Spread along the southwestern coast of Peru, the 9.9 to 8.9 million year old rock of the Pisco Formation has yielded some stunning fossils. Paleontologists working there have found the bones of enormous predatory whales, delicately-preserved shark jaws, and sea turtles, just to name a few highlights. But even those finds pale in comparison to a real rarity that was announced just last month: prehistoric whale vomit.

The unusual fossil, described by paleontologist Olivier Lambert and colleagues, is encased in a chunk of exceptionally-hard dolomite. The stone is so resilient, in fact, that preparing the rock away from the bones with tools and acid proved impossible. Nevertheless, the lower jaws of the early beaked whale Messapicetus gregarius can clearly be seen jutting from the rock, and surrounding those jaws are dozens of ancient sardines.

No one has found a fossil like this before. Fossil whale gut contents are extremely rare, and the sardines scattered across the fossil block had previously only been known from scales and other tattered remnants picked out of the Pisco Formation. And while it’s true that association doesn’t always equate with interaction when it comes to fossils, Lambert and coauthors make a solid case that the fossil wasn’t an accidental burial of a whale that came to rest on a bed of fish. A search in the same narrow layer around the whale failed to turn up any more fish, and, if they were anything like their modern counterparts, the ancient sardines were filter feeders that wouldn’t have been scavenging on the whale carcass.

View Images The remains of Messapicetus and prehistoric sardines. From Lambert et al., 2015.

The strongest scenario, Lambert and colleagues argue, is that this Messapicetus gorged itself on sardines just hours before its death. The fish fossils are preserved along the whale’s chest, throat, and mouth, showing little to no sign of digestion. Not that all of them got buried as gut contents. A large number of the 40-60 fish are scattered around the whale’s mouth. The cetacean heaved them up in death. This might be a clue as to what happened to the unfortunate Messapicetus.

The fact that Messapicetus ate filter-feeding fish doesn’t only end up being a useful indicator for the timing of whale evolution – Messapicetus patrolled coastal surface waters and was not a deep diver like its modern beaked whale relatives – but it also provides a pathway by which the cetacean could have been poisoned. There were toxic algal blooms during prehistory just as there are today, and the sardines could have fed on crustaceans that had in turn eaten the algae, eventually passing the toxins up through the food web to the Messapicetus.

Unfortunately, though, no sign of toxic algae has shown up in the same rock layer as the whale and the fish. A suspect fitting the profile has yet to appear. But the idea itself gives paleontologists something else to look for. Multiple other Messapicetus have been found nearby, not to mention the various other marine creatures, and toxic algal blooms have been blamed for other aggregations of prehistoric whales. The fatally-queasy whale could be the initial sign of ancient killers almost too small to see.

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