CORRELATED FAUNAL RICHNESS AND PRIMARY PRODUCTIVITY IN LATE JURASSIC TERRESTRIAL ENVIRONMENTS

Geochemical analysis of pedogenic carbonate and plant-derived organic matter from paleosol-bearing, Upper Jurassic strata indicates linkage between faunal richness and primary productivity in terrestrial paleoenvironments. Samples were collected from the Morrison Formation of the Western U.S., the Lourinhã Formation of Portugal, and the Stanleyville Group in the Democratic Republic of Congo. Soil CO₂ concentration, used as a proxy for paleoproductivity, was estimated using δ¹³C values of pedogenic carbonate and plant-derived organic material. Inferred relative paleoproductivity levels were then compared against richness of the local dinosaur fauna.

The measured δ¹³C value of fossilized plant material was used to represent the value of soil organic matter when pedogenic carbonates formed. Each carbonate sample was matched with the δ¹³C value of the stratigraphically-nearest organic material. When local plant material was unavailable, the δ¹³C value of soil organic matter was estimated by assuming a standard offset from the δ¹³C value of contemporary shallow marine carbonates. Soil pCO₂ estimates indicate that primary productivity was highest in the Late Jurassic coastal environments of Portugal and lowest in the continental interior paleoenvironments of Central Africa.

Using dinosaur occurrences compiled from the literature, the extrapolated total generic richness is 37 for Portugal and 32 for the Western U.S. A richness of zero was used for the Stanleyville Group, which yields no dinosaur fossils. Late Jurassic Portuguese environments were characterized by the highest primary productivity and the highest generic richness of dinosaurs. In contrast, Central Africa yields both the lowest relative paleoproductivity and faunal richness, and samples from the Western U.S. produce intermediate values. As in modern terrestrial environments, primary productivity levels and faunal richness in ancient ecosystems are inferred here to relate to water availability.