Equisetites pusillus sp. nov. from the Aptian of Patagonia, Argentina

: Compressions of fertile equisetacean branches belonging to the new species Equisetites pusillus are described using light microscopy. The specimens were collected at the Estancia Bajo Tigre locality in strata referred to the Anfiteatro de Ticó Formation, Baqueró Group (Aptian) from the Santa Cruz province, Argentina. The fertile stems bear an oval and solitary terminal cone with spores. The spores are alete, circular and without elaters. Equisetites pusillus s p. nov . is compared with other fossil equisetacean taxa from Argentina and elsewhere. The spores are compared with those of two extant Equisetum species from Argentina, and also with other mesozoic taxa. The compressions were found in sediments rich in organic matter, deposited in shallow waters surrounded by environments colonized by sphenophytes, ferns and bryophytes. This type of landscape was frequent during the Aptian originating a common type of deposit (lenticular) in the Anfiteatro de Ticó Formation.

The genus Equisetites was established by Sternberg (1833) for specimens of the European Triassic, preserved as casts, impressions and com-pressions having similar morphological harac-ters with the extant genus Equisetum L. c Since the early years of the last century, Equisetites was frequently reported in Mesozoic sediments of Argentina and Antarctica. Halle (1913) described the first equisetacean stems from the Middle Jurassic of Antarctica. Roots and stems of a new species (Equisetites fertilis) from the Triassic of San Juan and Mendoza provinces were described by Frenguelli (1944 a). Stems with leaf-sheaths (Equisetites frenguellii) were ana-lyzed by Orlando (1946) for the Lower Jurassic of Neuquén province. Archangelsky (1964) de-scribed new stems ( Equisetites sp.) from the Lower Cretaceous of Santa Cruz province; and Herbst (1964Herbst ( , 1965Herbst ( , 1966 found dif-ferent stems (Mendoza and Neuquén provinces), and created a new taxon (Equisetites patagonica) from the Liassic of Roca Blanca ormation, in Patagonia. F Menéndez (1958) described the first fertile specimens as Equisetites quindecimdentata from the Upper Triassic of San Juan province. The cones although incomplete show porangiophores and sporangia. s In the present paper, three fertile specimens of the new species Equisetites pusillus from the Aptian of Santa Cruz province are described using light microscopy. This is the first Equisetites species with spores found in the Mesozoic of Argentina.

MATERIALS AND METHODS
The fertile specimens occur at Estancia Bajo Tigre, Anfiteatro de Ticó Formation, Baqueró Group (Cladera et al., 2002). The site was first mentioned by Archangelsky (1967) and is located at 48º 30' 33" S and 69º 14' 11" W, in the Deseado Massif of the Santa Cruz province (Argentina), and is of Aptian ge (120 Ma; Corbella, 2001). a The specimens are well preserved compressions and were collected in organically rich sediments that were deposited to river banks or seasonal lagoons, and associated with ferns and bryo-phytes belonging to plant communities that ved near shallow waters. li The material was studied with a Leica MZ 12.5 stereoscopic microscope and photographed with a Canon camera. One specimen was partially treated with HF and HCl, and the residue was mounted in glycerine jelly for its observation with light microscopy. The spores were studied using a Leitz Diaplan microscope, measured using the Leica QWin software and photographed ith a Leica DFC 280 camera. w The material is deposited in the Paleobotanical Collection of the Argentine Museum of Natural Sciences «Bernardino Rivadavia» with the acronyms BA Pb (specimens) and BA Pb Pm (microscopic slides). Diagnosis. Stems 3 mm wide of unknown length, with 2 to 3 ridges, internodes 5 -10 mm long and leaf-sheaths with 12 -13 leaves at nodal diaphragms. Apical, single cones with rounded apex and convex sides supported by bracts. Spo-rangiophores helically arranged. Rhomboidal to hexagonal sporangiophore heads up to 1 mm in diameter. Alete spores, circular to subcircular outline, 46 -69 µm in diameter. Exine smooth to slightly scabrate, thinner in the central gion, without elaters. re Description. Equisetites pusillus sp. nov. is represented by sterile and fertile compressions with sporangiophores lacking cuticle. The fertile stem fragments are probably terminal branches, ut the main axis was not found. b Specimen BA Pb 13501 has a stem 2 mm wide and 20 mm long with three well defined ridges. At the base of the apical cone, the stem is en-larged bearing six imbricate and lanceolate bracts having rounded apexes (Fig. 1B). The stem is topped by an oval cone 5 mm wide and 6 mm long, with a rounded apex and convex sides (Fig. 1B). The helically arranged sporangiophores have a rhomboidal external head. Each shield has a de-pressed upper margin and a rising l wer margin (Fig. 1B). Sporangia are not visible. o Specimen BA Pb 13502 is a small stem fragment bearing a terminal cone. The branch is thin, up to 2.7 mm wide and 3.5 mm long. It has two longitudinal ridges, however nodes, internodes or leaf-sheaths are not seen. The branch ends with an irregular transversal margin (Fig. 1C). The cone is complete and oval, 5 mm wide and 6.5 mm long, with a rounded apex and convex sides. The sporangiophores are arranged helically. They are rhomboidal, 1 mm in diameter, with a slightly depressed central area (Fig. 1C).
porangia are not seen. S Specimen BA Pb 13503 is a fragment of stem bearing a terminal cone. The stem is thin, 2 mm wide and 32 mm long. It has two evident ridges, but leaf-sheaths are not observed (Fig. 1A). The cone is incomplete and wedge-shaped, 5 mm wide and 5 mm long, with truncate apex and convex sides. The sporangiophore heads are slightly marked and have a rhomboidal to hexagonal out-line (Fig. 1A). This cone fragment was chemically treated and in the residue inaperturate spores and few Classopollis pollen grains (conifer fam-ily Cheirolepidiaceae) ere found. w The spores are alete with a thinner exine (leptome) in the central region. They have a circular to subcircular outline and a diameter of 46.5 -68.2 µm. The exine is 1.30 -2.20 µm thick, smooth or slightly scabrate. Frequent folds oc-cur on the spore surface. Elaters are not present ( Fig. 1D -I). Abundant remnants of the tapetal membrane are adhered to the spores. Isolated Ubisch bodies are also irregularly distributed on the tapetal membrane and the spore surface, being more conspicuous on the former ( Fig. 1E -H).
Equisetites sp. (Archangelsky, 1964) from the Anfiteatro de Ticó Formation (Lower Cretaceous of Santa Cruz province) has sterile branches up to 3 mm wide, with internodes 5 -10 mm long and leaf-sheaths with 12 -13 leaves at the nodal diaphragms. This material was found at the Anfiteatro de Ticó locality in a kaolinic bed in which poorly preserved bennettitalean and filicinean remains also occur (Archangelsky, op. cit.) . Equisetites sp. is interpreted as the vegetative stems of Equisetites pusillus, and is included i the new species. n The palynological records of the Baqueró Group show the presence of equisetalean-type spores, being described as Calamospora aff. mesozoica Couper 1958. During Mesozoic times, there were equisetalean plants that have produced two types of spores, some with trilete laesura and others inaperturate (Archangelsky & illar de Seoane, 1994). V Comparisons with similar species. Several equisetacean species are known in the Mesozoic and Cenozoic of Argentina. Table 1 shows their sizes and morphological characters, the tiny stems of Equisetites pusillus sp. nov. being only omparable to Equisetites quindecimdentata c Menéndez, a fertile species with axes bearing ter-minal cones, found in the Upper Triassic of Hilario, San Juan province. The fertile stems are up to 37 mm wide, and have internodes, leafsheaths and stellate nodal diaphragms. The axes end in terminal, incomplete and large cones (up to 20 mm wide) with strongly umbonate sporangiophores and triangular to subtriangular sporania (Menéndez, 1958). This species differs from g Equisetites pusillus sp. nov. in having larger ones and sporangiophores. c Equisetites fertilis (Frenguelli) Frenguelli from the Upper Triassic of the Quebrada de Los Rastros (San Juan province) and Potrerillos For-mation (Mendoza province) includes specimens firstly referred to Macrotaenia fertilis Frenguelli (1943 a) and Equisetites scitulus Frenguelli (1943 b). A new interpretation of the fragments that were originally thought to be leaves bearing fer-tile bodies, allowed Frenguelli (1944 b) to pro-pose the new combination Equisetites fertilis. This is the most complete sterile species found thus far in Argentina. It has thin roots, stems up to 45 mm wide, leaf-sheaths and stellate nodal dia-phragms (Frenguelli, 1944 b Table 2). (s Watson (1983), and Watson & Batten (1990) found globose and alete spores (28 -48 µm in diameter) with a smooth or scabrate exine and without elaters in the cones of Equisetum lyellii Mantell, from the Wealden of Sussex (England). These spores are similar to Pilasporites allenii atten. B Batten (1968) described Pilasporites allenii from the soil-bed sediments from the Wadhurst Clay in the southern region of England (Lower Cretaceous), finding circular and alete spores with a smooth or scabrate exine and a mean iameter of 36.7 µm. These spores lack elaters. d Equisetites muensteri Sternberg from the Upper Triassic of Greenland (Harris, 1931) has rounded spores 40 -50 µm in diameter, with thin and smooth wall without elaters. He discussed the absence of elaters suggesting that they may have been destroyed during the fossilization or preparation of the spores. Halle (1908) and Batn (1968) shared this opinion. te Halle (1908) described two species of Equisetostachys from the Rhaeto-Liassic of Sweden: E. nathorstii and E. suecicus. The first species has globose and usually folded spores with a short trilete mark and smooth exine. They are 35 -40 µm in diameter. E. suecicus spores are lightly larger (40 -50 µm), alete and smooth. s The spores of Equisetites pusillus sp. nov. also differ from other species, in the size and the x-ine ornamentation (see Table 2). e Harris (1978) found spores with attached elatrs in the cones of Equisetum columnare e Brongniart from the Middle Jurassic of Yorkshire (England). These spores are globose, alete, about 40 -50 µm in diameter and have a scabrate exine.
Equisetum laterale Phillips from the Lower Jurassic of Australia has ovate and terminal cones with numerous spores. They are small (24 -41 µm in diameter), globose and alete with a cabrate to microrugulate exine (Gould, 1968 xine. e Equisetites arenaceus (Jaeger) Schenk from the Upper Triassic (Keuper) of Germany has three cones attached to the fertile branches. The cone has sporangiophore heads bearing a circle of sporangia on their lower surface. They include globose spores (50 -60 µm in diameter), alete or with a small trilete, and scabrate to microrugulate exine. None of these spores showed any attached elaters, except in one case in which there was a possible indication of this structure (Kelber & van Konijnenburg-van Cittert, 1998).
Discussion. The Equisetales include two families: Calamitaceae and Equisetaceae. Both have similar morphological and anatomical characters, such as the structure of the primary vascular tissue and the presence of spores with elaters (Tayr & Taylor, 1993). lo The Calamitaceae are Paleozoic and differ from the Equisetaceae (mostly Mesozoic to recent) in having arborescent habits. They present secondary vascular tissue, larger leaves and cones with a different morphology, size and position. The cones may be mono, bisporangiate or grouped in nodal sporangiophores, being axial in the branches or in terminal bracts (Taylor & aylor, op. cit.). T The geological history of the equisetalean plants begins in the Devonian and has its most   (Ponce, 1996) (Fig. 2). E. bogotense H.B.K. is a small plant (15 -20 cm high) with oblong cones 3 mm wide and 6-8 32º 32º URUGUAY mm long. The spores are 40 µm in diameter and have two elaters. This species occurs in Jujuy, Salta, Tucumán, Corrientes, Córdoba, San Juan, San Luis, Mendoza, Neuquén, Río Negro and Chubut provinces (Ponce, op. cit.) (Fig. 2)  The fertile branches and the cones of ous, where it is possible that both families coex-Equisetites pusillus are similar to those of the extant Equisetum bogotense, except that the fos-isted or the Equisetaceae evolved from the Calamitaceae. The first known equisetacean resil spores lack elaters. mains are Triassic, and in Argentina the oldest Equisetites species are of Early Triassic age. ACKNOWLEDGEMENTS Up to now, specimens of Equisetites have been found in Mesozoic strata of San Juan, Mendoza, Thanks are due to Dr. Sergio Archangelsky for Neuquén, Chubut and Santa Cruz provinces (Arvaluable suggestions that improved the original gentina), and in the Jurassic of Antarctica (Fig. 2). manuscript, and for providing the fossil material; At present, the Equisetaceae are represented Angel Fusaro for photographic assistance; Luis by one genus: Equisetum L., with approximately Lezama for technical support in the figure 1; and 30 herbaceous species, with terminal cones and Amalia González for the map drawing. This research has been supported by the Grant ANPCYT sporangiophores arranged in whorls, and a worldwide distribution, except Australia. The spores PICT 99 07-06044. are spheroidal, with a very small aperture, granulate surface with scattered spherules and two BIBLIOGRAPHY long and spathulate elaters coiled around the body (Tryon & Lugardon, 1991). Archangelsky, S. 1964. Notas sobre la Flora Fósil de la In Argentina, two extant species are known Zona de Ticó, provincia de Santa Cruz. to occur growing crowded in moist or wet places: