic time, but more importantly the fact that even 'supercarnivores' are not immune to extinction," adds Kenshu Shimada, a paleobiologist at DePaul University in Chicago, USA, and a coauthor of the new study. Previous studies, including one led by McCormack, indicated that, at least in part, the rise of the modern great white shark is to blame for the demise of Otodus megalodon.