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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 Podocarpaceae31,32,33,34.
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 Taxaceae31,32,33,34. 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. Compton31,34. 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 Taxaceae31,32,33,34. 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 & Philippe35. In fossil woods assigned to Podocarpaceae these structures have been reported by Chudajberdyev17 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 Podocarpaceae31,32,33,34. 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 Podocarpus acutifolius Kirk, Podocarpus macrophyllus (Thunb.) D. Don., Lagarostrobus (Dacrydium) franklinii (Hook. f.) Quinn, Halocarpus (Dacrydium) bidwillii (Hook. f. ex T. Kirk) Quinn, Phyllocladus glaucus Carr. and P. trichomanoides D. Don32,34,36,37,38. Axial parenchyma is also lacking in Dacrydium elatum (Roxb.) Wall. ex. Hook., D. colensoi Hook., D. biforme (Hook.) Pilg., D. kirkii F. Muell. ex Parl., D. intermedium Kirk, D. taxifolium Banks & Sol. ex D. Don, Halocarpus bidwillii Lepidothamnus intermedius (Kirk) Quinn., Manoao colensoi (Hook.) Molloy., Microcachrys tetragona (Hook.) Hook., Phyllocladus alpinus Hook.f., P. glaucus, P. trichomanoides, Podocarpus elongatus (Aiton) L’Hér. ex Pers., Prumnopitys harmsiana (Pilg.) de Laub., and P. taxifolia (Sol. ex D.Don) de Laub31,32,33,36,37,39,40. Although cross-fields with 1–2 pits are the most common condition in Podocarpaceae, the occurrence of cross-fields with up to 4 cupressoid or taxodioid pits has been reported for Dacrydium pierrei, D. intermedium, Podocarpus hallii, Microcachrys tetragona and Retrophyllum minor (Carrière) C. N. Page32,34,36,37,39. 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 Gothan31,32,41, as well as in the monospecific genus Prumnopityoxylon Franco & Brea that was recently described from the Upper Cenozoic of Argentina42. Moreover, this combination of wood traits has also been reported for Phyllocladoxylon annulatus Patton from the Oligocene of Australia43. The fossil sample from Donghuai exhibits similarities to Cenozoic species of Podocarpoxylon as well as with Prumnopityoxylon gnaedingerae Franco & Brea and Phyllocladoxylon annulatus Patton (Table 1).
Within seven species that have no axial parenchyma (Prumnopityoxylon gnaedingerae Franco & Brea, Podocarpoxylon aparenchymatosum Gothan, P. dusenii Kräusel, P. latrobensis Greenwood, P. sahnii (Ramanujam) Trivedi & Srivastava, P. tiruvakkaraianum (Ramanujam) Trivedi & Srivastava, and Phyllocladoxylon annulatus Patton), cross-fields with more than two pits occur only in P. aparenchymatosum and P. gnaedingerae. Podocarpoxylon aparenchymatosum from the Eocene deposits of Antarctica44,45 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’s42 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’s44 diagnosis of this genus. For this reason, we ascribe the new species to Podocarpoxylon rather than to Prumnopityoxylon.
Podocarpoxylon donghuaiense sp. nov. is the first record of podocarpoid fossil wood in China. Coeval macrofossils assigned to Podocarpaceae have already been reported from the South China: well-preserved leaves of Nageia have recently been described from the Eocene Changchang Formation of Hainan Island, and the Eocene Youganwo and Huangniuling formations of Guangdong Province25,26. Podocarpoxylon donghuaiense is distinct, however, from the wood of extant Nageia species by the absence of axial parenchyma32,33,34. Thus it is very unlikely that the fossil wood from Guangxi belonged to the same plant taxon as the fossil leaves from Hainan and Guangdong, despite the geographical proximity and nearly the same age of these three findings.
Jin et al.’s25 and Liu et al.’s26, 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 records7. As the results of molecular dating suggest8, 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 hemispheres46,47,48. 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 Tasmania49) 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.