Environmental drivers of the first major animal extinction across the Ediacaran White Sea-Nama transition

Abstract

The Ediacara Biota—the oldest communities of complex, macroscopic fossils—consists of three temporally distinct assemblages: the Avalon (ca. 575–560 Ma), White Sea (ca. 560–550 Ma), and Nama (ca. 550–539 Ma). Generic diversity varies among assemblages, with a notable decline at the transition from White Sea to Nama. Preservation and sampling biases, biotic replacement, and environmental perturbation have been proposed as potential mechanisms for this drop in diversity. Here, we compile a global database of the Ediacara Biota, specifically targeting taphonomic and paleoecological characters, to test these hypotheses. Major ecological shifts in feeding mode, life habit, and tiering level accompany an increase in generic richness between the Avalon and White Sea assemblages. We find that ∼80% of White Sea taxa are absent from the Nama interval, comparable to loss during Phanerozoic mass extinctions. The paleolatitudes, depositional environments, and preservational modes that characterize the White Sea assemblage are well represented in the Nama, indicating that this decline is not the result of sampling bias. Counter to expectations of the biotic replacement model, there are minimal ecological differences between these two assemblages. However, taxa that disappear exhibit a variety of morphological and behavioral characters consistent with an environmentally driven extinction event. The preferential survival of taxa with high surface area relative to volume may suggest that this was related to reduced global oceanic oxygen availability. Thus, our data support a link between Ediacaran biotic turnover and environmental change, similar to other major mass extinctions in the geologic record.

Nitrate limitation in early Neoproterozoic oceans delayed the ecological rise of eukaryotes

“…We would also like to note that δ 15 N data from the terminal Ediacaran Period (551 to 539 Ma) were not included in the analysis because this interval is marked by a brief return to extensive oceanic anoxia (64) and thus a likely return to a more broadly nitratelimited ocean. The terminal Ediacaran is characterized by rather low δ 15 N values (19,20) and witnessed the extinction of both macro-and microscopic eukaryotes (65,66), perhaps representing a transient return to a prokaryote-dominant world ecologically akin to pre-Cryogenian oceans.…”

Decline and fall of the Ediacarans: late‐Neoproterozoic extinctions and the rise of the modern biosphere

“…This hypothesis is currently undergoing a renaissance, fuelled in large part by the study of late Ediacaran proxies for massive geochemical alterations and their putative coincidence with biotic turnover (e.g. Yang et al ., 2021; Evans et al ., 2022). However, the geochemical record indicates that a consensus on environmental ‘catastrophes’ as the drivers of Ediacaran extinctions is premature.…”

New Ediacaran biota from the oldest Nama Group, Namibia (Tsaus Mountains), and re-definition of the Nama Assemblage

“…This assemblage is currently supposed to have largely disappeared by ca. 550 Ma, coincident with a marked decline in biodiversity (Grazhdankin, 2014; Yang et al 2021; Evans et al 2022). The updated compilation supports a dramatic drop in diversity ca.…”

“…An ecological shift in feeding mode and life habit accompanies the increase in generic diversity from the Avalon to White Sea assemblages (Evans et al 2022). Both the Avalon and White Sea assemblages largely disappeared by ca.…”

“…Both the Avalon and White Sea assemblages largely disappeared by ca. 550 Ma (Grazhdankin, 2014; Yang et al 2021), where analysis suggests that taxa with a high surface area to volume ratio may have preferentially survived, inferred to potentially reflect increased resilience to low environmental oxygen levels (Evans et al 2022). Geochemical proxies, however, suggest that this extinction pre-dates a decrease in global oxygen availability (Tostevin et al 2019).…”