The Dichotomus Horizon A01 PP1 CG2011
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Carnets de Géologie / Notebooks on Geology - Article 2011/01 (CG2011_A01)
The Dichotomus Horizon:
proposal for a new biochronologic unit of the Giraudi Zone
of the Upper Barremian of southeastern France,
and considerations regarding the genus Imerites ROUCHADZÉ
(Ammonoidea, Gassendiceratinae)
Didier BERT
1
Gérard DELANOY
Stéphane BERSAC
2
3
Abstract: Recent revisions of the genus Imerites ROUCHADZÉ make it possible to introduce a new biochronologic horizon to define more precisely the lower boundary of the Giraudi Zone: the Dichotomus
Horizon. Using the concept of 'interval zone', this new horizon maintains the current lower boundary of
the Giraudi Zone as accepted by authors, and thus contributes to the stabilization of the Barremian
zonal system. This stabilization is also strengthened by abandonment of the use of "Crioceras" cristatus
d'ORBIGNY (nomen dubium) that ought not be used as an index species in detriment of Imerites giraudi
(KILIAN). The classification, origin, and intraspecific variation of the genus Imerites ROUCHADZÉ are examined.
Key Words: Tethyan realm; Upper Barremian; biostratigraphy; Interval zone; southeastern France;
ammonites.
Citation: BERT D., DELANOY G. & BERSAC D. (2011).- The Dichotomus Horizon: proposal for a new biochronologic unit of the Giraudi Zone of the Upper Barremian of southeastern France, and considerations
regarding the genus Imerites ROUCHADZÉ (Ammonoidea, Gassendiceratinae).- Carnets de Géologie /
Notebooks on Geology, Brest, Article 2011/01 (CG2011_A01)
Résumé : L'horizon à Dichotomus : proposition d'une nouvelle unité biochronologique de la
zone à Giraudi du Barrémien supérieur du Sud-Est de la France, et considérations sur le
genre Imerites ROUCHADZÉ (Ammonitina, Gassendiceratinae).- Les récentes révisions du genre
Imerites ROUCHADZÉ permettent à présent d'introduire un nouvel horizon biochronologique afin de
mieux définir la limite inférieure de la Zone à Giraudi : l'horizon à Dichotomus. Par l'application du
concept de la zone d'intervalle, l'utilisation de cet horizon permet de conserver la limite inférieur
actuelle de la zone à Giraudi telle qu'elle a été acceptée par les auteurs, et ainsi de contribuer à la
stabilisation du schéma zonal du Barrémien. Cette stabilisation est aussi renforcée par l'abandon de
l'usage de "Crioceras" cristatus d'ORBIGNY (nomen dubium) qui ne devrait pas être utilisé comme
espèce indice au détriment de Imerites giraudi (KILIAN). La classification, l'origine et la variabilité
intraspécifique du genre Imerites ROUCHADZÉ sont aussi discutées.
Mots-Clefs : Domaine téthysien ; Barrémien supérieur ; biostratigraphie ; zone d'intervalle ; Sud-Est
de la France ; ammonites.
Introduction
After a period of instability, in recent years
several studies have helped to fix the calibration of the Upper Barremian biostratigraphy of
southeastern France that increased its value for
practical use and increased the degree of its re-
producibility by specialists working in discrete
localities (REBOULET et alii, 2007, 2009 - see
BERT et alii, 2008 for a historical account). It
now includes the Vandenheckei, Sartousiana
and Giraudi zones (Fig. 1), their limits based on
faunal changes commonly associated with sequence boundaries developed in relation to
1
* Corresponding author
Université de Bourgogne, Laboratoire Biogéosciences, UMR CNRS 5561, 6 bd Gabriel, F-21000, Dijon (France)
paleo-db@orange.fr
2
Département des Sciences de la Terre, Université de Nice-Sophia-Antipolis, Faculté des Sciences, 28 avenue
Valrose, 06108 Nice Cedex 2 (France)
delanoy@unice.fr
3
945 route de Gattières, F-06640 Saint Jeannet (France)
geosteber@yahoo.fr
Manuscript online since January 17, 2011
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Carnets de Géologie / Notebooks on Geology - Article 2011/01 (CG2011_A01)
eustasy (ARNAUD, 2005). In accord with the
recommendations of the IUGS Lower Cretaceous Ammonite Working Group, the KILIAN
Group, stabilization of the boundaries of these
zones led to a preference for the use of interval
zones rather than zones based on the range of
a taxon. With good reason, for the boundaries
of zones based on individual ranges are subject
to repeated change in range limits caused by
new finds or by tergiversation resulting from a
revision of index taxa. Thus it seems preferable
to choose stable and precise horizons for defining the base of zones (THIERRY, 1997). The
introduction of several biochronological horizons
(DELANOY, 1995, 1997, 1998; BERT et alii, 2008;
BERT & DELANOY, 2009; BERT et alii, 2010) has
contributed significantly to a refinement of the
biostratigraphic pattern of the Tethyan Upper
Barremian of southeastern France.
This work revises the definition of the lower
boundary of the Giraudi Zone [index species:
Imerites giraudi (KILIAN, 1888)] and is a manifestation of progress in the continuation of the
revision of Upper Barremian biostratigraphy in
southeastern France through emendation to the
ammonite faunas which are a major element of
its framework (here Imerites ROUCHADZÉ, 1933).
W
Figure 1: Zonal scheme
amended after BERT et alii, 2008,
and REBOULET et alii, 2009. In red
the Dichotomus Horizon (new).
Figure 1 : Schéma zonal modifié
d'après BERT et alii, 2008, et
REBOULET et alii, 2009. En rouge,
l'horizon à Dichotomus (nouveau).
The boundary between the
Sartousiana and Giraudi zones, and
the new Dichotomus Horizon
1- Geological setting
The Lower Cretaceous of southeastern France is marked by the evolution of a large intracratonic subsident area known as the Vocontian
Basin (PAQUIER, 1900). The area of the historical
Barremian stratotype (Angles-Barrême-Castellane area – Fig. 2), is in the southern part of
the Vocontian Basin which is less affected by
gravity remodelling and Alpine orogeny than its
northern portion. The Barremian here is characterized by pelagic sediments, mainly alternation of marls and limestones in decimetric to
metric beds. Given the relative continuity of
deposits and the paleontological record, it is
possible to track in considerable detail the succession and evolution of their ammonite faunas.
Thus, following up on the work of DELANOY
(1995, 1998), several sections of this area
expose the boundary between the Sartousiana
and the Giraudi zones well enough that they
can be examined minutely: they are the Vignon
section (VIG, Fig. 3), the Descouère section
(DES, Fig. 4), and the Grande-Terre section
(GT, Fig. 5).
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W
Figure 2: Barremian paleogeography of southeastern France
and location of the area studied
(from ARNAUD, 2005, amended).
Figure 2 : Cadre paléogéographique du Sud-Est de la France et
localisation du secteur d'étude
(modifiée d'après ARNAUD, 2005).
2- The boundary between the Sartousiana
and Giraudi zones in southeastern France
The base of the Giraudi Zone is characterized by an important phase of marine transgression (ARNAUD, 2005), marked lithologically
by the "vire marneuse à Heteroceras" of authors. It is associated with a major faunal turnover (DELANOY, 1990, 1995, 1997, 1998; BERT et
alii, 2008). This turnover is relatively progressive in strata at the top of the Feraudianus Subzone and at the base of the Giraudi Subzone. In
fact, all change occurs between the lower part
of the Feraudianus Subzone and the Emerici
Horizon of the Giraudi Subzone (Fig. 1) where
the Hemihoplitinae are progressively replaced
quantitatively by the Heteroceratidae. At the
base of the Feraudanus Subzone Hemihoplitinae
are the major components of the ammonite
fauna with a preponderance of the genus Hemihoplites SPATH. The Pulcheliidae and the Peirescinae are quite rare, as are the Gassendiceratinae (genera Gassendiceras BERT, DELANOY &
BERSAC, 2006, and Pseudoshasticrioceras DELANOY, 1998) which become more numerous at
the top of the Subzone (i.e. in the Bersaci and
Autrani horizons). The genus Heteroceras
d'ORBIGNY (with a turriculate morphology) is
present at the upper limit of the Feraudianus
Subzone (Autrani Horizon) but is extremely
rare, for at this level and time it is only a very
minor element of the ammonite fauna. In the
lower portion of the Giraudi Subzone the situation is reversed: the Hemihoplitinae and the
Pulcheliidae are gone and turriculate morphology dominates quantitatively, first briefly with
Imerites (Gassendiceratinae), and later with
Heteroceras which begin an increase in abundance at the base of the Giraudi Zone and proliferate accompanied by a morphological explosion in the Emerici Horizon (DELANOY, 1990,
1995, 1997, 1998; DELANOY & EBBO, 2000; DELANOY & BERT, 2006).
Above the Autrani Horizon (Feraudianus
Subzone, Sartousiana Zone, Fig. 1), the appearance of the genus Imerites ROUCHADZÉ is currently accepted by authors as the valid marker
of the lower limit of the Giraudi Zone (see
historical account in KAKABADZE, 1989; HOEDEMAEKER & BULOT, 1990; DELANOY, 1990, 1995,
1998; REBOULET et alii, 2006, 2007, 2009; BERT
et alii, 2008). Revisions of this genus by DELANOY (1998) and BERT et alii (2009), improved
understanding of the stratigraphic distribution
of the species of Imerites. Their development
over time is coincident with the evolutionary
framework of the last Gassendiceratinae BERT,
DELANOY & BERSAC, 2006. So in the succession
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Imerites dichotomus ERISTAVI, 1955 appears
before Imerites giraudi (KILIAN, 1888). The use
of an interval zone allows a redefinition of the
lower boundary of the Giraudi Zone without
change of level by use of the new Dichotomus
Horizon (defined below). This usage preserves
the integrity and stability of Barremian zonation
(BERT et alii, 2008). There is no need to move
the lower limit of the Giraudi Zone upward to
make it coincident with the appearance of its
index species, or to rename the Giraudi Zone as
the Dichotomus Zone, because I. dichotomus
ERISTAVI is the first species of Imerites of which
the appearance coincides with the accepted
definition of the lower limit of the Giraudi Zone.
In any event, any shift in level would dissociate
the base of the Giraudi Zone from the faunal
turnover and the sequence boundary with which
it is coincident (ARNAUD, 2005); application
either of these options would change the level
of the base of the zone and return to the instability that has long characterized Barremian
zonation and should cease.
3- The Dichotomus Horizon (new)
In France Imerites dichotomus ERISTAVI has a
very limited and precise stratigraphic position.
By anagenesis (BERT et alii, 2009) it is the successor of the index species Pseudoshasticrioceras autrani (DELANOY), and the ancestor of the
index species Imerites giraudi (KILIAN). In the
stratotype area (Vocontian Basin, southeastern
France – Fig. 2) its appearance in the stratigraphic succession is in agreement with its biologic
relationships. These facts, and the need for the
establishment of a high resolution biostratigraphy for the whole of the Barremian (REBOULET et alii, 2006; REBOULET et alii, 2007; BERT et
alii, 2008; REBOULET et alii, 2009), has impelled
us to propose Imerites dichotomus ERISTAVI as a
new biostratigraphic marker in the Vocontian
Basin. This species occurs in strata immediately
above the major beds of the Autrani Horizon at
the top of the Feraudianus Subzone (Sartousiana Zone), and immediately precedes those of
the Giraudi Horizon where Imerites giraudi (KILIAN) occurs (Figs. 3 - 4 - 5).
VI,
Index species: Imerites dichotomus ERISTArecently revised by BERT et alii (2009).
Status: This horizon is defined by the first
appearance of its index species (bed No. 436 in
the Vignon [VIG] section, Fig. 3), and its upper
limit is currently set at the base of the Giraudi
Horizon (bed No. 439 in the VIG section) with
the first appearance of Imerites giraudi (KILIAN).
The Dichotomus Horizon is also present in the
sections near La Baume (Castellane area): beds
151 to 152 in the Descouère section (DES, Fig.
4), and bed 679 in the Grande-Terre section
(GT, Fig. 5) [see also DELANOY, 1995, 1998].
Paleobiogeographic distribution: Imerites dichotomus ERISTAVI is present in southeastern France but also in Spain (oral commu-
nication of COMPANY, see DELANOY, 1998, p. 207),
Bulgaria, Romania and Georgia (see BERT et alii,
2009), a distribution that augers an extensive
application of the Dichotomus Horizon.
Faunal assemblages: The index species is
generally fairly well represented in the French'
sections. It is associated with (see DELANOY,
1995, 1998; BERT et alii, 2008): Macroscaphites
yvani (PUZOS) macro- and microconchs, Acantholytoceras pseudoaudouli (THOMEL) macroand microconchs, Jaubertites collignoni SARKAR,
Protetragonites crebrisulcatus (UHLIG), Eulytoceras phestus (MATHERON), Silesites seranonis
(d'ORBIGNY), Melchiorites melchioris (TIETZE),
Barremites difficilis (d'ORBIGNY), Barremites
strettostoma (UHLIG), Phyloceras ponticuli
(ROUSSEAU), and with Heteroceras coulleti DELANOY, Heteroceras baylei REYNES and Spinocrioceras trachyomphalus (UHLIG).
Considerations regarding the genus
Imerites ROUCHADZÉ, 1933
1- Remarks on the classification and origin
of Imerites
The genus Imerites ROUCHADZÉ has traditionally been classified as a Heteroceratidae, but it
is now recognized as a representative of the
family Hemihoplitidae (see BERT et alii, 2009 for
an historical account) despite the presence of a
turricone in the juvenile part of the shell (helicoidal coiling). SARKAR was the first to classify
Imerites (his Escragnolleites) in the Hemihoplitidae (1955, p. 24) because it has a hemihoplitid ancestor (1955, p. 18, 22), although KILIAN
(1907-1913) saw "some similarities" between
Heteroceras giraudi (=Imerites) and some
Hemihoplitidae [=Ancyloceras (Crioceras) heberti in KILIAN's time]. It has been recognized
for a long time that helicoidal coiling is not
restricted to the Heteroceratidae. Indeed, KILIAN
(1888, 1889) was the first to restrict the genus
Heteroceras d'ORBIGNY only to those Barremian
taxa that have a turricone. But d'ORBIGNY classified one Senonian species (1851, p. 222) as a
heteroceratid, and MEEK (1876) included helicoidal forms from the Upper Cretaceous of the
United States of America (Nostoceratidae) in
that group, thus indicating broader criteria for
their classifications. This type of coiling is
known to have developed repeatedly in the
evolutionary history of the ammonoids (e.g.
Triassic Cochloceras; some Bajocian Spiroceras;
Uppermost Barremian Kutatissites; some Lower
Cretaceous Leptoceratoidae; Albian Mariella,
Turrilitoides, Helicoceras or Pseudhelicoceras;
Cenomanian Turrilites, Hypoturrilites, Mesoturrilites and Ostlingoceras; Axonoceras, Jouaniceras, Anaklinoceras and some other Nostoceratidae from the Upper Cretaceous, etc. – see
ARKELL et alii, 1957 and WRIGHT et alii, 1996).
But these authors did not remark a connection
between these homeomorphs and the Heteroceratidae. More recently, KAKABADZE (2004, p.
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21) made the following observation about certain genera with helicoidal coiling and the
absence of any relationship to the Heteroceratidae: "the similarity in the mode of coiling
(helicoidal, planispiral or helicoidal, planospiral
and uncoiled) is not important for identifying
the systematic position as to family". And DELA(1998, p. 184) said about Imerites that "the
presence of the turricone would be only the
expression of a homeomorphism that affects
the early developments of these forms" (translation pars).
NOY
W
Figure 3: Distribution of the
ammonites in the Vignon (VIG)
section pars (Barrême area,
Alpes-de-Haute-Provence).
Figure 3 : Répartition des faunes
d'ammonites dans la coupe du
Vignon (VIG) pars (secteur de
Barrême, Alpes-de-Haute-Provence).
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BERT et alii (2009) were the first to distinguish and define evolutionary processes in the
origin and development of species in the genus
Imerites of the Hemihoplitidae. Their work
involved study of an evolved "Pseudoshasticrioceras" ornament in Imerites dichotomus
ERISTAVI (see BERT et alii, 2006). In that stage of
development weaker peri-ventral tubercules de-
monstrate that Imerites is a direct descendant
of the genus Pseudoshasticrioceras DELANOY
(Gassendiceratinae BERT, DELANOY & BERSAC).
This taxonomic distinction between genera of
the same lineage is based on the early appearance of a turricone during the growth of
Imerites (BERT et alii 2009).
Figure 4: Distribution of the ammonites in the Descouère (DES) section pars (La Baume in the Castellane area,
Alpes-de-Haute-Provence).
Figure 4 : Répartition des faunes d'ammonites dans la coupe du Descouère (DES) pars (La Baume, secteur de
Castellane, Alpes-de-Haute-Provence).
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Figure 5: Distribution of the ammonites in the Grande-Terre (GT) section pars (La Baume in the Castellane area,
Alpes-de-Haute-Provence).
Figure 5 : Répartition des faunes d'ammonites dans la coupe de la Grande-Terre (GT) pars (La Baume, secteur de
Castellane, Alpes-de-Haute-Provence).
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We welcome acceptance of the genus Imerites into the Gassendiceratinae but we do not
agree with the broader generic limits proposed
for Imerites by VERMEULEN & LEPINAY (2010, p.
18, 20). Their diagnosis includes some nonturriculate Hemihoplitidae, which would cause
problems that preclude its acceptance:
would be necessary: renaming the Giraudi Zone
as the Cristatus Zone [note that this is not obligatory, for the International Stratigraphic Guide
(SALVADOR, 1994, p. 67) states that "if it is desirable to continue use of a taxonomic term
which is no longer valid, the term should be in
quotation marks"].
1.
the large range of variation in morphology and ornament proposed for their Imerites by these authors requires a redefinition of this genus to include criteria
much broader in scope than those commonly acceptable to specialists. Their
proposal widely depasses the intent of
the author of the genus Imerites ROUCHADZÉ and that of subsequent workers.
This new definition would produce a
genus-group taxon which would greatly
lessen the precision of identification and
make correlation with previous work
difficult. So nomenclatural stability
would be threatened and Upper Barremian biostratigraphy again subject to
revision because of confusion regarding
the stratigraphic ranges of the species of
the genus Imerites ROUCHADZÉ;
2.
the
ornamental
and
morphological
characteristics specified in the emended
diagnosis of VERMEULEN & LEPINAY, would
include in their genus Imerites species of
Hemihoplitinae currently assigned to
other genera, although they lack any
phyletic link to them. So Imerites in VERMEULEN & LEPINAY'S usage, might well
include the taxa Hemihoplites SPATH,
Camereiceras DELANOY, Pachyhemihoplites DELANOY or Ancylezeiceras VERMEULEN.
However new data obtained in the recent
revision of Imerites by BERT et alii (2009, p. 3233) found the syntypes of "Crioceras" cristatus
d'ORBIGNY of the d'ORBIGNY's collection are too
fragmented to be identified at a specific level,
and the fragments may not be of just one
specimen but may even represent more than
one species. These facts render this taxon
unusable and a nomen dubium. Moreover,
d'ORBIGNY's (1842, Pl. 115, figs. 4-8) original
illustrations are probably a synthetic compilations (i.e. a picture based on several different
but fragmentary specimens assembled and
restored to appear as if they were a single
specimen). They were recognized as such by
KILIAN (1888, 1889). In the original illustration
the turricone is replaced by planispiral whorls
for the stages of growth of the actual syntypes
(particularly fragmented) when compared to the
original picture make this obvious. So the rehabilitation of "Crioceras" cristatus d'ORBIGNY,
1842, to replace Imerites giraudi (KILIAN, 1888),
a long-used, well defined taxon (recognized by
many authors), is not desirable. Moreover, the
purpose of the ICZN principle of priority is
explained by Article 23.2 which states very
explicitly that it must foster nomenclatural stability: "it is not intended to be used to upset a
long-accepted name in its accustomed meaning
by the introduction of a name that is its senior
synonym or homonym, or through an action
taken following the discovery of a prior and
hitherto unrecognized nomenclatural act"; a
fortiori when the older name is based on a synthetic compilations and can be considered as a
nomen dubium because it differs from characteristics found in its syntypes. On the other
hand, the proposal of invalidation by VERMEULEN
& LEPINAY (2010, p. 17-18) of the d'ORBIGNY's
syntypes in favour to a more complete neotype
(not among the syntypes) does not comply with
the provisions of the ICZN (Art. 75, and in
particular, § 75.3.4) and therefore cannot be
implemented. At most it might be possible to
designate one of the original syntypes as a
lectotype (n°5405-1 of the d'ORBIGNY's collection),
but
the
basic
problem
remains
unchanged...
This problem is particularly evident in the
taxon "I." stephaniae VERMEULEN & LEPINAY, 2010
(found in the Provincialis Subzone: One subzone separates it from the appearance of the
first Imerites s. str.) that they attribute to the
genus Imerites ROUCHADZÉ, but do not compare
it with contemporary Hemihoplitids. "I." stephaniae VERMEULEN & LEPINAY is a fragmentary specimen (VERMEULEN & LEPINAY, 2010, p. 21, Pl. 1,
fig. 7) particularly close in aspect to contemporary Hemihoplites of the group H. casanovai
DELANOY (=H. intermedius VERMEULEN - currently
under study).
2- The case of "Crioceras" cristatus d'ORBIGNY, 1842
As DELANOY (in GAUTHIER et alii, 2006, p. 138139), maintained "Crioceras" cristatus d'ORBIGNY
should be considered synonymous with Imerites
giraudi (KILIAN) and according to the principle of
priority of the International Code of Zoological
Nomenclature (ICZN - Art. 23.1) has seniority.
Consequently, for VERMEULEN & LEPINAY (2010, p.
20) another amendment of Barremian zonation
Whatever the nomenclatural possibilities
mentioned earlier (DELANOY in GAUTHIER et alii,
2006, p. 139), Imerites giraudi (KILIAN) is the
type-species of the genus Imerites ROUCHADZÉ
and we strongly recommend "Crioceras" cristatus d'ORBIGNY be considered invalid.
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Figure 6: Dimorphism and variability in shell morphology in Imerites.
Figure 6 : Dimorphisme et variabilité morphologique de la coquille chez Imerites.
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ERISTAVI seem quite rare, but adult
macroconchs are also few. The adult
form of the macroconch was unknown
prior to the recent revision by BERT et alii
(2009). Juvenile or fragmentary specimens of this species that are difficult to
place because of uncertainty regarding
which dimorph they represent are relatively common;
3 - Intraspecific variability in Imerites and
mechanical constraints associated with the
presence of turricone
Intraspecific variability in species of Imerites
involves ornamentation, some dimensional
parameters of the shell and the size and orientation of the turricone (Fig. 6c). The relationship of turricone size and placement to changes
in shell morphology has been studied by DELANOY (1998, p. 53) for the genus Heteroceras
and was illustrated by KAKABADZE (2004, Fig. 10)
for the Kutatissites. In Imerites, the phenomenon is the same: a larger turricone generally is
accompanied by a less regular or triangular
coiling before normal involution resumes. However, contrary to the views of VERMEULEN &
(2010)
these purely
mechanical
LEPINAY
constraints are not comparable with the dimorphic differentiation in Imerites dichotomus
ERISTAVI where tripartite adult microconch [m]
are associated with large planospiral adult
macroconch [M] (see BERT et alii, 2009, Pl. 2,
fig. 2 versus Pl. 1, fig. 1 – Fig. 6a) for several
reasons:
1.
The dimorphs of the adult stages of I.
dichotomum ERISTAVI are very different
and there are, to our knowledge, no
intermediaries (Fig. 6a);
2.
The presence of such intermediaries was
assumed by VERMEULEN & LEPINAY (2010)
because of the existence of specimens
with a triangular coil ("I. cristatus favrei
morphotype" of Pl. 1, figs. 5-6). These
specimens of which the whorl "may
represent a draft shaft" (p. 18) are too
small in size and incomplete. They are
only juvenile whorls with triangular coiling that wraps the turricone, but lack an
uncoiling of the outer whorls to form a
shaft (Fig. 6c). Only the specimens figured Pl. 1, figs. 1-2 of their publication
may be adult, but they are crioconic with
no shaft. Consequently, no adult specimen with a morphology intermediate
between those of micro- and macroconchs are known;
3.
The tripartite Imerites raricostatus (KAKABADZE) has a well expressed ornamental stage with fibular ribs (the type
specimen is refigured in KOTETISHVILI et
alii, 2005, Pl. 87, fig. 2) and its ontogenic sequence is very similar to the tripartite specimen figured in BERT et alii
(2009, Pl. 2, fig. 2 – Fig. 6a). Therefore
I. raricostatus (KAKABADZE) is a microconch of I. dichotomus ERISTAVI and not
a morphotype of I. giraudi (KILIAN) (VERMEULEN & LEPINAY, 2010). Complete adult
microconch specimens of I. dichotomus
4.
Variability in the size of the turricone
versus that of the morphology of the
shell is well expressed in the macroconchs of Imerites dichotomus ERISTAVI,
just as it is in Imerites giraudi (KILIAN)
(Fig. 6c). This is the same in the microconchs of I. dichotomus (Fig. 6b): the
type specimen of I. rarecostatus (=I.
dichotomus [m]) has a large turricone
and minute coil, whereas the specimen
figured Pl. 2, fig. 2 (in BERT et alii, 2009)
has a well-developed coil and a relatively
small turricone;
5.
The specimens of I. giraudi (=I. cristatus
in VERMEULEN & LEPINAY, 2010) cannot be
used to suggest the absence of dimorphism in I. dichotomus, since the tripartite coiling of the microconchs is currently recognized with certainty only in I. dichotomus.
Conclusions
Recent revisions of the genus Imerites ROUprovide an increased understanding of
its variability and its evolution, so now permit
the introduction of a new biochronologic horizon, the Dichotomus Horizon (Fig. 1), to define
the lower boundary of the Giraudi Zone more
precisely. Using the interval zone concept, the
addition of this horizon maintains the current
lower boundary of the Giraudi Zone as accepted
by authors (the appearance of the genus Imerites), and thus contributes to the stabilization
of the Barremian zonal scheme. This stabilization is also strengthened by the abandonment
of "Crioceras" cristatus d'ORBIGNY (nomen dubium) that ought not be used as index species
instead of Imerites giraudi (KILIAN). Following
THIERRY (1997), in general (as it is for the
Barremian) it is preferable to use interval zones
rather than the stratigraphic range of any one
taxon for example, to avoid constant changes of
boundaries that discoveries and systematic
reviews of index taxa might impose, thus threatening the stability of the zonation. The introduction of a new biochronologic horizon increases the precision of the biostratigraphic zonation of the Barremian stage in southeastern
France which is a requirement for the study in
detail of the evolution of ammonites and their
populations.
CHADZÉ
10
Carnets de Géologie / Notebooks on Geology - Article 2011/01 (CG2011_A01)
Acknowledgments
We wish to express our thanks to Mr. Raymond ENAY, and to the anonymous reviewer,
Mr. Stephane REBOULET and to Mr. Jaap KLEIN for
their valuable advice. We thank the Réserve
Géologique de Haute Provence and Mrs. Myette
GUIOMAR who have allowed us access to the
areas studied. One of us (D.B.) has interesting
discussions with Gerd E.G. WESTERMANN about
stratigraphical nomenclature, and we warmly
thank him. We want especially and sincerely to
thank Nestor SANDER who kindly corrected our
English in the final version.
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