,1,2,*, 朱敏1,2,3New material of thelodonts from Lochkovian (Lower Devonian) of Qujing, Yunnan, China
CUI Xin-Dong1,2,3, LI Qiang4, ZHU Min,1,2,*, ZHU Min1,2,3收稿日期:2019-04-25网络出版日期:2020-01-20
| 基金资助: |
Corresponding authors: *zhumin@ivpp.ac.cn
Received:2019-04-25Online:2020-01-20
摘要
采用高精度计算机断层扫描和三维重建技术,获取了大量来自云南下泥盆统洛霍考夫阶西屯组和西山村组下部花鳞鱼鳞片的三维重建模型。描述了花鳞鱼类的一个已知属——副花鳞鱼属(Parathelodus)的共7个种,三裂副花鳞鱼(P. trilobatus)、亚洲副花鳞鱼(P. asiaticus)、角状副花鳞鱼(P. cornuformis)、雅致副花鳞鱼(P. scitulus)、西屯副花鳞鱼(新种) (P. xitunensis sp. nov.)、王氏副花鳞鱼(新种) (P. wangi sp. nov.)和寥廓副花鳞鱼(新种) (P. liaokuoensis sp. nov.)。新材料的发现将副花鳞鱼的地层分布从洛霍考夫阶西屯组至西山村组上部延伸到西山村组下部,最低层位已非常靠近志留系与泥盆系的界线。西屯副花鳞鱼、王氏副花鳞鱼和廖廓副花鳞鱼3个新种的发现提高了我们对中国早泥盆世花鳞鱼类多样性的认识。
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Abstract
We reconstructed a wealth of three-dimensional virtual models of vertebrate micro-remains from the Xitun Formation and lower part of the Xishancun Formation (Lochkovian, Lower Devonian) of Qujing, Yunnan by means of high-resolution computed tomography. One genus (Parathelodus) and seven species (P. trilobatus, P. asiaticus, P. cornuformis, P. scitulus, P. xitunensis sp. nov., P. wangi sp. nov. and P. liaokuoensis sp. nov.) of thelodonts are described. The new discovery extends the stratigraphic distribution of Parathelodus from the Xitun Formation and the upper part of the Xishancun Formation to the lower part of the Xishancun Formation, the horizon close to the Silurian-Devonian boundary. The three new species, P. xitunensis, P. wangi and P. liaokuoensis, enhance our knowledge of the thelodont diversity from the Early Devonian of China.
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崔心东, 李强, 乔妥, 朱敏. 云南曲靖下泥盆统洛霍考夫阶花鳞鱼类新材料. 古脊椎动物学报[J], 2020, 58(1): 1-15 DOI:10.19615/j.cnki.1000-3118.190612
CUI Xin-Dong, LI Qiang, ZHU Min, ZHU Min.
1 Introduction
Thelodonts, a group of extinct agnathan (jawless) vertebrates, have been well studied since L. Agassiz in Murchison (1838) provided the first description of thelodonts based on disarticulated scales. However, most taxa of thelodonts are established on the basis of detached scales, and only 29 of 147 described thelodont species are represented by the articulated material (Turner, 1982; M?rss et al., 2007; Ferrón and Botella, 2017), which provides an important reference to the thelodont anatomy and the identification of disarticulated scales. Thelodont scales are useful to stratigraphic division and correlation because of their global distribution (Gross, 1947; M?rss, 1982; Karatajūt?-Talimaa and Predtechenskyj, 1995; M?rss et al., 1995; Turner, 1997; Sansom and Elliott, 2002; M?rss and Miller, 2004).So far, studies of the thelodonts from China are inadequate though there are extensive Middle Paleozoic strata that may potentially yield thelodont scales. Wang (1984) first described the thelodonts in China from the Xitun Formation, Lower Devonian, in Yunnan, based on micro-remains extracted by treatment with dilute acetic acid. The scales were referred to as Turinia asiatica including head, transitional and body scale types (Wang, 1984). Wang (1995,1997) erected Parathelodus with four new species, P. cornuformis, P. scitulus, P. catalatus and P. trilobatus, from the Xitun Formation and the upper part of the Xishancun Formation, and assigned Turinia asiatica (Wang, 1984:figs. 1, 2) as P. asiaticus.
Wang et al. (1986) described a new species, Turinia pagoda, from the Givetian Heyuanzhai Formation, and two morphological scale forms, Turinia sp. A and Turinia sp. B, from the Eifelian Malutang Formation of western Yunnan. Wang and Dong (1989) erected two Silurian thelodont species, Kawalepis comptus from the upper part of the Miaokao Formation and Thelodus sinensis from the upper part of the Kuanti Formation of eastern Yunnan. However, K. comptus with rhombic flat scales might be an osteichthyan taxon, and T. sinensis might be an endemic taxon reminiscent of Lanarkia scales, or broken placoderm tubercles or an acanthodian spine (M?rss et al., 2007; Zhao and Zhu, 2015). Accordingly, no definite thelodont has been known from the Silurian of China. Wang (1992) described Turinia sp. C and Nikoliviidae gen. indet. from the Emsian Yukiang Formation of Guangxi. Zhao et al. (2012) reported Parathelodus scitulus, P. asiaticus and P. cornuformis from the Lochkovian Xiaputonggou Formation of Sichuan. Zhu et al. (2015) systematically reviewed all the known thelodonts from China, including two genera, Parathelodus (five species) and Turinia (one species and three indeterminate species), and one indeterminate genus.
A wealth of early vertebrate fossils including numerous micro-remains were collected from the Xitun Formation and lower part of the Xishancun Formation (Fig. 1). The micro-remains from the Xitun Formation can be processed by the traditional method, the acid treatment. However, it is infeasible for the fossils from the siltstone beds of the Xishancun Formation. Hence, we resorted to the high-resolution computed tomography and three-dimensional (3D) virtual reconstruction to process these micro-remains and restored their virtual models. Here, we describe the thelodont micro-remains comprising Parathelodus trilobatus, P. asiaticus, P. cornuformis, P. scitulus, P. xitunensis sp. nov., P. wangi sp. nov. and P. liaokuoensis sp. nov.
Fig. 1
新窗口打开|下载原图ZIP|生成PPTFig. 1Locality map showing the fossil site and the fish-bearing horizons in this study
2 Material and methods
The specimens in this study include numerous thelodont scales from the muddy limestone of the Xitun Formation (late Lochkovian) and the siltstone of the lower part of the Xishancun Formation (early Lochkovian) near the northeast entrance of Liaokuo Park in Qujing, Yunnan, China (Fig. 1), in association with agnathans (Zhu, 1992), antiarchs (Zhu, 1996; Zhu and Janvier, 1996), petalichthyids (Zhu, 1991), and sarcopterygians (Zhu and Fan, 1995; Zhu and Schultze, 1997; Zhu et al., 1999). All of the specimens are housed at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), Chinese Academy of Sciences (CAS).The rock samples from the Xishancun Formation were cut into many cuboids, which were 1 cm long, 1 cm wide, and about 5 cm high. Then, they were scanned by the 225 kV micro-computerized tomography (developed by the Institute of High Energy Physics, CAS) at the Key Laboratory of Vertebrate Evolution and Human Origins, CAS. Each sample was scanned with beam energy of 100 kV and a flux of 100 μA at a resolution of 5.96 μm per pixel using a 360° rotation with a step size of 0.5°. A total of 720 projections were reconstructed in a 2048*2048 matrix of 1536 slices using the two-dimensional reconstruction software developed by the Institute of High Energy Physics, CAS.
Additional thelodont scales were found from the Xitun Formation by acid treatment, and then scanned by the Zeiss Xradia 520 Versa X-ray Microscope (Carl Zeiss X-ray Microscopy, Inc., Pleasanton, USA) at Yunnan Key Laboratory for Palaeobiology, Yunnan University, Kunming, China. The samples were scanned with scanning energy of 50 kV/4W at a resolution of 1.93 μm. 1353 TIFF images were output.
We reconstructed the 3D virtual models of thelodont scales by MIMICS (Materialise’s interactive medical image control system) 18.0.
3 Systematic paleontology
Subclass Thelodonti Jaekel, 1911Order Thelodontida Kiaer & Henitz, 1932
Family Coelolepidae Pander, 1856
Genus Parathelodus Wang, 1997
Type species Parathelodus scitulus Wang, 1997.
Diagnosis See the diagnosis by Wang (1997).
Parathelodus trilobatus Wang, 1997
(Figs. 2A, B, 4A, B)
Fig. 2
新窗口打开|下载原图ZIP|生成PPTFig. 23D models of Parathelodus trilobatus (A, B) and P. asiaticus (C-E)
A1, B1, C1, D1 and E1 in crown view; A2, B2, C2, D2 and E2 in lateral view; A3, B3, C3, D3 and E3 in basal view
A. IVPP V 12159.4; B. V 12159.5; C. V 25342.1; D. V 25342.2; E. V 25342.3. Scale bars = 0.1 mm
Fig. 4
新窗口打开|下载原图ZIP|生成PPTFig. 4Translucent 3D models of Parathelodus trilobatus (A, B), P. asiaticus (C), P. cornuformis (D), P. scitulus (E) and P. xitunensis sp. nov. (F)
A1, B1, C1, D1, E1 and F1 in crown view; A2, B2, C2, D2, E2 and F2 in lateral view
A. IVPP V 12159.4; B. V 12159.5; C. V 25342.2; D. V 25343.1; E. V 12156.10; F. V 26113.1 Scale bars = 0.1 mm
Holotype A complete body scale, IVPP V 12159.1 (Wang, 1984:figs. 3; pl. II, E).
Referred specimens IVPP V 12159.4-5.
Locality and horizon Qilin district, Qujing, Yunnan Province, China. Xitun Formation and Xishancun Formation, Lochkovian, Lower Devonian.
Diagnosis (emended) Elliptic scale crown with slightly bulged central part and notched posterolateral margin; flat or swollen base; straight pulp cavity with an opening.
Description Two new body scales (Fig. 2A, B) were identified. Their oval crown is slightly convex, of which the anterior edge is ornamented with weak tubercles and posterior edges with thorns (Fig. 2A1, B1). The neck is fairly high and distinct (Fig. 2A2, B2). Several large vertical ribs as well as many smaller ribs are present on the surface of the neck. The swollen base, irregular elliptical in shape, is smaller than the crown (Fig. 2A3, B3). The pulp cavity is straight and becomes thicker upwards (Fig. 4A, B). A small pulp cavity opening is positioned in the middle of the base (Fig. 2A3, B3).
Comparison The new specimens differ from the holotype (Wang, 1997) in having a more swollen base and lacking slender ridges on the anterior part of the crown. The holotype bears more prominent notches in the posterolateral margin than those of the new material. Compared with the other specimens, IVPP V 12159.4 (Fig. 2A) has a base that is much smaller than the crown. Furthermore, V 12159.4 (Fig. 4A) bears a more laterally compressed pulp cavity than V 12159.5 (Fig. 4B).
Parathelodus asiaticus Wang, 1984
(Figs. 2C-E, 4C)
Holotype A complete body scale, IVPP V 7215.3 (Wang, 1984:figs. 1E, 2D, F).
Referred specimens IVPP V 25342.1-5.
Locality and horizon Qilin district, Qujing, Yunnan Province, China. Xitun Formation and Xishancun Formation, Lochkovian, Lower Devonian.
Diagnosis (emended) Scale crown with small tubercles and fine or weak ridges on the anterior ledge, and several thorns on the posterolateral ledges; straight pulp cavity with an opening.
Description These new scales comprise a trunk scale (Fig. 2C) and four caudal scales (Fig. 2D, E). For the trunk scale, its pisiform crown has a downturned anterior edge with weak tubercles and an indented posterior edge with a large posterior cusp and 4 smaller thorns (Fig. 2C1). The neck is fairly high and distinct (Fig. 2C2). Four vertical ribs are present on the surface of the neck. The swollen base (Fig. 2C3), irregular elliptical in shape, is smaller than the crown. A small pulp cavity opening is positioned in the middle of the base (Fig. 2C3).
The caudal scale bears a rhombic or oval convex crown, which has indented posterolateral ledges and a posterior cusp (Fig. 2D1, E1). There is a pronounced ridge on the lower surface of the crown that extends to the neck (Fig. 2D3, E3). The neck is indistinct and becomes higher posteriorly (Fig. 2D2, E2). The base is rhombic or oval in shape and has a long anterior spur (Fig. 2D3, E3). The pulp cavity is thin, stretching out many branches forwards and upwards (Fig. 4C). According to the posterior extending crown, the pulp cavity curves backwards in the crown part and gradually becomes thinner (Fig. 4C). The pulp cavity opening is situated in the posterior part of the base (Fig. 2D3, E3).
Comparison The new specimens differ slightly from the previous material referred to as P. asiaticus (Wang, 1997) in having weaker tubercles or ridges and a smaller pulp cavity opening. As in P. catalatus and P. trilobatus, P. asiaticus bears an indented margin, while it is different from P. catalatus (Wang, 1997) in lacking laterally placed down-stepped rims. Like some trunk scales of Thelodus calvus (M?rss and Karatajūt?-Talimaa, 2002:fig. 14C, D), P. asiaticus has a slightly convex crown and a distinct neck, but differs from the former in having thorns on the posterolateral ledges and tubercles on the anterior ledge. The caudal scales resemble those of Thelodus trilobatus (M?rss and Miller, 2004) except for lacking ridges on the crown. In addition, the caudal scales are similar to those of P. trilobatus (Wang, 1997) except for a bulge on the anterior part of the base.
RemarksParathelodus asiaticus was first described as Turinia asiatica (Wang, 1984) and renamed as P. asiaticus based on some material presented by Wang (1997). The head scales described by Wang (1984:fig. 1A, B) might represent the trunk scale of Thelodus, not P. asiaticus. The caudal scales of P. asiaticus have large variations in their morphological features.
Parathelodus cornuformis Wang, 1997
(Figs. 3A, B, 4D)
Fig. 3
新窗口打开|下载原图ZIP|生成PPTFig. 33D models of Parathelodus cornuformis (A, B), P. scitulus (C) and P. xitunensis sp. nov. (D)
A1, B1, C1 and D1 in crown view; A2, B2, C2 and D2 in lateral view; A3, B3, C3 and D3 in basal view
A. IVPP V 25343.1; B. V 25343.2; C. V 12156.10; D. V 26113.1. Scale bars = 0.1 mm
Referred specimens IVPP V 25343.1-2.
Locality and horizon Qilin district, Qujing, Yunnan Province, China. Xitun Formation and Xishancun Formation, Lochkovian, Lower Devonian.
Diagnosis (emended) Horned scale in shape; high scale crown rising to a sharp posterior point and covered with long slender ridges; flat or swollen scale base.
Description Two new scales were identified (Fig. 3A, B). The horned crown, with prominent ridges, is high (Fig. 3A1, B1). The neck is smooth and distinct (Fig. 3A2, B2). The flat or bulged rhombic base has an anterior spur stretching out beyond the crown (Fig. 3A3, B3). The pulp cavity becomes thicker upwards to the neck part, then, it curves backwards and gradually becomes thinner (Fig. 4D). One or two very small pulp cavity openings are situated in the middle of the base (Fig. 3A3, B3).
Comparison The main difference between P. cornuformis and other species of Parathelodus is its horned and high crown (Fig. 3A1, B1). P. cornuformis are different from some species that have horned scales in the ornamentations of the crown (Gagnier et al., 1988; Karatajūt?-Talimaa, 2002; Sansom and Elliott, 2002; M?rss et al., 2002, 2006). Compared with the material of P. cornuformis (Wang, 1997), the new specimens are narrower, bearing a higher and more distinct neck. The slender base of the new scales has an anterior spur, unlike the holotype (Wang, 1997). The base of V 25343.1 is more swollen than that of the holotype and V 25343.2. It is interesting that V 25343.1 has two, rather than one, pulp cavity openings, unlike the holotype and V 25343.2.
Remarks Though the new specimens show some differences with the holotype V 12158.1, we refer to them as P. cornuformis based on their horned form, high crown rising to a sharply posterior point and covered with long slender ridges. M?rss et al. (2007) suggested the resemblance between P. cornuformis and Lanarkia or Thelodus traquairi, but P. cornuformis has a more swollen base.
Parathelodus scitulus Wang, 1997
(Figs. 3C, 4E)
Holotype A complete body scale, IVPP V 12156.1 (Wang, 1997:fig. 1, pl. I).
Referred specimen IVPP V 12156.10.
Locality and horizon Qilin district, Qujing, Yunnan Province, China. Xitun Formation and Xishancun Formation, Lochkovian, Lower Devonian.
Description One new well-preserved scale (Fig. 3C) is referred to as this species. The slightly convex crown has an arched anterior margin with short parallel ridges and a triangular posterolateral margin (Fig. 3C1). The distinct neck is ornamented by some short and weak vertical ridges, which become higher posteriorly (Fig. 3C2). The base is smaller than the crown, but is not very convex (Fig. 3C3). The pulp cavity is thick and straight, and ramifies at the top like a tree (Fig. 4E). A large pulp cavity opening lies in the middle of the base (Fig. 3C3).
Comparison Compared with the holotype (Wang, 1997), the new specimen has less developed ridges in the anterior margin of the crown (Fig. 3C1). Furthermore, its base is much flatter and the pulp cavity opening is larger than those of the holotype (Fig. 3C3). Thus, this scale may represent a juvenile scale.
Parathelodus xitunensis sp. nov.
(Figs. 3D, 4F)
Etymology After the Xitun Formation where the scales were collected.
Holotype A complete body scale, IVPP V 26113.1.
Locality and horizon Qilin district, Qujing, Yunnan Province, China. Xitun Formation, Lochkovian, Lower Devonian.
Diagnosis Scale having ellipse-shaped and flat crown with robust ridges around its margin; small bulged base with a large pulp cavity opening; vertical pulp cavity with a neck-like structure.
Description One scale (Fig. 3D) was identified. This scale has a flat crown, elliptic in shape. The crown is smooth, but its margin bears more than twenty robust ridges (Fig. 3D1). The neck is very prominent and devoid of any ornamentation (Fig. 3D2). The base, which is much smaller than the crown, is flat and bears an oval pulp cavity opening (Fig. 3D3). The pulp cavity is straight and laterally compressed (Fig. 4F). It becomes slimmer near the top creating a structure like a neck and broadens above the neck-like structure (Fig. 4F).
Comparison Both P. trilobatus and P. xitunensis have elliptic crown, but the crown of P. xitunensis does not become thinner from the center to the margin and is ornamented by ridges (Fig. 3D1). Moreover, P. xitunensis can be distinguished from the other species of Parathelodus by the possession of a neck-like structure near the top of the pulp cavity (Fig. 4F).
Parathelodus wangi sp. nov.
(Figs. 5, 6, 8A-C)
Holotype A complete body scale, IVPP V 12158.1 (Wang, 1997:fig. 5, pl. III, E).
Fig. 5
新窗口打开|下载原图ZIP|生成PPTFig. 53D models of Parathelodus wangi sp. nov.
A1, B1 and C1 in crown view; A2, B2 and C2 in lateral view; A3, B3, and C3 in basal view
A. IVPP V 25344.1 (holotype); B. V 25344.2; C. V 25344.3. Scale bars = 0.1 mm
Fig. 6
新窗口打开|下载原图ZIP|生成PPTFig. 63D models of Parathelodus wangi sp. nov.
A1, B1 and C1 in crown view; A2, B2 and C2 in lateral view; A3, B3, and C3 in basal view
A. IVPP V 25344.4; B. V 25344.5; C. V 25344.6. Scale bars = 0.1 mm
Etymology Specific name in honor of Wang Nianzhong for his contribution to the research of thelodonts in China.
Holotype A body scale, IVPP V 25344.1.
Paratype A body scale, IVPP V 25344.2.
Referred specimens IVPP V 25344.3-29.
Locality and horizon Qilin district, Qujing, Yunnan Province, China. Xitun Formation and Xishancun Formation, Lochkovian, Lower Devonian.
Diagnosis Scales small; crown quadrangular or oval; neck distinct and smooth; base swollen with one or two pulp cavity openings in the middle or slightly backwards.
Description These scales have a quadrangular, teardrop-shaped or triangular crown. The crown is convex, with a posterior point (Figs. 5A1, B1, C1, 6A1, B1, C1). The neck is high and smooth (Figs. 5A2, B2, C2, 6A2, B2, C2). The base, slightly smaller than the crown, is very swollen (Figs. 5A3, B3, C3, 6A3, B3, C3). A majority of the scales have a straight vertical pulp cavity with a medium-sized opening located in the middle of the base (Figs. 5A, C, 6A-C). However, one exceptional scale has a furcal pulp cavity with two small openings (Fig. 5B).
Comparison Compared with the other species of Parathelodus, the scales of P. wangi have a smooth crown with no sculptures (Figs. 5A1, B1, C1, 6A1, B1, C1). The new species is suggestive of Thelodus calvus (M?rss and Karatajūt?-Talimaa, 2002) in its scale profile and smooth crown, but the scales of the new species are evidently smaller than those of T. calvus, which are 0.3-0.9 mm in length (M?rss and Karatajūt?-Talimaa, 2002).
Parathelodus liaokuoensis sp. nov.
(Figs.7, 8D, E)
Fig. 7
新窗口打开|下载原图ZIP|生成PPTFig. 73D models of Parathelodus liaokuoensis sp. nov.
A1, B1 C1 and D1 in crown view; A2, B2, C2 and D2 in lateral view; A3, B3, C3 and D3 in basal view
A. IVPP V 25345.1 (holotype); B. V 25345.2; C. V 25345.3; D. V 25345.4. Scale bars = 0.1 mm
Fig. 8
新窗口打开|下载原图ZIP|生成PPTFig. 8Translucent 3D models of Parathelodus wangi sp. nov (A-C) and P. liaokuoensis sp. nov (D, E)
A1, B1, C1, D1 and E1 in crown view; A2, B2, C2, D2, and E2 in lateral view
A. IVPP V 25344.2; B. V 25344.3; C. V 25344.5; D. V 25345.1; E. V 25345.2. Scale bars = 0.1 mm
Etymology After the Liaokuo Hill where the fossil scales were collected.
Holotype A body scale, IVPP V 25345.1.
Referred specimens IVPP V 25345.2-15.
Locality and horizon Qilin district, Qujing, Yunnan Province, China. Xitun Formation and Xishancun Formation, Lochkovian, Lower Devonian.
Diagnosis Crown rhomboid or trapezoid with an angular or rounded anterior point and a sharp posterior cusp; surface of the crown smooth and slightly convex; neck distinct with serval developed vertical ribs; base pyramid-shaped; bifurcated pulp cavity.
Description The scales have a rhomboid or trapezoid crown with sharp posterior cusps (Fig. 7A1, B1, C1, D1). There is no sculpture on the slightly convex crown surface. The neck is high and has several developed vertical ribs (Fig. 7A2, B2, C2, D2). The base, which is smaller than the crown, is swollen and pyramidal in shape (Fig. 7A3, B3, C3, D3). The straight pulp cavity bifurcates anteriorly and posteriorly at the top (Fig. 8D, E). A small pulp cavity opening is in the middle of the base (Fig. 7A3, B3, C3, D3).
Comparison This species is similar to P. wangi in having a smooth crown with a posterior cusp of the crown, but the crown of P. liaokuoensis is trapezoid shaped (Fig. 7A1, B1, C1, D1). The smooth and trapezoid crown with a sharp posterior cusp is reminiscent of Thelodus parvidens (M?rss, 1986), but P. liaokuoensis has a pyramid-shaped base (Fig. 7A3, B3, C3, D3). P. liaokuoensis has a bifurcate pulp cavity, whose branches protrude anteriorly and posteriorly (Fig. 8D, E), unlike the other species of Parathelodus. Compared with the holotype, V 25345.2 bears a larger pulp cavity opening, a less swollen base and a pulp cavity possessing less developed branches (Figs. 7A, 8D, E), suggesting that it may represent a juvenile scale.
4 Discussion and conclusion
The thelodonts of China, including 2 genera (Parathelodus and Turinia) and an indeterminate genus, are only known from the Devonian of South China, and distributed in Yunnan, Sichuan and Guangxi provinces.Turinia is a cosmopolitan genus from the Devonian (Blieck and Goujet, 1978; Turner and Young, 1992; Blom and Goujet, 2002). The material of this genus in China is sparse. Only one species, T. pagoda (Wang et al., 1986), from the Givetian Heyuanzhai Formation of Yunnan and three morphological scale forms, Turinia sp. A (Wang et al., 1986) and Turinia sp. B (Wang et al., 1986), from the Eifelian Malutang Formation and Turinia sp. C (Wang, 1992) from the Emsian Yukiang Formation of Guangxi, have been reported. Wang (1992) also reported Nikoliviidae gen. indet. from the Emsian Yukiang Formation of Guangxi.
In previous studies, 5 species of Parathelodus, only known in China, were established. Parathelodus catalatus and P. asiaticus come from the Xitun Formation of Yunnan (Wang, 1984, 1995, 1997). P. trilobatus, P. cornuformis and P. scitulus come from the Xitun Formation and the upper part of the Xishancun Formations of Yunnan (Wang, 1995, 1997). Besides Yunnan, P. asiaticus, P. scitulus and P. cornuformis were only found in the Xiaputonggou Formation of Sichuan (Zhao et al., 2012). Stratigraphically, the horizons bearing Parathelodus in Yunnan and Sichuan are much higher than the Silurian-Devonian boundary, and their age is roughly late Lochkovian.
The fossil horizon of some new specimens from the Xishancun Formation in this study is located in the lower part of the Xishancun Formation, and very close to the Silurian-Devonian boundary (Zhu, 1992; Zhu et al., 1994). The new discovery extends the stratigraphic distribution of Parathelodus from the Xitun Formation and the upper part of the Xishancun Formation to the lower part of the Xishancun Formation, and prolongs the history of Parathelodus to the earliest Lochkovian. In addition, the three new species, P. xitunensis, P. wangi and P. liaokuoensis enhance the diversity of the thelodonts from the Devonian of China.
Data archiving statement Digital reconstructions of specimens used in this study are available through Admorph: http://www.admorph.org/. Image stacks are available on request from the authors.
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