About four million years ago, in the shallows of an ocean that once covered what is now southern Peru, a large shark bit into the jaw of a baleen whale. The whale had been dead for some time, but it was kept afloat but the gases building up in its body as it decomposed. It was absolutely rotten, but it still presented a free meal to the scavenging shark. As the shark bit down, however, one of its teeth became stuck in the whale's jaw bone and broke off. No matter. The lost tooth would soon be replaced by another.
The above scenario does involve a bit of speculation, but such events certainly did happen. Sharks fed on whale carcasses in ancient times just like they do today, and when sharks feed they often lose teeth. In the latest edition of the journal PALAIOS scientists Dana Ehret, Bruce MacFadden, and Rodolfo Salas- Gismondi describe a 4-5 million year old piece of whale jaw with a shark tooth stuck in it, direct evidence of interaction between predator and prey.
The fossil that is the subject of the study was discovered in 2007 from the Pisco Formation in Peru. It is a site well-known for its excellent preservation of fossils, enough to put together a fairly clear picture of what the area was once like. Around 4-5 million years ago the site was a shallow, coastal environment inhabited by turtles, crocodiles, fish, sharks, whales, and even a giant aquatic sloth. Of the shark species present there some got to be quite large; the older part of the Pisco Formation contains the remains of Carcharocles megalodon and Isurus hastalis while the younger part contains the fossils of a species of Carcharodon (the genus to which the living great white shark belongs).
The tooth that was broken off in the whale jaw most closely resembles that of Carcharodon, and this is supported by comparing it to a complete set of jaws from the same locality just described in the Journal of Vertebrate Paleontology. The question is, however, whether the whale was killed by the shark or already dead when the Carcharodon came along. It is not an easy question to answer, but there are ways to rule out some of the possibilities.
If the shark had attacked but the whale survived it would be expected that the whale bone would show signs of healing. No such signs were found. This leaves two main possibilities: that the shark killed the whale or the shark scavenged the whale carcass. The latter hypothesis seems more likely. Observations of sharks, even large ones, attacking living baleen whales are extremely rare, and when they do attack they often target the belly or sides of the whale. More often sharks are seen feeding on the decomposing carcasses of whales which often float on the surface of the ocean for extended periods of time. During these feeding events the postcranial part of the whale is often preferentially stripped of blubber and meat, but on at least one occasion juvenile great white sharks were seen to feed around the head region of a dead baleen whale. Perhaps the fossil from Peru is a remnant of a similar behavior.
Unfortunately the paper does not have much of a satisfying conclusion. The fossil probably represents a scavenging event but it is not possible to know for sure (unless, of course, you have a time machine, some scuba gear, and about a million years to kill). Perhaps the specimen is not as exciting as some other recently announced fossils but it is interesting for another reason. It is a specimen that illustrates an interaction between two organisms, a little piece of paleobiology frozen in time.
DANA J. EHRET, BRUCE J. MACFADDEN, and RODOLFO SALAS-GISMONDI (2009). CAUGHT IN THE ACT: TROPHIC INTERACTIONS BETWEEN A 4-MILLION-YEAR-OLD WHITE
SHARK (CARCHARODON) AND MYSTICETE WHALE FROM PERU PALAIOS, 24, 329-333 DOI: 10.2110/palo.2008.p08-077r
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It's a trace fossil. It's "a fossil of a living animal, rather than of a dead one".
I submitted a paper just like this to JVP recently but alas, it was rejected, though more conclusive than this one, I think.
I must admit I was expecting a little more from this paper, given the truly spectacular fossils from the Pisco Fm. There are certainly more spectacular cetacean fossils known from the fossil record - see Cigala-Fulgosi (1990). This specimen shows bite marks on nearly 1/2 of all the bones in the skeleton (and it is a nearly complete skeleton of Tursiops or Hemisyntrachelus - can't remember which, but I suppose the latter). Additionally, there's a paper on a fossil (or modern???) whale-fall from Japan that was discovered with over 550 shark teeth in close association (can't remember the ref, though).
Cigala-Fulgosi, F. 1990. Predation (or possible scavenging) by a great white shark on an extinct species of bottlenosed dolphin in the Italian Pliocene:. Tertiary Research 12:17â36.