Analía E. Artabe

Cuticular characters adapted to volcanic stress in a new Cretaceous cycad leaf from Patagonia, Argentina. Considerations on the stratigraphy and depositional history of the Baqueró Formation

Abstract The cuticle of a new cycad, Pseudoctenis ornata Archangelsky et al., sp. nov. is described and discussed in relation to the physical paleoenvironment in which the plant lived. The specimens occur in the Early Cretaceous Baquero Formation, near Estancia El Verano in the Santa Cruz Province, Argentina. A detailed stratigraphic section records four facies, namely (1) fluvial channel, (2) flood plain, (3) lacustrine, and (4) flat and extended plains. A detail of each facies is provided. Pseudoctenis cuticles are found in the flood plain facies; the other components of the plant association are Gleichenites, Araucaria and Taeniopteris . The depositional history of this succession is related to a braided river that periodically received volcanic ash. Plants grew until complete burial by ash. Leaves of Pseudoctenis are pinnate, hypostomatic, with polycyclic stomata that form ill-defined rows. Abundant papillae and hair bases are present, especially on the lower cuticle. Comparisons are made with other Pseudoctenis species found in the same formation, and in other regions of the world. It is suggested that the paleoenvironment had a strong influence on the vegetation, especially the ash fall, and that it may have played a role in the formation of xeromorphic structures that characterize several gymnosperms present in this stratigraphic unit, including Pseudoctenis ornata . The lack of burning features on the cuticle suggests a cold ash fall. This fall probably was responsible for changes in edaphic patterns and atmospheric conditions that governed the growth and distribution of plant communities during the deposition of the Baquero Formation.

Fossil Cycadales of Argentina

A survey of Cycadalean taxa of Argentina (including Antarctica) is presented. The record of leaves represented byNilssonia, Pseudoctenis, Ctenis, Mesodescolea, Ticoa, Almargemia,Kurtziana, andZamia genera are summarized. Recent investigations made of cuticles with transmission and scanning electron microscopy are included. In stems, a preliminary study of two forms found in the Upper Cretaceous of Rio Negro Province are incorporated intoMichelilloa, Bororoa, andMenucoa. The fossil record shows some characters of leaf morphology, presence of inverse xylem and medullary bundles, two kinds of leaf traces, and monoxylic and polyxylic steles of systematic importance when compared with both extant and fossil cycads. Affinities of fossil taxa related to extant families are suggested through analysis of the above-mentioned characters. The presence of medullary vascular rings related to the emission of terminal reproductive strobili are recognized inBororoa andMenucoa. This character associated with polyxyly makes it possible to assign these genera to the Zamiaceae-Encephalartoideae sensu Stevenson, widening the paleogeographical distribution of this subfamily.

Geological Factors and Evolution of Southwestern Gondwana Triassic Plants

Abstract A synergistic model based on reciprocal influences between biotic and abiotic factors is developed for the Triassic of southwestern Gondwana. Changes in physical environment exerted a strong influence on the characteristics and evolution of plant assemblages. The Permian-Triassic extinction, and the change from palaeophytic to mesophytic floras, is one of the most striking examples of direct influence of physical environment upon plant communities. Pangea coalescence, the distribution of land masses and seas, the spreading of continental climates (megamonsoonal conditions) and the waning polar glaciation determined the expansion of xeromorphic morphotypes that became dominant during the whole Mesozoic. In southwestern Gondwana, the introduction or invasion of immigrant lineages suggests a strong asymmetrical interchange from the Euroamerican realm to the Gondwana realm. In addition, generalised extensional volcanism, development of intracratonic rifts and the palaeolatitudinal location of climatic zones during the early-Middle Triassic favoured extinction of the Glossopteris flora and explosive diversification of endemic groups. From the chronological viewpoint, the Barrealian, Cortaderitian and Florian stages are recognised in the Triassic of southwestern Gondwana. These stages are respectively characterised by: (a) appearance of mesophytic elements, and coexistence of Palaeozoic and Mesozoic groups, (b) maximum diversification of the Dicroidium flora, and (c) Dicroidium flora decline and replacement by morphotypes with strong Jurassic affinity. These palaeofloristic changes seem to be strongly influenced by tectonic evolution of sedimentary basins, temporal and regional distribution of sedimentary environments, and intra-Triassic palaeoclimatic change.

Two New Petrified Cycad Stems, Brunoa gen. nov. and Worsdellia gen. nov., from the Cretaceous of Patagonia (Bajo de Santa Rosa, Río Negro Province), Argentina

Polyxylic columnar stems covered by persistent leaf bases and found in sediments assignable to the Upper Cretaceous of Bajo de Santa Rosa, Río Negro Province, Argentina, are described as two new generic entities in the Cycadales. Anatomical characters are the basis for their being assigned to the Encephalartoideae of the Zamiaceae.Brunoa santarrosensis gen. et sp. nov. is characterized by the presence of polyxyly, cone domes, mucilage cavities, and uniseriate to triseriate araucaroid, scalariform, or bordered intervascular pitting.Worsdellia bonettiae gen. et sp. nov. has polyxyly, anastomosing medullary vascular bundles, centripetal xylem, mucilage canals, and concentric extraxylary bundles. Some characters (polyxyly, medullary vascular bundles, and cone domes) were used to determine the systematic position, while other characters (mucilage reservoirs and centripetal xylem) were used to establish the relationship between polyxylic and monoxylic forms.

A new approach to Corystospermales based on Triassic permineralized stems from Argentina

Anatomically preserved mature stems of late Middle Triassic corystosperms from the Paramillo Formation of Argentina are described and assigned to Cuneumxylon spallettii gen. nov. et sp. nov. The silicified specimens show features of the pith, primary xylem and successive rings of secondary xylem and phloem. The most striking characteristic is the anomalous secondary growth, represented by secondary xylem bounded by arcs of secondary phloem probably derived from successive repositioned cambia. Cuneumxylon has two kinds of unusual centrifugal secondary growth. The first shows unequal activity of different portions of the cambium on the circumference of the axes; the consequent restriction of cambial activity to certain restricted areas develops wedged stems, which often split. The second produces polyxylic stems following supernumerary cambial activity. As in extant plants growing in arid regions, the included phloem and the associated parenchyma may have had functional value avoiding desiccation of the outer tissues of the stem during droughts. Anatomical features of other Corystospermaceae were used to determine systematic affinities and to establish relationships among medullosans, corystosperms and cycads.

Ecological reconstruction of a mixed Middle Triassic forest from Argentina

Brea, M., Artabe, A. & Spalletti, L.A., December, 2008. Ecological reconstruction of a mixed Middle Triassic forest from Argentina. Alcheringa 32, 365–393. ISSN 0311-5518. The palaeoecology of an in situ Middle Triassic forest known as the Darwin Forest, from the Paramillo Formation of Argentina, is described based on palaeobotanical, sedimentological and spatial analyses. The palaeoforest 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. The plant community was reconstructed by quantitative mapping of the fossilized stumps integrated with taxonomic and sedimentological information. The Darwin Forest had a tree density of 427–759 per ha, with an upper stratum (20–26 m) of corystosperms and a second stratum (16–20 m) of conifers. Estimated biomass is equivalent to modern dry monsoonal forest. The understorey was composed of ferns (Cladophlebis spp.). Quantitative growth-ring analysis of Araucarioxylon protoaraucana suggests that these trees were evergreen and, together with vegetation structure and sedimentary data, indicate that the forest developed under dry, subtropical, strongly seasonal conditions.

Estratigrafía, facies y paleoflora de la sucesión triásica de Potrerillos, Mendoza, República Argentina

Nuevas exposiciones en localidades triasicas clasicas como las de los cerros Bayo y Cocodrilo (Potrerillos, noroeste de la Provincia de Mendoza, Argentina) permitieron (1) describir y caracterizar las formaciones Rio Mendoza, Cerro de Las Cabras, Potrerillos y Cacheuta; y (2) mejorar el conocimiento bioestratigrafico de las sucesiones triasicas basandose en el hallazgo de nuevos registros de plantas fosiles. Dos secciones verticales fueron estudiadas en detalle, y el analisis de las litofacies ha sido usado para interpretar los sistemas de deposicion. Facies gruesas de abanico aluvial caracterizan a la Formacion Rio Mendoza y a la seccion inferior de la Formacion Potrerillos. Sistemas fluviales traccionales gravosos y arenosos aparecen hacia el techo de la Formacion Rio Mendoza y en distintos intervalos de la Formacion Potrerillos. En el tramo basal de la Formacion Cerro de Las Cabras y en la mayor parte de la Formacion Potrerillos se identificaron depositos de sistemas fluviales de alta sinuosidad y de planicies de inundacion. Finalmente, algunos intervalos de la Formacion Potrerillos y mas especificamente la Formacion Cacheuta estan dominados por sucesiones de lutitas negras generadas por procesos suspensivos en lagos meromicticos. En asociacion con los depositos mencionados, secuencias progradacionales discretas de areniscas y conglomerados sugieren el desarrollo de barras de desembocadura hipopicnales y de canales distributarios. Se individualizaron 16 niveles fosiliferos con 23 taxones: Equisetites fertilis, Neocalamites carrerei, Cladophlebis mesozoica, C. mendozaensis, Dicroidium argenteum, D. dubium, D. odontopteroides, Johnstonia coriacea, J. stelzneriana, Xylopteris elongata, Zuberia feistmanteli, Feruglioa samaroides, Pachydermophyllum praecordillerae, Kurtziana cacheutensis, Sphenobaiera argentinae, Baiera africana, B. cuyana, Yabeiella mareyesiaca, Y. spathulata, Y. brackebuschiana, Fraxinopsis andium y Gontriglossa sp. y Rochipteris sp. Se describieron 16 tafocenosis que se relacionaron con los respectivos ambientes de sedimentacion. El analisis de las tafocenosis y su vinculacion con las facies y paleoambientes de acumulacion permitio reconocer en la localidad tipo de la Formacion Potrerillos a dos biozonas de asociacion en continuidad estratigrafica (Yabeiella mereyesiaca-Scytophyllum bonettiae-Protophyllocladoxylon cortaderitaensis -MBC- y Yabeiella brackebuschiana-Scytophyllum neuburgianum-Rhexoxylon piatnitzkyi -BNP-). La Formacion Potrerillos en este sector de la cuenca se desarrollo durante el Triasico Medio tardio-Triasico Tardio temprano

Elchaxylon, a new corystosperm based on permineralized stems from the Late Triassic of Argentina

Artabe, A.E. & Zamuner, A.B., March 2007. Elchaxylon, a new corystosperm based on permineralized stems from the Late Triassic of Argentina. Alcheringa 31, 85‐96. ISSN 0311-5518. Anatomically preserved stems of Late Triassic corystosperms from the Río Blanco Formation of Argentina are described and assigned to Elchaxylon zavattieriae gen. et sp. nov. The polyxylic gymnospermous axes have two discontinuous cambial rings, which develop centrifugal secondary xylem and centripetal secondary xylem around the mesarch primary xylem bundles. The centrifugal undivided pycnoxylic secondary xylem incorporates uniseriate rays, uni- to biseriate radial pitting either alternate or opposite, and one or two simple pits in each cross-field. Atypical secondary growth is restricted to localized development of centripetal secondary xylem (inverted xylem). Elchaxylon could represent a basal member of the rhexoxyloid lineage. This basal clade is characterized by limited centripetal secondary xylem, lack of perimedullar bundles, and modest development of centrifugal xylem. Analia E. Artabe [aeartabe@museo.fcnym.unlp.edu.ar] and Alba B. Zamuner [azamuer@museo.fcnym.unlp.edu.ar]. División Paleobotánica, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Paseo del Bosque SN, 1900 La Plata, Argentina and Consejo Nacional de Investigaciones Científicas y Técnicas; received 20.5.2005, revised and accepted 14.9.2005.

A new peltasperm (Gymnospermopsida) from the Middle Triassic of Argentina

A new peltasperm, Scytophyllum bonettiae sp. nov., is described from the Middle Triassic Cortaderita Formation, San Juan, Argentina. The new species is characterised by having medium to small monopinnate, imparipinnate, lanceolate, bicoherent fronds with a free bipinnate pattern of venation, and lumps on the rachis. Recovered specimens display juvenile to adult foliage. The new taxon, together with other species, characteises late Middle Triassic-early Upper Triassic terrestrial flora from western Argentina.

Neochamberlainia, a new name for Chamberlainia Artabe, Zamuner & D. W. Stev. (Zamiaceae) non Chamberlainia Grout (Brachytheciaceae)

Artabe et al. (2005) described a new fossil cycad genus, Chamberlainia, from the Allen Formation (Late Cretaceous) of Baja de Santa Rosa, Rio Negro Province, Argentina. Unfortunately, this name is preoccupied by a moss genus in the Brachytheciaceae, described by Grout (1928). Thus, Chamberlainia Artabe, Zamuner & D. W. Stev. is a later homonym (Art. 53.1, ICBN Vienna) and a new name is required.