In modern ecosystems, it's known that animals flourish in regions where the climate and landscape produce lush vegetation.
A new study set out to discover whether that same relationship held true 150 million years ago during the Late Jurassic when dinosaurs roamed the Earth.
"The assumption has been that ancient ecosystems worked just like our modern ecosystems," said paleontologist and lead author Timothy S. Myers, Southern Methodist University, Dallas. "We wanted to see if this was, in fact, the case."
To test the theory, Myers analyzed fossil soils from the Late Jurassic by measuring the ratios of carbon isotopes. His analysis indicated that the Jurassic soils contained high levels of CO2 from vegetation.
From that, Myers was able to infer the presence of lush plant life in certain regions during the Jurassic. The soils came from locales where scientists previously have gathered animal fossils — North America, Europe and Africa. Combining the data with the known fossil sampling allowed Myers to confirm that the modern relationship between animals and vegetation held true even millions of years ago.
"Our analysis represents the first time that anyone has tried to apply ecological modeling to this relationship in the fossil record," Myers said.
Typically researchers count the number of animal species discovered in a region to determine how many different types of animals once lived there. Scientists call that a measure of faunal richness.
Myers took a different approach. Using a traditional method typically used to estimate carbon dioxide in the ancient atmosphere, Myers instead applied it to estimate the amount of CO2 in ancient soils.
Measurements were taken from nodules of calcite that form in soil as a result of wet and dry seasons. These nodules take on the isotopic signature of the CO2 gas around them, which is a mixture derived from two sources: the atmosphere, which leaves a more positive isotopic signature, and plants decaying in the soil, which leave a more negative isotopic signature.
A higher volume of CO2 from plants indicates a lusher, wetter environment.
"There's a lot more litter fall in an environment with a lot of plants, and that produces a lot of organic material in the soil, creating CO2. So we see more soil-produced CO2, displacing the atmospheric CO2. These are established relationships," Myers said.
"Our method can be used to infer relative levels of richness for areas where soils have been preserved, but where fossils are lacking because conditions were unsuitable for their preservation," he said.
The research demonstrates creative use of existing geological data, said co-author Tabor, an expert in ancient soil.
"Vertebrate paleontologists have been accumulating information about vertebrate fossils in the Jurassic for well over 100 years. In addition, geochemists have been systematically sampling the composition of ancient soils for several decades," Tabor said. "In these respects, the data that are the foundation of this study are not extraordinary. What is remarkable, though, is combining the paleontology and geochemistry data to answer large-scale questions that extend beyond the data points — specifically, to answer questions about ancient ecosystems."
yers tested Upper Jurassic soil nodules collected from the Morrison Formation in the western United States. The formation extends from Montana to New Mexico and has been the source of many dinosaur fossil discoveries.
He also analyzed Upper Jurassic soil nodules from Portugal, another location well-sampled for dinosaur fossils. The region's paleoclimate was broadly similar to that of the Morrison Formation.
In addition, Myers tested a small Upper Jurassic core sample from Central Africa, where there's no evidence of any major terrestrial life. Unique minerals in the rocks indicate that the region had an arid environment during the Late Jurassic.
Based on their hypothesis, the researchers expected to see regional variations in plant productivity — the amount of new growth produced in an area over time, which is an indirect measure of the amount of plant life in an environment. Forests, savannas and deserts all have different amounts of plant productivity, although those specific ecosystems can't be identified on the basis of plant productivity alone.
The researchers expected to see higher plant productivity for Portugal than for the Morrison Formation, with the lowest productivity in Central Africa.
"Essentially that's what we found," Myers said. "We understand it's tenuous and not a trend, but few places in the world are well-sampled. However, it's still a useful tool for places where all we have are the soil nodules, without well-preserved fauna."
Soil nodules are fairly common, Myers said. They form as a result of seasonally dry conditions and may be preserved in all but the wettest environments. Since they harden into mineralized clods, they are easy to spot and sample as they weather out of ancient soil profiles.