New Pitus and Eristophyton-type woods from the Tournaisian of Queensland, Australia: taxonomic, biogeographic, and palaeoclimatic implications

The fossil record of arborescent lignophytes shows an increasing anatomical diversity during the Tournaisian (360-347 Mya), suggesting a morpho-anatomical diversification following the extinction of the progymnosperm Archaeopteris Dawson, 1871 at the Devonian-Carboniferous boundary. This view has been partly constructed on recent investigations of Early Carboniferous deposits in eastern Australia. In this paper, we describe new silicified wood remains from the Tournaisian of the Burdekin Basin, northeastern Queensland, Australia, that are anatomically close to the genera Pitus Zalessky, 1911 and Eristophyton Gordon, 1935. While uncertain, the taxonomic affinities of these wood remains have several implications for future studies of Early Carboniferous arborescent lignophytes. First, the taxonomy of Eristophyton and Pitus has become increasingly problematic as reports of fossil wood assigned to these genera have extended beyond Laurussia. Second, Eastern Gondwanan and Laurussian floras display a convergent pattern of diversification from the Devonian, with the probable presence of a diversity of lignophyte trees in the Early Carboniferous. Third, convergence between Laurussia and Australia with respect to wood anatomy and discrete growth rings may be consistent with past suggestions of a monsoonal circulation during the Early Carboniferous.

Traditionally, the Early Carboniferous lignophyte trees were thought to have diversified only after the Late Tournaisian (353-347 Mya), with the extinction of Archaeopteris being followed by a period devoid of lignophyte trees (Behrensmeyer et al. 1992). Recently, however, a more transitional scenario has been proposed, based on the facts that: -lignophyte trees distinct from Archaeopteris are present and already diversified in the Middle Tournaisian (Decombeix et al. 2005(Decombeix et al. , 2006(Decombeix et al. , 2011a(Decombeix et al. , 2019; -some of these taxa might have been present during the latest Devonian (Matten et al. 1980;Prestianni et al. 2010); -most Early Carboniferous lignophyte trees are found in taphonomic conditions suggesting the colonization of welldrained "uplands", possibly disturbed by volcanism and/or fire (Scott & Galtier 1996;Falcon-Lang 2000b;Meyer-Berthaud et al. 2003;Prestianni et al. 2010;Decombeix et al. 2011a, b).
Permineralized plant remains from Early Carboniferous basins of Queensland, Australia, support this hypothesis. Arborescent lignophytes were probably already diversified in northeastern Gondwana during the Middle Tournaisian, as attested by the presence in this region of Dameria Decombeix et al., 2011and Protopitys Goeppert, 1850(Goeppert 1850, and at least one new taxon with a Pituslike wood (Decombeix et al. 2011b;Decombeix 2013). Interestingly, at least one of these genera (Protopitys) is also found in Laurussia at that time, which strengthens the idea of a pre-Middle Tournaisian origin for some of the arborescent lignophytes that diversified after the extinction of Archaeopteris.

MATERIAL AND METHODS
The four specimens described in this study are silicified decorticated trunk remains. They were collected in July 2008 at Mount Saint Michael, near Dotswood, in the Burderkin Basin, northeastern Queensland, Australia (19°37'S, 146°17'E). This locality was previously described by Hueber & Galtier (2002) and Decombeix et al. (2011b). It is located 50 km north of Charters Towers and about 60 km southwest of Townsville. The area includes Middle Devonian to Permian volcanic deposits (Mawson & Talent 1997;Talent et al. 2002;Decombeix et al. 2011b). The silicified specimens were found rafted in Cenozoid alluvial sediments, below Devonian-Carboniferous deposits dated from conodonts: Myrtlevale and Julia Formation for the Famennian, Hardwick Formation and Percy Creek Volcanics for the Tournaisian, and Carboniferous-Permian rhyolites (Mawson & Talent 1997). Although the stratigraphic origin of the specimens could not be directly assessed, the occurrence of similar fossil wood preserved in volcanoclastic sediments in the Tournaisian formations above the collection site indicates that the Tournaisian is the most plausible one (Hueber & Galtier 2002;Decombeix et al. 2011b).
The four specimens were prepared as classical thin sections in the transverse, tangential, and radial planes (Hass & Rowe 1999). Observation and photography were conducted using a Sony XCD-U100CR digital camera attached to an Olympus SZX12 stereomicroscope and an Olympus BX51 compound microscope. Images were captured using Archimed imaging software (Microvision Instruments, Evry, France). When necessary, the multifocus tool included in Archimed was used to provide a better image. Measurements were carried out using ImageJ software (U.S. National Institutes of Health, Bethesda MD, United States). Unless stated otherwise in the text, all average values and percentages are based on a minimum of 50 measurements. Images were processed using Photoshop (Adobe Systems, San Jose CA, United States). The specimens and corresponding slides are deposited in the AMAP Research Unit, Collections de Paléobotanique, Université de Montpellier, under accession numbers MSM08-14 (six slides), MSM08-20 (six slides), MSM08-171 (five slides), and MSM08-172 (eight slides).

DESCRIPTIONS
Two of the specimens, MSM08-171 and MSM08-172, have part of their pith and primary vascular system preserved. The other two, MSM08-14 and MSM08-20, are only composed of secondary xylem.

MSM08-171
This specimen is 3 cm long and 5.3 × 4 cm in diameter. Part of the central pith is preserved and its radius is estimated to be 1.1 cm, surrounded by 2.2 to 3.6 cm of wood (Fig. 1A, B). The pith shows variations in opacity but no conspicuous structures such as sclerotic nests or medullary strands ( Fig. 1A-D).

Primary xylem and lateral organs
Parenchyma cells in the pith are 105-174 µm with irregular shapes. Three types of cells can be distinguished: -dark and thick cells, interpreted as secretory cells; -brown cells with thick walls; -light cells with thin walls (e.g. Fig. 1C). In tangential section, pith cells are arranged in vertical to oblique files (Fig. 1D). Because only part of the stele's circumference is preserved only a few primary xylem strands have been observed. They range 90-115 µm in diameter (n = 5) (Fig. 1C, E). Primary xylem cells range 14-52 µm (n = 33) and the endarch vs mesarch maturation is not clear. The strands are typically not in contact with the secondary xylem but are separated by a few parenchyma cells and located 360-610 µm from the wood ( Fig. 1C: right arrow, E). One strand in contact with the secondary xylem ( Fig. 1C: left arrow) could represent a future departing trace to a lateral organ. The pithsecondary xylem outline is irregular, due to the presence of slightly dilated cells in the medullary rays ( Fig. 1A, C). Two departing vascular traces to lateral organs were observed, one on a transverse section that is 490 µm in diameter and one on a longitudinal section that is 325-360 µm.

Secondary xylem
The wood is composed of rays and tracheids. The later are polygonal to almost circular in transverse section (Fig. 1F). Their tangential diameter ranges 10-67 µm (average 35 µm) and the radial diameter 13-64 µm (average 41 µm). Rays separate 1-10 files of tracheids in transverse section. While there are some false rings, no true growth ring boundary was recognized.
In radial section, the wall of the tracheids bears bordered pits that are uniformly distributed in alternate rows (Fig. 1I). Most cells (85%) have two rows of pits, others have three. The pits are hexagonal to circular and 9-19 µm (average 14 µm) in diameter. Pit apertures are oval, oblique to horizontal. Ray cell length is 38-315 µm (average 152 µm). Very locally a few cross field pits are visible (Fig. 1J). They are araucarioid, oval oblique with oval apertures. Their maximum diameter is 10-15 µm (n = 8).

MSM08-172
This specimen is about 3 cm long and 6 × 6 cm wide. In transverse section it displays a 3 cm wide pith with conspicuous sclerotic nests, surrounded by a small amount of wood showing faint growth ring boundaries ( Fig. 2A, I).

Stele and leaf traces
The pith is composed of parenchyma cells that are 86 × 164 µm in diameter and contains 13 to 16 conspicuous sclerotic nests ( Fig. 2A-E). The later are composed of cells that have thicker Laloux B. & Decombeix A.-L. walls, are smaller, darker, and with a more circular outline than the parenchyma cells ( Fig. 2C-E). Only a few primary xylem strands have been observed in detail due to the poor preservation. Their diameter range 68-119 µm (n = 8) and they are composed of cells that are 9-32 µm (average 20 µm, n = 31) (Fig. 2F, G). Three of them have a mesarch maturation, while it is less clear in the others. Some are separated from the secondary xylem by a distance of 103-580 µm (e.g. Fig. 2F), while others are in contact with it (Fig. 2G). Medullary rays are less conspicuous than in the previous specimen and so the pith-wood boundary has a more regular outline ( Fig. 2A). Three vascular traces to lateral organs were observed in transverse section. Two of them are still very close to the stele (e.g. Fig. 2H). There is no primary xylem strand facing them, but one can be seen lying on the side close to the departing trace. The trace is 605 × 700 µm and surrounded by secondary xylem. A third trace can be seen within the secondary xylem (Fig. 2I), beyond a faint growth ring boundary.

Secondary xylem
In transverse section, the tracheids are more or less rectangular. Their tangential diameter is 17-95 µm (average 43 µm) and their radial diameter 24-58 µm (average 39 µm). Rays separate 2-25 files of tracheids (Fig. 2J). Two growth ring boundaries are visible; the rings are respectively 1.6 cm and 0.8 cm in thickness. Rays are one to four cells wide, with most of them (70%) being triseriate (Fig. 2J). Ray cells are 7-34 µm (average 17 µm) wide, 33-230 µm long (average 78 µm) and 28-59 µm high (average 43 µm). The radial wall of tracheids bears crowded alternate bordered pits that are hexagonal to circular and 9-19 µm (average 14 µm) in diameter (Fig. 2K). Pit apertures are elliptical to circular. About two thirds of the tracheids have two rows of pits, the others have three. A few cross-field pits were observed, they are about 10 µm wide and are oval and oblique.

MSM08-20
This specimen is 8 cm long and 1.1 × 2.3 cm in diameter.

CoMpariSon of the SpeCiMenS with previouSly deSCribed early CarboniferouS lignophyte treeS
We based our anatomical comparisons with previously described Early Carboniferous arborescent lignophytes on the taxonomically significant features reviewed by Galtier & Meyer-Berthaud (2006). We concluded that: -most of these morphospecies can be directly excluded from our study, except Eristophyton and Pitus; -specimens MSM08-14, 20 and 172 display Eristophytonlike wood, but each in an unusual combination; -the stele of specimen MSM08-172 has many typical characters of Eristophyton; -specimen MSM08-171 displays a Pitus-like pith with an original set of wood characters.
Specimens MSM08-14, MSM08-20 and MSM08-171 show a wood comprising small to medium-sized parenchymatous rays (up to 50 cells high and four cells wide) and narrow tracheids with multiseriate circular bordered pits.
All these wood characters are compatible with Eristophyton (Lacey 1953;Decombeix et al. 2007), but each specimen shows an unusual combination. Unfortunately, the primary vascular tissues are not preserved for specimens MSM08-14 and MSM08-20. Thus, the assignment of these specimens to Eristophyton is uncertain.
The stele of specimen MSM08-172 shows a parenchymatous pith with sclerotic nests, numerous mesarch primary xylem strands typically separated from the secondary xylem but coming in contact occasionally, and a small leaf trace undivided proximally. These features are found in Eristophyton beinertianum Scott, 1902and E. waltonii Galtier & Scott, 1990(Lacey 1953Decombeix et al. 2007). Integrating both stele and wood characters, specimen MSM08-172 could be tentatively diagnosed as Eristophyton sp.
Specimen MSM08-171 pith lacks intramedullary tracheids. However, it displays a pith with parenchymatous cells of three kinds, i.e. thin-walled ground parenchyma, secretory, and storage cells, revealing a strong similarity with Pitus (Gordon 1935). Interestingly, specimen MSM08-171 has a dense wood with straight and short rays (Zalessky 1911), which contrasts with large rays and multiseriate pits classically attributed to Pitus (Gordon 1935). Considered as a whole, specimen MSM08-171 could therefore be tentatively diagnosed as cf. Pitus.
taxonoMiC unCertaintieS Among the Early Carboniferous arborescent lignophytes, Pitus and Eristophyton are the two genera that combine the largest geographical distribution, stratigraphic distribution, and number of species (Decombeix et al. 2007(Decombeix et al. , 2011b. Several difficulties have appeared in the last thirty years as both genera were constructed based on remains from western and central Europe a century ago (Gordon 1935;Decombeix et al. 2007Decombeix et al. , 2008. In Laurussia, except Eristophyton feistii from Montagne noire, France (Decombeix et al. 2008), the latest described specimens attributed to Eristophyton could not be determined at the species level due to lack of preservation of the primary tissues (Galtier et al. 1998;Orlova 2010;Decombeix et al. 2017). In Pitus woods from Ireland and eastern Canada, taxonomic difficulties appear increasingly obvious when ontogenetic variability is considered (Falcon-Lang & Galtier 2010;Falcon-Lang et al. 2010;Henderson & Falcon-Lang 2011). In Australia, several Early Carboniferous woods have been assigned to Pitus primarily on the basis of multi seriate rays and multiseriate radial pitting on the tracheids but their primary vascular structures are unknown (Walkom 1928;Morris 1985). Recently investigated specimens with a Pituslike wood from Queensland revealed some differences with this genus when the bark or leaf trace emissions could be investigated in detail (Decombeix et al. 2007(Decombeix et al. , 2011b(Decombeix et al. , 2013(Decombeix et al. , 2019. In North Africa, only two Early Carboniferous arborescent lignophytes remains have been reported: -a Late Tournaisian axis from the Khenig Formation, Algeria, showing an original set of characters somewhat close to Eristophyton and Pitus but with a very distinct mode of leaf trace production and representing a new genus, Ahnetia (Decombeix & Galtier 2017); -an Early Serpukhovian wood from Tazekka, Morocco, close to Eristophyton but devoid of preserved primary vascular tissues (Chalot-Prat & Galtier 1989).
Besides, between-specimen comparisons from incomplete sets of characters impedes the assumption of similar habit, i.e., the Australian specimens could represent large trees but also smaller woody shrubs.
In the light of these studies and in order to avoid further confusion, we chose not to assign the new specimens described here to Pitus and Eristophyton. We strongly feel that revisions of Pitus and Eristophyton are needed before assigning any new fossils to these genera, especially those with no preserved primary tissues. Several taxa exhibiting ambiguous affinities with Eristophyton and Pitus, such as Aporoxylon, Cauloxylon, Paleoxylon, or Picnoxylon (Galtier & Meyer-Berthaud 2006), should also be included in these revisions.  -Berthaud et al. 2015-Berthaud et al. , 2021aEvreïnoff et al. 2017;Champreux et al. 2020) and Early Carboniferous localities (Sahni 1932;Hueber & Galtier 2002;Galtier et al. 2007;Decombeix et al. 2011aDecombeix et al. , 2019 suggest an apparent synchronism of diversification at the generic level with contemporaneous Laurussian floras. In contrast, fossil seeds from the Early Carboniferous of western Gondwana tentatively indicate a delayed diversification in that region (Prestianni et al. 2015). Morpho-anatomical connections between stem, foliage, and reproductive structures would allow undisputable identification of the taxa present and a better understanding of the degree of endemism between the different regions (Galtier & Meyer-Berthaud 2006;Decombeix et al. 2007).

iMpliCationS for paleoenvironMental and paleoCliMatiC reConStruCtionS
The presence of putatively large woody trees comparable to Pitus and Eristophyton in Eastern Gondwana does not support previous "megabiased" reconstructions (Behrensmeyer et al. 2000) toward rather uniform wetland landscapes, based on a fossil record dominated taxonomically by lycopsids and taphonomically by compressions-impressions (Gould 1975;Morris 1985;Pant 1996;Anderson et al. 1999;Evreïnoff et al. 2017). The discovery in the Tournaisian deposits of Australia of hydrasperman seed ferns and zygopterid tree ferns in the same region would rather indicate complex succession in volcanically disturbed habitats, as suggested from the study of different British sites (Galtier et al. 1993;Scott & Galtier 1996;Falcon-Lang 2000b;Phillips & Galtier 2005;DiMichele et al. 2006). Falcon-Lang (1999a, b) interpreted the presence of subtle growth rings in the lignophyte woods from the Viséan of the British Isles as indicating a global monsoonal circulation during the Early Carboniferous (Parrish 1990). This hypothesis is founded on two analogies regarding: -the phenology between the Carboniferous British taxa and extant Australian conifers (negative correlation between growth ring markedness and foliar persistence; Falcon-Lang 2000a); -an apparent symmetry between continental mass distribution during the Early Carboniferous and today (Falcon-Lang 1999b).
Because the important annual reversal of low and high pressure areas is influenced by the difference in heat capacity between oceans and the immense continental surface of Asia (Parrish 1993), it could be predicted from the surface of the supercontinent Gondwana that general air and oceanic circulation during the Early Carboniferous may have generated a similar monsoonal circulation (Kutzbach & Gallimore 1989;Parrish 1993;Falcon-Lang 1999b;Wang et al. 2005).
In our study, this hypothesis may be supported by: -the faint rings observed in our specimens, suggesting a tropical climate with alternating wet season and dry season (Creber & Chaloner 1984;Falcon-Lang 2005a, b); -the possible taxonomic affinities and/or convergent evolution with Laurussian woods.
We expect that the increasing wood fossil record from Gondwana, as well as further lithological, paleogeographical and paleontological evidence (Wang et al. 2005;Fielding et al. 2008;Fang et al. 2018), will enhance new opportunities to test the hypothesis of a global moonsonal climate regime in northeastern Gondwana during the Early Carboniferous.

CONCLUSION
In this paper, we described silicified fossil woods from the Tournaisian of the Burdekin Basin, northeastern Queensland, Australia, that are anatomically close to Eristophyton and Pitus. As part of recent investigations of Early Carboniferous deposits of this region, these descriptions are consistent with previous evidence for: -an increasing complexity of plant communities, including forests probably adapted to well-drained and/or volcanically disturbed uplands; -an apparently synchronous diversification of Eastern Gondwanan and Laurussian floras, in spite of a substantial geographic gap; -a rapid diversification of the arborescent lignophytes during the Tournaisian.
For future work, we suggest a focus on the following directions: -extending the geographic distribution of the fossil record of the arborescent lignophytes to other regions of Gondwana; -a revision of Pitus and Eristophyton, both defined in Laurussia, as well as potentially related morphotaxa; -testing the hypothesis of a monsoonal circulation in eastern Gondwana during the Early Carboniferous.