2.1 Geological setting and age

The present fossil wood specimens were collected from the Tiaojishan Formation in Lamaying and Toudaogou villages of Changgao town, Beipiao City, Liaoning Province, north-eastern China (Figure 1).

The Tiaojishan Formation (previously known as the Lanqi Formation) is widely distributed in western Liaoning Province and adjacent regions of northern Hebei Province. The age of the Tiaojishan Formation is considered to be between the late Middle Jurassic and the early Late Jurassic (approximately 165–153 Ma) according to laser ablation inductively coupled plasma mass spectrometry dating of volcanic rock (Zhang, Wang, & Liu, 2008), which corresponds to Callovian to Kimmeridgian ages of the Jurassic.

The Tiaojishan Formation, with a thickness of 2000 m, is lithologically composed of intermediate volcanic rocks and pyroclastic rocks, intercalated with basic volcanic rocks and sedimentary deposits, with fossil plant-bearing beds composed of fine-grained sandstones and shales (Figure 2) (Jiang, Cheng, & Yin, 2010; Wang et al., 1989).

2.2 Fossil preparation and imaging

This study examined two well-preserved silicified wood specimens (registration numbers PBLMY-297 and PBTDG-20). The technique used for the investigation is the classical thin-section method for silicified wood as described in Jones and Rowe (1999). The wood identification and terminology follow those of the IAWA Committee (2004) and Philippe and Bamford (2008). Images were obtained using ACT-1C DXM1200C software adapted to a Nikon E600 light microscope. All related fossil wood specimens and slides are housed in the Palaeobotanical collection of the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, in Nanjing (China).

2.3 Phenetic analysis [Please add here a new section title here] In order to assess phenetic relationships between the present fossil and related fossil and extant woods, we performed a cluster analysis by scoring six morphological characters of all known Sciadopitys-like species from the Jurassic and living species of Sciadopitys (Tables 1 and 2). All characters were unordered and equally weighted. The matrix was analysed in Phylogenetic Analysis Using Parsimony software, version 4.0a169 (Swofford, 2002) with an UPGMA (unweighted pair-group method with arithmetic) mean search to obtain a dendrogram. The settings include ties (if encountered) broken systematically, and distance measure equal to mean character difference.

3.1 Systematics

Family: Sciadopityaceae.

Genus: Zhangoxylon gen. nov. Jiang, Wang, Tian et Zheng.

Derivation of generic name. The generic name Zhangoxylon was named after the late Prof. Wu Zhang from the Shenyang Center of China Geological Survey (Shenyang Institute of Geology and Mineral Resources) for her contributions to fossil wood studies in China.

Generic diagnosis. Woody cylinder with pith, primary xylem, and secondary xylem. Pith abietinean type, stellate shape and heterogeneous, consisting of parenchyma cells and sporadic sclerenchyma cells. Sclerenchymatous nests are present in the pith. Primary xylem with endarch maturation. Secondary xylem with an araucarian radial pitting, uniseriate closely arranged or biseriate alternate. Cross-field pits sub-taxodioid, occasionally window-like, mostly one pit per field, occasionally two. Xylem rays uniformly uniseriate, 2–26 cells high, locally partially biseriate; horizontal walls and end walls of ray cells are smooth and unpitted. Axial parenchyma and resin canals are absent.

Comparisons and remarks. Generally, the new genus Zhangoxylon is anatomically characterized by heterogeneous pith, endarch primary xylem, araucarian radial pitting, sub-taxodioid and occasionally window-like cross-field pits. The combination of these wood characteristics, especially the cross-field pitting pattern, are comparable with those of living Sciadopitys wood.

Protosciadopityoxylon, and Sciadopityoxylon are represented only by secondary xylem, with no details of pith or primary xylem (Table 3).

The two genera mirror each other in gross wood anatomy, but mainly differ in radial pitting, that is, Protosciadopityoxylon and Sciadopityoxylon displayed transitional-type, xenoxylean-type radial pitting and abietinean type pitting, respectively. The genus Protosciadopityoxylon that Zhang et al. (1999) established was based on wood remains from the Lower Cretaceous of western Liaoning, north-east China.

When compared to Tertiary Sciadopitys wood (Philippe et al., 2002), the Tertiary sample bears abietinean radial pitting, with each cross-field having large podocarpoid to taxodioid oculipores in the early wood, whereas the late wood displays a single cupressoid oculipore. Compared to living Sciadopitys (Peirce, 1935), Zhangoxylon similarly demonstrates stellate pith, endarch primary xylem, sub-taxodioid or window-like cross-field pits, absence of pitting on both transverse and tangential walls of ray cells and absence of wood parenchyma as well as intercellular canals, but differs in its radial pitting pattern (Peirce, 1935; Zhou & Jiang, 1994). Zhangoxylon has araucarian radial pitting, while that of Sciadopitys is typical abietinean. For Sciadopitys, ray cells with unpitted horizontal and end walls (Zhou & Jiang, 1994) are strongly characteristic. A detailed comparison of secondary xylem of Zhangoxylon, Protosciadopityoxylon, Sciadopityoxylon, and Sciadopitys is given in Table 3.

3.2 Comparisons

Comparisons of fossil conifer woods with pith and primary xylem from China.

To date, about 16 species across 13 genera of fossil conifer woods have been described with pith, primary xylem, and secondary xylem in China (Table 4), ranging from the Late Palaeozoic to the Mesozoic.

The heterogeneous pith of the proposed genus Zhangoxylon distinguishes it from those taxa with homogeneous pith, such as Chapmanoxylon Wang, Haplomyeloxylon Zheng, Junggaropitys Shi, and Scotoxylon Zhang et Zheng (Shi, Yu, Broutin, & Pons, 2015; Wang, 2000; Zhang, Zheng, & Ding, 2000b; Zheng et al., 2008) (Table 4). Among those taxa with heterogeneous pith, the proposed genus varies from Sclerospiroxylon Zhang et Zheng, Liaoningoxylon Zhang et Zheng, Medulloprotaxodioxylon Wan, Yang, Tang, Liu et Wang, Phyllocladoxylon Cui et Liu, Tianoxylon Zhang et Zheng, and Damudoxylon Zhang et Zheng in cross-field pitting (Table 4). Sclerospiroxylon Zhang et Zheng had cupressoid pits; Liaoningoxylon Zhang et Zheng and Phyllocladoxylon Cui et Liu has a large and simple cross-field pit; Tianoxylon Zhang et Zheng contains numerous, small and bordered pits; while Medulloprotaxodioxylon Wan, Yang, Tang, Liu et Wang has taxodioid pits in cross-field. All these fossils differ from the proposed new genus Zhangoxylon. Additionally, axial parenchyma is absent in Zhangoxylon, whereas it was well developed in Damudoxylon, and the two differed from each other in that Damudoxylon had ray cells with pits on horizontal walls (Cui & Liu, 1992; Wan, Yang, Tang, Liu, & Wang, 2017; Zhang et al., 2007; Zheng et al., 2008).

Comparisons of fossil wood taxa with pith sclerenchymatous nests.

A remarkable feature of the proposed new genus is the presence of sclerenchymatous nests in the pith. Such a structure had also been found in Scleromedulloxylon Doubinger and Marguerier (1975), which was first described in the Permian of France. Generally, the two taxa resemble each other because of the presence of endarch primary xylem and sclerenchymatous nests. However, the secondary xylem of Scleromedulloxylon was of Dadoxylon schollianum type with araucarioid cross-field pits (Doubinger & Marguerier, 1975), which was quite different from that of Zhangoxylon. Furthermore, the pith composition of the two genera is also very different. The pith of the Scleromedulloxylon is large and discontinuous, and with irregular and horizontal diaphragms, unlike in Zhangoxylon.

Type species Z. yanliaoense Jiang, Wang, Tian et Zheng sp. nov. (Figures 3-6).

Derivation of specific epithet. The specific epithet is named after the fossil locality in western Liaoning Province, which is traditionally named as the Yanliao region.

Holotype. PBLMY-297 (Figures 3, 4), deposited in the repository of Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing.

Paratype. PBTDG-20 (Figures 5 and 6), deposited in the repository of Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing.

Type locality, stratigraphic horizon, and age. Lamaying and Toudaogou villages of Changgao town, Beipiao City, Liaoning Province (44°44′30″N, 121°4′30″E, and 44°45′30″N, 121°5′30″E, respectively); Taizishan Bed of the Tiaojishan Formation; Middle-Late Jurassic.

Specific diagnosis. Matching the generic diagnosis.

Detailed Description.

The holotype (PBLMY-297) is an irregularly elliptic cylinder ca. 3.5 cm in the widest direction and 5 cm high. The wood consists of pith, primary xylem, and secondary xylem. The anatomical characteristics are described as follows:

Pith. The pith, varying from 2.3 to 2.7 mm in diameter, is heterogeneous and irregularly stellate in shape, mostly consisting of parenchyma cells with scattered sclerenchyma cells (Figure 3a,b). Sclerenchymatous nests are present surrounded by radially orientated parenchymatous cells. Secretory cells are occasionally present (Figure 3d,f). In the transverse section, the parenchyma cells vary in form and size, from round to polygonal, and from 33 to 67 μm in diameter; these parenchyma cells are rectangular, 33 μm long, and 5 μm wide in radial section (Figure 3b,d). Many nodules are composed of clustered sclerenchyma cells, surrounded by radial parenchyma cells (Figure 3d,f). Nodules are dark-coloured (in radial section), horizontally arranged, and range in lengths from 1.5 to 5 mm. Separate sclerenchyma cells are ovate to polygonal in transverse view.

Primary xylem. The primary xylem is distributed at the apices of the secondary xylem. Tracheids of the primary xylem were small, crowded, and round to elliptical in shape (Figure 3b). The protoxylem extends centrifugally, giving way to the metaxylem, which has exocentric growth. The primary xylem strands show endarch maturation.

Secondary xylem. The secondary xylem has distinct growth rings in the transverse section (Figure 3e). Transitions from early wood to late wood are gradual. The early wood shows wide tracheids that are light in colour, thin-walled, and circular, polygonal, or elliptical, (where elliptical: 17.6–53.0 μm in major axis, 11.8–41.0 μm in minor axis). The late-wood zone is slightly narrower (mostly 3–4 tracheids wide). Tracheids in the late wood are thick-walled and darker in colour, flattened, and 35.3 μm × 7.8 μm in size. Bordered pits on the radial walls of tracheids are uniseriate (circular, separate, or continuous) to biseriate (alternate, sometimes opposite, circular, or polygonal), with diameters 11.4–18.6 μm (Figure 4ab). Both the horizontal and end walls of ray cells are smooth and unpitted (Figure 4c,e). Pits are only found on tangential walls of the early wood tracheids xylem were most and are small, uniseriate, circular, and separate. (Figure 4h).Cross-field pits are mostly sub-taxodioid (Figure 4ce) or occasionally window-like (Figure 4d). Each field has either one or occasionally two pits. The average size of the sub-taxodioid pit is 4.3 μm × 10.0 μm, while that of the window-like pit is 7.1 μm × 10.0 μm.

Xylem rays are usually uniseriate, occasionally partially biseriate (Figure 4f,h), and 2–26 cells high. The ray cells are longitudinally elliptic with long axes of 17.1 μm and short axes of 14.3 μm, respectively. Resin canals and wood parenchyma are absent.

The paratype (PBTDG-20), whose main characteristics generally resemble the holotype (PBLMY-297), is preserved with two piths, which were 9.5–14 mm and 7.6–10 mm in diameter, respectively (Figure 5a). Coinciding with those of the holotype, piths of the paratype are also heterogeneous and irregularly stellate in shape.

In the holotype (PBLMY-297), anatomical details of ray cells on the horizontal and end walls were not well preserved. Observation of the paratype (PBTDG-20) demonstrated that both horizontal and end walls were unpitted (Figure 6d,e).