Remarks

Nikiforova (Reference Nikiforova1978) established Schachriomonia based on the type species Schachriomonia schachriomonica from the Archalyk Beds (upper Katian) of Uzbekistan, but several other genera from other regions are now considered junior synonyms (Zhan and Cocks, Reference Zhan and Cocks1998; Popov et al., Reference Popov, Nikitin and Sokiran1999; Rong et al., Reference Rong, Zhan, Huang, Xu, Fu, Li, Rong, Jin, Shen and Zhan2017).

In China, these genera include Ovalospira Fu, Reference Fu1982 and Taoqupospira Fu, Reference Fu1982 (see Rong et al., Reference Rong, Zhan, Huang, Xu, Fu, Li, Rong, Jin, Shen and Zhan2017 for a description and discussion of these genera in English). In their remarks on Ovalospira, Zhan and Cocks (Reference Zhan and Cocks1998) synonymized Taoqupospira from the Upper Ordovician of the western margin of the North China Platform along with Zygospira (Kuzgunia) Klenina, Reference Klenina and Bandaletov1984 from the Ashgillian (middle Katian) Dulankara Formation of Kazakhstan based on similarities to their material. They tentatively considered Ovalospira part of the Zygospiridae (a family now synonymized under the Anazygidae; see Copper, Reference Copper and Kaesler2002) based on its primitive atrypide characteristics and the presence of ribs that would preclude it from the Cyclospiridae but noted that it was somewhat problematic to classify.

Popov et al. (Reference Popov, Nikitin and Sokiran1999) suggested that Ovalospira should be considered a junior synonym of Schachriomonia based on similarities in their costate radial ornamentation, the weakly uniplicate anterior commissure with shallow ventral sulcus originating near the anterior of the shell, and a weak dorsal median fold composed of two slightly accentuated ribs that is reversed from a sulcus near the posterior. They also suggested that the genus would be better classified under the subfamily Spirigerinidae alongside the similar early atrypides Eospirigerina, Boucot and Johnson, Reference Boucot and Johnson1967; Qilianotryma Xu, Reference Xu, Jin, Ye, Xu and Sun1979; and Sulcatospira Xu, Reference Xu, Jin, Ye, Xu and Sun1979. This is probably more appropriate, as Schachriomonia has separate jugal processes while Anazygidae (to which the Zygospiridae have since been assigned) possesses a dorsally to centrally located fused jugum (Copper, Reference Copper1977; Copper, Reference Copper and Kaesler2002). Popov et al. (Reference Popov, Nikitin and Sokiran1999) also provided a detailed discussion of Kuzgunia Klenina, Reference Klenina and Bandaletov1984 in English, summarizing the Russian references and correcting the authorship of the genus.

Copper (Reference Copper and Kaesler2002) considered Ovalospira, Kuzgunia, Taoqupospira, and the contemporaneous atrypide Antizygospira all junior synonyms of Sulcatospira. Rong et al. (Reference Rong, Zhan, Huang, Xu, Fu, Li, Rong, Jin, Shen and Zhan2017), however, noted that the evident V-shaped and well-defined sulcus and corresponding dorsal fold separate Sulcatospira from the broad and shallow U-shaped sulcus and low fold of Ovalospira. In addition, a median dorsal furrow, where the dorsal ribs fuse to become a single continuous ridge on the crest of the dorsal fold, present in Ovalospira is absent in Sulcatospira.

We tentatively agree with the opinions of Popov et al. (Reference Popov, Nikitin and Sokiran1999) and Rong et al., (Reference Rong, Zhan, Huang, Xu, Fu, Li, Rong, Jin, Shen and Zhan2017) in considering Ovalospira along with Taoqupospira and Kuzgunia to be synonyms of Schachriomonia but separate from Antizygospira and Sulcatospira. Rong et al. (Reference Rong, Zhan, Huang, Xu, Fu, Li, Rong, Jin, Shen and Zhan2017) considered Ovalospira Fu, Reference Fu1982 to be the senior synonym, but Schachriomonia Nikiforova, Reference Nikiforova1978 has priority given that it was established four years earlier.

Schachriomonia is very similar to Sulcatospira and could well be congeneric, but a study of additional collections that include well-preserved shells with intact brachidia from both genera is necessary to confirm this. According to Rong et al. (Reference Rong, Zhan, Huang, Xu, Fu, Li, Rong, Jin, Shen and Zhan2017), Sulcatospira differs from Schachriomonia in having a V-shaped, rather than a U-shaped, sulcus, but the description of Schachriomonia by Popov et al. (Reference Popov, Nikitin and Sokiran1999) includes mention of a V-shaped sulcus, so the division is apparently at least somewhat ambiguous. The fused ribs that form the fold in Sulcatospira could be a consequence of poor shell preservation, with abrasion and/or dissolution of the shell surface causing the apparent disappearance of the medial furrow of Schachriomonia. If these genera prove not to be congeneric, they are very closely related, as they are apparently similar in almost every other way.

Antizygospira can still be differentiated from the preceding genera largely on the basis of its ribbing and overall smaller size: while the preceding genera have simple costae with only rare bifurcations, Antizygospira exhibits prominent bifurcation and intercalation of ribs. In the shell interior, Antizygospira lacks true dental plates (Rong et al., Reference Rong, Zhan, Huang, Xu, Fu, Li, Rong, Jin, Shen and Zhan2017) while Schachriomonia has thin dental plates. Antizygospira is usually considerably smaller than the other atrypide genera reported from China.

Type specimens

No holotype was selected by Fang in the original description of the species, and the original type material cannot be located. We select NIGP 168782 from our collection as the neotype from the upper Katian Hadabulaktag Formation. The original collection locality of Liu and Fang (Reference Liu and Fang1990) is uncertain, but the current collection is from the same region near Uligezhitag and is at least equivalent to the original type material if not from the exact same locality.

Diagnosis

Small to medium-sized shell with subpentagonal to subcircular outline, ventribiconvex to biconvex, unisulcate anterior in smallest shells becoming rectimarginate and uniplicate in larger shells forming prominent tongue at anterior, prominent median furrow on dorsal fold, and prominent single median costae in dorsal sulcus, spiralia with seven to eight whorls, short jugal process.

Occurrence

Upper Hadabulaktag Formation (upper Katian), in the Kuruktag region of Xinjiang, Northwest China.

Description

(Emended from Liu and Fang, Reference Liu and Fang1990.) Shell small to medium-sized, with length and width approximately 5–15 mm and thickness approximately 2–10 mm; pentagonal to subcircular in outline, lateral profile ventribiconvex to biconvex. Shell equally as long as wide in small shells, becoming approximately 75% as long as wide in larger shells; 30% to 82% as thick as wide. Astrophic hinge line measuring approximately one-third of shell width. Anterior commissure unisulcate in small shells becoming rectimarginate, parauniplicate, and uniplicate in progressively larger shells. Ribs strong, simple, and rounded, mostly originating at umbo with rare bifurcation; one prominent rib in ventral umbo; two prominent ribs on dorsal fold; four to seven on shell flanks.

Ventral umbo large and prominent; ventral interarea anacline and with narrow delthyrium partially covered by elongate deltidial plates. U-shaped ventral fold becoming flattened about one-third of length from umbo and thereafter becoming U-shaped sulcus with one prominent median plication projecting upward at anterior commissure to form prominent tongue.

Dorsal umbo minute and incurved, covered by ventral umbo. U-shaped dorsal sulcus becoming flattened about one-third of length from umbo and thereafter becoming U-shaped fold with flattened top bounded by two prominent plications at each side forming a median groove.

Teeth relatively large with median groove in some shells supported by thin dental plates. Dental cavities triangular, becoming partially infilled by secondary shell material in some shells.

Dorsal median ridge in shell interior very small, extending to midlength of valve, not connected to hinge plates. Sockets relatively deep, with hinge plates separated medially, lacking septalium. Crura projecting anteriorly from crural bases embedded in shell material medial to hinge plates. Spiralia consisting of seven to eight whorls with short jugal process at posterior.

Remarks

These specimens are conspecific with Rhynchotrema pentagonia Fang in Liu and Fang, Reference Liu and Fang1990, also collected from the Hadabulaktag Formation in the Kuruktag region near Uligezhitag. In the description, Fang noted a prominent dorsal groove on the fold and a sulcus with a single prominent rib, indicating that those shells are not Rhynchotrema, which almost always has four ribs on the dorsal fold and three in the ventral sulcus (Sproat et al., Reference Sproat, Sohrabi and Jin2014). Many species that were described as Rhynchotrema in China are now being recognized as belonging to one of the atrypide genera (as in here) or the externally similar rhynchonellide Altaethyrella (Zhan and Li, Reference Zhan and Li1998; Sproat and Zhan, Reference Sproat and Zhan2018). The dimensions of the shells in the collection of Liu and Fang (Reference Liu and Fang1990) are within the range of the specimens we describe here (see Table 1 for summary statistics of each measurement), although our collection additionally includes shells that are much smaller than what they reported. Liu and Fang (Reference Liu and Fang1990) also mention the presence of narrow and elongate deltidial plates characteristic of this genus that partially cover the delthyrium, a diagnostic characteristic of Schachriomonia.

Liu and Fang (Reference Liu and Fang1990) included only four illustrations of sectioned material that are lacking interior details. Note that their first section, labeled ‘12’ on p. 142, is likely reversed and the apparent base of the ventral valve is actually the fused deltidial plates near the posteriormost part of the delthyrium, characteristic of the genus. In our sectioned specimens, six to eight whorls of the spiralia are preserved, but not in the smaller shell (Fig. 5). The larger shell figured here (Fig. 6) clearly shows at least seven whorls, and three incomplete shells in our collection have similar exposed spiralia (Fig. 7). The jugal process is also visible in the larger sectioned shell extending ventromedially from the left crura in the larger shell, although it is not preserved on the right side. The larger shell illustrated here seems to have a prominent midtooth furrow on the exterior-oriented side of the tooth that has not yet been described elsewhere, but this is apparently absent in the smaller specimen, indicating that it may develop over ontogeny through resorption of primary shell material.

Schachriomonia pentagonia is most similar to Schachriomonia parva described by Popov et al. (Reference Popov, Nikitin and Sokiran1999) from the Taldyboi Formation (lower Katian), River Taldyboi, Chingiz Range, including those synonymized by Popov et al. (Reference Popov, Nikitin and Sokiran1999) originally described by Klenina (Reference Klenina and Bandaletov1984). The shell outline is identical (0.97 mean shell length:width ratio in both species), but those S. pentagonia shells are slightly more convex overall than S. parva (0.62 mean shell thickness:length ratio here versus 0.55 in their collection) although the range of convexity is similar between these species. These shells also have slightly fewer ribs (11 to 17 here versus 14 to 21 in their collection).

The type species Schachriomonia schachriomonica Nikiforova (Reference Nikiforova1978) from the upper Katian Archalyk Beds in south Uzbekistan have more numerous ribs and less-pronounced ventral sulcus and dorsal fold. Serial sections for the species show much more substantial dental plates in that species, but the shell also appears to be somewhat distorted, possibly affecting the size and proportions of internal shell structures illustrated there (see also Copper, Reference Copper and Kaesler2002, p. 1433, fig. 4d).

Schachriomonia qinghaiensis (Xu, Reference Xu, Jin, Ye, Xu and Sun1979) from the Qaidam terrane in China is also very similar to S. pentagonia in terms of external morphology. Although some of the internal shell structures are shown in text figures accompanying the description of the genus, the brachidial structure is missing in their figured serial sections. Basic measurements to compare the dimensions and shape of the shell to other species are also lacking. Further study of additional material may show that it is conspecific with either S. pentagonia or S. parva.

Schachriomonia pentagonia differs from Schachriomonia dichotoma (Fu, Reference Fu1982) from the Xiazhen Formation (upper Katian) in the Zhejiang–Jiangxi border area of South China and central Shaanxi in North China in generally having a more prominent fold and sulcus at the anterior of the shell. S. pentagonia is generally larger in size and exhibits a slightly wider range in shell dimensions (Fig. 8).

Figure 8.

Length/width, thickness/width, and thickness/length linear regression plots of Schachriomonia pentagonia from Tarim in Northwest China and Schachriomonia dichotoma from the Zhejiang–Jiangxi border region in South China for comparison. Note that the shell outlines are very similar in these species, but S. pentagonia is generally more convex in profile than S. dichotoma from South China.

Many of the smaller shells in our collection are superficially similar to Antizygospira Fu, Reference Fu1982, especially in outline and the shape of the anterior commissure. However, these shells have the mostly simple costae of Schachriomonia rather than the bifurcating and intercalating costae of Antizygospira, and the smaller shells that have been sectioned all show definite, albeit relatively thin, dental plates that are often lacking in Antizygospira (see Rong et al., Reference Rong, Zhan, Huang, Xu, Fu, Li, Rong, Jin, Shen and Zhan2017).

Locating the Tarim paleoplate in the Late Ordovician

The precise location of the Tarim paleoplate during the Ordovician remains uncertain largely due to the lack of paleomagnetic data from the plate thus far, but it was probably located in the vicinity of the Kazakh terranes and North and South China paleoplates near the equator (Popov and Cocks, Reference Popov and Cocks2017; Torsvik and Cocks, Reference Torsvik and Cocks2017). A full comparison of the brachiopod faunas will be possible once the fauna from Tarim has been completely described, but a few preliminary remarks on the paleogeography of Tarim can be made based on commonalities in the atrypides of these plates.

Tarim, North and South China, and the Kazakh terranes were all parts of a low-latitude brachiopod paleobiogeographic province along with Laurentia, Siberia, and Australia (then part of northern Gondwana) during the Late Ordovician (Harper et al., Reference Harper, Rasmussen, Liljeroth, Blodgett, Candela, Jin, Percival, Rong, Villas, Zhan, Harper and Servais2013). Although there are many broad similarities across this province, there are significant differences within individual orders (e.g., dominance of rhynchotrematides in Laurentia in comparison with the dominance of ancistrorhynchides in the Kazakh terranes and North and South China and the evolution of a distinct epicontinental fauna in the intracontinental seas of Laurentia during the late Katian dominated by taxa such as Hiscobeccus and Hypsiptycha that do not occur elsewhere; see Jin, Reference Jin2001; Jin et al., Reference Jin, Sohrabi and Sproat2013, Sohrabi and Jin, Reference Sohrabi and Jin2013a, Reference Sohrabi and Jin2013b; Sproat et al., Reference Sproat, Sohrabi and Jin2014; Candela, Reference Candela2015) that can be used to distinguish two subprovinces within the larger low-latitude province (e.g., Harper et al., Reference Harper, Rasmussen, Liljeroth, Blodgett, Candela, Jin, Percival, Rong, Villas, Zhan, Harper and Servais2013, fig. 11.10).

Although Schachriomonia is known from the Upper Ordovician of the Kazakh terranes and North and South China, the Kazakh species Schachriomonia parva has thus far only been documented from the Chu Ili and Chingiz terranes (Popov et al., Reference Popov, Nikitin and Sokiran1999; Popov and Cocks, Reference Popov and Cocks2006, Reference Popov and Cocks2014). The similarity of this species to S. pentagonia in Tarim signals a close association between Tarim and these terranes during the Late Ordovician. In fact, S. parva may be a closely related ancestor of S. pentagonia given that S. parva is known from the lower to middle Katian strata in the Kazakh terranes and S. pentagonia is from the upper Katian of Tarim.

Such a relationship agrees with the findings from the rhynchonellides already published. The ancistrorhynchide Altaethyrella tarimensis Sproat and Zhan, Reference Sproat and Zhan2018 occurs alongside S. pentagonia at the collection locality in Tarim and is very similar in terms of shell shape and structures to Altaethyrella otarica (Rukavishnikova, Reference Rukavishnikova1956) found in the Chu-Ili Range, further solidifying the paleogeographic ties between these blocks.

Popov and Cocks (Reference Popov and Cocks2017, text-fig. 13) proposed a Late Ordovician paleogeographic reconstruction, based on similarities in fossil faunas, of the plates and terranes that now make up Central Asia and China. Given that the brachiopod fauna of Tarim is only poorly known, their treatment of Tarim was largely based on similarities of the trilobite faunas. The range of Schachriomonia across North and South China and the Kazakh terranes in addition to Tarim supports their notion that equatorial ocean currents could have enabled the dispersal of the brachiopod fauna across the plates that now comprise China and Central Asia. They projected Tarim to be located adjacent to the Chu-Ili terrane at 455 Ma (equivalent to the Sandbian, slightly earlier in age than the late Katian fauna being studied here), and thus far, the evidence from our work on the brachiopods of Tarim supports this arrangement. However, how the same species would have dispersed to or from the Chingiz terrane that they have plotted on the opposite side of their map closer to Gondwana is difficult to explain.

Central and East Asian atrypides in comparison to Laurentia

Schachriomonia and the other atrypides from Central Asia and China differ from those that evolved during the Sandbian and Katian in Laurentia. In Laurentia, the costate forms Anazyga, Catazyga, and Zygospira were common components of Katian brachiopod assemblages, with smooth-shelled Manespira, Protozyga, Idiospira, and Cyclospira being less common in the shallow marine platforms and basins (Copper, Reference Copper1977, Reference Copper1982, Reference Copper1986). Of these, only Cyclospira is known from the Upper Ordovician of South China (Zhan and Cocks, Reference Zhan and Cocks1998).

Many atrypides in China and Central Asia were initially classified under Zygospira, including those now considered Schachriomonia, and indeed these genera are similar in shell shape and ornamentation. However, the medial external shell structures are reversed—Zygospira has a dorsal sulcus and ventral fold while Schachriomonia has a dorsal fold and ventral sulcus. In the shell interior, the brachidial supports also differ significantly among these groups although these structures are often undocumented in older references. These Asian species are now mostly classified under the family Spirigerinidae, a group that would not reach Laurentia until the Hirnantian (Copper, Reference Copper1986). Thus, two different atrypide lineages occupied these regions during the Late Ordovician but apparently remained mostly isolated from one another until after the onset of the Late Ordovician Mass Extinction in the Hirnantian.

In the Late Ordovician, Australia was located at the northern tip of Gondwana between Laurentia and the plates that now comprise central and eastern Asia (Torsvik and Cocks, Reference Torsvik and Cocks2017). The Late Ordovician atrypides of Australia (Laurie, Reference Laurie1991; Percival, Reference Percival1991) seem to share closer affinities with those of Laurentia rather than with those of the Kazakh terranes, Tarim, and North and South China. In New South Wales, Zygospira carinata Percival, Reference Percival1991 shares the coarse, simple costae and overall shell shape of both Schachriomonia and Zygospira, but its dorsal sulcus and ventral fold suggest affinities to the latter. Protozyga, also found in Laurentia, was reported alongside the endemic forms Webbyspira Percival, Reference Percival1991 and Australispira Percival, Reference Percival1991. Laurie (Reference Laurie1991) reported a species of Protozyga from the Benjamin Limestone of Tasmania as well. This faunal similarity between Laurentia and Australia not only is apparent in the brachiopod fauna but was also shown to occur in Late Ordovician stromatoporoids (Webby, Reference Webby1991).

Variation in shell outline and convexity over ontogeny

The ratio of length (L) to width (W) corresponds to the outline of a shell—a larger L/W ratio indicates a more elongate shell while a smaller value indicates a wider shell. Most of the shells in our collection have a value near 1 (see Table 1), but the larger shells deviate from this trend, becoming significantly wider than long, with an L/W ratio as low as 0.73. A power function (Equation 1) best fits a regression line through the data points with an x and y intercept of zero (or c value).

(1)$${\rm Y}{\rm} = {\rm} {\rm a}{\rm x}^{\rm b} + {\rm} {\rm c}$$

A zero intercept is necessary assuming that the shell started growing from a single point (or 0, 0). Without this stipulation, a linear trend can be fitted to the data, but it is impossible to start growth from an initial stage that is 0 mm long but 1 mm wide, for example.

A growth trend defined by a power law may indicate a degree of allometry, or differential growth in terms of the shell outline. If both measurements increase linearly with regard to each other, the value of x in the power law equation should be equal to one, indicating isometric growth. A value larger or smaller than one indicates a differential change in one dimension relative to the other over ontogeny. The paleontological application of the power law and to allometric growth was thoroughly reviewed and discussed at length by Gould (Reference Gould1966). Literature is sparse on allometry in brachiopods, although Parkinson (Reference Parkinson1952) investigated the growth of Dielasma hasta Sowerby, Reference Sowerby1825 from the lower Carboniferous near Derbyshire, finding not only allometry in terms of length and thickness but also an unusual interruption in length and thickness growth trends midway through growth within the population.

To truly show allometry, the data should include specimens of all sizes, and although we have a wide range of shell dimensions here, the smallest specimens are missing. Nonetheless, the bivariate plots show the power law relationship between length and width, with an exponential (b) value of 0.777, confirming that large shells grow wider at a greater rate than they grow longer.

A similar power function can be fitted to the plot of shell thickness (i.e., shell depth, not thickness of the shell layers) versus shell width as a proxy for shell convexity. This trend is only weakly allometric, with an exponential value of 1.165, indicating that the shell became relatively more convex as it became larger, but not markedly so, as shown in Figure 9.

The widening and slight increase in convexity of the shell over its growth would laterally (and, to a far lesser extent, dorsoventrally) expand the mantle cavity as the shell became larger. Two main structures fill the mantle cavity; one is the spiralia that support the lophophore. As the brachiopod grew in size, its metabolic requirements also grew, necessitating the development of a larger lophophore. Given that the shells did not become significantly more convex over ontogeny, a wider shell would have allowed the lophophore to expand laterally as the energy requirements of the brachiopod grew, allowing for more efficient filter feeding as a greater volume of water would pass through the mantle cavity.

The other structures that would have occupied the mantle cavity are the gonads. These are soft tissues, and thus they are not amenable to preservation in fossil material; there is no direct evidence of their presence in these shells. However, given that only larger shells seem to have been strongly allometric in growth, this could be a signal of sexual maturity in the brachiopod. The spiralia appear to fill most of the mantle cavity even in large shells, but this does not preclude the development of sex organs from influencing shell shape as well.

Growth of the anterior tongue

A linear rather than a power function best fits the ratio of the depth of the sulcus at the anterior relative to the commissural plane (Ts) against the width of the shell (W), indicating that the extension of the tongue above the commissural plane occurs at a constant rate relative to overall shell growth. The smallest of shells in our collection have a dorsal sulcus and ventral fold, although these shells are not common and generally are not well preserved, perhaps owing to their small size. At a certain stage of ontogeny, the shells became rectimarginate without a fold or sulcus, subsequently developing a dorsal fold and a ventral sulcus. This phenomenon is illustrated best in the serial sections of S. pentagonia (Figs. 5, 6), where the sections close to the posterior in the larger shell show a shallow dorsal sulcus that becomes a dorsal fold toward the anterior. The inflection point occurs at approximately one-third to one-half of the shell length in fully grown shells. Unlike the functions fitted to L/W and T/W, this trend clearly has nonzero intercept values as the zero value for the depth of the sulcus (or the point at which the brachiopod would become rectimarginate in form) occurs midway through ontogeny rather than at the beginning of shell growth (see Ts/W graph in Fig. 9).

This change in shell form is difficult to explain. The prominent tongue in rhynchonellides has previously been interpreted to have separated the feeding incurrent generated by the lophophore from the outcurrent that expels waste from the mantle cavity (Rudwick, Reference Rudwick1970). Given that the lophophore in smaller shells is yet unknown, this shift in shell shape could reflect a change in the development of the lophophore or perhaps a change in the living position as the shell grows, necessitating a shift in the location of the incurrent and excurrent flows (such as a shift from resting on one valve on the seafloor to the other). This would be difficult to test, however, as these shells were probably not in their original living position.

Interior structures

The tooth and socket structures are also slightly different between the smaller and larger shells that were sectioned (Figs. 5, 6). In well-preserved shells where structures are not obscured by secondary calcite crystal growth, a prominent medial furrow oriented laterally usually occurs on each tooth. The development of this furrow would have strengthened the joint between the tooth and socket by locking the tooth into the socket and may have been beneficial as the shell became larger, placing more stress on the hinge. Cyrtomatodont teeth that are modified in shape as the shell grows are common in other groups with elaborate lophophore supports that necessitated the ability to resorb shell material to modify the shape of the brachidial supports (Carlson, Reference Carlson1989).