People: Astrobiology Graduate Students

Loren Ballanti
Biology

One of the fundamental aims of Astrobiology is to understand the long-term evolution of a biosphere, using Earth as a model for any world on which life may develop. However, our picture of terrestrial life's history is clouded by a lack of primary evidence that spans billions of years, its early ecology largely beyond reconstruction. Only since the evolution of metazoans large enough to be seen, and having developed the ability to secrete mineralized skeletons, has there been a fossil record of sufficient quality to illustrate how the biosphere changes over millions of years.

The most striking feature of this history preserved in stone is the periodic occurrence of mass extinctions. While species inherently have a finite shelf life and a background level of extinction occurs constantly, mass extinctions are catastrophic events that represent severe and prolonged disturbance on a global scale. I share an interest with my advisor Peter Ward in the most ruinous of these events, the Permian-Triassic cataclysm, a two part disaster that culminated about 252.5 million years ago with the demise of over 90% of marine species and similar estimated losses on land.

While all life clearly suffered at this time, the patterns of survival and recovery reveal a selectivity that is not easily explained. Articulate brachiopods, the world's most common macrofossils in Paleozoic rocks, dominated benthic marine suspension-feeding communities for 300 million years. However, they were decimated at the end of the Permian and never regained their former prominence. Their mollusc counterparts, bivalves, ultimately proved to be far more successful at surviving this event, and the early Triassic marks the first time in their history that clams and their ilk became more numerous and more diverse, a mantle they have not since relinquished. Over 30,000 described bivalve species live today in all aquatic environments, while less than 300 brachiopod species remain.

My research seeks to determine why bivalves were able to persist where articulates could not by testing their susceptibility to conditions relevant to the P/T extinction (and others), as inferred from paleoenvironmental observations. The most important are hypoxia/anoxia, hypercapnia, and hydrogen sulfide, in concert with global warming. Operating under the principle that the present is the key to the past, I hope to demonstrate that bivalves are significantly and systematically more resistant to these conditions than are brachiopods by using a taxonomically diverse assemblage of their living descendants as surrogates. I have many other interests as well.