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.

Nodules of ancient soil are fairly common in present day rock, forming as a result of seasonally dry conditions. They harden into mineralized clods, making them easy to spot and sample as they weather out of ancient soil profiles. (Credit: Myers)

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.”

Data from Morrison Formation, Central Africa and Portugal

Myers 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.

CO₂ in ancient calcite nodules offers key to ancient climate

From the analysis scientists can draw a more complete picture of the ancient landscape and climate in which prehistoric animals lived.

“The Jurassic is thought of as very warm, very wet, with lots of dinosaurs,” said Myers, research curator for SMU’s Shuler Museum of Paleontology. “But we see from our analysis that there was regional variability during the Late Jurassic in the climate and in the abundance of animals across the planet.”

The Late Jurassic extended from 160 million years ago to 145 million years ago.