New records of Late Triassic wood from Argentina and their biostratigraphic, paleoclimatic, and paleoecological implications

We report gymnospermous wood found in sandstone and siltstone beds of the Upper Triassic Hilario Formation, Sorocayense Group at Hilario Creek located in San Juan province, Argentina. The identified xylotaphoflora comprises Baieroxylon cicatricum (Ginkgoales) and a new species of Protophyllocladoxylon (Coniferales), it constitutes the first reports of these taxa from the Triassic in Argentina. Protophyllocladoxylon hilarioense sp. nov. differs from the other species by the following combination of anatomical characters: radial pits araucarian, mixed and some with abietinian tendency, uni-biseriate; contiguous, separated; tangential pits uni-biseriate; cross-field pits are simple elliptic, oblique, one to two in number and low uni-biseriate rays. The growth rings in the reported woods show a gradual transition from earlywood to latewood, suggesting little change in the climatic conditions experienced during their growth. The type of growth rings observed is consistent with a humid but seasonally dry subtropical climate. These woods are representatives of the arboreal stratum of a mesophytic association.

The stratigraphic framework of the Sorocayense Group in the Barreal (southern) area is overlapping on the discordance of Paleozoic sediments and Devonian diabases, and Hilario on "schists", graywacke and diabases of the Lower Paleozoic (Stipanicic 1972; Barredo et al. 2016).
The Triassic sequence in the Hilario depocenter (northern) of the Sorocayense Group begins with the Agua de los Pajaritos Formation, which represents the onset of the basin filling with fluvial-sandy elements and abundant pyroclastic input followed by levels of bituminous pellets from the Monina Formation (Baraldo and Guerstein 1984).
The filling is completed by the El Alcázar Formation characterized by clastic and pyroclastic sediments of fluvial and lacustrine environments alternating with tuff deposits (Baraldo and Guerstein 1984;Baraldo et al. 1990; Barredo et al. 2016; Table 1).
In the Barreal (southern) depocenter, the Hilario Formation correlates with the Don Raúl member of the Cortaderita Formation, which corresponds to an anastomosed fluvial system and flood plains where temporal ponds or lakes developed ).  Barredo et al. 2016). Pozzo (1948) Groeber and Stipanicic (1953) Stipanicic (1972) Baraldo and Guerstein (1984) Barredo et al. (2016) Hilario The Hilario Formation consists of sandstone-pelite deposits, and intercalations of tuffs, corresponding to a lacustrine environment that passes into a fluvial environment with flood plains and a palustrine environment.
The Hilario Formation was first described as a part of the Hilario Triassic Series (Pozzo 1948;Groeber and Stipanicic 1953) and later, within the Hilario Group, as an informal denomination of Stipanicic (1969). According to the nomenclature of the "Comité Argentino de Estratigrafía (1992)" (Argentine Committee of Stratigraphy), the denomination of "Hilario Triassic Series" and "Hilario Series" were not valid due to the use of the same toponym for two entities of different hierarchies such as Formation and Group (Stipanicic 2002; Table 1).
However, the detailed descriptions of fossils from the Hilario Formation are only those of the sphenopsids Neocalamites carrerei, Neocalamites sp. and the conchostracans Estheria sp. (Groeber and Stipanicic 1953).

Material and methods
The studied material consists of silicified, decorticated specimens, with good preservation of cellular elements, which are deposited in the Colecciones Paleontológicas de la Universidad Nacional del Nordeste "Dr Rafael Herbst", Sección Paleobotánica de Corrientes (CTES-PB). They were collected in various expeditions in the decades 1980-1990 by Rafael Herbst, Alicia Lutz, Susana Morton, Oscar Gallego and collaborators in Hilario Creek (Fig. 1). This site is located 122 m above sea level, and its coordinates are 31°20'S and 69°15'W.
The specimens were processed according to the standard methodology by making cuts in the transverse, radial longitudinal, and tangential longitudinal sections, and polishing with abrasives of different granulometry up to a thickness of 40 μm (Hass and Rowe 1999). The thin sections were studied in detail with a Leica microscope (DM500), the photomicrographs were taken with a digital camera (Leica ICC50). The scanning electron microscope (SEM Jeol 5800LV) from the Universidad Nacional del Nordeste, Corrientes, Argentina, has been used in pertinent cases. The measurements of the different anatomic elements were obtained after with a minimum of 30 measurements. The mean and, between parent, the minimum and maximum values were recorded in the descriptions.
The terminology used corresponds to the list of microscopic characters for the wood identification of the International Association of Wood Anatomists (Richter et al. 2004) and García Esteban et al. (2002. Systems of nomenclature and nomenclatural reviews are those of Philippe and Bamford (2008) and Zijlstra and Phillipe (2020).
In longitudinal tangential section, the rays are homocellular and uniseriate. The height of the rays is 1-12 cells; the average height is medium (6 cells). The average diameter of the cells is 23 µm high by 22 µm wide ( Fig. 2A 9 ). The density of rays is 23 per mm 2 . In this section, it is possible to visualize a rameal trace that has preserved a homogeneous medulla composed of parenchymatic cells with an oval to circular contour surrounded by the rays. Their average diameter is 13 µm (7-23 µm) ( Fig. 2A 10 ).
Remarks.-The anatomical characters of the wood branch under description allow to assign it to Baieroxylon, which is characterized by the presence on the radial walls of the tracheids of uni-biseriate pits, often araucarian, flattened, mixed type pitting with helical thickenings; and rays 1-15 cells high, mostly uniseriate (Greguss 1961;Philippe and Bamford 2008). Based on the radial pitting, Baieroxylon unites a group of heterogeneous woods; some species show an araucarioid type and others a mixed type (Gnaedinger 2012).
Baieroxylon has a wide stratigraphic distribution. It is present in sediments from the Permian to the Cretaceous (Crisafulli 2001;Gnaedinger 2012), and includes seven species: Baieroxylon multiseriale Prasad, 1982, from Kamthi   According to the comparison made between the species of Baieroxylon (SOM: table 1, Supplementary Online Material available at http://app.pan.pl/SOM/app67-Vallejos-Leiz_etal_SOM.pdf), the studied specimen of the Hilario Formation fits the diagnosis of B. cicatricum due to a similarity in types, size, shape, and seriation of radial pits, crossfields, and uniseriate woody rays in the secondary wood and numerous "eye-shaped scars".
The analyzed species is similar to B. graminovillae and B. cambodiense, due to similarities in radial pitting of tracheids and cross-fields (pits grouped in a cluster). However, B. graminovillae has tracheids with some triseriate pits and B. cambodiense has crassulae in the pits and a greater number of them in the cross-fields and also the presence of biseriate rays.
Our material supports the assignment of Baieroxylon to the woods of Ginkgoales due to the following anatomical characters: intercellular spaces between tracheids pointed ends in recurvate tracheids, simple or cupressoid pits in cross-fields, low rays and the variable size of the secondary wood tracheids (compare Greguss 1955;Prasad and Lele 1984;Crisafulli 2001;Leiva Verón et al. 2012;Gnaedinger 2012). It is also worth mentioning that Prasad and Lele (1984) found impressions of Baiera digitata (Brongniart 1828 Diagnosis.-Secondary pycnoxylic wood with growth rings slightly marked. Radial tracheid pitting of the earlywood uni-biseriate. Radial tracheid pitting of the latewood uniseriate. Uniseriate pits are circular, contiguous, flattened, and spaced with a tendency to abietinoid type. Biseriate pits circular, opposite and alternate, contiguous, and flattened. Cross-fields with 1-2 simple oblique to horizontal phyllocladoid oopores. Very low-medium uniseriate rays and some uniseriate rays with a short biseriate portion, 1-8 cells high. Tangential walls of tracheids with contiguous, circular and uniseriate pits. Description.-Decorticated, silicified, dark brown wood fragments. CTES-PB 11409 (Fig. 5A 1 ) has been selected as the holotype, and CTES-PB 11406, as the paratype. The holotype measures 3.5 cm in length, 5.5 cm in the largest diameter and 3.5 cm in the smallest diameter.
In transverse section, the growth rings are slightly marked (Fig. 5A 2 , A 3 ). The tracheids are circular, oval to quadrangular, and with wide lumens. The average tangential is 33 µm, and the radial diameter is 34 µm. The thickness of the tracheal wall is 5 µm. It presents "shearing zones" sensu Erasmus (1976). It shows false growth rings.
In longitudinal tangential section, the radial system is homogeneous. Some rays are uniseriate and some are uniseriate with a short biseriate portion. The rays are very low and medium; the average height is 4 (1-8) cells. Cells average is 26 μm high × 23 μm wide. The density of rays is 22 per mm 2 . The cells of the rays are oval in the central portion and triangular to ellipsoidal at the ends. The tangential walls of the tracheids have contiguous, circular, uniseriate areolate pits (Fig. 5A 11 , A 12 ).
Remarks.-The specimens present araucarioid, abietinoid and mixed pitting on the radial walls of the tracheids and simple phyllocladoid pits in the cross-fields, consistent with the diagnosis of the genus Protophyllocladoxylon (Kraüsel, 1939) Mussa, 1958. Vozenin-Serra (1970 classified Protophyllocladoxylon species according to four characters: types of radial pitting, various aspects of pits in cross-fields, pres- ence or absence of axial parenchyma and seriation of rays. Pant and Sing (1987) include it in the group of araucarioid woods for the Paleozoic and Bamford and Philippe (2001) in the mixed type for Mesozoic woods. Gnaedinger (2007) and Philippe and Bamford (2008) according to the described species show that Protophyllocladoxylon is characterized by species with an araucarioid woody plan and others with a mixed woody plant. Therefore, the key of the latter authors includes Protophyllocladoxylon in Group B (araucarian or xenoxylean radial pitting) and in Group D (with mixed type radial pitting). The described specimens of the Hilario Formation correspond to the mixed type or group D. Wan et al. (2019) point out that Protophyllocladoxylon species can have three types of pits in the cross-fields: phyllo cladoid type (Philippe and Bamford 2008), window-type (Richter et al. 2004) and others with small to medium-sized oval/circular pits, being all simple without borders. Some species have two types such as Protophyllocladoxylon owensii Fletcher, Cantrill, Moss, and Salisbury, 2014, showing phyllocladoid oopores and less circular pits. In the Hilario Formation specimens, the cross fields have only the phyllocladoid type of pits.
For the specific determination of the wood, the comparisons were established with species from the Triassic (SOM: table 2). It is worth mentioning that this specimen has similarities with P. korubaense Serra, 1966b, andwith P. xenoxyloides Serra, 1966b, both from Cambodia, regarding the pitting of the radial walls of the tracheids, and the cross-field pits. However, it differs from P. korubaense because the specimen from Argentina does not have axial parenchyma, nor tyloses, and presents flattened pits, a greater number of tangential pits and the pits of the cross-fields with elliptical oopores. Similarly, P. xenoxyloides is distinguished from the Hilario Formation specimen by presenting axial parenchyma, tyloses, spaced pits and the percentage of flattened pits is different from the specimens analyzed here. However, Boura et al. (2013) synonymise the species P. korubaense and P. thylloides (from Upper Triassic of Vietnam) with P. xeno xyloides Serra, 1966a (Triassic-Jurassic) based on a reassessment of quantitative anatomical parameters and the presence of tyloses.
According to the indicated differences with the two closest species shown in SOM: table 2, it is proposed to assign  This new taxon is added to others from South Ameri ca such as Protophyllocladoxylon dolianitii Mussa, 1958, Protophyllocladoxylon derby (Olivera, 1936 Maheshwari, 1972, and Pro to phyllo cladoxylon rosablancaense Pons, 1971. Pro to phyllo cladoxylon derbyi comes from the upper Carboni ferous of Brazil. Protophyllocladoxylon dolianitii is recorded in the Permian formations of Brazil, Paraguay, Antarctica, and Australia (Mussa 1958;Crisafulli and Herbst 2009;Mahe shwari 1972;Wan et al. 2020, respectively) and P. rosablancaense was found in Rosa Blanca Formation (Middle Valley of Rio Magdalena, Colombia) in Cretaceous sediments (Pons 1971). The SOM: table 3 compares the anatomical characters of these three taxa with the Hilario specimen. Protophyllocladoxylon rosa blancaense is the one that shows the most similarity with the material studied herein; although P. rosablancaense does not show spaced radial pitting or tangential pitting and the height of the rays is high.
Following the criteria of Gnaedinger (2007), Zhang et al. (2010), and Pujana et al. (2014Pujana et al. ( , 2015, Protophyllocladoxylon is most likely related to the basal forms of Podocarpaceae, due to the presence of cross-field pits of phyllocladoid-type, characteristic of some current members of Pinaceae and Podocarpaceae (Ritcher et al. 2004) and for presenting araucarian and mixed pitting on the radial walls of the tracheids. Likewise, in the Triassic, this type of woody axis was found close to Rissikia sp. (Podocarpaceae) leaves with female (Rissikistrobus sp.) and male (Rissikianthus sp.) cones (Townrow 1967;Anderson and Anderson 2003;Gnaedinger and Herbst 2008;Gnaedinger 2010;Holmes and Anderson 2013;Gnaedinger and Zavattieri 2017).  (Crisafulli 2001), and the Ecca Group in Africa (Crisafulli and Herbst 2010). It has also been found in the Upper Triassic of the Tiki Formation (Prasad and Lele 1984) and in the Cretaceous of the Gan gapur Formation (Muralidhar- Rao and Ramanujan 1986), both in India, and in Rio Grande do Sul, Brazil (Bardola et al. 2009). Here we present the first record from the Triassic of Argentina. The presence of Ginkgoales in the Sorocayense Group is furthermore corroborated by the record of leaf impressions.
Ginkgophyta had its acme in the Mesozoic times. Then, the group declined during the Cenozoic, with only Ginkgo biloba persisting to the present times (Taylor et al. 2009).
According to the considerations made by Gnaedinger (2012), Ginkgoales fossils present secondary xylem of the araucarian or mixed type from the Permian to the Cretaceous and an abietinian type from the Late Cretaceous to the Tertiary (similar to the living species of Ginkgo biloba). Moreover, the stratigraphic and paleogeographic distributions of these woods in terms of anatomical changes (from mixed type to abietinian type) are concurrent with the morphological changes in leaves (lamina and petiole differentiation) and reproductive structures (reduction of numbers of seeds and increase in their size) reported in Zhou and Wu (2006) and Zhou (2009).
This group is recorded throughout Gondwana, presenting a large number of endemic forms in the southwest. Meanwhile, the greatest biodiversity is recorded in the high paleolatitudes (around 60° south paleolatitude), in the Karoo Basin (South Africa) and El Tranquilo Group (Patagonia, Argentina).
Protophyllocladoxylon is a cosmopolitan taxon. Its oldest record is from the Carboniferous. Protophyllocladoxylon hilarioense sp. nov. is the thirty-sixth species of the genus and its first record from the Triassic of Argentina.
Paleoclimatic and paleoecological implications.-Baieroxylon and Protophyllocladoxylon species occur in various paleoclimates, both in warm and humid and temperate to cold conditions (Crisafulli and Herbst 2009;Zhang et al. 2010;Gnaedinger 2012).
The presence of variations in the growth rings of the analyzed woods may indicate slight seasonal changes characterized by the alternation of dry and humid periods in response to variables such as rainfall, temperature and light availability (Creber and Chaloner 1984;Taylor and Ryberg 2007;Pires and Guerra Sommer 2011;Yang et al. 2013).
The rings analyzed correspond to type "E" of the classification proposed by Creber and Chaloner (seen in Brison et al. 2001). It means that these plants went through relatively uniform growing seasons but each one with a terminal event represents a cessation or delay of cambium activity.
The presence of narrow latewoods, such as in these specimens (composed of 2-5 radially compressed cells), probably reflects a rapid onset of unfavourable growth conditions and/or the result of a water deficit during the summer. However, it is not ruled out that it could be associated with the genetic makeup load of the species (Creber and Chaloner 1984;Brea et al. 2005).
A value of 0.3 of mean sensitivity (SM) was obtained for Baieroxylon cicatricum, which corresponds to a complacent wood, equivalent to little marked climatic changes (Brea et al. 2005;Pires and Guerra Sommer 2011).
The presence of false growth rings observed in these woods could be attributed to a cold season, or also by drought, or by defoliation caused by insects (Jefferson 1982;Zamuner 1986). Another anatomical feature with paleoenvironmental significance is the value of the flattening coefficients of the pits. The numbers obtained here indicate openings of considerable size and tracheids with a wide lumen, which denotes that they grew in subtropical conditions with little marked seasons (Creber 1977).
According to these data, it can be inferred that these trees grew in a humid but seasonally dry subtropical climate, in an environment that corresponds to a fluvial system with flood plains and paludal environments. This is suggested by the sandy-limolithic sediments of the Hilario Formation inferred by Baraldo and Guerstein (1984) and Barredo et al. (2016) that are the hosts of the woods of this xylotaphoflora.

Conclusions
The first wood species found in the Hilario Formation are described and assigned to Baieroxylon cicatricum (Gink goales) and Protophyllocladoxylon hilarioense sp. nov. (Coni fera les: Podocarpaceae). The new specie contributes to the diversity and confirms the presence of the Podo carpaceae.
These taxa are added to the impressions of Neocalamites carrerei and Neocalamites sp. found in this Upper Triassic formation of San Juan province, described by Groeber and Stipanicic (1953). The wood of Baieroxylon cicatricum found in the Hilario Formation constitutes its first occurrence in the Triassic of Argentina, thus adding a new Gondwanan occurrence to those already known from Brazil and India. The species has wide both stratigraphic and geographic ranges occurring from the Permian in sediments of Paraguay, Uruguay, and Namibia to the Cretaceous of India.
From the paleoenvironmental point of view, the type of growth rings observed is consistent with a humid but seasonally dry subtropical climate. False growth rings were probably formed due to drought during the growing season, and/or arthropod damage during the life of these trees. These woods are probably representatives of the arboreal stratum of a mesophytic association where the Sphenophytes found would represent the understory. The described taxa extend the paleobotanical spectrum of the Sorocayense Group, composed of Corystospermales, Gingkoales and Coniferales (Podocarpaceae, Cupressaceae).