Original articleGinkgoalean woods from the Jurassic of Argentina: Taxonomic considerations and palaeogeographical distribution☆
Introduction
Fossil studies, based mainly upon leaves but also upon pollen and reproductive structures, have demonstrated that the existence of the Ginkgoales has extended from the Permian to the present (Scott et al., 1962, Kvacek et al., 2005, Zhou and Wu, 2006, Zhou, 2009). Although leaf remains are abundant, studies based upon identification of Ginkgo-like wood anatomy are rare. This may in part reflect the fact that, in contrast to the greater chemical resistance of the wood of many other gymnosperms, Ginkgo wood is particularly susceptible to cell wall degradation (Scott et al., 1962, Taylor et al., 2009).
This paper is based upon a study of the anatomy of permineralized Ginkgoalean wood fragments collected from sediments of the Roca Blanca Formation (Early Jurassic) and La Matilde Formation (Middle Jurassic), both located in Santa Cruz province, Argentina. Petrified woods previously reported from the Roca Blanca Formation include Taxales (Prototaxoxylon Kräusel and Dolianiti); Coniferales, Araucariaceae (Agathoxylon Hartig), and Ginkgoales (Baieroxylon sp. cf. Baieroxylon chilensis), while those identified from the La Matilde Formation include Taxales (Prototaxoxylon Kräusel and Dolianiti), Araucariaceae (Agathoxylon Hartig), cf. Cupressaceae (Protelicoxylon Philippe and Herbstiloxylon Gnaedinger), Protopinaceae group (Planoxylon Stopes), and Podocarpaceae (Circoporopitys Gnaedinger, Podocarpoxylon Gothan, and Circoporoxylon Kräusel) (Gnaedinger, 2001, Gnaedinger and Herbst, 2006, Gnaedinger and Herbst, 2009, Gnaedinger, 2007a, Gnaedinger, 2007b).
Another purpose of this paper is to report on analyses of the Ginkgoalean woods described for Gondwana and Laurasia, in terms of morphogenera and species identifications, and to combine diagnostic anatomical evidence with the stratigraphic and paleogeographical distributions of recorded taxa. As a result of this, a genus-level identification key is presented, as well as a summary of global species distributions during the Paleozoic-Mesozoic, as well as from the Cenozoic to the present.
Section snippets
Material and methods
The specimens described in this paper were collected in the Santa Cruz province, Argentina, from two formations: the Roca Blanca Formation and the La Matilde Formation (Fig. 1). The Roca Blanca Formation is present as outcrops in the central area of Santa Cruz province (approximately 48° S–68° W), and is made up of a series of sediments that are mainly sandstones, tuffaceous sandstones, and tuffs. The logs were found distributed in the upper levels of profile C, as described by Herbst (1965)
Systematic palaeobotany
Order GINKGOALES Engler, 1897
Genus Baieroxylon Greguss, 1961
Type species: Baieroxylon implexum Greguss, 1961
Baieroxylon rocablanquense nov. sp.
Fig. 2, Fig. 3
Derivatio nominis: The name of the species refers to the source formation.
Holotype: PMP-PB 2449, CTES-PB 14050 (CTES-PMP 2985-2987).
Paratype: PMP-PB 2450, CTES-PB 14051 (CTES-PMP 2988-2990).
Other material studied: PMP-PB 2452, CTES-PB 14052 (CTES-PMP 3047-3049).
Locality: Estancia Los Pirineos, Santa Cruz province, Argentina.
Stratigraphic
Quantitative analysis of the woods
Table 1 summarizes the quantitative data related to the anatomical characteristics of the analyzed wood fragments, as obtained from various specimens identified as B. rocablanquense nov. sp. and G. tanzanii. Based upon this, it was determined that the specimens from the Roca Blanca Formation represent a new species of Baieroxylon, since uniseriate pits are only present on the radial walls of the tracheids and with a typical range of 4-6 pits in the cross fields. These characteristics
Taxonomic considerations
Ginkgo biloba L. is the single extant species belonging to the Order Ginkgoales, with its native distribution today restricted to southeastern China. The pith of this species is irregularly “star-like” in form, and is made up of parenchyma cells, secretory cells, and smaller secretory canals. Primary xylem is endarch. In radial sections, the tracheids of the primary xylem show scalariform thickenings, with no bordered pits (Zhang et al., 2000). Giraud and Hankel (1986) have also noted that the
Stratigraphic and palaeogeographical distributions
Zhou (2009) and Süss et al. (2009) have discussed the paleogeographical distributions of woods from morphogenera with affinities to the Ginkgoales. This topic is addressed here in Table S1, which combines data regarding the set of diagnostic anatomical characters of Ginkgoalean woods with their known stratigraphic and paleogeographical distributions.
In Table 3, paleogeographic distribution data for the wood genera (araucarioid or mixed type and abietinoid type) are first presented along with
Conclusions
Table S1, Table 3 and Fig. 7 show the distribution of Ginkgoalean woods on a global scale, over the course of geological history. Through the analysis of anatomical, stratigraphic, and paleogeographic information related to the fossil Ginkgoales identified until now, the following inferences can be made:
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From the Permian to the Cretaceous, these woods have an araucarioid or mixed-type secondary xylem;
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From the Late Cretaceous to the Tertiary they show the abietinoid-type of secondary structure
Acknowledgements
The author is grateful to reviewers, Dr. Howard Falcon-Lang and Dr. Ronny Rössler, and Associate-Editor Dr. Marc Philippe for their constructive comments on the manuscript. Eva B. Acevedo is thanked, for the preparation of sections and peels, as well as Pedro Medina for his careful work in wood polishing. Financial support was partially provided by funding from ANCyT FONCyT PICT 2005, n° 33497, and PIP-AMZ-CONICET.
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Corresponding editor: Marc Philippe.