Georgina M. Del Fueyo

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.

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.

Leaf Cuticle Anatomy and the Ultrastructure of Ginkgoites ticoensis Archang. from the Aptian of Patagonia

The leaf cuticle of the Ginkgoites ticoensis Archang. type material from the Aptian Anfiteatro de Ticó Formation in Patagonia, Argentina, is fully characterized with additional scanning and transmission electron microscopic observations. Many new anatomical and ultrastructural cuticular features are identified in the four-lobed leaf of G. ticoensis: the leaf shows a hypostomatic and papillate laminae, straight and pitted anticlinal and granulate periclinal walls, actinocytic stomata with between five and seven papillate, striate subsidiary cells, and guard cells with anticlinal smooth walls. The TEM studies on ordinary epidermal cells, papillae, subsidiary cells, and guard cells reveal general ultrastructural features of Ginkgoaceae: an outer polylamellate layer A made with A1 and a granular inner layer A2; A1 with an upper part A1U with continuous and straight translucent lamellae; a lower part A1L with significantly disrupted and waving translucent lamellae; and the fibrillar cuticular layer B1 as the innermost part. Ten ultrastructural characters are detailed and ranked by the use of confidence intervals based on 30 statistical measurements. A three-dimensional reconstruction of the cuticle is also provided. Because of the anatomical and ultrastructural fine details shown in the G. ticoensis cuticle, new elements are given to suggest its probable family affinity and to enhance the specificities of Ginkgo and Ginkgoites.

Estudios cuticulares de Ginkgoites Seward del Cretácico Inferior de Patagonia

Cuticular studies of Ginkgoites Seward from the Lower Cretaceous of Patagonia. The anatomy and ultrastructure of the leaf cuticles of Ginkgoites ticoensis Archang. and G. skottsbergii Lund. is studied. Both taxa were collected in Santa Cruz province, G. ticoensis from the Anfiteatro de Tico Formation (Aptian) and G. skottsbergii from the Kachaike Formation (Albian). The cuticular studies were made by means of electronic microscopy (SEM, TEM). These observations showed that G. ticoensis has hypostomatic leaves, papillae on both cuticles, and monocyclic stomata with 5-7 papillate subsidiary cells; while G. skottsbergii presents amphistomatic leaves, and monocyclic to dicyclic stomata with 6-8 subsidiary cells partially papillate. At the ultrastructural level the cuticular membrane has three layers, which are clearly distinct in both species. Comparisons with the leaf cuticle of other ginkgoalean taxa are also made.

FOSSILIZATION MODEL FOR SQUAMASTROBUS TIGRENSIS FOLIAGE IN A VOLCANIC-ASH DEPOSIT: IMPLICATIONS FOR PRESERVATION AND TAPHONOMY (PODOCARPACEAE, LOWER CRETACEOUS, ARGENTINA)

Abstract The present study is a holistic approach to the relationship between volcaniclastic host rock characteristics and the fossilization processes of short leafy coniferous branches of Squamastrobus tigrensis, preserved as fossilized-cuticles (Lower Cretaceous, Baqueró Group, Patagonia, Argentina). The question of diagenetic influences of Aptian volcaniclastic sedimentation on preservation chemistry and taphonomic processes is addressed. Whereas infrared spectroscopy provided chemical information on the leaves, vitrinite reflectance and complementary thermal indicators provided data on the thermal maturity of the dispersed organic matter in the host rock. Three sample types were analyzed: fossilized-cuticle, macerated fossilized-cuticle (by infrared spectroscopy), and associated organic host rock matter (by light microscopy). Results clearly show chemical variability between, and within the fossilized-cuticle and cuticle, as well as a similarity to type I/II kerogen, i.e., high contents of both aliphatic groups and oxygen-containing compounds. Combined with the lower maturity of the host rock, the importance of the depositional environment during burial and taphonomic conditions that affected the fossilization of S. tigrensis are summarized in a general fossilization model.