# Environmental and evolutionary controls in animal-sediment interactions at the onset of the Cambrian explosion

Romain Gougeon, Luis A Buatois, M Gabriela Mángano, Guy M Narbonne, Brittany A Laing, Maximiliano Paz, Nicholas J Minter

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Romain Gougeon, Luis A Buatois, M Gabriela Mángano, Guy M Narbonne, Brittany A Laing, et al. Environmental and evolutionary controls in animal-sediment interactions at the onset of the Cambrian explosion. Current Biology, 2024, 35, pp.249 - 264.e4. 10.1016/j.cub.2024.11.028. hal-04938825

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# Current Biology

## Environmental and evolutionary controls in animal-sediment interactions at the onset of the Cambrian explosion

### Highlights

* The most extensive ichnologic dataset compiled from any Ediacaran-Cambrian section

* Time-environment matrices discriminate environmental from evolutionary signals

* Three ecological stages identified in shallow waters that are globally distributed

* The lower offshore was the cradle of animal behavior diversification

### Authors

Romain Gougeon, Luis A. Buatois, M. Gabriela Mángano, Guy M. Narbonne, Brittany A. Laing, Maximiliano Paz, Nicholas J. Minter

### Correspondence

gougeon.romain@gmail.com

### In brief

Gougeon et al. compile the most extensive dataset on trace fossils in the Ediacaran-Cambrian type section to decipher the structure of benthic seafloors at the dawn of animal evolution. Increasing bioturbation and diversification in early Cambrian animal behaviors rapidly led to modern-style mixgrounds in offshore marine environments.

<figure><img src="image_3.png" /></figure>
## Article

# Environmental and evolutionary controls in animal-sediment interactions at the onset of the Cambrian explosion

Romain Gougeon,<fnref n="1" />,<fnref n="2" />,<fnref n="6" />,* Luis A. Buatois,<fnref n="1" /> M. Gabriela Mángano,<fnref n="1" /> Guy M. Narbonne,<fnref n="1" />,<fnref n="3" /> Brittany A. Laing,<fnref n="1" /> Maximiliano Paz,<fnref n="1" />,<fnref n="4" /> and Nicholas J. Minter<fnref n="5" />

## SUMMARY

The Cambrian explosion was a time of groundbreaking ecological shifts related to the establishment of the Phanerozoic biosphere. Trace fossils, which are the products of animals interacting with their substrates, provide a key record of the diversification of the benthos and the evolution of behavioral complexity through this interval. The Chapel Island Formation of Newfoundland in Canada hosts the most extensive trace-fossil record from the latest Ediacaran to Cambrian Age 2, spanning about 20 million years continuously. To elucidate the relative roles of environmental changes as opposed to evolutionary trajectories, we gathered the largest trace-fossil dataset to date and designed fourteen high-resolution time-environment matrices on bioturbation intensity, burrow width and depth, tiering (i.e., the vertical partitioning of trace fossils within the substrate), ichnodiversity, ichnodisparity (i.e., the development of novel architectural designs in ichnod taxa), ecospace utilization (i.e., the development of ecological niches by benthic animals), and other trends related to specific trace-fossil types. Ecosystem engineering by early animals resulted in three stages identified in the Chapel Island Formation that are probably global—an Ediacaran matground ecology, a Fortunian matground/firmground ecology, and a latest Fortunian/Cambrian Age 2 mixground ecology. Time-environment matrices further imply that the lower offshore was the cradle of diversification for animal behavior, which later expanded inshore and led to a novelty evolutionary event, refining our understanding of the early stages of the Cambrian explosion.

## INTRODUCTION

The Cambrian explosion was a pivotal time in the history of shallow-marine ecosystems,<fnref n="1" />⁻<fnref n="5" /> and bioturbation has increasingly been recognized as a driving force in the evolution of the biosphere during this interval.<fnref n="6" />⁻¹⁶ Ichnofaunal changes at the dawn of the Phanerozoic are typically assessed either based on global datasets¹⁰, ¹³, ¹⁷, ¹⁸ or local studies.¹⁹, ²⁰ Thus far, local studies have tended to focus only on a few metrics, typically degree of bioturbation, but integration of other ichnologic data in a systematic fashion could potentially unravel finer evolutionary and ecological patterns of global significance.

The base of the Cambrian (ca. 538 Ma) is delineated by the demise of Ediacaran body fossils and the appearance of trace fossils of the *Treptichnus pedum* ichno-assemblage zone.²¹⁻²³ This biologic turnover is best observed at Fortune Head in the Burin Peninsula, Newfoundland, Canada, an outcrop of the Chapel Island Formation that was selected as the Cambrian Global Boundary Stratotype Section and Point (GSSP).²³ The

Chapel Island Formation is a 1,000+ m-thick continuous succession that is composed mostly of siliciclastic sedimentary rocks spanning ca. 20 Ma from late Ediacaran to Cambrian Age 2 and is renowned for its outstanding trace-fossil record.²¹⁻²³ The succession also has a good representation of small shelly fossils, but these are mostly recovered from late Fortunian and Cambrian Age 2 strata.²⁴ The Chapel Island Formation has served as the Cambrian GSSP for the past three decades, but concerns have also been raised intermittently about the global recognition and distribution of the events depicted in the succession.²⁵,²⁶

Assessing the utility of bioturbation in recording geobiological revolutions has been historically hampered by uncertainties in differentiating environmental from evolutionary controls. The fact that trace fossils have a narrow environmental tolerance has been raised as a supposed impediment,²⁵ but, as trace fossils display an intimate link with their associated substrates, their paleoenvironmental distribution can be framed within finely tuned depositional models. Environmental controls affecting

<footnote n="1">¹Department of Geological Sciences, University of Saskatchewan, Science Place, Saskatoon, SK S7N 5E2, Canada</footnote>

<footnote n="2">²Geo-Ocean, University of Brest, CNRS, Ifremer, UMR 6538, Place Nicolas Copernic, Plouzané 29280, France</footnote>

<footnote n="3">³Department of Geological Sciences and Geological Engineering, Queen's University, Union Street, Kingston, ON K7L 3N6, Canada</footnote>

<footnote n="4">⁴Oberlin College and Conservatory, W. Lorain Street, Oberlin, OH 44074, USA</footnote>

<footnote n="5">⁵School of the Environment and Life Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK</footnote>

<footnote n="6">⁶Lead contact</footnote>

<footnote>*Correspondence: gougeon.romain@gmail.com</footnote>

<footnote>https://doi.org/10.1016/j.cub.2024.11.028</footnote>
trace-fossil distribution can be discriminated using rigorous characterization of sedimentary facies.<sup>13,27</sup> In that respect, time-environment matrices represent a powerful tool that was originally designed to evaluate global evolutionary events of the Phanerozoic using the body-fossil record<sup>28,29</sup> and has since been adapted to trace-fossil analyses.<sup>13,19,27,30</sup> Although previous ichnologic matrices were built from the identification of individual ichnotaxa or bioturbation intensity within specific spatial and temporal loci, it is also possible to track animal behavior changes by integrating these with other proxies (e.g., burrow width [BW] and tiering). The Chapel Island Formation is ideally suited to perform an analysis of this nature because it hosts deposits with a broad marine environmental range that recur through time at a high frequency, therefore permitting the tracking of ichnologic changes at a high resolution across time and space.

Based on one of the largest ichnologic datasets gathered so far for any stratigraphic unit in the rock record (see Gougeon et al.<sup>31</sup> and STAR Methods), the objectives of this study are to: (1) discriminate temporal and spatial trends in animal-substrate interactions based on high-resolution time-environment matrices built from extensive ichnologic and sedimentologic datasets, (2) interpret ecologic changes and evolutionary innovations in the Chapel Island Formation, and (3) assess how these local patterns compared with those that have been suggested at a global scale.

## RESULTS

### Vertical bioturbation

Vertical bioturbation (i.e., biogenic disruption of the primary fabric) is evaluated using a bioturbation index<sup>32</sup> (BI) (see STAR Methods). After designing a template for time-environment matrices in the Chapel Island Formation (Figure 1), BI values highlight two major events in shallow-marine settings (Figure 2A). During the early Fortunian (parasequence 5, or PS 5), biogenic disturbance of the sedimentary fabric reached BI = 2 first in lower offshore and shelf subenvironments and later in the upper offshore (PS 7), offshore transition (PS 11), and lower shoreface (PS 12) by the middle Fortunian. Intensity of bioturbation remained low (BI = 1–3) for most of the Fortunian in all shallow-marine cells (PS 5–18). During the late Fortunian, BI first reached maximum values (BI = 6) in lower offshore (PS 19), and values were high (BI = 4–5) in the shelf (PS 19, 20). During Cambrian Age 2, maximum values of BI were also attained in lower shoreface (PS 21, 25), offshore transition (PS 25), upper offshore (PS 25), and shelf (PS 21, 22, 24). In intertidal environments, vertical bioturbation is absent at any time interval reported in the succession.

### Bedding plane bioturbation

Bedding plane bioturbation is reported from bed tops and bases using a bedding plane bioturbation index<sup>33</sup> (BPBI) (see STAR Methods). BPBI values highlight three conspicuous events in areal colonization by organisms (Figure 2B). Conversely to vertical bioturbation, bed surface disruption started early in the succession during the Ediacaran, both in intertidal and shallow-marine environments (PS 1–5), although with low values (BPBI = 2–3). However, maximum values of BPBI were first

attained in the lower shoreface during the early Fortunian (PS 9). BPBI is also high (BPBI = 4) in other early Fortunian cells, notably in the sand-flat (PS 9), upper offshore (PS 7), and lower offshore (PS 9). The remainder of the Fortunian was characterized by moderate bioturbation values (BPBI = 1–3) in most depositional settings. By the late Fortunian, bed surface bioturbation mimicked trends in vertical bioturbation (see Figure 2A) with maximum BPBI values in the lower offshore (PS 19). The Cambrian Age 2 was marked by widespread and intense bed surface disruption across the shallow-marine depositional profile, from lower shoreface to shelf (PS 20, 21, 23, 25).

### Trace-fossil width

Trails and burrows were relatively small during the Ediacaran (0.1–0.3 cm), whereas slightly larger sizes (0.4–0.9 cm) were reached in lower and upper offshore during the early Fortunian (PS 5) (Figure 2C). Besides significantly high values in the early Fortunian upper offshore (PS 7), resulting from the appearance of plug-shaped burrows *Bergaueria* cf. *radiata* and *Conichnus conicus* (2.6–3.9 cm), and in the lower shoreface (PS 9) due to larger *Palaeophycus* isp. (1.6–2.0 cm) (see also Figures S2D, S3A, and S5A), BW remained low to moderate in intertidal and shallow-marine environments during most of the Fortunian (0.1–1.2 cm in PS 5–18). However, the late Fortunian was marked by a sharp increase in BW in lower offshore and shelf settings (up to 2.5 cm in PS 19 and 20) due to larger *Psammichnites gigas circularis* (see also Figure S5B). Cambrian Age 2 displays higher BW values in shallow-marine environments, with peaks in middle shoreface (PS 25, due to *Rosselia erecta*), lower shoreface (PS 25), and shelf (PS 21 and 24, the latter two due to large *Psammichnites gigas circularis*) (see also Figures S5B and S5C). In intertidal environments, BW was low (0.1–0.6 cm) both during the Ediacaran (PS 1–4) and Fortunian (PS 8, 9).

### Burrow depth and tiering

Burrow depth (BD) reached higher values at the base of the Fortunian (up to 6.0 cm in lower offshore of PS 5) in comparison with the Ediacaran (maximum of 1.0 cm in lower offshore of PS 5), whereas values were low to moderate for most of the overlying Fortunian strata in shallow-marine environments (0.1–2.0 cm in PS 6–18) (Figure 2D). The late Fortunian is marked by BD attaining 6.0 cm (PS 19, 20), whereas the most significant peak took place during Cambrian Age 2 in upper offshore to shelf settings (PS 23, 24) with the appearance of deep, pyritized, simple vertical *Trichichnus* isp. (maximum reported depth of 57.0 cm).

Tiering corresponds to the vertical partitioning of trace fossils within the substrate (see STAR Methods). Trends in tiering are more conspicuous than in BD (Figure 3C) because of the early appearance of *Gyrolithes gyratus*, *G. scintillus*, and *Trichichnus linearis*, which are vertical burrows typically reported in shallow-marine lower and middle Fortunian strata and considered of shallow-tier position (see also Figures S4C and S6C; Table S1). The appearance of *Teichichnus rectus*, first sparsely noted in offshore transition to lower offshore middle Fortunian strata and then becoming very common during the late Fortunian and Cambrian Age 2 (PS 19–25) (see also Figure S5F), played an important role in tiering structure as it was made by organisms colonizing the mid-tier. *Arenicolites* isp. and *Rosselia erecta*
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Template for time-environment matrices within the Chapel Island Formation of Newfoundland, Canada</figcaption></figure>

A composite section was created based on data from Grand Bank Head, Fortune Head, and Little Dantzic Cove, allowing the delineation of 25 parasequences (y axis in the matrix). Depositional subenvironments (x axis in the matrix) were defined in this study from detailed sedimentary facies characterization (see STAR Methods; u. s., upper shoreface, shoref., shoreface, and transit., transition). A total of 100 cells are identified. Cells with solid black lines are representative of data recovered in siliciclastic strata; cells with dotted black lines are related to two carbonate beds. From left to right: stratigraphic boundaries of parasequences (acronyms and numbers correspond to section names and stratigraphic levels in meters, respectively; this information can be found on higher-resolution logs of Gougeon et al.³¹); time interval; member of the Chapel Island Formation; scale; parasequence number; sedimentary log (dark and light gray colors for fine and medium mudstone, respectively, yellow color for sandstone, red-colored intervals for red strata, and blue color for limestone beds and nodules); biostratigraphic zone (Z) or ichno-assemblage zone (IAZ); and time-environment matrix.
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Figure 2. Time-environment matrices on bioturbation intensities and depth, trace-fossil size, and ichnodiversity and ichnodisparity per cell within the Chapel Island Formation of Newfoundland, Canada</figcaption></figure>

Parasequence (PS) numbers are indicated on the right side.

(A) Maximum bioturbation index (BI) reached BI = 2 during the early Fortunian (PS 5–7) and BI = 6 during the late Fortunian to Cambrian Age 2 (PS 19–25). Crossed cells indicate absence of data despite the record of strata from the related sedimentary facies (see Figure S7A).
also reached down into the mid-tier during Cambrian Age 2 (see also Figures S2C and S5C; Table S1). Finally, vertical burrows assigned to ?Diplocraterion isp. and Trichichnus isp. in Cambrian Age 2 strata further contributed to tiering structure complexity by reaching the deep tier (see also Figures S4A and S6C; Table S1). In intertidal environments, data on BD is absent, and tiering is limited to very shallow-tier structures both during the Ediacaran and the Fortunian (Figures 2D and 3C).

## Ichnodiversity and ichnodisparity

Trace-fossil diversity per cell corresponds to the number of ichnotaxa present at a single temporal and spatial locus (see STAR Methods). Ichnodiversity per cell reached high values at the base of the Fortunian in lower offshore settings (n = 10 in PS 5) (Figure 2E). Ichnodiversity remained high in offshore subenvironments during the early and middle Fortunian (e.g., n = 12 in upper offshore of PS 17), whereas it was noticeably lower in coeval lower shoreface and shelf (maximum of n = 4). During the late Fortunian, ichnodiversity per cell shows a burst in the shelf (n = 11 in PS 19), but the overlying strata demonstrate a sharp decrease in shoreface, offshore, and shelf environments (maximum of n = 5 in PS 20–25). Trends in ichnodisparity per cell closely follow ichnodiversity, with maximum values in offshore subenvironments during the early and middle Fortunian (n = 8 in six cells of PS 11–17) (Figure 2F). However, very high values in ichnodisparity per cell are characteristic of the three offshore subenvironments at the early to middle Fortunian transition (n = 7–8 in PS 11). In intertidal environments, both ichnodiversity (maximum of n = 5) and ichnodisparity per cell (maximum of n = 4) were at lower values than in their shallow-marine counterparts during the Ediacaran and the Fortunian (Figures 2E and 2F).

Ichnodiversity and ichnodisparity cumulated through time per subenvironment represent the added values, through time and within specific subenvironments, of individual data per cell (see STAR Methods). They depict slightly different tendencies than per cell (Figures 3A and 3B). Cumulative ichnodiversity and ichnodisparity both show that the lower offshore was the first subenvironment to be significantly colonized at the start of the Fortunian (n = 10 and 7 in PS 5, respectively). Lower offshore ichnodiversity then slowly increased for the remainder of strata analyzed (n = 19 in PS 25). Conversely, cumulative ichnodiversity in upper offshore remained low during the early Fortunian (n = 8 in PS 9), increasing first slightly later in the Fortunian (n = 12 in PS 10) and subsequently during the middle Fortunian (n = 19 in PS 17). In offshore transition and shelf, cumulative ichnodiversity increased slowly during the early and middle Fortunian and reached moderately high values by the end of Cambrian Age 2 (n = 18 in offshore transition and n = 17 in shelf in PS 25). Trends

in cumulative ichnodisparity (Figure 3B) follow trends in cumulative ichnodiversity. However, higher values of cumulative ichnodisparity all appeared at the same time in the three offshore subenvironments (n = 9–10 in PS 11). A very high value was then reached early in offshore transition during the middle Fortunian (n = 12 in PS 14).

## Ecospace utilization

A multidimensional ecospace utilization cube analysis evaluates the development of modes of life (MOL) in time and space in the succession (Figure 4, see also STAR Methods and Table S1). An occupancy ratio (OR) is also introduced to compare the proportion of Chapel Island Formation MOL with global MOL for each time interval and specific environment (see STAR Methods). The Ediacaran interval of the Chapel Island Formation has an extensive record of intertidal MOL (n = 7, OR = 1.00), notably with various very shallow-tier feeding strategies present (i.e., suspension feeding, deposit feeding, and predation). MOL in Ediacaran offshore are more limited (n = 5, OR = 0.50) partly because of the restricted stratigraphic record in this interval (Figure S8). Comparatively, early Fortunian offshore (within the Treptichnus pedum ichno-assemblage zone) has a higher number of MOL (n = 8), notably with the development of mid-tier chemosymbiotic strategies (with Trichichnus linearis). Overall, though, data from the Treptichnus pedum ichno-assemblage zone remain limited in all depositional settings (n = 3, OR = 0.20 in intertidal; n = 5, OR = 0.31 in shoreface; OR = 0.26 in offshore; and n = 5, OR = 0.33 in shelf). During the middle to late Fortunian (within the *Rusophycus avalonensis* ichno-assemblage zone), MOL significantly expanded in offshore settings, with the development of deposit feeding (both non-specialized and specialized) and predation strategies in many surficial to shallow-tier unit cubes (n = 15, OR = 0.48). The number of ichnotaxa per mode of life was also the highest in middle to late Fortunian offshore (up to 6). Shoreface (n = 7, OR = 0.41) and shelf (n = 12, OR = 0.75) environments also display higher values. During Cambrian Age 2, offshore (n = 10, OR = 0.32) and shelf settings (n = 9, OR = 0.60) have more limited data from surficial and semi-infaunal tiers in comparison with the previous time interval analyzed. The shoreface has higher values with the development of mid-tier non-specialized deposit feeding and mid- and deep-tier suspension-feeding strategies by facultatively motile animals (n = 8, OR = 0.42).

## Ecosystem engineering

To assess ecosystem engineering in the past using trace fossils, a multidimensional ecosystem engineering cube analysis is performed to plot the different styles of engineering (SOE)

<footnote>(B) Maximum bedding plane bioturbation index (BPBI) reached BPBI = 5 during the early Fortunian (PS 9) and later during the late Fortunian to Cambrian Age 2 (PS 19–25). Crossed cells indicate absence of data despite the record of strata from the related sedimentary facies (see Figure S7B).</footnote>

<footnote>(C) Maximum burrow width (BW) attained 1.0–1.2 cm during the early Fortunian (PS 5) and then burrows reached much larger sizes during the late Fortunian to Cambrian Age 2 (up to 4.3 cm in PS 19–25).</footnote>

<footnote>(D) Maximum burrow depth (BD) first reached 3.1–6.0 cm during the early Fortunian (PS 5) and later attained 57.0 cm during the Cambrian Age 2 (PS 23, 24) with the appearance of deep *Trichichnus* isp.</footnote>

<footnote>(E) Ichnodiversity per cell reached high values (n = 10–12) during the early and middle Fortunian (PS 5–19), then reduced to n = 0–5 after the late Fortunian (PS 20–25).</footnote>

<footnote>(F) Trends in ichnodisparity per cell mimic those of ichnodiversity per cell, albeit bursts in disparity can be less pronounced in specific cells (e.g., lower offshore in PS 5 and offshore transition in PS 11).</footnote>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Figure 3. Time-environment matrices on cumulative ichnodiversity and ichnodisparity per subenvironment, tiering, and other trends within the Chapel Island Formation of Newfoundland, Canada</figcaption></figure>

Parasequence (PS) numbers are indicated on the right side.

(A) Ichnodiversity cumulated per subenvironment reached high values (n = 10) early in Fortunian lower offshore (PS 5), and high values are observed only later in more proximal (e.g., n = 12 in upper offshore of PS 10) and distal settings (n = 14 in the shelf of PS 19).
developed at a certain time and space in the Chapel Island For-
mation, which can be compared with global data reported by
Buatois et al.¹³ (Figure 5, see also STAR Methods and
Table S1). During the Ediacaran, styles of ecosystem engineer-
ing in intertidal environments were moderately represented (n =
4, OR = 1.00), whereas offshore data are more limited (n = 2,
OR = 0.40). In the Chapel Island Formation, Ediacaran trace fos-
sils remained surficial to very shallow infaunal, sediment interac-
tion modes were limited to compression and backfilling, and
sediment modification modes were diverse (i.e., biodiffusor, gal-
lery biodiffusor, and conveyor). During the early Fortunian (within
the *Treptichnus pedum* ichno-assemblage zone), SOE were
limited in comparison with the global Terreneuvian dataset (n =
2, OR = 0.18 in intertidal; n = 2, OR = 0.17 in shoreface; n = 5,
OR = 0.26 in offshore; and n = 3, OR = 0.27 in shelf). In the Chapel
Island Formation, early Fortunian trace fossils were constrained
within surficial to shallow-tiers, and sediment interaction and
modification modes were like those of the Ediacaran.
Conversely, the middle and late Fortunian (within the *Rusophy-
cus avalonensis* ichno-assemblage zone) show more diverse in-
teractions: the offshore displays the highest number of SOE of
the succession (n = 10, OR = 0.53), with up to 5 ichnotaxa per
style of engineering, and the shelf has an elevated OR (n = 9,
OR = 0.82). In addition, excavators and regenerators (i.e., *Cruzi-
ana*) represented novel sediment interaction and modification
modes, respectively, in the offshore. Finally, during Cambrian
Age 2, the shoreface (n = 6, OR = 0.46), offshore (n = 8, OR =
0.42), and shelf (n = 8, OR = 0.67) have moderately diverse
SOE. Although discrete surficial and semi-infaunal trace fossils
are rarely preserved, tiering ranges down to mid- and deep-
infaunal realms in all shallow-marine environments. However,
sediment interaction and modification modes do not display
any novelty in comparison with previous time intervals.

Comparison of the Chapel Island Formation MOL and
SOE with global trends

A series of indices were calculated to evaluate the relationship
between the Chapel Island Formation and global data in the eco-
space utilization and ecosystem engineering analyses (Table S3,
see also STAR Methods and Figures S10 and S11). There are
some MOL and SOE that are exclusive to the Chapel Island For-
mation in certain settings and points in time, but they tend to be
minor in relation to the global dataset (MOL b < 0.29; MOL
y < 0.18; SOE b < 0.25; SOE y < 0.11). The MOL and SOE that
are shared by the rest of the world and the Chapel Island Forma-
tion have the greatest effect across Ediacaran intertidal

(MOL c = 0.86; MOL z = 0.94; SOE c = 0.75; SOE z = 0.89) and Terreneuvian shelf settings (MOL c = 0.53; MOL z = 0.71; SOE c = 0.83; SOE z = 0.96) and are in similar proportion to those that are exclusive to the rest of the world in Ediacaran offshore, Terreneuvian offshore, and Terreneuvian shoreface settings. In Terreneuvian intertidal settings, MOL and SOE are strongly influenced by those that are exclusive to the rest of the world (MOL a = 0.80; MOL x = 0.53; SOE a = 0.82; SOE x = 0.50).

Other trends

Simple horizontal trails that are typical of a matground ecology
were widespread in intertidal and shallow-marine environments
of the Ediacaran and early to middle Fortunian (52% of cells are
occupied in PS 1–19) (Figure 3D). However, simple horizontal
trails noticeably disappeared from those environments in the
upper Fortunian and Cambrian Stage 2 (8% of cells are occu-
pied in PS 20–25). Arthropod trace fossils comprise scratch im-
prints with fine incisive elements diagnostic of arthropod ap-
pendages interacting with the substrate. These trace fossils
appeared during the early Fortunian (PS 7) but mostly devel-
oped in offshore and shelf environments during the middle For-
tunian (62% of cells are occupied in PS 10–19) (Figure 3E).
Later, arthropod trace fossils became less common (12% of
cells are occupied in PS 20–25). Sediment bulldozers are an
informal grouping of animals that process large amounts of
sediment through deposit feeding and are illustrated in the
Chapel Island Formation by *Curvolithus*, *Didymaulichnus*, and
*Psammichnites*. Sediment bulldozing trace fossils appeared in
lower shoreface and offshore environments during the early
Fortunian (PS 10) and then expanded into the shelf by the
late Fortunian (PS 19) (Figure 3F). These trace fossils became
common and diverse immediately after their first appearance
(61% of cells are occupied in PS 10–25).

Correlations of metrics

Several sets of the above metrics have trends that are correlated
with each other. Principal Components Analysis (Figure S9)
demonstrates that all metrics correlate positively with PC1. The
analysis shows two sets of metrics with similar vectors accord-
ing to PC2. The first set comprises BI, BPBI, BW, BD, ichnodis-
parity (cumulated), ichnodiversity (cumulated), and tiering, which
has a negative correlation with PC2. The second set comprises
arthropod trace fossils, ichnodisparity (per cell), ichnodiversity
(per cell), sediment bulldozing trace fossils, and simple horizon-
tal trails, which has a positive correlation with PC2.

(B) Ichnodisparity cumulated per subenvironment reached high values (n = 9–10) for the three offshore subenvironments close to the start of the *Rusophycus avalonensis* ichno-assemblage zone (PS 11).

(C) Development of the deepest tier per cell showing attainment of the intermediate infaunal realm during the early Fortunian (PS 11) and of the deep infaunal realm during the late Fortunian to Cambrian Age 2 (PS 20–25). Values correspond to surficial (0.0 cm); semi-infaunal (0.1–0.5 cm); very shallow infaunal (0.6–3.0 cm); shallow infaunal (3.1–6.0 cm); intermediate infaunal (6.1–12.0 cm), and deep infaunal (12.1+ cm). Crossed cells indicate absence of data despite the record of strata from the related sedimentary facies (see Figure S8C).

(D) Occurrence and diversity of simple horizontal trails (i.e., *Archaeonassa*, *Circulichnis*, *Cochlichnus*, *Gordia*, *Helminthoidichnites*, and *Helminthopsis*) suggest that a matground ecology typical of the Ediacaran was still widespread during the early and middle Fortunian (PS 5–19), but then became rare (PS 20–25) as a result of the development of mixgrounds (see Figure 2A).

(E) Occurrence and diversity of arthropod trace fossils (i.e., *Allocotichnus*, *Cruziana*, *Dendroidichnites*, *Dimorphichnus*, *Monomorphichnus*, and *Rusophycus*) showing their dominance in Fortunian firmgrounds of the *Rusophycus avalonensis* ichno-assemblage zone (PS 10–19).

(F) Occurrence and diversity of sediment bulldozing trace fossils (i.e., *Curvolithus*, *Didymaulichnus*, and *Psammichnites*) showing their appearance during the early to middle Fortunian (PS 10), which later promoted the development of mixgrounds by the late Fortunian (see Figure 2A).
<figure><img src="image_9.png" /><figcaption>Figure 4. Time-environment-ecospace utilization matrix within the CIF of Newfoundland, Canada</figcaption></figure>

Global data from the Ediacaran and Terreneuvian are based on Buatois et al.¹³ See Figure S10 for an evaluation of the relationship between Chapel Island Formation and global data. Ediacaran modes of life are already extensive in intertidal strata of the Chapel Island Formation, whereas numbers are lower in the offshore counterpart because of the thinner stratigraphic interval recovered there (Figure S8). Fortunian data of the *Treptichnus pedum* ichno-assemblage zone do not show significant advances overall beside the appearance of shallow-tier chemosymbiotic strategies (with *Trichichnus linearis*) in offshore and shelf. Conversely, the middle to late Fortunian (i.e., *Rusophycus avalonensis* ichno-assemblage zone) displays a burst in offshore and shelf modes of life, with advances in deposit feeding (both non-specialized and specialized) and predation strategies per tier. Cambrian Age 2 data are slightly more extensive in shoreface with the appearance of mid- and deep-tier suspension and non-specialized deposit-feeding strategies, whereas the reduction in modes of life in surficial and semi-infaunal tiers of offshore and shelf environments is likely to be a consequence of the establishment of mixgrounds.

DISCUSSION

The issue of facies control masking the timing of evolutionary breakthroughs has been consistently raised in the literature dealing with the Ediacaran-Cambrian transition,²⁴⁻²⁶,³⁴ but this formerly intractable problem can be addressed through input from detailed sedimentary facies and sequence stratigraphic architecture in the form of time-environment matrices. The Chapel Island Formation depositional profile comprises proximal

environments under the influence of tidal processes and shallow-marine settings dominated by storm and wave activity. As such, the same environments may be tracked through time and at high resolution so that evolutionary changes may be disentangled from environmental factors.

Typical environmental controls of tidal flats are variations in hydrodynamic energy, grain size, substrate cohesiveness, salinity, oxygen of interstitial waters, temperature, and subaerial exposure, denoting stressful conditions.³⁵,³⁶ Ediacaran tidal flats
<figure><img src="image_10.png" /><figcaption>Figure 5. Time-environment-ecosystem engineering matrix within the CIF of Newfoundland, Canada</figcaption></figure>

Global data from the Ediacaran and Terreneuvian are based on Buatois et al.¹³ See Figure S11 for an evaluation of the relationship between Chapel Island Formation and global data. The Ediacaran is characterized by low numbers of styles of engineering. During the early Fortunian, the number of styles of engineering

(legend continued on next page)
of the Chapel Island Formation were colonized by epibenthic organisms grazing on microbial mats as well as very shallow endobenthic animals (Figures 4, 5, and 6). In comparison, early Fortunian tidal flats differ by the appearance of burrow systems portraying advances in infaunal ecospace colonization (Figures 2B and 6). Burrowing deeper within the substrate is a strategy developed by modern animals to dampen temperature fluctuations, avoid desiccation, serve as anchorage for stationary feeding, and buffer salinity changes in the intertidal zone.³⁶,³⁷ Global datasets and information from other successes showed that major changes in intertidal animal-sediment interactions took place only later, at the turn of the Terreneuvian to Cambrian Epoch 2 (ca. 521 Ma), with the appearance of new styles of ecosystem engineering.¹³

Shorefaces are characterized by strong storm and wave action aiding in the formation of dunes and hummocks and keeping smaller particles in suspension.³⁸ Accordingly, animals of Phanerozoic shoreface typically lived in robust vertical burrows and gallery systems to withstand hydrodynamic stress.³⁹ Fortunian lower shoreface of the Chapel Island Formation was colonized during rare periods of quiescence by semi-infaunal and shallow-infaunal grazers and suspension feeders/predators (Figures 3D, 3F, and 6). Surficial trace fossils made by arthropods are notably absent (Figure 3E). By contrast, Cambrian Age 2 middle and lower shoreface display more modern aspects with the appearance of mid- and deep-tier burrows made by detritus- and suspension-feeding animals (Figures 3C and 6).

Offshore subenvironments represent the dominant settings encountered in the Chapel Island Formation and are extensively recorded from Ediacaran to Cambrian Age 2 (Figure 1). Below fair-weather wave base and above storm-weather wave base, fluctuating hydrodynamic conditions facilitate the continuous renewal of nutrients in the system, with organic particles primarily carried in suspension by currents and oscillatory flows and later accumulated on the seafloor during moments of quiescence.⁴⁰ Therefore, stress factors on the physiology of animals are significantly mitigated. Below storm-weather wave base and above the slope break, the shelf is characterized by low energy, high food supply, and oxygen levels dependent on distal currents and the position of the oxygen minimum zone.³⁹,⁴¹ Fortunian shelf deposits in the Chapel Island Formation are not well represented in comparison with other settings (Figure S8), but late Fortunian to Cambrian Age 2 data advocate for the settlement of a modern-style community of animals permitting the development of mixed and transition layers (Figure 6).¹¹ In summary, shallow-marine settings—notably the offshore—are ideal to track evolutionary innovations in animal behavior and are extensively available in the Chapel Island Formation.

Overall, the Chapel Island Formation depicts three ecologic stages that illustrate evolutionary innovations in animal-substrate interactions. The Ediacaran is typified by a matground ecology⁶ recorded both in intertidal and shallow-marine environments (Figure 6). Trace fossils are surficial to shallow infaunal,

and bioturbation is only visible on bed surfaces. Morphologic disparity in trace fossils is therefore restricted to simple horizontal trails, with rare penetrative burrows in the latest phase of the Ediacaran.²² The low number of SOE (Figure 5) suggests that early burrowing animals lacked the physiologic adaptations to process sediment efficiently and at depth, therefore keeping ecosystem engineering and trophic relationships to their simplest.

During the early and middle Fortunian, a firmground ecology⁴² was characterized by highly cohesive seafloors with low water content. This resulted from limited vertical bioturbation and aided in the preservation of surficial to shallow-tier, highly diverse trace-fossil assemblages in shallow-marine environments (Figure 6). The sharp increase in bed surface disruption in lower shoreface deposits of the early Fortunian resulted from the appearance of deposit-feeding sediment bulldozers that displaced relatively higher volumes of sediment (Figure 3F). Hence, enhanced spatial colonization in the Fortunian (e.g., PS 9) was linked with the appearance of new strategies in animal-substrate interactions that are depicted by higher ichnodiversity and ichnodisparity values (e.g., PS 8–10) and suggests higher competition for space and food by animals. Endobenthic colonization and incipient ecospace partitioning became slightly more complex, and the shallow-infaunal niche was firmly established, as represented by *Gyrolithes* and *Trichichnus*. During the Fortunian, *Gyrolithes* became progressively less common, and *Teichichnus* became dominant in the mid-tier, the latter being a classic component of the transition layer that develops below fully bio-mixed sediments in modern settings.⁴³ However, the ecosystem engineering cube analysis reveals that the most significant advances took place within the *Rusophycus avalonensis* ichno-assemblage zone (Figure 5), supporting the case that the early Fortunian *Treptichnus pedum* ichno-assemblage zone represented a transitional phase toward more typical Cambrian ecosystems.⁴⁴ This is also evidenced by the persistence of a matground ecology on the shallow-marine seafloor during the early and middle Fortunian (Figure 6).⁴⁵

By the late Fortunian and continuing into Cambrian Age 2, a dramatic change in seafloor character is demonstrated by the establishment of a mixground ecology⁶ in shallow-marine environments. The appearance of *Psammichnites* in the Chapel Island Formation, a trace fossil that depicts the progressive dominance of larger sediment bulldozers onto Fortunian seafloors, ultimately led to increased sediment churning and a more complex partitioning of the infaunal ecospace (Figures 4, 5, and 6).⁴⁶ Because of high bioturbation intensities, surficial and very shallow-tier structures made both by mobile and stationary animals were more typically erased.⁴⁷ This taphonomic effect explains the rarity of simple horizontal trails and the overall decline in ichnodiversity and ichnodisparity per cell recorded after the Fortunian in the Chapel Island Formation (Figures 3A, 3B, and 3D). Mixgrounds first appeared in distal settings and are later observed in shallower-water environments (Figure 6) in an

<footnote>slightly increased in concomitance with the development of tiering into the shallow-infaunal realm. The middle to late Fortunian (within the *Rusophycus avalonensis* ichno-assemblage zone) displays the highest number of styles of engineering in the offshore, notably with the appearance of excavators and regenerators and the deeper colonization of the substrate. The Cambrian Age 2 is marked by the development of bioturbation down to the deep-tier in all shallow-marine environments, the limited importance of surficial and semi-infaunal styles of engineering, and the absence of novel sediment interaction and modification modes in comparison with previous time intervals.</footnote>
<figure><img src="image_11.png" /><figcaption>**Figure 6.** Summary of ecologic and evolutionary events taking place within the Chapel Island Formation of Newfoundland, Canada</figcaption></figure>

Proximal (left) to distal (right), and oldest (base) to youngest (top) block diagrams are based on datasets from previous matrices (Figures 2 and 3, with the template of the matrix in the background). Yellow color corresponds to sand deposition, gray color to mud deposition, and white color to trace fossils and bioturbation. The Ediacaran was dominated by simple trails and burrows associated with microbial mats and corresponds to a matground ecology. On the shallow-marine seafloor, the early to middle Fortunian was characterized by a combination of both surficial trace fossils and burrows emplaced more deeply within the substrate and
expansion that cannot be fully deciphered in the Chapel Island Formation because shoreface and offshore strata are either absent or not well recorded across the late Fortunian to Cambrian Age 2 interval (Figures 1 and S8). A subtidal carbonate bed in PS 22 also displays intense bioturbation (Figure 2A), suggesting the development of mixgrounds in more proximal environments. In the shoreface, the presence of mid and deeply emplaced burrows strongly resembles younger nearshore endogenous communities.³⁹

In our study, time-environment matrices also demonstrate their full potential for tracking trends in ichnodiversity and ichno-disparity (Figures 2E, 2F, 3A, and 3B). In the *Treptichnus pedum* ichno-assemblage zone (PS 5–9), the ichnodiversity burst in early Fortunian lower offshore (PS 5) (Figures 2E and 3A) resulted from a marked increase in the diversity of simple horizontal trails (Figure 3D) that did not affect ichnodisparity. In the offshore transition, reduced stratal thickness may have negatively affected the number of trace fossils observed and would have potentially created a bias (Figure S8). In the upper offshore, however, reported numbers of data points are high (Figure S8), and the lower ichnodiversity found within the *Treptichnus pedum* ichno-assemblage zone is interpreted as a genuine evolutionary signal. Therefore, we regard the progressive landward diversification of behaviors from the lower offshore to upper offshore as an environmental expansion sensu Bottjer and Jablonski⁴⁸ that may be related to a lack of adaptation by early burrowing animals to withstand intense storm energy and avoid drifting. The colonization of early Fortunian lower offshore is in striking departure from the preferential settling of Ediacaran organisms in shoreface and offshore transition environments⁴⁹,⁵⁰ and demonstrates the physiologic abilities developed by early Fortunian animals to exploit new infaunal niches (although a preservational bias could be invoked as well). At the start of the *Rusophycus avalonensis* ichno-assemblage zone (PS 10–25), high ichnodisparity in the three offshore subenvironments (Figure 3B) was due to the rapid appearance of deposit-feeding sediment bulldozers (Figure 3F) that were living within the substrate and, therefore, were protected from the hazards of the water column hydrodynamics. Highly mobile and robust arthropods (Figure 3E) can also withstand higher energetic conditions and avoid passive dispersal.⁵¹ This “disparity-first” pattern taking place at the start of the *Rusophycus avalonensis* ichno-assemblage zone is typical of a novelty evolutionary event⁵² and represents the most common evolutionary radiation in the fossil record.⁵³,⁵⁴ Although a rapid trace-fossil diversification has already been noted at a global scale for the Cambrian explosion,¹⁷ our data from the Chapel Island Formation demonstrate a two-step diversification event—first with an inshore environmental expansion and second with a novelty event. Detailed studies elsewhere are needed to evaluate if this is a local, regional, or global phenomenon.

The three ecologic stages are identified for the first time in a single continuous succession spanning ca. 20 Ma across the Ediacaran-Cambrian Age 2 interval, and their timing and spatial trends can only be deciphered accurately through a high-

resolution analysis as presented herein. Although no other Ediacaran-Cambrian section worldwide has been studied at the same level of detail, a summary of global data suggests that the trends observed in the Chapel Island Formation were not local (Figure 7; Table S2).¹⁰ The fact that the patterns observed in the Chapel Island Formation can be broadly detected elsewhere suggests that this unit is representative of global trends in similar shallow-marine environments. During the Ediacaran, matgrounds were globally distributed (Figure 7A) and were present in broad depositional environments.⁶ Microbial and bacterial bindings provided a medium for organisms to feed and anchor⁶ and played a taphonomic role aiding the preservation of body fossils in sandy shorelines.⁵⁵ Low to moderate bioturbation intensity, preservation of surficial scratch imprints and discrete shallow-infaunal burrows, and high ichnodiversity suggesting altogether the presence of firmgrounds are also typical of Fortunian successions in northwestern and eastern Canada, China, Iran, Mongolia, Norway, Russia, and the USA, for instance, with a broad paleogeographic distribution covering Avalonia, Baltica, western and eastern Gondwana, Laurentia, Siberia, and the Zavkhan terrane (Figure 7B; Table S2). The later appearance of mixgrounds in Cambrian Age 2 rocks can be identified in other areas, such as China, England, Mongolia, Norway, Russia, the USA, and Wales (Figure 7C; Table S2),¹⁰ pointing to their global distribution across a wide range of latitudes. During Cambrian Age 2, matgrounds and firmgrounds locally formed patches in the sea floor (Figure 7C), revealing that although animals developed the ability to thoroughly process the sediment at depth, their interplay with environmental conditions was already complex and led to heterogeneous seascapes like those observed today.⁵⁶

Discriminating between evolutionary and environmental controls has been a persistent problem in paleobiology in general and in the study of the Ediacaran-Cambrian boundary interval in particular. Time-environment matrices allow differentiating between these two sets of factors by summarizing a large volume of information in a systematic and articulated fashion. We suggest that their use to scrutinize key episodes in the history of life, including during the Ediacaran-Cambrian geobiological revolution, will yield valuable insights.

## RESOURCE AVAILABILITY

### Lead contact

Further information and requests should be directed to and will be fulfilled by the lead contact, Romain Gougeon (gougeon.romain@gmail.com).

### Materials availability

All trace fossils and sedimentary rocks analyzed in this study have been observed in place without the collection of specimens. Localities investigated are mentioned in the text, and their geographic position is provided in Figure S1.

### Data and code availability

*   **Ichnologic and sedimentologic data have been deposited at Mendeley Data and are publicly available as of the date of publication. The DOI is listed in the key resources table.**

<footnote>demonstrates the mutual persistence of matground and firmground ecologies. During the late Fortunian, maximum bioturbation intensities and advances in substrate colonization resulted in the development of a mixground ecology, which dominates shallow-marine Cambrian Age 2 seafloors. In tidal flats, data are scarcer but still demonstrate the development of infaunal burrowing during the early Fortunian. Arrows at the tips of lines on the right side indicate that matgrounds developed earlier during the Ediacaran and mixgrounds extended later during the Phanerozoic while considering global data (see also Table S2).</footnote>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 7.** Global distribution of the three ecologic stages identified originally in the Chapel Island Formation in shallow-marine waters</figcaption></figure>

The late Ediacaran (A) is characterized by mat-
grounds and simple trophic interactions world-
wide. While the Fortunian (B) demonstrates a co-
dominance of matground and firmground ecolo-
gies, the Cambrian Age 2 (C) shows the appear-
ance of mixgrounds at all latitudes.

Base maps are from the PALEOMAP Project (http://scotese.com). See Table S2 for detailed dataset. 1, Chapel Island and Random formations, eastern Canada; 2, Ståhpogieddi and Breidvika formations, northern Norway; 3, Lublin, Wlodawa, and Mazovia formations, southeast Poland; 4, Basa Formation, western Russia; 5, Brioverian Series and Le Rozel Formation, northwestern France; 6, Floyd Church Formation, eastern USA; 7, Cijara Formation, central Spain; 8, Fowlers Gap Formation, southeast Australia; 9, Urusis and Nomtsas formations, southern Namibia; 10, Denyging, Zhujiaqing, and Shiyantou formations, southern China; 11, Miette Group, southwestern Canada; 12, Ingta Formation, northwestern Canada; 13, MacCodrum Formation, eastern Canada; 14, Voosi Formation, northwestern Estonia; 15, Torneträsk Formation, northern Sweden; 16, Khmelnitsky Formation, western Ukraine; 17, Dolkraals Formation, northwestern South Africa; 18, Puncoviscana Formation, northwestern Argentina; 19, Wood Canyon Formation, western USA; 20, Hinojosas Formation, southern Spain; 21, Kocayayla Group, western Turkey; 22, Arumbera Sandstone, central Australia; 23, Uratanna Formation, southern Australia; 24, Nagaur Group, northern India; 25, Stelkuz Formation, northwestern Canada; 26, Deep Spring and Campito formations, western USA; 27, Syhargalakh and Mattaia formations, northern Russia; 28, Soltanieh Formation, northern Iran; 29, Ratcliffe Brook Formation, eastern Canada; 30, Hartshill Formation, central England; 31, western USA (specific successions not available); 32, Purple Slate, Mottled Blue, and Green Slate groups, and St. Non's Sandstone, Wales; 33, Bayan Gol Formation, western Mongolia; 34, Bastion Formation, Greenland; and 35, Swithland Formation, central England.

* This paper does not report original code.

* Any additional information required to reanalyze the data reported in this paper is available from the [lead contact](#) upon request.

ACKNOWLEDGMENTS

We thank B. Novakovski for his help with sample processing. P. Myrow is
thanked for providing his field notes and for discussing different aspects of the
Chapel Island Formation. We are grateful to A. Liu and three anonymous referees

for their valuable critical feedback. We also thank R. Thomas for facilitating our
work in the Fortune Head Ecological Reserve under a Scientific Research Permit
from Parks and Natural Areas, Newfoundland and Labrador. Funding was pro-
vided by Natural Sciences and Engineering Research Council (NSERC) Discov-
erygrants 311726-13/422931-20 (L.A.B.), 311727-15/20 (M.G.M.), and 2020-
06741 (G.M.N.). M.G.M. is thankful for additional funding from the George J.
McLeod Enhancement Chair in Geology. R.G. acknowledges financial support
from the Geological Society of America (student geoscience grant 2018), the In-
ternational Association of Sedimentologists (postgraduate grant scheme 2016),
the Paleontological Society (student research grant 2018), the Society for
Sedimentary Geology (student assistance grant 2018), and the Interdisciplinary graduate School for the blue planet (ISblue). B.A.L. was funded by an NSERC Postgraduate Scholarship—Doctoral and a Cotutelle Research Excellence Scholarship from Macquarie University (Australia).

## AUTHOR CONTRIBUTIONS

R.G., L.A.B., M.G.M., and G.M.N. conceptualized the study and developed the methods. Field investigations were performed by R.G., B.A.L., and M.P., with data curated by R.G. L.A.B., M.G.M., and G.M.N. supervised the study. R.G. wrote the original draft and designed the illustrations. R.G. and N.J.M. did the quantitative analyses and evaluation of potential biases. All authors participated in the review and editing process of the manuscript.

## DECLARATION OF INTERESTS

The authors declare no competing interests.

## STAR★METHODS

Detailed methods are provided in the online version of this paper and include the following:

* **KEY RESOURCES TABLE**

* **EXPERIMENTAL MODEL AND STUDY PARTICIPANT DETAILS**

* Outcrops investigated

* **METHOD DETAILS**

* Ichnologic and sedimentologic data collection

* Description of depositional environments

* Parasequence (PS) delineation

* Construction of time-environment matrices

* Tiering analysis

* Ecospace cube analysis

* Ecosystem engineering cube analysis

* **QUANTIFICATION AND STATISTICAL ANALYSIS**

* Comparison of patterns of modes of life and styles of engineering in the Chapel Island Formation to global trends

* Principal component analysis

* **ADDITIONAL RESOURCES**

## SUPPLEMENTAL INFORMATION

Supplemental information can be found online at https://doi.org/10.1016/j.cub.2024.11.028.

Received: April 25, 2024

Revised: June 19, 2024

Accepted: November 15, 2024

Published: December 23, 2024

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49. Grazhdankin, D. (2014). Patterns of evolution of the Ediacaran soft-bodied biota. J. Paleontol. 88, 269-283. https://doi.org/10.1666/13-072.

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54. Minter, N.J., Buatois, L.A., Mángano, M.G., Davies, N.S., Gibling, M.R., MacNaughton, R.B., and Labandeira, C.C. (2017). Early bursts of diversification defined the faunal colonization of land. Nat. Ecol. Evol. 1, 0175. https://doi.org/10.1038/s41559-017-0175.

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61. MacEachern, J.A., Zaitlin, B.A., and Pemberton, S.G. (1999). A sharp-based sandstone of the Viking Formation, Joffre Field, Alberta, Canada: criteria for recognition of transgressively incised shoreface complexes. Journal of Sedimentary Research 69, 876-892. https://doi.org/10.2110/jsr.69.876

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## STAR★METHODS

### KEY RESOURCES TABLE

<table><thead><tr><th>REAGENT or RESOURCE</th><th>SOURCE</th><th>IDENTIFIER</th></tr></thead><tbody><tr><td>Deposited data</td><td></td><td></td></tr><tr><td>Raw ichnologic and sedimentologic data</td><td>Mendeley Data</td><td>https://doi.org/10.17632/g2g74cj4x7.1</td></tr></tbody></table>

### EXPERIMENTAL MODEL AND STUDY PARTICIPANT DETAILS

#### Outcrops investigated

Sedimentologic and ichnologic datasets were gathered during four field seasons (summers of 2016, 2017, 2019, and 2021, thirteen weeks in total) from sections that crop out in coastal cliffs at Burin Peninsula (Figure S1). The different nature of outcrops in the localities studied has been systematically assessed.²² Sections at Grand Bank Head, Fortune Head, and Little Dantzic Cove are renowned for their excellent exposure of strata, which are continuous stratigraphically and laterally extensive (with outcrops from a few meters to ca. forty meters wide).²¹,²² No significant variation in quality and size of outcrop for the different sedimentary facies is apparent, and the three sections were then used to create the composite stratigraphic column of Figure 1. In addition, three coeval sections of the Chapel Island Formation were investigated at Fortune North, Lewin's Cove, and Point May (Figure S1). Although these outcrops are smaller and of lower quality of exposure,²² they can still be used to record ichnologic and sedimentologic attributes.

### METHOD DETAILS

#### Ichnologic and sedimentologic data collection

In total, twenty-five sedimentary logs were drawn at 1:40, representing 1,708.2 m of strata.³¹ Nine of these logs coming from Grand Bank Head, Fortune Head, and Little Dantzic Cove were used to build the composite stratigraphic column of Figure 1 which is 846.8 m thick. For the sedimentologic dataset, focus was placed on: (1) bed geometry; (2) bed thickness; (3) grain size; (4) sandstone/mudstone ratio; and (5) sedimentary structures. In addition, six high-resolution logs, representing 7.13 m of strata in total, were measured at 1:1.67 from different intervals to support general observations.³¹ Along each log, a trace-fossil dataset was reported and consists of: (1) bioturbation index (BI); (2) bedding plane bioturbation index (BPBI); (3) burrow width (BW); (4) burrow depth (BD); and (5) ichnotaxonomic affiliation. BI evaluates vertical bioturbation³² and can be summarized as: BI = 0 for no bioturbation (0%); BI = 1 for sparse bioturbation (1–5%); BI = 2 for low bioturbation (6–30%); BI = 3 for moderate bioturbation (31–60%); BI = 4 for high bioturbation (61–90%); BI = 5 for intense bioturbation (91–99%), and BI = 6 for complete bioturbation (100%). BPBI evaluates bed surface bioturbation³³ and can be summarized as: BPBI = 1 for no bioturbation (0%); BPBI = 2 for low bioturbation (1–10%); BPBI = 3 for low to moderate bioturbation (11–40%); BPBI = 4 for moderate to high bioturbation (41–60%), and BPBI = 5 for intense bioturbation (61–100%). BW and BD were recorded in the field using a manual caliper with a 1 mm accuracy. Ichnotaxonomy follows standard procedures in ichnology which consists of the description of ichnotaxobases and other significant morphologic features.³⁹,⁴⁷,⁵⁷ Total reports on each trace-fossil metric are: (1) 824 data points for BI; (2) 892 data points for BPBI; (3) 1,886 data points for BW; (4) 784 data points for BD, and (5) 2,121 ichnotaxonomic identifications. Reports per cell for each trace-fossil metric show that: (1) 92% of cells have at least one data point on BI, and 68% at least four; (2) 94% of cells have at least one data point on BPBI, and 62% at least four; (3) 88% of cells have at least one data point on BW, and 70% at least four, and (4) 61% of cells have at least one data point on BD, and 36% at least four (Figures S7A–S7D). Cells with no data points usually result from the limited stratigraphic extent of the corresponding sedimentary facies (Figure S8). Overall, this is the largest dataset of this sort compiled so far for any succession worldwide (846.8 m of strata studied at 1:40; 1 BI value reported every 1.03 m; 1 BPBI value reported every 0.95 m; 1 BW value reported every 0.45 m; 1 BD value reported every 1.02 m, and 1 ichnotaxon reported every 0.40 m).

#### Description of depositional environments

Sedimentary facies of the Chapel Island Formation were described in refs.⁵⁸,⁵⁹ and their characteristics are detailed in Table S4. In the Chapel Island Formation, the subdivision of intertidal subenvironments follows previous models⁶⁰ and corresponds to: (1) mud-flat; (2) mixed-flat; (3) sand-flat, and (4) tide dominated- or influenced embayments. Shallow-marine subenvironments also follow previous models¹³,³⁸,⁶¹ and are subdivided into: (5) middle shoreface; (6) lower shoreface; (7) offshore transition; (8) upper offshore; (9) lower offshore, and (10) shelf (sensu stricto). The shoreface is located below the breaker zone and above fair-weather wave base. In the Chapel Island Formation, the upper shoreface has not been recorded but was added to matrices as it represents the intermediate environment between the distal part of intertidal environments and the proximal part of shallow-marine ones. The offshore corresponds to the zone below fair-weather wave base and above storm-weather wave base. The shelf is found below
storm-weather wave base and above the continental shelf break. Two limestone beds are found within M4 at Little Dantzic Cove.<sup>58,59</sup> The first one, located 47.4 m above the base of M4, is interpreted as being deposited in a shallow-marine subtidal environment. The second limestone bed is located at the top of M4 and was deposited in an intertidal environment.

## Parasequence (PS) delineation

A PS corresponds to a stratigraphic unit composed of stacked, genetically related beds or bedsets forming a shallow-upward succession and bounded by flooding surfaces.<sup>62</sup> These consist of fining-upward successions in tide-dominated settings and of coarsening-upward successions in wave-dominated shallow-marine environments.<sup>38,60</sup> Twenty-five PS were identified on high-resolution logs (see Gougeon et al.<sup>31</sup> for logs). PS are 15.5 to 74.5 m thick, with boundaries controlled by the position of distinct flooding surfaces, and each encompass two to six subenvironments. The Ediacaran–Cambrian boundary is located within PS 5. Therefore, PS 5 is further subdivided into two intervals to discriminate precisely Ediacaran and Fortunian datasets and to acknowledge the presence of offshore deposits in the latest Ediacaran. The Fortunian–Cambrian Age 2 boundary, marked by the first appearance of the small shelly fossils *Aldanella attleborensis* and *Watsonella crosbyi* (although still pending formal decision<sup>34</sup>), is located within PS 20. Consequently, PS 20 is split into two intervals.

## Construction of time-environment matrices

Extensive ichnologic and sedimentologic datasets were compiled<sup>31</sup> and are required for the creation of high-resolution time-environment matrices (see also MacNaughton and Narbonne<sup>27</sup> for a comparable approach in Ediacaran–Cambrian strata of north-western Canada). A composite section of the Chapel Island Formation was created based on measured sedimentologic logs and following stratigraphic correlations established by previous authors.<sup>21,58</sup> Twenty-five PS (plotted on the y-axis of the matrix) are delineated based on the identification of flooding surfaces in the Chapel Island Formation and encompass late Ediacaran (PS 1–5), Fortunian (PS 5–20), and Cambrian Age 2 (PS 20–25) strata (Figure 1). Ten subenvironments (plotted on the x-axis of the matrix) are described from siliciclastic strata in the Chapel Island Formation, encompassing intertidal settings (mud-flat, mixed-flat, sand-flat, and tidal embayment), shallow-marine shoreface (middle and lower shoreface), shallow-marine offshore (offshore transition, upper offshore, and lower offshore), and shelf (sensu stricto) (Figure 1, see previous section). Limestone beds in Cambrian Age 2 deposits have broader environmental interpretations and were formed under subtidal (PS 21) and intertidal (PS 22) conditions. Therefore, an upper shoreface was added to the matrices to account for the lateral equivalence of the subtidal zone with the shoreface.<sup>63</sup> Intertidal deposits are present in the Ediacaran (PS1–4), Fortunian (PS 8, 9), and Cambrian Stage 2 (PS 22), although the latter is represented by a single medium-bedded carbonate interval that provides limited sedimentologic and ichnologic information. Wave-dominated shallow-marine deposits are also present in the Ediacaran (PS 5), Fortunian (PS 5–7, 9–20), and Cambrian Stage 2 (PS 20–25). In total, one hundred cells (i.e., specific time-environment loci in the matrix) are identified in the Chapel Island Formation, with a greater number in shallow-marine (eighty-three cells) than intertidal (seventeen cells) environments. Lower offshore deposits are the most common (recorded in twenty PS), whereas tidal bay deposits are the least represented (recorded in one PS). The matrices were used to track changes in individual ichnotaxa (Figures S2–S6) and in other trace-fossil proxies (Figures 2 and 3), and were accommodated to evaluate ecospace utilization and ecosystem engineering (Figures 4 and 5).

## Tiering analysis

Tiering corresponds to the vertical partitioning of trace fossils within the substrate.<sup>39,47</sup> Deciphering the original position of a trace fossil at its time of formation depends on understanding its three-dimensional morphology, its dimension (notably its depth), and cross-cutting relationships with other trace fossils. For instance, *Rosselia erecta* and ?Diplocraterion isp. that are observed on bed surfaces in Cambrian Stage 2 strata of the Chapel Island Formation do not provide insights on precise burrow depth but still inform on the development of mid- and deep-tier trace fossils.<sup>13</sup> Tiering can be subdivided into<sup>44</sup>: (1) surficial (0.0 cm deep); (2) semi-infaunal (0.1–0.5 cm deep); (3) very shallow infaunal (0.6–3.0 cm deep); (4) shallow infaunal (3.1–6.0 cm deep); (5) intermediate infaunal (6.1–12.0 cm deep), and (6) deep infaunal (12.1+ cm deep). In this study, tiering interpretations follow refs.<sup>13,44,54</sup> (with exceptions detailed below) and are summarized in Table S1. Delicate scratch imprints (*Allocotichnus*, *Dimorphichnus*, and *Monomorphichnus*) are formed on the sea bottom and are considered surficial. In the Chapel Island Formation, *Archaeonassa* and *Dendroidichnites* are trace fossils that involve plowing the sediment surface and are considered surficial. Simple horizontal trails (*Circulichnis*, *Cochlichnus*, *Gordia*, *Helminthoidichnites*, and *Helminthopsis*) are semi-infaunal as they were made by organisms ploughing the sediment surface or displacing the sediment just below the surface; they are also typically associated with shallow endobenthic microbial mats. In the Chapel Island Formation, *Cruziana* is small (up to 0.5 cm in width and depth), left opened to the surface, and is semi-infaunal. *Rusophycus* is made of scratch imprints organized in two lobes that bulge from the bed surface. Therefore, *Rusophycus* is regarded as semi-infaunal. In the Chapel Island Formation, *Bergaueria* extends within the reach of the very shallow infaunal (0.2–1.6 cm deep reported for *Bergaueria perata*, and 1.3–1.8 cm deep for *Bergaueria cf. radiata*), and *Conichnus* specimens from Cambrian Stage 2 even extend deeper into the intermediate infaunal (0.3–10.5 cm deep for *Conichnus conicus*). Therefore, *Bergaueria* is here considered as a very shallow infaunal burrow, whereas *Conichnus* is either considered as very shallow infaunal or intermediate infaunal on a case-by-case basis. *Palaeophycus* is an open subhorizontal burrow that is typically 1–2 cm deep and is very shallow infaunal in the Chapel Island Formation. *Torrowangea* is a horizontal backfilled burrow associated with microbially modified surfaces and is here regarded as very shallow infaunal. *Treptichnus*
is a burrow system that is emplaced close to the sediment-water interface and is considered very shallow infaunal. *Saerichnites*, a gallery system made of a basal horizontal burrow from which depart vertical shafts, is larger than *Treptichnus* but has not been found extending at great depth in the Chapel Island Formation; it is therefore very shallow infaunal. Sediment bulldozing trace fosssils (*Curvolithus*, *Didymaulichnus*, and *Psammichnites*) are made infaunally but did not go deep within the substrate. *Curvolithus* tends to display a slight vertical undulation,⁶⁴ whereas *Psammichnites* tracemaker kept in contact with the sediment-water interface using a snorkel-like device.⁴⁶ Therefore, sediment bulldozing trace fossils are best regarded as shallow infaunal. Helical burrows of *Gyrolithes* extend as deep as 5.0 cm in the Chapel Island Formation and are shallow infaunal. Although an incipient *Trichichnus linearis* was reported to develop as deep as 2 m in modern sediments,⁶⁵ Chapel Island Formation material only reaches 4.1 cm in depth and is considered shallow infaunal. *Rosselia* is a spindle-shaped vertical burrow that can be up to 9 cm deep in the Cambrian⁶⁶ and, therefore, is considered an intermediate infaunal structure. *Arenicolites* is a vertical U-shaped burrow and is intermediate infaunal. *Teichichnus* is a vertical spreiten burrow formed by the displacement of a subhorizontal burrow and is intermediate infaunal. *Diplocraterion* is only expressed on bed tops in the Chapel Island Formation, but it is traditionally regarded as a deep infaunal trace fossil on well exposed vertical specimens. Finally, large vertical pyritized burrows of *Trichichnus* isp. are common in Cambrian Stage 2 strata of the Chapel Island Formation (see Gougeon⁵⁹ for details on their morphology). These burrows can extend down to 57.0 cm and are deep infaunal.

## Ecospace cube analysis

Block diagrams are created based on refs. ¹³,⁴⁴,⁵⁴,⁶⁷ after Bambach et al.,⁶⁸ and information is summarized in Table S1. A mode of life represents an individual ecospace utilization niche that is characterized by a unique combination of tiering, feeding mode, and motility. In our analysis, 126 modes of life are depicted on each block diagram, although in reality not all of these modes are represented in the natural world. In addition, the trace-fossil record in soft sediments is biased towards motile and facultatively motile organisms, and none of the non-motile unit cubes are filled (Figure 4). To evaluate the breadth of our data within each time interval and specific environment, the occupancy ratio (OR) is introduced. For each block diagram, OR = "number of modes of life per time interval and subenvironment in the Chapel Island Formation"/"number of modes of life per time interval and subenvironment from global data", where the latter is provided in the supplementary material of Buatois et al.¹³ and is depicted on Figure 4.

## Ecosystem engineering cube analysis

Ecosystem engineering is a fairly modern concept in ecology that deals with animals that change their environment actively by modifying, maintaining, and creating habitats, ultimately impacting on resource availability to themselves and others.⁶⁹ Block diagrams are designed following the method of Minter et al.⁶⁷ Information is summarized in Table S1. A style of engineering corresponds to an individual ecosystem engineering niche resulting from a unique combination of tiering, sediment interaction mode, and sediment modification mode. Although 96 styles of engineering are displayed on each three-dimensional cube, not all those styles can actually be realized in the natural world. For instance, a gallery biodiffusing structure requires a three-dimensional interaction with the substrate and, hence, cannot be realized on the surficial tier. As for the multidimensional ecospace cube analysis, an occupancy ratio (OR) is calculated to evaluate the breadth of our data within each time interval and specific environment. For each block diagram, OR = "number of styles of engineering per time interval and subenvironment in the Chapel Island Formation"/"number of styles of engineering per time interval and subenvironment from global data", where the latter is provided in the supplementary material of Buatois et al.¹³ and is depicted on Figure 5.

## QUANTIFICATION AND STATISTICAL ANALYSIS

### Comparison of patterns of modes of life and styles of engineering in the Chapel Island Formation to global trends

To evaluate whether patterns of modes of life and styles of engineering from the Chapel Island Formation could be related to global trends, two sets of three indices were calculated (Table S3; see also Figures S10 and S11). The first set is based on the presence of a mode of life or style of engineering unit cube: a = "number of occupied unit cubes exclusive to the rest of the world"/"total number of occupied unit cubes"; b = "number of occupied unit cubes exclusive to the Chapel Island Formation"/"total number of occupied unit cubes"; and c = "number of occupied unit cubes shared by the rest of the world and Chapel Island Formation"/"total number of occupied unit cubes". Together, a + b + c = 1. The closer the index for a particular dataset is to 1, the greater influence it has on the global presence-absence dataset of modes of life and styles of engineering. The second set of indices incorporates how common or rare a particular mode of life or style of engineering is across the global dataset of assemblages: x = "sum of number of occurrences of each occupied unit cube exclusive to rest of world"/"sum of number of occurrences of all occupied unit cubes"; y = "sum of number of occurrences of each occupied unit cube exclusive to the Chapel Island Formation"/"sum of number of occurrences of all occupied unit cubes"; and z = "sum of number of occurrences of each occupied unit cube shared by the rest of the world and Chapel Island Formation"/"sum of number of occurrences of all occupied unit cubes". Together, x + y + z = 1. Values of indices for a particular dataset that are closer to 1 indicate that the dataset contains the most common modes of life or styles of engineering and so is more consistent with the global trends, whereas indices for a particular dataset that have values closer to 0 indicate that the dataset has modes of life or styles of ecosystem engineering that are rare and not especially representative of global trends.
**Principal component analysis**

A principal component analysis was performed to evaluate correlations between all the metrics investigated in the study using the PAST software (Figure S9). Raw data and results based on correlation and variance-covariance matrices are available in Tables S5, S6, and S7.

**ADDITIONAL RESOURCES**

There are no additional resources to report.