Fossil Site Shows Impact of Early Jurassic Period’s Low Oxygen Oceans

Using a combination of fossils and chemical markers, scientists have tracked low ocean-oxygen in early Jurassic marine ecosystem that could have led to survival of only a few species.

The research, led by Rowan Martindale of the University of Texas at Austin Jackson School of Geosciences, zeroes in on a recently discovered fossil site in Canada located at Ya Ha Tinda Ranch near Banff National Park in southwest Alberta. The site records fossils of organisms that lived about 183 million years ago during the Early Jurassic in a shallow sea that once covered the region.

The oxygen level of the surrounding environment during the Early Jurassic influences the type and amount of carbon preserved in rocks, making the geochemical record an important method for tracking an anoxic event.

“We have this beautiful geochemical record that gives us a backbone for the timing of the Oceanic Anoxic Event,” said Martindale. “So with that framework we can look at the benthic community, the organisms that are living on the bottom of the ocean, and ask ‘how did this community respond to the anoxic event?”

The fossils show that before the anoxic event, the Ya Ha Tinda marine community was diverse, and included fish, ichthyosaurs (extinct marine reptiles that looked like dolphins), sea lilies, lobsters, clams and oysters, ammonites, and coleoids (squid-like octopods). During the anoxic event the community collapsed, restructured, and the organisms living in it shrunk. The clams that were most abundant in the community before the anoxic event were completely wiped out and replaced by different species.

Crispin Little, a senior lecturer in paleontology at the University of Leeds who was not involved with the research, said that the similarity between the sites underscores the widespread nature of the anoxic event. “This confirms previous work suggesting that the T-OAE (anoxic event) was genuinely a global event,” Little said.

However, while other sites were recovering from the anoxic event, the environment at Ya Ha Tinda continued to face stress. Even for small, hardy bivalves, life was tough. “One of the interesting things about the recovery is that we actually see fewer individuals at a time when we’re supposed to be seeing community recovery,” Martindale said.

The paper was published in Palaeogeography, Palaeoclimatology, Palaeoeconology. The co-authors include Martin Aberhan, a curator at the Institute for Evolution and Biodiversity Science at the Natural History Museum in Berlin, Germany.