Abstract: Re-examination of newly collected topotype material confirms that the type species of Yangtzeella, Y. poloi, a widespread Early and Mid Ordovician syntrophiidine brachiopod in South China, has a finely costellate shell rather than being smooth as previously thought. Thus, the subgenus Yangtzeella (Vadimella) Nikitina etal., established on the basis of fine costellae, is invalidated. Among 15 species of Yangtzeella, five species are recognized as valid based on multivariate analyses: Y. poloi, Y. unsulcata, Y. songziensis, Y. kueiyangensis and Y. igori, among which the type species was the oldest known. Six are synonymized: Y. septata, Y. reticulata, Y. lensiformis, Y. depressa, Y. yichangensis and Y. minuta. Four are rejected from Yangtzeella: Y. extensa, Y. similior, Y. yohi and Y. poloi var. minor. Regional biostratigraphy indicates that Yangtzeella first appeared in a relatively deep-water setting on the Lower Yangtze Platform (South China palaeoplate) during late Tremadoc time (Scolopodus warendensis conodont biozone) and then expanded to the deeper Jiangnan Slope as well as to the shallower Upper Yangtze Platform. The genus experienced two episodes of heightened abundance and diversity on the Upper Yangtze Platform during late Dapingian and mid Darriwilian times, respectively. Outside South China, Yangtzeella occurs sporadically in a few microplates or terranes, such as Tarim, Chu-Ili (southern Kazakhstan) and Taurides (southern Turkey) during the Dapingian and Darriwilian. Worldwide, Yangtzeella became extinct by the end of the Darriwilian.
YangtzeellaKolarova, 1925, a syntrophiidine pentameride genus, is one of the characteristic taxa of the Early–Mid Ordovician brachiopod faunas of South China. The genus was established on the basis of Schizophoria poloiMartelli, 1901 from the Dawan Formation (Floian–Dapingian, late Early to early Mid Ordovician) of Yichang district, western Hubei Province, central China. Its relatively large, strongly uniplicate shell and wide distribution on the Yangtze Platform (shellbed-forming at numerous localities) make this globally endemic but regionally ubiquitous genus easily recognizable in numerous Ordovician outcrops in South China.
Several new species of Yangtzeella were proposed during the 1970s–1980s, mostly associated with publications of palaeontological atlases for various regions of China. During the past 10 years, Yangtzeella was also reported from several other palaeoplates or terranes outside South China, such as Tarim (Xu and Sun 1998), Taurides (South Caspian, southern Turkey, Cocks 2000; Torsvik and Cocks 2004), and Chu-Ili Range, southern Kazakhstan (Nikitina et al. 2006). Since the late 1980s, some Chinese workers attempted to refine the taxonomy of Yangtzeella by paying attention to detailed morphological characters (e.g. Zeng 1986). From its initial establishment (Kolarova 1925) to the revised brachiopod volumes of the Treatise on Invertebrate Palaeontology (Carlson 2002), Yangtzeella has been thought to have a smooth shell devoid of any radial ribbing, a misinterpretation owing to poor preservation and a lack of careful observation. Occasional reports of the presence of fine costellae in this genus (e.g. Schuchert and Cooper 1932; Zeng 1986) have been either ignored or treated as rare variations. This has led to the proposal of a new subgenus, Yangtzeella (Vadimella) by Nikitina et al. (2006).
Since 2000, the authors have re-examined many classic Lower–Middle Ordovician sections on the Yangtze Platform, with new fossil collections. As a result of these field investigations, numerous specimens of several species of Yangtzeella have been collected with fine biostratigraphical control, as graptolites are commonly preserved together with, or sandwiched between, shelly beds in all sections measured on the Yangtze Platform. Our observation on large collections suggests that well-preserved shells (or parts of a shell) of Y. poloi invariably have very fine, low-relief costellae. Because of the new fossil and biostratigraphic data, it is now both possible and necessary to attempt a taxonomic revision of Yangtzeella using multivariate analyses of key biometric characters and to discuss the temporal and spatial evolution of Yangtzeella in South China.
Yangtzeella has been studied using thin sections or serial sections by several researchers (e.g. Kolarova 1925; Schuchert and Cooper 1932; Zeng 1977, 1986; Wang and Yan 1978; Xu et al. 1978; Xu and Liu 1984), although microstructures of the spondylium and the septalium-like structure have not been clearly illustrated. It has been uncertain whether Yangtzeella has a spondylium simplex or duplex, a true septalium or a pseudoseptalium, and whether or not crura are present (Carlson 2002). Schuchert and Cooper (1932) sectioned a topotype specimen of Yangtzeella poloi and Yoh (1945) made serial sections from a specimen of Yangtzeella kueiyangensis from Wudang, Guiyang. These authors mentioned the presence of a spondylium simplex, but unfortunately their illustrations do not show clearly the microstructure of the spondylium. Similarly, Nikitina et al. (2006, text-fig. 38) reported that Yangtzeella (Vadimella) has a spondylium simplex, but their serial sections do not show this convincingly.
With regard to the various morphological uncertainties surrounding Yangtzeella, a genus with important implications for pentameride evolution and palaeobiogeography, one of the main objectives of this study is to elucidate its shell morphology and microstructure including:
1 whether fine costellae are always present;
2 whether the spondylium is simplex or duplex;
3 whether the genus has a true septalium;
4 whether the genus possesses true crura.
Both disarticulated valves exposing shell interiors and serial sections of conjoined shells will be used in an attempt to find answers to these long-standing questions.
The Finely Costellate Shells of Yangtzeella
Since its initial description, the shell of Yangtzeella has been thought to be smooth, devoid of radial ornament. Schuchert and Cooper (1932) and Zeng (1986), however, pointed out that some individuals of Yangtzeella have very weak costellae. In South China, Yangtzeella most commonly occurs in mudrocks, and its shells tend to be either abraded to various degrees or covered by a fine mud matrix. In most previous studies, the common presence of fine and delicate costellae has been overlooked because such radial ornament is clearly visible only after the shells are carefully cleaned and examined under a stereomicroscope, preferably with the shell coated with sublimated magnesium oxide or ammonium chloride. A casual observation in the field with a hand lens is usually inadequate to reveal the fine costellae. This has led to the persisting, but erroneous, description of the Yangtzeella shell being smooth. Even when studying large collections of Yangtzeella, Zeng (pers. comm., 2007) only noted rare specimens with fine and low costellae and treated them as an undescribed new genus related to Yangtzeella. Nikitina et al. (2006) named a new subgenus, Yangtzeella (Vadimella) from the Middle Ordovician of Kazakhstan on the basis that the shells have ‘very fine flattened costellae’.
During the course of this study, examination of several large collections of Yangtzeella from the Dawan Formation at the Huanghuachang section (the type locality of Yangtzeella poloi) and its vicinity in the northern Yichang district (Text-fig. 1) indicates that nearly all specimens have fine, delicate, rarely branching, uneven-sized costellae (Text-figs 2, 3). The fine ornament is usually best preserved in the sulcus where abrasion tends to be rare, although a few specimens have costellae on the entire shell surface. In the coarsely silicified shells of Yangtzeella kueiyangensis from the Kuniutan Formation of Wudang area, Guiyang, central Guizhou Province, the fine costellae appear to have been obliterated, with only relatively strong, concentric growth lines or lamellae preserved (Rong et al. 2005).
The Spondylium of Yangtzeella
There has been debate regarding whether Yangtzeella has a spondylium simplex or duplex, although the division of such spondylial structures has been regarded to have dubious taxonomic utility for pentamerides (Williams 1997). In establishing the new genus, Kolarova (1925) regarded Yangtzeella as having a spondylium supported by a median septum (Text-fig. 4) which contains two separate thin plates. Subsequently, Yangtzeella was considered to have a spondylium simplex by several workers (e.g. Schuchert and Cooper 1932; Amsden and Biernat 1965; Carlson 2002; Nikitina et al. 2006). Through his observation using serial sections, however, Zeng (1986) supported the interpretation of a spondylium duplex for the genus, although his published illustrations show only limited details of the spondylial microstructure.
In this study, serial sections clearly demonstrate that the spondylium is formed by two dental plates, composed of dense lamellar layers, that descend from the hinge line for slightly more than half of the valve depth, then converge rapidly along the medial line to become the flanking layers of the median septum (Text-fig. 5). The core of the median septum is a thin layer that consists of granular (and possibly prismatic) calcite (Text-fig. 5A). In this respect, the median septum resembles that of some stricklandiid brachiopods in the suborder Pentameridina (compare Text-fig. 2 to Rong et al. 2005, p. 1146, figs 4.3, 4.4). This implies that Yangtzeella has a spondylium duplex.
The Pseudoseptalium of Yangtzeella
The ‘septalium-like structure’ used for Yangtzeella in the Treatise (Carlson 2002), or ‘cruralium’ (sensuZeng 1986), is treated herein as a pseudoseptalium, as the structure is not formed by a union of the hinge plates (Text-fig. 4C, D). The floor of such a trough structure consists mainly of a muscle pad or myotest (Text-fig. 6).
The pair of septa supporting the ‘septalium-like structure’ (Carlson 2002, p. 946) is analogous, and perhaps even homologous, to the inner hinge plates of other pentamerides, as they are located between the crural base and the dorsal valve floor. Between the crural base and the hinge line, however, the plate structure is more complex in Yangtzeella than in most other pentamerides (Syntrophiidina or Pentameridina). The socket plates (sensuCarlson 2002) that descend from the hinge line are parallel to each other in their posterior portions, becoming medially inclined (but not united) to each other only distally (Text-fig. 6C). The socket plates sit on a pair of relatively small crural plates, which extend ventro-laterally from the crural bases at a very wide angle and extend anteriorly for nearly as far as the crura (Text-fig. 7). The crural plates originate in the lateral cavities and are clearly separate brachidial elements from the socket plates. Judging from their shape and spatial relationship to the crura, the crural plates are probably most analogous (or homologous) to the outer hinge plates of other pentamerides. Serial sections of several specimens indicate that none of these plate structures medially coalesce to form a true septalium. Rather, a prominent muscle pad (myotest) forms a lining along the inner sides of the sockets and crural plates, creating a septalium-like trough structure (Text-fig. 6A, B). For this reason, this structure is called a pseudoseptalium in Yangtzeella.
Species Groups of Yangtzeella
Differentiation among species of Yangtzeella has been based mainly on shell outline (usually measured by length/width and hinge width/width ratios) and the configuration of sulcus and fold, particularly their starting positions (distance from beak) and amplitude. In this study, a quantitative analysis is carried out to help define the various species of Yangtzeella, using a series of biometric measurements, including ventral valve length (L), dorsal valve length (L1), shell width (W), position of maximum width (L2), hinge width (W1), shell thickness (T), sulcus width (W2), sulcus height (H), ventral interarea height (L3), dorsal interarea height (L4) and starting position of sulcus (L5) (Text-fig. 8). Ratios of L/W, L2/L, L5/L, W1/W, W2/W and H/W2 are used for multivariate analyses for all 13 known species of Yangtzeella. Principal component analysis (PCA) and cluster analysis (CA) suggest that five species groups can be recognized (Text-fig. 9). Group C comprises a single taxon, Clarkella extensa Wang inWang and Yan, 1978 which has been regarded as Yangtzeella by some Chinese workers but is excluded in this study. Thus, four species groups of Yangtzeella are recognized in the multivariate analyses.
Group A includes Yangtzeella kueiyangensis from South China and Y. igori from Kazakhstan. Yangtzeella kueiyangensisYoh, 1945 (pl. 1, figs 1–6) occurs in the Kuniutan Formation (Darriwilian) at Wudang near Guiyang, central Guizhou Province, the Dawan and Meitan formations (mainly Dapingian) in Yichang, western Hubei Province, and Yanhe, northeastern Guizhou (Xu and Liu 1984), the Dongguashan Formation (Dapingian) in Hetian and Pishan, Karakorum, southwestern Xinjiang (Xu and Sun 1998), and the Dashaba Formation (Darriwilian) in Changning, southern Sichuan (Zhan and Jin 2005). It is characterized by a nearly equidimensional shell (L = W), strong concentric growth lamellae, a fold and sulcus originating near the umbo, and the sulcus about two-thirds as deep as wide (see Appendix). The shells described as Y. kueiyangensis by Xu and Sun (1998) from Xinjiang differ from the type material from the Yangtze Platform because of their weaker shell convexity and the starting position of fold and sulcus close to middle length. It may be more closely related to Y. poloi.
Yangtzeella igori from the Uzunbulak Formation (Darriwilian) of Chu-Ili Range, southern Kazakhstan is similar to Y. kueiyangensis in having the fold and sulcus originating just anterior of the shell umbo, but differs in having a much wider shell (L/W ratio average at 0.69, compared to 0.9 for Y. kueiyangensis).
Group B comprises two species: Yangtzeella songziensis and Y. yichangensisZeng, 1977; the former occurs in the basal Dawan Formation of Hubei Province (Zeng 1977; Wang and Yan 1978), and the latter is known only from Floian beds of the lower Dawan Formation, Yichang district (Zeng 1977; Wang 1984). These two species have a similarly equidimensional shell (L/W ratio = 0.97 and 0.96 respectively) and a strong ventral sulcus that originates at level of maximum width (L2/L and L5/L ratios are nearly equal; see Appendix). Other diagnostic characters include a globular profile because of its strongly convex dorsal valve, and a deep sulcus (deeper than wide) originating between one-third and one-half length of the shell.
Rong et al. (2005) regarded these two species as junior synonyms of Yangtzeella poloi. The numerical analysis in this study, however, indicates that Y. poloi has a different shell outline with L/W ratio around 0.78. Thus, Yangtzeella songziensis can be treated as a valid species and Y. yichangensis as its junior synonym.
Group C consists of a single species, Clarkella extensa Wang inWang and Yan, 1978, initially described from the Dawan Formation of Zigui, southwestern Hubei Province but subsequently regarded as a species of Yangtzeella by Rong et al. (2005). Compared to typical forms of Yangtzeella, Clarkella extensa differs in several aspects: (1) the maximum width of the shell is attained largely at the position of the hinge line; (2) the gently biconvex shell lacks a fold or sulcus, resulting in a rectimarginate anterior commissure; (3) presence of spondylium simplex; and (4) septalium-like structure is absent. Rong et al. (2005) treated this species as a junior synonym of Yangtzeella poloi, but here it is rejected from Yangtzeella. Its taxonomic identity requires further study.
Group D contains three species, including the type species (Text-fig. 9). Yangtzeella poloi is widespread on the Yangtze Platform but known also from Dapingian rocks of the Tarim terrane (Xu and Sun 1998) and the Sobova Limestone (upper Dapingian) of Seydişehir, southern Turkey (Cocks 2000).
Yangtzeella septata Xu inLiu et al. (1983, p. 282, pl. 93, figs 18–21) from the upper Upper Ouchong Formation (late Tremadoc) of Chuxian, eastern Anhui Province (lower Yangtze Platform) is the oldest species known for the genus and has not been found elsewhere. This species is similar to Y. poloi in its general shell outline and commencement of fold and sulcus, but its shell is much smaller, and its L/W ratio (0.91) is greater than the maximum value for Y. poloi. Its hinge width is shorter than that of Y. poloi. It might be the primitive form of Y. poloi.
Yangtzeella reticulataEndo (1932, p. 50, pl. 36, fig. 9) was based on a dorsal valve from the Ordovician rocks near Huangbayi of Ningqiang, southern Shaanxi Province, central China, and is similar to Y. septata in several biometric parameters (see Appendix). Rong et al. (2005) proposed to reject this species from Yangtzeella because of its fine radial ornamentation. Both Y. septata and Y. reticulata are possibly junior synonyms of Y. poloi, but a further taxonomic revision requires more material.
Group E includes Yangtzeella unsulcata, Y. lensiformis, Y. minuta and Y. similior (Text-fig. 9). Y. unsulcata (Xu, Rong and Liu, 1974) was first reported as Clarkella from the lower Meitan Formation (Floian) of Sinan, northeastern Guizhou and was found later also in the coeval strata in Yanhe, NE Guizhou by Xu and Liu (1984, p. 214, pl. 17, figs 12–14) when they reassigned this species to Yangtzeella. It is characterized by a small to medium-sized shell, about six-sevenths as long as wide, with very weak fold and sulcus originating at about two-thirds of shell length, the maximum shell width at about three-tenths of shell length from apex, and longitudinal side septa absent in both valves.
Yangtzeella lensiformis Wang inWang and Yan, 1978 is known only from the Dawan Formation (Floian to Dapingian) of Fenxiang, northern Yichang district, western Hubei. It has several biometric parameters similar to those of Y. unsulcata (see Appendix), particularly its shell outline and weak fold and sulcus originating in anterior one-third of the shell. It is interpreted as a possible junior synonym of Y. unsulcata (Rong et al. 2005).
Yangtzeella minuta Xu inXu et al., 1978 was described initially from the lower Dawan Formation (Floian) at Rongxi, Xiushan, southern Chongqing District, and reported later also from the lower Meitan Formation (Floian) at Baisha, Shiqian, northeastern Guizhou Province (Xian and Jiang 1978). It is characterized by a small to medium-sized shell, with several biometrics similar to those of Y. unsulcata, such as the ratio of L/W, L2/L, L5/L, W1/W and H/W2. Therefore, it is treated herein as a junior synonym of Y. unsulcata rather than of Y. poloi as suggested by Rong et al. (2005), as the values of their L/W ratio are significantly different (0.93 vs 0.78).
Yangtzeella similiorEndo, 1935 was reported from the Lower Ordovician of northeastern China, making it the only occurrence of Yangtzeella so far reported from the North China palaeoplate. In detailed morphology, however, it has a small, rather transversely extended shell, with relatively strong costellae and concentric rugae. No information is available about its internal structures. In this study, it is provisionally excluded from Yangtzeella.
Yangtzeella depressaXu and Liu (1984, p. 214, pl. 17, figs 5–8, 10, 11, text-fig. 18) from the lower Meitan Formation (Floian) in Yanhe, northeastern Guizhou Province is characterized by a small shell with poorly developed fold and sulcus and the absence of side septa in both valves. These characters make Y. depressa similar to Y. unsulcata and Y. lensiformis, which corroborates the study of Rong et al. (2005) in treating it as a junior synonym of Y. unsulcata. Y. depressa is not included in the multivariate analyses because of the lack of well-preserved material for biometric measurements.
Yangtzeella? sp. Zhan and Jin (2008) from the lower Tonggao Formation (early Floian) is very small in size (rarely larger than 10 mm) and shares most of the external and internal characters with Y. unsulcata, Y. depressa and Y. lensiformis. It is the only known representative of Yangtzeella found in a slope facies of early Floian age. More recent study suggests that this form is most similar or even conspecific to Y. unsulcata (Xu, Rong, and Liu, 1974).
The illustrated specimens and the acetone peels of the sectioned specimens in this article are deposited in Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences, Nanjing, China, and the University of Western Ontario (W), London, Ontario, Canada.
Remarks. Popov et al. (2005) noted that taxa of the family Porambonitidae do not have a spondylium and thus differ from those of other families that used to be assigned to the superfamily Syntrophioidea Ulrich and Cooper, 1936 but are now included in the superfamily Porambonitoidea (see Carlson 2002). Yangtzeella and other genera of the family Clarkellidae have a well-developed spondylium and, according to the concept of Popov et al. (2005), should be assigned to the resurrected superfamily Syntrophioidea, rather than to the superfamily Porambonitoidea as in the revised brachiopod volumes of the Treatise on Invertebrate Paleontology (Carlson 2002). However, the problem of superfamilial taxonomy of the Porambonitoidea and the Syntrophioidea has not been resolved and is beyond the scope of this study. Thus, the classification scheme of the Treatise is followed tentatively herein.
1978 Yangtzeella Kolarova; Xian and Jiang, p. 276.
1978 Yangtzeella Kolarova; Xu et al., p. 314.
1982 Yangtzeella Kolarova; Fu, p. 126.
1983 Yangtzeella Kolarova; Liu et al., p. 282.
1984 Yangtzeella Kolarova; Xu and Liu, p. 212.
1986 Yangtzeella Kolarova; Zeng, p. 79.
2005 Yangtzeella Kolarova; Rong et al., p. 1232.
2005 Yangtzeella Kolarova; Zhan and Jin, p. 48.
2006 Yangtzeella (Vadimella) Nikitina et al., p. 204.
Type species. Schizophoria poloiMartelli, 1901, p. 302, pl. 4, figs 17–22. Dawan Formation, Dapingian–Darriwilian, Huanghuachang, Yichang district, Hubei Province, central China.
Diagnosis. Shell medium to large, finely costellate, biconvex, usually with strong ventral sulcus and weaker dorsal fold. Spondylium duplex broad, shallow, bearing pair of longitudinal ridges on its floor, supported by low median septum in its anterior part; several low septa on valve floor, supporting spondylium posteriorly. Socket plates descending dorsally, parallel to each other posteriorly, inclined dorso-medially (but not united) anteriorly; crural plates (analogous to outer hinge plates) long, each being fused posteriorly between crural base and socket plate, becoming free-hanging anteriorly but supporting crus from outside; inner hinge plates relatively high, divergent from each other dorso-laterally. Thick muscle pad (myotest) lining between socket and crural plates, forming pseudoseptalium. Floor of dorsal valve with numerous low, longitudinal septa (emended herein).
Remarks. In the light of the consistently developed fine costellae now recognized for the type species of Yangtzeella, the subgenus Yangtzeella (Vadimella) Nikitina et al., 2006 is no longer necessary because it was distinguished on the basis of its fine costellae and our previously inadequate knowledge of the typical Yangtzeella.
Species assigned. There have been 14 species of Yangtzeella reported from South China. Biometric analysis in this study partly corroborates Rong et al.’s (2005) revision in recognizing four Chinese species, with some differences in the grouping of synonymous species (see detailed discussion in the previous section).
1Schizophoria poloiMartelli, 1901; Dawan Formation (Dapingian–Darriwilian), Yichang, western Hubei Province, central China. Junior synonyms include: Yangtzeella septata Xu inLiu et al., 1983; upper Upper Ouchong Formation (late Tremadoc), Chuxian, eastern Anhui Province, E. China. Yangtzeella reticulataEndo, 1932; Ordovician rocks near Huangbayi of Ningqiang, southern Shaanxi Province, central China. Yangtzeella cf. kueiyangensis Yoh (see Xu and Sun 1998); Dongguashan Formation (Dapingian), Hetian and Pishan counties, Karakorum region, Xinjiang, NW China.
2Clarkella unsulcataXu, Rong and Liu, 1974; lower Meitan Formation (Floian), Sinan, northeastern Guizhou, SW China. Junior synonyms include: Yangtzeella lensiformis Wang inWang and Yan, 1978; Dawan Formation (Floian–Dapingian), Fenxiang, northern Yichang, western Hubei, central China. Yangtzeella minuta Xu inXu et al., 1978; lower Dawan Formation (Floian), Rongxi, Xiushan, southern Chongqing District and lower Meitan Formation (Floian), Baisha, Shiqian, northeastern Guizhou Province, SW China. Yangtzeella depressaXu and Liu, 1984; lower Meitan Formation (Floian), Yanhe, northeastern Guizhou Province, SW China. Yangtzeella sp., Zhan and Jin 2008; lower Tonggao Formation (early Floian), Sandu, southeastern Guizhou Province, SW China.
3Yangtzeella songziensisZeng, 1977; basal Dawan Formation (Floian), Songzi, southwestern Hubei Province, central China. Junior synonym includes: Yangtzeella yichangensisZeng, 1977; lower Dawan Formation (Floian), northern Yichang, western Hubei Province, central China.
1925 Yangtzeella poloi (Martelli); Kolarova, p. 215, pl. 1, figs 2–13.
1932 Yangtzeella reticulata Endo, p. 50, pl. 36, fig. 9.
1936 Yangtzeella poloi (Martelli); Wirth, p. 301, pl. 20, figs 20–22.
1955 Yangtzeella poloi (Martelli); Wang, p. 128, pl. 68, figs 15–18.
1964 Yangtzeella poloi (Martelli); Wang et al., p. 166, pl. 22, figs 27–30.
1975 Yangtzeella poloi (Martelli); Fu, p. 107, pl. 12, figs 10–11.
1977 Yangtzeella poloi (Martelli); Zeng, p. 52, pl. 17, figs 15–17.
1978 Yangtzeella poloi (Martelli); Xian and Jiang, p. 276, pl. 98, figs 1–4.
1978 Yangtzeella poloi (Martelli); Xu et al., p. 314, pl. 117, fig. 12.
1978 Yangtzeella poloi (Martelli); Wang and Yan, p. 226, pl. 57, figs 1–4; pl. 58, figs 1–4.
1982 Yangtzeella poloi (Martelli); Fu, p. 126, pl. 36, figs 8–9.
1983 Yangtzeella poloi (Martelli); Liu et al., p. 282, pl. 93, fig. 17.
1983 Yangtzeella septata Xu; Liu et al., p. 282, pl. 93, figs 18–21.
1984 Yangtzeella poloi (Martelli); Xu and Liu, p. 213, pl. 16, figs 12–18; pl. 18, figs 1–4.
1986 Yangtzeella poloi (Martelli); Zeng, p. 79, pl. 1, figs 1–16; pl. 2, figs 1, 2, 5.
1998 Yangtzeella poloi (Martelli); Xu and Sun, p. 155, pl. 4, figs 3–7.
Description. Shell medium- to large-sized, transversely subelliptical to subquadrate in outline, with average length/width ratio of 0.78 in adults, dorsibiconvex, with strongest convexity at about mid-length of shell. Hinge line straight, approximately seven-tenths shell width (Text-fig. 10), with rounded cardinal extremities. Maximum shell width at about one-half to three-fifths shell length. Anterior commissure prominently uniplicate. Ventral sulcus originating at about one-half to three-fifths valve length, widening and deepening rapidly towards anterior margin to occupy about three-fifths shell width (Text-fig. 11). Dorsal fold rounded, without clearly delimited boundaries from shell flanks, best developed in anterior one-fifth of shell. Ventral interarea apsacline, planar or slightly concave with curved beak, with average height at 11 per cent of valve length. Delthyrium open, with weak delthyrial ridges. Dorsal interarea about half as high as ventral (see Appendix), anacline to nearly orthocline; notothyrium open. Costellae fine, low; concentric growth lines weak. Concentric lamellae unevenly spaced in anterior half of shell.
Delthyrial cavity large, relatively shallow. Teeth stout, wedge-shaped, with rounded crests. Massive secondary shell thickening present in umbonal area. Spondylium duplex relatively broad, shallow, supported in its anterior part by low, thick median septum and apically by several side septa, which become low and separated from spondylium anteriorly (Text-fig. 7). Dental plates converging medially to form outer layers of spondylium and median septum, continuous with lamellar layer of valve floor (Text-figs 5, 7); median portion of spondylial floor lined with thick myotest; muscle field clearly differentiated into medially located adductor scars and flanking diductor scars, separated by pair of delicate ridges on spondylial floor (Text-fig. 5). Posterior portion of valve floor bearing up to 20 low radiating ridges (septa).
Sockets slender, open antero-laterally; outer socket ridges low, thick; inner socket ridges strong; socket plates relatively small, descending dorsally in subparallel orientation to each other in their posterior portions, becoming medially inclined (but not united) to each other in their anterior portions (Text-figs 6, 7). Crural plates (‘outer hinge plate’) well developed, strongly divergent ventro-laterally from each other at about 130 degrees, extending anteriorly from lateral cavities of valve to become prominent plates, which support crura from the sides. Inner hinge plates high, weakly divergent from each other towards valve floor. Thick muscle pad (myotest) lining inner sides of socket plates and crural plates, and bridging inner edges of crural plates, thus forming pseudoseptalium (Text-figs 6, 7). Crura strong, platy, extending for slightly more than one-third of valve length. Dorsal median septum thin, low, not supporting pseudoseptalium, originating from umbonal cavity and terminating anteriorly before mid-length of valve. Septa on valve floor similar to those of ventral valve in number and strength, but shorter. Muscle field poorly impressed. Mantle canal system unknown.
Remarks. The Turkish shells, reported as Yangtzeella poloi by Cocks (2000, p. 6, fig. 7b), are noticeably more elongate than the typical material from South China. Also, the Turkish material shows a dorsal fold that originates near the umbo, resembling that of Yangtzeella kueiyangensis.
Examination of large collections of the Y. poloi reveals several significant intraspecific variations:
1Shell outline. Normally transverse subrectangular with length/width ratio at 0.78, but some individuals relatively elongate about two-thirds as long as wide, and some others more transverse with L/W ratio at 0.86.
2Hinge width. Most shells have their maximum width at about three-fifths shell length while their hinge width about seven-tenths shell width, but some shells with much wider hinge line up to five-sixths shell width.
3Dorsal convexity. Dorsal valve of Yangtzeella poloi is normally evenly convex when the shell thickness is a little more than half shell width. There are also a certain amount of individuals having a dorsal valve strongly convex in central-medial part causing the thickness/width ratio up to 0.63.
4Ventral sulcus. More than half the individuals of Y. poloi in the studied collection have their sulcus originating near the maximum width of the shell (L2 about the same as L5), but there are some ontogenetic changes. In relatively small shells, the sulcus originates posterior to the maximum shell width (i.e. L5<L2), whereas in some large individuals the sulcus commences significantly anterior to the maximum shell width (i.e. L5>L2) (Text-fig. 12). The shape of the sulcus also changes from one individual to another. It may be broad, shallow, nearly one-third as deep as wide in some shells, but narrow, deep, nearly as deep as wide in others.
Temporal and Spatial Evolution of Yangtzeella
In South China, the oldest known Yangtzeella-bearing brachiopod fauna appeared in a relatively deep-water environment. Yangtzeella poloi from the grey, thin- to medium-bedded dolomitic limestone in the upper part of the Upper Ouchong Formation, Chuxian, eastern Anhui Province (Liu et al. 1983) (Text-fig. 13), for example, occurs in a rich shelly fauna comprising mainly brachiopods (such as Nanorthis, Archaeorthis, Imbricatia) and nautiloids (e.g. Rectroclintendoceras, Proterocameroceras, Kirkoceras, Chuxianoceras; see Zhu et al. 1986; Anhui Geological Bureau 1997). Palaeogeographically, the hosting lithofacies was located on the Lower Yangtze Platform, far removed from any exposed landmasses, in a BA4 or slightly deeper environmental setting (Zhu et al. 1986; Chen et al. 1995). Associated conodonts indicate that the shelly fauna is of late Tremadoc age (Scolopodus warendensis conodont biozone, correlative to the Glyptoconus quadraplicatus-Paltodus deltifer conodont biozone on the Upper Yangtze Platform (An et al. 1985; An 1987; Chen et al. 1995; Anhui Geological Bureau 1997)).
Shortly after its first appearance, Yangtzeella became widespread over the Lower Yangtze Platform, where it is known from numerous localities of the Dawan Formation of Floian–Dapingian age (Jiangsu Geological Bureau 1988; Anhui Geological Bureau 1997). However, the greatest expansion of the genus took place in the vast area of the Upper Yangtze Platform, resulting in an increase in both its general abundance and species diversity.
In the eastern-central part of the Upper Yangtze Platform, Yangtzeella first occurs in the basal part of the Meitan Formation (Acrograptus filiformis Biozone) at the Shatuo section near Yanhe, northeastern Guizhou (Text-fig. 13). Here, Yangtzeella is associated with other brachiopods such as Nocturnellia, Sinorthis, Xinanorthis, Leptella and Tarfaya to form a Nocturnellia subcommunity of the Paralenorthis Community in a lower BA3 palaeoenvironmental setting (Zhan and Rong 2006). Through much of the Meitan Formation (up to the top of the Expansograptus hirundo Biozone), Yangtzeella is a common, but rarely the predominant, component of all samples (up to 10 per cent of the individuals per sample; Text-fig. 13; see also Zhan et al. 2006).
In the Yichang district of western Hubei Province, palaeogeographically the northeastern part of the Upper Yangtze Platform, Yangtzeella poloi first occurs near the base of the Dawan Formation (lower part of the Didymograptellus eobifidus Biozone, Text-fig. 13) and has been found in all samples collected up to the top of the formation (see Zhan et al. 2007). In the lower Dawan Formation, Y. poloi is usually common, making up 10–20 per cent of the brachiopod individuals per sample. It becomes predominant in the upper part of the formation, making up 50 per cent or more of total individuals in many samples, which were recognized as a characteristic Yangtzeella-Nereidella Community (Zhan et al. 2007). In the overlying lower–middle Kuniutan Formation (Darriwilian), Y. poloi occurs sporadically, commonly associated with more abundant shelly fossils such as nautiloids and trilobites, where microfossils (e.g. ostracodes and conodonts) are also common (Zeng et al. 1987; Zhang 1998).
In the Tongzi area of northern Guizhou, palaeogeographically the central part of the Upper Yangtze Platform, the Ordovician sequence is complete, thick and dominated by siliciclastic deposits, in which Yangtzeella is uncommon in the otherwise rich brachiopod fauna of Floian-Dapingian age (Zhang et al. 1964; Zhan et al. 2005, 2006). Here, it first appears in the lower Meitan Formation (lower part of the Didymograptellus eobifidus Biozone, Text-fig. 13) and extends upwards to the upper part of the Corymbograptus deflexus Biozone (see Zhan et al. 2005). It usually forms a subordinate taxon (<10 per cent) of the Sinorthis, Paralenorthis and Desmorthis communities that occupied mid-shelf equivalent environments (lower BA2 to lower BA3; see Zhan and Rong 2006).
In the Ordovician sequence at Dabaochang, near the Qiaojia county town of northeastern Yunnan Province (palaeogeographically proximal to the Dianzhong and Xichang highlands (Text-fig. 14)), Yangtzeella was found recently in the argillaceous limestone of the upper Lower Qiaojia Formation (Exigraptus clavus-Undulograptus austrodentatus biozones), in a brachiopod assemblage that contains also Leptellina, Martellia and Porambonites. Yangtzeella constitutes approximately 20 per cent of the individuals in the collection. Preliminary study suggests that the shelly fauna lived in a BA2 setting on the basis of high abundance but low diversity of shelly fauna, and the grainstone substrate usually with some poloids and oolites (corresponding to a shallow subtidal sedimentological environment; Liu Jianbo, pers. comm. 2008).
In the Kuniutan Formation (Darriwilian) at Wudang, 20 km northeast of Guiyang city proper, central Guizhou Province (Text-fig. 14), Yangtzeella kueiyangensis occurs as a dominant species in a silicified brachiopod fauna, making up approximately one-third of the individuals in two large collections that define a Yangtzeella kueiyangensis–Orthambonities delicata association of mid-shelf origin (Rong et al. 2005). In this area, Yangtzeella is largely absent in the underlying Meitan Formation (Floian to Dapingian) or the overlying Huanghuachong Formation (early Sandbian), except for a putative report of Y. kueiyangensis from the top part of the Meitan Formation (Guizhou Geological Bureau 1987).
The occurrence of Yangtzeella in near-shore depositional environments is typified by Y. kueiyangensis in the Dashaba Formation (Darriwilian) of the Changning area, southern Sichuan Province, palaeogeographically located close to a chain of emergent islands (known as ‘oldlands’ in Chinese literature) along the western margin of the Upper Yangtze Platform (Zhan and Jin 2005, 2007; Text-fig. 14). In this area, however, Yangtzeella is absent from both the underlying Daguanshan Formation (Floian to earliest Darriwilian) and the overlying Pagoda Formation (early to mid Sandbian; see Zhan et al. 2005). The brachiopod collections from the Dashaba Formation usually contain Orthambonites, Saucrorthis, Parisorthis, Calyptolepta, Leptastichidia, Pentagomena, Heteromena, Glyptomena, etc., with Yangtzeella making up less than 10 per cent of the individuals. Two Yangtzeella-bearing brachiopod associations are recognized: the Saucrorthis–Parisorthis association (lower BA3 environment) in the main part of the Dashaba Formation and the Orthambonites–Pentagomena association (lower BA2 benthic environment) near the top of the formation.
Yangtzeella is represented by its presence in the lower Tonggao Formation (middle Tetragraptus approximatus Biozone) in the Sandu area of southeastern Guizhou Province, as part of the deep-water Paralenorthis–Nereidella brachiopod association (BA5 to 6); see Zhan and Jin 2008). Within the association, Y. unsulcata constitutes about 19 per cent of the brachiopod individuals. Preserved together with the shelly fossils are some graptolites, such as Dictyonema, Expansograptus and Tetragraptus. Several brachiopod taxa in the Tonggao Formation are the oldest known representatives of some important genera in South China, such as Paralenorthis, Nereidella and Euorthisina. These taxa first appeared in a BA5 to 6 setting (continental shelf-slope transitional zone) during early Floian time and subsequently invaded the shallower-water Yangtze Platform to flourish and develop into successful brachiopod communities by late Floian time (Zhan and Jin 2008).
In summary, Yangtzeella first appeared in South China in a relatively deep-water environment on the Lower Yangtze Platform during late Tremadoc time. Subsequently, the genus seems to have expanded into both deeper and shallower habitats. Its occurrence in the deep-water (BA5 to 6) Tonggao Formation, however, was short-lived, being confined to the early Floian. Its palaeoecological expansion into the vast area of the Yangtze Platform was much more successful. Yangtzeella first reached some mid-shelf equivalent settings in the central part of the Upper Yangtze Platform (e.g. Yanhe, Yichang and Tongzi; Text-fig. 14) and flourished particularly in the northeastern part of the Upper Yangtze Platform (today’s Yichang district), where the depositional setting was characterized by a calcareous muddy substrate that was favourable for the thriving Y. poloi. From late Tremadoc to late Darriwilian time, Yangtzeella migrated progressively westward from offshore to nearshore localities. Available data suggest that Yangtzeella became extinct by the latest Darriwilian in South China. During the Dapingian (early Mid Ordovician), Yangtzeella started to appear outside the South China palaeoplate, concomitant with its palaeogeographical and palaeoecological expansions on the Yangtze Platform. In the other regions (e.g. the Chu-Ili, Taurides and Tarim terranes), Yangtzeella remained a minor taxon of each particular brachiopod fauna, generally rare and of low species diversity.
Acknowledgements. Several seasons of field work were conducted jointly with our colleagues Rong Jiayu, Zhou Zhiyi, Wang Yi, Liu Jianbo, Zhang Yuandong, Yuan Wenwei, Cheng Jinhui, Luo Tiantian, Chen Jinchun, Zhang Jin, Wu Rongchang and Li Guipeng. The careful reviews of Ian Percival, an anonymous reviewer and the Editor Svend Stouge greatly improved the clarity of our discussions. Financial support for this project was provided by the Chinese Academy of Sciences (KZCX2-YW-Q05-01), the National Natural Science Foundation of China (40825006), the Ministry of Science and Technology of China (2006CB806402), the State Key Laboratory of Palaeobiology and Stratigraphy (20081103) and the Natural Sciences and Engineering Research Council of Canada (JJ).
Editor. Svend Stouge
Statistical data of shell measurements. See Text-figure 8 for abbreviations of all parameters. AVG (A), average; STD (S), standard deviation; MIN (N), minimum; MAX (X), maximum. All measurements are in mm.