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Home , Ediacaran, Ediacaran biota, Pteridinium, Swartpuntia

Received: 14 June 2018 | Revised: 6 October 2018 | Accepted: 19 October 2018 DOI: 10.1111/ter.12368

PAPER

New high‐resolution age data from the – boundary indicate rapid, ecologically driven onset of the

Ulf Linnemann1 | Maria Ovtcharova2 | Urs Schaltegger2 | Andreas Gärtner1 | Michael Hautmann3 | Gerd Geyer4 | Patricia Vickers-Rich5,6,7 | Tom Rich6 | Birgit Plessen8 | Mandy Hofmann1 | Johannes Zieger1 | Rita Krause1 | Les Kriesfeld7 | Jeff Smith7

1Senckenberg Collections of Natural History Dresden, Museum of Mineralogy Abstract and Geology, Dresden, Germany The replacement of the late by morphologically disparate 2Département des Sciences de la Terre, at the beginning of the was a key event in the history of on University of Geneva, Genève, Switzerland ‐ 3Paläontologisches Institut und Museum, , the mechanisms and the time scales of which are not entirely understood. A Zürich, Switzerland composite section in providing biostratigraphic and chemostratigraphic data 4 Bayerische Julius-Maximilians-Universität, bracketed by constrains the Ediacaran–Cambrian boundary to Lehrstuhl für Geodynamik und Geomaterialforschung, Würzburg, Germany 538.6–538.8 Ma, more than 2 Ma younger than previously assumed. The U–Pb‐CA‐ID 5Department of Chemistry and TIMS ages demonstrate an ultrashort time frame for the LAD of the Ediacaran Biotechnology, Swinburne University of Technology, Melbourne (Hawthorne), biota to the FAD of a complex, burrowing Phanerozoic biota represented by trace fos- Victoria, Australia sils to a 410 ka time window of 538.99 ± 0.21 Ma to 538.58 ± 0.19 Ma. The extre- 6 Museums Victoria, Melbourne, Victoria, mely short duration of the faunal transition from Ediacaran to Cambrian biota within Australia less than 410 ka supports models of ecological cascades that followed the - 7School of Earth, and Environment, Monash University, ary breakthrough of increased mobility at the beginning of the Phanerozoic. Melbourne (Clayton), Victoria, Australia 8Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany

Correspondence Ulf Linnemann, Senckenberg Collections of Natural History Dresden, Museum of Mineralogy and Geology, Dresden, Germany. Email: [email protected]

Funding information University of Geneva; Swiss National Science Foundation; GeoPlasmaLab Dresden; Senckenberg Naturforschende Gesellschaft; Deutsche Forschungsgemeinschaft, Grant/ Award Number: GE 549/22-1

1 | INTRODUCTION a key event in the , which has been explained by environ- mental, evolutionary or ecological factors, possibly superimposed by a The replacement of the enigmatic Ediacaran biota by morphologically major taphonomic bias (e.g., Darroch et al., 2015; Laflamme, Darroch, disparate animals (metazoans) at the beginning of the Phanerozoic was Tweedt, Peterson, & Erwin, 2001; Muscente, Boag, Bykova, &

| Terra Nova. 2019;31:49–58. wileyonlinelibrary.com/journal/ter © 2018 John Wiley & Sons Ltd 49 50 | LINNEMANN ET AL.

Schiffbauer, 2018; Schiffbauer et al., 2016). These hypotheses predict rapid appearance of complex trace , formally defined by the different time‐scales for this turnover; however, age data with sufficient lower boundary of the pedum Assemblage Zone in the time resolution have been lacking to date. Ediacaran–Cambrian bound- GSSP at , . There, T. pedum appears for ary sections on the Swartpunt and Swartkloofberg farms in Southern the first time (Brasier, Cowie, & Taylor, 1994; Geyer & Landing, Namibia (Saylor & Grotzinger, 1996) combine rich palaeontological data 2016; Landing, 1994; Peng et al., 2012), which is otherwise recog- (Darroch et al., 2015) with the presence of tuff layers that allow high‐ nizable by the first pronounced in earth history (Bua- precision radio‐isotopic age determination. Grotzinger, Bowring, Saylor, tois, Almond, Mángano, Jensen, & Germs, 2018; Buatois & Mángano, and Kaufman (1995) published pioneering work in dating these tuffs, 2016). This concept appears to conflict with bilaterian trace fossils in using multigrain air‐abrasion U–Pb on zircon ID‐TIMS technique. Their the Ediacaran (e.g., Chen, Chen, Zhou, Yuan, & Xiao, 2018), albeit analytical precision was insufficient to quantify the time‐scale necessary considered subhorizontal traces of undermat miners. This boundary for establishing new metazoans. Here, we present new high‐precision was previously noted to occur around 541.00 ± 0.81 Ma (Amthor et U–Pb‐CA‐ID TIMS zircon ages from these tuff layers that provide a al., 2003; Bowring et al., 2007), based on a U–Pb zircon age of an much refined age datum for the beginning of the Cambrian and the first ash sample from the Ara Group of Oman. This ash occurs just below absolute ages for the evolutionary transition from the Ediacaran biota a sequence that records a negative δ13C isotope excursion termed to the existence of complex Phanerozoic trace makers. the Basal Cambrian Isotope Excursion (BACE; Zhu, Zhang, Li, & Yang, 2004) and immediately above strata hosting the biomineral- ized tubular Cloudina. However, biostratigraphic data are com- 2 | DEFINITION AND TIME FRAME OF THE pletely lacking from the strata above the ash layer. The GSSP EDIACARAN–CAMBRIAN BOUNDARY section for the Ediacaran–Cambrian boundary in Avalonian New- foundland lacks the soft‐bodied Ediacaran biota as well as dateable The Ediacaran–Cambrian boundary is characterized by the appear- ash beds (Brasier et al., 1994; Geyer & Landing, 2016). From the ance of advanced (i.e., ). Evidence is the rather boundary sections of the Yangtze/South China sequence, the

Karibib Okahandja

Windhoek Swakopmund Gobabis Walvis Bay

Witvlei Subbasin

Rehoboth

24°

Zaris Subbasin Mariental

Osis Arch

Atlantic Ocean Helmeringhausen 26°

Keetmannshoop Aus Lüderitz Witputs Subbasin

Thrust working Fault area Gariep belt Cover Fish River Subgroup Rosh Karasburg 28° Schwarzrand Subgr. Pinah Nama Kuibis Subgroup Group Naukluft Nappe Complex Gariep Group FIGURE 1 Geological map of southern 100 km Damara Supergroup Oranjemund Namibia and the Nama basin (Grotzinger Pre-Damara 16° 18° and Miller, 2008) LINNEMANN ET AL. | 51

Alluvium Pinnacle reefs Nomtsas Formation Upper Middle Lower Thrust fault thrust thrust quarry thrust Spitskop Member plate plate plate Feldschuhhorn Member (A) (B) Swartkloofberg(C) Road Huns Member

Nudaus Formation incl. Nasep Member

S Kuibis Subgroup F Swartkloofberg section Rocks older than (tillite etc.) (unit G)

Swartpunt section (units A-F)

rg tbe un tp war S

Swartpunt Swartkloof

North Witpütz N Swartpunt State Witpütz Land F 16°30´

ierkloof 27°35´ T North Witpütz Aub

5 km South Witpütz Kolke

FIGURE 2 Geological map of the farms Swartkloof, Swartpunt and Nord‐Witpütz. Note location of the Swartpunt section (units A–F) and the Swartkloofberg section (unit G) (modified from Saylor & Grotzinger, 1996)

Ediacaran–Cambrian boundary was dated between 542.6 ± 3.7 Ma the Swartpunt section into units A–F, whereas the nearby Swartk- and 536.3 ± 5.5 Ma (Chen, Zhou, Fu, Wang, & Yan, 2015). Ash beds loofberg section represents a terminal unit G (Figures 2 and 3). Units related to this boundary are yet unknown from other relevant sec- A–F comprise a 139 m thick intercalation of limestone, and tions globally, such as those in the (Australia), of the upper Spitskop Member. Rapid regional uplift led to , the Ukraine (Brasier et al., 1994), the White Inyo–Death Val- a hiatus in caused by incision of canyons into the Spitskop ley–Mojave regions (California) (Hagadorn, Fedo, & Waggoner, 2000) Member. Sedimentary infilling of these valleys forms unit G of the or the Mount Dunfee section (Nevada) (Smith et al., 2016). Nomtsas Formation (Saylor & Grotzinger, 1996). Ash beds in the Swartpunt and Swartkloofberg sections have been dated at 545.1 ± 1 Ma (middle part of the Spitskop Member, 3 | NAMIBIAN KEY SECTIONS AND NEW upper Ediacaran), 543.3 ± 1 Ma (Spitskop Member, unit A of this ZIRCON AGES paper) and 539.4 ± 1 Ma (Nomtsas Formation, unit G of this paper) (Bowring et al., 2007; Grotzinger et al., 1995). These ages were Sections of the upper Ediacaran Spitskop Member (Urusis Formation, recalculated to 542.68 ± 2.8 Ma for the middle part of the Spitskop Schwarzrand Group) and the lower Cambrian Nomtsas Formation Member, 540.61 ± 0.67 Ma for the upper part of the Spitskop Mem- (Nama Group) are preserved in the Witputs Subbasin of the Nama ber and 538.18 ± 1.11 Ma for the base of the Nomtsas Formation Basin (Figure 1) and exposed on Swartpunt and Swartkloofberg farms (Schmitz, 2012). in southern Namibia (Figure 2; Saylor & Grotzinger, 1996). These sec- The Ediacaran /erniettomorph biota, including such tions are invaluable for studying the development of complex life dur- forms as germsi and simplex (Figures 3, 4a and ing the Ediacaran–Cambrian transition as they contain a unique 6) (Narbonne, Saylor, & Grotzinger, 1997; Narbonne, Xiao, & Shields, association of ash beds suitable for radiometric dating, carbonates 2012; Saylor & Grotzinger, 1996), occur in siliciclastic, storm‐domi- useful for stable isotope chemostratigraphy, and imprints of soft‐bod- nated shelf deposits of unit D but are absent above metre 107. The ied and biomineralized remains of the Ediacaran biota (Figure 3). Ediacaran–Cambrian transition interval (ECTI; Figure 3) is represented Importantly, this sequence also includes trace fossils indicative of by an 18 m thick limestone (unit E). Metres 125 to 128 of unit F con- bilaterian metazoans that are significant for biostratigraphic analysis. tain an association of diverse trace fossils, including branched forms Recent field studies of these outcrops have resulted in the division of such as Streptichnus narbonnei (Jensen & Runnegar, 2005) and 52 | LINNEMANN ET AL.

<= T g <= ash 6 (538.58 ± 0.19 Ma) T - Treptichnus pedum

(section 70 m, not to scale) T cf - Treptichnus cf. pedum

Nomtsas Fm. St - Streptichnus narbonnei 13C (carb ‰)

N - δ [m] C - Cloudina m139 P - Pteridinium simplex f Sw - Swartpuntia germsi Cambrian 14m 130 <= C + N <= St + T cf m125

120 boundary e

18m ECTI

m107 9 <= P 100 8 d 7 <= Sw 16m 90 6

m81 80

<= ash 5 (538.99 ± 0.21 Ma)

70 <= ash 4 (539.64 ± 0.19 Ma)

c 60 <= ash 3 (539.52 ± 0.14 Ma) Ediacaran 42m <= ash 2 (539.58 ± 0.34 Ma)

Spitskop Member 50

5 40 FIGURE 3 m39 4 Geological sections at Swartpunt (units a–f) and Swartkloofberg 3 (unit g) indicating the Ediacaran–Cambrian

b late Ediacaran Positive Carbon Isotope Plateau (EPIP) 30 15m 2 boundary interval in the Swartpunt and m24 Swartkloofberg sections. Precise U–Pb 1 ‐ ‐ 20 ages obtained by CA ID TIMS techniques with uncertainties given at 2 sigma level, carbon isotope values and fossil horizons. a 10 1—debris flow, shale, olistoliths; 2—shale, 24m <= ash 1 (540.095 ± 0.099 Ma) sandstone, conglomerate; 3—grey‐green sandstone, 4—greenish shale; 5—grey m0 0 thick‐bedded micrite; 6—grey thin‐bedded <= Sw micrite; 7—black thick‐bedded micrite; 8— -2 -1 0 1 2 black thin‐bedded micrite; 9—ash bed

Treptichnus cf. pedum (Figures 4 (e‐f) and 4 (h–j), and other ichnofos- 2016; Smith, Nelson, Tweedt, Zeng, & Workman, 2017). These fossil sils of and Phanerozoic aspect (Figure 4e–j). A recent study assemblages indicate a progressive rise of more complex organisms, also emphasizes the presence of trace fossils produced by sediment peaked by the advent of complex and burrowing metazoans responsi- bulldozers in this part of the section, which “may in fact be regarded ble for the successive reduction in the extent of microbial mats above as a representative of Cambrian‐style bioturbation” (Buatois et al., a 547.36 ± 0.23 Ma old ash (Bowring et al., 2007). The 2018, p. 3). Simple, Ediacaran‐type traces are represented by interval is overlain by a black, thin‐bedded micrite with biomineralized Helminthopsis, which are also known from older strata of the Huns Ediacaran taxa such as Cloudina and Namacalathus. Member and the Nudaus Formation, as are the non‐ or poorly miner- The trace fossil assemblage in the 3 m thick unit F in the Swartpunt alized body fossils Gaojiashania and Shaanxilithes (Darroch et al., section marks the emergence of Cambrian (and Phanerozoic‐type) LINNEMANN ET AL. | 53

FIGURE 4 Fossils from the Swartpunt (h) and Swartkloofberg sections. (a) Pteridinium simplex Gürich, 1930; Spitskop Member, unit D, metre 104. (b) Streptichnus narbonnei Jensen & Runnegar, 2005; Spitskop Member, unit F, metre 126. (c) (i) (g) Unusually small specimen assigned to Streptichnus narbonnei Jensen & Runnegar, 2005; Spitskop Member, unit F meter 127. (d) Treptichnus cf. pedum (Seilacher, 1955), garlands crossing each other; Spitskop Member, unit F, meter 127. (e) (j) Incompletely preserved vertical burrows resembling Bergaueria; unit F meter 127. (f) Branched traces with secondary weak mineralization of outer surface (arrows (d) point to branching points), tentatively assigned to Olenichnus; Spitskop Member, unit F, metre 127. (g) Shallow horizontal burrows with different types of annulations (arrows) suggesting a spiral burrow; Spitskop Member, unit F, metre (c) (e) (f) 127. (h) Cochlichnus isp.; from Nomtsas Formation, unit G. (i) Trace fossil (b) (a) assemblage with simple ‐type horizontal traces crossing each other, associated with a string‐of‐pearl‐type or pelleted trace (arrow in lower left corner) and delicate traces composed of elongate probes (arrow near right margin); Nomtsas Formation, unit G above ash 6. (j) Irregularly sinuous pelleted trace with subregular constrictions; Nomtsas Formation, unit G. White scale bars (in b, d, e, g, h, j) equal 5 mm, black scale bars (in c, f) 5 mm. Coin diameter 22.6 mm (in a, i)

advanced bilaterians, represented by Streptichnus narbonnei and Trep- table see Supporting Information Data S1, Table S1; the results are tichnus cf. pedum. Streptichnus narbonnei reflects the complex behaviour presented in Figure 5). Ash 1, located in unit A, has yielded an age of of its producer, which corresponds to that displayed by T. pedum.Speci- 540.095 ± 0.099 Ma. Up‐section in unit C in ascending stratigraphic mens assigned to T. pedum from the younger, , Rosenhof order, ashes 2–5 have depositional ages of 539.58 ± 0.34 Ma, Member of the Fish River Subgroup (Geyer & Uchman, 1995) show a 539.52 ± 0.14 Ma, 539.64 ± 0.19 Ma and 538.99 ± 0.21 Ma. In unit transition between the two ichnospecies. The rather sudden appear- G of the lower Nomtsas Formation, the 25 cm thick ash 6 ance of complex traces in the Ediacaran–Cambrian boundary section at (538.58 ± 0.19 Ma) exhibits features similar to those of older ashes the GSSP in Newfoundland (Geyer & Landing, 2016), therefore, and has been ripped into metre‐sized fragments. Due to the wide dis- matches a similar appearance of such traces with Phanerozoic aspect in tribution of related fragments over several decametres, we assume ash the Namibian sections (Figure 4e–j), where the lowest occurrence of 6 is a primary ash bed in the Nomtsas Formation, which has been frag- T. cf. pedum is in unit F (Figures 3 and 4d). mented during sediment deposition. Alternatively and less probable, In the Swartpunt section (Figure 3), ash beds crop out as 8 to ash 6 could be reworked material from the underlying Spitskop Mem- 80 cm thick, whitish‐greenish, splintery, silicified and weathering‐resis- ber. If so, its age of 538.58 ± 0.19 Ma provides a maximum deposi- tant layers. U–Pb age determinations were performed applying CA‐ID‐ tional age of the Nomtsas Formation. In any case, this age provides a TIMS to zircon grains, using the EARTHTIME 205Pb–233U–235U tracer minimum age for the base of the Cambrian. Even if ash 6 occurred pri- solution (ET 535, http://www.earthtime.org) (for methods and data mary in the Spitskop Member, it must be younger than ash 5 54 | LINNEMANN ET AL.

FIGURE 5 Concordia diagrams of the CA‐ID‐TIMS U‐Pb zircon data (for position of ashes in the section see Figures 3 and 6)

(538.99 ± 0.21 Ma) and also younger than the Cambrian fossil‐bearing Isotope Plateau (EPIP) and seemingly below the BACE, according to bed in unit F, because no additional ash bed exists between ash 5 and Zhu, Zhuravlev, Wood, Zhao, and Sukhov (2017). The BACE does not the Cambrian fossil assemblage at metre 127 of the Swartpunt section. occur in the section (Figure 3). Reasons could be (a) a shallow bathy- It should be noted that another ash bed aged 538.18 ± 1.11 Ma is metry of the section, (b) that the BACE is possibly not global, or (c) reported from unit G in the Nomtsas Formation (Grotzinger et al., the Ediacaran–Cambrian boundary, at least in the Swartpunt section, 1995; recalculated by Schmitz, 2012). is far below the BACE. However, the age determinations suggest that Slight positive uniform δ13C ratios around +1 (Figure 3, Support- units A–F lie well above the strong positive δ13C excursion detected ing Information Table S3), combined with biostratigraphic constraints, in the Ara Group of Oman (Amthor et al., 2003). Similarly, S. narbon- place units A–F into the range of the late Ediacaran Positive Carbon nei and T. pedum, indicative for Cambrian age as produced by LINNEMANN ET AL. | 55

[Ma] Cambrian complex 538.30 traces

538.40 Treptichnus pedum

538.50

538.60 ash 6

538.58 ± 0.19 Treptichnus 538.70 cf. pedum + Streptichnus narbonnei 538.80 Boundary

538.90 Gordia

539.00 , and others ash 5 538.99 ± 0.21 539.10 Treptichnids, , Treptichnids, 539.20 Helminthopis

539.30

539.40

539.50 ash 3 , Namacalathus 539.52 ± 0.14

539.60 ash 2 539.58 ± 0.34 ash 4

539.70 Trace fossils 539.64 ± 0.19 Cloudinids Swartpuntia, Pteridinium 539.80

FIGURE 6 Range of life forms and suggested biological developments versus 539.90 age including new geochronological data (this study) in a time window ranging from 540.00

540.1 to 538.3 Ma indicating the ash 1 Ediacaran–Cambrian boundary interval in the Swartpunt and Swartkloofberg sections 540.10 540.095 ± 0.099 advanced bilaterians, occur in all known sections above the BACE, by the advent of complex bilaterian trace‐makers, and demonstrates but reliable changes in trace fossil assemblages that are believed to that Ediacaran biomineralized taxa extended for a short time beyond be indicative of phylogenetic changes are only known from the For- this key event, at least locally. tune Head section and the sections presented here. Our new age data provide for the first time a precise, absolute tim- ing for this evolutionary turnover during the Ediacaran–Cambrian tran- sition (Figure 6). Accordingly, the age of ash 5 (538.99 ± 0.21 Ma) 4 | IMPLICATIONS FOR THE TIMING AND predates the termination of the erniettomorph Pteridinium simplex and OF BIOTIC CHANGES rangeomorph Swartpuntia germsi in unit D at metre 104. The first appearance of Cambrian‐type indicators, including Strep- The stratigraphic sequence at Swartpunt confirms that the disap- tichnus narbonnei, can now be dated at between 538.99 ± 0.21 Ma pearance of and erniettomorphs was rapidly followed (ash 5) and 538.58 ± 0.19 Ma (ash 6). Thus, the extinction of the 56 | LINNEMANN ET AL.

FIGURE 7 Three models (a, b, c) for the displacement of the Ediacara‐type biota (ETB, blue) by the Cambrian‐type fauna (CTF, red); x‐axis represents diversity; shaded interval indicates hypothetical environmental disturbances in (a) rangeomorphs/erniettomorphs and the beginning of the Cambrian sporadic reports of rangeomorphs from the Cambrian (Hagadorn et al., radiation occurred within a short period of 410 ± 400 ka, given by the 2000; Jensen, Gehling, & Droser, 1998). However, survivorship of age difference between ashes 5 and 6. It should be noted that this rangeomorphs into the Cambrian is questionable (Laflamme et al., duration is overestimated, because it includes an erosional unconfor- 2013) and represents at best an exception. Generally, rangeomorphs mity in the basal Cambrian (Figure 3). Furthermore, ash 6 predates the are not found above the lowest occurrence of Cambrian trace‐makers, first appearance of T. pedum, part of a moderately diverse assemblage. and there is no evidence for a successive decline during an extended We therefore suggest the age of the Ediacaran–Cambrian boundary period of co‐existence in Namibia, or globally. between ash 5 and ash 6 needs to be c. 538.8 Ma, thus about It has frequently been proposed that the extinction of the ETB c. 2.4 Ma younger than previously suggested. was ecologically driven, e.g. by destruction of the matground envi- The new timeframe allows testing of different evolutionary mod- ronment by newly evolved sediment‐mixing metazoans, competition els for the replacement of the Ediacaran‐type biota (ETB) by the with ecologically more successful animals, or (Bengtson & Cambrian‐type fauna (CTF) (Darroch et al., 2015; Laflamme et al., Yue, 1992; Schiffbauer et al., 2016; Seilacher & Pflüger, 1994). Eco- 2013; ; Muscente et al., 2018; Smith et al., 2016). We herein discuss logical effects of newly evolved key adaptations that enhanced com- three models, which differ significantly in the time‐scale they predict. petitiveness, predatory skills or the ability to alter the habitat would These include: (a) the CTF expanded in response to increased eco- appear geologically suddenly, analogous at a larger scale to the pro- logical opportunities after extinction of the ETB and/or in response found alterations of some present‐day following the to the environmental changes that supposedly caused this extinction; introduction of invasive species (Lowe, Browne, Boudjelas, & De (b) the extinction of the ETB was the endpoint of a long‐term Poorter, 2004). We suggest that an adaptive breakthrough, such as demise due to competition with the expanding CTF; (c) the end‐Edia- the evolution of advanced mobility, could shift the process of caran mass extinction reflects the tipping point at which the devel- replacement from the evolutionary to the ecological time‐scale, - opment of mobility allowed the acquisition of new feeding strategies ing to a situation that is herein referred to as the truncated double in the CTF, negatively affecting the ETB by the destruction of the wedge model (Figure 7c): one clade declines progressively in response vital microbial matground food source or by direct interference (e.g. to the expansion of another until an adaptive breakthrough acceler- predation). ates this replacement by orders of magnitude, leading to the trunca- The first model predicts that the expansion of the CTF began after tion of the shrinking “wedge”. The progressive decline of the the onset of the hypothesized changes in environmental conditions rangeomorphs/erniettomorphs during the late Ediacaran, contrasted (Figure 7a). However, it is unlikely that the short duration of the ECTI, by the short interval of their final disappearance benchmarked by as benchmarked by our new age data, was sufficient for a de novo evo- the new age data, is predicted by this model. A possible preservation lution of the morphological complexity of Cambrian trace‐makers with bias against ETB in the Cambrian (Gehling, 1999) may have accentu- its advanced grade of organization comparable to that of priapulid ated the abruptness of this transition. However, the persistence of worms (Vannier, Calandra, Gaillard, & Żylińska, 2010). The second microbial mats into the basal Cambrian, and uncertainties in the tax- model, which conforms to Benton's metaphoric ‘double wedge’ (Ben- onomic identity of potential Cambrian rangeomorphs/erniettomorphs, ton, 1987; Sepkoski, 1996), predicts an extended period of replace- casts the empirical evidence for this model into doubt (Laflamme et ment, during which the CTF expanded at the expense of the ETB al., 2013). (Figure 7b). The diversity decline from the older White Sea assem- Summarized, we found the best model‐to‐data fit for the trun- blages to the younger Nama assemblages has been depicted in this cated double wedge model, although our data are not completely light (Darroch et al., 2015; Muscente et al., 2018), as have the incompatible with an environmentally driven scenario, at least if (a) LINNEMANN ET AL. | 57 the maximum duration within the error of the age data is assumed Chen, Zh., Chen, X., Zhou, Ch., Yuan, X., & Xiao, Sh. (2018). Late Edi- and (b) the geological time represented by the erosional unconfor- acaran trackways produced by bilaterian animals with paired appen- dages. Science Advances, 4, eaao6691, 8 pp. mity was short. However, evidence for possible environmental Chen, D., Zhou, X., Fu, Y., Wang, J., & Yan, D. (2015). New U‐Pb zircon changes is currently weak, which also detracts from a possible sce- ages of the Ediacaran‐Cambrian boundary strata in South China. Terra nario in which environmental and evolutionary aspects worked in Nova, 27,62–68. concert during the Ediacaran–Cambrian biotic transition. Darroch, S. A. F., Boag, T. H., Racicot, R. A., Tweedt, S., Mason, S. J., Erwin, D. H., & Laflamme, M. (2016). A mixed Ediacaran‐metazoan assemblage from the Zaris Subbasin, Namibia. Palaeogeography, – ACKNOWLEDGEMENTS Palaeoclimatology, Palaeoecology, 459, 198 208. Darroch, S. A., Sperling, E. A., Boag, T. H., Racicot, R. A., Mason, S. J., Three unknown reviewers are thanked for their helpful comments, Morgan, A. S., & Laflamme, M. (2015). Biotic replacement and mass extinction of the Ediacara biota. Proceedings of the Royal Society B, discussions and corrections. Introduction to the field area by K.H. 282, 20151003. Hoffmann (Windhoek, Namibia) and fruitful discussions including Erwin, D. H. (2001). Lessons from the past: biotic recoveries from mass important suggestions by B. Saylor (Case Western Reserve Univer- extinction. Proceedings of the National Academy of Sciences, 98, 5399– sity, Cleveland, USA) are greatly acknowledged. We further thank L. 5403. and B. Roemer, L. Gressert and B. Boehm‐Ernie from Aus, Namibia, Gehling, J. G. (1999). Microbial mats in terminal siliciclastics; Ediacaran death masks. Palaios, 14,40–57. for support during our fieldwork. Sincere thanks go to the Geological Geyer, G., & Landing, E. (2016). The Precambrian–Phanerozoic and Edi- Survey of Namibia, particularly to G. Schneider, for facilitating our acaran–Cambrian boundaries: a historical approach to a dilemma. work, and to the National Geographic Society for support of field- Geological Society (London) Special Publications, 448, 311–349. work in southern Namibia since 2004. We acknowledge long‐term Geyer, G., & Uchman, A. (1995). Ichnofossil assemblages from the Nama Group (‐Lower Cambrian) in Namibia and the Protero- funding of the geochronology facility at the University of Geneva zoic‐Cambrian boundary problem revisited. Beringeria, Special Issue, through the Swiss National Science Foundation. Furthermore, we 2, 175–202. appreciate long‐term funding of the GeoPlasmaLab Dresden by the Grotzinger, J. P., Bowring, S. A., Saylor, B. Z., & Kaufman, A. J. (1995). Senckenberg Naturforschende Gesellschaft and the Deutsche Biostratigraphic and geochronologic constraints on early evo- lution. Science, 270, 598–604. Forschungsgemeinschaft. The contribution of GG was made possible Grotzinger, J. 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