Fossil woods (Coniferales) from the Baqueró Group (Aptian), Santa Cruz Province, Argentina

Two new conifer morphospecies represented by a secondary xylem are reported for the Lower Cretaceous Baqueró Group (Santa Cruz Province, Argentina). Agathoxylon sp. is characterized by poorly defined growth rings, predominantly uniseriate pitting in the radial wall of the tracheids, cross fields with 1-6 pits and uniseriate rays. These features are shared with the leafy branches described for the unit as Araucaria grandifolia Feruglio emend. Del Fueyo and Archangelsky. The second taxon, Brachyoxylon sp. cf. B. boureaui , possesses poorly defined growth rings, mixed wood with predominantly uniseriate radial pitting, cross fields with 8-26 pits and uniseriate rays. These taxa represent the first fossil woods described for Baqueroan strata.


INTRODUCTION
The Baqueró flora represents one of the most exhaustively studied fossil communities of the Early Cretaceous of Gondwana.The research made since the early twenties of the twentieth century has resulted in an extensive list of taxa, as well as in several biostratigraphical and palaeoecologial studies (see Archangelsky (2003), Del Fueyo et al. (2007), and Limarino et al. (2012) for a detailed review).Most megafloral remains of the unit are preserved as impression fossils, whereas compressions of leaves and branches with preserved cuticles are abundant in some fossiliferous levels.
Fossil woods have been reported from Baqueroan sediments since the beginning of the field studies in the area (Archangelsky 1967).However, for the time being, no description has been provided in the literature for these remains.In this contribution, several fossil wood specimens collected from several fossiliferous localities are described in detail and referred to the conifer genera Agathoxylon Hartig 1848 sensu Philippe 1995 andBrachyoxylon Hollick andJeffrey 1909.The Baqueró Group occurs in the central region of the Santa Cruz Province (Argentina), in the Macizo de Deseado Basin.Deposits included in this Group are mostly continental, consisting of lacustrine and fluvial sediments (Cladera et al. 2002).Cladera et al. (2002) recognize three formational units in this group, named the Anfiteatro de Ticó, Bajo Tigre and Punta del Barco formations.Recently, Limarino et al. (2012) recognized within this Group three depositional sequences, and identified 13 fossiliferous stratigraphic levels.Radiometric ages obtained for these deposits revealed ages of 111.8 +/-7.40Ma and 118.56 +/-1.40Ma for the Anfiteatro de Ticó (Corbella, 2001(Corbella, , 2006) ) and 114.67 +/-0.18Ma for the Punta del Barco Formation (Césari et al. 2011), placing the deposition of the Baqueró Group during the late Aptian.
For the generic classification of the studied woods we followed the key for fossil morphogenera proposed by Philippe and Bamford (2008).The use of the form-genera Agathoxylon Hartig 1848 and Araucarioxylon kraus (in Schimper 1870) for fossil woods with the anatomy similar to the extant Araucariaceae is long established.However, several views regarding the legitimity of these genera have been published elsewhere.In particular, some authors concluded that these fossil woods should be included in Agathoxylon, leaving Araucarioxylon as a junior synonym of Pinites Witham 1833 (Philippe 1993, 1995, Bamford and Philippe 2001, Philippe and Bamford 2008).Although the original diagnosis of the genus Agathoxylon Hartig (Hartig 1848, Philippe andBamford 2008) includes the presence of axial parenchyma as a diagnostic character (a feature absent in several specimens referred to Agathoxylon), Philippe (1995) reformulated the original diagnosis proposed by Hartig (1848), allowing the inclusion of woods with and without axial parenchyma.We believe this taxonomic decision is adequate since a high number of genera have already been proposed, and erecting another one for including Agathoxylon-like woods without axial parenchyma would lead to a more confuse taxonomy.

Age: Early Cretaceous/Aptian
Description: This description is based on three specimens composed exclusively of secondary xylem.

Comparisons:
The fossil record of Agathoxylontype wood is extensive, with more than 60 species (and many referred as Agathoxylon sp.) listed by Philippe et al. (2004a) for Gondwana.As previous authors have expressed (e.g.Bamford and Philippe 2001, Poole and Mirzaie Ataabadi 2005), a comprehensive revision of the species referred to this genus is pending.Since such revision exceeds the objectives of the present work, no attempt was made to place the studied specimens in one of the currently recognized species.
Leafy araucarian branches have been previously reported for the Baqueró Group, first by Feruglio (1951) and later by Del Fueyo and Archangelsky (2002), and referred to Araucaria grandifolia Feruglio emend.Del Fueyo and Archangelsky.These specimens, collected in the volcaniclastic strata of the Punta del Barco Formation, have anatomical features similar to the ones present in Agathoxylon sp., such as the presence of uniseriate pitting in the radial walls of the tracheids, uniseriate rays and cross fields with 5-6 pits.Since both taxa were found in the Baqueró Group, it is possible that they may have been originated from a single biological entity.
Affinities: Among extant conifers, Agathoxylontype wood (characterized by the presence of araucarian tracheid pitting and araucarioid cross-field pits) is recorded exclusively in the Araucariaceae (Philippe et al. 2004a).However, Pteridospermales and Cheirolepidiaceae also possessed this type of wood (Philippe et al. 2004a).As pointed above, Agathoxylon sp. and Araucaria grandifolia share many anatomical features, and they may have been part of the same biological species (referable to the Araucariaceae).

Brachyoxylon Hollick and Jeffrey 1909
Type species: Brachyoxylon notabile Hollick and Jeffrey 1909 Protopodocarpoxylon Eckhold includes in its diagnosis the presence of podocarpoid cross fields.However, Protopodocarpoxylon blenvillense, the first species referred to this genus (and thus, it's Type Species), possesses araucarioid cross fields.If this species remained as the type species, this genus should be regarded as a junior synonym of Brachyoxylon (Bamford and Philippe 2001).Recently, Philippe et al. (2002) have proposed a nomina conservanda for the genus Protopodocarpoxylon using as neotype the species Protopodocarpoxylon bedfordense Stopes, which has podocarpoid cross fields.This proposal, accepted by the Committee for Fossil Plants (Skog 2003), is here followed, and the studied specimens are referred to (Brachyoxylon Hollick and Jeffrey 1909).In transverse section, growth rings are indistinct, sometimes with a narrow zone (1-3 cells in width) of late wood (Fig. 3A, B).Early wood tracheids are 33 (17-64) µm in diameter, with thick (4.7 (2.8-6.6)µm) walls.Approximately 6 (2-12) series of tracheids are present between the parenchymatic rays.

Comparisons:
The studied specimens are comparable with Brachyoxylon boureaui Serra 1966 identified from the Late Jurassic Phu Kradung Formation in Thailand and coeval units in Cambodia (Serra 1966, Philippe et al. 2004b).This species is characterized by a high (up to 37) number of pits per cross fields, mostly uniseriate radial pitting of the tracheids, and low rays, features shared with the Baqueroan specimens.Based on spatial and temporal differences, we prefer to refer our specimens to Brachyoxylon boureaui with reserves.
Affinities: Brachyoxylon type of wood has been found in association with conifer leaves and Classopollis-bearing cones (zhou 1983), suggesting that this xylotype is closely related to the Cheirolepidiaceae, a group of plants already identified in Baqueroan strata (e.g.Archangelsky 2003).

CONCLUSIONS
In this work, the number of taxa reported for the Baqueró Groups is expanded with the description of Brachyoxylon sp.cf.B. boureaui and Agathoxylon sp..They represent the first fossil woods described EzEQUIEL I. VERA and SILVIA N. CéSARI for the Baqueroan deposits, although the presence of wood had been reported previously in the literature.Agathoxylon sp., characterized by the presence of predominantly uniseriate pitting on the radial walls of the tracheids and one to six pits per cross field, could be adscribed to Araucaria grandifolia Feruglio emend.Del Fueyo and Archangelsky, previously reported as impressions/ compressions of leafy branches.On the other hand, Brachyoxylon sp.cf.B. boureaui represents one of the scarce records of this genus in South America (Philippe et al. 2004a, Gnaedinger et al. 2009) and provides new information about the presence of this xylotype in the Mesozoic of Western Gondwana.

ACKNOWLEDGMENTS
The authors wish to express their gratitude to Drs.Oscar Limarino, Magdalena Llorens, Mauro Passalía and Valeria Perez Loinaze for their invaluable help in the field.Dr. Marc Philippe kindly provided helpful information regarding taxonomic aspects of fossil woods.Dr. Douglas Riff kindly translated the abstract to Portuguese.The comments made by three anonymous reviewers improved the quality of the paper.Thanks are extended to Lic.Fabián Tricárico, who assisted with SEM photographs at the MACN.This work is a contribution to the PICT 32320 and PIP 512.