Trilobite palaeobiodiversity during the Devonian in the Ardennes Massif

Abstract

Trilobite occurrences and biodiversity from Devonian sections of the Ardennes Massif are presented. Fluctuations in diversity
reflect environmental changes and event-stratigraphic turnovers throughout the Devonian. Peak diversity is attained during
the Eifelian (Middle Devonian) when the carbonate system is developed. Six trilobite associations have been delineated based
on cluster analysis. Distribution of these associations is correlated with the deposition environments and age of the strata.

Full text

Bull. Soc. géol. Fr., 2007, n
o
6
Trilobite palaeobiodiversity during the Devonian in the Ardennes Massif
CATHERINE CRÔNIER
1
and ALLART van VIERSEN
2
Key-words. – Trilobites, Devonian, Ardennes Massif, Biodiversity, Assemblages
Abstract. – Trilobite occurrences and biodiversity from Devonian sections of the Ardennes Massif are presented. Fluc
-
tuations in diversity reflect environmental changes and event-stratigraphic turnovers throughout the Devonian. Peak di
-
versity is attained during the Eifelian (Middle Devonian) when the carbonate system is developed. Six trilobite
associations have been delineated based on cluster analysis. Distribution of these associations is correlated with the de
-
position environments and age of the strata.
Paléobiodiversité des trilobites en Ardenne occidentale au Dévonien
Mots-clés. – Trilobites, Dévonien, Ardenne occidentale, Biodiversité, Assemblages
Résumé. – Dans les environnements favorables du Dévonien de l’Ardenne occidentale, les trilobites sont des compo
-
sants majeurs du macrobenthos. Une analyse des changements de la paléobiodiversité et une modélisation de la réparti
-
tion des communautés à trilobites du massif des Ardennes au Dévonien sont proposées. Le pic de diversité est atteint à
l’Eifélien (Dévonien moyen) quand le système carbonaté est développé. Six associations ont été reconnues à l’aide des
analyses de classification hiérarchique. La distribution de ces associations est corrélée aux conditions écologiques et
temporelles.
INTRODUCTION
The Ardennes Massif is part of a complex of Palaeozoic
outcrops between the English Channel in the west and the
Rhine River and beyond in the east. Originally part of Ava-
lonia, a terrane located north of the Gondwana, it became an
element of the southern margin of the Old Red Sandstone
Continent in the Devonian and Carboniferous, when it suf
-
fered the effects of the Hercynian Orogeny. These global
tectonic events, linked to climatic changes due to continen
-
tal drift, had profound consequences on the living orga
-
nisms of the Ardennes Massif.
Recent works [Meilliez, 1989; Cibaj, 1992; Goffette,
1991; Lacquement, 2001] have reported knowledge of the
tectonic and geodynamical context of the Ardennes area;
they have allowed revision of the stratigraphy [Bultynck et
al., 1991; Bultynck and Dejonghe, 2002] and of the geolo
-
gical map of Givet [Lacquement et al., 2003]. In addition,
these works have shown the need to improve knowledge of
the palaeoenvironments represented by numerous stratigra
-
phic units such as the formation cropping out at the ‘Mur
des douaniers’ in the French Ardennes [Crônier and van
Viersen, 2008]. This knowledge must be based on a revision
of the sedimentological and faunal contents. Such fossilife
-
rous outcrops must be studied in the contexts not only of the
taxa present where the biodiversity can be estimated but
also of the paleoecology and paleoenvironments.
Trilobites are important elements of the Devonian ma-
crobenthos. During this period, one of the most severe res-
trictions in trilobite diversity on a global scale occurred: at
the end of the early Late Devonian, during the global Kell-
wasser Crisis [Schindler, 1990]. At that time, most trilobites
that were adapted to outer - shelf level - bottom habitats af
-
ter the mid-Givetian Taghanic Onlap, i.e. the major trans
-
gressive event of the Givetian Age [Johnson, 1970], were
affected by repeated rises of sea-level leading to a
world-wide ‘drowning’ of reef ecosystems and reductions
of shallow water habitats [Schlager, 1981; Feist, 1991].
Long-lived orders such as the Corynexochida, Harpetida
and Lichida disappeared, and the two orders surviving the
crisis, the Phacopida and Proetida, were severely affected.
In this context, we focussed our investigations on repre
-
sentative trilobite-bearing sections of the Devonian Ardennes
Massif, in order to evaluate this biodiversity. This work,
which started in the spring of 2003 by one of us (A.vanV.), is
an account of numerous observations in the Belgian Arden
-
nes and a first estimate of Belgian Devonian trilobite paleo
-
biodiversity. These Belgian faunas appear to be highly
diverse but are poorly documented, not being as well known
as those from the German Eifel and French Ardennes.
Bull. Soc. géol. Fr., 2007, t. 178, n
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6, pp. 473-483
1. Université de Lille 1, Sciences de la Terre, UMR 8014 du CNRS, Laboratoire de Paléontologie et Paléogéographie du Paléozoïque, F-59655 Villeneuve
d’Ascq cedex. Catherine.Cronier@univ-lille1.fr
2. Graaf van Loonstraat 25, NL-6121 JS Born, The Netherlands. apvanviersen@hotmail.com
Manuscrit déposé le 30 novembre 2006; accepté après révision le 21 mai 2007.
Séance spécialisée
Sédimentation et structuration
du front nord varisque et évolution
méso-cénozoïque : exemple de l’Ardenne
Givet, 4-6 mai 2006

GEOLOGICAL SETTING AND MATERIAL
Several sections from the Devonian Ardennes Massif, loca
-
ted in the ‘Allochthonous Ardennes’ [Meilliez and Mansy,
1990; Mansy and Meilliez, 1993], along the southern and
northern-eastern marging of the Dinant synclinorium (fig. 1)
are known to yield trilobites. These sections include the fa
-
mous fossiliferous site known as the ‘Mur des douaniers’
[van Viersen, 2006a; Crônier and van Viersen, 2008]. The tri
-
lobites from these sites have remained largely undocumen
-
ted. Moreover, most of these trilobite-bearing sections have
not yet been described with respect to their sedimentologi
-
cal/faunal content and bioevent stratigraphy. Here, ten sec
-
tions with trilobite records of varying quality – some of the
sample sizes are small – are documented in order to obtain a
preliminary assessment of paleobiodiversity in the western
Ardennes (fig. 1). The main focus is on the evolution of trilo
-
bite associations from the Early to Late Devonian and pos
-
sible correlations with sea-level variations.
BIODIVERSITY ANALYSIS THROUGH DEVONIAN
TIME
The study of palaeobiodiversity, especially at regional and
local scales, is meaningful even though estimate of Phane-
rozoic global biodiversity is limited by biases that may be
present in the fossil record. Important biases include un-
der-representation of some areas, inaccuracies in taxonomic
data and uneven sampling intensity across groups of orga
-
nisms, environments, and time intervals [Badgley, 2003].
Recognition of these methodological problems has resulted
in notable improvements in the Phanerozoic diversity data
-
base [Adrain and Westrop, 2000; Alroy et al., 2001].
Fluctuations in biodiversity
In the Ardennes Massif, trilobites are mainly represented by
nine families: Styginidae (Corynexochida), Lichidae and
Odontopleuridae (Lichida), Homalonotidae, Phacopidae
and Acastidae (Phacopida), Proetidae, Tropidocoryphidae
and Aulacopleuridae (Proetida). Another family, Harpetidae
(Harpetida), is rare. For comparison, trilobite paleobiodi
-
versity from Early to Late Devonian at the global and regio
-
nal scales (Ardennes Massif) is given in figure 2A-B.
Lower Devonian
Trilobites seem to be poorly diverse macrofaunal consti
-
tuents in the Lower Devonian of the Ardennes Massif, in
contrast with their worldwide diversity (fig. 2A-B). At the
global scale, trilobite diversity is relatively poor (fig. 2A,
2C). Only a marginal decrease in diversity during this time
seems to exist. In fact, according to Chlupá [1994], this lo
-
wered trilobite diversity persists from the Silurian into the
Lochkovian with no drastic changes, only minor extinctions
and more important migrations. The Basal Pragian Event of
regressive character, i.e. an eustatic sea level lowering
[Chlupá and Kukal, 1986] contributed to widening of shal-
474
CRÔNIER C. and VA N VIERSEN A.
Bull. Soc. géol. Fr., 2007, n
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FIG. 1. – Geological map showing the location of several Devonian fossiliferous sections in the Ardennes Massif, southern Belgium [modified after De
-
jonghe, 2001].
F
IG.1.–Localisations géographique et géologique des différents sites fossilifères du Dévonien en Ardennes occidentale, Wallonie, Belgique [modifié
d’après Dejonghe, 2001].

low marine realms with carbonate sedimentation favourable
for trilobites. This led to an increase of the generic diversity
[Chlupá, 1994] and, at least, to a preservation of the family
diversity which lasted with some changes until the early Ei
-
felian.
For the regional scale, at the beginning of the Devonian
Period, a major marine transgression from the south-west
reached the northern borders of the Ardennes and Eifel
[Ziegler, 1982; Steemans, 1989]. Erosion of several ancient
massifs, such as the Rocroi Massif, led to important shaly
Bull. Soc. géol. Fr., 2007, n
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TRILOBITE PALAEOBIODIVERSITY IN THE ARDENNES MASSIF
475
FIG. 2. – Total occurrences of trilobite taxa (A) at the global scale [modified from Feist, 1991; Chlupá1, 1994], (B) in the Ardennes Massif; total family
occurrences for each trilobite order (C) at the global scale [modified from Feist, 1991; Chlupá1, 1994], (D) in the Ardennes Massif; total (E) subfamily
and (F) genus occurrences of each trilobite order in the Ardennes Massif, from Early to Late Devonian.
F
IG.2.–Nombre total de trilobites (A) à l’échelle mondiale [modifié d’après Feist, 1991 ; Chlupá1, 1994], (B) dans le massif des Ardennes ; nombre total
de familles pour chaque ordre de trilobites (C) à l’échelle mondiale [modifié d’après Feist, 1991 ; Chlupá1, 1994], (D) dans le massif des Ardennes ;
nombre total de (E) sous-familles et (F) genres pour chaque ordre de trilobites dans le massif des Ardennes, au Dévonien.

and sandy deposits [Godefroid et al. 1994] in which trilobi
-
tes are rare. The paleobiodiversity is low during the Loch
-
kovian-Pragian, characterized exclusively by Phacopida
(fig. 2D-F). Only two families occur, and those are rare:
Acastidae and Homalonotidae. New discoveries in the eas
-
tern Ardennes (Treveropyge, Dunopyge and Acastava;un
-
pub. data from van Viersen) corroborate the occurrences of
two subfamilies of the Acastidae: Acastavinae and Astero
-
pyginae, as mentioned by Maillieux [1933] and Asselberghs
[1946]. Homalonotinae are represented by Digonus ornatus
disornatus R
ICHTER &RICHTER and Scabrella asselberghsi
(M
AILLIEUX), among others. Wenndorf [1990] resolved
many of the synonyms within this subfamily in the Arden
-
nes and Eifel areas. Additional study and fieldwork are re
-
quired to revise and complete the list of Early Devonian
trilobites in the Ardennes.
Trilobite diversity is greater in the Emsian than in the
Lochkovian and Pragian but is not particularly high
(fig. 2D-F). Increase in diversity is linked to the opening of
the marine habitat during the Early Devonian. During the
Middle Emsian, a regressive episode occurred which resul
-
ted in characteristic red shales [Godefroid et al., 1994].
Three additional families were recorded by Maillieux
[1933]: Proetidae, Phacopidae and Odontopleuridae. So far,
none of these records could be corroborated although it
seems likely that these families actually occur in the Bel-
gian Emsian. Treveropyge is among the most common Aste-
ropyginae of the Ardennes-Eifel area during this time.
Locally, another common trilobite genus is Digonus (Homa-
lonotinae).
Middle Devonian
At the global scale (fig. 2A, 2C), the general decline in tri-
lobites, marked by stepwise diversity falls, was strongly ac-
celerated by the Middle Devonian Kacák and Taghanic
events in the late Eifelian and middle Givetian respectively
[Chlupá, 1994].
At the regional scale, a more major transgressive phase
[Ziegler, 1982; Bultynck et al., 1991] leads to more diversi
-
fied trilobites (fig. 2D-F). Middle Eifelian limestones in
particular yield a rich trilobite fauna, including a number of
species known also from the German Eifel [Struve, 1982;
Basse, 1996, 1997, 1998; Magrean and van Viersen, 2005;
van Viersen, 2007]. The initiation of carbonate deposition,
characterised by shallow, warm and clear seas, favoured the
appearance of Corynexochida, Proetida and Lichida (Li
-
chidae being generally linked to neritic and peri-reefal envi
-
ronments [Feist, 1991]), and the diversification of
Phacopida among trilobites. Stromatoporids [Lecompte,
1951-1952], corals [Tourneur, 1985], crinoids and brachio
-
pods [Godefroid, 1995] also occur. Peak diversity was attai
-
ned during the Eifelian when typically Rhenish faunas
flourished. Their generic diversity is three times that during
the Emsian (fig. 2D-F). All orders present at the global
scale are present also in the Ardennes Massif, with Phacopi
-
da and Proetida dominant. Harpetida are rare.
Eifelian-Givetian transitional sequences are comparati
-
vely well exposed in some places, especially in the Roche
-
fort area. Trilobites from the Givetian are mostly poorly
documented (fig. 2B, D-F). Calycoscutellum, Dechenella,
Gerastos and Nyterops have been identified, and notably
Dechenella, Gerastos and Nyterops in the Rochefort biota
during the Early Givetian (see the next section, fig. 4).
Upper Devonian
The Belgian Upper Devonian is not particularly rich in tri
-
lobites although fragmentary material may be abundant lo
-
cally. The basal Frasnian is represented by a mainly shaly
unit that corresponds to a major transgressive phase [Bul
-
tynck et al. 2000]. At some localities on the southern border
of the Dinant Synclinorium, reef cycles may be observed
[Boulvain, 2006]; each period of reefal growth is interrup
-
ted by a phase of sea-level increase leading to terrigenous
deposits [Boulvain et al., 1999]. Trilobites are mostly res
-
tricted to Middle Frasnian units, especially for Proetida and
Phacopida (fig. 2D-F). A third remaining order, Corynexo
-
chida seems to be present (figs. 2D-F). Indeed, Richter and
Richter [1926] and Maillieux [1927] pointed out some re
-
cords of Scutellum. Nevertheless, no new samples were col
-
lected in this study.
The Frasnian-Famennian crisis led to the extinction of
almost all reefs and their associated trilobites (Lichidae),
and a significant decrease in other trilobite taxa.
The Famennian is associated with an important regres
-
sive phase with terrigenous deposits on a shallow platform
in the process of filling [Thorez et al. 2006; Bultynck and
Dejonghe, 2002]. A record of a Famennian trilobite, Pha-
cops (Omegops) accipitrinus maretiolensis R
ICHTER &
R
ICHTER, 1933, seems to exist [Rome, 1936; Struve, 1976].
Nevertheless, no new samples were collected in this study.
Associative and ecological structures in biodiversity
In order to understand the biodiversity better, a preliminary
analysis of trilobite associations from the Devonian of the
Ardennes Massif has been undertaken. The term association
is used here for assemblages of trilobites with similar taxo
-
nomic composition [Brenchley and Harper, 1998; Thomas
and Lane, 1999; Chlupá, 1987]. Assemblages are generally
characterised both by their species composition and the re
-
lative abundance of individuals of the different species pre
-
sent. This analysis has focused on the quantitative aspect,
especially cluster analyses, which are commonly applied to
palaeoecological problems [Cugny, 1988; Brenchley and
Harper, 1998; Botquelen et al. 2006], notably on Cam
-
brian-Ordovician trilobites [Ludvigsen and Westrop, 1983],
and helped in the identification of associations.
The biodiversity of sampled faunas is represented in our
study by 33 genera and two subfamilies that refer to a
single, undetermined genus (fig. 4), for 19 samples from
Lochkovian to Frasnian strata (fig. 1). Data have been ana
-
lysed with hierarchical cluster analyses. The relative abun
-
dances of the 35 genera/subfamilies have been quantified in
a separate way in eight classes (fig. 4). Hierarchical cluster
analysis based on the trilobite abundance classes was achie
-
ved using PAST 1.04 software [Hammer et al., 2001]. Clus
-
ter analysis is an agglomerating technique that groups
together objects by level of similarity [Davis, 1986; Harper,
1999]. Results of Q-mode clustering (grouping of samples)
and R-mode clustering (groupings of taxa) are presented
as dendrograms, which were achieved using paired-group
method with the Pearson Similarity Coefficient. Groups of
Bull. Soc. géol. Fr., 2007, n
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476 CRÔNIER C. and VA N VIERSEN A.

Bull. Soc. géol. Fr., 2007, n
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TRILOBITE PALAEOBIODIVERSITY IN THE ARDENNES MASSIF
477
FIG. 3. – (A) Changes in trilobite faunal diversity,
dominance and evenness through 19 sites from Early
to Upper Devonian of the Ardennes Massif. (B) Re
-
lative abundance of trilobite taxa in the ‘Rochefort’
site from the Middle Eifelian.
F
IG.3.–(A)Changement dans la diversité, la do
-
minance et la régularité des faunes trilobitiques de
19 sites provenant du Dévonien des Ardennes.
(B) Abondance relative des taxons de trilobites dans
le gisement de ’Rochefort’ de l’Eifélien moyen.

Bull. Soc. géol. Fr., 2007, n
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478 CRÔNIER C. and VA N VIERSEN A.

genera with a high probability of mutual occurrence are
thus grouped together.
In order to complete the palaeoecological information
of these assemblages, three ecological indices have been
used (fig. 3): diversity, dominance and evenness indices
[Brenchley and Harper, 1998]. Diversity measures (Marga
-
lef Div = S-1/log N; S = number of species, N = number of
specimens) are usually standardized against the sample size
whereas dominance measures (Dom = S (ni/N)
2
; ni = num
-
ber of i
th
species), based on relative abundance, have high
values for assemblages with a few common elements, and
low values where species are more or less evenly represen
-
ted; measures of evenness (Even = 1/S (ni/N)
2
), are thus
usually the inverse of dominance.
Symbols have been used to identify each site and age: C
= Couvin, F = Ferrières, MF = Marche-en-Famenne, N =
Nassogne, R = Rochefort, SR = Sougné-Remouchamps, T =
Treignes,V=Vireux-Molhain,W=Wellin; L = Lochko
-
vian, P = Pragian, Es = Emsian, Ef = Eifelian,G=Givetian,
Fr = Frasnian;l=lower,m=middle, u = upper.
Ecological structure of samples
Cluster analysis performed on palaeoecological parameters
has permitted recognition of assemblage types on the basis
of the relative abundance and diversity of the trilobite ma-
crofauna. Thus, four specific assemblage types are defined
as follows (fig. 3A):
– type A characterised by only 1 species, from the Lo-
wer Devonian: SR-L, F-uP, N-uEs, T-uEs;
– type B characterised by an important dominance from
the Lower Devonian, lower and upper parts of the Middle
Devonian and Upper Devonian: MF-lEs, MF-uEs, W-lEf,
R-uEf, R-lG, C-mFr. For example, R-uEf biota is dominated
by Calycoscutellum, R-lG by Nyterops and C-mFr by Bra-
docryphaeus;
– type C characterised by a moderate diversity and
evenness: MF-uP/lEs, V-lEf, C-lEf, C-mEf, C-uEf/lG;
– type D characterised by an important diversity and
evenness: Rochefort site from Middle Devonian (R-mEf).
The distinction between ecological assemblages and
those that arise from transport can be difficult. Assemblages
that are either in-situ or nearly in-situ because they show no
signs of prolonged transport could represent a former com
-
munity or a palaeocommunity [Brenchley and Harper,
1998]. Species within a community are associated partly be
-
cause they have similar tolerances of physical aspects of the
environment and partly because they interact with one ano
-
ther through a food chain or through complementary niche
requirements [Brenchley and Harper, 1998]. Moreover, in
‘pioneer’ communities, there occur one or two very abun
-
dant, opportunistic species. By contrast, in ‘equilibrium’
communities, there is a relatively high diversity of taxa
more or less equally present [Brenchley and Harper, 1998].
Some sites from the Eifelian (types C-D) could represent
‘equilibrium’ communities: for example, V-lEF [Crônier
and van Viersen, 2008] or R-mEf (see the next section).
The only well known Devonian trilobite fauna from the
Ardennes is that of the famous ‘Mur des douaniers’ near Vi
-
reux-Molhain (V-lEf), southern Ardennes, France [van
Viersen, 2006a; Crônier and van Viersen, 2008]. Neverthe
-
less, of particular interest is the Middle Eifelian Rochefort
site yielding a rich, diverse trilobite fauna that shows a re
-
markable resemblance to the trilobite fauna of the German
Eifel [van Viersen, 2007]. This fauna is dominated by mem
-
bers of the Proetida, which is represented by six genera of
trilobites, representing half the total number of specimens
collected (51%, fig. 3B). The other groups with a moderate
relative abundance are Phacopida (27%) with five genera,
Odontopleuridae (15%) and Lichidae (6%). Corynexochida
is uncommon (abundance 3%).
Associative structure of samples
The hierarchical cluster analysis performed on all taxa al
-
lows us to delineate six discrete trilobite associations re
-
flecting their taxonomic affinities and partly related to
spatial and temporal gradients (fig. 4).
I) The Digonus Association characterises the Lower De-
vonian: Praguian-Emsian (Nassogne, Ferrières, Marche-
en-Famenne). This genus is typically found in sandstones
that were deposited near-shore. Because of this turbulent
environment favoured by Digonus, articulated specimens
are particularly rare. Mass occurrences are not exceptional
in homalonotines, as is also the case in other regions
[Gandl, 1972; Wenndorf, 1990]. Nevertheless, Sandford
[2005] described a complex relationship between homalo-
notid trilobite faunal composition and taphonomy. Sandford
recognized a recurrent relationship between homalono-
tid-dominated low diversity assemblages and high diversity
assemblages in relatively shallower-water facies.
II) The mixed association characterises the Middle De
-
vonian, and especially the Lower-Middle Eifelian. Three
main sub-associations can be observed:
(IIa) association dominated by Calycoscutellum repre
-
sented at the Couvin, Lower Eifelian (C-lEf) and Rochefort,
Upper Eifelian (R-uEf) sites. The C-lEf biota, confined to
argillaceous limestone beds, is characterised by mostly di
-
sarticulated exoskeletons with most specimens of Calycos
-
cutellum also being fragmented. Complete specimens are
rare, but surprisingly they are of taxa that are usually not
found complete elsewhere, such as Tropidocoryphe, Cy
-
phaspis and Dohmiella [van Viersen, 2006b]. Accumulation
of small individuals occurs locally in the bed, with other
macrofaunal elements. The R-uEf is dominated by Calycos
-
cutellum , which is known from numerous fragments and
Bull. Soc. géol. Fr., 2007, n
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TRILOBITE PALAEOBIODIVERSITY IN THE ARDENNES MASSIF
479
FIG. 4. – R and Q mode cluster analysis of data set (35 genera and 19 samples) of Devonian trilobites from the Ardennes Massif. The R and Q mode den
-
drograms have been plotted against the original data, modified to show relative abundance classes of taxa, to indicate the generic and locality group be
-
hind the clusters. 6 clusters or associations (I to VI) are recognized.
F
IG.4.–Classification hiérarchique ascendante réalisée sur 35 genres et 19 sites provenant du massif des Ardennes. La proportion des taxa a été quan
-
tifiée de manière disjonctive en 8 classes d’abondance relative. 6 clusters ou associations ont été identifés (I à VI).

disarticulated exoskeletons, rarely accumulated. No articu
-
lated trilobites are known from this locality.
Calycoscutellum is possibly among the most common
trilobites in the Upper Eifelian although usually represented
by disarticulated sclerites. So far, complete specimens have
only been reported from the Lower and Middle Eifelian;
(IIb) association with dominant Geesops, represented
by the Vireux-Molhain biota [see Crônier and van Viersen,
2008];
(IIc) association with dominant Cyphaspis (Aulaco
-
pleuridae), Radiaspis (Odontopleuridae), Kettneraspis
(Odontopleuridae), Cornuproetus (Proetidae) and Geras
-
tos (Proetidae), represented by the Rochefort biota. Ecolo
-
gical indices and preservation state indicate that the
Rochefort biota is an equilibrium community probably
found in situ. Trilobites are abundant and diverse, compri
-
sing both disarticulated exoskeletons and a significant por
-
tion of fully articulated ones. Accumulation, especially of
small odontopleurid and aulacopleurid parts (librigenae),
seems to occur locally in the bed where trilobites are trans
-
ported together with other typical macrofaunal constituents
such as bryozoans, crinoid ossicles and especially brachio
-
pods. For the Couvin Middle Eifelian biota, the faunal
content is less rich than the Rochefort biota but has the
same state of preservation.
III) The Asteropyginae Association, dominated by Tre
-
veropyge, is representative of the Lower Devonian. Only di
-
sarticulated exoskeletons of asteropygines are usually found
Bull. Soc. géol. Fr., 2007, n
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480 CRÔNIER C. and VA N VIERSEN A.
FIG. 5. – Localisation of samples through time and a bathymetric profile and comparison with diversity index curve, the global sea-level curve and the re
-
gressions/transgressions at the regional scale.
F
IG.5.–Positionnement des affleurements au cours du temps et le long d’un profil bathymétrique et comparaison avec la courbe obtenue d’après l’indice
écologique de diversité, la courbe eustatique globale et les régressions/transgressions connues à l’échelle régionale.

in the Ardennes-Eifel; articulated specimens are represented
only by one complete enrolled Comura and a fully articula
-
ted Treveropyge thoracopygidium from Marche-en-Famenne
(MF-uEs), confined to sandstone with calcareous cement. In
older strata at the same locality (MF-uP/lEs), trilobites are
fragmented and confined to sandy layers intercalated with
dark blue slates. These represent post-mortem accumula
-
tions of trilobites and macrofaunal elements such as bra
-
chiopods, crinoids and corals.
IV) The Dechenella (Dechenellinae, Proetidae) and Ny
-
terops (Phacopinae, Phacopidae) Association characterises
the Upper Eifelian-Lower Givetian. Dechenellinae appear
to be slightly more common during the Upper Eifelian in
the Ardennes than they are in contemporaneous strata in the
adjacent German Eifel [Basse, 2002].
At the Couvin locality (C-uEf/lG) all trilobites are
known from disarticulated exoskeletons and fragments from
mainly crinoidal limestones, as is also the case at the Wellin
(W-uEf) and Rochefort localities (R-lG).
V) The Kayserops ‘occurrence’ is restricted to the
Upper Emsian of the Treignes area. Only one well-preser
-
ved cranidium has been sampled for this locality. Conse
-
quently, this single ‘endemic’ individual cannot constitute
an association. In this study, the term of ‘occurrence’ has
been used until the discovery of new material.
VI) The Bradocryphaeus Association, dominated by
Bradocryphaeus supradevonicus (69%), is restricted here to
greenish shales at the Middle Frasnian Couvin. The remai-
ning species are Bradocryphaeus maillieuxi (17%), Otarion
cf. ibergium (2%) and the highly endemic Heliopyge helios
(5%). Bradocryphaeus supradevonicus occurs in both Bel-
gium and Germany, whereas Heliopyge helios and Brado-
cryphaeus maillieuxi are only found in Belgium. Otarion is
extremely rare in the Ardennes, being known only from the
Frasnian in the area between Couvin and Olloy-sur-Viroin
[Richter and Richter, 1926; Maillieux, 1927]. In this area,
specimens are usually fragmented, complete specimens are
very rare, and accumulations often occur.
Relative paleobathymetric position of samples
Each of the trilobite associations has a distinctive taxono
-
mic composition and abundance structure that reflects some
ecological conditions, ranged across shelves. Trilobite com
-
munities have already proved useful in recognizing marine
environments of different depths [Fortey, 1975].
Palaeocological indices, state of preservation and nature
of sediment allow us to locate fossiliferous sites along a ba
-
thymetric profile [modified from Fortey and Owens, 1997
and Brenchley and Harper, 1998]. Additionally, Erben
[1964] recognized two dominant magnafacies, i.e. nearshore
clastic and offshore carbonates, in the Devonian of Europe.
The shoreline environment (0-15 m), characterised by
shallow, proximal and agitated deposits includes possibly a
Digonus-dominated biota always known from fragmented
exoskeletons in sandstones. Diversity, though generally low,
may be increased by introduction of allochthonous faunas
by wave activity.
The upper offshore or inner shelf environment is cha
-
racterised by mudstones interbedded with sandstones. It in
-
cludes possibly the Bradocrypheus-dominated biota known
from disarticulated and fragmented exoskeletons and/or as
coquinas.
The median offshore or middle shelf environment cha
-
racterised by relatively deep deposits includes moderately
to highly diverse assemblages. Their abundance is generally
high but decreases with depth. Mudstones may contain in
situ trilobites with well-developed eyes, clumped or disper
-
sed sclerites that are commonly articulated, and various sta
-
ges. Quieter hydrodynamic conditions are favourable to the
development of faunas and/or to conditions for their preser
-
vation. Mixed biotas are typically found in such environ
-
ments.
The lower offshore or outer shelf environment is loca
-
ted below effective storm wave-base (from 50-60 m). Trilo
-
bites are rare, often blind or with reduced-eyes and occur as
articulated exoskeletons. No sampled locality in the Arden
-
nes represents this environmental type.
Spatial and temporal interpretation of associations
The distribution of these associations recognised in the
Ardennes Massif and the diversity curve (fig. 5) are correla
-
ted with the transgressive/regressive trends in the Ardennes
Massif [from Godefroid et al. 1994; Steemans, 1989; Gou
-
wy and Bultynck, 2000]. Three trends can be observed: 1)
the Digonus-dominated association spreads out during a
phase of low relative sea-level and is known from the shore
-
line domain; 2) the mixed associations flourished during a
period of rising sea-level and they are known from the me-
dian offshore domain; 3) peaks of diversity coincide with
deepening of the environment (rising sea-level).
The distribution of the trilobite associations is related to
palaeobathymetric domains and sea-level variations. An
association is potentially a rather precise palaeoenviron-
mental indicator. Trilobites are considered as vagrant epi-
benthos, living mobile above the substratum where
conditions are little uniform. Trilobites are sensitive to
sea-level variations, resulting in numerous association re-
placements due to rapid alteration of the ecosystems. There
is a continuous turnover in species, so the composition of
palaeocommunities changes with time and their taxonomic
composition and environmental ranges.
CONCLUSION
Fluctuation in biodiversity
At all sampled localities, trilobites increase in both number
and diversity in the Middle Devonian. The relative variation
of trilobite abundance may be related to changes in ecologi
-
cal conditions influenced by substratum and palaeobathy
-
metry. The data show that the trilobites are most diversified
and abundant in a peri-reefal context.
Taxonomic structuration
Five trilobite associations and one ‘occurrence’ delineated
among the 35 genera/subfamilies were recognised from the
Early to Late Devonian, across environmental gradients, re
-
lated to water depth and substrate. Trilobites that mostly be
-
long to the vagrant epibenthos seem to be sensitive to
palaeoenvironmental fluctuations. Sections of the same en
-
vironmental gradient are very similar to each other concer
-
ning the trilobite content, even if faunal replacement seems
to exist through time (for example, possible replacement of
the Dechenella and Nyterops Association from the Early Gi
-
Bull. Soc. géol. Fr., 2007, n
o
6
TRILOBITE PALAEOBIODIVERSITY IN THE ARDENNES MASSIF
481

vetian by the Bradocrypheus Association from the Frasnian
for the same environmental gradient).
Lower Devonian strata are characterised by abundant
Asteropyginae or Homalonotinae. Subsequently, these sub
-
families are replaced by Phacopinae and Proetinae in the
Middle Devonian. The mixed middle Eifelian fauna at Cou
-
vin is much less diversified than that at Rochefort and re
-
presentatives of Cyphaspis, Radiaspis and Kettneraspis
have not been found by us. This is due to sampling bias
since a rich trilobite fauna was reported from the middle Ei
-
felian of this area by Maillieux [1904, 1907, 1912]. The
‘Mur des douaniers’ Lagerstatte offers a glimpse of what is
absent from the Lower Eifelian.
At the Ardennes regional scale, changes in species rich
-
ness and ecological diversity of trilobites are in relation to
physical environmental gradients. Changes in climate and
topography limit most species ranges, resulting in conside
-
rable spatial turnover in species richness and ecological
structure.
Acknowledgments. – With this paper, the authors would like to pay homage
to the eminent Professor J.L. Mansy (University of Lille) who was working
on the structural geology of the French Ardennes Massif; we miss him.
This is a contribution of UMR 8014 of Centre National de la Recherche
Scientifique.
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