Sergio Archangelsky

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.

Early cretaceous angiosperm leaves from southern South america.

Early angiosperm leaves from the Aptian (113 to 119 million years ago) Baquer� Formation of Patagonia have been found in a fossil flora dominated by more than 100 species of gymnosperms and pteridophytes. They may be the first early Cretaceous angiosperm leaves to be reported from southern South America and one of the few reported in the Southern Hemisphere. The leaves are large, lobate, craspedodromous, and dentate (A-1 teeth) and have ramified tertiary veins and random fourth-order venation. Several of these features have been found in coeval and younger strata elsewhere, but not in the same combination. They were probably a marginal component of the flora.

Spore and pollen types of the lower cretaceous in Patagonia (Argentina)

The Baquero Formation of Santa Cruz Province, Argentina, has proved to be rich in spore-pollen content. So far, 46 species have been determined, 10 of them being new. Three other groups of species are recognized: (1) those which are common only with Australia (and Antarctica); (2) those common with northern provinces only; and (3) those which are cosmopolitan (present in northern as well as in southern provinces). The age of the formation, judging by the whole assemblage, is Lower Cretaceous, more precisely Barremian to Aptian. No true angiosperms occur in this assemblage.

Squamastrobus gen. N., A fertile podocarp from the early cretaceous of Patagonia, Argentina

Abstract Squamastrobus tigrensis nov. gen. et sp. has been found in early Cretaceous strata (Baquero Formation), Santa Cruz Province, Argentina. It is based on branches with squamiform leaves, laterally attached pollen cones and terminally disposed seed cones. The cuticle of leaves and female bract-scale complexes was studied in their general structure and ultrastructure. The pollen cones contain bisaccate pollen grains that are similar both structurally and ultrastructurally with extant and fossil members of the Podocarpaceae. The seed cones are compact structures resembling some taxa found in Mesozoic Gondwana strata. Comparisons made with living and extinct members of the Podocarpaceae suggest some phylogenetic links between Mesozoic cone-bearing forms with the extant genera Microcachrys and Pherosphaera . Finally, some considerations on the origin of Mesozoic podocarps are also included in regard to recently discovered Permian conifers from Gondwana.

Coniferous Ovulate Cones from the Lower Cretaceous of Santa Cruz Province, Argentina

Two types of coniferous ovulate cones borne on leafy twigs are described from the Lower Cretaceous Kachaike Formation in Santa Cruz Province, Argentina. The fossils are impressions and compressions with well‐preserved cuticles. Morphology, anatomy, and ultrastructure were studied using LM, SEM, and TEM. Ovulate cones were assigned to Athrotaxis ungeri (Halle) Florin of the taxodiaceous Cupressaceae and Kachaikestrobus acuminatus gen. et sp. nov. of the Cheirolepidiaceae. This latter taxon is characterized by terminal elongate cones borne on twigs with Brachyphyllum leaves, cone scales densely and helically disposed, broad bracts with an accentuated acuminate apex fused at their bases to eight‐lobed ovuliferous dwarf shoots shorter than the bracts, and an epimatium covering at least one ovule that has preserved only the outer integument and the megaspore membrane. The ultrastructure of the bract and ovuliferous dwarf shoot cuticle is composed of three layers. Comparisons of K. acuminatus with other cheirolepidiaceous ovulate cones showed closest resemblance to Hirmeriella muensteri (Schenk) Jung. These Patagonian cone scales appear to have some of the most ancestral characters in the family. By the early Albian, A. ungeri and K. acuminatus were part of a plant assemblage dominated by ferns and a few subordinate angiosperms. The finding of A. ungeri in the Kachaike Formation extends its distribution during the Lower Cretaceous in Patagonia. These fossils also show that at that time, the taxodiaceaous Cupressaceae and the Cheirolepidiaceae still were well represented in southern South America.

Pollen grains found in coniferous cones from the lower cretaceous of patagonia (Argentina)

Abstract Two coniferasof the Baquero Formation (Lower Cretaceous), Santa Cruz Province, Argentina, have been found with male cones organically attached to leafy twigs. The first, Tomaxellia biforme archangelsky has cones with Classopollis type of pollen. The second, Brachyphyllum irregulare , has cones with Inaperturopollenites limbatus pollen. A brief description of both pollen types is given.

Aspects of Gondwana paleobotany: gymnosperms of the Paleozoic—Mesozoic transition

During the late Paleozoic and early Mesozoic the Gondwana Supercontinent underwent dramatic geographic and climatic changes. Geologic and biologic factors concurrently played an important role modelling the vegetation of that time. The gymnospermic component of plant assemblages shows significant variations in composition and in the distribution of different taxa. Analysis of the assemblages shows that some plant groups dominated the scenario, such as the pteridosperms, glossopterids, corystosperms and, to a lesser degree, cordaites and conifers. Ginkgophytes, bennettites or cycads were less important in the Paleozoic but their numbers increased in the Triassic. Paleozoic assemblages were extensively dominated by glossopterids that became extinct in the earliest Mesozoic. Pteridosperms crossed the P—M barrier and became dominant during the Triassic, at a time when corystosperms evolved and radiated. Other groups became extinct in the Paleozoic, namely dicranophylls and cordaites. Conifers were represented by different families, restricted either to the Paleozoic or the Mesozoic. They were not conspicuous in the analysed assemblages. In some areas of Gondwana, taxa of the Euramerican alliance are present through a migrational mechanism that occurred during continental displacements which produced global climatic changes. Recent studies have shown that there are far more common elements between Euramerica and Gondwana than suspected up to now. These elements find their distribution especially in the western part of Gondwana (Africa—South America).

Comparative ultrastructure of three early cretaceous gymnosperm pollen grains: Araucariacites, Balmeiopsis and Callialasporites

Abstract Transmission electron microscope studies of three types of pollen grains, i.e. Balmeiopsis, Callialasporites and Araucariacites, found in Early Cretaceous strata of Patagonia have shown that they possess several similar ultrastructural patterns. The nexine is uniformly lamellate and the sexine is composed of granular to vermiculate sporopollenin units. Sometimes granular elements may form ill defined collumella-like structures oriented radially near the surface of the grains. Balmeiopsis has a thickened equator that may contain a small empty space, while Callialasporites has rudimentary sacci formed by the separation of both exine layers. However, the inner surface of the sexine does not form an endoreticulum and there are no thin sporopollenin threads that fill the cavities between nexine and sexine. This type of primitive saccus is different from the eusaccate or protosaccate types found in gymnosperms. Araucariacites has the thinnest exine of the three and no equatorial thickening or rudimentary sacci. All three types belong to conifers and comparisons are made with fossil and extant members of the Podocarpaceae and Araucariaceae, two families that are now represented in the Andean region.

Polyspermophyllum, a new Permian gymnosperm from Argentina, with considerations about the dicranophyllales

Abstract Polyspermophyllum gen. nov., a new primitive seed plant, is described. The material was found in early Permian strata, Chubut province, Argentina. It includes twigs that bear long, linear, once dichotomized leaves with two marginal longitudinal grooves in a compact helix. The fertile structures have proximal portions similar to sterile leaves and ultimate trusses with axes being several times dichotomized (or trichotomized) and bearing subsphaerical bodies composed of a distally curved segment (peduncle) enclosing a single oval structure (ovule). The vegetative organs ar closely comparable to taxa that have been referred to different genera while the fertile organs resemble those of the late Palaeozoic Trichopitys and Dicranophyllum . It is suggested that the Dicranophyllales are a primitive order of seed plants, including two families, the Trichopityaceae and the Dicranophyllaceae, in which Polyspermophyllum is placed. Possible relationships with Paleozoic pteridosperms and Mesozoic ginkgophytes are discussed, especially with regard to a putative “pre-ginkgophytic” line, already established during the Carboniferous in all palaeofloristic regions, viz. Angara, Equatorial Belt and Gondwana.

Early Cretaceous riparian vegetation in Patagonia, Argentina

Early Cretaceous riparian vegetation in Patagonia, Argentina. An autochthonous plant as-semblage found at the Bajo Grande locality, Anfiteatro de Tico Formation, Baquero Group, Early Aptian of Santa Cruz province, Argentina, is described. Sedimentological data show an alternation of sandstone and lime-stone, with recurrent fossil levels associated to a fluvial system. The frequent flood in areas close to the levee had an influence on the vegetation growing along the banks of the channels. A plant assemblage composed of Ricciopsis grandensis nov. sp. (bryophyte), Adiantopteris tripinnata nov. sp., Schizaeaceae pinnules (ferns), and the gnetalean fructification Ephedra verticillata nov. sp. was found in this paleoenvironment. The taphonomical studies suggest that the plant assemblage was autochthonous, and it grew and was buried in an area near a levee. This result is congruent with environmental data provided by living representative of the fossil taxa analyzed.