Tracking Cambrian environmental perturbation using a new bradoriid arthropod database

The Cambrian Period (~539 to 487 Ma) was a pivotal time for biotic innovation. Extinction events played an important role in shaping evolutionary patterns throughout this interval, yet first order questions remain regarding the extent, severity, and mechanisms behind Cambrian extinctions. To address this we present a new global database of bradoriids, microscopic bivalved arthropods, spanning the Cambrian to early Ordovician, which we analyse in terms of biotic response to environmental change. 

Our analyses reveal that most bradoriid fossils come from well-oxygenated shallow marine facies deposited above storm wave base. Therefore, sea surface temperature, oxygen concentration, and mode of dispersal likely exerted strong controls on distribution of these taxa. A smaller proportion of bradoriid occurrences are from deeper water oxic and dysoxic facies, with the relative proportions of deep water oxic and dysoxic varying throughout the Cambrian. The occurrences of some bradoriid fossils in dysoxic facies suggests that these were either benthic taxa, more resilient to marginal oxygen conditions, or pelagic taxa, that lived in overlying better-oxygenated waters.

From Cambrian Age 3 (~521 to ~514.5 Ma) to Age 4 (~514.5 to 506.5 Ma) there was a poleward shift in bradoriid occurrences, away from equatorial latitudes, coupled with a sharp decrease in the proportion of dysoxic deeper water occurrences and increase in well-oxygenated shallow water occurrences. From Cambrian Age 4 to the Wuliuan Age (~506.5 to 504.5 Ma) there was a further decrease in the proportion of low latitude occurrences but an increase in dysoxic deeper water occurrences, returning to proportions similar to Age 3.

Hyperthermal events have been proposed as the driver for the early Age 4 Sinsk extinction and terminal Age 4 extinction of redlichiid and olenellid trilobites. The apparent stepwise poleward shifts in bradoriid occurrences are consistent with hyperthermal temperature rise exceeding thermal tolerance limits in lower latitudes. From Age 3 to Age 4, the decrease in occurrences from deeper dysoxic facies and proportional increase in shallow oxygenated water facies occurrences is consistent with shoaling of anoxic waters forcing benthic taxa, perhaps already close to their minimum oxygen tolerance, to migrate to shallower waters or perish. Shoaling of dysoxic waters is well evidenced as a kill mechanism during the Sinsk event on the Siberian Platform. Coming out of Age 4 into the Wuliuan, the increased proportion of occurrences in deeper waters suggests that these settings may have become more tolerable again, with a lower risk of exposure to lethally low oxygen concentrations. The similar proportional depth distributions of Age 3 and the Wuliuan contrasts with the distribution observed in intervening Age 4, an interval with known extinction events. The combination of palaeolatitude, depth, and seafloor oxygen concentration influence on bradoriid occurrences suggest that Cambrian Age 4 may have been an interval of profound biotic crisis caused by multiple hyperthermal events.