The increasing interest
of geologists in nonmarine sediments has revealed in
the last 20 years the importance of charophyte algae
in the stratigraphy and correlation of freshwater and
brackish deposits. The charophyte calcified fructifications
(gyrogonites) that are preserved as fossils are present
in carbonate sediments dating from the Late Silucian
to Holoceme. These gyrogonites undergo phylogenetical
changes that, in certain periods, are of short duration
and thus provide a tool for precise biostratigraphical
dating. Some species, widely distributed, are useful
in distant correlations. Further advantages of gyrogonites
are their small size (diameter ranging from 0,2 to
2 mm) and the fact that they are found frequently in
abundance and can be recovered from borehole material.
Because of the large amount of information
that has been collected on 100 charophyte genera, specialists
from around the world have needed to gather in order
to discuss evolutionary processes, compare biozonations,
and debate the significance of paleoenvironments. The
first such meeting of specialists in both extant and
fossil charophytes was held in Montpellier, France,
in July 1989. Sponsored by the International Palaeontological
Association and by the International Geological Correlation
Programme's project on biological events in Earth history,
the meeting drew 60 participants from 15 different
countries. A total of 54 communications were presented
as lectures or poster sessions, änd they covered
a wide scope of biological and paleontological studies.
One major theme of the symposium was
biostratigraphy. New results presented on the Mesozoic
and Tertiary charophyte ftoras of Asia (India, Republic
of Korea, PRC), South America (Argentina), and Europe
(France, Spain, Switzeriand) showed that the different
zonations determined so far for each of the separate
land masses can be correlated, in some cases, by means
of cosmopolitan or widely distributed forms. Examples
include Atopochara trivolovis triquetra (early
Barremian), the species of Platychara at the Cretaceous-Tertiary
boundary, and Maedleriella monolifera in the early
Eocene. Of great importance is the integration of the
charophyte zonation into the standard cominental (ostracode-,
palynoflora-. mammal-based) zonations and, as far as
possible, marine (ammonoid-, foraminifer-, or calpionellid-based)
zonations.
Marine-continental correlations provide
information on the evolution of sedimentary basins,
as was illustrated by lectures reporting on Early Cretaceous
floras from Spain and Switzerland. Several contributions,
including the comparison of charophyte studies with
data from other fossil groups and comparison with radiometric
and paleomagnetic data, all focused on the Cretaceous-Tertiary
boundary, in particular the dating of the Infra-Trappean
Beds of the Godävari area (southwest India). These
beds appear likely to be of Danian age. If confirmed
by further research. this discovery would be of great
importance in dating the Deccan Traps volcanism, which
is thought to be at the origin of the Cretaceous-Tertiary
boundary crisis. The duration of the charophyte zones
varies according to their age. The average duration
of each zone is thought to be 5.5 m.y. for the BerriasianCenomanian
interval and 1.6-1.7 m.y. for the Late Cretaceous (late
Campanian / Maastrichtian) and Paleogene Periods.
Charophyte ecology and paleoecology
are gaining interest. The influence of latitude, climate,
altitude, water depth, and salinity on the distribution
of living species dcew special attention. The position
of charophyte populations within limnic ecosystems
was also a subject of discussion. Charophytes are pioneers
in the colonization of new ecological niches. As demonstrated
by research on the Scanian lakes in Sweden, charophytes
are particularly sensitive to lake eutrophication as
a result of chemical pollution and drainage of lakes
and ponds. The distribution patterns of living species
can be extrapolated, in some cases, to fossil species.
Thus, a Holocene thanatocoenosis consisting of the
unusual association of Chara species with marine formaminifers
and mollusks, found in a coastal saline lake in Languedoc
in the south of France, resulted from periodic incursions
of the sea. This example could be used to support the
perhaps-rash interpretation that ancient charophyte
environments were, therefore, marine habitats. An example
of a semiprotected, open-marine oligohaline habitat
containing Sycidium was presented from the Devonian
carbonate complexes of southern Potand. In this case,
however, the environmental conditions differed only
slightly from che usual environment of charophyte species,
which grow where sheltered from violent waves and currents.
The oldest charophyte assemblage described from the
Upper Silurian deposits of Podolia (USSR) has been
found in Pridoli deposits together with terrestrial
plants (notably, the first vascular plant Cooksonia),
apparently a nonmarine environment.
The use of stable oxygen and carbon
isotopes of gyrogonites as indicators of paleoclimatic
variations during the Oligocene/Miocene transition
is a new avenue of research. Data presented on the
calcified wall and oospore membrane also constitute
a potential for future investigations, particularly
conceming the relationship between extant and fossil
gyrogonites.
Severallectures focused on phylogeny and evolutionary
processes. Polymorphism resulting from environmental variations,
which has been observed in some of the Porocharaceans from the
Swiss Berriasian, is found also in tiving species, where it appears
to correspond not to genetic differentiation but to ecotypic
variation. This has been demonstrated by using electrophoresis,
applied recently to the study of charophytes for the first time.
An example of fossil evidence that provides possible clues to
the present and future was given by the cycle extinction-crisis-speciation,
a cycle demonstrated by a multidisciplinary study of the Cretaceous-Tertiary
boundary in the western Mediterrancan area.
Two excursions in the south of France
followed the charophyte symposium. One excursion to
the Camargue Natural Park took participants along coastal
saline lakes and freshwater paddy fields containing
living characean species. The other excursion presented
participants the opportunity to cotlect abundant. well-preserved
fossil specimens first in the Upper Cretaceous-Paleocene
sequences of the Villeveyrac basin and second in a
succession of brackish and lacustrine sediments of
lower Eocene age in the Aude valley and Minervois.
The numerous collaborations inspired
by the charophyte symposium prove the value of such
a meeting. The creation of an International Research
Group of Charophyte Specialists and the publishing
of a review, the IRGC News, will facilitate contact
among charophyte researchers. The second symposium
is planned for 1992 and will take place in Nanjing,
PRC. Contributions from the charophyte symposium witl
be published in a future issue of the Bulletin de la
Societe Botanique de France.
Monique Feist, Montpellier
