Occurrence of Brachyoxylon Hollick et Jeffrey from the Lower Cretaceous of Zhejiang Province, southeastern China

Anatomically-preserved petrified woods play significant roles in revealing the floristic composition and evolution of plant kingdom in geological history (Zhang et al. 2006). China is one of the most significant fossil localities for petrified woods with a remarkable high diversity. Until 2006, totally 181 species referred to 106 genera of petrified woods have been reported in China, ranging from Late Paleozoic to Cenozoic in time (Zhang et al. 2006). Xylofloras of China received progressively greater attention over the past decades. An increasing number of Mesozoic petrified woods have been reported from series of horizons in all over China (e.g., Jiang et al. 2008; Wang et al. 2009, 2017; Jiang et al. 2012, 2016; Yang et al. 2013; Feng et al. 2015; Shi et al. 2015; Tian et al. 2015, 2016; Zhang et al. 2015, 2016; Ding et al. 2016; Wan et al. 2016). For the Cretaceous records of petrified woods in China, the spatial-temporal distribution pattern is notably unbalanced. It is noted that 52 species have been described from the Early Cretaceous deposits in China (Yang et al. 2013); whereas, only five species were reported from the Late Cretaceous deposits (Sze 1951; Du 1982; Wang et al. 1996; Terada et al. 2011). Palaeogeographically, most of the Cretaceous petrified woods were described from northern China (especially Northeast China), whereas they are very scarce and limited in southern China (Yang et al. 1990; Duan et al. 2002).

The Lower Cretaceous Guantou Formation is known as an important horizon for the Cretaceous petrified woods in southern China. A large number of petrified woods have been found in this formation; however, only one species has been recognized, i.e., Araucarioxylon xinchangense Duan (Duan et al. 2002). In this paper, a new species of coniferous wood, Brachyoxylon zhejiangense sp. nov. is described based on a fossil wood fragment collected from the Lower Cretaceous Guantou Formation in Xinchang County of Zhejiang Province, southeastern China. This new finding enriches the diversity of Early Cretaceous petrified woods in southern China, and sheds new light on further understanding the floral composition, especially the forest vegetation type of the southern phytoprovince in the late Early Cretaceous.

The new fossil material consists of a well-preserved silicified wood fragment found from Lower Cretaceous Guantou Formation in Suqin Village of northwestern Xinchang Basin of Zhejiang Province, southeastern China (29°30′20″N, 120°47′37″E) (Fig. 1). In the Xinchang Basin, the Early Cretaceous strata are well-developed and widely distributed, and can be sub-divided into the Guantou Formation and Chaochuan Formation, ascendingly (Dong et al. 2002). The Guantou Formation mainly crops out in the western edge of the basin (Dong et al. 2002). In the Xinchang Basin, the Guantou Formation is conformably overlain by the Chaochuan Formation, and unconformably overlies on the Upper Jurassic Jiuliping Formation (Fig. 2). Lithologically, the lower part of the Guantou Formation consists of purple pebbly sandstone and conglomerate, and then ascendingly consists of interbeds of variegated sandstones, siltstones, silty mudstones, and shales. The whole Guantou Formation can be divided into six layers, and the present fossil wood specimen was collected from the layer 4 (Fig. 2). Isotopic dating data of volcanic rock (110–103 Ma) suggest the age of the Guantou Formation to be Early Cretaceous (Albian) (Han et al. 1992; Yu and Xu 1999).

The petrified wood specimen was cut transversely, longitudinally and tangentially into several thin sections. These sections were prepared by standard methods, including cutting, grinding and polishing preparations (Hass and Rowe 1999). Photographs were taken with Scope Image 9.0 (H3D) Software adapted to a Yongxin BM2000 Microscope. The fossil wood specimen and six correlated thin slides described in this paper are housed in the Palaeontological Museum of Liaoning, Shenyang, China, with a registration on number PMOL-B-01476. Fossil wood description follows the standardized terminologies of IAWA Committee (2004) and Philippe and Bamford (2008).

Class: Coniferopsida

Order: Coniferales

Genus: Brachyoxylon Hollick and Jeffrey 1909

Type species: Brachyoxylon notabile Hollick and Jeffrey 1909

Species: Brachyoxylon zhejiangense sp. nov. Tian, Zhu et Wang

3.7 Description

The present fossil wood specimen (PMOL-B-01476) is preserved as a fragment of secondary xylem. No anatomical details of the pith and primary xylem are preserved. In the transverse section, the growth rings are weakly defined (Fig. 3a). The transition from the early to late wood is abrupt (Fig. 3a). The late wood zone is very narrow with only 3–5 layers of tracheids. Tracheids of the early wood are large and polygonal and thin-walled, while those of the late wood are rectangular and relatively thick-walled (Fig. 3a).

In the longitudinal radial section, the bordered pits on the tracheid walls belong to the mixed type. Most of them are uniseriate continuous (Fig. 3b–c) or biseriate alternated (Fig. 3f), partly uniseriate distant (Fig. 3d-e), and very rarely biseriate opposite or sub-opposite (Fig. 3g). In order to give a clear explanation of the radial pitting type, we counted the percentages of different type of radial pits. We assign each tracheid to a specific type of radial pitting, according to its predominant pitting. Considering that the biseriate opposite pits are rare and only occupy part of a tracheid, as long as biseriate opposite pits are observed in one tracheid, we define the tracheid having biseriate opposite pits. The results show that the uniseriate continuous and biseriate alternated pits are dominant (84.4% of the total number of tracheids, n = 141), the uniseriate distant ones account for 11.3%, the biseriate opposite and sub-opposite ones account for 4.3%. If only the typical biseriate opposite pits with a square outline is considered, they are with a percentage of 2.1%. Such a result shows that the present fossil wood meets the definition of mixed type of radial pitting defined by Philippe and Bamford (2008). The bordered pits are mostly rounded or slightly flattened, 13–17 μm in diameter (Fig. 3b–g). The pit-apertures are oval or rounded, ca. 4.5 μm in diameter (Fig. 3b–g). Rays are homogeneous, with both the horizontal and end walls smooth and unpitted (Fig. 3i–k). Cross-field pit arrangement is of the araucarioid type sensu IAWA Committee (2004) (Fig. 4a–c). Each cross-field bears 2–7 bordered pits with circular or oval outline, 23–28 μm in diameter, contiguous and arranged in 1–3 rows (Fig. 4a–c). Individual pits are of cupressoid type sensu IAWA Committee (2004). Pit aperture is horizontal or oblique, 6–11 μm in diameter (Fig. 4a–c).

In the longitudinal tangential section, rays are homocellular, mostly uniseriate and rarely partially biseriate, 1–16 (mostly 2–7) cells high (Figs. 4d–e, 5). Generally ca. 35 rays per mm² are observed. No axial parenchyma and tangential pits were identified in the studied specimen.

As a fossil wood morphogenus, the systematic affinity of Brachyoxylon is still undetermined. It has been linked to various coniferous families, such as Araucariaceae (Hollick and Jeffrey 1909), and Cupressaceae (Seward 1919). However, Brachyoxylon has been more frequently related to the distinct family Cheirolepidiaceae (e.g., Alvin et al. 1981; Alvin 1982; Machhour and Pons 1992; Limarino et al. 2012). The finding of Brachyoxylon woods in association with conifer leaves and Classopollis-bearing cones (Zhou 1983) suggests that at least some Brachyoxylon woods may be representatives of the Cheirolepidiaceae. In fact, the type species B. notabile Hollick et Jeffrey was related to Brachyphyllum macrocarpum Newberry 1896, a possible Cheirolepidiaceae (Alvin 1982). It is of interest that compressed leaf fossils of possible Cheirolepidiaceae, e.g., Pseudofrenelopsis parceramosa (Fontaine) Watson, Pagiophyllum delicatum Cao, P. obtosior Cao, and P. xinchangense Cao, have also been described from the Lower Cretaceous Guantou Formation in the same fossil locality (Suqin Village of Xinchang County, Zhejiang Province) with Brachyoxylon zhejiangense sp. nov. (Cao 1989, 1991). Additionally, Pseudofrenelopsis papillosa Chow et Tsao and P. parceramosa have also been found from the Guantou Formation in adjacent Jingling Village of Xinchang County (Ren et al. 2008). Generally, the co-occurrence of Brachyoxylon wood and leaf fossils of Cheirolepidiaceae in the Lower Cretaceous Guantou Formation further implies that the fossil wood genus Brachyoxylon may be systematically referable to the conifer family Cheirolepidiaceae.

Fossils of Brachyoxylon have been widely documented from both the boreal and Gondwana regions range from the Permian to the Late Cretaceous (Philippe et al. 2004a; Bodnar et al. 2013). Palaeogeographically, Brachyoxylon is much more common in boreal areas (e.g., the United States, Tunisia, Japan, Cambodia, Thailand, France and Romania) than in Gondwana region (Philippe et al. 2004a; Bodnar et al. 2013). Though abundant and diverse Mesozoic wood remains have been described, fossil records of Brachyoxylon are very scarce in China. Vozenin-Serra and Pons (1990) assigned a wood sample from the Lower Cretaceous in Lhunzhub of Tibet, southwestern China to Protopodocarpoxylon orientale Serra. However, due to its araucarioid cross-field pits, Philippe et al. (2004a) proposed it as a new combination Brachyoxylon orientale (Serra) Philippe et al. Then recently, this specimen is reassigned to Shimakuroxylon japonicum (Shimakura) Philippe et al. for having radial pitting of “japonicum-type” (Philippe et al. 2014). Additionally, Brachyoxylon sp. has been reported from the Lower Cretaceous in Lhasa of Tibet (Vozenin-Serra and Pons 1990; Zhang et al. 2006). Generally, the new finding of Brachyoxylon zhejiangense sp. nov. enriches the diversity of the Early Cretaceous petrified wood in southern China, and contributes to further understanding the floral composition, especially the forest composition of the southern phytoprovince in the late Early Cretaceous.

A new coniferous wood, Brachyoxylon zhejiangense sp. nov. was is described from the Lower Cretaceous Guantou Formation of Zhejiang Province, southeastern China, which represents the first record of Brachyoxylon wood in southeastern China. The new finding provides new clues for understanding the palaeogeographical distribution of this genus, as well as the forest composition of the Early Cretaceous flora in southeastern China. Additionally, the co-occurrence of Brachyoxylon wood and cheirolepidiaceous leaf fossils in the Guantou Formation further implies that Brachyoxylon may be systematically referable to the Cheirolepidiaceae.