Clam Fossils Divulge Secrets of Ecologic Stability

In May 2013, scientists at Cornell and Cincinnati universities announced that 380-million-year-old clam fossils show evidence of how the natural ecosystem can stay surprisingly stable even during periods of severe environmental change. The research was published in the journal PLOS One.

The geological period in question is called the Devonian period. At the time, much of the eastern seaboard of the United States – where the clam fossils for this study were collected – lay under the Devonian Sea. According to the University of California Museum of Paleontology, life during this period consisted mainly of small plants like ferns, land animals called tetrapods (the first land-based vertebrates) and sea animals called brachiopods, in addition to a growing number of fish.

One of the surprising features of the Devonian period is that species commonly persist unchanged in the fossil record for an unusually long amount of time – millions of years – rather than undergoing evolutionary change. This is so despite the fact that shifts in the abundance of fossils over time show the overall plant and animal populations rose and fell significantly over time.

A team of researchers at the Paleontogical Research Institution of Cornell University and at the University of Cincinnati set out to explain this puzzle. Instead of simply counting the number of fossilized clams recovered at different times, they decided to examine the shells more closely, looking for evidence of tooth marks left by predators. “To understand how these species fared in the Devonian,” explains one of the study’s co-authors, Gregory Dietl, “you have to look at how they interacted with other species. There is more to ecology than just the abundance and distribution of species.”

Surprisingly, the level of predation of clams seemed to remain steady over the period, as did the clam’s body shape – even though the overall number of clams fluctuated over time. That suggests that even though animal population levels rose and fell significantly, the ecosystem as a whole remained quite stable, with the same types of animals persisting, just in different numbers. This level of ecologic stability was not expected.

Lead author Judith Nagel-Myers said her team still isn’t quite sure how to explain this, but does say it calls into question the ways in which paleobiologists have normally interpreted the fossil record. “Our results,” she says, “raise serious doubt as to whether ecological stability can be tested meaningfully, based solely upon the abundance of species, which has been the standard metric used to test for ecological stability in paleoecology.” Biologists will need to go back to the drawing board to devise new ways of studying population dynamics in this period of the history of life on Earth.

The article was co-written by Judith Nagel-Myers, Gregory P. Dietl, John C. Handley, and Carlton E. Brett. It is called “Abundance is Not Enough: The Need for Multiple Lines of Evidence in Testing for Ecological Stability in the Fossil Record,” and can be read in full, free of charge, at the PLOS One website. Cornell University has also published a readable summary for non-experts.