An upper Paleocene radiolarian-bearing succession (ZN1) located in the southern part of the north subzone of the Tethyan Himalaya is composed of siliceous siltstones and claystones. In this paper, 23 species belonging to 13 genera are reported. Three new species, Lychnocanium? pyramis Li and Matsuoka sp. nov., Lychnocanium? stypticum Li and Matsuoka sp. nov., and Pterocyrtidium sinense Li and Matsuoka sp. nov., are described. Radiolarian assemblages from this succession can be compared with the Bekoma campechensis Zone (RP6), indicating a time interval of 61.5–58.23 Ma, i.e., late Paleocene. The radiolarian assemblages from the Yamdrok mélange, the Zheba section, and the Jiazhu section indicate that they are of the same age as those from the Zhinadibu 1 section. The lack of calcareous and coarse-grained terrestrial materials in the ZN1 section proves that the strata of this section were deposited in a relatively deep marine environment below the calcium carbonate compensation depth (CCD) and more distal to the continents during the late Paleocene. Coeval deepwater sediments near Saga were accumulated near the CCD.
Introduction
The Yarlung-Tsangpo Suture Zone (YTSZ) is the southernmost and youngest amongst the sutures which subdivide the Tibetan Plateau into five east-west trending blocks. The YTSZ marks where the Neo-Tethys was consumed as the Indian continent approached northward and finally collided with the Eurasian continent.
Methods such as tectonic deformation, paleomagnetism, and magmatic analyses have been applied in interpreting the initial contact between the Eurasian and Indian continents. However, the timing of the initial contact is still controversial and ranges from the Late Cretaceous (≈70 Ma) to the early Oligocene (≈34 Ma) (e.g. Beck et al., 1995; Aitchison et al., 2000, 2007; Yin and Harrison, 2000; Ding, 2003; Najman, 2006; Ali and Aitchison, 2008).
The Paleocene is the final stage of the subduction of the Neo-Tethyan oceanic lithosphere beneath the southern margin of the Lhasa terrane (Yin and Harrison, 2000; Ding, 2003). Radiolarian biostratigraphic studies on the Paleocene pelagic strata along the YTSZ will provide direct clues for the subduction and collision.
Reports on radiolarian-based upper Paleocene marinesediment succession along the YTSZ are relatively scarce. Liu and Aitchison (2002) found a late Paleocene radiolarian fauna from the green mudstone matrix of the Yamdrok mélange (Loc. 1 in Figure 1). Ding (2003) reported Paleocene radiolarians from flysches near Saga (Loc. 2 in Figure 1). Hu et al. (2015) reported late Paleocene radiolarians and nannofossils from the same section as Ding (2003). An early Eocene radiolarian assemblage from a section near Saga was reported (Li et al., 2007) (Loc. 3 in Figure 1). Liang et al. (2012) reported a late Paleocene radiolarian assemblage from the sedimentary mélange west of Zhongba County (Loc. 4 in Figure 1). The previous reports on radiolarian-bearing upper Paleocene strata along the YTSZ are isolated and scattered. Because the strata are seriously disrupted and diachronous spatially, the detailed distribution and extension of Paleocene deep-marine sediments along the YTSZ remain unclear.
Radiolarians have been widely reported from upper Paleocene and lower Eocene sedimentary strata. Upper Paleocene deep-sea sediment cores containing a continuous succession of well preserved radiolarians are known from Northwest Atlantic (Nishimura, 1987, 1992; Sanfilippo and Blome, 2001; Jackett et al., 2008), Gulf of Mexico (Foreman, 1973; Sanfilippo and Riedel, 1973; Jackett et al., 2008), Caribbean (Riedel and Sanfilippo, 1973), Northeast Atlantic (Liu et al., 2011), Indian Ocean (Blome, 1992), South Pacific (Dumitrica, 1973; Hollis, 1993; O'Connor, 2001), and eastern New Zealand (O'Connor, 2001; Hollis, 2002). Sparse late Paleocene assemblages have been reported from land sections: Western Kuban in the former USSR (Borisenko, 1958), northern Cyprus (Sanfilippo et al., 2003), southern Tibet (Liu and Aitchison, 2002; Ding, 2003; Li et al., 2007; Liang et al., 2012; Hu et al., 2015), Japan (Nanayama, 1992; Oyaizu et al., 2002), North America (Frizzell and Middour, 1951), western Cuba (Sanfilippo et al., 1999), Canary Islands (Bustillo et al., 1994), Southwest Pacific (Cluzel et al., 2001), New Zealand (Hollis and Hanson, 1991; Hollis, 1993, 2006; Strong et al., 1995), and South Africa (Jannou, 2007). Only samples from southern Tibet and New Zealand contain well preserved radiolarians. However, new species, which are abundant and typical in southern Tibet during the late Paleocene, have not been formally described.
Figure 1.
Geological map around the Yarlung-Tsangpo Suture Zone showing the major tectonic units (modified from Zhou et al., 2007) and the locations of previous research: 1, Liu and Aitchison (2002); 2, Ding (2003) and Hu et al. (2015); 3, Li et al. (2007); 4, Liang et al. (2012).
In this paper, we present the results of a detailed study of upper Paleocene radiolarian biostratigraphy of a siliceous mudstone succession near Zhongba County and clarify the tectonic attributes of this succession. Three new species, Lychnocanium? pyramis Li and Matsuoka sp. nov., Lychnocanium? stypticum Li and Matsuoka sp. nov., and Pterocyrtidium sinense Li and Matsuoka sp. nov., are described. The age assignments of radiolarian assemblages are based on low-latitude zonation updated by Norris et al. (2014). We discuss the distribution of upper Paleocene radiolarian-bearing sediments and their depositional environments.
Geological setting
Four lithotectonic units are discriminated along the YTSZ from north to south: (1) Gangdese arc, (2) Gangdese forearc, (3) Yarlung-Tsangpo Suture Zone, and (4) Tethyan Himalaya (Figure 1). The Gangdese arc consists of Late Jurassic to early Paleogene granitoid intrusions (Debon et al., 1986) and non-marine volcanic sequences of the Linzizong Formation of 68 to 43 Ma (Maluski et al., 1982; Coulon et al., 1986). The Gangdese forearc basin is located to the south of the Lhasa terrane. The forearc basin is occupied mostly by a sequence of Cretaceous to Paleogene turbiditic rocks (Einsele et al., 1994). The YTSZ is characterized by a pile of ophiolitic bodies (e.g. Nicolas et al., 1981; Aitchison and Davis, 2004).
The Tethyan Himalaya incorporates Mesozoic to lower Paleogene marine sequences deposited on the Indian passive continental margin. The strata have been roughly separated into two tectonic subzones by a line along the Gyirong-Kangmar Intracrustal Thrust, which was formed during the collision of the Indian and the Eurasian continents.
The northern subzone of the Tethyan Himalaya (NTH) consists of sandstones, siltstones, and limestones (Liu and Einsele, 1994; Hu et al., 2008). Jurassic to lower Paleogene strata of the northern subzone exposed in the Gyangze area are relatively well studied and divided into five formations (Liu and Einsele, 1994; Li et al., 1999; Wang et al., 2000; Liu and Aitchison, 2002; Hu et al., 2008): the Weimei Formation (Upper Jurassic quartz sandstones), the Rilang Formation (Lower Cretaceous volcaniclastic litharenites intercalated with siltstones), the Gyabula Formation (Lower to Upper Cretaceous black siltstones with pyrite nodules intercalated with sandstones), the Chuangde Formation (Upper Cretaceous violet-red limestones intercalated with thin marlstone beds), and the overlying Zongzhuo Formation (Upper Cretaceous to Paleocene dark gray to black shales enclosing various olistoliths of sandstones, limestones, and bedded cherts).
Figure 2.
Strata distribution and the location of the ZN1 section in our research area (after 1:50,000 geological mapping investigations).
The southern subzone of the Tethyan Himalaya (STH) is composed mainly of carbonate and clastic sediments (Wan et al., 2000). Well studied sections are located in the Gamba-Tingri area where the youngest shallow marine strata of the middle Eocene (Lutetian) have been reported (Wen, 1987; Willems et al., 1996; Xu, 2000).
Material and methods
In our research area near Zhongba County, a previous 1:250,000 geological map defined the strata south of the Yarlung-Tsangpo River as the Upper Cretaceous to middle Eocene Sangdanlin Formation, Denggang Formation, Guoyala Formation, and Yanduo Formation (Institute of Geological Survey of Hebei Province, 2006). According to our field investigations from 2010 to 2013, the strata are divided into mélange and sedimentary strata. The sedimentary strata are located to the south of the mélange (Figure 2).
The sedimentary strata consist of quartz sandstones, lithic sandstones, siliceous mudstones interbedded with claystones, and limestones. The strata trend NW-SE and dip to the north or south. These strata are comparable with the strata exposed in the Gyanze area and subdivided into the Weimei Formation, the Rilang Formation, the Duobeng Formation, the Chuangde Formation, and the Denggang Formation (Du et al., 2015).
The ZN1 (Zhinadibu 1) section (29°46′33″N, 83° 16′40″E) is located in the southern part of the Mesozoic to Paleogene sedimentary strata (Figure 2). It is a succession of red siliceous siltstone and grayish-green claystone (Figures 3, 4). The strata trend NW-SE and dip to the northeast. The strata of the section measured in this paper display a continuous, northeast-dipping succession. Laminations are visible in radiolarian-bearing beds. However, no sedimentary structures indicative of younging direction were observed. Twenty-one samples were collected from the 70 m-thick section. An inferred fault is located between ZN1-13 and ZN1-14.
Rock samples were disaggregated by leaching in 4% hydrofluoric acid for 20–24 hours. Samples were sieved and the >74 µm fraction was examined. Light microscope, stereoscopic microscope, and scanning electron microscope were used for observation, picking up, and identification of radiolarians. Of the 21 samples, six (ZN1-4, ZN1-6, ZN1-15, ZN1-16, ZN1-20, and ZN1-21) contain moderately to well preserved radiolarians. The fossil specimens reported in this paper are deposited in the Department of Geology, Niigata University.
Radiolarian assemblages and age assignments
Of the six samples, which contain moderately to well preserved radiolarians, two (ZN1-4 and ZN1-6) contain a radiolarian assemblage which is comparable with the Costata Subzone of the Turbocapsula Zone (O'Dogherty, 1994), indicating an Early Cretaceous (Aptian) age. Radiolarian assemblages from Lower Cretaceous strata in our research area have been reported by Li et al. (2017). The other four samples (ZN1-15, ZN1-16, ZN1-20, and ZN1-21) yield late Paleocene radiolarian assemblages (Figure 4).
Radiolarian zonal schemes for the Paleocene and lower Eocene are summarized below (Figure 5). Foreman (1973) defined four radiolarian zones for the uppermost Paleocene to lower-middle Eocene based on samples from the Gulf of Mexico: the Bekoma bidartensis Zone, the Buryella clinata Zone, the Phormocyrtis striata striata Zone, and the Theocotyle cryptocephala Zone (from the oldest to youngest). The stratigraphic interval, which is lower than the Bekoma bidartensis Zone, was expressed as “unzoned interval”. Nishimura (1987, 1992) defined this interval as the Bekoma campechensis Zone and subdivided it into three subzones based on materials from the northwest Atlantic. Hollis (1993, 1997, 2002) proposed a zonation for the lower Paleocene to middle Eocene in the mid-latitude of the South Pacific. Sanfilippo and Nigrini (1998a, b) proposed a tropical radiolarian zonation. Jackett et al. (2008) established an upper Paleocene to lower Eocene low-latitude radiolarian zonation based on re-sampled materials from DSDP Leg 43 Site 384 (Nishimura, 1992), ODP Leg 171B Hole 1051A (Sanfilippo and Blome, 2001), and DSDP Leg 10 Sites 86, 94, 95, and 96 (Foreman, 1973; Sanfilippo and Riedel, 1973). Norris et al. (2014) updated the low-latitude radiolarian zonal scheme of the lowermost Eocene through Upper Cretaceous (Sanfilippo and Nigrini, 1998b; Hollis, 2002) to the geological timescale 2012 (Gradstein et al., 2012) based on correlation with calcareous microfossil datums. These publications on the Paleocene to lower Eocene radiolarian biostratigraphy (Figure 5) provide a sufficient standard for comparison and age assignment. In this study, the age assignment is based on low-latitude radiolarian zonation that was updated by Norris et al. (2014). Radiolarian assemblages in this study are comparable with the Bekoma campechensis Zone (RP6). The base of the RP6 is defined by the first appearance (FA) of Bekoma campechensis Foreman. The top of this zone is defined by the evolutionary transition from B. campechensis Foreman to Bekoma bidartensis Riedel and Sanfilippo (Nishimura, 1987, 1992; Sanfilippo and Nigrini, 1998b). The RP6 is divided into three subzones by the differences of the associated species. Our radiolarian assemblages are assigned to the zonal scheme according to the co-occurrence of some diagnostic species instead of zonal markers. The composite stratigraphic ranges of the diagnostic species referred to in this study are based on Foreman (1973), Nishimura (1992), and Sanfilippo and Nigrini (1998a, b).
Figure 4.
Columnar section with radiolarian range chart (scale bar is 50 µm). Zonation of Early Cretaceous radiolarians is based on O'Dogherty (1994) and zonation of late Paleocene radiolarians is based on Sanfilippo and Nigrini (1998b).
Sample ZN1-15 yields Lychnocanoma? costata Nishimura, Bekoma? cf. demissa robusta Nishimura, Phormocyrtis striata praexquisita Nishimura, Phormocyrtis turgida (Krasheninnikov), Lychnocanium? pyramis Li and Matsuoka sp. nov., Pterocyrtidium sinense Li and Matsuoka sp. nov., Buryella tetradica Foreman s.s., Buryella tetradica Foreman var. A, Buryella pentadica Foreman, Buryella sp. A, Stylosphaera coronata coronata Ehrenberg, Stylosphaera coronata macrosphaera Nishimura, Stylosphaera goruna Sanfilippo and Riedel, Amphisphaera minor (Clark and Campbell), Orbula sp. A, Stylotrochus sp., Cromyechinus sp., and Tripocalpis sp. (Figures 4, 6–9). Buryella tetradica Foreman s.s., B. pentadica Foreman, S. coronata coronata Ehrenberg, and S. goruna Sanfilippo and Riedel are common species during the late Paleocene to the early Eocene. Phormocyrtis striata praexquisita Nishimura and L.? costata Nishimura are species restricted to the Bekoma campechensis Zone, RP6a Subzone of Sanfilippo and Nigrini (1998b), indicating an age of 61.5-60.4 Ma (Norris et al., 2014) (Figure 10).
Sample ZN1-16 yields Bekoma? cf. demissa robusta Nishimura, B.? cf. demissa demissa Foreman, Phormocyrtis striata praexquisita Nishimura, Lychnocanoma carinatum Ehrenberg, Phormocyrtis turgida (Krasheninnikov), Pterocyrtidium sinense Li and Matsuoka sp. nov., Lychnocanium? pyramis Li and Matsuoka sp. nov., Stylosphaera coronata coronata Ehrenberg, S. coronata macrosphaera Nishimura, Pseudostaurosphaera? aff. perelegans Krasheninnikov, and S. goruna Sanfilippo and Riedel (Figures 4, 6–9). Stylosphaera coronata coronata Ehrenberg, S. coronata macrosphaera Nishimura, and S. goruna Sanfilippo and Riedel are common species during the late Paleocene to the early Eocene. The occurrence of Phormocyrtis striata praexquisita Nishimura indicates that the radiolarian assemblage is comparable with the Bekoma campechensis Zone, RP6a Subzone of Sanfilippo and Nigrini (1998b) with an age of 61.5–60.4 Ma (Norris et al., 2014) (Figure 10).
Figure 5.
Correlations of Paleocene radiolarian, planktonic foraminifera, and calcareous nannofossil zonal schemes to the geochronologic time scale of Gradstein et al. (2012). Modified from Norris et al. (2014) and Sanfilippo and Nigrini (1998a, b).
Figure 6.
Scanning electron microscopic images of Paleocene radiolarians. 1, 2, Phormocyrtis turgida (Krasheninnikov), ZN1-15; 3–5, Pterocyrtidium sinense Li and Matsuoka sp. nov., transitional forms between P. turgida (Krasheninnikov) and P. sinense Li and Matsuoka sp. nov., ZN1-15; 6–8, Pterocyrtidium sinense Li and Matsuoka sp. nov.; 6, holotype, ZN1-16; 7, paratype, ZN1-20; 8, ZN1-21; 9, 10, Pterocyrtidium aff. sinense Li and Matsuoka sp. nov.; 9, ZN1-20; 10, ZN1-21; 11, Lychnocanium? pyramis Li and Matsuoka sp. nov., holotype; 11b, basal view, showing the constricted aperture on the fifth segment, ZN1-15; 12–14, Lychnocanium? stypticum Li and Matsuoka sp. nov., ZN1-21; 14, holotype; 15, Buryella tetradica Foreman var. A, ZN1-15; 16, Lychnocanoma sp., ZN1-20; 17, Lychnocanoma anacolum Foreman, ZN1-20; 18, Lychnocanium carinatum Ehrenberg, ZN1-20; 19, Phormocyrtis striata praexquisita Nishimura, ZN1-15; 20, Orbula sp. A, ZN1-15; 21, Bekoma? cf. demissa robusta Nishimura, ZN1-15; 22, Bekoma? cf. demissa demissa Foreman, ZN1-20; 23, Lychnocanoma? costata Nishimura, ZN1-15; 24, Stylosphaera coronata macrosphaera Nishimura, ZN1-15; 25, 26, Stylosphaera coronata coronata Ehrenberg; 25, ZN1-15; 26, ZN1-20; 27, Stylosphaera goruna Sanfilippo and Riedel, ZN1-15. Arrows indicate small wings.
Figure 7.
Scanning electron microscopic images of Paleocene radiolarians. 1, Pseudostaurosphaera? aff. perelegans Krasheninnikov, ZN1-20; 2, Pseudostaurosphaera sp., ZN1-20; 3, 4, Stylotrochus sp., ZN1-15; 5, Amphisphaera minor (Clark and Campbell), ZN1-15; 6, Cromyechinus sp., ZN1-15; 7, Tripocalpis sp., ZN1-15.
Sample ZN1-20 yields Bekoma? cf. demissa robusta Nishimura, B.? cf. demissa demissa Foreman, Lychnocanoma carinatum Ehrenberg, Lychnocanoma anacolum Foreman, Phormocyrtis turgida (Krasheninnikov), Lychnocaniuml stypticum Li and Matsuoka sp. nov., Pterocyrtidium sinense Li and Matsuoka sp. nov., P. aff. sinense Li and Matsuoka sp. nov., Stylosphaera coronata coronata Ehrenberg, S. coronata macrosphaera Nishimura, S. goruna Sanfilippo and Riedel, Pseudostaurosphaera? aff. perelegans Krasheninnikov, and Stylotrochus sp. (Figures 4, 6–9). The co-occurrence of Lychnocanoma anacolum Foreman, Stylosphaera coronata coronata Ehrenberg, and S. coronata macrosphaera Nishimura indicates that the radiolarian assemblage is restricted to the Bekoma campechensis Zone of Sanfilippo and Nigrini (1998b) with an age range of 61.5–58.23 Ma (Norris et al., 2014) (Figure 10).
Sample ZN1-21 yields Bekoma? cf. demissa demissa Foreman, Phormocyrtis turgida (Krasheninnikov), Lychnocanoma carinatum Ehrenberg, Lychnocanium? pyramis Li and Matsuoka sp. nov., Lychnocanium? stypticum Li and Matsuoka sp. nov., Pterocyrtidium sinense Li and Matsuoka sp. nov., P. aff. sinense Li and Matsuoka sp. nov., Pseudostaurosphaera? aff. perelegans Krasheninnikov, Stylosphaera coronata coronata Ehrenberg, S. coronata macrosphaera Nishimura, and S. goruna Sanfilippo and Riedel (Figures 4, 6–9). The co-occurrence of S. coronata coronata Ehrenberg and S. coronata macrosphaera Nishimura indicates the radiolarian assemblage is restricted to the Bekoma campechensis Zone of Sanfilippo and Nigrini (1998b) with an age of 61.5–58.23 Ma (Norris et al., 2014) (Figure 10).
Within the species stated above, the three new species (Lychnocanium? pyramis Li and Matsuoka sp. nov., L.? stypticum Li and Matsuoka sp. nov., and Pterocyrtidium sinense Li and Matsuoka sp. nov.) that occur in our samples (Figures 4, 6–8) have been sparsely reported in previous studies of other areas.
Discussion
Tectonic attributes and distribution
In the previous 1:250,000 geological map, the strata located to the south of the Yarlung-Tsangpo River near Zhongba County was mapped as Upper Cretaceous to middle Eocene sediments. As the map shows, upper Paleocene strata of the Denggang Formation are widely distributed in this area. According to our field investigations and radiolarian analyses, the strata are divided into the Weimei Formation (quartz sandstones), the Rilang Formation (volcaniclastic sandstones), the Duobeng Formation (siliceous mudstones interbeded with claystones), the Chuangde Formation (red limestones), and the Denggang Formation (siliceous siltstone and claystone). Geochronological data show that the youngest detrital zircons in the lowest volcaniclastic sandstones of the Rilang Formation have a U-Pb age of (134 ±4 Ma) (Du et al., 2015). Radiolarians of Late Jurassic and Early Cretaceous age can be found in the upper part of the Rilang Formation and the Duobeng Formation (Li et al., 2017). The majority of the strata are sediments belonging to these four Mesozoic formations instead of Upper Cretaceous to middle Eocene flysches. The upper Paleocene radiolarian-bearing siliceous mudstones, as the map shows, are not widely distributed, but only preserved in the southern part of these four formations. The upper Paleocene siliceous mudstone (20 m thick) is in fault contact with the Lower Cretaceous siliceous mudstone (Figures 3, 4).
Figure 8.
Light microscopic images of Paleocene radiolarians. 1, 2, Buryella pentadica Foreman, ZN1-15; 3–5, Buryella sp. A, ZN1-15; 6, Buryella tetradica Foreman s.s., ZN1-15; 7–10, Buryella tetradica Foreman var. A, ZN1-15; 11–15, Buryella sp., 11–13, ZN1-20; 14, 15, ZN1-21; 16, 17, Phormocyrtis turgida (Krasheninnikov), ZN1-15; 18–23, Pterocyrtidium sinense Li and Matsuoka sp. nov.; 18, ZN1-16; 19–23, ZN1-20.
Figure 9.
Light microscopie images of Paleocene radiolarians. 1–4, Lychnocanium?pyramis Li and Matsuoka sp. nov., ZN1-15; 4, paratype; 5–10, Lychnocanium?stypticum Li and Matsuoka sp. nov.; 5, ZN1-20; 6–10, ZN1-21; 7, paratype; 11, Pterocyrtidium aff. sinense Li and Matsuoka sp. nov., ZN1-21; 12, Lychnocanoma? costata Nishimura, ZN1-15; 13, Stylosphaera coronata coronata Ehrenberg, ZN1-20; 14, Stylosphaera coronata macrosphaera Nishimura, ZN1-20; 15, Stylotrochus sp., ZN1-20.
Depositional environment of upper Paleocene radiolarian-bearing strata
Radiolarians are planktonic protozoa that are widely distributed in the oceans, throughout the water column from the near surface to the bottom.
Upper Paleocene radiolarian-bearing strata are the latest marine sediments recorded near Zhongba County. Mesozoic to Paleogene deep-marine sediments (Liu and Einsele, 1994) change southward into shallow-water calcareous and terrigenous rocks (Willems et al., 1996; Ding, 2003) on the Indian passive margin. The upper Paleocene radiolarian-bearing strata near Zhongba County stand for deep-marine sediments.
Liu and Aitchison (2002) remarked that the radiolarian faunas from the Yamdrok mélange correspond to the Bekoma campechensis Zone of Nishimura (1992). Ding (2003) assigned radiolarian assemblages from the Zheba section as the Buryella tetradica Zone (RP5) and the Bekoma campechensis Zone (RP6), inferring a time interval of 61–55 Ma. Liang et al. (2012) assigned the radiolarian assemblage from the Jiazhu section to the Buryella pentadica Zone (RP6) of Hollis (2002), indicating an equivalent age of 59–56.5 Ma. Radiolarian assemblages from the three locations are comparable with the Bekoma campechensis Zone (RP6) of Sanfilippo and Nigrini (1998b), indicating an age range of 61.5–58.23 Ma based on the updated zonal scheme of Norris et al. (2014) (Figures 5, 10).
The Jiazhu section is 4 km west of the ZN1 section, which is 200 km west of the Zheba section (Figure 1). The ZN1 section and Jiazhu section (Liang et al., 2012) consist of siliceous mudstones and cherts, which lack any calcareous and coarse-grained terrestrial materials. These strata stand for deep-marine sediments deposited below the calcium carbonate compensation depth (CCD). The Zheba section consists of gray-white cherts, porcellanites, siliceous shales, siliceous limestones, and conglomerates. The coexistence of limestones and radiolarian-bearing porcellanites is evidence that sediments near Zheba were accumulated in a relatively shallower depth near the CCD. The conglomerate might be related to the initial collision between the Indian and the Eurasian continents. The lack of coarse-grained terrestrial sediments near Zhongba provides additional information that the marine basin here was more distal to the continents than that near Zheba.
Further to the east near Gyangze, olistostromes and chaotic deposits, which contain clasts from the passive continental margin, the oceanic crust, and the active continental margin, were accumulated during the late Paleocene (Liu and Einsele, 1996; Liu and Aitchison, 2002). The olistostromes and chaotic deposits stand for the trench basin sediments developed during the collision.
The lateral variations in the lithology indicate that the strata near Zhongba, Zheba, and Gyangze were deposited in marginal areas with different water depths and distances to the continents during the late Paleocene.
Conclusions
Radiolarian assemblages from a deep-marine succession of siliceous siltstone and claystone can be correlated to the Bekoma campechensis Zone (RP6) of Sanfilippo and Nigrini (1998b) with an age range of 61.5–58.23 Ma. This succession represents a record of deep-marine sediments on the Indian passive margin near Zhongba County. The distribution of the late Paleocene radiolarian-bearing strata is limited to the southern part of the deep-marine sediments. The radiolarian assemblages of the Yamdrok mélange (Liu and Aitchison, 2002), the Zheba section (Ding, 2003), and the Jiazhu section (Liang et al., 2012) indicate the same age as those of the ZN1 section. The sediments were deposited below the CCD near Zhongba. Compared with coeval sediments of the NTH, the deep-marine realm near Zhongba was deeper and more distal than that near Saga and Gyanze.
Systematic paleontology
Full description is given for new species and species for which taxonomic clarification is required. For other species, reference to the author, the first illustration, and the currently adopted species concept are given. Genera and species are arranged in alphabetical order within orders Spumellaria and Nassellaria, respectively.
Superorder Polycystina Ehrenberg, 1838, emend. Riedel, 1967
Order Spumellaria Ehrenberg, 1875
Genus Amphisphaera Haeckel, 1881, emend.
Petrushevskaya, 1975
Type species.—Amphisphaera neptunus Haeckel, 1887.
Amphisphaera minor (Clark and Campbell, 1942)
Figure 7.5
Stylosphaera minor Clark and Campbell, 1942, p. 27, pl. 5, figs. 1, 2, 12.
Amphisphaera minor (Clark and Campbell). Sanfilippo and Riedel, 1973, p. 486, pl. 1, figs. 1–5, pl. 22, fig. 4.
Remarks.—Our specimen is identified as A. minor (Clark and Campbell) by having a shell with two opposite similar basally three-bladed, narrow, needlelike polar spines. The polar spines of the specimen examined here are broken. The cortical shell of our specimen is larger than those reported by Clark and Campbell (1942), which is 100 µm in diameter.
Occurrence.—Upper Paleocene in North Atlantic (Nishimura, 1987; Jackett et al., 2008; Liu et al., 2011), Gulf of Mexico (Sanfilippo and Riedel, 1973), New Zealand (Hollis, 2006), South Africa (Jannou, 2007), California (Clark and Campbell, 1942), and Tibet.
Genus Pseudostaurosphaera Krasheninnikov, 1960
Type species.—Pseudostaurosphaera perelegans Krasheninnikov, 1960.
Pseudostaurosphaera? aff. perelegans Krasheninnikov, 1960
Figure 7.1
Pseudostaurosphaera perelegans Krasheninnikov, 1960, p. 276, pl. 1, fig. 6.
Pseudostaurosphaera? aff. perelegans Krasheninnikov. Nishimura, 1992, pl. 1, fig. 5.
Remarks.—The specimens in this study should be conspecific with Pseudostaurosphaera? aff. perelegans Krasheninnikov reported by Nishimura (1992, pl. 1, fig. 5). Specimens of this species in Tibet reported by Liang et al. (2012, fig. 3a, 3b) were assigned to Hexacontium? palaeocenicum Sanfilippo and Riedel, 1973. This species differs from P. perelegans Krasheninnikov and H.? palaeocenicum Sanfilippo and Riedel by having more numerous and smaller pores.
Occurrence.—Upper Paleocene in Northeast Atlantic (Nishimura, 1992) and Tibet (Liang et al., 2012).
Genus Stylosphaera Ehrenberg, 1847
Type species.—Stylosphaera hispida Ehrenberg, 1854 (designated by Campbell, 1954).
Stylosphaera coronata coronata Ehrenberg, 1873
Figures 6.25, 6.26, 9.13
Stylosphaera coronata Ehrenberg, 1873, p. 258; Ehrenberg, 1875, pl. 25, fig. 4.
Stylosphaera coronata coronata Ehrenberg. Sanfilippo and Riedel, 1973, p. 520, pl. 1, figs. 13–17, pl. 25, fig. 4.
Remarks.—Our specimens are identified as this subspecies by having a spherical or elliptical outer shell with two three-bladed polar spines.
Occurrence.—Upper Paleocene in North Atlantic (Nishimura, 1992; Jackett et al., 2008; Liu et al., 2011), Japan (Oyaizu et al., 2002), New Zealand (Hollis, 2006), and Tibet (Liu and Aitchison, 2002; Liang et al., 2012).
Stylosphaera coronata macrosphaera Nishimura, 1992
Figures 6.24, 9.14
Stylosphaera coronata macrosphaera Nishimura, 1992, p. 325, pl. 1, figs. 3, 4, pl. 11, fig. 1.
Remarks.—Specimens examined here identified as S. coronata macrosphaera Nishimura differ from S. coronata coronata Ehrenberg by having a larger outer shell and shorter polar spines.
Occurrence.—Upper Paleocene in North Atlantic (Nishimura, 1992; Jackett et al., 2008; Liu et al., 2011), and Tibet (Liu and Aitchison, 2002; Liang et al., 2012).
Stylosphaera goruna Sanfilippo and Riedel, 1973
Figure 6.27
Stylosphaera goruna Sanfilippo and Riedel, 1973, p. 521, pl. 1, figs. 20–22, pl. 25, figs. 9, 10.
Remarks.—Our specimens are identified as S. goruna Sanfilippo and Riedel by having a cortical shell with two or more spines at or near each pole. This species includes variable forms with different size and thickness of the outer shell and different length of radial spines.
Occurrence.—Upper Paleocene and lower Eocene in North Atlantic (Nishimura, 1992; Jackett et al., 2008; Liu et al., 2011), Japan (Nanayama, 1992; Oyaizu et al., 2002), Canary Islands (Bustillo et al., 1994), New Zealand (Hollis and Hanson, 1991; Hollis, 1993; Strong et al., 1995), South Africa (Jannou, 2007), and Tibet (Liu and Aitchison, 2002; Liang et al., 2012).
Order Nasselaria Ehrenberg, 1875
Genus Bekoma Riedel and Sanfilippo, 1971
Type species.—Bekoma bidartensis Riedel and Sanfilippo, 1971.
Bekoma? cf. demissa demissa Foreman, 1973
Figure 6.22
Bekoma? demissa Foreman, 1973, p. 432, pl. 3, fig. 22, pl. 10, fig. 5.
Bekoma? demissa Foreman. Nishimura, 1992, p. 333, pl. 6, fig. 1.
Remarks.—Specimens examined in this study are doubtfully identified as Bekoma? cf. demissa demissa by lacking typical long conical feet that extend out from the lower part of the thorax. Specimens examined in this study is similar to B.? cf. demissa demissa in having a hemispherical cephalis and a campanulate, thick-walled thorax with small pores regularly arranged.
Occurrence.—Upper Paleocene in Tibet.
Bekoma? cf. demissa robusta Nishimura, 1992
Figure 6.21
Bekoma? demissa robusta Nishimura, 1992, p. 333, pl. 6, figs. 2, 3.
Remarks.—Our specimens are doubtfully identified as Bekoma? cf. demissa robusta by lacking typical long conical feet that extend out from the lower part of the thorax. This specimen is different form B.? cf. demissa demissa (Figure 6.21) by having a rather campulate, not roundish thorax.
Occurrence.—Upper Paleocene in Tibet (Liang et al., 2012).
Genus Buryella Foreman, 1973
Type species.—Buryella tetradica Foreman, 1973.
Buryella pentadica Foreman, 1973
Figures 8.1, 8.2
Buryella pentadica Foreman, 1973, p. 433, pl. 8, fig. 8, pl. 9, figs. 15, 16.
Remarks.—This species differs from Buryella tetradica Foreman s.s. by the number and size of segments. Buryella pentadica has an inverted truncate-conical fifth segment, while B. tetradica has four segments. They both have a small cephalis. The thorax of B. pentadica is smaller than that of B. tetradica Foreman s.s. The abdomen of B. pentadica is truncated-conical, while the abdomen of B. tetradica Foreman s.s. is inflated cylindroidal. The fourth segment of B. pentadica is similar to the abdomen of B. tetradica Foreman s.s. The fourth segment of B. tetradica is inverted truncate-conical. The thorax and abdomen of this species is smaller compared with B. tetradica Foreman s.s., B. tetradica Foreman var. A, and B. sp. A. The fourth segment is more inflated and bigger than that of B. tetradica Foreman s.s., B. tetradica Foreman var. A, and B. sp. A.
Occurrence.—Upper Paleocene in Gulf of Mexico (Foreman, 1973), North Atlantic (Nishimura, 1992; Liu et al., 2011), South Pacific (O'Connor, 2001), Cuba (Sanfilippo et al., 1999), Canary Islands (Bustillo et al., 1994), and Tibet (Ding, 2003; Liang et al., 2012).
Buryeiia tetradica Foreman, 1973 sensu stricto
Figure 8.6
Buryella tetradica Foreman, 1973, p. 433, pl. 8, figs. 4, 5, pl. 9, figs. 13, 14.
Buryella tetradica Foreman s.s. Sanfilippo and Blome, 2001, p. 210.
Remarks.—Buryella tetradica Foreman s.s. and B. tetradica Foreman var. A are two subspecies of B. tetradica. These two subspecies have similar cephalis, thorax, and abdomen.
Occurrence.—Upper Paleocene to lower Eocene in Gulf of Mexico (Foreman, 1973; Jackett et al., 2008), South Pacific (O'Connor, 2001), North Atlantic (Nishimura, 1992; Sanfilippo and Blome, 2001; Liu et al., 2011), Cuba (Sanfilippo et al., 1999), northern Cyprus (Sanfilippo et al., 2003), Canary Islands (Bustillo et al., 1994), New Zealand (Hollis, 2006; Strong et al., 1995), and Tibet (Liang et al., 2012).
Buryella tetradica Foreman var. A
Figures 6.15, 8.7–8.10
Bwyella tetradica Foreman var. A. Sanfilippo and Blome, 2001, p. 210, figs. 8d, 8e.
Remarks.—Buryella tetradica Foreman var. A differs from B. tetradica Foreman s.s. by having a fourth segment which is transversely segmented. The differences of this species from B. pentadica Foreman and B. sp. A are described under those species.
Occurrence.—Upper Paleocene in North Atlantic (Sanfilippo and Blome, 2001) and Tibet.
Buryella sp. A
Figures 83–8.5, 8.11–8.15
Buryella sp. A. Jackett et al., 2008, p. 47, pl. 1, figs. 21–23.
Remarks.—This species differs from Buryella tetradica Foreman s.s. by having five segments. This species differs from B. tetradica Foreman var. A and B. pentadica Foreman by the third and fourth segments displaying similar proportions. The third segment of B. tetradica Foreman var. A is more inflated and larger than the fourth segment. The third segment of B. pentadica Foreman is smaller than the fourth segment.
Occurrence.—Upper Paleocene in Northwest Atlantic (Jackett et al., 2008) and Tibet.
Genus Lychnocanium Ehrenberg, 1847
Type species.—Lychnocanium falciferum Ehrenberg, 1854.
Lychnocanium carinatum Ehrenberg, 1875
Figure 6.18
Lychnocanium carinatum Ehrenberg, 1876, p. 78, pl. 8, fig. 5.
Remarks.—For the specimen of Ehrenberg (1875), there are no pores in the upper part of the thorax. Our specimen has pores and costae in the whole thorax.
Occurrence.—Upper Paleocene to lower Eocene in Northeast Atlantic (Liu et al., 2011), Gulf of Mexico (Jackett et al., 2008), and Tibet (Liang et al., 2012).
Lychnocanium? pyramis Li and Matsuoka sp. nov.
Figures 6.11, 9.1–9.4
Lithochytris sp. Oyaizu et al., 2002, fig. 6.11.
Diagnosis.—Five-segmented Nassellaria characterized by having a trigonal pyramidal shell with an aperture. Pores are circular to subcircular, variable in size, and closely spaced on the post-cephalic segments. Three solid ribs extend externally from the lower part of the thorax.
Description.—Trigonal-pyramidal shell with five segments. Cephalis small, conical, with a short apical horn. Pores small, circular to subcircular, irregularly distributed. Collar stricture not expressed externally. Thorax truncated pyramidal. Lumbar stricture slightly distinct in contour. Abdomen and the fourth segment truncated-pyramidal. The fifth segment inverted truncated-pyramidal with a constricted triangular aperture on more complete specimens (Figure 6.11b). The fifth segment very delicate, generally only seen as small remnants (Figures 9.2, 9.3). All the post-cephalic segments with circular to subcircular, variable in size, and closely spaced pores. All the segments except the fifth one increase their size gradually and constitute a large trigonal pyramid. Three solid ribs distally diverging and externally visible from the lower part of the thorax along the edges of the abdomen and the fourth segment. As the fifth segment is inverted truncated-pyramidal and constricted distally, the three ribs extend below the fourth segment as three short feet.
Measurements (in µm, based on 6 specimens).—Total height, 131–163 (mean, 147; holotype, 157; paratype, 136); Height of cephalis, 18–25 (mean, 22; paratype, 18); Height of thorax, 15–21 (mean, 19; paratype, 20); Height of abdomen, 27–43 (mean, 34; paratype, 27); Height of the fourth segment, 28–36 (mean, 33; paratype, 35); Maximum width, 119–160 (mean, 131; holotype, 160; paratype, 133); Angle between two ribs in lateral view, 41°–56° (mean, 48°; holotype, 56°; paratype, 48°).
Remarks.—This species is questionably assigned to the genus Lychnocanium, because of its special trigonal-pyramidal morphology, more segments, and smaller thorax. This species is distinguished from species of the genus Lithochytris by having an open aperture. This species is distinguished from Dictyocephalus middouri Nishimura, 1992 by having a trigonal-pyramidal shell, constricted fifth segment, and closely spaced pores on the post-cephalic segments. This species has a close phyletic relationship with Lychnocanium? stypticum Li and Matsuoka sp. nov. Their relationship is described under the latter species.
Etymology.—This species name is derived from Greek pyramis, meaning the pyramids of Egypt.
Type specimens.—Holotype NU MR 0120 (Figure 6.11); paratype NU MR 0121 (Figure 9.4).
Occurrence.—Upper Paleocene in Japan (Oyaizu et al., 2002) and Tibet.
Lychnocanium? stypticum Li and Matsuoka sp. nov.
Figures 6.12–6.14, 9.5–9.10
Diagnosis.—Five-segmented Nassellaria characterized by possessing a trigonal pyramidal shell with an aperture. The lumbar stricture is distinct in contour. Pores are circular to subcircular, variable in size, and closely spaced on the post-cephalic segments. Three solid ribs are externally visible from the lower part of the thorax.
Description.—Trigonal-pyramidal shell with five segments. Cephalis small conical, with a short apical horn, generally sooth surfaced. Pores, if present, small in number, circular to subcircular, irregularly distributed. Collar stricture slightly distinct in contour. Thorax inflated truncated-conical. Lumbar stricture marked by a distinct change in contour. Abdomen and the fourth segment truncated-pyramidal. The fifth segment very delicate, generally only seen as small remnants (Figures 9.5, 9.7). All the post-cephalic segments with circular to subcircular, variable in size, and closely spaced pores. All the segments except the fifth one increase their width gradually and constitute a large trigonal pyramid. Three solid ribs externally visible from the lower part of the thorax to the distal postabdomen. Shell terminated in a ragged margin.
Measurements (in µm, based on 24 specimens).—Total height, 115–173 (mean, 143; holotype, 166; paratype, 148); Height of cephalis, 17–25 (mean, 22; paratype, 22); Height of thorax, 29–42 (mean, 34; paratype, 29); Height of abdomen, 27–45 (mean, 36; paratype, 37); Height of the fourth segment, 25–40 (mean, 34; paratype, 38, based on 8 specimens); Maximum width, 86–137 (mean, 109; holotype, 122; paratype, 125); Angle between two ribs in lateral view, 23°–42° (mean, 34°; holotype, 42°; paratype, 38°).
Remarks.—The wall of the fourth and fifth segments is fragile and not preserved in some specimens (Figures 9.9, 9.10). This species is distinguished from Lychnocanium? pyramis Li and Matsuoka sp. nov. by having a larger thorax that is obviously inflated, well expressed lumbar stricture in contour, and smaller angle between two ribs in lateral view. This species differs from Lychnocanium auxilla Foreman in having five segments, a robust abdomen that is generally preserved, and thoracic ribs which are slightly visible. This species differs from Dictyocephalus middouri Nishimura in having a cross section which is triangular instead of circular and having pores which are smaller and closer distributed. This species may have evolved from L.? pyramis Li and Matsuoka sp. nov. by developing a larger and more inflated thorax and constricting the angles between adjacent ribs.
Etymology.—This species name is derived from Latin stypticus, meaning astringent.
Type specimens.—Holotype NU MR 0122 (Figure 6.12); paratype NU MR 0123 (Figure 9.7).
Occurrence.—Upper Paleocene in Tibet.
Genus Lychnocanoma Haeckel, 1887, emend. Foreman, 1973
Type species.—Lychnocanium clavigerum Haeckel, 1887 (designated by Campbell, 1954)
Lychnocanoma anacolum Foreman, 1973
Figure 6.17
Lychnocanoma anacolum Foreman, 1973, p. 437, pl. 1, figs. 19, pl. 11, fig. 7.
Remarks.—Our specimens are identified as L. anacolum Foreman by having a thorax with large irregular pores, three-bladed feet with convexity outwards, and a horn.
Occurrence.—Upper Paleocene in Gulf of Mexico (Foreman, 1973) and Tibet.
Lychnocanoma? costata Nishimura, 1992
Figures 6.23, 9.12
Lychnocanoma? costata Nishimura, 1992, p. 342, pl. 6, figs. 4–6.
Remarks.—Specimens examined in this study are identified as L.? costata Nishimura by having well developed costae and pores regularly arranged in longitudinal rows on the wide thorax.
Occurrence.—Upper Paleocene in Northwest Atlantic (Nishimura, 1987, 1992) and Tibet (Liu and Aitchison, 2002; Ding, 2003).
Genus Phormocyrtis Haeckel, 1887
Type species.—Phormocyrtis longicornis Haeckel, 1887 (designated by Campbell, 1954).
Phormocyrtis striata praexquisita Nishimura, 1992
Figure 6.19
Phormocyrtis striata praexquisita Nishimura, 1992, p. 346, pl. 9, figs. 1–3.
Remarks.—Nishimura (1992) defined forms having well developed abdomen and convexly curved keels as this subspecies. This subspecies is located in a lower horizon than Phormocyrtis striata exquisita (Kozlova, 1966) (Foreman, 1973).
Occurrence.—Upper Paleocene in Northwest Atlantic (Nishimura, 1987, 1992) and Tibet.
Phormocyrtis turgida (Krasheninnikov, 1960)
Figures 6.1, 6.2, 8.16, 8.17
Lithocampe turgida Krasheninnikov, 1960, p. 301, pl. 3, fig. 17.
Phormocyrtis turgida (Krasheninnikov). Foreman, 1973, p. 438, pl. 7, fig. 10, pl. 12, fig. 6; Nishimura, 1987, p. 727, pl. 2, fig. 12; Sanfilippo and Nigrini, 1998a, p. 273, pl. 13.2, fig. 3; Kozlova, 1999, ? pl. 15, fig. 7, pl. 20, figs. 13; Sanfilippo et al., 1999, pl. 2, fig. 6; Nishimura, 2001, pl. 6, figs. 5, 25; Liu and Aitchison, 2002, p. 150, pl. 1, fig. 23; Sanfilippo et al., 2003, p. 56, pl. 2, fig. 15; Hollis, 2006, pl.2, figs. 13, 14; Jackett et al., 2008, p. 57, pl. 2, fig. 15.
Phormocyrtis cf. turgida (Krasheninnikov). Kozlova, 1999, ? p. 149, pl. 18, fig. 21; pl. 46, figs. 7, 8.
Phormocyrtis striata striata (Kozlova. 1999). Cluzel et al., 2001, pl. 2. fig. D.
Phormocyrtis aff. turgida (Krasheninnikov). Nishimura, 2001, pl. 13, fig. 8.
Podocyrtis turgida Krasheninnikov. Ding, 2003, fig. 4.18.
Description.—Four-segmented, fusiform shell narrows in the aperture. No strictures expressed externally. Cephalis small, spherical, with a small and slightly bladed to conical apical horn. Pores circular to subcircular, various in size, and irregularly distributed. Collar stricture slightly distinct externally as change in contour. Thorax truncated conical. Lumbar stricture externally invisible. Abdomen large and inflated truncated-conical. The fourth segment inverted truncated-conical. Pores on the postcephalic segments, circular to subcircular, various in size, and with thick ribs between adjacent longitudinal rows of pores. Eleven to thirteen ribs in lateral view. Some ribs separate into two branches on the abdomen.
Remarks.—This species is assigned to the genus Phormocyrtis because it bears an apical horn and ribs from the thorax to the distal part. Specimens of this species from Tibet are characterized by having a larger cephalis, shorter thorax, and a more inflated overall shape than specimens reported in Foreman (1973, pl. 7, fig. 10; pl. 12, fig. 6) and Jackett et al. (2008, pl. 2, fig. 15), which are restricted to the early Eocene. Specimens of this species from Tibet are the same as the specimens reported by Nishimura (2001, pl. 6, figs. 5, 25; pl. 13, fig. 8), which are also of late Paleocene age. Our specimens differ from Podocyrtis papalis Ehrenberg, 1847 (Nishimura, 1992, pl. 10, fig. 1–2, pl. 13, fig. 18; Sanfilippo and Blome, 2001, fig. 10f) in having a less inflated overall shape and longer fourth segment. This species appears to be an early form of P. papalis Ehrenberg. This species has a close phyletic relationship with Pterocyrtidium sinense Li and Matsuoka sp. nov. Their relationship is described under that species.
Occurrence.—Upper Paleocene to lower Eocene in Cyprus (Sanfilippo et al., 2003), Gulf of Mexico (Foreman, 1973; Jackett et al., 2008), Atlantic (Nishimura, 1987), Southwest Pacific (Cluzel et al., 2001), Caucasus (Krasheninnikov, 1960), Russia (Kozlova, 1999), Cuba (Sanfilippo et al., 1999), New Zealand (Hollis, 2006), and Tibet (Liu and Aitchison, 2002; Ding, 2003).
Genus Pterocyrtidium Bütschli, 1882, sensu
Petrushevskaya and Kozlova, 1972
Type species.—Pterocanium barbadensis Ehrenberg, 1873 (designated by Petrushevskaya and Kozlova, 1972)
Pterocyrtidium sinense Li and Matsuoka sp. nov.
Figures 63–6.8, 8.18–8.23
Calocycletta tibeta Ding, 2003, figs. 4.4, 4.5; Liang et al., 2012, figs. 4.a, 4.b.
Diagnosis.—A Pterocyrtidium species characterized by possessing a four-segmented, fusiform shell. Three latticed, triangular wings extend outwards and downwards from the lower part of the abdomen. Thick ribs are distributed between adjacent longitudinal rows of pores on the post-cephalic segments.
Description.—Four-segmented, fusiform shell narrow distally. Cephalis small, spherical, sparsely perforate with circular to subcircular, various in size, and irregularly distributed pores. Apical horn small and slightly bladed to conical. Collar stricture externally distinct as change in contour. Thorax small truncated conical. Lumbar stricture usually externally invisible. Abdomen large and inflated truncated-conical. Three latticed, triangular wings spaced equally around the lower part of the abdomen, extending outwards and downwards. The fourth segment inverted truncated-conical and terminated in a ragged margin. Pores on the post-cephalic segments, circular to subcircular, various in size, and with thick ribs between adjacent longitudinal rows of pores. Eleven to thirteen ribs in lateral view. Some ribs separating into two branches on the abdomen.
Measurements (in µm, based on 35 specimens).—Total height, 147–227 (mean, 188; holotype, 193; paratype, 166); maximum width, 77–103 (mean, 91; holotype, 92; paratype, 88).
Remarks.—Pterocyrtidium sinense (Figure 6.8) differs from Pterocyrtidium genriettae Nishimura, 1992 by having obvious costae on the thorax and a shorter abdomen. This species is similar to Phormocyrtis turgida (Krasheninnikov) in overall shape, pore arrangement, and ribs. This species differs from P. turgida (Krasheninnikov) by having three latticed, triangular wings in the thorax. Pterocyrtidium sinense Li and Matsuoka sp. nov. can be regarded as a descendant of P. turgida (Krasheninnikov) tentatively and seems to be derived from the latter species by acquiring three latticed, triangular wings on the abdomen. Transitional forms between these two species have smaller wings than P. sinense Li and Matsuoka (Figures 6.3–6.5). These P. sinense Li and Matsuoka with smaller wings are found only in sample ZN1-15. Phormocyrtis turgida (Krasheninnikov) is dominant in this sample. In samples ZN1-16, ZN1-20, and ZN1-20, P. sinense Li and Matsuoka becomes dominant, while rare specimens of P. turgida (Krasheninnikov) are found.
Type specimens.—Holotype NU MR 0124 (Figure 6.6); paratype NU MR 0125 (Figure 6.7).
Occurrence.—Upper Paleocene in Tibet (Ding, 2003; Liang et al., 2012).
Pterocyrtidium aff. sinense Li and Matsuoka sp. nov.
Figures 6.9, 6.10, 9.11
Pterocyrtidium cf. genriettae Nishimura. Liang et al., 2012, figs, 11v, 11w.
Remarks.—This species is different from Pterocyrtidium sinense Li and Matsuoka sp. nov. by having a larger and more inflated cephalis. The apical horn of this species lacks grooves. The wings of this species are solid cylindrical instead of latticed triangular. This species differs from Pterocyrtidium genriettae Nishimura (1992, pl. 8, figs. 11, 12, pl. 13, fig. 21) by having more closely spaced pores and more prominent ribs between adjacent longitudinal rows of pores.
Occurrence.—Upper Paleocene in Tibet (Liang et al., 2012).
Acknowledgments
We sincerely thank Christopher Hollis, an anonymous reviewer, and the editor Yasunari Shigeta for carefully and critically reviewing our paper. This study was financially supported by the National Natural Science Foundation of China Grant Number 41572188 and the Project of Geological Survey of China Grant Number 1212011086037. This study was financially supported in part by the Japan Society for the Promotion of Science KAKENHI Grant Numbers 15K05329 and 15H02142E. We sincerely thank Li Xiaohan and Zhang Xiaolong (China University of Geosciences, Beijing) for their cooperation in the field work.
