First record of Podocarpoid fossil wood in South China

A new species of fossil conifer wood, Podocarpoxylon donghuaiense sp. nov., is described from the late Eocene of Nadu Formation in Baise Basin of the Guangxi Province, South China. This fossil wood is characterized by distinct growth rings, circular to oval tracheids in cross section, 1–2-seriate opposite pits on radial tracheid walls, uniseriate (rarely biseriate) rays, smooth end walls of ray parenchyma cells, and the absence of resin ducts, suggesting its affinity to Podocarpaceae. The new species is distinctive from other Cenozoic woods ascribed to this family by the combination of distinctive growth rings, the absence of axial parenchyma, the occurrence of bordered pits on tangential tracheid walls, and the occurrence of 3–4 cuppressoid or taxodioid pits on cross-fields. This represents the first record of podocarpoid fossil wood in South China and provides fossil evidence for the early dispersal and diversification of Podocarpaceae in eastern Asia as well as for mild temperate seasonal climate in this region during the late Eocene.

As the second largest family within the conifers, the modern Podocarpaceae largely comprises evergreen trees and shrubs belonging to 194 species within 19 genera 1 . This family is mainly distributed in tropical and subtropical regions from central to South America, Africa (include Madagascar), Indochina through Malesia to Australia and Oceania 1,2 , extending as far north as China and Japan as well as to Mexico and the Caribbean [3][4][5] . Podocarpaceae are most abundant in montane tropical rainforests, but occasionally occur in lowland forests 6 . In comparison with extensive investigations on extant phylogenetics and geography of the family, its early evolution and migration in deep time are still poorly known, mainly due to the lack of megafossils, especially of Cenozoic age.
Fossil records of podocarps from the Mesozoic were well documented in both the Northern and Southern hemispheres. Molecular and fossil evidence suggests that the podocarps originated during Triassic-Jurassic time in Gondwana and apparently spread to the Northern Hemisphere during the Jurassic 7 . Although the family appears to be of ancient origin, molecular dating analysis suggests that the majority of extant genera have arisen relatively recently in the Upper Cretaceous to Cenozoic 8 .
Although modern Podocarpaceae are widespread mostly in the Southern Hemisphere, there are numerous reports of Cenozoic megafossils and palynomorphs attributable to this family from the Northern Hemisphere. Podocarpus shoots were found in Eocene deposits of Tennessee 9 , whereas pollen seemingly belonging to this genus is known from the Miocene of Oregon and Idaho 10 and late Eocene of Colorado 11 . In Europe, leaves of Prumnopitys have been reported from the Eocene of England 12 , while leaf fossils assigned to Podocarpus have been found in Eocene to Oligocene deposits of Ukraine 13,14 and of the southeastern regions of European Russia 15 . A phylloclade of Phyllocladus has also been described by Krasnov 13 from Eocene deposits of the Kharkov region (Ukraine). The assignment of these fossils to Podocarpus and Phyllocladus need special examination, however. In particular, several records of Podocarpus-like leaves from Eocene to Miocene localities of central and eastern Europe ascribed to the species Podocarpus eocenica Ung. have been reconsidered by Ferguson et al. 16 as members of Amentotaxus, a genus of the Taxaceae. In addition, podocarpoid fossil wood has been reported from the Oligocene-Miocene of southern Ural 17 . Podocarpaceae palynomorphs occur widely in Europe from the Paleocene to the Miocene 18-20 , whereas the records of Podocarpus pollen from the Pliocene of Iceland 21 and Pleistocene of Belgium 21,22 possibly result from redeposition from older beds 19 .

Discussion
Comparison with modern materials. Within the conifer families, the fossil wood from Donghuai may not be placed into Pinaceae, Cupressaceae or Cephalotaxaceae, as it has no resin ducts, axial parenchyma and ray tracheids. This late Eocene wood is also distinctive both from Araucariaceae by the absence of two or more seriate alternate intertracheary pitting and by the lacking crowded araucarioid cross-field pits, and from Sciadopityaceae by the absence of window-like cross-field pits. Therefore, it remains for us to consider the relationships of this fossil wood to Taxaceae or Podocarpaceae [31][32][33][34] .
The presence of spiral and branched thickenings on the walls of latewood tracheids in combination with smooth horizontal and tangential walls of ray cells suggests the fossil wood from Donghuai may be considered as a member of Taxaceae [31][32][33][34] . After careful examination of the thickenings on tracheid walls in this sample we suggest, however, that these structures are not of the same nature as the spiral tertiary thickenings that occur in all modern genera of this family, with the exception of Austrotaxus R.H. Compton 31,34 . Extant Taxaceae show finer spiral thickenings, more widely spaced and tilted at lower angles (usually not exceeding 45°) in respect to the tracheid axes than the thickenings observed in the late Eocene wood from Donghuai. Such features of the fossil wood as tracheids bearing spiral thickenings confined only to the latewood and their absence in the earlywood has also not been reported in any modern Taxaceae [31][32][33][34] . Therefore, the spiral and branched structures occurring in tracheids of the Donghuai wood are more likely artifacts (probably, effects of wood compression) than true tertiary thickenings. The occurrences of similar artifacts in fossil woods seemingly belonging to Taxaceae have been surveyed by Afonin & Philippe 35 . In fossil woods assigned to Podocarpaceae these structures have been reported by Chudajberdyev 17 in tracheids of Podocarpoxylon uralense Chudajb.
As far the thickenings on tracheid walls are recognized as artifacts rather than diagnostic traits, the suite of other characters (smooth horizontal and tangential walls of ray cells without indentures, cupressoid and taxodioid pits on cross fields) indicates that the fossil wood reported here has greatest affinity to the Podocarpaceae 31-34 . This family, however, shows considerable diversity of wood structure that does not allow distinction between its genera using wood anatomical traits. The late Eocene wood from Donghuai is different from most modern Podocarpaceae in growth rings with conspicuous latewood, lacking axial parenchyma and the occurrence of 1-4 pits on the cross-field. However, the presence of prominent latewood portion has been reported in growth rings of Therefore, the fossil wood from Donghuai shows most resemblance to some species of Prumnopytis (especially P. taxifolia) as well as to some members of Podocarpus, Dacrydium and Microcachrys, but it cannot be convincingly placed in any extant genus of Podocarpaceae on the basis of its anatomical traits.
Comparison with fossil materials. The fossil wood described here is characterized by an absence of axial parenchyma and by the cross-fields with cupressoid and/or taxodioid pits. Within fossil woods ascribed to Podocarpaceae, these traits are found in some species of the genus Podocarpoxylon Gothan 31 Patton), cross-fields with more than two pits occur only in P. aparenchymatosum and P. gnaedingerae. Podocarpoxylon aparenchymatosum from the Eocene deposits of Antarctica 44,45 differs, however, from the Donghuai wood in possessing 3-seriate pitting on radial tracheid walls. Unlike Prumnopityoxylon gnaedingerae, the Donghuai wood sample shows distinct growth rings, and by higher rays height with the occasional occurrence of bi-seriate portions.
It follows from its unique character combinations that the late Eocene wood from Donghuai can be recognized as a new species named here as Podocarpoxylon donghuaiense. Although this fossil wood shows certain similarity to Prumnopityoxylon, Franco & Brea's 42 diagnosis of this genus lacks sufficient detail to separate it from Podocarpoxylon. Any of the wood traits, that have been considered by these authors as diagnostic for Prumnopityoxylon (i.e. "slightly distinct or indistinct growth rings; absence of axial parenchyma; uniseriate and homocellular rays; uniseriate or biseriate, opposite or sub-opposite, separate or contiguous tracheid pitting; taxodioid or cuppressoid cross-field pitting, with 1-5 bordered pits per field") 42 , can be also found elsewhere in Podocarpoxylon, and their occurrences are consistent with Gothan's 44 diagnosis of this genus. For this reason, we ascribe the new species to Podocarpoxylon rather than to Prumnopityoxylon.  26 , as well as our data confirm that the common occurrence of the Podocarpaceae species in the late Eocene vegetation of South China, that is consistent with the age of initial appearance of this family in Southeast Asia estimated by palynological records 7 . As the results of molecular dating suggest 8 , diversification of Podocarpaceae during Eocene gave rise to such modern genera distributed now in this region, as Dacrydium, Podocarpus and Nageia. Although Podocarpoxylon donghuaiense shares some wood traits with some extant species of Podocarpus and Dacrydium, this fossil wood can be convincingly ascribed to neither of them, probably because these taxa had not yet been emerged as distinct entities in the late Eocene.
Most modern species of Podocarpaceae have growth rings with indistinct to very narrow latewood, or growth rings lacking. Contrastingly, the growth rings of Podocarpoxylon donghuaiense show relatively high proportion of latewood with gradual transition from earlywood to latewood. These traits are indicative for plants with long growing period without rapid shift to seasonal dormancy that are encountered mainly in the middle latitudes of both hemispheres [46][47][48] . Within modern Podocarpaceae, this type of growth rings has been reported only in six species (Podocarpus macrophyllus ranged from Myanmar through mainland China and Taiwan to Japan, Podocarpus acutifolius, Halocarpus bidwillii, Phyllocladus glaucus and P. trichomanoides from New Zealand, and Lagarostrobus franklinii from Tasmania 49 ) which are restricted to the temperate regions without dry season, with hot or warm summer (Cfa and Cfb climate types of Köppen's classification) 50 . The occurrence of Podocarpaceae wood with prominent latewood in distinctive growth rings may therefore be an indicator that South China had mild temperate seasonal climate during the late Eocene.

Conclusion
A new species of fossil conifer wood, Podocarpoxylon donghuaiense sp. nov., is described from the late Eocene of Baise Basin of Southern China. This is the first Cenozoic record of podocarpoid fossil wood in Southern China. The new species is distinguished from other Cenozoic woods ascribed to Podocarpaceae by the unique combination of distinctive growth rings, the absence of axial parenchyma, the lack of bordered pits on tangential tracheid walls, and the occurrence of 3-4 cuppressoid or taxodioid pits on cross-fields. The tightly spaced thickenings on latewood tracheid walls of the fossil wood are recognized as artifacts (probably, effects of wood compression) rather than true tertiary thickenings. Therefore, Podocarpoxylon donghuaiense sp. nov. provides the first robust physical evidence for the early steps of dispersal and diversification of Podocarpaceae in eastern Asia, as well as for warm and wet climate in this region, and the presence of clear growth rings also suggest this was a seasonal climate during the late Eocene.

Methods
The fossil woods described here were collected from the Nadu Formation outcropping at Donghuai Coal-mine in the west part of the Baise Basin (transliterated also as the Bose Basin by some authors), Guangxi, South China (23° 52′ 14.84″ N, 106° 34′ 49.27″ E, Fig. 1). The geological age of the formation is believed to be late Eocene based on the well-studied Nadu Mammalian Fauna 51 .
The well-preserved wood specimens examined represent portions of a main trunk (DHW001-013). The holotype DHW006 is about 10 cm diameter by 15 cm in length. Thin-sections were prepared according to standard methods of cutting, grinding, and polishing using different grades of carborundum powder 52 . Wood slides were examined using a Carl Zeiss Axio Scope A 1 light microscope. Microphotographs were taken using a Cool Snap digital camera fitted with QCapture Pro 6.0 photographic software. Wood anatomical characters were measured and described according to the IAWA list of microscopic features for softwood identification 53 . The systematic position of the fossil wood was determined by consulting the key to morphogenera of fossil conifer woods 54 and by carefully comparing with similar modern and fossil woods. Fossil wood generic nomenclature and circumscription followed the criteria of Philippe & Bamford 54 . Fossil wood specimens and thin sections are housed in the Museum of Biology, Sun Yat-sen University, Guangzhou, China.