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Palaeobiodiversity and Palaeoenvironments

, Volume 99, Issue 1, pp 45–62 | Cite as

Emsian (Lower Devonian) conodonts from the Lufengshan section (Guangxi, South China)

  • Jianfeng LuEmail author
  • José Ignacio Valenzuela-Ríos
  • Chengyuan Wang
  • Jau-Chyn Liao
  • Yi Wang
Original Paper

Abstract

The Lufengshan section (Guangxi, South China) is one of the best representative sections of the Xiangzhou facies (benthic facies), from which a few Emsian conodont studies have been accomplished in the past. In this paper, we present conodonts from the Upper Member of the Ertang Formation at the Lufengshan section. The conodont fauna is characterised by a distinctly low diversity with only six species reported and is assigned to the nothoperbonus Zone. ‘Ozarkodina’? chenae, ‘O.’? wuxuanensis and Polygnathus praeinversus are newly described species herein. A comparison of the contemporaneous conodont faunas between the Lufengshan, Liujing and Daliantang sections indicates that the conodont biodiversity during the time of the nothoperbonus Zone in South China is mainly bathymetrically controlled. Moreover, P. praeinversus, which was previously thought to be transitional between P. inversus and P. serotinus, is interpreted to represent an intermediate form between P. nothoperbonus and P. inversus on the basis of stratigraphical and morphological analyses. The outline of the basal cavity of this new polygnathid species better documents the evolutionary process of the inversion of the basal cavity of Polygnathus: the basal cavity in the middle part of the platform starts to invert on the inner side at first, whereas the flat or slightly elevated outer flank of basal cavity is laterally extended to form a shelf-like protuberance on the outer side of the pit.

Keywords

Ertang Formation nothoperbonus Zone Wuxuan Biodiversity Inversion of basal cavity 

Introduction

The marine Devonian strata in South China can be subdivided into the Xiangzhou facies, the Nandan facies and the transitional facies based on the lithology and faunal compositions (Wang et al. 1974; Xian et al. 1980; Wang and Ziegler 1983; Hou and Wang 1988; Ma et al. 2009). The Xiangzhou facies represents an oxygen-rich, high-energy and shallow sea environment with abundant benthic fossils, and the Nandan facies represents a deeper and quiet sea environment abounding in pelagic fossils. The transitional facies generally refers to the ramp and platform marginal environment that is geographically distributed between the benthic and pelagic facies. In the last nearly 40 years, the Emsian conodonts from the transitional facies have been extensively investigated, whereas only a few conodont studies from the Xiangzhou facies have been accomplished (Bai et al. 1979a, b, 1982, 1994; Wang 1979, 1981, 1989; Wang and Ziegler 1983).

The Lufengshan section mainly exposing the Ertang Formation is one of the best representative sections of the Xiangzhou facies. Abundant and well-preserved benthic fossils such as brachiopods and rugose corals from the Ertang Formation have been simply reported (Yu and Yin 1978) or systematically studied (Yu and Kuang 1980; Chen 1983). Wang (1981) first described the conodont fauna that includes Polygnathus perbonus (Philip, 1966) and P. cf. serotinus Telford, 1975 from one sample (BD304) in the Upper Member of the Ertang Formation. Later, a preliminary study on the conodont biostratigraphy of the entire Ertang Formation was conducted by Wang and Ziegler (1983, p. 80, fig. 6), who reported the occurrences of P. gronbergi Klapper and Johnson, 1975, P. perbonus and P. aff. perbonus and assigned the Ertang Formation to the then perbonus Zone (= upper part of the excavatus Zone plus the nothoperbonus Zone now). The conodonts were subsequently described by Wang (1989).

According to the lineages of the Emsian polygnathids recognised in different places around the world (Bardashev 1986; Mawson 1987; Bultynck 1989; Yolkin et al. 1994, 2011; Bardashev et al. 2002; Martínez-Pérez et al. 2011; Martínez-Pérez and Valenzuela-Ríos 2014; Baranov and Blodgett 2016), the nothoperbonus Zone witnessed a relatively high conodont biodiversity with many species of Polygnathus first arising or becoming extinct within this interval (a subjective synonymy is not discussed herein). In contrast, the previously reported conodont fauna from the Ertang Formation at the Lufengshan section is marked by a distinctly low diversity. Therefore, the main purposes of this paper are to further our understanding about the Emsian conodont fauna of the Xiangzhou facies, to describe two new species of the genus ‘Ozarkodina’ and one new species of Polygnathus, to figure out the environmental factors that are mainly responsible for the remarkable difference on the conodont diversity between different facies during the time of the nothoperbonus Zone and finally to discuss the evolutionary processes of the inversion of polygnathid basal cavity based on the Lufengshan material.

Geological setting

The Lufengshan section (Fig. 1a; GPS: N. 23° 44' 30", E. 109° 44' 55") is located about 500 m northeast of the Tuntou Village, Wuxuan County, Laibin. A continuous succession of the Upper Member of the Lower Devonian Ertang Formation is well exposed along the zigzag road at this section (Fig. 1b). Yu and Yin (1978) first named and described the Ertang Formation at the west slope of the Liubuling Mountain, approximately 600 m west of the Tuntou Village, and subdivide it into three lithological parts in ascending order. The lowermost part consisting of dolomite is named Shanglun Dolomite, and the middle and upper parts which are mainly represented by limestone and shale are divided into the Lower and Upper members (Fig. 2). The Ertang Formation in the Wuxuan area is conformably underlain and overlain by the Yujiang Formation and Guangqiao Dolomite, respectively. The Upper Member of the Ertang Formation studied in the present research is about 107.68 m thick and is mainly composed of thin to medium-bedded limestone intercalated by several thin mudstone beds (Fig. 1b, c). A total of 67 limestone samples were collected at the Lufengshan section (Fig. 2).
Fig. 1

a Locality of the Lufengshan section at Wuxuan County, Guangxi, South China (black star). b General view of the Upper Member of the Ertang Formation at the Lufengshan section. c Outcrop view of the interval from which samples AGP-LFS-1 and AGP-LFS-2 were collected (the length of geological hammer is about 31.5 cm)

Fig. 2

Stratigraphical column of the Lufengshan section showing the levels sampled for conodonts and ranges of conodont taxa. Dl. dolomite, Fm. formation, P. Polygnathus, ‘O.’ ‘Ozarkodina

Material and methods

Limestone samples, weighing approximately between 2.4 and 4.5 kg, were reduced mechanically to a size of approximately 125 cm3 and then dissolved in dilute acetic acid (5–10%). The insoluble residues were subsequently washed, air-dried and later concentrated by heavy liquid separation using sodium polytungstate (Merrill 1987; Krukowski 1988; Savage 1988; Jeppson and Anehus 1999). Concentrated residues were finally handpicked under a stereo microscope. Selected specimens were gold-coated and photographed using a scanning electron microscope (LEO 1530 VP and Zeiss EVO 18) in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. Only 27 samples yielded conodont elements, from which six species were identified (Table 1).
Table 1

Occurrence of conodont species from the Upper Member of the Ertang Formation at the Lufengshan section. ‘O.’ ‘Ozarkodina’, P. Polygnathus

Sample no. (AGP-LFS)

1

2

6

9

15

16

23

24

32

33

34

35

36

37

38

39

40

42

44

45

47

50

53

66

67

Total number

Weight (kg)

4.15

3.10

3.44

3.67

3.88

3.42

4.11

3.83

3.20

3.50

3.34

3.52

3.46

3.59

3.26

3.01

3.10

2.44

3.10

3.31

2.28

3.23

2.89

3.13

2.98

 

Weight (kg)

4.15

3.10

3.44

3.67

3.88

3.42

4.11

3.83

3.20

3.50

3.34

3.52

3.46

3.59

3.26

3.01

3.10

2.44

3.10

3.31

2.28

3.23

2.89

3.13

2.98

 

O.’? wuxuanensis (Pa)

5

2

                       

7

P. perbonus

  

1

 

1

4

1

 

4

 

1

14

3

 

1

  

1

  

5

1

  

1

38

P. nothoperbonus

   

1

5

6

  

5

2

 

8

3

3

1

1

2

   

6

  

5

28

76

P. praeinversus

       

1

2

           

1

  

2

2

8

P. aff. gilberti

        

1

                

1

O.’? chenae (Pa)

                  

1

2

  

2

  

5

Indeterminate elements

Pb

                  

1

1

     

2

M

                   

1

     

1

Sa

                  

1

1

     

2

Total number

5

2

1

1

6

10

1

1

12

2

1

22

6

3

2

1

2

1

3

5

12

1

2

7

31

140

All specimens described and illustrated herein are deposited in the Collection of Nanjing Institute of Geology and Palaeontology (NIGP 166416–166452), Chinese Academy of Sciences.

Systematic palaeontology

(By Lu, J. F., Valenzuela-Ríos, J. I. and Wang, C. Y.)

Order Ozarkodinida Dzik, 1976

Family Spathognathodontidae Hass, 1959

Genus ‘Ozarkodina’ Branson and Mehl, 1933

Ozarkodinachenae sp. nov. (Figs. 3a, f)

Holotype: NIGP 166416 (Pa element, Fig. 3a, b).
Fig. 3

(a–f) ‘Ozarkodina’? chenae sp. nov.: (a, b) lateral and lower views of the Pa element, holotype, NIGP 166416, sample AGP-LFS-44; (c, d) lateral and lower views of the Pa element, paratype, NIGP 166417, sample AGP-LFS-53; (e, f) lateral and lower views of the Pa element, paratype, NIGP 166418, sample AGP-LFS-53. (g) Posterior view of an indeterminate Sa element, sample AGP-LFS-44. (h) Lateral view of an indeterminate Pb element, sample AGP-LFS-44. (i–r) ‘Ozarkodina’? wuxuanensis sp. nov.: (i, j) lateral and lower views of the Pa element, NIGP 166419, sample AGP-LFS-1; (k, l) lateral and lower views of the Pa element, holotype, NIGP 166420, sample AGP-LFS-1; (m, n) lateral and lower views of the Pa element, paratype, NIGP 166421, sample AGP-LFS-1; (o, p) lateral and lower views of the Pa element, paratype, NIGP 166422, sample AGP-LFS-1; (q, r) lateral and lower views of the Pa element, NIGP 166423, sample AGP-LFS-2. Scale bars = 100 μm

Paratype: NIGP 166417 (Pa element, Fig. 3c, d) and NIGP 166418 (Pa element, Fig. 3e, f).

Material: Five Pa elements, AGP-LFS-44 (1), AGP-LFS-45 (2) and AGP-LFS-53 (2).

Type locality: Lufengshan section, approximately 500 m northeast of the Tuntou Village, Wuxuan County, Guangxi, China.

Type stratum: Devonian, Emsian, Ertang Formation.

Etymology: Named in honour of Prof. Dr. Xiuqin Chen (Nanjing Institute of Geology and Palaeontology) for her outstanding works on the Devonian System and brachiopods in China.

Diagnosis: A species of ‘Ozarkodina’ in which the Pa element has a straight or slightly curved blade with numerous, irregular, triangular and strongly compressed denticles. One to two denticles directly behind the anterior margin are remarkably enlarged and high. Cusp(s) above the basal cavity is specially stout and prominent, from which the height decreases drastically toward the posterior end. Semicircular and more or less symmetrical lobes of the basal cavity are laterally expanded and centrally located. Basal cavity tapers anteriorly as a narrow and shallow groove, but is slightly or clearly inverted beneath the posterior blade.

Description: The Pa element is usually dominated by a few remarkably large and high denticles, one to two at the anterior end of the blade, and one to three in the middle part of the blade. Cusp(s) is centrally situated, anterior of which are five to seven irregular denticles and posterior of which are four to six irregular denticles. Denticles are erect in the anterior two thirds of the blade (Fig. 3c) or posteriorly reclined throughout the unit (Fig. 3a, e). The posterior part of the blade bows slightly downward, and the height declines drastically, forming a convex arc posteriorly. The shallow basal cavity, which is centrally located with semicircular and laterally flared lobes, tapers anteriorly as a shallow or flat groove and extends posteriorly as a keel flanked by parallel striations (Fig. 3b, d, f). The lower margin of the blade is straight or weakly concave, resulting in the anterior and posterior ends of the blade being more or less at the same height.

Remarks: The irregularity of denticles in the Pa element of ‘Ozarkodina’? chenae makes this new species somewhat similar to the Pa element of ‘O.buchanensis (Philip, 1966), from which, however, it differs by the shallower basal cavity that is less clearly and abruptly delimited anteriorly and posteriorly. Moreover, the anterior one or two denticles in ‘O.’? chenae are usually remarkably enlarged and high. ‘Ozarkodinaprolata Mawson, 1987 differs from ‘O.’? chenae in the presence of a longer blade, a comb-like anterior part of blade with more enlarged and high denticles and more uniform denticles in the Pa element. Although one of the paratypes (Fig. 3c, d) slightly differs from the holotype and the other paratype (Fig. 3a, b, e, f) in having several erect denticles in the anterior part of the blade, a weakly concave lower margin and a more laterally expanded and rounded lobes, it is provincially assigned to this new species because of the lower number of similar specimens.

In addition to several Pa elements of ‘Ozarkodina’? chenae in samples AGP-LFS-44 and AGP-LFS-45, two Pb elements, one M element and two Sa elements, all of which cannot be doubtlessly assigned to this new species because of the lower number of specimens found, were also obtained. The indeterminate Pb element (Fig. 3h) has a posteriorly reclined and prominent cusp that is two to three times wider than other denticles. Anterior of the cusp are nine closely spaced, irregular and palisade-like denticles, with those closest to the cusp similarly reclined posteriorly as the cusp. The posterior process is broken, but arched downward to form an angle of about 145° with the anterior process in lateral view. Basal cavity is completely inverted with a tiny lip on both sides. The indeterminate M element (not illustrated here) is characterised by a long and steeply descending posterior process, whose denticles are discrete, widely spaced and rounded in cross section. Cusp is enlarged and erect with a rounded cross section. Anterior process is broken. Basal cavity is totally inverted with a tiny lip on the inner side, extending beneath the posterior process as a narrow keel flanked by parallel striations. The indeterminate Sa element (Fig. 3g) is trichonodelliform with a thin anterior arch, the processes of which diverge at an angle of approximately 115°. Cusp is large and rounded in cross section. The discrete and widely spaced denticles of lateral processes are similarly rounded in cross section as the cusp and tend to increase in size distally. Basal cavity is completely inverted, and a tiny posterior lip may be developed. However, due to the few specimens of other elements collected in the present study, this new taxon is provisionally assigned to the genus ‘Ozarkodina’ with a question mark.

Stratigraphic range: The nothoperbonus Zone.

Ozarkodina’? wuxuanensis sp. nov. (Figs. 3i, r)

Holotype: NIGP 166420 (Pa element, Fig. 3k, l).

Paratype: NIGP 166421 (Pa element, Fig. 3m, n), NIGP 166422 (Pa element, Fig. 3o, p) and NIGP 166423 (Pa element, Fig. 3q, r).

Material: Seven Pa elements, AGP-LFS-1 (5) and AGP-LFS-2 (2).

Type locality: Lufengshan section, approximately 500 m northeast of the Tuntou Village, Wuxuan County, Guangxi, China.

Type stratum: Devonian, Emsian, Ertang Formation.

Etymology: Named after the type locality, Wuxuan County.

Diagnosis: Representative Pa elements of ‘Ozarkodina’? wuxuanensis have a short and straight blade with numerous, irregular, triangular and compressed denticles. Denticles anterior of the remarkably stout and high cusp are much lower and more or less at the same height. Denticles posterior of the cusp are slightly high and posteriorly reclined. A wide space exists between the prominent cusp and the posterior set of denticles. Basal cavity with two rounded and laterally flared lobes is centrally situated.

Description: Cusp that is two to three times wider than other denticles is posteriorly reclined and markedly high in the middle part of the blade. Anterior of the cusp are four to six erect, small and triangular denticles tending to be uniform in height. A slightly large denticle may be developed in the anterior-most part of the blade (Fig. 3o, q). Three to five posteriorly reclined denticles in the posterior part of the blade are separated from the prominent cusp by a wide space. The anterior-most denticle in the posterior part of the blade is slightly high and enlarged (Fig. 3k, o); however, a small denticle may germinate from the base of this large denticle (Fig. 3i, m, q). Basal cavity is abruptly expanded in the middle part of the blade and is restricted to the area under the two rounded, symmetric or asymmetric and laterally flared lobes (Fig. 3j, l, r). Beneath the blade anterior and posterior of the lobe, the basal cavity is replaced by a narrow groove that extends to the ends of the blade. The lower margin anterior of the basal cavity is straight or convex. The anterior end of the blade is higher than (Fig. 3m, o, q) or at the same height with (Fig. 3i, k) the anterior end of basal cavity expansion. The lower margin posterior of the basal cavity is oblique with the posterior end of the blade being higher than the posterior end of basal cavity expansion.

Remarks: The Pa elements of ‘Ozarkodina’? wuxuanensis are mainly characterised by an extraordinarily wide space between the cusp and posterior set of denticles and a remarkably high and stout cusp. It differs greatly in the arrangement of denticles from other contemporaneous spathognathodontid species. In addition to several Pa elements figured here, other elements of the apparatus (especially the Sa element) were not obtained in the present study. As a result, a precise and determinate identification of this species at the generic level is difficult, and this taxon is provisionally assigned to the genus ‘Ozarkodina’ with a question mark.

Compared with the holotype and other paratypes, one of the paratypes (Fig. 3q, r) has a more enlarged cusp, a distinctly oblique posterior lower margin and more stout anterior and posterior blades. Taking into consideration of the length of total blade, this individual probably is an ontogenetically senile specimen.

Stratigraphic range: The nothoperbonus Zone.

Family Polygnathidae Bassler, 1925

Genus Polygnathus Hinde, 1879

Polygnathus nothoperbonus Mawson, 1987 (Figs. 4a, r, 5a, j)

1981 Polygnathus perbonus (Philip); Wang, p. 403, pl. 1, figs. 20–21.

1987 Polygnathus nothoperbonus Mawson, p. 276, pl. 32, figs. 11–15; pl. 33, figs. 1, 2; pl. 36, fig. 7.

2016 Polygnathus nothoperbonus Mawson; Lu et al., p. 288, figs. 5O–R, 6A, B [with synonymy list].

2016 Polygnathus nothoperbonus Mawson; Lu and Chen, figs. 4.11–4.12.

Material: Seventy-six Pa elements, AGP-LFS-9 (1), AGP-LFS-15 (5), AGP-LFS-16 (6), AGP-LFS-32 (5), AGP-LFS-33 (2), AGP-LFS-35 (8), AGP-LFS-36 (3), AGP-LFS-37 (3), AGP-LFS-38 (1), AGP-LFS-39 (1), AGP-LFS-40 (2), AGP-LFS-47 (6), AGP-LFS-66 (5) and AGP-LFS-6 (28).

Description: Free blade is about one fifth to one fourth of the total length and consists of four to five close and palisade-like denticles (Figs. 4d, e and 5d, e, j). Carina, flanked by well-developed adcarinal grooves in the anterior half of the platform, varies from stout and fused denticles in the anterior part of the platform to a row of small discrete denticles in the middle part. Adcarinal grooves are unequally developed, the outer one being wider, deeper and extending far more posteriorly than the inner one. In most specimens, the anterior platform margins meet the blade at the same position. One specimen (Fig. 4m, n) has its inner margin reaching the free blade more anteriorly than the outer one; whereas another specimen (Fig. 5a, b) has its outer margin joining the free blade more anteriorly than the inner one. Parallel platform margins in the anterior half are at the same height and are generally ornamented by short transverse ridges, which are separated from the carina by adcarinal grooves. In some specimens (Figs. 4g, i, l and 5d), the anterior platform margins are relatively smooth. The posterior part of the platform (or tongue) which is distinctly deflected inward is generally crossed by numerous and continuous transverse ridges, resulting in the carina not extending to the posterior termination. In one juvenile specimen (Fig. 5d), the carina extends two denticles posterior of the platform termination. On the lower side, an asymmetric, medium-sized and shallow basal cavity is located in the central part of the platform, the posterior end is flat (Fig. 4b, j, o) or clearly inverted (Fig. 4c, e, h, k, n, q and 5b, c, f, h, i, k, l).
Fig. 4

(a–r) Polygnathus nothoperbonus Mawson, 1987: (a, b) upper and lower views of the Pa element, Early form, NIGP 166424, sample AGP-LFS-9; (c, d) lower and upper views of the Pa element, Late form, NIGP 166425, sample AGP-LFS-15; (e, f) lower and upper views of the Pa element, Late form, NIGP 166426, sample AGP-LFS-35; (g, h) upper and lower views of the Pa element, Late form, NIGP 166427, sample AGP-LFS-36; (i, j) upper and lower views of the Pa element, slightly large immature specimen, Early form, NIGP 166428, sample AGP-LFS-47; (k, l) lower and upper views of the Pa element, Late form, NIGP 166429, sample AGP-LFS-47; (m, n) upper and lower views of the Pa element, Late form, NIGP 166430, sample AGP-LFS-47; (o, p) lower and upper views of the Pa element, juvenile specimen, Early form, NIGP 166431, sample AGP-LFS-66; (q, r) lower and upper views of the Pa element, Late form, NIGP 166432, sample AGP-LFS-67. Scale bars = 100 μm

Fig. 5

(a–l) Polygnathus nothoperbonus Mawson, 1987: (a, b) upper and lower views of the Pa element, Late form, NIGP 166433, sample AGP-LFS-67; (c, d) lower and upper views of the Pa element, juvenile specimen, Late form, NIGP 166434, sample AGP-LFS-67; (e, f) upper and lower views of the Pa element, juvenile specimen, Late form, NIGP 166435, sample AGP-LFS-67; (g–i) upper, lower and enlarged views of the Pa element, Late form, NIGP 166436, sample AGP-LFS-32; (j–l) upper, lower and enlarged views of the Pa element, Late form, re-illustrated from Wang (1981, pl., figs. 20–21), NIGP 49604, sample BD304. (m–r) Polygnathus perbonus (Philip, 1966): (m, n) upper and lower views of the Pa element, slightly large immature specimen, NIGP 166437, sample AGP-LFS-6; (o, p) lower and upper views of the Pa element, NIGP 166438, sample AGP-LFS-23; (q, r) upper and lower views of the Pa element, slightly large immature specimen, NIGP 166439, sample AGP-LFS-32. Scale bars = 100 μm

Remarks: Polygnathus nothoperbonus differs from P. perbonus mainly by the shallow or flat basal cavity that may be inverted posteriorly. On the basis of the outline of the posterior basal cavity, this taxon can be further subdivided into the Early and Late forms (Yolkin et al. 1994). The Early form is characterised by the flat or slightly convex posterior basal cavity, whereas the posterior basal cavity of the Late form is clearly inverted with a well-developed basal furrow. Both forms are present in our specimens, but the Late form dominates.

Stratigraphic range: Ranging from the nothoperbonus Zone to inversus Zone (Yolkin et al. 1994, text-fig. 4b), but also recorded in the serotinus Zone at the Sihongshan section (Wang and Ziegler 1983; Ziegler and Wang 1985; Wang 1989).

Polygnathus perbonus (Philip, 1966). (Figs. 5m, r, 6a, i)

1966 Roundya perbona Philip, p. 449, pl. 4, figs. 7–8, text-fig. 6 [Sa element].

1966 Polygnathus linguiformis Hinde; Philip, pp. 448, 449, pl. 2, figs. 29–40.

1967 Polygnathus linguiformis foveolatus Philip and Jackson, p. 1265, text-figs. 2d–h, 3b.

1970 Polygnathus linguiformis foveolatus Philip and Jackson; Philip and Jackson in Pedder et al., p.216, pl.40, figs. 11–12?, 13–14.

1971 Polygnathus perbonus (Philip); Klapper and Philip, fig. 11.

1973 Polygnathus foveolatus Philip and Jackson; Boersma, pp. 285–287, pl. 1, figs. 1–3.

1978 Polygnathus perbonus (Philip); Mashkova and Apekina in Kim et al., pl. 74, fig. 5; pl. 75, figs. 7, 9–10; pl. 76, figs. 1–2.

1978 Polygnathus foveolatus Philip and Jackson; Wang and Wang, pl. 340, pl. 41, figs. 31–33.

1979b Polygnathus perbonus (Philip); Bai et al., p. 69, pl. 3, figs. 3, 6. pars 1981 Polygnathus perbonus (Philip); Wang, p. 403, pl. 1, figs. 9–17 [non 18–19 = P. inversus; 20–21 = P. nohoperbonus].

1999 Polygnathus perbonus (Philip); Talent and Mawson, pl. 10, figs. 1, 6–8, 10–11, 14, 16.

2016 Polygnathus perbonus (Philip); Lu et al., pp. 288, 289, figs. 6C–N [with synonymy list].

2016 Polygnathus perbonus (Philip); Lu and Chen, fig. 4.4.

Material: Thirty-eight Pa elements, AGP-LFS-6 (1), AGP-LFS-15 (1), AGP-LFS-16 (4), AGP-LFS-23 (1), AGP-LFS-32 (4), AGP-LFS-34 (1), AGP-LFS-35 (14), AGP-LFS-36 (3), AGP-LFS-38 (1), AGP-LFS-42 (1), AGP-LFS-47 (5), AGP-LFS-50 (1) and AGP-LFS-67 (1).

Description: Free blade consisting of four to five close and palisade-like denticles is about one fourth of the total length and decreases gradually in height posteriorly (Figs. 5q and 6c, f, j). Carina, flanked by well-developed adcarinal grooves in the anterior half of the platform, varies from stout and fused denticles in the anterior part of the platform to a row of small discrete denticles in the middle part. Platform is more or less asymmetric with the carina being generally centrally situated in the anterior part of the platform, but approaching the inner margin in the middle part. Adcarinal grooves are unequally developed, the outer one being wider, deeper and extending more posteriorly than the inner one. Platform margins in the anterior half are parallel, and at the same height, they are somewhat smooth (Figs. 5m and 6b, c) or ornamented by short transverse ridges that grow longer posteriorly (Fig. 5p, q, f, g, j, k). The posterior part of the platform (or tongue) is distinctly deflected inward. In adult specimens (Figs. 5p and 6b, j, k), the long and well-developed tongue is ornamented by numerous (more than four) continuous transverse ridges that hinder the carina from reaching the posterior end of the platform. Some slightly larger immature specimens (Figs. 5m, q and 6g) possess a short or weakly developed tongue bearing only two to three transverse ridges, the anterior-most one of which is always continuous, but the posterior-most one of which is semicrossed; the relatively small juvenile specimens (Fig. 6c, f) are characterised by having a carina extending one to two denticles posterior of the platform termination and a posterior part of platform ornamented by one to two semicrossed transverse ridges. On the lower side, an asymmetric, medium-sized and V-shaped basal cavity is located in the central part of the platform, and the posterior end is flat or in a shallow groove.
Fig. 6

(a–l) Polygnathus perbonus (Philip, 1966): (a, b) lower and upper views of the Pa element, NIGP 166440, sample AGP-LFS-32; (c, d) upper and lower views of the Pa element, juvenile specimen, NIGP 166441, sample AGP-LFS-35; (e, f) lower and upper views of the Pa element, juvenile specimen, NIGP 166442, sample AGP-LFS-35; (g, h) upper and lower views of the Pa element, slightly large immature specimen, NIGP 166443, sample AGP-LFS-35; (i, j) lower and upper views of the Pa element, NIGP 166444, sample AGP-LFS-35; (k, l) upper and lower views of the Pa element, NIGP 166445, sample AGP-LFS-42. (m–r) Polygnathus praeinversus sp. nov.: (m, n) upper and lower views of the Pa element, NIGP 166446, sample AGP-LFS-24; (o–q) upper, lower and enlarged views of the Pa element, paratype, NIGP 166447, sample AGP-LFS-32. Scale bars = 100 μm

Remarks: The Pa elements of Polygnathus perbonus are mainly characterised by a medium-sized and V-shaped basal cavity on the lower side and a deflected posterior part of the platform bearing crossed transverse ridges on the upper side (Philip and Jackson 1967; Klapper and Johnson 1975; Mawson 1987; Yolkin et al. 1994). However, as pointed out by Mawson (1987, p. 277), the transverse ridges crossing the tongue of P. perbonus are not always continuous. In the present study, the Lufengshan material shows that the ornamentation of the tongue in P. perbonus is actually quite variable. Specimens during different ontogenetic stages have different ornamentations on the tongue. This can be further demonstrated by the collection of Philip (1966). The juvenile specimen (Philip 1966, pl. 2, figs. 29–31) from the Buchan Group in Eastern Australia develops a carina running to the posterior end of the platform and only one semicrossed transverse ridge in the posterior part of the platform; the adult specimens (Philip 1966, pl. 2, figs. 32–35, 39–40) have a well-developed tongue ornamented by four to six continuous transverse ridges, whereas the transverse ridges in larger or senile specimen (Philip 1966, pl. 2, figs. 36–37) are as much as nine and sometimes discontinuous at the posterior end of the platform. A similar ontogenetic change of the ornamentation of the tongue in P. perbonus can also be observed in specimens figured by Carls and Gandl (1969, pl. 18, figs. 15, 17–18, 22), Wang (1981, pl. 1, figs. 9–17) and Jin et al. (2005, pl. 4, figs. 7–8, 10–11; pl. 12, figs. 17–18; pl. 13, figs. 1–2, 5–10, 17–18). Additionally, the juvenile and immature specimens of P. perbonus are also highly distinguishable by a deep and V-shaped basal cavity on the lower side, which differs greatly from the shallow or flat basal cavity that may be even inverted at the posterior end in the juvenile and immature specimens of P. nothoperbonus (Figs. 4j, o and 5c, f).

Stratigraphic range: Ranging from the upper part of the excavatus Zone to the lower part of the inversus Zone (Yolkin et al. 1994, text-fig. 4b).

Polygnathus praeinversus sp. nov. (Figs. 6m–q, 7a–n)

1975 Polygnathus inversus Klapper and Johnson, p. 73, pl. 3, figs. 19–22, 24–31.

1980 Polygnathus inversus Klapper and Johnson; Uyeno and Klapper., p. 89, pl. 8.1, figs. 13–16; pl.8.3, figs. 4–5.

1981 Polygnathus perbonus (Philip); Wang, p. 403, pl. 1, figs. 18–19.

1981 Polygnathus cf. serotinus Telford; Wang, pp. 403, 404, pl.1, figs. 31–32.

1983 Polygnathus aff. perbonus (Philip); Wang and Ziegler, pl. 5, fig. 5.

1987 Polygnathus inversus Klapper and Johnson; Mawson, p. 274, pl. 33, figs. 6–8; pl. 36, fig. 9.

1990 Polygnathus inversus Klapper and Johnson; Uyeno, p. 83, pl. 7, figs. 41–42; pl. 9, figs. 7–8, 15–16.

2003 Polygnathus inversus Klapper and Johnson; Mawson and Talent, p. 346, pl. 1, figs. 8–10.

Holotype: NIGP 166451 (Figs. 7g, h).
Fig. 7

(a–n) Polygnathus praeinversus sp. nov.: (a, b) lower and upper views of the Pa element, NIGP 166448, paratype, sample AGP-LFS-32; (c, d) upper and lower views of the Pa element, NIGP 166449, sample AGP-LFS-47; (e, f) upper and lower views of the Pa element, paratype, NIGP 166450, sample AGP-LFS-67; (g, h) upper and lower views of the Pa element, holotype, NIGP 166451, sample AGP-LFS-67; (i–k) upper, lower and enlarged views of the Pa element, re-illustrated from Wang (1981, pl., figs. 18–19), NIGP 49603, sample BD304; (l–n) upper, lower and enlarged views of the Pa element, re-illustrated from Wang (1981, pl., figs. 3132), NIGP 49609, sample BD304. (o–q) Polygnathus aff. gilberti Bardashev, 1986, upper, lower and enlarged views of the Pa element, NIGP 166452, sample AGP-LFS-32. Scale bars = 100 μm

Paratype: NIGP 166447 (Figs. 6o–q), NIGP 166448 (Fig. 7a, b) and NIGP 166450 (Fig. 7e, f).

Material: Eight Pa elements, AGP-LFS-24 (1), AGP-LFS-32 (2), AGP-LFS-47 (1), AGP-LFS-66 (2) and AGP-LFS-67 (2).

Type locality: Lufengshan section, approximately 500 m northeast of the Tuntou Village, Wuxuan County, Guangxi, China.

Type stratum: Devonian, Emsian, Ertang Formation.

Etymology: prae (Lat). = before, and inversus; points to the earlier appearance in reference to Polygnathus inversus.

Diagnosis: Representative Pa elements of Polygnathus praeinversus have a small basal pit just anterior of the sharp inward deflection of the keel. Basal cavity posterior of the pit is completely inverted. The inner flank of the basal cavity close to the pit is inverted, whereas the outer flank of the basal cavity in the middle part is laterally expanded to form a semicircular and shelf-like protuberance on the outer side of the pit. Anterior inner and outer platform margins are at the same height and are separated from the carina by wide and deep adcarinal grooves. Transverse ridges crossing the posterior part of the platform are generally continuous.

Description: Free blade consisting of five close and palisade-like denticles is approximately one fifth to one fourth of the total length and decreases gradually in height posteriorly (Figs. 6o and 7g, i, l). Carina, flanked by well-developed adcarinal grooves in the anterior half of the platform, varies from stout and fused denticles in the anterior part of the platform to a row of small discrete denticles in the middle part. Platform is asymmetric with the carina positioned close to the inner margin. Adcarinal grooves are unequally developed, the outer one being wider and deeper than the inner one. Platform margins in the anterior part are relatively smooth (Figs. 6o and 7b, c, e) or ornamented by short transverse ribs (Figs. 6m and 7g, i, l). Most specimens develop more or less parallel anterior platform margins that are at the same height; however, in one specimen, the outer platform margin in the middle part is marked by a clear convex curve (Fig. 6o). The posterior part of the platform is distinctly deflected inward to form a well-developed tongue ornamented by numerous and continuous transverse ridges, some of which may be slightly discontinuous at the end of the platform (Fig. 7e, g, i). On the lower side just anterior of the inward deflection of the keel is situated a small basal pit, posterior of which the basal cavity is completely inverted. The inner flank of the basal cavity in the middle part of the platform is totally inverted, whereas the outer flank of the basal cavity in the middle part is slightly elevated or flat and laterally extended to form a semicircular and shelf-like protuberance suspended below the lower surface (Figs. 6n, p, q and 7a, d, f, h, j, k, m, n). A narrow groove extends from the pit to the anterior end of the platform.

Remarks: Polyganthus praeinversus shows a close morphological affinity with P. inversus Klapper and Johnson, 1975, in having a basal pit just anterior of the keel’s inward deflection, a totally inverted basal cavity posterior of the pit, and wide and deep adcarinal grooves in the anterior part of the platform. However, the former species differs from the latter mainly by the relatively small basal pit and the shelf-like protuberance on the outer side of the pit. The new species also has a close similarity to the Late form of P. nothoperbonus, from which it differs by the inverted inner flank of basal cavity in the middle part of the platform. In contrast, the Late form of P. nothoperbonus has a flat or slightly elevated inner flank of basal cavity that converges with the flat or slightly elevated outer flank to form a shallow or totally flat basal cavity in the middle part of the platform (compare Fig. 5g–l with Figs. 6o–q and 7l–n). Polygnathus serotinus resembles P. praeinversus in the development of the protuberance on the lower side, but is highly distinguishable by the flange-like anterior outer margin that is distinctly higher than carina and inner margin.

Stratigraphic range: Ranging from the nothoperbonus Zone to the base of the serotinus Zone.

Polygnathus aff. gilberti Bardashev, 1986

Material: One Pa element, AGP-LFS-32 (1).

Description: Free blade is broken and lost. Carina, flanked by well-developed adcarinal grooves in the anterior half of the platform, is centrally situated and varies from stout and fused denticles in the anterior part of the platform to a row of discrete denticles in the middle part (Fig. 7o). Adcarinal grooves, which are slightly deep only at the anterior end of the platform, are narrow and flanked by broad transverse ridges that grow longer posteriorly. The outer adcarinal groove is somewhat wider and deeper than the inner one. Platform narrows smoothly toward the anterior end and widens in the middle part, resulting in a more laterally extended outer platform than the inner one. The posterior part of the platform that is distinctly deflected inward is crossed by approximately 12 continuous transverse ridges. On the lower side just anterior of the inward deflection of the keel is situated a small basal pit, posterior of which the basal cavity is completely inverted (Fig. 7p). The outer flank of basal cavity in the middle part is slightly elevated and wide, forming a semicircular and shelf-like protuberance on the outer side of the pit (Fig. 7q). A narrow groove extends from the pit to the anterior end of the platform.

Remarks: Representative Pa elements of Polygnathus gilberti are characterised by a wide platform narrowing smoothly in the anterior part, shallow and narrow adcarinal grooves flanked by broad marginal ridges, an inward deflected tongue ornamented by crossed transverse ridges and a totally inverted basal cavity posterior of the pit. All of these features can be observed in our specimen. Although the basal cavity in the middle part of the platform is inverted on the inner side, however, our specimen has a slightly elevated outer flank of basal cavity in the middle part. The outer flank is laterally extended to form a semicircular and shelf-like protuberance that is supported in its main extent by the solid shaft from the platform lower surface but still has its rim suspended below the lower surface. This differs greatly from P. gilberti.

Stratigraphic range: The nothoperbonus Zone.

Conodont biostratigraphy and biodiversity

Polygnathus perbonus has its lowest occurrence in sample AGP-LFS-6 (Fig. 5m, n) in the lowermost part of the Upper Member of the Ertang Formation and ranges upward to sample AGP-LFS-67 (Fig. 5o–r and 6a–l) in the uppermost part of the investigated strata herein. Polygnathus nothoperbonus that first appears in sample AGP-LFS-8 is characterised by a shallow and flat basal cavity which is not inverted posteriorly (Fig. 4a, b), thus belonging to the Early form proposed by Yolkin et al. (1994). The Late form with an inverted posterior basal cavity entries subsequently in sample AGP-LFS-9 (Fig. 4c, d). Both forms have a long range from the lower to uppermost parts of the Upper Member of the Ertang Formation (Figs. 4e–r and 5a–i). The first entry of P. praeinversus is recorded in sample AGP-LFS-24 (Fig. 6m, n), and its last occurrence is recorded in sample AGP-LFS-67 (Figs. 6o–q and 7a–n) together with P. nothoperbonus and P. perbonus. According to Yolkin et al. (1994), the nothoperbonus Zone could be informally subdivided into the Lower and Upper subzones on the basis of the successive appearances of the Early and Late forms of P. nothoperbonus. Therefore, the first entry of the Late form of P. nothoperbonus in sample AGP-LFS-9 indicates that the interval from samples AGP-LFS-9 to AGP-LFS-67 is assigned to the Upper nothoperbonus Subzone. Considering that the lower boundary of the nothoperbonus Zone or the Lower nothoperbonus Subzone was previously demonstrated in the middle part of the underlying Shanglun Dolomite by Wang and Ziegler (1983), the lowermost part of the Upper Member of the Ertang Formation ranging from samples AGP-LFS-1 to AGP-LFS-8 belongs to the Lower nothoperbonus Subzone.

On the basis of the conodont biostratigraphy, the middle part of the Shanglun Dolomite to the Upper Member of the Ertang Formation at the Lufengshan section can be correlated with the Daliancun Member of the Yujiang Formation to the Moding Formation at the Liujing section (Henxian, Central Guangxi), and the lower and middle parts of the Daliantang Formation at the Daliantang section (Guangnan, Western Yunnan) (Lu and Chen 2016, Fig. 3). However, the conodont faunas during the time of the nothoperbonus Zone at these three sections are somewhat different. The Ertang Formation, which represents a near-shore, oxygen-rich and shallow sea environment abounding in various benthic fossils (the Xiangzhou facies), contains relatively few conodonts with a distinctly low diversity. In contrast, the Daliancun Member of the Yujiang Formation and the Moding Formation at the Liujing section, both of which were deposited in a slightly deeper environment, yield abundant conodonts that can be assigned to six to eight species or subspecies (Lu et al. 2016). However, only four (re-examined) taxa were reported from the Liujing Member of the Yujiang Formation, a lithological unit also characterised by abundant and various benthos and representing a stable and shallow carbonate shelf environment (Kuang et al. 1989). The highest conodont biodiversity up to 13 species or subspecies (only restricted to spathognathodontids and polygnathids) is recorded in the lower and middle parts of the Daliantang Formation at the Daliantang section (manuscript in preparation). In addition to conodonts, extremely abundant pelagic tentaculites were collected from several thin beds in the lower part of the Daliantang Formation. Different from the Ertang and Yujiang formations, the Daliantang Formation at the Daliantang section refers to a relatively deeper sedimentary environment that was previously viewed as a transitional facies between the Xiangzhou facies (neritic facies) and the Nandan facies (pelagic facies) (Yun 1978; Liao et al. 1978; Xian et al. 1980; Hou and Wang 1988). Accordingly, the comparison of conodont faunas at the Lufengshan, Liujing and Daliantang sections suggests that the conodont biodiversity during the time of the nothoperbonus Zone in South China was mainly bathymetrically controlled.

Phylogenetic implications on the inversion of basal cavity of Polygnathus

Due to their wide geographical distribution and their rapid morphological evolution, the Emsian polygnathids have been demonstrated to be excellent index fossils to date and correlate marine rocks (especially limestone) around the world. Therefore, the early evolution of the genus Polygnathus have been extensively studied, and several lineages showing the phylogenetic relationships between the Emsian polygnathids have been proposed in the past 50 years (Philip and Jackson 1967; Klapper and Johnson 1975; Weddige 1977; Weddige and Ziegler 1979; Bardashev 1986; Mawson 1987; Bultynck 1989; Yolkin et al. 1994; Bardashev et al. 2002; Martínez-Pérez et al. 2011; Yolkin et al. 2011; Baranov et al. 2014; Baranov and Blodgett 2016). Although modes of the phylogeny of Polygnathus floated by different researchers differ slightly or greatly from each other, a widely accepted agreement is arrived at that the inversion of basal cavity represents one of the most important and remarkable morphological changes in the early evolution of Polygnathus during the Emsian Age. According to Yolkin et al. (1994), the whole process of the inversion of basal cavity starts from the phylogenetically Early and Late forms of P. nothoperbonus and is accomplished with P. inversus by developing a large pit in the central part of the platform and a posterior keel flanked by parallel striations. In other words, this morphological change takes place during the time of the nothoperbonus Zone. Polygnathids from the Lufengshan section that is mostly assignable to the nothoperbonus Zone provide us more details about the progressive inversion of the basal cavity, which mainly deals with the stratigraphical and morphological analyses of the Late form of P. nothoperbonus and P. praeinversus.

Polyganthus praeinversus was initially interpreted as transitional forms between P. inversus and P. serotinus by Klapper and Johnson (1975) because of the development of a protuberance on the outer side of the pit. Later, similar specimens were reported by Uyeno and Klapper (1980, pl. 8.1, figs. 13–16; pl. 8.3, figs. 4–5), Mawson (1987, pl. 33, figs. 6–8; pl. 36, fig. 9), Uyeno (1990, pl. 7, figs. 41–42; pl. 9, figs. 7–8, 15–16), and Mawson and Talent (2003, pl. 1, figs. 8–10), and Klapper and Johnson’s (1975) opinion was adopted. According to Klapper and Johnson (1975), P. inversus shows its first occurrence in sample LM 24 in the upper part of the Bartine Member, whereas P. praeinversus entries in the stratigraphically higher levels (samples LM 27 and LM 29) in the lowermost part of the overlying Coils Creek Member. Conodont distribution of the Blue Fiord Formation at the type and adjacent areas in southwestern Ellesmere Island also suggests a relatively higher first occurrence of P. praeinversus than that of P. inversus (Uyeno and Klapper 1980). A similar situation is observed at Buchan and Bindi (Mawson 1987) and the Vendom Fiord area (Uyeno 1990). In contrast, P. praeinversus is recorded from a relatively lower level in the nothoperbonus Zone (Upper nothoperbonus Subzone) at the Lufengshan section rather than the inversus Zone like at the Lone Mountain, southwestern Ellesmere Island area, Buchan and Vendom Fiord area; and P. inversus has not been collected from the Upper Member of the Ertang Formation in the present study, neither did Wang and Ziegler (1983). Apparently, P. praeinversus occurs much earlier than P. inversus at the Lufengshan section. Specimens of P. inversus illustrated by Mawson and Talent (2003, pl.1, figs. 8–10) from the Douglas Creek Limestone at the LIL-1 and LIL-2 sections that possesses a remarkably well-developed protuberance on the outer side of the pit are synonymised with P. praeinversus herein. They do also occur in the nothoperbonus Zone. As a result, P. praeinversus cannot be an intermediate taxon between P. inversus and P. serotinus in the point of view of stratigraphic range.

The Late form of Polygnathus nothoperbonus at the Lufengshan section possesses a basal cavity that is inverted posteriorly from the beginning of the inward deflection of the posterior platform (Figs. 4c–h, k–n, q–r and 5a–l). In the middle part of the platform, the basal cavity is flat or shallow, and slightly or clearly asymmetric with a more laterally extended outer flank suspended below the lower surface of the platform (Figs. 4c, e, h, k, q and 5b, c, f, h, i, k, l). The inversion of posterior basal cavity in P. praeinversus also takes place from the beginning of the inward deflection of the posterior platform (Figs. 6n, p, q and 7a, d, f, h, j, k, m, n), which resembles closely to the Late form of P. nothoperbonus; moreover, a semicircular and shelf-like protuberance suspended below the lower surface is also developed on the outer side of the pit in P. praeinversus. However, compared with specimens of P. praeinversus in samples AGP-LFS-24 and AGP-LFS-32 (Figs. 6n, p–q and 7a) from the lower part of the Upper Member of the Ertang Formation, specimens in sample AGP-LFS-67 (Fig. 7f, h) from the uppermost part of the Ertang Formation have a more inverted outer flank of basal cavity in the middle part of the platform and a slightly smaller protuberance on the outer side of the pit. Taking into account the stratigraphic ranges of both species, we suggest that P. praeinversus evolves directly from P. nothoperbonus by the inversion of the inner flank of basal cavity in the middle part of the platform and that the protuberance on the outer side of the pit in P. praeinversus represents the remnant of shelf-like extension the outer flank of basal cavity. A similar stage of the inversion of basal cavity can be similarly corroborated by P. aff. gilberti (Fig. 7o, q), which also develops a shelf-like protuberance on the outer side of the pit and an inverted inner basal cavity in the middle part of the platform. Accordingly, it is indicated by the Lufengshan material that P. praeinversus represents a transitional form between P. nothoperbonus and P. inversus (Fig. 8).
Fig. 8

Proposed lineage of the Emsian polygnathids showing the progressive inversion of the basal cavity on the lower side based on data from the sequence at the Lufengshan section. The phylogenetic relationship between P. perbonus and P. nothoperbonus suggested by Klapper and Johnson (1975) and Yolkin et al. (1994, 2011) is adopted herein

Notes

Acknowledgements

We thank Chanming Yu and Wenkun Qie (Nanjing Institute of Geology and Palaeontology) and Jiangsi Liu (China University of Geosciences, Wuhan) for their assistances in the field. Thomas J. Suttner and an anonymous reviewer read critically this paper and provided many important suggestions and comments from which we have profited largely. This research is financially supported by the National Natural Science Foundation of China (41702009, 41290260 and 41530103) and State Key Laboratory of Palaeobiology and Stratigraphy (Y721053109 and Y726040107). J.C. Liao was supported by the Juan de la Cierva Program.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

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Copyright information

© Senckenberg Gesellschaft für Naturforschung and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • Jianfeng Lu
    • 1
    • 2
    Email author
  • José Ignacio Valenzuela-Ríos
    • 2
  • Chengyuan Wang
    • 1
  • Jau-Chyn Liao
    • 2
  • Yi Wang
    • 1
  1. 1.Nanjing Institute of Geology and PalaeontologyChinese Academy of SciencesNanjingPeople’s Republic of China
  2. 2.Department of Botany and GeologyUniversity of ValenciaBurjassotSpain

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