High-precision 40Ar/39Ar age constraints on the basal Lanqi Formation and its implications for the origin of angiosperm plants
Introduction
Well-preserved Mesozoic terrestrial fossils, mainly including plants and insects, were discovered in the Haifanggou Formation and the overlying Lanqi Formation in northeastern China. Since the late 1990s, the fossil-bearing beds have been excavated in Inner Mongolia, Hebei and Liaoning (Fig. 1, Fig. 2). Some previous studies referred to fossils within the Haifanggou Formation in Liaoning (also called the Jiulongshan Formation in Hebei Province) as the Yanliao Biota. Similar fossil components were found from the overlying Lanqi Formation. The name of the Yanliao Biota, however, has not been widely accepted and is not well defined yet. In this study we refer to these assemblages collectively as the Haifanggou and Lanqi fossils (HLF), rather than the Yanliao Biota. The geographic distributions of the less-studied HLF and the well-known Jehol Biota consisting of terrestrial vertebrates including the famous feathered dinosaurs (Chen et al., 1998, Ji et al., 1998, Xu et al., 1999) are similar but the fossils were discovered from distinct formations now believed to be of different ages (Yang and Li, 2008). The lateral extension of the HLF and its correlation with the Daohugou Biota of Inner Mongolia is controversial; however, the unearthed fossils from the HLF have greatly increased our knowledge of Mesozoic terrestrial ecosystems.
The origin and rapid diversification of angiosperms (flowering plants) in the fossil record, referred to by Charles Darwin as the “abominable mystery”, has long been debated (Davies et al., 2004). As more floral fossils have been found since then, evidence suggests that angiosperm characteristics may have been acquired in a series of steps (Soltis and Soltis, 2004, De Bodt et al., 2005). Despite much research on the fossil record and phylogenetic analyses, the origin of the angiosperms and their rise to ecological dominance remain unclear. Recently, most paleobotanists accept an Early Cretaceous origin for angiosperms because fossil pollen provides the oldest evidence of angiosperms at roughly 136 Ma, about 10 Ma ealier than the oldest unknown flower fossil (Friis et al., 2005, Friis et al., 2006, Rydin et al., 2006, Frohlich and Chase, 2007). However, there is some circumstantial evidence that angioperms may have existed much earlier than the current fossil record. Molecular clock data suggested that the crown node of the angiosperms is from the Jurassic (145–208 Ma) (Sanderson et al., 2004).
The recent discovery of Schmeissneria, previously identified as a member of Ginkgoales (Kirchner and Van Koniinenbura-Van Cittert, 1994), from the Haifanggou Formation provided evidence that the origin of angiosperms could be earlier than Cretaceous and several theories regarding early seed plants should be reappraised. The well-preserved internal structures of female reproductive organs indicated that Schmeissneria sinensis could be considered as an angiosperm or a new seed plant group parallel to angiosperms and other known seed plants (Wang et al., 2007). The features of the vertical complete septum and the closed apex of the central unit showed that Schmeissneria sinensis was distinct from any known gymnosperms, i.e. definitely not a member of Ginkgoales. Moreover, the appearance of the closed carpel, a character only found in flowering plants, suggested that Schmeissneria sinensis should be classified as an angiosperm. On the other hand, Schmeissneria sinensis, with some unknown features, does not look like any known angiosperm (Wang et al., 2007). If Schmeissneria sinensis is accepted as an angiosperm, the origin of the angiosperms would be pushed back to much earlier than Cretaceous due to Schmeissneria microstachys from the Early Jurassic sediments in Germany and Poland (Kirchner and Van Koniinenbura-Van Cittert, 1994). If Schmeissneria sinensis is classified as a new seed plant, the finding could challenge our perspective on the plant evolution, diversity and systematics.
In addition to the Haifanggou Formation, the overlying Lanqi Formation yielded a rich and varied terrestrial flora which includes 92 taxa of bryophytes, ferns, cycads, ginkgos and conifers (Jiang et al., 2008). The high diversity and abundance of the paleoflora provides a unique window to understand the floral evolution and its diversification in Mesozoic. Discovery of the wood fossil Sahnioxylon rajmahalense, the first ever in China, indicated that during the Mesozoic Sahnioxylon had a much wider distribution than expected. Furthermore, examination of Sahnioxylon demonstrated it could be an ancestral type of angiospermous wood devoid of vessels which represented a transition between gymnosperm and angiosperm (Zheng et al., 2005). The anatomical study of Millerocaulis sinica, a new species of fern from the Lanqi Formation, provided phylogenetic evidence that Millerocaulis sinica was an intermediate link between living fern Osmunda and its ancestor (Cheng and Li, 2007). In addition to its importance for floral evolution, the fossil assemblage of silicified woods and compressed plants from the Lanqi Formation indicated a subtropical, humid and seasonal climate (Jiang et al., 2008).
Determining the age of the HLF is critical to establishing its role in floral evolution, particularly of the angiosperms. However, reliable radioisotopic age determinations for these fossil-bearing formations are lacking. Previous age studies of the Haifanggou Formation and the overlaying Lanqi Formation were mainly focused on the tectonic evolution of the North China Craton. Previously reported ages are scattered and the uncertainties of these ages were fairly large. Here, we present new 40Ar/39Ar data for two tuffs from the lowest part of the Lanqi Formation near the main outcrop of floral fossils in Beipiao City, Liaoning, China. These data serve to precisely define the age of the HLF, and may provide the oldest constraint yet reported for the origin of the angiosperm plants.
Section snippets
Geological setting and previous geochronology
During the middle-late Mesozoic period, the sampling location and the surrounding area were located near the northern margin of the North China Craton, in a terrestrial environment at a paleolatitude of approximately 45°N (Smith et al., 1994). Geochemical and isotopic studies provide evidence for lithospheric thinning beneath the North China Craton during that period (Xu, 2002, Zhang et al., 2005a, Yang and Li, 2008). Interbedded tuffs from recurrent volcanism were responsible for the
Methodology
We collected two tuff samples from the Lanqi Formation near Yujiagou village of Beipiao City, Liaoning, China (N 41°50.568', E 120°46.592'). Tuff LQ07-1 is from the lowermost Lanqi Formation and Tuff LQ07-2 is about 10 m higher than LQ07-1. Examination of petrographic thin sections reveals that both tuff samples contain phenocrysts of sanidine, plagioclase, biotite and quartz. All minerals from LQ07-1 are more coarse-grained and fresher than those from LQ07-2. Biotite and sanidine from LQ07-2
Result
Seven single grains of LQ07-1 sanidine yielded plateau ages of 160.8 ± 0.9 Ma, 161.1 ± 0.7 Ma, 161.3 ± 0.8 Ma, 160.5 ± 0.8 Ma, 161.0 ±0.8 Ma, 160.1 ± 0.6 Ma and 160.6 ± 0.6 Ma (2 σ errors are presented here and throughout this paper) (Fig. 3). The weighted mean age of all plateau ages is 160.7 ± 0.4 Ma with an MSWD of 0.91, indicating that data are not significantly scattered in excess of analytic errors (MSWD > 1) and that analytic errors are not overestimated (MSWD < 1). The spectra are mostly flat and show
Previous geochronology
Previous radioisotopic results for the Lanqi Formation obtained by different methods were discordant. Most recently, Yang and Li (2008) reported a 40Ar/39Ar plateau age of 166.14 ± 0.89 Ma, including 92.2% of 39Ar released, for a basalt sample from the lower part of the Lanqi Formation near our sampling site. However, the standard used as neutron fluence monitor, the criteria for the determination of this plateau and MSWD for this so-called plateau age are unknown. The age spectrum showed
Conclusions
Two tuff samples collected from the basal Lanqi Formation near Yujiagou village of NE China yield robust high-precision 40Ar/39Ar ages of 160.7 ± 0.4 Ma and 158.7 ± 0.6 Ma, providing accurate age calibration for the Lanqi Formation and the underlying Haifanggou Formation. Although further paleontological and stratigraphic studies are needed, our age result indicates the Daohugou beds of Inner Mongolia might be correlated to the Haifanggou Formation as well as the overlying Lanqi Formation. In other
Acknowledgements
This research was supported by the National Science Foundation of China (40872015), the Ann and Gordon Getty Foundation, and the Leroy Hill Opportunity Fund. We thank Pei-ji Chen for suggesting the sampling location, Tim Becker for mass spectrometry assistance and three anonymous reviewers for their useful comments. Su-chin Chang thanks her dissertation committee member Kevin Padian for discussion.
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