Increase in metazoan ecosystem engineering prior to the Ediacaran–Cambrian boundary in the Nama Group, Namibia

Abstract

The disappearance of the soft-bodied Ediacara biota at the Ediacaran–Cambrian boundary potentially represents the earliest mass extinction of complex life, although the precise driver(s) of this extinction remain unresolved. The ‘biotic replacement’ model proposes that an evolutionary radiation of metazoan ecosystem engineers in the latest Ediacaran profoundly altered marine palaeoenvironments, resulting in the extinction of Ediacara biota and setting the stage for the subsequent Cambrian Explosion. However, metazoan ecosystem engineering across the Ediacaran–Cambrian transition has yet to be quantified. Here, we test this key tenet of the biotic replacement model by characterizing the intensity of metazoan bioturbation and ecosystem engineering in trace fossil assemblages throughout the latest Ediacaran Nama Group in southern Namibia. The results illustrate a dramatic increase in both bioturbation and ecosystem engineering intensity in the latest Ediacaran, prior to the Cambrian boundary. Moreover, our analyses demonstrate that the highest-impact ecosystem engineering behaviours were present well before the onset of the Cambrian. These data provide the first support for a fundamental prediction of the biotic replacement model, and evidence for a direct link between the early evolution of ecosystem engineering and the extinction of the Ediacara biota.

Regional nutrient decrease drove redox stabilisation and metazoan diversification in the late Ediacaran Nama Group, Namibia

“…Indeed, recent quantification of bioturbation intensity in the Nama Group shows an overall increase in the percentage of bedding plane bioturbated from a mean value of 1.94% in the Kliphoek Member to 3.34% in the lower Urusis Formation, and a further increase to 5.61% in the Spitskop Member, immediately beneath the basin-wide unconformity that marks the top of the Urusis Formation (Fig. 2g) 55 . The increase in bioturbation intensity occurs coincident with the stabilisation of oxic conditions in this location (Fig.…”

“…2g). There is a further increase in bioturbation intensity from the lower Schwarzrand Subgroup to the basal Cambrian unconformity in the Nama Group 26,55 . Indeed, recent quantification of bioturbation intensity in the Nama Group shows an overall increase in the percentage of bedding plane bioturbated from a mean value of 1.94% in the Kliphoek Member to 3.34% in the lower Urusis Formation, and a further increase to 5.61% in the Spitskop Member, immediately beneath the basin-wide unconformity that marks the top of the Urusis Formation (Fig.…”

“…Particularly prominent fossiliferous quartzites appear in the Aar, Nasep and Spitskop members (see Supplementary Information Table S1 for references) 23,57 . However, a notable trend of increasing bioturbation intensity is present in the shallow inner to mid-ramp during deposition of the Nudaus Formation 55 , within the middle part of the Nama Group succession (Fig. 2g).…”

Quantifying ecospace utilization and ecosystem engineering during the early Phanerozoic—The role of bioturbation and bioerosion

“…The marked increase in all metrics of animal-substrate interactions in siliciclastic offshore settings was coincident with an inferred rise of ocean oxygenation ( 16 , 17 ) and the appearance of the sediment mixed layer ( 7 ) during the early Cambrian, further emphasizing the impact of bioturbation during the CE. Ichnologic evidence from Namibia indicates that an increase in trace fossil complexity and ecosystem engineering took place in siliciclastic offshore settings by the end of the terminal Ediacaran ( 13 , 14 , 18 ). However, this increase cannot be confirmed with our global analysis because when the Ediacaran is split into Vendian and Nama assemblages ( 19 ), there are significant correlations between these parameters and the total number of lithostratigraphic units (table S3).…”

Complex marine bioturbation ecosystem engineering behaviors persisted in the wake of the end-Permian mass extinction

“…Ultimately, 164 unique ichnogenera were entered into the database given the parameters of the study (confident tiering assignment, one occurrence per tier, one occurrence per time period, and shallow marine). These data were analyzed using two frameworks: the ecosystem engineering occupation cube method 22 and the ecosystem engineering impact (EEI) values method 14,31 . Trace fossils and their categorical assignments for both ecosystem engineering analyses were grouped into stages based on stratigraphic descriptions in the primary literature.…”