Type species

Producta modesta Say in Hall, Reference Hall1847.

Other species

Copper (Reference Copper1977) briefly reviewed species assigned to Zygospira and restored Anazyga Davidson, Reference Davidson1883, to include species with smaller, medially directed spiralia. Using the shell morphology and stratigraphic ranges of the type species as a guide, he then assigned described species of Zygospira either to Zygospira or Anazyga. Some ambiguity remains regarding the spiralia in many species due to the time-intensive nature of serial sectioning necessary to examine the internal morphology of the shells and the lack of density contrast between the shell material and surrounding matrix that makes imaging using modern CT techniques difficult to impossible. That said, several species can confidently be assigned to Zygospira based on their ventribiconvex lateral profile, wider than long outline, strong simple ribs, and known configuration of the spiralia. These include:

Zygospira modesta (Say in Hall, Reference Hall1847).—Say in Hall (Reference Hall1847, p. 141–142, pl. 33, fig. 15); type specimens initially identified as Producta modesta by Say (see Hall, Reference Hall1847).

Zygospira kentuckiensis Nettelroth, Reference Nettelroth1889.—Nettelroth (Reference Nettelroth1889, p. 138–139, pl. 34, 21–23; not pl. 24, 25).

Zygospira resupinata Wang, Reference Wang1949.—Wang (Reference Wang1949, p. 18–19, pl. 10A, figs. 1–12); only a holotype and a single paratype remain in the SUI collections but Jin et al. (Reference Jin, Caldwell and Norford1997) illustrated a specimen of the subspecies Zygospira resupinata multicostata Howe, Reference Howe1965, that clearly possesses dorso-medially directed spiralia consistent with Zygospira (see Jin et al., Reference Jin, Caldwell and Norford1997, pl. 30, fig. 21).

Several species can be synonymized with the above species based on strong similarities in shell morphology including:

Zygospira concentrica Ulrich, Reference Ulrich1879.—Ulrich (Reference Ulrich1879, p. 14, pl. 7, figs. 10a, b); likely a synonym of Z. modesta.

Zygospira meafordensis Foerste, Reference Foerste1924.—Foerste (Reference Foerste1924, p. 125, pl. 15, figs. 3a–c); herein considered a subspecies of Z. kentuckiensis.

Zygospira raymondi Foerste, Reference Foerste1924.—Foerste (Reference Foerste1924, p. 127, 128); not figured, but based on specimens described as Zygospira uphami by Raymond (Reference Raymond1921); likely a synonym of Z. kentuckiensis with a slightly flattened ventral valve in comparison to the type collection.

Zygospira richmondensis Caley, Reference Caley, Wilson, Caley, Sproule and Okulitch1936.—Caley (Reference Caley, Wilson, Caley, Sproule and Okulitch1936, p. 60, 78, pl. 1, figs. 4, 6); likely a synonym of Z. kentuckiensis, although more evenly convex than typical Z. kentuckiensis.

Most of the early (early Katian, pre-Maysvillian) species previously assigned to Zygospira have been reassigned to Anazyga (see also Copper, Reference Copper1977, for a list with minor differences of opinion). Their taxonomic assignment should be critically reassessed after their spiralia and associated structures have been documented. These are not examined in detail here, but include:

Atrypa recurvirostra Hall, Reference Hall1847.—Type species of Anazyga, sometimes referred to in literature as Zygospira recurvirostris.

Zygospira calhounensis Fenton and Fenton, Reference Fenton and Fenton1922.—Fenton and Fenton (Reference Fenton and Fenton1922, p. 76–77, pl. 2, figs. 4–6).

Zygospira circularis Cooper, Reference Cooper1956.—Cooper (Reference Cooper1956, p. 670, pl. 141C, figs. 18–21, pl. 142B, figs. 6–10, pl. 142D, fig. 16).

Zygospira elongata Cooper, Reference Cooper1956.—Cooper (Reference Cooper1956, p. 670–671, pl. 268G, figs. 29–32).

Zygospira gutta Oraspõld, Reference Oraspõld1956.—Oraspõld (Reference Oraspõld1956, p. 64–65, pl. 4, figs. 14, 15).

Zygospira lebanonensis Cooper, Reference Cooper1956.—Cooper (Reference Cooper1956, p. 671–672, pl. 142C, figs. 11–15).

Zygospira matutina Cooper, Reference Cooper1956.—Cooper (Reference Cooper1956, p. 672, pl. 141B, figs. 13–17).

Zygospira maynei Roy, Reference Roy1941.—Roy (p. 102–103, fig. 69).

Zygospira mediocostellata Cooper, Reference Cooper1956.—Cooper (Reference Cooper1956, p. 672–673, pl. 143D, figs. 13–18).

Zygospira recurvirostris aequivalvis Twenhofel, Reference Twenhofel1928.—Twenhofel (Reference Twenhofel1928, p. 214, pl. 19, figs. 10–12).

Zygospira recurvirostris noquettensis Hussey, Reference Hussey1926.—Hussey (Reference Hussey1926, p. 162–163, pl. 11, figs. 1–3).

Zygospira recurvirostris turgida Foerste, Reference Foerste1917.—Foerste (Reference Foerste1917, p. 103, pl. 5, fig. 15 a–c).

Zygospira variabalis Fenton and Fenton, Reference Fenton and Fenton1922.—Fenton and Fenton (Reference Fenton and Fenton1922, p. 75–76, pl. 2, figs. 7–9).

Zygospira variabalis fountainensis Fenton and Fenton, Reference Fenton and Fenton1922.—Fenton and Fenton (Reference Fenton and Fenton1922, p. 76, pl. 2, figs. 1–3).

A few species previously assigned to Zygospira share similarities with other atrypide genera or have already been assigned to other genera, including the species below.

Athyris headi Billings, Reference Billings1862.—See Meek (Reference Meek1873, p. 127, pl. 11 a–d) for description of species as Zygospira; assigned to Catazyga as type species by Hall and Clarke (Reference Hall and Clarke1894).

Zygospira uphami Winchell and Schuchert, Reference Winchell, Schuchert, Lesquereux, Woodward, Thomas, Schuchert, Ulrich and Winchell1893.—Convex profile suggests affinities with Catazyga, but spiralia shape and configuration remain unknown.

Zygospira maynei Roy, Reference Roy1941.—See Bolton (Reference Bolton, McCracken and Bolton2000); very convex profile suggests affinities with a different lineage. Idiospira was suggested by Bolton, but the illustrated specimen would be unusual for Idiospira. It seems likely that the shells described by Roy (Reference Roy1941) belong to a slightly older collection and may be Anazyga while the shells described and figured by Bolton (Reference Bolton, McCracken and Bolton2000) are an unusual species of Zygospira or a different lineage altogether.

Zygospira putilla Hall and Clarke, Reference Hall and Clarke1894.—Hall and Clarke (Reference Hall and Clarke1894, p. 157, fig. 150, pl. 54, figs. 35–37; not plate 55 as indicated in text); considered Eospirigerina by Amsden (Reference Amsden1974, p. 72), the elongate shell form would be unusual for Zygospira and possesses a distinctive plate in the interior (see Amsden, Reference Amsden1974, text-figs. 49, 50) not known in other Zygospira species and more variation in rib bifurcation (Amsden, Reference Amsden1974, text-fig. 48).

The affinities of the following species remain uncertain because they are only known from a few specimens, sometimes are poorly preserved, and/or their external morphology is unusual for the genus.

Zygospira sulcata Howe, Reference Howe1965.—Howe (Reference Howe1965, p. 655–656, pl. 81, figs. 9–12); fine ribs indicate affinities with Anazyga, but species is known only from poorly preserved fragmentary material.

Zygospira tantilla Bradley, Reference Bradley1921.—Also resembles Anazyga in shape, but if specimens are truly Richmondian in age, this is by far the youngest known species of Anazyga.

Two species of Zygospira from Scotland and Norway cannot be confidently assigned to Zygospira and require further material to make any definitive assignment.

Zygospira orbis Reed, Reference Reed1917.—Reed (Reference Reed1917, p. 944, pl. 24, figs. 24–27); was assigned to Zygospira from Scotland (part of Laurentia during the Ordovician) alongside an unnamed questionably assigned specimen (Zygospira?, sp. Reed, Reference Reed1917, pl. 24, figs, 28, 29). Described to have a broad dorsal depression on the ventral valve and a fold containing two ribs on the dorsal valve, suggesting affinities with Zygospira. The questionably assigned specimens are unusually elongate for Zygospira, however, and may belong to the Atrypinidae. Additional material will need to be examined before anything definitive can be said about either species.

Zygospira meldalensis Reed, Reference Reed and Kiarer1932.—Reed (Reference Reed and Kiarer1932, p. 144, pl. 22, figs. 12, 12a); from the Upper Hovin Group of Norway, only known from a single poorly preserved specimen that cannot definitively be located, although a single specimen within a limestone fragment bearing the same specimen number contains an external mold of the specimen consisting of only the anterior was described by Neuman et al. (Reference Neuman, Bruton and Pojeta1997); however, the configuration of the brachidial structures remains unknown; resembles Z. kentuckiensis in size and shape.

Several species from the Kazakh terranes and North and South China previously have been assigned to Zygospira, but these have almost all since been reassigned to other genera. These all differ from Zygospira in lacking a complete jugum, although the spiralia of many species are still inadequately known. Most also possess a ventral sulcus and dorsal fold rather than the ventral fold and dorsal sulcus that is typical of Zygospira. These species include:

Zygospira parva Rukavishnikova, Reference Rukavishnikova1956.—Rukavishnikova (Reference Rukavishnikova1956, p. 162–163, pl. 5, figs. 14–16); assigned to Schachriomonia by Popov et al., Reference Popov, Nikitin and Sokiran1999.

Zygospira qinghaiensis Xu in Jin et al., Reference Jin, Ye, Xu and Sun1979.—Xu in Jin et al. (Reference Jin, Ye, Xu and Sun1979, p. 108, pl. 21, figs. 10–15, 19–21, text fig. 51; see also Rong et al., Reference Rong, Jin, Shen and Zhan2017, p. 188).

Zygospira shaanxiensis Fu, Reference Fu1982.—Fu (Reference Fu1982, p. 145, pl. 39, fig. 9a–c; see also Rong et al., Reference Rong, Jin, Shen and Zhan2017, p. 192).

Zygospira (Kuzgunia) bankanasensis Klenina, Nikitin, and Popov, Reference Klenina, Nikitin and Popov1984.—Klenina et al. (Reference Klenina, Nikitin and Popov1984, p. 116); assigned to Sulcatospira by Popov et al., Reference Popov, Nikitin and Sokiran1999.

Zygospira (Sulcatospira) plicata Xu in Jin et al., Reference Jin, Ye, Xu and Sun1979.—Xu in Jin et al. (Reference Jin, Ye, Xu and Sun1979; see also Rong et al., Reference Rong, Jin, Shen and Zhan2017, p. 190).

Zygospira carinata Percival, Reference Percival1991 (p. 165, 167, 169; figs. 20, 27) is one of the few probable Zygospira species from outside Laurentia. It is unusual for the genus in that it has a prominent ventral medial rib rather than an interspace, and generally has fewer and coarser ribs. The shell of this species is more elongate than typical Zygospira, which are usually shorter in length than width. The spiralia and associated structures remain unknown for this species, so it can only be assigned to the genus provisionally until additional specimens with intact shell interiors are found. The unusual morphology may reflect the relative isolation of Australia (part of Gondwana in the Ordovician) from Laurentia and lends the species considerable paleobiogeographic significance.

Zygospira modesta kagawongensis Caley, Reference Caley, Wilson, Caley, Sproule and Okulitch1936 (Caley, Reference Caley, Wilson, Caley, Sproule and Okulitch1936, p. 58) was published without description or figures and thus should be considered a nomen nudum (see also Copper, Reference Copper1977). A search of the database of the Royal Ontario Museum where the other specimens described by Caley (Reference Caley, Wilson, Caley, Sproule and Okulitch1936) were deposited revealed no results.

Diagnosis

See Copper, Reference Copper and Kaesler2002.

Occurrence

Common in middle to upper Katian (Maysvillian–Richmondian) rocks of eastern North America, but its range outside of Laurentia is limited. Species from lower Katian strata are almost certainly Anazyga or another early atrypide genus, but need re-examination.

Remarks

Zygospira has referred to a number of different species in literature. Here, the Treatise on Invertebrate Paleontology (Copper, Reference Copper and Kaesler2002) is followed in referring to only the coarsely ribbed species with mediodorsally directed spiralia. Other authors have included species more recently considered to belong to Anazyga (see above). These species generally have spiralia directed medially rather than dorso-medially (although the internal shell morphology has not yet been documented for all species) and can be recognized externally by a generally more convex dorsal valve, weaker ribs, and often is significantly smaller. Zygospira also characteristically has one or two very prominent ribs on each side of the ventral fold, a feature that is usually absent from species that tentatively have been assigned to Anazyga.

Anazyga has been reported only from rocks that are older than Zygospira, being particularly abundant in the lower Katian Trenton Limestone and equivalent strata in eastern North America (Chatfieldian and Maysvillian in North American terminology). The species referred to Zygospira above are all middle to late Katian (Maysvillian–Richmondian in North American terminology) in age. This led Copper (Reference Copper1977) to suggest that Anazyga was a likely ancestor to Zygospira, but this hypothesis has yet to be tested in a broader cladistic analysis of the lineage.

The paleoecology of Zygospira and other early atrypides has not yet been examined in detail (although see Copper, Reference Copper1977, for a brief discussion), but the genus is known to occur in dense clusters (Fig. 3). These dense accumulations could reflect an opportunistic life habit where Zygospira may have been able to multiply rapidly and take advantage of regular disruptions in the environment, such as regular storm events in eastern North America during the Ordovician (e.g., Brookfield and Brett, Reference Brookfield and Brett1988; Kerr and Eyles, Reference Kerr and Eyles1991; Jennette and Pryor, Reference Jennette and Pryor1993). Alternatively, these dense shell beds could represent periods of quiescence with reduced sedimentation that enabled Zygospira to proliferate (Dattilo et al., Reference Dattilo, Brett, Tsujita and Fairhurst2008, Reference Dattilo, Brett and Schramm2012). McFarland et al. (Reference McFarland, Westrop and Cheel1999) analyzed shell beds in the lower Katian Verulam Formation of Ontario where the older atrypide Anazyga is known to be locally abundant and concluded that most of the shell beds must have been formed from allogenic processes such as winnowing by storms. They mentioned that the dense accumulations of atrypide shells may have been at least partially derived from autogenic processes given their common preservation in apparent life position.

Figure 3.

Zygospira modesta attached to bryozoans from Tanner's Creek Formation in Indiana. (1) PE 16630, dense cluster of Zygospira modesta from near Brookville; (2, 3) FM PE16583 Zygospira modesta preserved attached to bryozoan; (4–6) FM PE16631 Zygospira modesta attached to Spatiopora corticans (Nicholson, Reference Nicholson1874) from near Clifton; (5) and (6) magnified views of (4) in blue and green, respectively. Scale bars for (1–4) = 1 cm; (5, 6) = 1 mm.

These dense clusters are also sometimes associated with filter feeders, such as bryozoans and crinoids (Fig. 3). The smaller Z. modesta are preserved in positions that suggest that they may have been attached to these filter feeders when alive via their pedicle (Fig. 3.2, 3.3). There is no evidence of such associations with the larger Zygospira species thus far, however, and the extent of this phenomenon across the Anazygidae has not yet been investigated.

Zygospira modesta Say in Hall, Reference Hall1847
 Figures 4, 5

Reference Hall1847

Atrypa modesta Say in Hall, p. 141, pl. 33, fig. 15a, b; not pl. 33, fig. 15c.

Reference Hall1862

Zygospira modesta; Hall, p. 155, text-figs. 1, 2.

Reference Ulrich1879

Zygospira concentrica; Ulrich, p. 14, pl. 7, figs. 10, 10a, b.

Reference Hall and Clarke1894

Zygospira modesta; Hall and Clarke, pl. 54, figs. 7–10, 12.

Reference Foerste1910

Zygospira modesta; Foerste, p. 29, pl. 2, fig. 15a, b.

Reference Foerste1924

Zygospira modesta; Foerste, p. 127, pl. 10, fig. 21a, b.

Reference Copper1977

Zygospira modesta; Copper, p. 303, pl. 37, figs. 1–8, text-figs. 3, 4.

Reference Howe1988

Zygospira modesta; Howe, p. 205, fig. 2.1–2.7.

Figure 4.

(1–10) Zygospira modesta types from the Hudson River Group (Katian) near Cincinnati, Ohio. (1–5) Lectotype AMNH 29835, dorsal, ventral, lateral, anterior, and posterior views; (6–10) paratype AMNH 29836, dorsal, ventral, lateral, anterior, and posterior views. Scale bars = 5 mm. This is part of Hall's type collection at the AMNH along with several unfigured paratypes in the collection (labeled AMNH 1356d–o).

Figure 5.

Tracings of serial sections of a Zygospira modesta shell from the Grant Lake Formation near Sharonville in the region around Cincinnati, Ohio (CMC IP96903). Numbers represent distance from posterior of ventral umbo in mm at which the section was ground. Scale bar = 1 mm.

Types

Lectotype AMNH 29835 (formerly AMNH 1356A) selected by Foerste (Reference Foerste1910) from Hall's collection at the AMNH (Fig. 3, AMNH 1356a–d). The specimen tags associated with the type collection lack specific information on the location of the original type locality and strata from which the specimens were collected. The older, typed tags indicate that specimens were collected from the Hudson River Group at Cincinnati, Ohio. These are unlikely to be the original tags, however, because they refer to the specimens as Zygospira while Hall (Reference Hall1847) described the specimens as a species of Atrypa. The newer tags suggest that the specimens are from the Trenton Limestone, but this is highly unlikely. Foerste (Reference Foerste1910) suggested that similar specimens are known from the Fairmount beds near Hamilton, Ohio, which should be considered the type locality. Although this is a reasonable inference, in the absence of any further information regarding their origin, all that can be said is that the types were likely, but not definitively, collected from Cincinnatian-aged strata in the area around Cincinnati.

One of the types, now labeled AMNH 29837 but formerly labeled AMNH 1356C, is characteristic of Z. cincinnatiensis (see below). We tentatively assign it to that species, but a more extensive review is needed to determine the full range of morphological variability within Z. cincinnatiensis, along with a more extensive examination of the internal morphology of both species.

There are 12 unfigured paratypes deposited in the collection along with 2 casts under the collection number AMNH 1356d–1356o (probably following the original numbering scheme—no updated numbers are associated with the specimens).

Diagnosis

See Copper, Reference Copper and Kaesler2002.

Occurrence

Zygospira modesta is most common in middle Katian (Maysvillian) strata of eastern North America near Cincinnati in Ohio, Kentucky, Indiana, and Illinois, but has been reported from upper Katian (Richmondian) strata as well. It is considered the most widespread species of Zygospira, but other species are commonly misidentified as Z. modesta in historical collections.

Description

Shell small, measuring on average 6.4 mm in length, 7.4 mm in width, and 3.7 mm in depth (Fig. 6, Table 1); outline subpentagonal and always significantly wider than long (mean of 87% as long as wide) with ventribiconvex lateral profile with ventral valve ~1.5 times as deep as dorsal valve. Astrophic hingeline with rounded lateral flanks. Anterior margin strongly unisulcate forming prominent flat-topped tongue. Ribs strong and rounded, simple with rare bifurcations and expanding towards anterior, numbering between 14–20, but usually 16–18. Fine growth lamellae on well-preserved specimens cover the shell.

Figure 6.

Bivariate plots of length vs. width and thickness vs. width for measured Zygospira modesta specimens. Black = middle Katian (Maysvillian) specimens (n = 92); blue = late Katian (Richmondian) specimens (n = 50); circles indicate length vs. width; plus signs represent thickness vs. width. The r² values shown (red lines) represent values for combined ages; r² values for individual datasets are as follows: length/width Maysvillian = 0.85759; thickness/width Maysvillian = 0.70637; length/width Richmondian = 0.82215; thickness/ width Richmondian = 0.75109.

Table 1.

Summary statistics for Zygospira modesta specimens measured for this study (n = 92). L = length; W = width; T = thickness (depth); Ad = deflection at anterior of the commissure (all in mm); Aa = apical angle in degrees; R = number of ribs counted on the ventral valve; L/W = length/width ratio; T/W = thickness/width ratio.

Ventral umbo strongly curved featuring prominent apical foramen. Strongly anacline interarea that becomes nearly perpendicular to commissural plane towards tip, projecting over hingeline. Delthyrium mostly open, flanked by minute deltidial plates visible on well-preserved specimens. Carinate, with flat-topped ventral fold with medial interspace flanked by a pair of prominent ribs on each side.

Dorsal umbo minute and incurved, obscuring open dorsal interarea. Broad dorsal sulcus usually containing single prominent medial rib flanked by two smaller ribs beginning near umbo with a single rib forming each flank of the sulcus. Medial rib larger than other ribs on shell, but similar in prominence to ribs flanking ventral fold.

Dental plates thin, thickening slightly towards anterior, especially at bases, with small dental cavities infilled with secondary shell material towards posterior. Small teeth pointed dorso-medially with weak medial grooves forming accessory lobes.

Minute myophragm near posterior of dorsal valve. Sockets small with weak medial ridge forming tight interlocking hinge with teeth. Hinge plates fuse to form cruralium, rising off the base of the ventral valve towards anterior and supported by a medial septum. Crural bases flat with crura becoming rod-like towards anterior. Dorsomedially directed spiralia consisting of up to four whorls connected by ventro-anteriorly pointed jugum high in dorsal valve near posterior of spiralia.

Remarks

Zygospira modesta appears to be the most common and long ranging species of Zygospira in the eastern United States. Middle Katian (Maysvillian) specimens of Z. modesta have been widely reported as occurring alongside Z. cincinnatiensis James in Meek, Reference Meek1873, and Z. concentrica Ulrich, Reference Ulrich1879 (the latter species herein synonymized with Z. modesta) in this region while late Katian (Richmondian) specimens occur in strata of similar age to Z. kentuckiensis. The younger late Katian specimens are notably longer than wide in comparison to the middle Katian specimens based on the collections measured herein (Fig. 6).

The late Katian (Richmondian) Z. modesta primarily differ from Z. kentuckiensis based on their much smaller size, a more prominent and angular dorsal sulcus and ventral fold, and fewer ribs on the flanks of the shell than Z. kentuckiensis. Given that ribs generally do not increase in number as the shell grows (other than a few uncommon examples of bifurcation in some specimens), this seems a reliable diagnostic character to differentiate the species.

The earlier species are more problematic to differentiate. Zygospira cincinnatiensis was differentiated from Z. modesta in having fewer ribs on the lateral flanks of the shell (typically five) and generally larger size (Foerste, Reference Foerste1910). It would be difficult to differentiate Z. cincinnatiensis and Z. modesta of similar sizes, however, perhaps indicating that Z. cincinnatiensis is more common than reports would suggest. Zygospira cincinnatiensis is tentatively retained here, but the division between these specimens remains somewhat indistinct.

Zygospira concentrica was differentiated from Z. modesta based on having stronger concentric striae, no radiating plications (ribs), straighter posterior margins, and a more pointed ventral beak (Foerste, Reference Foerste1910). The apparent lack of ribs in specimens of Z. concentrica seems to reflect preservation rather than any real diagnostic difference because examination of collections of species from the Cincinnati region revealed weak ribs that appear to have been abraded in most specimens identified as Z. concentrica. Furthermore, the strong concentric striae probably represent a prominent interruption in the growth of the shell and should not be considered a reliable diagnostic character. Zygospira concentrica is thus herein synonymized with Z. modesta.

An in-depth analysis of the configuration of the spiralia is outside the scope of this study, but it is perhaps notable that the jugum appears slightly closer to the posterior in our sectioned specimen than in the sections illustrated by Copper (Reference Copper1977). It is still located postero-dorsally, however.

Zygospira cincinnatiensis James in Meek, Reference Meek1873
 Figure 7

Reference Hall1847

Atrypa modesta Say in Hall, p. 141, pl. 33, fig. 15c; not pl. 33, fig. 15a, b.

Reference Meek1873

Zygospira cincinnatiensis James in Meek, p. 126, pl. 11, fig. 5a–c.

Reference Hall and Clarke1894

Zygospira cincinnatiensis; Hall and Clarke, pl. 54, figs. 13, 14.

Reference Foerste1910

Zygospira cincinnatiensis; Foerste, p. 30, pl. 6, figs. 16a, b.

Figure 7.

Zygospira cincinnatiensis types from the Fairmount Beds (Cincinnatian = Katian) near Cincinnati, Ohio. (1–5) Lectoype FM UC16-a: dorsal, ventral, lateral, anterior, posterior views; (6–10) figured paratype FM UC164-b: dorsal, ventral, lateral, anterior, posterior views. Zygospira cincinnatiensis from the Hudson River Group near Cincinnati, Ohio (from Hall's type collection for Atrypa modesta). (11–15) AMNH 29837, dorsal, ventral, anterior, posterior, and lateral views. Scale bars = 5 mm.

Types

Foerste (Reference Foerste1910) explained that the original types (collected by James and described by Meek, but credited to James by Meek) were not deposited in a known institution and suggested that the collection numbered UC164 from the Fairmount Beds near Cincinnati, Ohio, be regarded as “typical” (Foerste, Reference Foerste1910, p. 30). He reasoned that Meek would have received the original type specimens from James whose collection was then housed at the University of Chicago but is now located at the Field Museum in Chicago. One of the specimens from the original UC164 collection has been selected here as a more typical neotype (FM UC164-a).

A single specimen in the Field Museum collection is marked as a plesiotype (FM UC12391) and was apparently figured by Hall and Clarke (Reference Hall and Clarke1894), but this specimen has far more ribs than is typical for Z. cincinnatiensis. It is unlikely that this specimen was collected from the same locality, although this is not clear from the label associated with the specimen. In museum records, this is noted as belonging to the Hall collection while the other specimens are part of the James collection. This may represent a single, unusual example of Z. cincinnatiensis or could represent a subspecies, but it is difficult to draw any conclusions based on a single shell. Although it has a similar number of ribs to Z. kentuckiensis, it more closely resembles Z. cincinnatiensis in having a more prominent fold and sulcus.

Occurrence

Zygospira cincinnatiensis seems to be restricted to the middle Katian (Maysvillian) in the Cincinnati area, although further detailed study of larger collections of Zygospira may extend its range into the late Katian (Richmondian). Foerste (Reference Foerste1910) mentioned that ancestral forms of the species are found in the Edenian (lower to middle Katian) that have less conspicuous bifurcation of ribs on the fold. Given the doubtful utility of rib bifurcation as a diagnostic feature in this genus, these specimens should likely be considered Z. cincinnatiensis as well.

Remarks

Zygospira cincinnatiensis was distinguished from Z. modesta by Meek based on the presence of bifurcating ribs on the ventral fold, but bifurcating ribs are sometimes present in large collections of Z. modesta as well and may not be a reliable diagnostic character (also noted by Foerste, Reference Foerste1910). The elevated ventral fold that was mentioned, however, is more diagnostic, and Z. cincinnatiensis is, on average, larger than Z. modesta (Fig. 8). Both differ from the similar-sized Z. kentuckiensis in having a significantly more prominent fold with a matching prominent medial rib in the dorsal sulcus. Zygospira cincinnatiensis also has consistently fewer ribs than both Z. modesta and Z. kentuckiensis when large collections are compared.

Figure 8.

Bivariate plots of length vs. width and thickness vs. width for measured Zygospira cincinnatiensis specimens. Circles indicate length vs. width; plus signs represent thickness vs. width.

It is difficult to determine the precise abundance and stratigraphic range of this species. Although apparently easily differentiated from Z. modesta in isolated collections (especially when the Z. cincinnatiensis are larger), it can be difficult to differentiate these species when specimens of similar sizes are compared. When Z. cincinnatiensis is small, the shells very closely resemble large Z. modesta in every way other than having fewer ribs on average (14–16 in Z. cincinnatiensis rather than 16–18 in Z. modesta). These smaller shells are nearly identical in terms of length, width, and depth to Z. modesta and lack the distinctive raised fold of larger shells (Table 2). Larger shells are easily differentiated from Z. kentuckiensis shells, which have more ribs and much less prominent dorsal sulcus and ventral fold.

Table 2.

Summary statistics for Zygospira cincinnatiensis specimens measured for this study (n = 32). L = length; W = width; T = thickness (depth); Ad = deflection at anterior of the commissure (all in mm); Aa = apical angle in degrees; R = number of ribs counted on the ventral valve; L/W = length/ width ratio; T/W = thickness/width ratio.

One of the paratypes of Z. modesta from the American Museum of Natural History (AMNH 29837) is herein considered Z. cincinnatiensis. It is unclear whether all the Z. modesta type specimens were collected from the same locality, but if that was the case, Z. modesta co-occurs with Z. cincinnatiensis as the species are currently defined.

Zygospira kentuckiensis James, Reference James1878
 Figures 9, 10.1–10.10, 11

Reference James1878

Zygospira modesta var. kentuckyensis James, p. 7.

Reference Nettelroth1889

Zygospira kentuckiensis James in Nettelroth, p. 138, pl. 34, figs. 21–25.

Reference Hall and Clarke1894

Zygospira kentuckiensis; Hall and Clarke, pl. 54, figs. 11, 15, 16.

Reference Foerste1924

Zygospira kentuckiensis; Foerste, p. 127, pl. 10, fig. 20a–c, pl. 15, figs. 1a–p, 2a, b, 4a–c.

Figure 9.

Zygospira kentuckiensis. (1–10) Types from Cincinnatian (= Katian) of Jefferson County, 18 miles east of Louisville, Kentucky; (1–5) neotype FM UC96-a dorsal, ventral, lateral, anterior, posterior views; (6–10) figured paratype FM UC96-b, dorsal, ventral, lateral, anterior, posterior views; (11–15) illustrated plesiotype FM UC 12390 in Hall and Clarke (Reference Hall and Clarke1894) from Cincinnatian (=Katian) of Oldham County, Kentucky, dorsal, ventral, lateral, anterior, posterior views. Scale bars = 5 mm.

Figure 10.

(1–10) Zygospira kentuckiensis from the Queenston Shale (late Katian) at Big Bay near Owen Sound, Ontario. (1–5) ROMIP 66852 dorsal, ventral, lateral, anterior, posterior views; (6–10) ROMIP 66853 dorsal, ventral, lateral, anterior, posterior views. (11–15) Zygospira kentuckiensis meafordensis from the Queenston Shale near Meaford, Ontario (GSC 8514); (11) small slab with several shells on surface; (12) dorsal valve attached to piece of rock; (13) exterior of isolated dorsal valve attached to slab in (11); (14) interior of dorsal valve attached to slab in (11); (15) magnified view of (14). Scale bars = 5 mm (1–14); = 1 mm (15).

Figure 11.

Tracings of serial sections of Zygospira kentuckiensis from the Queenston Formation at Big Bay near Owen Sound, Ontario (ROMIP 66854). Numbers represent distance from posterior of ventral umbo in mm at which the section was ground. Figure represents two sectioned shells to show the complete internal morphology of species (above and below line). Scale bar = 1 mm.

Types

The species was first mentioned by James (Reference James1878) in brief, describing fossils from Nettelroth from the upper part of the Cincinnati Group, Jefferson County, Kentucky, 18 miles east of Louisville. Nettelroth (Reference Nettelroth1889) later described the species in more detail himself, explaining that the species is found in different places in Oldham County, Kentucky, and that it is found in great abundance at Taylor's Station but also on the Shelby Railroad in the shales of the Hudson River Group. Unfortunately, there is no indication of where Nettelroth may have deposited his collection, although a large collection of Nettelroth's fossils was apparently donated to the US National Museum (now Smithsonian Institution) after his death.

A small collection of six shells within James’ collection at the Field Museum labeled UC96 apparently were collected from the exact locality mentioned in James’ (Reference James1878) initial description of the species (Jefferson County, 18 miles east of Louisville, Kentucky) and is thus likely the original type collection. A single specimen from UC96 (FM UC96-a) is herein selected as a lectotype for Z. kentuckiensis.

Another collection of Z. kentuckiensis from the Field Museum labeled IP 12390 consists of a single well-preserved and carefully cleaned specimen. This shell apparently was illustrated by Hall and Clarke (Reference Hall and Clarke1894), but the specimens from the James collection (FM UC96) have priority as types.

Diagnosis

Shells large for the genus, usually wider than long, and ventribiconvex in profile; simple ribs numbering >18 but less prominent than other species; relatively weak ventral fold with medial interspace accentuated by two ribs on each flank and broad and shallow dorsal sulcus with weak medial ridge; thick dental plates and hinge plates for the genus with simple dorso-posteriorly located jugum; prominent teeth with hook-like transverse profile towards anterior.

Occurrence

Upper Katian (Richmondian) of eastern North America. Common in the upper Fairmount (and correlated units) in the Cincinnati area and the Queenston Shale in Ontario.

Description

Shell medium-sized, with a mean length of 9.8 mm, width of 10.9 mm, and a thickness of 5.5 mm (Table 3); outline subpentagonal with outline wider than long (mean of 91% as long as wide) and ventribiconvex in lateral profile with ventral valve ~1.5 times as deep as dorsal valve. Astrophic hingeline with rounded lateral flanks. Anterior margin unisulcate forming prominent tongue. Ribs strong and rounded, simple with rare bifurcations and becoming larger towards anterior, typically numbering from 18–22 but can be more numerous (up to 28 in collections examined herein). Fine growth lamellae visible especially near anterior margin on well-preserved shells.

Table 3.

Summary statistics for Zygospira kentuckiensis specimens measured for this study (n = 110). L = length; W = width; T = thickness (depth); Ad = deflection at anterior of the commissure (all in mm); Aa = apical angle in degrees; R = number of ribs counted on the ventral valve; L/W = length/ width ratio; T/W = thickness/width ratio.

Ventral umbo strongly curved, featuring prominent apical foramen. Strongly anacline interarea that becomes nearly perpendicular to commissural plane towards tip, projecting over hingeline. Delthyrium open, flanked by minute deltidial plates visible on some well-preserved specimens. Carinate, with less prominent flat-topped ventral fold with medial interspace flanked by a pair of prominent ribs on each side beginning near umbo.

Dorsal umbo minute and incurved, obscuring dorsal interarea. Broad, weak dorsal sulcus containing single prominent medial rib flanked by two smaller ribs beginning near umbo with no obvious prominent flanking ribs. Medial rib generally most prominent.

Dental plates moderately thick, divergent near posterior of ventral valve and convergent and thinner towards anterior. Dental cavities small. Teeth prominent, thickened, and curved, becoming increasingly curved towards anterior with variable weak accessory lobes separated from main lobe by weak medial groove. Ventral muscle scars poorly impressed but serial sections show evidence of small divergent adductor scars separated by slightly raised diductor platform.

Minute myophragm variably present near posterior of dorsal valve. Sockets curved with weak medial ridges, forming tight interlocking fit with teeth in dorsal valve. Median septum forms cruralium with hinge plates. Crural bases flat but crura become rod-like and laterally directed towards the anterior. Spiralia with dorso-medially directed apices consist of up to five whorls. Jugum located high in dorsal valve posterior to spiralia and curved towards anterior.

Remarks

Zygospira kentuckiensis is similar in shape to the type species Z. modesta, but Z. kentuckiensis is significantly larger (Fig. 12) and has more numerous ribs (18–22 in Z. kentuckiensis in comparison to the 16–18 in Z. modesta). The two ribs that flank each side of the ventral medial fold are less prominent in this species in comparison to Z. modesta and form a shallower medial interspace on the ventral valve. The corresponding medial rib in the dorsal sulcus is also less prominent in Z. kentuckiensis.

Figure 12.

Bivariate plots of length vs. width and thickness vs. width for measure Zygospira kentuckiensis specimens. Circles indicate length vs. width; plus signs represent thickness vs. width.

There are slight differences in the interior of the shell as well. Zygospira kentuckiensis has significantly thicker dental plates and hinge structures in comparison to Z. modesta, as well as a more thickened posterior shell. This thickening may simply reflect the growth of a larger shell in the case of Z. kentuckiensis, but Z. modesta appears to have consistently thinner shells (based on the 5 specimens of each species sectioned here).

New material (Figs. 10.1–10.10, 11) was collected from the Queenston Formation at Big Bay near Owen Sound, Ontario (44.796537°N, 80.924807°W). These shells are similar to typical shells from the Cincinnati area but are slightly less convex. Zygospira kentuckiensis shells are common and numerous at this locality, forming low-diversity shell beds that form packstones comprised of this one species. Some of the exposed bedding planes show wave ripples, implying that Zygospira may have been able to tolerate relatively high energy environments above fair-weather wave base.

Zygospira richmondensis Caley, Reference Caley, Wilson, Caley, Sproule and Okulitch1936, which was described from the Kagawong Formation (Richmondian, latest Katian) on Manitoulin Island, is represented by a single shell in the collections of the Royal Ontario Museum in Toronto (ROMIP 12448). This specimen resembles Z. kentuckiensis in size but seems to have a shallower fold and sulcus than is typical from the species. Given that all other characteristics are similar, however, it is here synonymized with Z. kentuckiensis.

Types

Lectotype GSC 8514A selected from GSC 8514 collected from Concession VIII, lot 24, ~4 miles northwest of Meaford (Foerste, Reference Foerste1924). Although these shells are illustrated as isolated shells, all three illustrated specimens are embedded in matrix. GSC 8514A is a ventral valve, GSC 8514B is a dorsal interior, and GSC 8514C is wider than a typical ventral valve.

Occurrence

Specimens in the GSC collection were collected from only a single locality near Meaford, Ontario, from the Queenston Formation, which was described by Foerste (Reference Foerste1924) as being at Concession VIII, lot 24, 4 miles northwest of Meaford. Two other localities are noted from the same region.

Remarks

This species very closely resembles Z. kentuckiensis differing only in the much more numerous ribs (26–30 in this subspecies versus 18–22 in Z. kentuckiensis) and in its slightly smaller size, based on the limited number of specimens available for study. The fold and sulcus are broad and shallow, as is typical of Z. kentuckiensis, but Foerste noted that the slope between the fold and sulcus and the shell flanks is more gradual in these specimens in comparison to Z. kentuckiensis. Foerste (Reference Foerste1924) described a cardinal process, but no cardinal process is visible in the type material (GSC 8514, Fig. 10). Given the extremely limited range of this species and overall similarity in shell size and shape, Z. meafordensis is herein regarded as a s subspecies of Zygospira kentuckiensis with which it is found in Ontario.

Zygospira resupinata Wang, Reference Wang1949
 Figure 13

Reference Wang1949

Zygospira resupinata Wang, p. 18, pl. 10, fig. A (1–12).

Figure 13.

(1–10) Zygospira resupinata types from Brainard Shale, Maquoketa Formation in Jackson County, Iowa. (1–5) Holotype SUI 1874, dorsal, ventral, lateral, anterior, posterior views; (6–10) paratype SUI 1873, dorsal, ventral, lateral, anterior, posterior views. Scale bars = 5 mm.

Types

SUI 1874 (holotype), SUI 1873 (paratype) from the Cornulites zone of the Brainard member; south corner of section 29, Fairfield township, Jackson County, Iowa (Wang, Reference Wang1949).

Occurrence

Upper Katian (Richmondian) of central North America.

Description

See Wang (Reference Wang1949).

Remarks

Wang only deposited two specimens in the State University of Iowa collection. Based on these two specimens (Table 4) and his descriptions, this species differs from all other species of Zygospira in being longer than wide and having far fewer costae (12 versus the typical 18–22 of Z. modesta). It also differs from other species in having a smaller and narrower ventral fold and dorsal sulcus, creating a less prominent deflection of the commissure near the anterior. The interior of this species remains unknown, but the external characteristics suggest affinities with Zygospira, although this species shares the more elongate shell shape and narrower fold and sulcus of the earlier Anazyga. A single damaged Zygospira resupinata multicostata Howe, Reference Howe1965, shell from the Hudson Bay Lowlands (Jin et al., Reference Jin, Caldwell and Norford1997, pl. 30, fig. 21) exhibits typical Zygospira-style spiralia, but this has not been described in the material from Iowa. If this feature is confirmed in Zygospira resupinata sensu stricto, the species may represent an earlier divergence from earlier atrypide stocks (such as Anazyga), although the lack of early occurrences of this species at a time when species such as Z. modesta were becoming widespread makes this unlikely.

Table 4.

Measurements of Zygospira resupinata types. L = length; W = width; T = thickness (depth); Ad = deflection at anterior of the commissure (all in mm); Aa = apical angle in degrees; R = number of ribs counted on the ventral valve; L/W = length/ width ratio; T/W = thickness/width ratio.

Large collections of this species are currently unavailable for study, but Z. resupinata and Z. resupinata multicostata Howe, Reference Howe1965, from the Montoya Group in Texas and the Surprise Creek and Caution Creek formations in the Hudson Bay Lowlands represent the farthest that Zygospira was able to infiltrate into the inland seas of Laurentia. No atrypides are yet known from the paleoequatorial seas of southern Manitoba despite a relatively diverse brachiopod fauna being reported there (Jin and Zhan, Reference Jin and Zhan2001).