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\DOI{10.5802/crgeos.294}
\datereceived{2025-01-13}
\daterevised{2025-04-25}
\dateaccepted{2025-05-12}
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\dateposted{2025-06-16}
\begin{document}

\begin{noXML}

\TopicFR{\'Ecosyst\`emes terrestres et aquatiques}
\TopicEN{Terrestrial and aquatic ecosystems}

\CDRsetmeta{articletype}{research-article}

\title{The Asselian palynoflora from the Autunian series (Muse
oil-shale beds, Autun Basin, France): new insights on early Permian
equatorial palaeoflora and palaeoenvironments}

\alttitle{La palynoflore ass\'{e}lienne de la s\'{e}rie autunienne
(Couche de Muse, bassin d'Autun, France) : nouvelles perspectives sur
la pal\'{e}oflore \'{e}quatoriale et les pal\'{e}oenvironnements du
Permien inf\'{e}rieur}

\author{\firstname{Manuel Antonio} \lastname{Juncal}\CDRorcid{0000-0002-3344-1342}}
\address{CINTECX, Grupo de Tecnolog\'{i}a Energ\'{e}tica (GTE),
Universidade de Vigo, 36310 Vigo, Spain}
%\email{majuncales@gmail.com}

\author{\firstname{Iv\'an Rodriguez} \lastname{Barreiro}\CDRorcid{0000-0002-0938-662X}\IsCorresp}
\address{Centro de Investigaci\'{o}n Mari\~{n}a, Universidade de Vigo,
BASAN, 36310 Vigo, Spain}
\address{Departamento de Xeociencias Mari\~{n}as e Ordenaci\'{o}n do
Territorio, Universidade de Vigo, 36310 Vigo, Spain}
\address{Museum of Nature South Tyrol, Bindergasse/Via Bottai 1, 39100
Bozen/Bolzano (BZ), Italy}
\email[I. R. Barreiro]{ivan.rodriguez.barreiro@uvigo.gal}

\author{\firstname{Jean} \lastname{Galtier}}
\address{CIRAD, AMAP TA 40/PS2, Boulevard de la Lironde, F-34398
Montpellier, France}
%\email{galtierjean@wanadoo.fr}

\author{\firstname{Jean} \lastname{Broutin}}
\address{Sorbonne Universit\'{e}s, Pal\'{e}obotanique \&amp;
Pal\'{e}o\'{e}cologie, CR2P, UPMC Paris 6-MNHN-CNRS, F-75005 Paris,
France}
%\email{colette.broutin@wanadoo.fr}

\author{\firstname{Pierre}\nobreakauthor\lastname{Pellenard}\CDRorcid{0000-0002-1452-2569}}
\address{Universit\'{e} Bourgogne Europe, Biog\'{e}osciences UMR
CNRS/uB/EPHE 6282, 6 boulevard Gabriel, 21000 Dijon, France}
%\email{Pierre.Pellenard@u-bourgogne.fr}

\author{\firstname{Jean-S\'ebastien} \lastname{Steyer}\CDRorcid{0000-0003-1835-7852}}
\address{Centre de Recherches en Pal\'{e}ontologie de Paris, UMR 7207,
CNRS-MNHN-SU, Mus\'{e}um national d'Histoire naturelle, CP 38, 8 rue
Buffon, 75005 Paris, France}
%\email{jean-sebastien.steyer@mnhn.fr}

\author{\firstname{Jos\'e Bienvenido} \lastname{Diez}\CDRorcid{0000-0001-5739-7270}}
\addressSameAs{2}{Centro de Investigaci\'{o}n Mari\~{n}a, Universidade
de Vigo, BASAN, 36310 Vigo, Spain}
\addressSameAs{3}{Departamento de Xeociencias Mari\~{n}as e
Ordenaci\'{o}n do Territorio, Universidade de Vigo, 36310 Vigo, Spain}
%\email{jbdiez@uvigo.gal}

\author{\firstname{Georges}\nobreakauthor\lastname{Gand}}
\addressSameAs{7}{Universit\'{e} Bourgogne Europe, Biog\'{e}osciences
UMR CNRS/uB/EPHE 6282, 6 boulevard Gabriel, 21000 Dijon, France}
%\email{georgesb.gand@orange.fr}

\shortrunauthors

\keywords{\kwd{Lacustrine
palaeoecosystem}\kwd{Cisuralian}\kwd{Palaeobotanical
reconstruction}\kwd{Late Palaeozoic}\kwd{Explosive volcanic activity}}

\altkeywords{\kwd{Pal\'{e}o\'{e}cosyst\`{e}me
lacustre}\kwd{Cisuralien}\kwd{Reconstruction
pal\'{e}obotanique}\kwd{Pal\'{e}ozo\"{i}que
sup\'{e}rieur}\kwd{Volcanisme explosif}}

\thanks{Ministerio de Ciencia y Innovaci\'{o}n of the Spanish
Government (ref.: PID2022-141050NB-I00), Promotion of Educational
Policies, University and Research Department of the Autonomous Province
of Bolzano - South Tyrol (CUP H33C23001340003).}

\begin{abstract} 
The Muse oil-shale beds (Muse OSB) of the Permian Muse Formation in the
Autun Basin (France) have received attention for their exceptionally
well-preserved flora and fauna throughout many decades. Recent
CA-ID-TIMS U/Pb radiometric datings place the Muse section in the lower
Asselian (\tralicstex{\textasciitilde{}}{${\sim}$}298.6 Ma), leading to a re-examination of the
palynoflora using an alternative palynological method. Twelve
productive samples provide a detailed taxonomic and quantitative
evaluation of the palynoflora. The new data revealed now two distinct
palynological assemblages, Muse-A in the lower part and Muse-B in the
middle-upper part of the Muse section, assigned to a peat swamp forest
environment dominated by cordaitalean conifers, alongside medullosalean
cycads and callistophytalean seed ferns, with an undergrowth of
lycophytes and ferns. Additionally, a forest dominated by walchian
conifers (Voltziales) would be present but more distant from the lake.
Vegetation dynamics between these associations are generally stable,
except for an increase in Cordaitales at the expense of Voltziales.
This variation, observed throughout the Autun Basin, seems to be
affected by seasonal climate-driven changes. The co-occurrence in both
assemblages of Permian xerophytic \mbox{elements}
\mbox{accompanied} by Carboniferous
wetland flora suggests a seasonally dry climate, supporting hygrophyte
plant survival, persisting for at least 300 ky after the
Carboniferous--Permian boundary. The phytoplankton community
consisted of freshwater green algae, increasing towards the upper part
of the section, coeval with increased explosive volcanic activity.
Wind-transported volcanic ash likely provided nutrient input, enhancing
lacustrine phytoplankton growth.
\end{abstract} 

\begin{altabstract} 
Les argilites carbon\'{e}es permiennes de la couche de Muse ({\og}
couche de schistes bitumineux de Muse {\fg} ; Muse OSB) appartenant
\`{a} la Formation de Muse du Bassin d'Autun (France) ont attir\'{e}
l'attention pour leur pr\'{e}servation exceptionnelle de flore et faune
depuis plusieurs d\'{e}cennies. Des datations radiom\'{e}triques
r\'{e}centes par la m\'{e}thode CA-ID-TIMS U/Pb, pla\c{c}ant la couche
de Muse dans l'Ass\'{e}lien inf\'{e}rieur (\tralicstex{\textasciitilde{}}{${\sim}$}298,6 Ma), conduit au
r\'{e}examen de la palynoflore \`{a} l'aide d'une m\'{e}thode
palynologique alternative. L'analyse de douze \'{e}chantillons permet
une \'{e}valuation taxonomique et quantitative d\'{e}taill\'{e}e de la
palynoflore. Ces nouvelles donn\'{e}es r\'{e}v\`{e}lent deux
assemblages palynologiques distincts, nomm\'{e}s Muse-A (base de la
coupe) et Muse-B (partie haute de la coupe). Ceux-ci refl\'{e}tent un
premier milieu domin\'{e} par des for\^{e}ts de tourbi\`{e}res \`{a}
conif\`{e}res cordaitales, aux c\^{o}t\'{e}s de cycadales
m\'{e}duddales et de foug\`{e}res \`{a} graines callistophytales, avec
un sous-bois de lycopodes et de foug\`{e}res. Un second milieu
forestier domin\'{e} par des conif\`{e}res walchiens (Voltziales)
serait pr\'{e}sent \`{a} une distance plus importante du lac. Les
dynamiques de v\'{e}g\'{e}tation entre ces associations sont
g\'{e}n\'{e}ralement stables, \`{a} l'exception d'une augmentation des
Cordaitales au d\'{e}triment des Voltziales au cours du temps. Cette
modification, observ\'{e}e dans l'ensemble du Bassin d'Autun, semble
\^{e}tre li\'{e}e \`{a} des changements climatiques saisonniers. La
pr\'{e}sence d'\'{e}l\'{e}ments floraux x\'{e}rophytes accompagn\'{e}e
par des \'{e}l\'{e}ments floraux hygrophytes caract\'{e}ristiques de
mar\'{e}cages du Carbonif\`{e}re dans les deux assemblages t\'{e}moigne
d'un climat sec pr\'{e}sentant une saisonnalit\'{e}, qui se poursuit
pendant au moins 300 000 ans apr\`{e}s la limite
Carbonif\`{e}re-Permien, permettant la survie des plantes hygrophytes.
La communaut\'{e} du phytoplancton est constitu\'{e}e d'algues vertes
lacustres, augmentant au sommet de la section, co\"{i}ncidant avec une
activit\'{e} volcanique explosive plus intense. Les cendres
volcaniques transport\'{e}es par le vent pourraient avoir fertilis\'{e}
le milieu par l'apport en nutriments, favorisant la prolif\'{e}ration
du phytoplancton lacustre.
\end{altabstract} 

\maketitle
%\vspace*{.8pc}

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\end{noXML}

\defcitealias{Gandetal2015b}{ibid.}
\defcitealias{Traverse2007}{ibid.}
\defcitealias{DoubingerElsass1975}{ibid.}
\defcitealias{Maysetal2021}{ibid.}

\section{Introduction}

The Pennsylvanian--Cisuralian transition took place at the end of a long
period of globally cold climates marked by glacial activity in the
South Pole and during the early tectonic post-assembly of Pangaea
\citep{Ziegleretal1979, Simancasetal2005, Isozaki2009}. In the tropical
regions, the early Permian was marked by a general drying trend during
a transition from a maximum glacial coverage (Late Palaeozoic Ice Age
or LPIA) to greenhouse conditions following a glacial recession at the
Mississippian--Pennsylvanian boundary \citep{Fieldingetal2008,
MontanezPoulsen2013}. In western Europe, a progressive aridification
reaches its maximum around the Kungurian--Roadian boundary and was
followed by a seasonal dry climate during the Guadalupian and until the
Permo--Triassic boundary \citep{Schneideretal2006, Montanezetal2007,
TaborPoulsen2008, Lopezetal2008, Gulbransonetal2015, Micheletal2015}.

This global climate evolution is recorded worldwide in the
palaeofloras. For example, in western Europe, the Pennsylvanian
continental vegetation (i.e., ``Stephanian flora'') composed of
pteridosperms, marattialean ferns, lycopsids, calamiteans, and
cordaiteans trees \citep{Doubingeretal1995, DiMicheleetal2006,
ThomasCleal2017} is mostly replaced by conifers and other gymnosperms
corresponding to a meso-xerophytic flora (the ``Autunian flora''),
better adapted to drier conditions which became progressively
predominant in the Permian palaeoenvironments
\citep[e.g.,][]{LemoigneDoubinger1984, DiMicheleetal2001,
Looyetal2014}.

New radiometric data based on U--Pb analyses by Chemical
Abrasion-Isotopic Dilution Ionisation Mass Spectrometry (CA-ID-TIMS) on
zircon grains from tonsteins interbedded in lacustrine deposits have
been recently published by \citet{Pellenardetal2017} in the Autun Basin
(Figure~\ref{fig1}A) with the aim to resolve the puzzling
stratigraphical location of the Carboniferous--Permian boundary and to
date the lower Autunian (i.e., Igornay and Muse formations).
Subsequently, the Autunian series within the Autun Basin (France)
provides an essential framework to study the changes in the vegetation
succession observed during the Carboniferous--Permian transition. The
objective of this paper is therefore to complete the previous
palynological data after new detailed investigations in the Muse
oil-shale beds (MOSB) within the Muse Formation (lower Autunian) using
(1)~the new established stratigraphical framework and (2)~an
alternative palynological method based on hydrogen peroxide
\citep{Ridingetal2010}. Our new results lead to a detailed
palaeoenvironmental reconstruction based on palynology for the Autunian
continental series in the global climate and geodynamic changes
affecting the Carboniferous--Permian \mbox{transition}.

\begin{figure*}[t!]
\includegraphics{fig01}
{\vspace*{-.2pc}}
\caption{\label{fig1}(A)~Geographical and geological map of the Autun
Basin \citep[modified from][]{Gandetal2007}. (B)~Lithostratigraphy of
the lower and upper Autunian in the Autun Basin with approximate
thickness of the main formations, including oil shale beds (OSB) as
valuable key markers  \citep[modified from][]{Pellenardetal2017}.
(C)~Stratigraphic log of the Muse OSB \citep[modified
from][]{Pellenardetal2017}. The palynological samples with the asterisk
are low in relative abundance (counting ${&lt;}$200 palynomorphs).
Radiometric datings reported with full uncertainties.}
{\vspace*{-.6pc}}
\end{figure*}

\section{Stratigraphical and Geological setting} \label{sec2}
\subsection{Stratigraphy of the Regional Continental{\hfill\break}
Autunian Stage} \label{sec2.1}

In the Autun Basin (Figure~\ref{fig1}A), the continental Autunian
stratotype is divided into lower and upper Autunian
\citep[Figure~\ref{fig1}B; sensu][]{Pruvost1942}. The lower
Autunian units encompass the Igornay and Muse formations, while the
upper Autunian corresponds to the Surmoulin, Millery and Curgy
formations \citep{FeysGreber1972, Marteau1983, Chevremontetal1999}.\ A
synthesis of the stratigraphy and palaeobotany was proposed by
\citet{Chateauneufetal1992a, Chateauneufetal1992b}. Additionally, the
history of the Autunian stratotype and its geochronology according to
the different ``stages'' or ``formations'' since the early 19th century
was developed by \citet{Gandetal2017}.

After previous palaeobotanical studies (on both macroflora and
palynomorphs), a late Gzhelian--early Sakmarian age was inferred for
the regional continental Autunian Stage \citep[e.g.,][]{Doubinger1956,
FeysGreber1972, BourozDoubinger1977, DoubingerElsass1979,
ChateauneufFarjanel1989, Broutinetal1999, Gandetal2013}. Moreover,
\citet{Davydovetal2012}, \citet{Hendersonetal2012}, and
\citet{Gradsteinetal2012}, considered the Autunian as equivalent to the
middle Gzhelian--Kungurian stages as proposed in the Geologic Time
Scale 2012 (GTS2012). In contrast, \citet{WagnerAlvarezVazquez2010}
proposed to include the whole ``Autunian regional substage'' in the
Gzhelian Stage (Upper Pennsylvanian) based on their findings in
macroflora content succession. However, recent radiometric datings
using the CA-ID-TIMS U/Pb method indicate an upper Gzhelian age for the
Igornay Formation and a lower Asselian age for the Lally and Muse OSBs
interval \citep{Pellenardetal2017}. This work also suggests to fix the
Carboniferous--Permian boundary in the Lally OSB, near the base of the
Muse Formation, using a CA-ID-TIMS U/Pb age of $298.91 \pm 0.08$~Ma
(0.36 full uncertainty) from interbedded tonstein dating. Therefore, a
likely Asselian age can be inferred in the lower section of the Muse
Formation. Conversely, the Igornay Formation must be ascribed to the
upper Gzhelian.

\subsection{Geology of the Autun Basin} \label{sec2.2}

The Autun Basin (Figure~\ref{fig1}A) holds significant prominence
within the global array of Permian basins due to its well-known status
as a reference stratigraphical sequence for the lower Permian
continental strata, housing the renowned Autunian stratotype
(Figure~\ref{fig1}B). The Autunian deposits are mainly composed of
coarse siliciclastic sediments (i.e., sandstones and conglomerates),
with intercalated organic-rich claystone intervals and singular organic
matter-rich horizons called oil-shale beds (OSBs), corresponding to a
strictly deltaic-lacustrine system 
\citep[e.g.,][]{Mercuzotetal2021,Mercuzotetal2022}.
It is a well-studied area which was the subject of numerous
stratigraphic and cartographic investigations \citep{Manes1844,
Delafond1889, Pruvost1947, ElsassDamon1977, Chateauneufetal1980,
Marteau1983, MarteauFeys1989, ChateauneufFarjanel1989, Feys1988,
Feys1991, Chevremontetal1999}. Since~the~1980s, all these studies were
mainly carried out during the mining inventory of the basin by the
Bureau de Recherches G\'{e}ologiques et Mini\`{e}res (BRGM). Among
them, the study of \citet{Marteau1983} provides a tectonic,
stratigraphical, and sedimentological synthetic framework of the
Carboniferous--Permian deposits of the Autun\break Basin.

The Autun Basin is located in Morvan, northern Massif Central (France),
with a W--E direction, elliptical shape and depressed to the granitic
base of a volcano-sedimentary depositional environment
\citep[e.g.,][]{Marteau1983, Pellenardetal2017}. This basin is a small
half-graben with a present-day outcropping extension of about
260~km$^{2}$ and an average altitude of 320~m. Middle Triassic
sandstones overlie the Permian deposits in the centre of the basin,
reaching a thickness of 458~m \citep[e.g.,][]{Gandetal2007,
Gandetal2015a, Pellenardetal2017}. It is limited to the south by the
Autun extensional fault, which was active during the Permian
sedimentation in an extensive geodynamic context \citep{Marteau1983,
ChateauneufFarjanel1989}. A second fracturing system, with radial
faults perpendicular to the southern edge of the basin, caused
progressive subsidence of the blocks towards the west, leading to a
displacement of the depocenter in the same direction. This
post-orogenic (late Variscan) extension affected many other French
Permian and Carboniferous basins, including Blanzy-Le Creusot, Aumance,
Lod\`{e}ve, Saint-Affrique and the Provence \citep{ToutinMorin1980,
ChateauneufFarjanel1989, Faure1995, Faureetal2009}.

The Muse OSB, located north of Autun (Figure~\ref{fig1}A), has been a
focus of study since the 19th century thanks to mining activities. The
studied outcrop consists of 3.5~m of grey to black claystone and
siltstone, with intercalations of fine-grained sandstone layers
(Figure~\ref{fig1}C). The internationally known ``\textit{couche \`{a}
poissons}'' (literally ``fish bed''), discovered in 1811
\citep{Brignon2014}, is located at the base of this outcrop. It does
not correspond to a single fossilised fish layer but small
fossiliferous sequences \citep{Gandetal2015a}. In addition, the Muse
OSB became famous due to the discovery of the first temnospondyl of the
Autun Basin: \mbox{\textit{Onchiodon}} (``\textit{Actinodon}'')
\textit{frossardi} (Gaudry) Werneburg and Steyer, 1999;
``\textit{Protriton petrolei}'' ${=}$ \textit{Apateon pedestris} (Gaudry)
Schoch 1992 \citep{Steyeretal1998} and thousands of complete
actinopterygian fish remains. Between 2010 and 2015, systematic
excavations were headed by some of the authors of this paper (GG and
JSS) to complete the knowledge of the palaeontological content and
taphonomy of the Muse OSB \citep{Gandetal2010, Gandetal2012,
Gandetal2014, Gandetal2015a}. These annual and international
excavations led to the discovery of thin tonstein layers
\citep{Pellenardetal2017}, as well as hundreds of fossil specimens.
Concerning the palaeofauna, insects, possible annelids, acanthodians,
sharks (orthacanthids), bony fish (aeduelliforms) and rare amphibian
remains (a phalanx and a possible dermal bone) have been collected and
are under study \citep{Gandetal2010, Gandetal2012, Gandetal2014,
Gandetal2015a, Gandetal2017, Luccisanoetal2021, Luccisanoetal2023}.

\subsection{Muse palaeobotanical background} \label{sec2.3}

In the Muse OSB, a detailed sampling through the sequence
(Figure~\ref{fig1}C) has been carried out by several authors during the
last decades, and has documented vegetation dominated by marattialean
ferns and cordaiteans, with subsidiary pteridosperms (peltasperms),
medullosalean cycads, and sphenopsids. The Muse OSB contain plant
remains identified by Desa Dordjevic in 2010 \citep{Gandetal2010} and
Isabel van Waveren \citep{Waverenetal2012} and eventually completed by
Galtier in \citet{Gandetal2015b}: Lycopsida (\textit{Sigillaria} sp.,
?\textit{Sigillariophyllum}), Equisetopsida (\textit{Asterophyllites}
sp., \textit{Calamites} sp., \textit{Calamostachys} sp.,
\textit{Sphenophyllum} sp., \textit{Sphenophyllostachis} sp.),
Filicopsida (\textit{Pecopteris} (${=}$\textit{Asterotheca})
\textit{arborescens}, \textit{Pecopteris unita}, \textit{Pecopteris}
sp., \textit{Dizeugotheca} sp., \textit{Scolecopteris} sp.),
Pteridospermopsida (\textit{Neuropteris planchardii},
\textit{Neuropteris} cf.\ \textit{heterophylla}, \textit{Neuropteris}
sp., \textit{Alethopteris} sp., \textit{Callipteridium} sp.,
\textit{Callipteris} sp. (\textit{Autunia} sp.), \textit{Linopteris}
sp., \textit{Odontopteris} sp., \textit{Sphenopteris} sp.) Cordaitales
(\textit{Cordaites principalis, Poacordaites sp., ?Cordaianthus})
Coniferales (\textit{Walchia piniformis}, ?\textit{Pseudovoltzia},
?\textit{Walchiostrobus}), Cycadopsida and reproductive structures
(medullosan and cordaitean) and seeds (\textit{Trigonocarpus} sp.,
\textit{Rhabdocarpus} sp., \textit{Carpolithes} sp.,
\textit{Cardiocarpus} cf.\ \textit{expansus}, \textit{Cardiocarpus} sp.,
\textit{Samaropsis} sp., \textit{Pachytesta} sp.). The presence of
\textit{Taeniopteris multinervis} (\textit{incertae sedis}) is also
reported.

During the inventory at the end of five successive palaeontological
excavations, ca.~600 plant specimens have been collected in the Muse
OSB \citepalias{Gandetal2015b}. The quantitative analysis showed lycopsids
(1\%), equisetopsids (14\%), ferns (20\%), pteridosperms (9\%),
\textit{Cordaites} (18\%), conifers (2\%), and cycadoids (1\%), as well
as \textit{incertae sedis} (1\%), seeds (30\%), and woods (3\%). During
the palaeontological excavations carried out in 2010, the following
taxa were observed in the upper levels of the outcrop (between tonstein
GI and GV, Figure~\ref{fig1}C): \textit{Calamites} (38\%),
\textit{Cordaites} (9\%), ferns and pteridosperms (15.5\%), of which
\textit{Pecopteris} was dominant, while \textit{Walchia piniformis} and
\textit{Alethopteris} were both uncommon (1\% or less)
\citepalias{Gandetal2015b}.

Few works were focused on the palynology of the Muse OSB before:
\citet{Doubinger1960} first identified a preliminary assemblage with a
more detailed study later in \citet{Doubinger1969}. 
\citet{DoubingerElsass1975, DoubingerElsass1979} applied a quantitative
analysis to the latter palynological assemblage \citep{Doubinger1969}
and compared it with others from the Autun Basin. Unfortunately, these
are isolated samples lacking a detailed stratigraphic location. In all
these, the dominant elements are the monosaccate (in particular
\textit{Potonieisporites}, \textit{Wilsonisporites}, and
\textit{Florinites}) and bisaccate (i.e., \textit{Vesicaspora})
gymnosperms, while the main pteridophyte representative is
\textit{Lundbladispora}, frequently found in tetrads. Although scarce,
prasinophycean algae (i.e., \textit{Tasmanites} sp.) were also
identified, which, according to the authors, may have
\mbox{problematic} palaeoenvironmental implications
\citep{DoubingerElsass1975}. 

\section{Material and methods} \label{sec3}

Fifteen samples were collected from the Muse OSB (Figure~\ref{fig1}C).
The samples were processed at the laboratory of the University of Vigo
using the standard palynological technique described by
\citet{Woodetal1996}, consisting in HCl--HF--HCl acid digestion. This
method consists in the addition of HCl and HF to dissolve carbonate and
silicate minerals.

The bituminous characteristic of the samples hindered palynomorph
isolation because these were usually overlapped by flocculent organic
remains. For this reason, we decided to use an alternative method of
palynological preparation procedure, including hydrogen peroxide
\citep{Ridingetal2010}. This experimental technique consisted of
boiling in H$_{2}$O$_{2}$ (30\%) the rock sample previously fragmented
between 1 and 2~mm to disaggregate the organic matter, leading to
obtain different fractions after successively 2, 5, 10, and 20~min.
Then, evaluating the sample under the microscope is necessary to choose
the fraction with a better balance between conservation and the number
of palynomorphs. The oxidising character of the H$_{2}$O$_{2}$ and the
heat of boiling are adequate to eliminate organic remains, yet less
aggressive than the nitric acid technique used for these cases
\citep{Traverse2007}. After 5--10~min, the sample fraction is diluted
in 1~litre of water. A dispersing agent (sodium hexametaphosphate,
(NaPO$_{3}$)$_{6}$) was added to facilitate filtering and sieving at
$10~\upmu$m, and the residue was eventually smeared in glass slides
with a mounting medium of acrylic adhesive Loctite AA~350. These
palynological slides were analysed under a Leica DM 2000 LED
incorporated with a Leica ICC50~W camera at the University of Vigo.

A quantitative analysis was also carried out on all positive samples
(see Supplementary Material). At least 200 specimens per sample were
counted to estimate the relative abundance of each taxon in the whole
Muse OSB section, a standard practice in palynology that ensures
statistically meaningful results for common taxa and minimises errors
in relative abundance estimates \citepalias{Traverse2007}. Samples with less
than 200 palynomorphs were excluded for the palaeoecological
interpretation. Although the data are shown in percentages, they should
be considered qualitative since taphonomic sorting might have obscured
the signature of the original biocenoses. The assignation of each taxa
to major taxonomic groups was based on \citet{Balme1995}, 
\citet{DoubingerGrauvogelStamm1971}, \citet{Dimitrovaetal2011},
\citet{LooyHotton2014}, and \citet{DiMicheleetal2018}.

\section{Results} \label{sec4}

After the application of the alternative methodology, twelve
palynological samples were productive (Figures~\ref{fig2}--\ref{fig4}),
considering them as two different \mbox{palynological} assemblages
according to their composition: Muse-A in the lower part and Muse-B in
the middle-upper part of the Muse section. Both associations were as
diversified as the ones from previous works \citep{Doubinger1960,
Doubinger1969, DoubingerElsass1975, DoubingerElsass1979} and throughout
the entire Muse section, allowing, for the first time, a taxonomical
(Figure~\ref{fig5}) and quantitative evaluation (Figure~\ref{fig6}).
Moreover, several new species were identified belonging to 12 different
genera that were found in the Muse OSB for the first time, including
\textit{Crassispora}, \textit{Densosporites}, \textit{Endosporties},
\textit{Firmysporites}, \textit{Knoxisporites}, \textit{Lophotriletes},
\textit{Lycospora}, \textit{Raistrickia}, \textit{Cycadopites},
\textit{Plicatipollenites}, \textit{Protohaploxypinus}, and
\textit{Vittatina} (Figure~\ref{fig5}). The preservation grade was
generally moderate except for the upper interval of the Muse-B (samples
AUT/03 to AUT/01) which was low.

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Muse-A and Muse-B palynological assemblages:
(A)~\textit{Triquitrites minutus}. (B)~\textit{Triquitrites} sp.
(C)~\textit{Deltoidospora adnata}. (D)~\textit{Deltoidospora priddyi}.
(E)~\textit{Deltoidospora levis}. (F)~\textit{Deltoidospora
sphaerotriangula}. (G)~\textit{Granulatisporites minutus}.
(H)~\textit{Granulatisporites parvus}. (I)~\textit{Granulatisporites
microgranifer}. (J)~\textit{Granulatisporites} sp.
(K)~\textit{Lophotriletes microsaetosus}. (L)~\textit{Lophotriletes}
sp. (M)~\textit{Pilosisporites microspinosus}. (N)~\textit{Thymospora
thiessenii}. (O)~\textit{Thymospora pseudothiessenii}.
(P)~\textit{Punctatosporites minutus}. (Q)~\textit{Punctatosporites
granifer}. (R)~\textit{Spinosporites spinosus}.
(S)~\textit{Raistrickia} sp. (T)~\textit{Lycospora} sp.
(U)~\textit{Knoxisporites glomus}. (V)~\textit{Firmysporites
irregularis}. (W)~\textit{Cyclogranisporites} sp.
(X)~\textit{Punctatisporites} sp. (Y)~\textit{Verrucosisporites
verrucosus}.}
\end{figure*}

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Muse-A and Muse-B palynological assemblages:
(A--B)~\textit{Calamospora} sp. (C)~\textit{Densosporites annulatus}.
(D)~\textit{Densosporites} sp. (E)~\textit{Crassispora kosankei}.
(F)~\textit{Latensina trileta}. (G)~\textit{Lundbladispora} sp.
(H)~\textit{Candidispora candida}. (I)~\textit{Endosporites} sp.
(J)~\textit{Schopfipollenites sinuosus}. (K)~\textit{Schopfipollenites
signatus}. (L)~\textit{Florinites millotti}. (M)~\textit{Florinites
mediapudens}. (N)~\textit{Florinites florini}. (O)~\textit{Vesicaspora
wilsonii}. (P)~\textit{Vesicaspora ovata}.
(Q)~\textit{Schopfipollenites ellipsoides}. (R)~\textit{Wilsonites
vesicatus}. (S)~\textit{Potonieisporites novicus}.
(T)~\textit{Potonieisporites} sp. (U)~Undetermined monosaccate.
(V--W)~\textit{Plicatipollenites} sp.}
\end{figure*}

\begin{figure*}[p!]
\includegraphics{fig04}
\caption{\label{fig4}Muse-A and Muse-B palynological assemblages:
(A)~\textit{Columnisporites peppersi}. (B)~\textit{Laevigatosporites
perminutus}. (C)~\textit{Columnisporites} sp.
(D)~\textit{Laevigatosporites vulgaris}. (E)~\textit{Cheiledonites} sp.
(F)~\textit{Cycadopites} sp. (G)~\textit{Protohaploxypinus
microcorpus}. (H)~\textit{Vittatina} sp. (I--J)~Undetermined
monosaccate pollen. (K--M)~Undetermined bisaccate pollen.
(N)~\textit{Limitisporites} sp. (O)~Undetermined freshwater algae.
(Q)~\textit{Pilasporites} sp. (R)~\textit{Leiosphaeridia} sp.
(P--S)~Fungal structures.}
\end{figure*}

\begin{figure*}[t!]
\includegraphics{fig05}
\caption{\label{fig5}Compilation of previously published and new taxa
from the Muse OSB, Autun Basin, France. Labels, (a)~synonym taxa,
(b)~confer.}
{\vspace*{-1.35pc}}
\end{figure*}

The Muse-A assemblage is dominated by voltzialean and cordaitalean
conifers (mainly \textit{Florinites}, \textit{Potonieisporites}, and
other monosaccate pollen). To a lesser extent, pteridophytes
(Lycopsida, Filicopsida, and Marattiopsida) and medullosalean cycads
(\textit{Schopfipollenites}) are also present.

The Muse-B assemblage highlights a clear dominance of cordaitalean
conifers with \textit{Florinites} (ca.~40--50\%) to the detriment of
the voltzialean palynomorphs. The presence of pteridophytes and
pteridosperms remains relatively constant compared to Muse-A. It is
also noteworthy the increase in lacustrine elements (i.e.,
prasinophycean algae) towards the upper part of the assemblage
(Figure~\ref{fig6}).

\section{Discussion} \label{sec5}
\subsection{The Muse OSB palaeoecology} \label{sec5.1}
\subsubsection{Terrestrial plant community} \label{sec5.1.1}

According to these new results, conifers and pteridosperms are the main
components of the Muse palynofloras with a high diversity and relative
abundance of ca.~50\% and ca.~20\%, respectively (Figure~\ref{fig6}).
Although less abundant, pteridophytes are well represented through both
lycophytes and ferns with a relative abundance of ca.~15\%. As stated
above, a division between the Muse-A and Muse-B assemblages has been
established mainly due to significant differences in the type
of{\pagebreak} conifers.{\unskip\break}

\onecolumngrid
\begin{sidewaysfigure*}
{\vspace*{20pc}}
\includegraphics{fig06}
\caption{\label{fig6}Quantitative analysis of the MUSE-A and MUSE-B
assemblages from the Muse OSB (Muse section, Autun, France), focusing
on the main palynological groups. Samples AUT/01-AUT/03 were not
plotted due to its low relative abundance ($&lt;$200 palynomorphs). Legend
for stratigraphic log in Figure~\ref{fig1}.}
\end{sidewaysfigure*}
\twocolumngrid

\noindent
In the Muse-A assemblage, Cordaitales (i.e., \textit{Florinites}) and
Voltziales (i.e., \textit{Potonieisporites}) are the main conifer
representatives.\ By constrast, in the Muse-B assemblage, Voltziales
decrease dramatically, giving way to Cordaitales sole dominance.
Although the observed increase in mean particle size in the Muse-B
assemblage (see Supplementary Material) may suggest a taphonomic
overprint, we consider this an unlikely explanation for the observed
shift in palynological composition. Notably, cordaitalean
pollen---typically smaller than voltzialean pollen---becomes more
abundant in the upper part of the section, while voltzialean pollen
virtually disappears. Were hydrodynamic sorting the primary driver, one
would expect a relative enrichment of larger pollen types, such as
voltzialean \textit{Potonieisporites}, in the upper levels of the
section. However, this pattern is not observed. This inverse
relationship suggests that the compositional change is unlikely to be
taphonomically controlled and may instead reflect broader environmental
or vegetational changes. In any case, the Cordaitales conifers, seed
ferns, cycads, and pteridophyte spores apparently constitute the
autochthonous elements of the palynoflora, while the Voltziales
conifers and other bisaccate pollens (such as \textit{Pityosporites} or
\textit{Alisporites}) may be the allochthonous elements transported to
the palaeo-lake corresponding to the Muse OSB. 

These new results are consistent with previous palynological studies
\citep{Doubinger1960, Doubinger1969, DoubingerElsass1975,
DoubingerElsass1979}: in the Muse section, isolated samples also
recorded assemblages with a predominance of monosaccate pollen
corresponding to cordaitalean and voltzialean conifers and putative
peltasmermalean seed ferns (\textit{Wilsonites}), as well as lycophytes
represented by \textit{Lundbladispora} spores, frequently found in
tetrads). It is noteworthy that several palynological zones were also
differentiated by \citet{Chateauneufetal1992a} based on comparative
palynological data from the Autun Basin \citep{DoubingerElsass1979}.

\looseness=-1
The previous vegetal macro-remains found in the Muse OSB are generally
similar to the palynological data 
\citep[e.g., records of \textit{Sigillaria}
and \textit{Pecopteris};][]{Doubinger1994}. However,
contrary to the palynological findings, conifer remains in the
macroflora associations are few in the Muse OSB. A taphonomic bias
could explain why silicified conifer wood has been discovered in the
Lally upper sandstones Member within the base of the Muse
Fm.\ \citep[Figure~\ref{fig1}B;][]{DoubingerMarguerier1975,
MarguerierPacaud1980,Gandetal2015b}.
These fossil trunks show similar orientations and exhibit traces of
abrasion on their surface{\break} \citep{MarguerierPacaud1980},
indicative of transport in running water \citep{Gandetal2015b}. It is
suggested that the silicification process occurred near the growth
site, and, in some cases, the trunks were preserved in the growth
position \citepalias{Gandetal2015b}. According to \citetalias{Gandetal2015b},
these remains belong to the \textit{Dadoxylon} group of type~II, which
would correspond to walchian conifers 
\citep[Voltziales;][]{DoubingerMarguerier1975}, \textit{Metacordaites
rigollotii} and \textit{Scleromedulloxylon varollense}, reflecting a
high diversity. Also, close to the Muse locality, another type of
\textit{Dadoxylon} wood (\textit{Cordaixylon}; related to Cordaitales)
and silicified fragments of \textit{Psaronius}, \textit{Arthropitys},
and \textit{Sigillaria} have been found \citep{Broutinetal1999,
Gandetal2015b}. Moreover, the compilation of the macroflora findings
resulted in a classification of the floral associations by different
ecological groups \citep{Waverenetal2012}: hygrophytic (e.g.,
\textit{Equisetopsida}), hygro-mesophytic (\textit{Cordaites
principalis}, \textit{Neuropteris planchardii}, and \textit{Asterotheca
arborescens}), and mesophytic plants (\textit{Walchia piniformis},
\textit{Sphenopteris} sp., \textit{Dicranophyllales}, and Cycadopsida).

Therefore, after the integration of our new palynological data with the
previous palaeobotanical studies, the Muse OSB plant community
corresponds to a peat swamp forest dominated by cordaitalean and
(occasionally) walchian conifers, with the presence of medullosalean
cycads and callistophytalean seed ferns, as well as an understory of
lycophytes and ferns (primarily marattialean). Surrounding this
community, distant from the lake, a forest dominated by walchian
conifers would also be present (Figure~\ref{fig7}). According to the
quantitative palynological analysis, the vegetation dynamics of the
Muse OSB were apparently stable except for the Cordaitales, which
increased towards the upper part of the unit in to the detriment of the
Voltziales (Figure~\ref{fig6}).

\begin{figure*}
\includegraphics{fig07}
\vspace*{-.1pc}
\caption{\label{fig7}Palaeoecological reconstruction of the plant and
phytoplankton communities in the Muse OSB.}
\vspace*{-.5pc}
\end{figure*}

\subsubsection{Lacustrine phytoplankton community} \label{sec5.1.2}

Several aquatic palynomorphs were also found in both Muse-A and Muse-B
assemblages with an increasing relative abundance upwards
(Figure~\ref{fig6}). Some of these elements resembling
\textit{Leiosphaeridia} (Figure~\ref{fig4}R) and \textit{Pilasporites}
(Figure~\ref{fig4}Q)
are present throughout the Muse OSB. Both taxa are
polyphyletic due to their simple morphology but can be
\mbox{considered} non-marine algae.\ In the case of
\textit{Leiosphaeridia} sp., it corresponds to green algae, which
includes phycomatas of pyramimonadalean prasinophytes (primarily
marine) or vegetative cells of Trebouxiophyceae (primarily non-marine)
\citep{Maysetal2021}. \textit{Pilasporites} have been considered as
vascular plant spore (Equisetales and Isoetales) or as zygospores of
non-marine green algae (Zygnematophyceae), the latter affinity being
the most likely according to algae co-occurrence and its morphology
\citepalias{Maysetal2021}.

In previous palynological studies, the presence of prasinophycean algae
was also observed. Due to their marine affinity, these algae had
controversial implications for the palaeoenvironmental interpretation
\citep[i.e., \textit{Tasmanites} sp.;][]{DoubingerElsass1975}.
However, non-marine
prasinophycean species also exist \citep{Traverse2007}, allowing the
presence of \textit{Tasmanites}-like palynomorphs without marine
influence. Moreover, the exclusively marine nature of
\mbox{\textit{Tasmanites}} is currently under debate, as these \mbox{algae}
have recently been found in Permian freshwater lacustrine
deposits in East Timor \citep{Lelono2019}.\ In any case, we consider
the \textit{Tasmanites} present in \citet{DoubingerElsass1975} as
\textit{Pilasporites} instead, after comparing their illustration
\citepalias[][plate~1, Figure~15]{DoubingerElsass1975} to ours
(Figure~\ref{fig4}Q). This re-assignation would be consistent with the
purely freshwater lacustrine environment corresponding to the Autun
Basin \citep[e.g.,][]{Gandetal2007, Gandetal2015a, Mercuzotetal2022,
Luccisanoetal2023}.

\subsection{Palaeoecological and palaeoenvironmental implications}
\label{sec5.2}

Beginning with the Carboniferous--Permian transition, the early to late
Permian exhibits a progressive climate shift towards more arid
conditions and a subsequent succession of floras adapted to \mbox{wetland}
first, seasonally dry then, and, eventually, arid with a wet season
\citep{Broutinetal1990, DiMicheleAronson1992, FalconLang2003,
DiMicheleetal2006}. However, the earliest Permian plant communities
usually present transitional floras with Carboniferous and Permian
characteristics. Several pteridophytes belonging to the
Carboniferous-type wetland biome \citep{Juncaletal2019} were found in
the Muse palynofloras. In addition, medullosalean cycads and
callistophytalean seed ferns, also typical Carboniferous elements, have
a common occurrence in the Muse palynological \mbox{assemblages}.
\looseness=-1

The presence of these wetland elements together with xerophytic Permian
elements (i.e., Voltziales) indicates that the Autun Basin remained
under the influence of a seasonally dry climate, but maintaining enough
moisture for the persistence of these hygrophytes during at least
${\sim}$300~ky in the early Asselian. These environmental conditions
allowed plants producing \textit{Densosporites},
\textit{Punctatosporites}, \textit{Spinosporites}, and
\textit{Thymospora} to survive in a context of instability and gradual
evolution to drier conditions throughout the Permian
\citet{Broutin1986, Juncaletal2019}.

However, the main plants in the Muse section are xerophytic or tolerant
to drier climates, as is the case of the conifers
\citep{LyonsDarrah1989}. The best example is walchian conifers
(Voltziales), which were restricted to seasonally dry habitats and
considered as a marker of semi-arid floras \citep{Ziegleretal2002,
DiMicheleetal2010}. The Cordaitales, also typical of seasonally dry
conditions \citep{Kerp1990, AlgeoScheckler1998}, were usually adapted
to wetter environments than walchian conifers
\citep{Bashforthetal2014}. In this case, they inhabited lowland peat
mires where they formed mostly monotypic stands or were part of
heterogeneous vegetation thriving interspersed among arborescent ferns
(such as \textit{Calamites}) and lycophytes \citep{RaymondPhillips1983,
TrivettRothwell1991, Raymond1988, DiMichelePhillips1994}.

Walchian conifers were exclusively related to topographically high
environments \citep[e.g.,][]{RothwellMapes1988}. However, recent
Palaeozoic studies on vegetation dynamics noticed that variation in
walchian dominance was not affected by topographical changes but by
climatic shifts between wet and dry conditions
\citep{FalconLangetal2009, Dolbyetal2011, DiMichele2014}.\ Therefore,
the \mbox{differences} between the Muse-A and Muse-B assemblages
related to the conifer composition may reflect climate change. The
walchian conifers may be established as a permanent community in
distant parts, which may be topographically higher (allochthones in
Muse-A and Muse-B assemblages; Figure~\ref{fig7}), while they expanded
towards the communities closer to the palaeo-lake during drier phases
due to more marked seasonality (parautochthonous in Muse-A assemblage;
Figure~\ref{fig7}).

This trend is observed throughout the latest Carboniferous--early
Permian transition, based on the palynological biozones established for
the Autun Basin \citep{Chateauneufetal1992a}. After a pronounced
decrease of spores since the late Carboniferous, walchian-pollen
(\textit{Potonieisporites}) settles as one of the main components of
the early Permian palynofloras. In the case of Cordaitales pollen
(\textit{Florinites}), it was present since the latest Carboniferous
remaining solid as a main component throughout the early Permian. There
is an inversely proportional relationship between these two floral
components in the same ecological niche. The higher the proportion of
Walchian pollen, the lower the proportion of Cordaitales, and vice
versa. As we have seen, this was most likely influenced by climate.
This pattern was also found during the Pennsylvanian Coal Age,
suggesting an increase in seasonality when walchian conifers dominated
\citep{DiMichele2014}. Therefore, in the Autun Basin, the
walchian-cordaitalean pollen relation may be used as a proxy for
seasonality.

Another significant aspect of Muse palaeoecology is related to the
lacustrine phytoplankton community. As mentioned before, the
quantitative analysis of the lacustrine elements in the Muse OSB
exhibits an increase towards the upper part of the Muse-B assemblage
(Figures~\ref{fig6}--\ref{fig7}). This green algae maximum interval
corresponds to a sequence of tonstein occurrence (Figure~\ref{fig1}C)
and, therefore, to a period of active explosive volcanism
\citep{Pellenardetal2017}. Volcanic ash affects the nutrient and light
availability in marine and freshwater algae communities, frequently
producing phytoplankton blooms or enhanced growth
\citep[e.g.,][]{Hammeetal2010, Modenuttietal2013, Browningetal2014}.
Consequently, the increase in the relative abundance of green algae in
the Muse section may be related to enhanced growth of the lacustrine
phytoplankton \mbox{community} after a more active volcanic period. It
is worth noting that sample AUT/07A, located directly above one of the
volcanic layers (GVIII), shows a notably low proportion of lacustrine
palynomorphs. This exception may reflect local taphonomic or
environmental conditions (e.g., limited water availability or rapid
burial), which may have restricted phytoplankton development despite
the potential nutrient input. However, the overall trend across Muse-B
remains consistent with a volcanically induced productivity pulse.

\section{Conclusions} \label{sec6}

The understanding of the Muse Formation in the Autun Basin, France, has
significantly advanced through ongoing palaeontological investigations
and geological analyses spanning back to the 19th century. In this
regard, a comprehensive reassessment of prior palaeobotanical inquiries
has been undertaken alongside a novel taxonomic and quantitative
palynological examination across the entire Muse Formation. These
endeavours aim to elucidate the fluctuations in floral composition
within the predominantly lacustrine sediments of this formation.

The plant community corresponding to the Muse OSB is identified as a
peat swamp forest characterised by a prevalence of cordaitalean
conifers. Additionally, this community gathers medullosalean cycads and
callistophytalean seed ferns, complemented by an undergrowth comprising
lycophytes and ferns (including marattialean species). Adjacent to the
lake and topographically higher, a forest dominated by walchian
conifers (Voltziales) would also be present.

The vegetation dynamics of the Muse OSB would be relatively stable,
except for the Cordaitales, which exhibit an increment towards the top
of the unit to the detriment of the Voltziales. This pattern, also
observed throughout the Autun Basin deposits, could be related to an
increase of wetter conditions. However, taphonomical or preservation
factors should not be excluded. Moreover, the persistence of
palynomorphs belonging to the Carboniferous wetland biome indicates the
presence of environments with enough moisture for the survival of
hygrophyte plants in the Autun Basin for at least ${\sim}$300~ky after the
Carboniferous--Permian boundary.

The phytoplankton community consisted of green algae, including
Trebouxiophyceae and Zygnematophyceae. A higher relative abundance was
observed in the upper part of the Muse OSB, corresponding to enhanced
growth of this community that was likely affected by increased nutrient
availability due to enhanced distal explosive volcanism.

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organisation that could benefit from this article, and have
declared no affiliations other than their research organisations.

\section*{Acknowledgements}

We gratefully acknowledge the constructive suggestions of the editor
Dr.~Sylvie Bourquin, as well as the two anonymous reviewers. This
research was partially financed by the Ministerio de Ciencia y
Innovaci\'{o}n of the Spanish Government (ref.: PID2022-141050NB-I00),
and the research project ``CALDERA - CALibrating a Deglaciation ERA:
Decline of the Late Palaeozoic Ice Age and its consequences for
tropical terrestrial ecosystems'' financed by the Promotion of
Educational Policies, University and Research Department of the
Autonomous Province of Bolzano - South Tyrol (CUP H33C23001340003).

\CDRGrant[MICIU]{PID2022-141050NB-I00}
\CDRGrant[URD]{CUP H33C23001340003}

\back{}

\section*{Supplementary data}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crgeos-294-suppl.pdf}}

\printbibliography
\refinput{crgeos20250032-reference.tex}

\end{document}
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