Darwin forest at agua de la zorra: the first in situ forest discovered in South America by Darwin in 1835
Mariana Brea1,4, Analía E. Artabe2,4 and Luis A. Spalletti3,4
1Laboratorio de Paleobotánica, Centro de Investigaciones Científicas, Diamante. CICYTTP-Diamante, Consejo Nacional de
Investigaciones Científicas y Técnicas, Diamante, Entre Ríos, Email: cidmbrea@infoaire.com.ar
2División Paleobotánica, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, La Plata. Email:
aeartabe@museo.fcnym.unlp.edu.ar
3Centro de Investigaciones Geológicas (CONICET-UNLP), La Plata. Email: spalle@cig.museo.unlp.edu.ar
4Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).
ABSTRACT
The Agua de la Zorra area (near Uspallata, Mendoza, Argentina) is one of the best renowned fossil localities of the country because of its spectacular in situ fossil forest. This forest was discovered by Charles Darwin in 1835, who described this forest as monotypic and assigned it a Tertiary age. Nowadays, this fossil locality is known as the Darwin Forest. Over a century and a half later it was reinterpreted as a mixed Middle Triassic forest and a new fossil monotypic palaeocommunity of horsetails was discovered. This palaeovegetation is included in the Paramillo Formation (i.e., lower section the Potrerillos Formation) of northwestern Cuyo Basin, Mendoza province (69°12' W and 32°30' S). The sediments were deposited in a sinuous fluvial system, in which channel-filling sand bodies were associated with mud-dominated floodplain deposits. The palaeoforest grew on an andisol soil that developed on volcaniclastic floodplain deposits. It had a density of 427 -759 trees per hectare, and was constituted by conifers and corystosperms distributed in two arboreal strata. The highest reached 20-26 m tall, and was dominated by corystosperms, but it also included the tallest conifers. The second stratum, mainly composed of conifers, ranged between 16-20 m tall. The forest has also emergent corystosperms, which reached 30 m tall. The understorey was composed of ferns. Growth ring anatomy suggests that conifers could have had an evergreen habit. Structure of vegetation, growth ring analyses and sedimentation suggest that the forest developed under dry, subtropical, and strongly seasonal conditions.
Keywords: Palaeovegetation; Palaeoecology; Palaeoenvironments; Middle Triassic; Southwestern Gondwana.
RESUMEN: El Bosque Darwin en Agua de la Zorra: El primer bosque in situ descubierto en América del Sur por Darwin en 1835. El área de Agua de la Zorra (cerca de Uspallata, Mendoza, Argentina) es uno de los sitios fósiles más espectaculares y renombrados del país porque aflora un bosque in situ. Este bosque descripto como monotípico y asignado al Terciario fue descubierto por Charles Darwin en 1835. Un siglo y medio más tarde, se reinterpretó como un bosque mixto del Triásico Medio y se reconoció una nueva paleocomunidad monotípica de esfenófitas. La paleovegetación proviene de la Formación Paramillo (= sección inferior de la Formación Potrerillos) de la región noroeste de la Cuenca Cuyana, provincia de Mendoza (69°12'O y 32°30'S). Los sedimentos fueron depositados por un sistema fluvial de alta sinuosidad. El bosque fósil creció sobre suelos del tipo andisol que se desarrollaron sobre depósitos de planicies volcaniclásticas. El bosque Darwin tiene una densidad de 427-759 árboles por hectárea. Este está constituido por coníferas y corystospermas y tiene dos estratos arbóreos. El más alto se desarrolla entre los 20-26 m, y es dominado por corystospermas, pero también por las coníferas más altas. El segundo estrato arbóreo, principalmente compuesto de coníferas, con un rango entre 16-20 m. El bosque tiene también algunos emergentes de corystospermas las cuales alcanzan hasta 30 m de altura. El sotobosque estaba compuesto de helechos. La anatomía de los anillos de crecimiento sugiere que las coníferas podrían haber tenido hábito siempreverde. La estructura de la vegetación, el análisis de los anillos de crecimiento y la sedimentología sugieren que este bosque se desarrollo bajo condiciones climáticas subtropicales, secas y estacionales.
Palabras clave: Paleovegetación; Paleoecología; Paleoambientes; Triásico Medio; Sudoeste de Gondwana.
INTRODUCTION
The first fossil plants in South America
were recorded by European naturalists
during the 18th and 19th centuries (Ottone
2005). The British naturalist Charles Robert
Darwin (1809- 1882) was the first
who provided palaeontological and geological
observations of the Agua de la
Zorra area, Uspallata, near Villavicencio,
Mendoza province in Argentine territory
(Darwin 1839a, 1839b, 1845, 1846). Later, this famous fossiliferous locality was
visited by other European naturalistexplorers
such as Burmeister 1861, Doering
1882, Stelzner 1885, Avé-Lallemant
1891 (see Ottone 2005).
The expeditions of Charles Darwin -on
board HMS Beagle- during his historic
journery around the world between 1831
and 1836, contributed valuable information
and observations on the geology,
plant and animal fossils, and extant organisms
of South America. Darwin also
collected a huge number of specimens
and samples, many of them new to science.
Darwin arrived in Argentina in 1833 and
visited the Río de La Plata area, La Bajada
(now Paraná city, Entre Ríos), the
Santa Cruz River and many other sites in
Patagonia and Tierra del Fuego. Then,
after crossing the Cordillera de los Andes
from Chile, he arrived in Mendoza province
and discovered the first in situ forest
in South America, at Agua de la Zorra,
located about 25 km from the town
of Uspallata (Conwentz 1885, Rusconi
1941, Harrington 1971, Brea 1995, Ottone
2005). He recorded the presence of
52 fossil tree trunks measuring 90-152
cm in diameter, buried in sandstones or
volcanic sandstones and standing out as
columns several meters high (Darwin
1846). This discovery by Darwin is remembered
by a monolith (Fig. 1).
Figure 1: a. Charles Darwin's tribute plaque at Agua de la Zorra, near Uspallata, Mendoza, Argentina. This photo was taken in 1993. b. Panoramic
view of Triassic sequences, the arrow shows the plaque.
On 30 March 1835 he collected the first specimens of fossil wood from Agua de la Zorra. The samples were sent to London and were referred by Robert Brown to the genus Araucarites (Darwin 1846). Recently, one of the authors (M. B.) carried out three research field trips as part of work towards her PhD dissertation. Brea (1995) characterized and studied the Triassic units at Agua de la Zorra from a palaeobotanical and sedimentological viewpoint. She defined two palaeocommunities in the Paramillo Formation, together with their associated palaeoenviroments (Fig. 2). The first paleocommunity, the in situ Darwin Forest (Fig. 2), appears at four fossiliferous sites (Brea 1995), while the second was found at only one locality and included only fossil horsetails (Brea and Artabe 1999, Brea et al. 2008).
Figure 2: The Darwin Forest at Agua de la Zorra area (near Uspallata, Mendoza). a-b-c-e. Large fossil tree stumps in growth position in Triassic
sequences; d-f-h. Petrified standing trees at fossiliferous sites C; g. Large fossil log exposed in Triassic sediments at fossiliferous site B.
The Darwin Forest was first referred to
the Tertiary by Darwin (1839a, 1839b),
but new data now available suggest a late
Middle Triassic age (Spalletti et al. 1999,
Brea et al. 2008). The most important feature
of the Darwin Forest is that the
trees are still preserved at the sites on
which they grew (Fig. 2); it is the best
renowned of its age in southwest Gondwana.
Although petrified forests in
growth position are very scarce in the
geological record, the Triassic fossil record
of SW Argentina shows four in situ
fossil forests (Paramillo, Cortaderita,
Ischigualasto, and Río Blanco Formations)
and their occurrences have contributed
significantly to our understanding
of Triassic Gondwana ecosystems (Zamuner
1992, Artabe et al. 2001, 2007a,
Brea et al. 2008).
The Darwin Forest was reinterpreted as a
subtropical dry seasonal forest (Brea et al.
2005, 2008). It grew on an andisol soil
that developed on volcaniclastic floodplain
deposits. The volcanic detritus and
the rhythmic amalgamation of upper
flow-regime tractional deposits overlying
the andisol indicate that the forest was
buried rapidly by a subaerial, cool and
wet pyroclastic base surge flow (Poma et
al. 2004, Brea et al. 2008).
The continental Triassic succession of
southwestern Gondwana occurs in a series
of narrow rifts produced as a result
of Triassic continental extension. These
rift basins are composed of a continental
clastic infilling, and record complex
interactions between alluvial, fluvial, deltaic
and lacustrine depositional systems
with intercalations of volcaniclastic sequences
in most of these basins. The rich
floristic record allowed recognition of
several assemblages, biozones and stages
characterized by floristic events (Spalletti
et al. 1999, 2003, Artabe et al. 2001, 2003,
2007b, Morel et al. 2003).
Over the last two decades, most investigations on the Argentinian Triassic have
focused on the gross stratigraphy and
taxonomy of fossil plants (Stipanicic
2001, Stipanicic and Marsicano 2002, Zamuner
et al. 2001, and references therein).
However, recently published studies
(Spalletti et al. 1999, 2003, Artabe et al.
2001, 2003, 2007b, Morel et al. 2003) provided
an increasing understanding of
how the Triassic palaeofloras developed
and changed over time as a response to
variations in depositional systems, tectonism
and climate.
The most spectacular fossil sites of the
Paramillo Formation- in the Agua de la
Zorra area -, are located in the northernwestern
sector of the Cuyo Basin (between
32º and 36º SL). The Paramillo Formation
is composed of a 140 m thick
succession of clast-supported conglomerates,
pebbly sandstones, tuffaceous
sandstones and mudstones (Figs. 3 and 4). These deposits have been previously
described by Harrington (1971), Strelkov
and Álvarez (1984), Kokogian and
Mancilla (1989), Ramos and Kay (1991),
Massabie (1985), Massabie et al. (1985),
Linares and González (1990), Ramos
(1993), Kokogian et al. (1993). These authors
all agreed in interpreting the Paramillo
Formation as deposited in highly
sinuous fluvial systems.
Figure 3: Location map showing Agua de la Zorra fossiliferous locality, Mendoza province,
Argentina indicating the four fossiliferous sites: A, B, C, D respectively.
Figure 4: a. Lithostratigraphic
section of the Paramillo and
Agua de la Zorra Formations
at locality A, showing the main
lithofacies and the position of
the fossil forest level. b.
Lithostratigraphic section of
the Paramillo and Agua de la
Zorra Formations at locality
C. PF: Paramillo Formation,
AZF: Agua de la Zorra
Formation. FL I - V: fossiliferous
levels.
Previous palaeobotanical studies, comprising mostly lists of fossil plants, were published by Darwin (1846), Conwentz (1885), Stappenbeck (1910), Kurtz (1921), Du Toit (1927), Groeber (1939), Windhausen (1941), Harrington (1971) and Stipanicic et al. (1996), while modern systematic contributions were offered by Brea (1995, 1997, 2000), Brea and Artabe (1999), Artabe and Brea (2003) and Brea et al. (2005, 2008).
THE PARAMILLO FORMATION
The Paramillo Formation is a volcaniclastic
unit composed of yellowish lithic
sandstones, brownish and yellowish tuffaceous
sandstones, dark gray and green
shales and mudstones, and pink to reddish
ash fall tuffs (Fig. 4). The sedimentary
record of the overlying Agua de la
Zorra Formation is dominated by bituminous
shales and marls with subordinate
intercalations of yellowish fine-grained
sandstones and mudstones (Fig. 4).
Both units are intruded by several sills of
olivine diabase dated 235 + 5 Ma (K/Ar
whole age) by Ramos and Kay (1991). At
that time Mendoza was a part of the vast
supercontinent called Gondwana and it
was placed approximately at the same
geographic latitude as it is nowadays.
Brea (1995) studied in more detail the
upper part of the Paramillo Formation
and the lowermost Agua de la Zorra
Formation and defined several lithofacies
and facies associations. The Paramillo
Formation consists of cross-bedded
conglomerates, cross-bedded, plane-bedded
and massive pebbly sandstones,
cross-bedded, massive, plane bedded and
ripple-laminated sandstones and intercalations
of laminated mudstones and shales.
Brea (1995) interpreted that these
sediments were deposited in a highly
sinuous (meandering) fluvial system, in
which channel-filling sand bodies are
associated with mud-dominated floodplain
deposits. The cyclic arrangement of
well preserved soil horizons and sedimentary
deposits lacking evidence of soil
formation indicates alternation of periods
with strong sediment aggradation
produced by non-channelized high-regime
flows with periods characterized by
very low accommodation rates that
favoured the development of immature
soil profiles (Brea et al. 2008). The volcanic
nature of detrital components suggests
that the highly aggrading non-channelized
flows that produced the burial of
the Darwin Forest could be related to
pyroclastic events (Poma et al. 2004). The forest might have died as a result of a
diluted, subaerial, cool and wet base
surge pyroclastic flow (Poma et al. 2004,
Brea et al. 2008).
The Paramillo Formation, where the
Darwin Forest emerges, has been correlated
with the lower section of the Potrerillos
Formation (Spalletti et al. 1999,
Morel et al. 2003) on the basis of Assemblage
Biozone chronology, analysis of
main stratigraphic unconformities, and
evolution of basin infill.
ECOLOGICAL RECONSTRUCTION OF THE DARWIN FOREST
The Triassic landscape at the Agua de la
Zorra area was very different in comparison
with the modern-day scenery. Sphenopsids
dominated the flooded areas and
conifers and corystosperms were the
most important components of the arboreal
vegetation.
Four exposures of the Darwin Forest
were found in the Paramillo Formation
during fieldwork carried out in 1993-1994 (Figs. 2 and 3). This unit was thoroughly
examined at two localities: Darwin
and El Sauce (Figs. 3 and 4), where
the lithology was logged bed-by-bed in
vertical sections at a 1:100 scale (see Brea
et al. 2008).
Imperfect carbonization processes (Poma
et al. 2004) preserved the stumps and
in just over 2 km2 one hundred and
twenty stumps in growth position and
fallen logs were counted (Fig. 2). The fossiliferous
levels (FL) with stumps were
found at four localities belonging to the
same in situ forest. Thus, FL IV (at A
locality), FL V (at B and C localities) and
FL VI (at D locality) belong to a single
stratigraphic level (Figs. 2 and 4). Fossiliferous
levels FL I and FL III found at
locality A (Fig. 4) have ferns preserved as
impressions-compressions (Brea 2000).
The sedimentary sequences in which this
forest is preserved consist of continental
volcaniclastic units, resulting from deposition
on a highly sinuous fluvial system
associated with river flood-plains (Fig. 4).
The palaeoecological reconstruction of
the Darwin Forest was based on the
quantitative data of the mapped forest
(mean separation of trees, basal area per
ha, species distribution) integrated with
the taxonomic and sedimentological information
(Brea et al. 2008). The mixed
forest was composed by corystosperms
(30-40%) and conifers (60-70%). The
corystosperms are a group of extinct
plants with mostly fern-like foliage with
ovules borne on modified leaves or cupules.
This group was the dominant seed
plant in Gondwana during the Triassic
(Stewart 1983). All conifers are a diverse
group of trees and shrubs that underwent
a major radiation during the Triassic
period, when the first occurrence and radiation
of the eight conifer families occurred
(Willis and McElwain 2002). The
conifers have a pyramidal arborescent
growth form, with cone-bearing seed
plants; the foliage is either needle-like or
scale-like. The plants are mainly small,
long, and thin arranged spirally and bearing
the male and female reproductive
organs in separate cones on different
trees, or indifferent parts of the same.
The average density of the tree stumps in
this forest is 556 trees/ha. The spatial autocorrelation
for the species variable
indicates that in some places the corystosperms
and conifers are intermingled
while in others the corystosperms and
conifers seem to be aggregated into cohesive
social groups (Fig. 5). The Darwin
Forest reveals two strata: the highest,
developed between 20-24 m, has a preponderance
of corystosperms but also
the tallest conifers; the second stratum,
mainly composed of conifers, ranges between
16-20 m. The forest also has emergent
corystosperms which reach 30 m
tall (Brea et al. 2008). The corystosperms
were assigned by Artabe and Brea (2003)
to Cuneumxylon spallettii and constitute the
dominant species in the Darwin Foest.
Figure 5: Reconstruction of the Triassic Darwin Forest landscape in a high sinuosity fluvial system, in which channel-filling sand bodies are associated
with mud-dominated floodplain deposits. The canopy is integrated by two arboreal strata and emergent trees with conifers and corystosperms.
The understorey is formed by ferns. Cinodonts are characteristic tetrapods during the Triassic of the Cuyo Basin (drawing by Jorge Gonzalez).
This fossil forest presents values of biomass
and stand basal area comparable to
those of the current subtropical seasonal
forests (Brea et al. 2008). These forests
develop under a climatic regime that
includes an annual cycle with one season
in which water is unavailable to plants
because of lack of precipitation. The dry
season alternates with another in which
there is abundant water. In addition to
structural data of the Darwin Forest that
match those of an extant monsoon forest,
the polyxyly found in Cuneumxylon
has been considered an important adaptive
wood character to avoid water stress.
As suggested by the functional anatomy
and distribution of living groups (Fahn
1990), the included phloem associated
with great amounts of parenchyma could
be a strategy of subtropical plants to
fight against water stress in arid regions
during drought seasons.
During Permian-Triassic biosphere reorganization,
aridity of the Earth increased
and in the Triassic the Pangea was characterized
by strongly seasonal climates
in a warm-house period (Parrish et al.
1982, Dubiel et al. 1991, Parrish 1993,
Scotese et al. 1999). The spreading of
continental climates caused the extension
of semiarid belts into middle latitudes
and, partly, into high latitudes too
(Chumakov and Zharkov 2003). Recently,
general circulation models (GCMs)
were developed by Sellowood and Valdes
(2006) to simulate Mesozoic climatic patterns.
In this scheme the Triassic Southwestern
Gondwana is modelled as seasonal
and winter-wet against the opinion of
Robinson (1973) who stated that it was
seasonal and summer-wet.
Growth-ring analysis of coniferous
wood - assigned by Brea (1997) to Araucarioxylon
protoaraucana - was used to evaluate
climatic conditions (Fritts 1976,
Holmes 1985). Conifer growth rings are
narrow and subtly demarcated; they are
characterized by a relatively wide zone of
large, thin-walled early-wood cells terminated
by ten to seventeen thick-walled
late-wood cells. The presence of narrow
growth rings indicates poor conditions
for cell division and expansion during
growth season (Creber and Chaloner
1984). In addition, a thin late-wood zone
within a ring can be the result of water
shortage at the end of the growing season,
a sharp photoperiod indicating end
of growth season or abrupt leaf shedding.
Therefore, A. protoaraucana growth
rings suggest strongly seasonal conditions.
The average ring width in these
fossil woods is 1.49 mm (0.80-2.53 mm),
the narrowest ring is 0.12 mm and the
widest ring is 4.44 mm. Mean sensitivity
(MS) values range between 0.14 and 0.38.
Average MS values (0.30) indicate that
the growing environment was stressed
and not uniform. Although available data
are limited, the complicated growth
trends suggest that competition, disturbance
events, or climatic stress influenced
the growth of trees in the Darwin
Forest. Furthermore, Falcon Lang's method
(Falcon-Lang 2000a, 2000b) was
used to distinguish between evergreen
and deciduous species (see Brea et al.
2008). The quantitative growth ring anatomy
analysis method (Falcon-Lang
2000a, 200b) indicates that the conifer
fossil woods of the Darwin Forest probably
belong to an evergreen gymnosperm.
The understorey of the Darwin
Forest includes Cladophlebis mesozoica
Frenguelli 1947 (Fig. 6, b), Cladophlebis
mendozaensis (Geinitz) Frenguelli 1947
(Fig. 6, c), and Cladophlebis kurtzi Frenguelli
1947 (Fig. 6 a) (Brea 2000). Cladophlebis
is an extinct genus of ferns characterized
by the presence of large sterile
bipinnate foliage and it was widely distributed
in southern Gondwana during
the Mesozoic.
Figure 6: a. Cladophlebis kurtzii Frenguelli (LPPB 12618); b. Cladophlebis mesozoica Frenguelli (LPPB 12614); c. Cladophlebis mendozaensis (Geinitz)
Frenguelli (LPPB 12621); d-e. Neocalamites carrerei (Zeiller) Halle (LPPB 12584 and LPPB 12581); f. aff. Nododendron suberosum Artabe et Zamuner
(LPPB 12591); g-h. Neocalamostachys arrondoi Brea et Artabe (LPPB 12593 and 12565). Scale bars, e =2 cm, f, g and h= 1cm.
Fossiliferous level FL II found at locality A (Figs. 3 and 4) records another monotypic palaeocommunity dominated by sphenopsids (Brea and Artabe 1999).The sedimentary sequences in which this community is preserved consist of continental clastic units composed of black shales and mudstones resulting from deposition on the flood plains of a fluvial system. This autochtonous taphocenosis is constituted by stems, nodal diaphragms and reproductive structures assigned to Neocalamites carrerei (Zeiller) Halle 1908 (Fig. 6 d-e), aff. Nododendron suberosum Artabe and Zamuner 1991 (Fig. 6 f), and Neocalamostachys arrondoi Brea and Artabe 1999 (Fig. 6 g-h). The stem of Neocalamites and nodal diaphragm of Nododendron found in close association with reproductive Neocalamostachys were probably based on parts of individuals of a single taxon (Brea and Artabe 1999). Because plants growing in wet habitats are hygrophytes, this monotypic horsetail palaeocommunity conformed a waterdependent, bamboo-like thicket with hygrophytic adaptations associated to the flood plains of a fluvial environment (Fig. 7).
Figure 7: Reconstruction of Triassic horsetail landscape in the flood-plain of a fluvial environment.
In the background are groups of conifers (drawing by Jorge
Gonzalez).
COMPARISON WITH OTHERS TRIASSIC PETRIFIED FORESTS
Three other in situ Triassic petrified forests
dominated by corystosperms have
been discovered in Argentina, i.e., in the
Cortaderita (late Middle Triassic), Ischigualasto
(early Late Triassic) and Río
Blanco (Late Triassic) formations (Zamuner
1992, Spalletti et al. 1999, Artabe et
al. 2001, Artabe et al. 2007b). Presently,
the only available structural data are from
the Late Triassic corystosperm forest
found at La Elcha Mine (Río Blanco Formation,
Cuyo Basin). This is an evergreen
and monotypic community composed
of 150 stumps in life position, and the
trees colonized well-drained proximal
flood-plain areas, close to channel belts
(Artabe et al. 2007b). The permineralized
stumps were described and assigned to
Elchaxylon zavattieriae (Corystospermaceae)
by Artabe and Zamuner (2007).
Vegetation analysis of the La Elcha forest
shows that it has a clustered distribution
pattern, with a forest density of 727-1504 tree /ha. The deduced height of
Elchaxylon and the distribution of class
diameters suggest that the canopy in the
forest community would have had the
majority of specimens ranging 13-21 m.
Growth ring analysis indicates that the
forest community colonized stressed
ecosystems (Artabe et al. 2007b).
At very high latitudes, in the central
Transantarctic Mountains, another in situ
corystosperm riparian forest was found
and assigned to the Middle Triassic (Cúneo
et al. 2003). The permineralized
stumps were described as Jeffersonioxylon
Del Fueyo et al. (1995) and later assigned
to the Corystospermaceae by Cúneo et al.
(2003). This monotypic forest is interpreted
as deciduous and the tree density and
the basal area are around 274 tree/ha and
20.83 m2/ha respectively. The Antarctic
plant has been reconstructed as a tree of
~20-30 m with coniferous-like habit
(Cúneo et al. 2003).
Petrified remains of a Late Triassic forest
were preserved in the Paraná Basin,
Brazil (Pires et al. 2005). This forest was
dominated by Sommerxylon spiralosus, a
morphotaxon with taxacean affinities
(Pires and Guerra Sommer 2004).
Growth ring analysis indicates that the climate was not equable but rather with
marked seasonal variations. The fossil
wood shows a distinctive seasonal pattern
related to growth cycles, with extensive
favourable and restricted unfavourable
growth periods. The external factors
that affected the cycles were mainly related
to cyclic restrictions of water supply
and irregular changes in environmental
conditions, probably linked to occasional
droughts during the growing season
(Pires et al. 2005).
The Petrified Forest National Park in
Arizona is a better known Late Triassic
fossil forest in the southwestern United
States (Ash and Creber 1992). This fossil
forest lies within the Chinle Formation
and is mainly dominated by Woodworthia
arizonica and Schilderia adamanica (Creber
and Ash 2004). The fossil woods of this
forest do not show annual growth rings
but contain irregular growth interruptions
similar to those found in trees
nowadays growing in humid tropics.
These interruptions could be due to endogenous
hormonal effects or to occasional
local variations in water supply
(Ash and Creber 1992).
SUMMARY
Two late Middle Triassic palaeocommunities
were preserved in the Paramillo
Formation, at Agua de la Zorra, northwestern
Cuyo Basin, in southwestern
Gondwana, i.e., the Darwin Forest associated
with banks of highly sinuous fluvial
systems, and second including fossil
horsetails that grew on flood plains of
fluvial systems.
The Darwin Forest was a mixed forest,
integrated by corystosperms (Cuneumxylon
spallettii) and conifers (Araucarioxylon
protoaraucana). Species distribution shows
that in some places corystosperms and
conifers are intermingled, whereas in
others corystosperms and conifers appear
to be aggregated into cohesive monotypic groups. The canopy is formed by
two arboreal strata with a small number
of emergent; the understorey with ferns.
The included phloem and associated
parenchyma present in Cuneumxylon spallettii
may be an important adaptative
strategy to avoid water stress. Thus, polyxyly
as a functional anatomy characters
correlates with a seasonal climatic regime.
Morpho-functional analysis, structural
parameters, biomass and basal stand
area of the Darwin Forest allows classifying
it as a mainly dry evergreen subtropical
seasonal forest. The anatomy of the
growth rings of Araucarioxylon protoaraucana
suggests strongly seasonal conditions.
Moreover, growth ring analysis indicates
that this species was an evergreen
gymnosperm.
The second palaeocommunity integrated
by sphenopsids shows herbaceous-arbustive
vegetation with hygrophytic adaptations.
A fluvial flood-plain in an open
landscape was the environment in which
this fossil vegetation developed.
ACKNOWLEDGEMENTS
The authors would like to express their thanks to Guillermo Ottone, Greg Retallack and one anonymous reviewer for their critical and constructive comments on an earlier version of this paper. We would like to thank the financial support provided by the Consejo Nacional de Investigaciones Científicas y Técnicas (Project PIP 5430) and Universidad Nacional de La Plata (11/N535). The charts of the Triassic vegetation reconstruction were made by Jorge Gonzalez.
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Recibido: 8 de agosto de 2008
Aceptado: 28 de octubre de 2008