Low oxygen caused early Jurassic's marine ecosystem to shrink

Scientists, using a combination of fossils and chemical markers, have found that a period of low ocean-oxygen globally caused the Early Jurassic marine ecosystem to collapse, shrinking diversity to just a few species.
The study, led by Rowan Martindale from the University of Texas at Austin in the US, 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 fossil site broadens the scientific record of the Toarcian Oceanic Anoxic Event, a period of low oxygen in shallow ocean waters which is hypothesised to be triggered by massive volcanic eruptions.

The Oceanic Anoxic Event was identified at this site by the geochemical record preserved in the rocks.
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.
With that framework the researchers looked at the benthic community, the organisms that are living on the bottom of the ocean, and found 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.
The clams that survived during and after the event were much smaller than the clams from before the event, suggesting that low oxygen levels limited their growth.

The sea life recorded at Ya Ha Tinda before and during the anoxic event is similar to fossils found at European sites.
The fossils suggest that the environment was undergoing local stresses that kept oxygen low, Martindale said.
More research is needed to untangle why life at Ya Ha Tinda did not recover at the same rate as other places.
Since the oceanic anoxic event was a side-effect of climate change, looking back at ancient marine communities could be a window into the potential impacts of ongoing and future climate change, researchers said.
The research was published in the journal Palaeogeography, Palaeoclimatology, Palaeoeconology.