Correlation of the Cambrian Evolutionary Radiation: geochronology, evolutionary stasis of earliest Cambrian (Terreneuvian) small shelly fossil (SSF) taxa, and chronostratigraphic significance

Abstract

Early faunas with
Watsonella crosbyi
with or without
Aldanella
spp. have been equated with the Siberian Tommotian Stage (uppermost Terreneuvian) and used to define a proposed Cambrian Stage 2 base. Much earlier Terreneuvian occurrences are now shown by recovery of these micromolluscs below the I’ carbon excursion in the Siberian ‘Nemakit-Daldynian’ Stage and comparable δ
13
C excursions in the middle Meishucunian (China) and middle Chapel Island Formation (Avalonia). This δ
13
C excursion, a reliable Stage 2 marker, lies in a
c
. 10 Ma interval in the Cambrian Radiation in which long-ranged small shelly fossil taxa provide limited biostratigraphic resolution.

Full text

Geol. Mag. 153 (4), 2016, pp. 750–756. c
Cambridge University Press 2016 750
doi:10.1017/S0016756815001089
RAPID COMMUNICATION
Correlation of the Cambrian Evolutionary Radiation:
geochronology, evolutionary stasis of earliest Cambrian
(Terreneuvian) small shelly fossil (SSF) taxa, and
chronostratigraphic significance
ED LANDING∗†& ARTEM KOUCHINSKY‡
∗New York State Museum, 222 Madison Avenue, Albany, NY 12230, USA
‡Department of Palaeobiology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden
(Received 22 September 2015; accepted 2 December 2015; first published online 1 February 2016)
Abstract
Early faunas with Watsonella crosbyi with or without Aldan-
ella spp. have been equated with the Siberian Tommotian
Stage (uppermost Terreneuvian) and used to define a pro-
posed Cambrian Stage 2 base. Much earlier Terreneuvian
occurrences are now shown by recovery of these micromol-
luscs below the I’ carbon excursion in the Siberian ‘Nemakit-
Daldynian’ Stage and comparable δ13C excursions in the
middle Meishucunian (China) and middle Chapel Island
Formation (Avalonia). This δ13 C excursion, a reliable Stage 2
marker, lies in a c. 10 Ma interval in the Cambrian Radiation
in which long-ranged small shelly fossil taxa provide limited
biostratigraphic resolution.
Keywords: Cambrian Evolutionary Radiation, Terreneuvian,
Tommotian, Meishucunian, Siberia.
1. Introduction
The Terreneuvian, the earliest and longest Cambrian epoch,
lasted over 20 Ma (e.g. Landing et al.1998,2007)and
spanned the three stages of the Cambrian Evolutionary Ra-
diation (CER) (Landing et al.1989,2013; Landing 1992;
Zhu et al.2008). CER stage 1 (terminal Ediacaran – earli-
est Cambrian) featured increasingly diverse biomineralized
metazoans and soft-bodied animals that produced deep, be-
haviourally complex burrows (e.g. Geyer & Uchman, 1995;
Zhuravlev et al.2012). The Ediacaran–Cambrian boundary
global stratotype section and point (GSSP) lies in CER stage
1attheTrichophycus pedum Assemblage Zone base (ich-
nofossils) in eastern Newfoundland (Narbonne et al.1987;
Landing, 1992,1994; Brasier, Cowie & Taylor, 1994;see
T. pedum Assemblage Zone of Landing et al.2013;Fig. 1).
This report focuses on CER stage 2 with the radiation
and likely intertidal origin of many biomineralized metazo-
ans, such as molluscs, hyoliths and tommotiids (Landing &
Westrop, 2004). CER stage 3 featured the offshore diver-
sification of mineralized trilobites and was a ‘non-event’ in
peritidal communities, where trilobite remains are sparse and
the mollusc-hyolith-linguloid assemblages from CER stage
†Author for correspondence: ed.landing@nysed.gov
2 persisted (Landing et al. 1989,2013; Landing & Westrop,
2004).
The Terreneuvian includes the Fortunian Stage and a later
Stage 2 (the Laolinian Stage of Landing et al. 2013;Fig. 1).
The Siberian Tommotian Stage with diverse SSFs (small
shelly fossils) is latest Terreneuvian in age (Ogg, Ogg &
Gradstein, 2008) but, as concluded here, the earliest appear-
ances of key ‘Tommotian-aspect’ SSF taxa are older (by c.
5 Ma) and referable to the latest Fortunian – early Age 2.
2. Early correlations of Terreneuvian micromollusc-rich
assemblages
CER stage 2 featured the FAD (first (actually ‘lowest’) ap-
pearance datum) of the micromolluscs Watsonella crosbyi
(e.g. discussed in Landing et al.1989,2013; often reported
as Heraultipegma sibirica in Siberia and H. varensalensis
in South China) and Aldanella attleborensis (i.e. A. yangji-
aheensis in South China). The FADs of these two forms have
been regarded as Tommotian and global correlation aids (e.g.
Steiner et al.2007;Liet al. 2011; Demidenko & Parkhaev,
2014). The W. crosbyi and A. attleborensis FADs have been
used to correlate the Tommotian and middle Meishucunian
Stage of South China (e.g. Brasier, 1989; Khomentovsky &
Karlova, 1993; Steiner et al.2007,2011; Devaere et al.2013;
taxonomy of both forms in Landing, 1988; Landing et al.
1989,2013; Parkhaev & Karlova, 2011; Parkhaev, Karlova
& Rozanov, 2011).
This correlation seemed justifiable as the very lowest Cam-
brian has low diversity biotas with the triradiate tubes of Ana-
barites trisulcatus and pseudoconodont elements of Proto-
hertzina anabarica in the lower ‘Nemakit-Daldynian’ Stage
and lower Meishucunian. Overlying strata have early mol-
luscs as Purella, other monoplacophoran-like forms, and
coelosclerotophorans in the upper ‘Nemakit-Daldynian’ and
lower middle Meishucunian (Fig. 1). ‘Nemakit-Daldynian’
and ‘Manykayan’ are essentially synonymous, Siberian low-
est Cambrian stage names (e.g. Missarzhevsky, 1989;Brasier
et al. 1994b; Zhuravlev & Riding, 2001, tab. 1.1). Most
Russian reports regard ‘Nemakit-Daldynian’ as terminal
Proterozoic (e.g. Rozanov, Khomentovsky & Shabanov,
2008). ‘Nemakit-Daldynian’ and ‘Manykayan’ are impre-
cisely defined units without type sections or formally defined
bases. ‘Nemakit-Daldyn’ and ‘Manykay’, first used for

RAPID COMMUNICATION 751
Figure 1. Lower Cambrian correlation. Avalonian carbon isotope stratigraphy (Brasier, Anderson & Corfield, 1992) and bio- and
lithostratigraphy (Landing et al. 2013); Fosters Point Formation thickness increased four-fold for figure; carbon isotope stratigraphy of
Lena–Aldan river region (Kirschvink et al. 1991;Brasier et al. 1994a,b), Bol’shaya Kuonamka (Kouchinsky et al.2001) and Selinde
(Kouchinsky et al. 2005); coeval Siberian and Avalonian stratigraphic units and isotope excursions aligned; South China Platform
succession (Li et al. 2009). Fm – Formation; Mbr – Member; M.L. – Mystery Lake Member; T. –Trichophycus;W. –Watsonella.
Siberian zones: N.–Nochoroicyathus sunnaginicus;D. r. – Dokidocyathus regularis;D. lenaicus – Dokidocyathus lenaicus;P. –
Profallotaspis jakutensis;R. –Repinaella;D. a. –Delgadella anabara;J. – Judomia. Modified from Landing et al. (2013,fig.
5); addition of Heraultia and Ary-Mas-Yuryuakh sections and deletion of several Siberian and Chinese sections. Laolinian Stage,
Lenaldanian Series and Zhurinskyan Stage proposals (Landing et al. 2013), submitted to Cambrian Subcommission in 2013, follow
Remane et al.(1996) guidelines.
lithostratigraphic units, cannot designate chronostratigraphic
units (e.g. Salvador, 1994). Without an appropriate stage-
level name, a so-called ‘Nemakit-Daldynian’ Stage is used
herein for the Siberian lowest Cambrian (Fig. 1).
Emphasis on Watsonella crosbyi and Aldanella attleboren-
sis in correlation led to proposal of an A. attleborensis FAD
as a guide for global Tommotian correlation or a Cambrian
Stage 2 base (Rozanov, Khomentovsky & Shabanov, 2008;
Parkhaev, Karlova & Rozanov, 2011; Devaere et al. 2013;
Demidenko & Parkhaev, 2014). A W. crosbyi FAD was also
argued as a potential (e.g. Li et al.2011; Steiner et al.2011)
or ‘preferred’ Stage 2 base (Peng & Babcock, 2011).
3. Tommotian concepts
The Tommotian Stage stratotype base is the base of the
Nochoroicyathus sunnaginicus Zone (archaeocyathans) at
‘Dvortsy’ on the Aldan River, SE Siberia (Rozanov & Mis-
sarzhevsky, 1966;Fig. 1). Another ‘Tommotian concept’ re-
garded all diverse lowest Cambrian SSF assemblages with or
without archaeocyathans, and which are possibly older than
the Tommotian stratotype, as Tommotian (e.g. Rozanov et al.
1969). The ‘Tommotian Stage’ was equated to an ‘evolution-
ary event’ (Rozanov & Zhuravlev, 1992).
The ‘Nemakit-Daldynian’–Tommotian contact in SE
Siberia is a change from lower sparsely fossiliferous Yudoma
Formation dolostone with karstic or extensional fissures and
channels. Glauconitic remanié sedimentary deposits with
the lowest Tommotian fossils fill these depressions and are
overlain by reddish and green, siliciclastic and calcareous
mudstones (e.g. Khomentovsky, Val’kov & Karlova, 1990;
Fig. 1). A sub-Tommotian hiatus in SE Siberia (Landing in
Narbonne et al.1987; Moczydłowska & Vidal, 1988)has
been considered to be short (Khomentovsky & Karlova,
1993), but the abrupt appearance of over 174 named taxa,

752 RAPID COMMUNICATION
as well as high-level groups such as archaeocyathans and
brachiopods, without a record of their earlier diversification
suggests a long hiatus (Landing, 1994).
4. The Tommotian and carbon isotope stratigraphy
Development of a Siberian terminal Precambrian – lower
Cambrian carbon isotope stratigraphy (Magaritz, Holser &
Kirschvink, 1986; Magaritz et al.1991; Kirschvink et al.
1991;Brasier et al. 1994a,b) allowed evaluation of the dura-
tion of the pre-Tommotian hiatus in SE Siberia, and its rela-
tionship to an ‘evolutionary event.’ An important extension
of this work was Knoll et al.’s ( 1995b) synthesis of lowest
Cambrian biostratigraphy and carbon isotope stratigraphy on
the NW Anabar Uplift.
Knoll et al. (1995b)recognized a sub-Tommotian inter-
val in the Medvezhya Formation at the Kotuikan (Kotujkan,
herein) River section (Fig. 1) that had earlier been considered
sub-Tommotian biostratigraphically by A.K. Val’kov and V.V.
Missarzhevsky (e.g. Khomentovsky & Karlova, 2002). This
interval has a strong δ13C positive excursion (I’) distinct
from the negative excursion of the lowest Tommotian. Pos-
itive excursions in the underlying Manykay Formation were
compared to the I and Z excursions in the sub-Tommotian
Yudoma Formation in SE Siberia (Knoll et al. 1995b;Fig. 1).
The diverse, ‘Tommotian’ SSFs in the Medvezhya Forma-
tion at Kotujkan River of Rozanov et al.(1969, exposures
M 418–419, units 13–16) were therefore shown to be pre-
Tommotian (Knoll et al. 1995b; also Parkhaev & Karlova,
2011;Fig. 1), and must be referred to the upper ‘Nemakit-
Daldynian’. Knoll et al. (1995b,fig. 2) concluded this ‘pre-
viously unknown’ interval showed an earlier, more staggered
appearance of ‘Tommotian’ SSFs, and that the local FADs of
supposed Tommotian ‘index’ SSFs did not offer unambigu-
ous Tommotian correlations.
Knoll et al.’s (1995b) report did not seem to alter Cambrian
biostratigraphic practices. Aldanella attleborensis, with a
sub-Tommotian FAD at Kotujkan River (Knoll et al. 1995b),
was later proposed as a guide to the Tommotian base, to Tom-
motian equivalents globally and a Stage 2 base (Rozanov,
Khomentovsky & Shabanov, 2008; Parkhaev & Karlova,
2011; Parkhaev et al.2011; Rozanov et al. 2011; Demidenko
& Parkhaev, 2014). The A. attleborensis FAD lies below the I’
carbon excursion peak and just above the sequence boundary
under the Medvezhya Formation on Kotujkan River (Knoll
et al. 1995b). These data mean that the known lowest Siberian
range of the species is likely truncated, sub-Tommotian and
inappropriate for defining a Stage 2 base.
Knoll et al. (1995b)noted that the presence of archae-
ocyathans indicates a Tommotian or younger age. Landing
et al. (1987, 2013) complemented this by concluding that
(a)rchaeocyathans, lingulate (b)rachiopods and (h)yolithid
hyoliths (‘abh taxa’) do not occur below the Tommotian or
globally below Tommotian-equivalent successions. Bimem-
brate tommotiids (e.g. Camenella kozlowskii) appear just
above the I’ excursion peak (Knoll et al. 1995b)andtheir
lowest occurrence may be a correlation aid to the upper
‘Nemakit-Daldynian’, although the Russian literature gener-
ally assigns their oldest occurrences to the Tommotian (e.g.
Missarzhevsky, 1989; Khomentovsky & Karlova, 1993). Ar-
chaeocyathans show habitat limitations in the Tommotian to
tropical shelves, but genera and species of bimembrate tom-
motiids, lingulates, hyoliths and micromolluscs allow inter-
continental correlation with their tropical Siberia cool-water
Avalonia range and are not limited to archaeocyathan-bearing
facies (e.g. Landing, 1988; Landing et al. 1989; Landing &
Westrop, 2004).
5. Revised correlation of Terreneuvian
micromollusc-rich assemblages
An alternative interpretation emphasizes a pre-Tommotian
appearance of early assemblages with Watsonella crosbyi
with or without Aldanella attleborensis (e.g. Landing, 1988;
Landing et al.1989,2013). Indeed, correlations of lower
Tommotian and middle Meishucunian faunas with the two
species are problematical as this means that very dissimilar
assemblages are regarded as coeval. Middle Meishucunian
faunas with W. crosbyi and A. attleborensis lack any ‘abh
taxa’ species, although ‘abh taxa’ appear in similar carbonate
platform facies in the higher lower Cambrian strata of South
China (e.g. Zhang, 2003). Another problem with many cor-
relations based on W. crosbyi and A. attleborensis FADs is
that these taxa have diachronous ranges that are often brack-
eted by stratigraphic breaks of unknown duration (e.g. SE
Siberia) or are lithofacies-associated, with the two species
appearing with marine shoaling in Avalonia (Landing et al.
1989,2013; Landing, 2004).
In South China, diverse micromolluscs with Watsonella
crosbyi and rare Aldanella specimens appear above local
unconformities in the middle Meishucunian strata and with
a change from a phosphatic or dolomitic into an overlying
more calcareous unit (e.g. Zhongyicun and Dahai members;
e.g. Landing, 1994;Yanget al. 2014;Fig. 1). This middle
Meishucunian fauna disappears upwards at an unconformity
or a change to poorly fossiliferous sedimentary deposits (e.g.
Steiner et al.2007;Yanget al. 2014;Fig. 1), which means the
upper W. crosbyi and Aldanella ranges are truncated and do
not show the long Avalonian biostratigraphic ranges where
both taxa extend into Atdabanian-equivalent rock (Landing
et al. 1989,2013;Fig. 1).
Other successions with Watsonella crosby have been cor-
related with the Tommotian. Esakova & Zhegallo (1996)re-
ported five Tommotian faunal zones, with a W. crosbyi Zone at
their top, from the Bayan Gol Formation of Mongolia. How-
ever, most of Esakova & Zhegallo’s (1996, fig. 3 range chart)
‘Tommotian’ taxa from the upper Bayan Gol Formation (their
W. crosbyi Zone; 31 named species with 14 described only
from the Bayan Gol) have ranges limited to the lower–middle
Meishucunian and upper ‘Manykayan-Nemakit-Daldynian’
strata, and no ‘abh’ taxa occur. The exception was W. crosbyi,
reported as W. sibirica from one section, with a supposed
Tommotian FAD as then known. Demidenko & Parkhaev
(2014) referred the upper Bayan Gol Formation to the Tom-
motian based on the occurrence of W. crosbyi with the mono-
placophoran Bemella jakutica, although Knoll et al. (1995b)
reported B. jakutica below the I’ excursion. Lapworthella tor-
tuosa was cited as a Tommotian index for the upper Bayan
Gol. However, Esakova & Zhegallo’s (1996) specimens lack
the granulose transverse growth ridges of L. tortuosa, while
elongate, non-granulose sclerites (i.e. L. ludvigseni)thatre-
semble L. tortuosa’s have sub-Tommotian FADs in Avalonia
and possibly in the middle Meishucunian. (Landing (1984,
p. 1394; Maloof et al. 2010, fig. A3) noted Jaing’s (1982,
pl. 18) illustrations of L. tortuosa-like specimens from Dahai
Member localities, while Qian & Bengtson (1989, p. 145)
and Li et al.(2007) questioned these middle Meishucunian
reports.)
The Watsonella crosbyi-bearing faunas of the Heraultia
Limestone in France were regarded as Tommotian (Devaere
et al.2013;Fig. 1). The Heraultia Limestone shows litho-
facies control of its 25 known taxa, with W. crosbyi present
in limestone, not dolostone. Heraultia faunas include taxa
mostly limited to the lower–middle Meishucunian strata
(Devaere et al.2013), and no ‘abh taxa’ are present. The
presence of W. crosbyi provided the basis for a Tommotian-
equivalency, and a sub-Tommotian recovery of W. crosbyi

RAPID COMMUNICATION 753
Figure 2. Ary-Mas-Yuryakh section (70°08.5ʹN, 114°00ʹE, left bank of Kotuj River upstream from mouth of Ary-Mas-Yuryakh
Creek), northern Siberia. Upper Manykay Formation (Koril Member, thrombolithic boundstones) overlain at sequence boundary by
Medvezhya Formation limestone. Lower horizon with Aldanella crassa (90 conchs), middle with A. attleborensis (80 conchs), and
upper with Watsonella crosbyi (7 specimens) and A. attleborensis (10 conchs).
(this report) would remove any certainty for a Tommotian
correlation of the Heraultia and Bayan Gol faunas.
6. Ary-Mas-Yuryakh
Rozanov et al.(1969, section M 410) described the lithology
and biota of the Ary-Mas-Yuryakh section (Fig. 2), which
is located 14 km SSW of Knoll et al.’s (1995b) Kotujkan
River section, northern Siberia. Kotujkan River and Ary-
Mas-Yuryakh have similar, supposedly ‘Tommotian’ SSFs in
the lower Medvezhya Formation, and lack ‘abh taxa’ (faunas
in Rozanov et al.1969;Knoll et al. 1995b,fig. 2). The sec-
tions are almost identical, with the Medvezhya’s base a se-
quence boundary on Kotujkan River (Knoll et al. 1995b,
fig. 1) and Ary-Mas-Yuryakh (Landing, 1994). Our new data
show the strong I’ excursion at both Kotujkan River and
Ary-Mas-Yuryuakh (Knoll et al. 1995b; this report) and sub-
Tommotian Watsonella crosbyi and Aldanella attleborensis
FADs at Ary-Mas-Yuryakh (Figs 1,2).
7. Conclusions
The I’ excusion in the ‘Nemakit-Daldynian’ above the Wat -
sonella crosbyi and Aldanella attleborensis FADs (Knoll
et al. 1995b; this report) is critical in lowest Cambrian
correlation. Carbon isotope stratigraphy has long supported

754 RAPID COMMUNICATION
correlation of the upper ‘Nemakit-Daldynian’ I’ excursion
and middle Meishucunian L4 excursion (also P3 excursion,
although the Zhujiang carbon isotope excursion (ZHUCE)
may include several excursions) (Knoll et al. 1995b;Kauf-
man et al.1996; Zhu, Li & Zhang, 2001;Liet al.2009).
Samples through the W. crosbyi Zone in SE Newfoundland
show increasingly positive δ13C values (Brasier, Anderson &
Corfield, 1992) that suggest the I’ excursion (Landing et al.
2013;Fig. 1). Finally, the Bayan Gol Formation and Heraultia
Limestone faunas with W. crosbyi are best regarded as pre-
Tommotian. The Bayan Gol’s δ13 C succession suggests cor-
relation of its F excursion or, pending further study, its D and
E excursions, with L4 and I’ (Brasier, Dorjnamjaa & Lindsay,
1996;Brasieret al.1996; Landing et al.2013).
The earliest SSF assemblages with Watsonella crosbyi
with or without Aldanella attleborensis are demonstrably pre-
Tommotian as earlier proposed (Landing, 1988,1992,1994,
1996; Landing et al.1989,2013) and occur worldwide (east-
ern Newfoundland, southern New Brunswick, South China,
NW Siberia, Mongolia and France.) Assemblages with di-
verse micromolluscs, orthothecid hyoliths, W. crosbyi and
Aldanella spp. appear to bracket a time of relative evolution-
ary stasis for many metazoans. Landing & Westrop (2004;
also Landing et al.1989) showed that once benthic genera
and species appeared, they persisted through much of the
Avalonian lower Cambrian strata (i.e. W. crosbyi–top Ca-
menella baltica zones; Fig. 1). Geochronologic data suggest
these early occurrences of W. crosbyi and A. attleborensis at
c. 530 Ma (Landing et al.2013), and their persistence into
Atdabanian-equivalent strata younger than c. 520 Ma in Ava-
lonia (e.g. Landing et al.1998). A duration of SSF elements
of c. 10 Ma of these assemblages emphasizes the suggestion
of Knoll et al. (1995b, p. 1142) that SSFs are not ‘robust
markers’ for recognition and correlation of the Tommotian.
A 530–529 Ma volcanic ash date on the upper Kessy-
usa Formation in northern Siberia was assigned to the lower
Tommotian Nochoroicyathus sunnaginicus Zone (Kaufman
et al.2012). Earlier, Missarzhevsky (1989) referred the upper
Kessyusa Formation to the N. sunniginicus Zone. However,
archaeocyathans and Tommotian ‘abh taxa’ do not appear
in the upper Kessyusa, and its fauna (Missarzhevsky, 1989;
Knoll et al. 1995a, table 2; Rogov et al. 2015,fig. 2j) includes
Watsonella crosbyi and other taxa that are not necessarily
Tommotian as they appear below the I’ carbon excursion
peak at Kotujkan River and Ary-Mas-Yuryakh. Only Came-
nella kozlowskii (discussed above) from the upper Kessuysa
(Missarzhevsky, 1989) suggests correlation as high as the
upper I’ excursion.
The strong positive δ13C values of the upper Kessyusa
Formation (Knoll et al. 1995a; Kaufman et al.2012;A.
Kouchinsky, unpubl. data from Khorbusuonka) do not occur
in the lower Tommotian stratotype and argue against a Tom-
motian correlation. The positive carbon isotope excursions in
the upper Kessyusa of Knoll et al. (1995a)correlate best into
the Medvezhya (Figs 1,2;alsoKnoll et al. 1995b; Kaufman
et al.1996)andlowerEmyaksin(Fig. 1; Kouchinsky et al.
2001) formations of the Anabar uplift, and are older than the
lowest Tommotian’s negative shift (Kirschvink et al.1991;
Knoll et al. 1995b; Kouchinsky et al.2001,2007). A a 530–
529 Ma date on Watsonella crosbyi-bearing assemblages of
the upper Kessuya Formation is therefore biostratigraphic-
ally and geochemically pre-Tommotian, and older than an
estimated Tommotian base of c. 525.5 Ma (Maloof et al.
2010).
The earliest biotas with Watsonella crosbyi may pre-date
530 Ma. Moczydłowska (1991, p. 35) reported Aldanella
attleborensis,A. crassa and Anabarella sp., which occur
elsewhere with W. crosbyi, from the Asteridium tornatum-
Comasphaeridium velvetum Zone (acritarchs) in Poland
(SSF taxonomy in Landing, 1988; Parkhaev & Karlova,
2011). This very early Cambrian occurrence emphasizes that
SSF FADs without non-biostratigraphic brackets should not
define Cambrian stage bases. Landing et al.(2013) proposed
that the globally correlatable L4-I’ carbon isotope excursion,
with a lower bracket provided by the W. crosbyi and/or A.
attleborensis FADs, best defines a Stage 2 GSSP base. How-
ever, the A. attleborensis FAD is only a few metres above
the sequence boundary at Kotujkan River and, at a change
into more calcareous lower Medvezhya Formation facies (A.
Kouchinsky, unpub. data), is in the lowest Cambrian Polish
acritarch succession. These biostratigraphically problemat-
ical A. attleborensis FADs suggest that a sub-Tommotian-
equivalent horizon based on a W. crosbyi FAD b e l o w t h e
L4-I’ excursion or, perhaps preferably, on the L4-I’ excur-
sion peak above a W. crosbyi FAD, may provide more robust
definitions of a Stage 2/Laolinian Stage basal GSSP horizon.
Acknowledgements. K. Ziegler (University of California,
Los Angeles) provided carbon isotope analyses.
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