A typical Euramerican floral element from the Shanxi Formation (Cisuralian, lower Permian) in the Wuda Coal Field, Inner Mongolia, North China
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- Wan, M., Zhou, W., He, X. et al. Palaeobio Palaeoenv (2016) 96: 507. doi:10.1007/s12549-016-0246-z
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Autunia conferta (Sternberg) Kerp is one of the best known, earliest and most widespread callipterids in Euramerica during the Cisuralian. Unambiguous A. conferta foliage is for the first time recorded from Cathaysia in the Shanxi Formation of Wuda, Inner Mongolia, North China. Together with the find of Autunia naumannii (Gutbier) Kerp in the lowermost Permian strata of the same region, it proves that the radiation and spatio-temporal distribution of peltasperms was apparently not hampered by the floral provincialism, and it also indicates a mutual migration of floral elements between Cathaysia and Euramerica. Their long stratigraphic range, from the Cisuralian to the Lopingian, demonstrates that callipterids played an important role in the palaeotropical landscape and had a long history in North China during the Permian. The nearly absence of callipterids in typical Cisuralian wetland floras indicates their preference for well-drained settings and uplands. It is inferred that A. conferta lived contemporaneously with the wetland floral elements in Cathaysia during the Cisuralian, but commonly outside the window of preservation.
KeywordsAutunia conferta (Sternberg) KerpPeltaspermEarly PermianWudaCathaysia floraNorth China
Callipterids are significant components of Cisuralian floras throughout Europe (e.g. Kerp and Fichter 1985; Kerp 1988; Opluštil et al. 2013) and the southwestern USA (DiMichele et al. 2005, 2013b; Chaney and DiMichele 2007; Tabor et al. 2013). It is assumed that most callipterids were peltasperms (DiMichele et al. 2013b) based on related reproductive organs that have been demonstrated for a couple of species (Kerp 1988; Poort and Kerp 1990; Naugolnykh and Kerp 1996) as well as the overall morphological and cuticular characteristics of other species (Krings et al. 2005; Wan and Wang 2015). The peltasperms originated in tropical extrabasinal areas during the Late Pennsylvanian (Doubinger 1979; Kerp 1988; Doubinger et al. 1995). During the Cisuralian through Lopingian, they spread widely and rapidly throughout the Pangaean tropics and northern temperate regions (Naugolnykh and Kerp 1996; Naugolnykh 1999). During the Permian, they were the most common and widespread groups of pteridosperms. A recently published record of Autunia naumannii (Gutbier) Kerp from a wetland flora of earliest Permian age from the Taiyuan Formation of Wuda, Inner Mongolia, indicates that callipterids also played an important role in the earliest Permian landscape of Cathaysia (Wang et al. 2014). In general, all available records indicate that the peltasperms flourished in the Permian palaeotropics (Wang et al. 2014). However, their early presence in the Cathaysia Flora is very remarkable.
Typical Cathaysian elements are very abundant in the Shanxi Formation of North China as listed by Shen (1995), including the so-called oriental lepidophytes (e.g. Lepidodendron oculus-felis, L. posthumii), sphenophytes (e.g. Sphenophyllum, Lobatannularia), ferns (e.g. Psaronius), seed ferns (e.g. Emplectopteris, Emplectopteridium), tingialeans (e.g. Tingia) and Cordaitales. Callipterids, of which were the first occurrences were thought to be in the Guadalupian Lower Shihhotse Formation (Shen 1995), have never been found in the Shanxi Formation. The flora of the Shanxi Formation in Wuda has been collected extensively and many of its common elements have been illustrated (Sun et al. 1998; Chen et al. 2001; Zhou et al. 2015). There is no illustration of callipterids.
In this contribution, we describe and illustrate material identified as Autunia conferta (Sternberg) Kerp collected from the Cisuralian Shanxi Formation of the Wuda Coal Field, Inner Mongolia. It was thought that the absence of callipterids in the early stage of the development of the Cathaysia Flora was due to the edaphic and/or phytogeographic conditions (Sze 1935). The occurrence of A. conferta in Cisuralian deposits of North China, together with the previous records of peltaspermalean plants in the similar interval (Wang et al. 2014), confirms that the evolutionary radiation and spatio-temporal distribution of the peltasperms was not hampered by floral provincialism (Kerp et al. 2001).
Geological setting, material and method
Sun and Deng (2003) proposed a Caulopteris wudaensis-Paratingia datongensis assemblage for Cisuralian fossil plants from Wuda. However, these typical species were recorded from the tuff flora in the uppermost part of the Taiyuan Formation (Pfefferkorn and Wang 2007; Wang et al. 2012) and rarely occur in the Shanxi Formation. The elements of the flora of the Shanxi Formation were recently reported by Zhou et al. (2015). Fourteen species belonging to 12 fossil plant genera compose five sequential vegetational communities preserved in the delta plain wetland settings. This flora consists of Lepidodendron posthumii Jongmans et Gothan, Lepidodendron incertum Sze et Lee, Stigmaria ficoides (Sternberg) Brongniart, Sphenophyllum spathulatum Sze, Calamites suckowii Brongniart, Calamites cisti Brongniart, Annularia orientalis Kawasaki, Sphenopteris (0ligocarpia) gothanii Halle, Pecopteris taiyuanensis Halle, Alethopteris norinii Halle, Emplectopteris triangularis Halle, Taeniopteris serrulata Halle, Cordaites principalis (Germar) Geinitz and Carpolithus sp., all of which are common elements of the Cathaysia Flora and indicate a Cisuralian age.
The specimens of A. conferta (Sternberg) Kerp are preserved as impressions. They were photographed with a Nikon D800 digital single-lens reflex camera. When taking photos, the specimens were submersed in ethanol to increase the contrast between the fossil and the rock matrix as suggested by Kerp and Bomfleur (2011). All illustrated specimens are stored in the Palaeobotanical Collection of the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences in Nanjing.
Genus Autunia Krasser emend. Kerp
Type species Autunia conferta (Sternberg) Kerp, 1988
The frond is bipinnate with eight pairs of pinnae arranged suboppositely to alternately. Apical and basal parts of the frond are not preserved. The primary rachis is massive, straight and rough with longitudinal striae. The primary rachis can reach a width of 25 mm in the lowermost preserved part of the frond to 8 mm in the middle. Pinnae are lanceolate in outline, inserted to the primary rachis at angles of 45–60°. Pinnules are somewhat vaulted, decurrent and entire-margined. The ovate to tongue-shaped pinnules are slightly obliquely inserted to the pinnae rachides. The sunken and decurrent midvein is conspicuous and extends up to the pinnule apex. The lateral veins are commonly difficult to discern due to the imperfect preservation. Between two successive pinnae, two or three intercalary pinnules are inserted to the primary rachis.
Callipteris Brongniart non Bory is a widespread fossil genus of foliage from the Northern Hemisphere. It was originally used as a biostratigraphic and a biogeographic marker for the lower Permian in Europe (Jongmans 1928; Jongmans and Gothan 1937), but is now known from the latest Carboniferous and Permian (Vasilevskaya 1987; Kerp 1988; Doubinger et al. 1995; Kerp et al. 2001; DiMichele et al. 2013b; Wang et al. 2014). The generic name Callipteris Brongniart, 1849 is illegitimate as it is a later homonym of Callipteris Bory, 1804, a genus of extant ferns (Kerp 1981, 1986). A number of the European species have been re-examined and placed into other fossil genera of callipterid foliage, such as Rhachiphyllum Kerp, Lodevia Haubold et Kerp, Arnhardtia Haubold et Kerp, Gracilopteris Kerp, Naugolnykh et Haubold and Dichophyllum Elias (Kerp 1988; Kerp and Haubold 1988; Kerp et al. 1991). Kerp and Haubold (1988) placed Callipteris conferta type foliage and the reproductive organs associated with it (both ovulate and pollen organs) in the genus Autunia Krasser, which was emended to include all relevant organs and they formally assigned Autunia to the Peltaspermaceae. The previous records of C. conferta from North China (Sze 1935; Stockmans and Mathieu 1939) can be confirmed; the similar frond architecture and morphology justifies identification as A. conferta (Sternberg) Kerp. In Europe, North America and northwestern China, foliage of A. conferta shows considerable morphological variability (Kerp 1988; Zhang and Wu 1989, 1991; DiMichele et al. 2013b). However, the variation is not apparent from the current material due to the limited number of specimens.
Due to the bipinnate frond and ovate to tongue-shaped pinnules, the specimen described here can easily be distinguished from other genera of callipterids, including Lodevia, Arnhardtia, Gracilopteris and Dichophyllum. The current specimen shares many similarities with A. conferta and Rhachiphyllum schenkii. Autunia (al. Callipteris) conferta was often confused with another common callipterid that was originally described as C. conferta (Heyer 1884) and is now known as R. schenkii (Kerp 1988; Kerp and Haubold 1988). There are several differences between these two species (Kerp 1988). A conferta is characterised by commonly bipinnate, rarely tripinnate fronds with an overtopping or pseudodichotomous apex. Rhachiphyllum schenkii can be distinguished from A. conferta by its monopodial apex. In some cases, the pinnules of these two species are quite similar. However, A. conferta usually has thick, vaulted pinnules, whereas those of R. schenkii are very thin. In addition, the variation in pinnule length within a pinna is different in these two species. In A. conferta, pinnules in the basal part of the pinna are relatively smaller, and they become longer in the upper part of the pinna. R. schenkii has the longest pinnules in the lower part of the pinna, progressively shorter ones towards the pinna apex. A. conferta has a straight sunken midvein that runs to shortly before the pinnule apex, whereas R. schenkii has a much thinner, more flexuous midvein that is not sunken. In A. conferta, the lateral veins are densely spaced and straight, whereas the venation R. schenkii is more open and the lateral veins are more flexuous. The apical part of the frond and the lateral veins of the current specimen are not preserved. However, based on the shape of the pinnules, the variation of pinnule length in the pinna, and the sunken midvein, it is easily distinguishable from R. schenkii; it is very similar to the foliage of A. conferta described illustrated in Kerp (1988). Therefore, we assign the specimen described here to A. conferta.
Wide distribution of A. conferta and peltasperms during the Cisuralian
The peltasperms are a group of pteridosperms that were originally thought to be exclusively Mesozoic seed ferns. The group has proven, however, to be widespread and abundant in the Permian tropics (Kerp 1988; Kerp and Haubold 1988; DiMichele et al. 2005, 2013a, b; Wang et al. 2014), appearing at least as early as the Late Pennsylvanian (Doubinger et al. 1995; Lausberg and Kerp 2000; Kerp et al. 2001). Traditionally, most records of peltasperms were known from Europe and North America (listed in Jongmans and Dijkstra 1958, 1967; Dijkstra and Van Amerom 1981). In addition, there are many records of Artinskian, Kungurian and Tatarian peltasperms from the mid-latitude Angaran regions (e.g. Zalessky 1913, 1918, 1927, 1939; Fefilova 1973; Gomankov and Meyen 1979, 1986; Naugolnykh and Kerp 1996). The previously described Cisuralian flora from the Tarim Basin of western China contains a number of peltasperms (Zhang and Wu 1989, 1991; Shen et al. 1990; Sun 1995; Wu et al. 1997). Liu and Yao (2000) described a peltaspermalean reproductive organ with radially symmetrical ovuliferous discs from the lowermost Permian of the Turpan Basin, China. Booi et al. (2009) reported Comia variformis Booi et al., which is of peltaspermalean affinity, the callipterid Rhachiphyllum, Supaia-like specimens and an Autunia type fructification from the lower Permian of Jambi, Sumatra. The Jambi flora was interpreted as an early Permian Cathaysian flora distributed in the palaeotropics of the Southern Hemisphere (Van Waveren et al. 2007). Protoblechnum Lesquereux, a genus of which the natural affinity is still unclear but might be peltaspermalean, was reported from the lower Permian Palmario Formation of Venezuela, which was a part of northern Gondwanaland (0°–10° S) (Ricardi-Branco 2008). Recently, leaves and peltaspermalean reproductive organs were found in the lower Permian Barakar Formation of the Satpura Basin, central India, co-occurring with various glossopterids (Srivastava et al. 2011). Although the early history of the peltasperms is still insufficiently understood, all known records indicate that peltasperms probably originated in the tropical extrabasinal areas in Euramerica (Doubinger et al. 1995; Lausberg and Kerp 2000; Kerp et al. 2001), subsequently migrated within the tropical belt (DiMichele et al. 2005; Wang et al. 2014) and rapidly extended to the Cathaysia, Angara and Gondwana during the Cisuralian. Their rapid radiation was apparently not hampered by the floral provincialism that characterised this part of the world (Kerp et al. 2001).
The peltasperms consist of several major groups: the callipterids, the comioids, the supaioids, probably the “American” gigantopterids (DiMichele et al. 2005, 2006), the tatarinoids (Meyen 1984) and the glenopterids (Wan and Wang 2015). Callipterid fronds are imparipinnate and have two orders of branching, entire-margined to profoundly lobed pinnules, and usually pinnate venation (Kerp 1988; Kerp and Haubold 1988; Poort and Kerp 1990; DiMichele et al. 2005, 2006; 2013a, b). Most typical are the intercalary pinnules attached to the primary frond rachis. The callipterids consist of a number of different genera based on frond and pinnule morphology and epidermal anatomy (Kerp 1988; Kerp and Haubold 1988). A. conferta and A. naumannii have been associated with reproductive organs unequivocally. A. conferta is one of the best known (Kerp 1988) and earliest members (Doubinger et al. 1995) of the callipterid lineage.
Callipterids were long considered as index fossils for the base of the Permian (Jongmans 1928). After a thorough revision of the material from the West- and Central European, Kerp (1988) concluded that the traditional concept of the importance of callipterids in chronostratigraphy and biostratigraphy could not be maintained. The earliest record of A. conferta is from the upper Stephanian of the St. Étienne Basin, Central France (Doubinger et al. 1995). The species is abundant in Cisuralian Euramerican floras (Kerp 1988; Chaney and DiMichele 2007; DiMichele et al. 2013b). However, the stratigraphic range of A. conferta in the Cathaysia Flora of North China seems not well constrained. It was first recorded from the Guadalupian to Lopingian Upper Shihhotse Series in Central Shanxi Province (Sze 1935). The ovuliferous organ of A. conferta with megasporophylls was reported from the Guadalupian Shenhou Formation in Henan Province (Yang 2006). In addition to A. conferta, another species of Autunia, A. naumannii was reported from the Asselian in Wuda, Inner Mongolia (Wang et al. 2014). Autunia-like reproductive organs were found in the Upper Tianlongsi Formation (Lopingian), associated with Supaia type foliage (Wang 1997). Nevertheless, the specimens of A. conferta from the Guadalupian and Lopingian of Cathaysia are of fragmentary state of preservation, and do not permit a justified identification.
A. conferta was one of the most widespread callipterids during the Cisuralian. Large numbers of A. conferta with smaller pinnules commonly occur in the Rotliegend of Europe (Kerp 1988) and from the Asselian to the Kungurian red beds of central New Mexico (DiMichele et al. 2013a). Recently, abundant A. conferta specimens with considerable variation in the apparent lamina thickness have been recorded from the Dunkard Group, a stratigraphic interval of controversial age in the Central Appalachian Basin (DiMichele et al. 2013b). Specimens with medium-sized to large, bluntly pointed or rounded pinnules that are overall similar to some Autunia specimens have been reported from the Asselian to Kungurian red beds of North-Central Texas (Chaney and DiMichele 2007). Zhang and Wu (1989) for the first time reported A. conferta showing a considerable morphological variability, together with Dichophyllum flabellifera (Weiss) Kerp et Haubold and Rhachiphyllum schenkii (Heyer) Kerp from the Cisuralian Kaipaizileike Formation from the northwestern Tarim Basin, western China. Since then, the contemporaneous flora from the northwestern Tarim Basin has been intensively collected and A. conferta, a common element, has been illustrated (Shen et al. 1990; Zhang and Wu 1991; Sun 1995; Wu et al. 1997). Based on the occurrence of A. conferta, the Cisuralian flora from this region was treated as Euramerican (Shen 1995). Stockmans and Mathieu (1939) reported A. conferta from the upper part of the Zhaogezhuang Group in Tangshan, Hebei, North China. Its age, however, remains uncertain. The occurrence of A. conferta in the Shanxi Formation in this study is the first unambiguous record of this widespread species in the Cathaysia Flora from the Cisuralian. Together with the discovery of A. naumannii in the lowermost Permian wetland flora of Wuda (Wang et al. 2014), it proves that there was a migration of floral elements from Euramerica into Cathaysia during the early Permian. All these occurrences indicate that great caution is necessary when using the fossil record of callipterids as crucial elements for the biogeographic analysis.
During the Wuchipingian to Lopingian, peltasperms were abundant in North China (Halle 1935; Sze 1954a, 1954b; Durante 1983, 1992; Wang and Wang 1986; Wang 1985, 1996, 1997; Yang 2006; Zhang et al. 2012). Previously, they were thought to originate from the Angaran and Euramerican provinces (Halle 1935; Wang 1985, 1996). It might have been easier for plant to migrate within the same palaeogeographic region (Wan and Wang 2015). Therefore, the occurrence of megafossils (current material; Wang et al. 2014) of callipterids from the Cisuralian deposits of central North China indicates some of the later Permian peltasperms in China very likely immigrated from central North China and not necessarily from the Angaran and Euramerican provinces (Wang et al. 2014; Wan and Wang 2015).
It was inferred that although callipterids started to radiate in North China from the earliest Permian (Wang et al. 2014), they never played a major role in the Cathaysia Flora from the North China Block prior to the Lopingian when peltasperms became abundant (Halle 1935; Sze 1954a, 1954b; Wang and Wang 1986; Durante 1992; Wang 1996, 1997; Wu et al. 1997; Yang 2006; Zhang et al. 2012). However, it is undeniable that callipterids were significant tropical landscape elements somewhere (Wang et al. 2014), but have been incorporated into the fossil record rarely, and thus their early history in Cathaysia remains largely unknown. Located in the easternmost parts of the tropical belt, Cathaysia remained ever-wet (Rees et al. 2002) and wetland floras very similar to those of the middle Pennsylvanian continued to survive into the Permian (Guo 1990; Tian et al. 1996; Hilton et al. 2001). Nearly all information on Cisuralian fossil plants from the research area is from wetland floras. The earliest Permian coal-swamp flora from the vegetational Pompeii in the current research area was dominated by lycopsids, sphenopsids, tree ferns, noeggerathiales, possible early cycads and cordaitaleans (Pfefferkorn and Wang 2007, 2009; Wang et al. 2009, 2012). Recently, Zhou et al. (2015) reported five sequential wetland vegetation communities from the Shanxi Formation. The coal-swamp flora was dominated by lycopsids, whereas the clastic swamp flora consisted of lycopsids and sphenopsids. Characteristic elements of the plants from the floodplains included tree ferns, possible early cycads and cordaitaleans. The near absence of callipterids in typical Cisuralian wetland floras suggests their preference for well-drained settings and uplands.
In Europe and North America, A. conferta occurs in deposits indicating that they generally inhibited seasonally dry environments (Kerp 1988; Galtier and Broutin 1995; DiMichele et al. 2013b), although a few examples of occurrences in ever-humid, in one case even in a coal-forming environment, are known (Barthel and Haubold 1980; Kerp 1988). A. naumannii occurred in an earliest Permian roof shale; Wang et al. (2014) suggested that peltasperms could exist in a variety of environments including those with ever-wet conditions of many Cathaysian basins. The depositional environment of the Shanxi Formation in Wuda district, analysed in detail by Peng and Zhang (1995), was dominated by a delta plain and delta front. The plant fossils occur in thin-bedded units, consisting of siltstones and fine-grained sandstones with parallel bedding. It is inferred that the current material is from sediments representing interdistributary floodplain environments. The plant fossils of this interfluve setting comprise L. posthumii, S. ficoides, upright lycopsid stems, Annularia sp., Calamites sp., Neuropteris ovata and Cordaites sp. It appears that A. conferta occurs as a rare element within the lycopsid-dominated wetland flora. Based on the ecology of callipterids of this type, inferred from other studies, it is presumed that A. conferta lived contemporaneously with the wetland floral elements, such as lycopsids and cordaites in Cathaysia during the Cisuralian, but commonly outside the window of preservation (Looy et al. 2014). DiMichele et al. (2013b) concluded that peltasperms occurring in the clastic deposits indicate that they usually grew in seasonally dry environments. The spatio-temporal sparseness of the appearances of callipterid fossils, in light of the high density of plant remains from sites where they do occur, indicates that their fossil record also is highly incomplete (Looy et al. 2014; DiMichele et al. 2015).
We appreciate Mr. Shengwu Mei of the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, who helped us doing lots of the field work in the research areas. We thank Mr. Xiaoyi Fan of the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, who made the photographs. We are grateful to Prof. Hans Kerp of Forschungsstelle für Paläobotanik, Westfälische Wilhelms-Universität Münster for discussing the identification of the material and sharing his collection. We thank Prof. Hans Kerp and Dr. William A. DiMichele whose critical and detailed comments greatly improved this paper. This research was jointly supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB18030404), the Basic Project of State Key Laboratory of Palaeobiology and Stratigraphy and the National Nature Science Foundation of China (41502006, 41530101, 41372011, 41372010, J1210006 and 41472005). Financial support was also provided by the China Scholarship Council to Mingli Wan.
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Conflict of Interest
The authors declare that they have no conflict of interest.