Older Ediacaran White Sea Assemblage Fossils Discovered in Canada

Most Ediacaran animals had soft bodies and lacked hard skeletons, which means that their preservation as fossils was already an unusual event. For this reason, any well-preserved fossil community of the Ediacara Biota represents a major discovery. In the lower Blueflower Formation near Sekwi Brook in the Mackenzie Mountains of the Northwest Territories, Canada, researchers have identified an Ediacaran fossil community belonging to the White Sea assemblage. Here, "assemblage" refers to a grouping defined by the taxonomic composition of fossil organisms. Species of the White Sea assemblage were previously known mainly from relatively shallow-water deposits, such as those in the White Sea region of Russia and the Flinders Ranges of South Australia. Now, however, they have been recovered from deep-water slope deposits on the paleocontinent of Laurentia, and they may be older than previously recognized. This discovery forces us to rethink how early animals dispersed, how they entered different marine environments, and whether the three major assemblages of the Ediacara Biota simply replaced one another through time.

Previous studies have often divided the Ediacara Biota into three major phases. The earliest is the Avalon assemblage, which appeared roughly between 575 and 559 million years ago and is represented mainly by sessile, deep-water frondose organisms. This was followed by the White Sea assemblage, from about 559 to 550 million years ago, when biodiversity and morphological disparity increased substantially and more motile or structurally complex forms began to appear, with shallow-marine settings traditionally regarded as typical. The final phase is the Nama assemblage, from about 550 to 538 million years ago, which is generally associated with more proximal marine environments and includes some of the earliest biomineralized animals. This framework appears straightforward, but it has always been constrained by the small number of fossil localities. More than three quarters of global Ediacaran diversity comes primarily from a handful of famous sites: Mistaken Point in Newfoundland, Canada; the White Sea region of Russia; the Flinders Ranges of South Australia; and the Nama basin of Namibia. As a result, our view of the spatial and temporal pattern of early animal evolution has long been affected by substantial sampling bias.

The fossils discovered at Sekwi Brook help fill part of this gap. The research team recovered more than one hundred fossil specimens from mixed carbonate-siliciclastic deposits in the lower Blueflower Formation. These strata formed in a deep-water slope setting below storm wave base. Their sedimentary features include Bouma subdivisions in turbidite deposits, slump folds, soft-sediment deformation, matrix-supported conglomerates, thick intervals of lime mudstone, and calcareous shale. Many fossils are preserved on discrete bedding planes and occur together with textured organic surfaces, which may reflect structures associated with microbial mats.

The most common fossil in the assemblage is the tubular organism Sekwitubulus annulatus. These tubular fossils often occur in dense concentrations, suggesting that they may have lived gregariously. The researchers also identified attachment disks of sessile organisms such as Aspidella, as well as a specimen of Mawsonites in which a stalk extends from the central disk. This supports the interpretation that Mawsonites was probably related to frondose sessile organisms. Although its precise classification remains uncertain, the discovery suggests that this deep-water environment still preserved body plans resembling the sessile communities characteristic of the Avalon assemblage.

(A to C) Sekwitubulus annulatus; (D) Aspidella; (E) Mawsonites; (F) probable Arboreomorpha; (G to I) unassigned taxa; (J) Swartpuntia（Image source：Evans SD et al. (2026)， CC BY-NC 4.0 ）

The study also confirmed several typical members of the White Sea assemblage: Funisia dorothea, Dickinsonia, Aulozoon soliorum, Kimberella quadrata, and Eoandromeda octobrachiata. These fossils occur in strata above the termination of the Shuram Carbon Isotope Excursion and can be correlated with Re-Os dated horizons of approximately 567.3 and 566.9 million years ago. This means that the assemblage may be at least close to 567 million years old, making it older than the commonly cited global appearance of the White Sea assemblage at around 559 million years ago.

(A and B) Funisia dorothea: white triangles in (A) indicate Funisia specimens that may have become disarticulated along body boundaries; the white box in (A) is magnified in (B). In (B), the white arrow indicates Sekwitubulus annulatus, and the white triangle indicates a potentially branching Funisia specimen. (C) Funisia holdfasts, with white triangles indicating these discoidal attachment structures. (D to G) Dickinsonia: the white box in (D) is magnified in (E); (F) and (G) show additional Dickinsonia specimens. (D) also preserves Aulozoon soliorum, indicated by the white arrow. (H and I) Aulozoon soliorum: the white box in (H) marks a rounded termination, which is magnified in (I). (J) Kimberella quadrata. (K and L) Kimberichnus: the white box in (K) is magnified in (L), showing arrays of parallel grazing traces. (M) Eoandromeda octobrachiata. (N) possible new taxon, not yet formally named（Image source：Evans SD et al. (2026)， CC BY-NC 4.0 ）

This discovery also changes how we understand the relationship among the three major Ediacaran assemblages. The underlying Nadaleen Formation at Sekwi Brook is already known to preserve deep-water frondose organisms characteristic of the Avalon assemblage, while the lower Blueflower Formation contains typical members of the White Sea assemblage. The two may therefore have overlapped in both time and environment, suggesting that the Avalon and White Sea assemblages were not necessarily sharply separated, with one disappearing before the other appeared. A more reasonable interpretation is that early animal communities gradually increased in diversity and morphological disparity over time, with different groups coexisting in deep-water environments before later expanding into shallower marine settings. The White Sea assemblage, therefore, was not simply a shallow-water biota; it may have had an earlier deep-water origin.

After the Cambrian, many marine animal groups commonly show an evolutionary pattern of expansion from nearshore environments into offshore settings. During the Ediacaran, however, the pattern may have been the reverse. Early animals, or at least their ecosystems, appear to have first become established in relatively stable deep-water environments before gradually moving into middle-outer shelf, inner shelf, and nearshore habitats. The greater thermal stability of deep-water settings may have provided a safer starting point for early complex animals.

Paleoenvironmental distribution of Ediacaran faunal assemblages through time. Each bar represents the number of described fossil genera from a given time interval and depositional environment; numbers above the bars indicate the total number of genera in each category. Depositional environments are arranged from nearshore, inner shelf, and middle-outer shelf to slope settings, allowing comparison of how the Avalon, White Sea, and Nama assemblages varied through time and across marine environments. The inset in the upper left shows a paleoecological reconstruction of the lower Blueflower Formation at Sekwi Brook, Canada（Image source：Evans SD et al. (2026)， CC BY-NC 4.0 ）

However, this discovery should not be read as a simple statement that "after the Shuram Carbon Isotope Excursion ended, ocean oxygen increased, and the White Sea assemblage appeared." The two may be temporally related, but the current evidence is not direct enough to support such a simple conclusion. Some geochemical data suggest that oxygenated marine environments suitable for animal life may have expanded at that time. Yet chemical signals from the Rackla Group in Canada do not clearly show a synchronous shift in oxygen conditions during the faunal transition. Increased oxygen availability may therefore have been one background factor in the expansion of early animals, but it cannot be treated as the sole cause. In addition, specimens of Aspidella, Aulozoon, Dickinsonia, and Funisia from the lower Blueflower Formation are all relatively small, which may imply that these organisms still lived under environmental stress, such as oxygen limitation, temperature constraints, or other factors that remain unresolved. These fossils clearly show that the White Sea assemblage may have appeared earlier in deep-water settings, but why it emerged at that particular time is still not a question that can be answered by a single cause.

Author: Shui-Ye You

Reference:

Evans SD et al. (2026). Discovery of White Sea assemblage fossils from Laurentia. Science Advances.