How did the organisms in the shallow Devonian seas change in response to environmental changes?

This question has occupied paleontologists for over a century. The Devonian was a time of high global sea level, and low parts of continental surfaces became flooded with sea water. As sediments settled to sea bottoms, the bottoms subsided in proportion to the weight of the sediments, allowing hundreds of meters of sediment to accumulate in water bodies less than a hundred meters deep. As sea level, climate, and sediment type oscillated over periods of tens of thousands of years, the sea bottoms changed, and with them the nature of the organisms thriving in, upon, and above the sea sediments.

These resulting thick sequences of layers of sedimentary rock, with their rich fossil records, have become natural laboratories for paleontologists. What happens to organisms when environments change? Do species persist, migrate, go extinct, or evolve? Do different kinds of organisms respond differently? Do species respond as a group, or does each species seem to react independently? Answering these questions is interesting and potentially important, because observing how faunas changed in the past may help us understand how faunas are likely change in the future. But even if Devonian faunas respond differently from modern faunas, they help us understand the dynamics of faunas more generally. We cannot do experiments on entire oceans, of course, but we can look into the past to see what has actually happened to understand how these dynamics work.

The research focused upon in Fossil Finders involves layers of sediment from the middle part of the Devonian Period, from central New York State. The layers, known collectively as the “Hamilton Group,” are composed of fine sediment eroded from mountains to what is now the east of New York State. Superficially, the rocks from this interval look rather homogenous – all are made of some quantity of clay, silt, and sometimes find sand, and are colored some shade of gray from organic matter trapped in the rock. A closer look reveals that the grain size and quantity of organic matter vary among the layers. Research by people such as Carleton Brett (University of Cincinnati) and Gordon Baird (SUNY Fredonia) have suggested that these variations in the middle Devonian are cyclic, with sedimentary rocks showing evidence of shallowing and then deepening again over a dozen times over the course of about 5 million years.

Our Fossil Finders study with teachers and students complements previous research by collecting detailed reconnaissance data on stratigraphic sections that have not been studied in detail. We also focus on data not typically collected that may provide new insight into biotic responses, specimen size in particular. Our study uses higher level taxonomic data, such as brachiopods, bivalves, and trilobites, rather than individual species. Though this practice lumps together varied ecological preferences of individual species, it also summarizes the response of evolutionarily related groups that broadly overlap in their ecological adaptations. Interesting anomalies in this “higher level” data can be (and have been) followed up with more detailed counts of morphogroups (species of a particular shape) or species.

We are using our data to answer questions such as to what degree are changes in Hamilton Group faunas influenced by food availability. Changes in sea level and environment are likely to influence distance from shore, nutrients at the surface for phytoplankton, and bottom energy, all of which impact how much organic matter is floating in the water or mixed in the sediment at the bottom. Other variables influencing specimen abundance and size that can be approximated by observing rocks include dissolved oxygen, softness of the surface, and amount of sediment suspended in the water. Our measurement of specimens, organic matter in the rock, and fragmentation of shells thus potentially inform us about changing nutrient conditions and food webs.

Why haven’t such data been previously collected by other researchers? Carl and Gordon and many of their colleagues have spent decades observing the Hamilton Group the field, walking up streams and along road cuts, observing how rock layers and their fossils vary from place to place, to piece together the history of the sea in what is known as the Appalachian Basin. Dozens of their graduate students have collected the detailed data to flesh out the details of ecology and evolution of the ancient organisms found in the rocks. But layers of Hamilton Group rocks are at the surface for thousands of square miles in New York State alone, and each locality contains meters of rock and countless thousands of fossils. It is time consuming to measure and document individual specimens, and numerous scientific questions remain unanswered for lack of data. Therefore involving teams of students makes available data that most researchers could not otherwise have time to collect.