Fish was fossil frog’s last meal

The skeleton of Palaeobatrachus from Lake Enspel, Germany. From Wuttke and Poschmann, 2010.

In On the Origin of Species, Charles Darwin said of the fossil record:

For my part, following out Lyell’s metaphor, I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of the slowly-changing language, in which the history is supposed to be written, being more or less different in the interrupted succession of chapters, may represent the apparently abruptly changed forms of life, entombed in our consecutive, but widely separated formations.

The relatively few pages which naturalists had to work with by the time of Darwin’s 1859 writing had been cobbled together from numerous discoveries in many places, and even today paleontologists must often sift through scraps as they update and revise the story of life on earth. Nevertheless, there are a few places in which a greater number of fossil “words” and “sentences” have been kept safe – sites of exceptional preservation which record tales of ancient life in exquisite detail. One such place is Germany’s Lake Enspel, and in the 24 million year old vestiges of the ancient lake paleontologists have found clues to an ancient interaction between predator and prey.

As described by paleontologists Michael Wuttke and Markus Poschmann in Palaeobiodiversity and Palaeoenvironments, 24 million years ago the Lake Enspel was a body of water which had formed inside the caldera of a small volcano. This created a sort of natural trap which preserved organisms which died in the lake or slid through the forests anchored on the crater walls. Among the fossils found in the sediments from the lake bottom are skeletons of the frog Palaeobatrachus, an aquatic frog closely related to living African clawed frogs. These frogs are very rare, but one particular specimen contained a bonus.

a Close-up of the pelvic area of the frog which preserved the fish bones. b A crescent-shaped bone from the head of the fish. c Fish vertebra and bits of the fins. From Wuttke and Poschmann, 2010.

Preserved between the hips of one frog were a small accumulation of bones which, upon close inspection, turned out to belong to a fish. There were only a few bits of fin and backbone preserved in a “brown mass” representing former soft parts of the frog, but their position and preservation led Wuttke and Poschmann to suggest that the bones were from one of the frog’s last meals. Since its extant relatives catch and consume fish when they can, there was little reason to think that Palaeobatrachus wasn’t doing the same.

The discovery of more skeleton of frogs with fish bones preserved inside would certainly bolster this hypothesis, but the likelihood of finding such specimens is slim. The parts of lake bottom which became preserved were far from the margins of the lake where the frogs would have captured their prey, and the tangle of water plants along the edge of the ancient lake may have even acted as a filter which prevented dead frogs from drifting out further to where they could be preserved. Even so, the exquisite level of preservation at Lake Enspel records the details of a long-lost ecosystem, and finds such as the one discussed here are helping scientists flesh out the relationships of the animals which once lived within it.

Wuttke, M., & Poschmann, M. (2010). First finding of fish in the diet of a water-dwelling extinct frog Palaeobatrachus from the Upper Oligocene Fossil-Lagerstätte Enspel (Westerwald Mountains, Western Germany) Palaeobiodiversity and Palaeoenvironments, 90 (1), 59-64 DOI: 10.1007/s12549-009-0019-z