Age of the Edgewood-type Hirnantian fauna in Laurentia

In North America and North Greenland, most of the Edgewood fauna intervals have not been dated precisely because of the lack of age-diagnostic graptolites and conodonts. Thus, the exact range of these strata within the Hirnantian remains to be determined.

In the ongoing investigations regarding the early versus late Hirnantian age for the Edgewood fauna in Laurentia, three factors should be taken into consideration. (1) On Anticosti Island, the debate a decade ago was whether or not the lower part of the Ellis Bay Formation was upper Katian or Hirnantian. More recently, studies (see summaries in Achab et al., Reference Achab, Asselin, Desrochers and Riva2013; Copper et al., Reference Copper, Jin and Desrochers2013; Mauviel and Desrochers, Reference Mauviel and Desrochers2016; Mauviel et al., Reference Mauviel, Sinnesael and Desrochers2020) have shown that not only is the entire Ellis Bay Formation Hirnantian, but a few meters of strata below and above the Ellis Bay Formation may also be Hirnantian. (2) In the Edgewood region of the American Midcontinent, the chemostratigraphic and paleontological data of Farnam et al. (Reference Farnam, Brett, Shoemaker, Jin and Elias2023) show that some stratigraphic units overlying the classic Edgewood Group, previously thought to be lower Rhuddanian (e.g., the Wilhelmi Formation in northeastern Illinois, the Centerville Member of the Whippoorwill Formation in Ohio and Indiana) are Hirnantian in age. This would place the traditional Edgewood strata lower in the Hirnantian. (3) The diagnostic brachiopod taxa of the Edgewood fauna, at present, are not reliable for determining early versus late Hirnantian. In western Laurentia, for example, Brevilamnulella occurs as low as the uppermost Katian (Jin and Blodgett, Reference Jin and Blodgett2020). Thus, its common occurrence in Laurentia in the Edgewood fauna cannot be constrained to the early or late Hirnantian. Similarly, other taxa of the Mackenzie Mountains examined in this study, such as Eospirigerina, Epitomyonia, Glyptorthis, Gnamptorhynchos, and Parastrophinella, can all be traced back to the late Katian or early Hirnantian elsewhere in Laurentia.

In light of the above, we have tentatively assigned a broad Hirnantian age to the Edgewood fauna in Laurentia in this study, although the Cathay fauna of South China, which may share some common taxa of the Edgewood fauna (e.g., Brevilamnulella, Eospirigerina, Epitomyonia, Glyptorthis), has been assigned a late Hirnantian age (Rong and Huang, Reference Rong and Huang2023).

Type species

Orthis insculpta Hall, Reference Hall1847, upper Katian (Richmondian) strata, Ohio, USA.

Types

Holotype. GSC 131852 (Fig. 5.1–5.3), locality S-2, Hirnantian strata of the lower Whittaker Formation, central Mackenzie Mountains, NWT, Canada. Figured paratypes, GSC 131853–131855 (Fig. 5.4–5.11), same locality.

Figure 5. (1–11) Glyptorthis papillosa new species, locality S-2, Hirnantian strata of the lower Whittaker Formation, central Mackenzie Mountains. (1–3) GSC 131852, holotype, exterior, interior, and details of papillae-bearing growth lamellae of anteriorly damaged ventral valve (largest specimen available for the new species). (4–7) GSC 131853, paratype, exterior, interior, enlarged image of dental plates, and papillae-bearing growth lamellae of small, incomplete ventral valve. (8, 9) GSC 131854, paratype, interior and detailed view of delthyrium and teeth of small, laterally damaged ventral valve. (10, 11) GSC 131855, paratype, exterior and interior of dorsal valve, showing weak, blade-like cardinal process. (12–15) Skenidioides sp. from Hirnantian strata of Whittaker Formation, Avalanche Lake area, southern Mackenzie Mountains; (12, 13) GSC 131856, exterior and interior view of ventral valve, section AV1, 77.5 m above base of section; (14, 15) GSC 131857, interior and tilted apical views of incomplete ventral valve, showing spondylium supported by short median septum (15).

Diagnosis

Small shells of Glyptorthis with fine, closely spaced growth lamellae bearing tubercular papillae.

Description

Shell small, nearly plano-convex, or with slightly convex dorsal valve, wider than long, rarely exceeding 7 mm in length or 10 mm in width (Fig. 5). Shell widest close to its mid-length. Cardinal extremities rounded to subangular, not extending into ears. Hinge line wide, straight, slightly shorter than maximum width of shell. Fold and sulcus absent, with rectimarginate anterior commissure. Costae strong, rounded, regular in strength and spacing, increasing in number anteriorly by asymmetrical bifurcation, averaging 3 ribs per 1 mm at 5 mm length growth stage, and 2 ribs per 1 mm at 7 mm length growth stage (Fig. 5.1, 5.4, 5.7, 5.10). Growth lamellae well defined and regularly spaced, averaging 7 per 1 mm, bearing tubercular papillae along their crests (Fig. 5.3, 5.7).

Ventral umbo moderately and uniformly convex, with rounded, suberect beak. Ventral interarea straight along its width, apsacline, with flat surface close to hinge line, becoming arched apically (Fig. 5.2, 5.5, 5.9). Delthyrium large, open. Teeth knobby, strong; dental plates short and low, not extending anteriorly beyond hinge line, but may extend into antero-medially curved low ridge to bound small muscle field (Fig. 5.5, 5.6, 5.9). Adductor and diductor scars not clearly differentiated. Vascular markings in medio-lateral areas adjacent to muscle field marked by low, radiating ridges (Fig. 5.2).

Dorsal umbo flat as in other parts of valve. Dorsal interarea lower than ventral, nearly orthocline (Fig. 5.11). Hinge sockets sitting on valve floor, bounded by strong inner socket ridges. Notothyrium slightly thickened into platform, with low, thin-ridged cardinal process. Posterior pair of adductor scars slightly larger and wider than anterior pair, but overall weakly impressed. Brachiophores short, blunt.

Etymology

From the Latin adjective, papillosus (feminine, papillosa), having nipples or pimples, denoting the tubercles on the growth lamellae in the new species.

Measurements

The two valves are variously damaged, with approximate measurements in millimeters.

Remarks

Glyptorthis has been shown to be a relatively common taxon of the Hirnantian fauna in several regions, such as in South China and the UK (Rong et al., Reference Rong, Huang, Zhan and Harper2013, Reference Rong, Harper, Huang, Li, Zhang and Chen2020). Glyptorthis papillosa n. sp. can be distinguished easily from other congeneric species by its generally small shell, very fine and closely spaced growth lamellae that bear well-developed, rounded, tubercular papillae.

Type species

Skenidioides billingsi Schuchert and Cooper, Reference Schuchert and Cooper1931, Rockland Formation (lower Katian), Ottawa River, Quebec.

Remarks

Two ventral valves from the lower Whittaker Formation of Avalanche Lake section AV1 (75.5–77.5 m above base) were listed, but not illustrated, by Jin and Chatterton (Reference Jin and Chatterton1997) under Skenidioides operosa Johnson, Boucot, and Murphy, Reference Johnson, Boucot and Murphy1976, which has a long stratigraphic range (Llandovery to Ludlow) in the Mackenzie Mountains and other regions of Arctic Canada (Lenz, Reference Lenz1977; Zhang, Reference Zhang1989). The two ventral valves, however, differ from those of typical S. operosa from higher stratigraphic levels in being transversely elliptical, more uniformly convex, with a moderately inclined apsacline interarea, and rounded, less-numerous costae (Fig. 5.12–5.14), although the short median septum (Fig. 5.15) is typical of the genus. Typical S. operosa, predominantly of Wenlock–Ludlow age in North America, commonly have a strophic shell, more-numerous and frequently bifurcating costae, and a catacline to strongly inclined apsacline ventral interarea to give the ventral valve a pyramidal appearance (see Zhang, Reference Zhang1989; Jin and Chatterton, Reference Jin and Chatterton1997). In these respects, the two ventral valves illustrated here resemble more closely “Skenidioides cf. S. scoliodus” reported by Lenz (Reference Lenz1977) from lower Llandovery (likely Hirnantian in modern stratigraphy) strata of the Road River Formation in his locality 4, Mount Sekwi area, central Mackenzie Mountains. The Mount Sekwi shells also have a moderately inclined apsacline ventral interarea, a hinge line that is slightly shorter than maximum shell width, rounded costae showing only weak bifurcation, unlike S. scoliodus from the UK, which tends to have a strophic shell with prominent ears (e.g., Temple, Reference Temple1968, pl. 5, figs. 7, 8, 22, 23).

Unfortunately, no dorsal valves have been found in samples from AV1 75.5–77.5 m. This precludes a specific identification of Skenidioides.

Type species

Gnamptorhynchos regularis var. globata Twenhofel, Reference Twenhofel1928 (= Gnamptorhynchos inversum Jin, Reference Jin1989), Ellis Bay Formation (Hirnantian), Anticosti Island, eastern Canada (see Jin and Zhan, Reference Jin and Zhan2000, for detailed discussion).

Gnamptorhynchos orbiculoidea (Jin and Chatterton, Reference Jin and Chatterton1997)
Figure 6

Reference Amsden1974

Platystrophia sp. Amsden, p. 32, p. 6, figs. 5a–e.

Reference Jin and Chatterton1997

Platystrophia orbiculoidea Jin and Chatterton, p. 21, pl. 3, figs. 6–24.

Reference Stott and Jin2007

Platystrophia cf. P. daytonensis; Stott and Jin, p. 453, figs. 6–10.

Figure 6. Gnamptorhynchos orbiculoidea (Jin and Chatterton, Reference Jin and Chatterton1997). (1–4) UA 10499, holotype, dorsal, ventral, posterior views, and enlarged view of tubercular shell surface (4) of incomplete, conjoined shell, section AV4B, 111.3–111.6 m above base of section, lower Whittaker Formation, Hirnantian, Avalanche Lake, southern Mackenzie Mountains. (5–18) Four specimens from locality S-2, Hirnantian strata of the lower Whittaker Formation, central Mackenzie Mountains; (5–7) GSC 131858, dorsal, ventral, and lateral views of conjoined shell (slightly offset between two valves posteriorly); (8–11) dorsal, GSC 131859, dorsal, ventral, posterior, and anterior views of small shell; (12–14) GSC 131860, exterior, posterior interior showing anteriorly raised notothyrium and ridge-like cardinal process, and shell surface tubercles (some preserved as long filaments) of dorsal valve; (15, 16) GSC 131861, exterior and interior of ventral valve, showing typical platystrophiid muscle field; (17, 18) GSC 131862, exterior and interior of dorsal valve.

Holotype

UA 10499, by original designation (re-illustrated herein, Fig. 6.1–6.4); section AV4B, 111.3–111.6 m above base of section, lower Whittaker Formation, Hirnantian, Avalanche Lake, southern Mackenzie Mountains, northwestern Canada.

Remarks

A detailed description of the Avalanche Lake material was provided by Jin and Chatterton (Reference Jin and Chatterton1997). The newly discovered shells from northern Mackenzie Mountains (locality S-2) resemble the types from Avalanche Lake in their small, weakly transverse, subelliptical, nearly equibiconvex shell. S-2 specimens reach a maximum size of 7.8 mm in length and 9.8 mm in width, comparable to the maximum length (8.8 mm) and width (9.8 mm) for the Avalanche Lake material, although the S-2 shells seem slightly more extended transversely. Both the Avalanche Lake and S-2 shells have a relatively low (barely attaining 1 mm in height), narrow (about one-third shell width), apsacline, ventral interarea (Fig. 6.3, 6.11, 6.16); moderately convex ventral umbo with a small, suberect beak; and a well-delimited dorsal fold and ventral sulcus leading to an uniplicate anterior commissure. The strong, subrounded to subangular costae are typical of shells from both regions, usually simple, two in the sulcus, three on the fold, six to eight on each flank, with only rare intercalation in the sulcus (Fig. 6.15). Microscopic tubercles or spines are present on shells from both regions (Fig. 6.4, 6.8), as is typical of Gnamptorhynchos (Jin and Zhan, Reference Jin and Zhan2000), Platystrophia, and other closely related platystrophiids (Zuykov and Harper, Reference Zuykov and Harper2007; Jin and Zhan, Reference Jin and Zhan2008).

Internally, the ventral valves from both regions possess strong teeth, supported by prominent dental plates, which extend anteriorly beyond the hinge line as low ridges to bound an elongate-oval, antero-medially raised muscle field (Fig. 6.16); the muscle field floor is smooth, without clearly defined adductor or diductor muscle scars. The notothyrial platform in the dorsal valve is raised at its anterior margin, laterally bounded by brachiophore plates to form a pseudocruralium-like structure that bears a low median ridge resembling a cardinal process (Fig. 6.14, 6.18). As is typical of this genus, the dorsal adductor scars are poorly impressed.

In their revision of Gnamptorhynchos, Jin and Zhan (Reference Jin and Zhan2000) proposed to assign this small-shelled platystrophiid species to the genus. Since then, more specimens similar to G. orbiculoidea have been discovered from coeval strata of other areas, such as the S-2 collection from northern Mackenzie Mountains, and the Laframboise Member of uppermost Ellis Bay Formation, Anticosti Island (the latter will be reported in a separate study). On Anticosti Island, the type species, Gnamptorhynchos globatum (Twenhofel, Reference Twenhofel1928), is confined to the aulaceridid biostrome unit of the Prinsta Member, now considered the basal Ellis Bay Formation (see Copper et al., Reference Copper, Jin and Desrochers2013), where it occurs together with Hirnantia notiskuani Zimmt and Jin, Reference Zimmt and Jin2023. Gnamptorhynchos orbiculoidea has a much smaller shell size than the type species (average width ~16 mm, and maximum width 20 mm), although the shells of both species attain a similarly strong, globular biconvexity, commonly an indication of an adult growth stage.

Amsden (Reference Amsden1974) reported Platystrophia sp. from the Edgewood fauna, noting its rhynchonellide-like (or Plectothyrella-like) shell morphology with a narrow hinge line and interarea. Its overall morphology is similar to Gnamptorhynchos orbiculoidea, especially in its simple costae, with two in the ventral sulcus and three on the fold. Plectothyrella, which is a true rhynchonellide and a common taxon of the Hirnantia fauna in the Kosov faunal province, is characterized by strong, angular, and commonly bifurcating costae. The presence of Gnamptorhynchos in the type area of the Edgewood fauna emphasizes the significance of this genus in the Hirnantian fauna of Laurentia.

The three specimens illustrated by Stott and Jin (Reference Stott and Jin2007) from the non-reefal facies of the lower Manitoulin Dolomite of Manitoulin Island, Ontario, are virtually identical to the Mackenzie Mountains shells, especially in their “rhynchonellide-like” posterior. This fauna has been considered to be Hirnantian in age, which is supported by chemostratigraphic dating of the lower Manitoulin Dolomite (Bergström et al., Reference Bergström, Kleffner, Schmitz and Cramer2011).

Type species

Epitomyonia glypha Wright, Reference Wright1968, Boda Limestone (uppermost Katian), Sweden.

Epitomyonia paucitropida Chen, Jin, and Lenz, Reference Chen, Jin and Lenz2008
Figure 7

Reference Chen, Jin and Lenz2008

Epitomyonia paucitropida Chen, Jin, and Lenz, p. 95, figs. 9A–S.

Figure 7. Epitomyonia paucitropida Chen, Jin, and Lenz, Reference Chen, Jin and Lenz2008, locality S-2, Hirnantian strata of the lower Whittaker Formation, central Mackenzie Mountains. (1–5) GSC 131796, paratype, dorsal, ventral, lateral, posterior, and anterior views. (6, 7) GSC 131794, holotype, exterior and interior; note transverse ridges located close to mid-length of valve. (8–12) GSC 131798, dorsal, ventral, lateral, posterior, and anterior views.

Types

Holotype, GSC 131794, Hirnantian beds, lower Whittaker Formation, section S-2 (63°06′N, 127°24′W), 45 km west of Dal Lake, Mackenzie Mountains, northwestern Canada.

Remarks

This species has been described in detail by Chen et al. (Reference Chen, Jin and Lenz2008). Epitomyonia paucitropida resembles most closely E. sekwiensis Lenz, Reference Lenz1977, from lower Llandovery strata of Mount Sekwi (~50 km northwest of locality S-2) in its shell size (average length 4.7 mm compared to 4.4 mm for the holotype of E. sekwiensis), rather strong costae for the genus, and weak transverse ridges in the dorsal valve. As noted by Chen et al. (Reference Chen, Jin and Lenz2008), E. paucitropida can be distinguished easily from E. sekwiensis in having a pair of wavy transverse ridges located at about mid-length of the dorsal valve (Fig. 7.7), whereas in E. sekwiensis the transverse ridges are located near the anterior margin of the dorsal valve.

The dicoelosiid specimens from the Bowling Green Dolomite (Edgewood Group) of Missouri, reported by Amsden (Reference Amsden1974) as Dicoelosia sp., were poorly preserved as molds in coarse-grained dolomite, yielding little information on internal structures. Amsden (Reference Amsden1974, p. 42) commented on the possibility that these specimens were Epitomyonia because of their wide hinge line, very shallow anterior emargination, and overall similarity to Epitomyonia glypha in external morphology. The affinity of these specimens to Epitomyonia also was suggested by Lenz (Reference Lenz1977). In its general shell shape and relatively even costae (due to weak bifurcation), the Missouri Dicoelosia sp. is most similar to Epitomyonia paucitropida, and is thus assigned to Epitomyonia rather than Dicoelosia in this study.

Epitomyonia paucitropida is similar also to E. glypha from the Boda Limestone (uppermost Katian) in shell size (usually < 5 mm in length), with the dorsal transverse ridges located close to mid-length of the valve. The Boda species, however, differs in having notably sharper costae (and an especially strong medial costa in the ventral valve) that bifurcate more intensely, and much stronger, anteriorly tilting transverse ridges in the dorsal valve (Wright, Reference Wright1968, pl. 1, figs. 5, 14, 15). In this respect, the specimens reported as E. glypha by Rubel (Reference Rubel2011, pl. 33, figs. 1, 2) from Rhuddanian strata of Estonia are more closely allied with E. paucitropida. Another Hirnantian species of Epitomyonia, E. americana Sheehan and Lespérance, Reference Sheehan and Lespérance1979, from Percé, Quebec, differs from E. paucitropica in having a distinctly wider, planoconvex shell, with a notably shorter dorsal median septum.

Holotype

GSC48019, lower Llandovery beds, Road River Formation, Mount Sekwi (locality 4 of Lenz, Reference Lenz1977, p. 1523, 1530) Mackenzie Mountains, Northwest Territories.

Remarks

Detailed description and illustration of this species were provided by Jin and Chatterton (Reference Jin and Chatterton1997). Specimens from Avalanche Lake are assigned to E. sekwiensis based on their similarity to those described by Lenz (Reference Lenz1977) in the development of subtriangular ventral valves in some shells, coarse costae, and a relatively deep anteromedial emargination for the genus. As noted by Lenz (Reference Lenz1977), some triangular forms with prominent lobes are externally homeomorphic with the Late Ordovician Dicoelosia lata Wright, Reference Wright1968. Epitomyonia glypha from lower Llandovery rocks Estonia (Rubel, Reference Rubel1971, pl. 8, fig. 6; Rubel, Reference Rubel2011, p. 51, pl. 33, figs. 1, 2) also has a strongly bilobate shell with anteromedial emargination and, importantly, a pair of simple transverse septa in the antero-middle to anterior part of the dorsal valve. Lenz (Reference Lenz1977) distinguished E. sekwiensis from the approximately coeval E. glypha by a more strongly convex ventral valve in the Sekwi species.

Type species

Orthis testudinaria Dalman, Reference Dalman1828. Dalmanitina Beds (Hirnantian), Västergötland, Sweden (for taxonomic update see Jin and Bergström, Reference Jin and Bergström2010).

Lectotype

UI X-865, selected by Amsden (Reference Amsden1974, pl. 8, figs. 2a–c), Hirnantian–Rhuddanian boundary interval, Edgewood Group, near Edgewood, Missouri.

Remarks

In the Mackenzie Mountains, this species has been found only in the Avalanche Lake locality, as described by Jin and Chatterton (Reference Jin and Chatterton1997). The Avalanche Lake specimens resemble those from the Edgewood Group of Missouri and Illinois (Amsden, Reference Amsden1974) in their subrounded, ventribiconvex shell with rounded multicostellae or weakly developed fascicostellae. The affinity of these specimens to typical Dalmanella (as assessed by Jin and Bergström, Reference Jin and Bergström2010, based on type specimens from the Hirnantian strata of Sweden) is supported by the presence of an interspace (instead of a rib) along the medial line in the dorsal umbonal area, a relatively small ventral muscle field (relative to valve size), and a relatively delicate cardinal process with a bilobed and crenulated myophore. Relatively large Edgewood shells may reach 15 mm long and develop a fairly deep dorsal valve. In comparison, the Mackenzie shells rarely exceed 10 mm in length and are typically ventribiconvex, as are the smaller Edgewood forms. The maximum shell size may be related to another minor difference—the Edgewood shells have a slightly wider ventral muscle field with prominently arched laterally bounding ridges, compared to the slightly narrower ventral muscle field with subparallel to weakly arched lateral bounding ridges (compare Amsden, Reference Amsden1974, pl. 7, figs. 1v, 1zz and pl. 8, figs. 1a, 3f, with Jin and Chatterton, Reference Jin and Chatterton1997, pl. 10, figs. 1, 3, 19, 21, 23). However, the ventral muscle field in specimens from both regions shows notable variations in its outline in terms of its relative size (especially width) relative to the valve, as well as the curvature and strength of bounding ridges.

Type species

Orthis redux Barrande, Reference Barrande1848, Letná Formation (Sandbian), Czech Republic.

Drabovia noixella (Amsden, Reference Amsden1974)
Figure 8

Reference Amsden1974

Hirnantia noixella Amsden, p. 45, pl. 10, figs. 1a–y.

Figure 8. Drabovia noixella (Amsden, Reference Amsden1974), locality S-2, Hirnantian strata of the lower Whittaker Formation, central Mackenzie Mountains. (1–5) GSC 131863, dorsal, ventral, lateral, posterior, and anterior views of small, ventribiconvex shell, with strong growth lamellae near anterior margin; (6) GSC 131864, interior of incomplete ventral valve, showing muscle field; (7–10) GSC 131865, exterior view of incomplete dorsal valve showing interspace along medial line of valve (7), interior showing cardinalia and small adductor scars (8), and detailed views of cardinalia from anterior (9) and posterior (10), showing thin shaft of cardinal process sitting directly on valve floor (9) and bilobed, crenulated myophore (10); (11–14) GSC 131866, exterior, interior of dorsal valve, and two views of cardinalia from posterior (13) and anterior (14); (15–18) GSC 131867, dorsal, posterior, anterior, and tilted anterior views of incomplete, strongly biconvex, conjoined shell.

Holotype

By original designation, OU 6780, Noix Limestone, Edgewood Group, Hirnantian, Missouri.

Remarks

Amsden (Reference Amsden1974) originally assigned the species to Hirnantia based mainly on dorsal internal structure. This Edgewood draboviid, however, has a considerably smaller shell (generally < 6 mm in length or 7 mm in width) but strong biconvexity and very coarse ribs for shell size. Amsden (Reference Amsden1974, p. 46) incorrectly compared the ventribiconvex shell of D. noixella to a “ventribiconvex” Hirnantia sagittifera because the latter is characterized by a distinctly dorsibiconvex shell; he also incorrectly contrasted the Edgewood species with “Drabovia and Pionodema with dorsibiconvex shells” since the latter two are typically ventribiconvex. The type species of Hirnantia has fine, fairly even-sized multicostellae, which clearly differ from the coarse, unevenly bifurcating ribs (fascicostellae) of D. noixella. In its small, ventribiconvex shell and such coarse ribs (with wide interspaces), the Edgewood species is regarded here to have a much stronger affinity to Drabovia than to Hirnantia.

The Mackenzie specimens are assigned to D. noixella on account of their similarly small ventribiconvex shells, relatively coarse ribbing, and strong growth lamellae near the anterior margin. An average-sized complete shell measures 6.5 mm in length, 6.9 mm in width, and 4.1 mm in thickness (Fig. 8.1–8.5), matching those reported by Amsden (Reference Amsden1974). In shell ribbing, the Mackenzie Mountains specimens resemble Amsden's (Reference Amsden1974) illustrations from the Noix Limestone in several aspects: (1) the ribs are strong and coarse relative to shell size; (2) the medial line is occupied by an interspace in the dorsal valve, and a strong costa in the ventral valve (compare Fig. 8.2 with Amsden, Reference Amsden1974, pl. 19, fig. 1k; and Fig. 8.7 with Amsden's, Reference Amsden1974, pl. 10, fig. 1b); and (3) the bifurcating ribs range from multicostellate (compare Fig. 8.1 with Amsden's, Reference Amsden1974, pl. 10, fig. 1s) to weakly fascicostellate (compare Fig. 8.7 and 8.11 with Amsden's, Reference Amsden1974, pl. 10, fig. 1k). Dorsal valves from both Missouri and the Mackenzie Mountains show the diagnostic internal characters of Drabovia, especially the antero-laterally divergent brachiophore plates along the valve floor, and the slender cardinal process, with a bilaterally crenulated myophore, sitting directly on the valve floor because of the lack of a notothyrial platform (Fig. 8.8–8.10, 8.12–8.14). Despite their generally small size, the shells show a relatively strong biconvexity for Drabovia, a truncated anterior margin with densely spaced growth lamellae, and robust and high hinge plates (Fig. 8.10, 8.14, 8.18), suggesting that these are mature individuals. The anterior and posterior pairs of adductor scars are small and narrow relative to valve size, bearing a gently developed myophragm (medial ridge; see Fig. 8.8, 8.12).

Drabovia? minuta Hints, Reference Hints2012, from Porkuni strata (Hirnantian) of Estonia is similar to D. noixella in its small shell with a fairly strong biconvexity for this genus, relatively short hinge line, relatively coarse fascicostellae, and strong growth lamellae in the anterior part of the shell. The Estonian shells seem to have a proportionally small cardinalia (relative to shell size) in comparison to the North American specimens.

Type species

Strophomena antiquata var. woodlandensis Reed, Reference Reed1917, Woodland Formation (Rhuddanian), Girvan, Scotland.

Remarks

Typical species of Katastrophomena are usually considered common in the Ordovician–Silurian boundary interval. A review of the genus by Cocks (Reference Cocks2008) demonstrated its fairly long stratigraphic range, from upper Katian (Cautleyan) to Ludlow.

Katastrophomena mackenzii new species
Figures 9, 10.12–10.15

Reference Lenz1977

Katastrophomena sp Lenz p. 1538, pl. 7, figs. 1–6.

Reference Jin and Chatterton1997

Katastrophomena cf. K. woodlandensis; Jin and Chatterton, p. 28, pl. 18, figs. 7–13.

Figure 9. Katastrophomena mackenzii new species. (1–13) Five specimens from locality S-2, Hirnantian strata of the lower Whittaker Formation, central Mackenzie Mountains; (1–3) GSC 131870, paratype, exterior, interior, and posterior views of incomplete ventral valve; note fascicostellae and semi-tubular pseudodeltidium (3); (4–6) GSC 131869, paratype, dorsal, ventral, and posterior views of incomplete, conjoined shell; note strongly developed fascicostellae; (7, 8) GSC 131868, holotype, exterior and interior views of dorsal valve; note short central pair of trans-muscle septa, and knobby lateral septa (8); (9, 10) GSC 131871, paratype, exterior and interior of relatively small dorsal valve; (11–13) GSC 131872, paratype, interior and exterior views of posterior fragment of relatively large dorsal valve, and detailed view of cardinalia (13), note well-developed fascicostellae and oval-shaped adductor scars; (14–18) two specimens from section AV4B, 111.3–111.6 m above base of section, lower Whittaker Formation, Hirnantian, Avalanche Lake, southern Mackenzie Mountains; (14–16) UA 10636, paratype, exterior, interior of dorsal valve, and detailed view of cardinalia (16); (17, 18) UA 10635, paratype, exterior and detailed view of ventral muscle field.

Figure 10. (1–11) Biparetis paucirugosus Amsden, Reference Amsden1974, two specimens from locality S-2, Hirnantian strata of the lower Whittaker Formation, central Mackenzie Mountains. (1–4) GSC 131874, dorsal, ventral, lateral, and posterior apical views of immature, concavo-convex shell, showing presence of concentric rugae; (5–11) GSC 131875, dorsal, ventral (slightly tilted to show trail), ventral (low-angle lighting to show weak concentric rugae), lateral, and anterior views of concavo-convex shell with sharp geniculation and prominent trail (8), and details of epibionts on dorsal valve (10, 11). (12–15) Katastrophomena mackenzii new species, GSC 131873, paratype, dorsal, lateral, ventral, and apical views of incomplete shell showing biconvexity, locality S-2, Hirnantian strata of the lower Whittaker Formation, central Mackenzie Mountains.

Types

Holotype, GSC 131868 (Fig. 9.7, 9.8), dorsal valve, from locality S-2, lower Whittaker Formation, central Mackenzie Mountains. Figured paratypes, GSC 131869–131873, 2 conjoined shells (Figs. 9.4–9.6, 10.12–12.15), 2 ventral valves, (Fig. 9.1–9.3, 9.17, 9.18), and 3 dorsal valves, also from Hirnantian strata in the lower Whittaker Formation, including two from locality S-2 (Fig. 9.9–9.13) and one from the Avalanche Lake section AV4B (Fig. 9.14–9.16). All type specimens have various degrees of damage.

Diagnosis

Shell relatively small for genus, weakly biconvex, with rectimarginate anterior commissure; costae coarse, uneven, commonly sinuous, and crooked, strongly fascicostellate. Internal structures same as in type species.

Description

Shell small to medium-sized for genus, with estimated average width of ~15 mm, and maximum width of ~20 mm (Fig. 9.1–9.3, 9.11–9.13); wider than long, but variable in length/width ratios (e.g., Fig. 9.7, 9.9, 9.14, 9.17); plano-convex to weakly biconvex (Fig. 10.12–10.15), attaining ~5 mm thickness in an incomplete shell estimated at ~20 mm wide. Hinge line wide, straight, but usually shorter than maximum shell width, forming rounded cardinal extremities (Fig. 9.1, 9.4, 9.11). Anterior commissure largely rectimarginate. Costae coarse for strophomenids, averaging 3 per 2 mm in anterior parts of relatively large shells, commonly sinuous, crooked with knotted or otherwise irregular crests (e.g., Fig. 9.7, 9.9, 9.14), increasing anteriorly by frequent, asymmetrical bifurcation to form clearly defined fascicostellae (Fig. 9.1, 9.4, 9.7, 9.12).

Ventral valve moderately convex in umbonal area, becoming weakly convex to flat laterally and anteriorly, without fold or sulcus; beak low but clearly defined, erect to suberect. Ventral interarea moderately high in apical area (up to 2 mm in height), becoming lower gradually towards cardinal extremities, with predominantly planar surface (Fig. 9.2, 9.4). Delthyrium covered by arched to semi-tubular pseudodeltidium, bearing rounded apical foramen (Fig. 9.3).

Dorsal valve flat to weakly convex; umbonal area flattened, marked by strong costae in umbonal area along medial line (Fig. 9.4, 9.7, 9.12). Dorsal interarea notably lower than ventral, close to being orthocline. Notothyrium covered apically by arched chilidium, anteriorly occupied by bilobed crests of cardinal process.

Ventral interior: dental plates well developed, extending anteriorly into high, laterally arched, and tiled bounding ridges of muscle field (Fig. 9.2, 9.18). Muscle field small, occupying about one-quarter of shell length or width, subrounded in outline, antero-medially open, bearing strong medial ridge, but without sharply differentiated adductor and diductor scars.

Dorsal interior: hinge sockets elongated along hinge line, bounded anteriorly by “semi-tubularly” curved and suspended hinge plates (Fig. 9.11, 9.13). Cardinal process bilobed, sitting on raised notothyrial platform; each lobe with posterior-facing myophore and spoon-like crest (Fig. 9.11, 9.13, 9.15, 9.16). Adductor muscle field oval in outline, raised slightly above valve floor, bounded postero-laterally by stubby, rod-like lateral trans-muscle septa, laterally by pair of weak, slender central-lateral septa, and postero-medially by pair of rounded septa that converge and merge onto notothyrial platform. Brachiophores poorly preserved.

Etymology

After the Mackenzie Mountains, where the new species occurs.

Remarks

The new species is most similar to Katastrophomena woodlandensis (Reed, Reference Reed1917) from Rhuddanian rocks of Scotland and Wales in its coarse, sinuous, and crooked costae and in overall internal structure, but differs in having a predominantly biconvex shell with strongly developed fascicostellae. Temple (Reference Temple1970, Reference Temple1987) observed that ribbing density is the only consistent character for distinguishing Katastrophomena woodlandensis from K. scotica (Bancroft, Reference Bancroft and Lamont1949). In the Scottish material, K. woodlandensis has coarser, more crooked, and uneven costae than K. scotica. Cocks (Reference Cocks2008) agreed with Temple (Reference Temple1987) that K. scotica should be treated as a subspecies of K. woodlandensis. The crooked and knotted ribbing in the two specimens (ventral and dorsal valves, both convex) from section AB4B is nearly identical to that of K. woodlandensis (e.g., Cocks, Reference Cocks1968, pl. 2, figs. 1, 3) in addition to their similarly small ventral muscle field with prominent bounding ridges and a rounded median ridge, a dorsal median septum that abuts against the notothyrial platform and bifurcates immediately to form the central trans-muscle seta, and a pair of rod-like lateral trans-muscle septa. The fascicostellae in AV4B specimens are less strongly developed than in those from locality S-2 but are more like the less-distinct fascicostellae in the paratype of K. woodlandensis scotica illustrated by Cocks (Reference Cocks1968, pl. pl. 3, fig. 5) and Temple (Reference Temple1987, pl. 7, fig. 6). The British specimens reach a large size of over 20 mm long and 30 mm wide and are variably resupinate at adult stage (from flat to nearly geniculate). The dorsal valve resembles that illustrated in Cocks (Reference Cocks1968, pl. 2, fig. 6) in its relatively strong convexity.

In his description of Katastrophomena woodlandensis from the Rhuddanian of Estonia, Rubel (Reference Rubel2011) commented that they are identical to conspecific material from Wales and Norway (Temple, Reference Temple1968, Reference Temple1987; Baarli, Reference Baarli1995), but observed two significant variations in the Estonian specimens: (1) the shell curvature changes from convexo-planar to weakly resupinate; and (2) the ribbing ranges from fairly regular (non-crooked) and somewhat unequally parvicostellate to strongly crooked (Rubel, Reference Rubel2011, pl. 1, fig. 5). The Mackenzie shells most closely resemble the variants with relatively coarse and crooked ribs, or a convexo-planar lateral profile. The internal structures in shells from Estonia and the Mackenzie Mountains are nearly identical, especially in its small ventral muscle field with prominent, semicircular, lateral bounding ridges and a medial ridge (compare Fig. 9.2, 9.18 with Rubel, Reference Rubel2011, pl. 1, figs. 8), and in the blunt, rod-like lateral trans-muscle septa (compare Fig. 9.8, 9.10, 9.11, 9.15, 9.16 with Rubel, Reference Rubel2011, pl. 1, figs. 10–14). The Mackenzie Mountains species differs from the Estonian K. woodlandensis in having a weak biconvex shell and distinct fascicostellae.

Lenz (Reference Lenz1977) reported a doubtful and unidentified species Katastrophomena from lower Llandovery strata of the Road River Formation, Mount Sekwi (Mackenzie Mountains); it is similar to the Avalanche Lake and S-2 specimens in having a biconvex shell and strong and irregular costae, as well as similar internal structures and, therefore, it is considered conspecific with the new species described here.

Holotype

UA 10673 (Jin and Chatterton, Reference Jin and Chatterton1997, pl. 23, figs. 13, 14), section AV1, 95.5 m above base, upper Hirnantian beds of the lower Whittaker Formation.

Remarks

In the Avalanche Lake area of southern Mackenzie Mountains, Katastrophomena parvicardinis occurs in section AV1 and is stratigraphically coeval with K. cf. K. woodlandensis from section AV4. This is another species of Katastrophomena with a biconvex shell, although shells are generally very small and only weakly convex, with a thickness < 1 mm, average length of 4 mm (max. 5.5 mm) and average width of 5 mm (max. 7.5 mm). Despite the small shell size, K. parvicardinis shares some similarities with K. cf. K. woodlandensis, such as the well-developed apsacline ventral interarea, relatively large pseudodeltidium, and relatively strong rounded ribs that tend to become fascicostellate (see Jin and Chatterton, Reference Jin and Chatterton1997, pl. 23, figs. 6, 18), although not as distinctly fascicostellate as in K. cf. K. woodlandensis. Katastrophomena parvicardinis has a rounded ventral muscle field with curved lateral bounding ridges and a low median ridge, which are typical of the early forms of Katastrophomena described by Cocks (Reference Cocks1968) from the UK, but the Avalanche Lake species differs from those British species in its notably smaller and thinner-walled shell, finer costellae, poorly developed growth lamellae and lack of lateral trans-muscle septa in the dorsal valve. K. woodlandensis from the British type area, for example, commonly reach sizes of more than 20 mm in length and 30 mm in width, with well-defined lateral trans-muscle septa in the dorsal valve.

In the Avalanche Lake locality (section AV1), resupinate shells of Katastrophomena sp. (see Jin and Chatterton, Reference Jin and Chatterton1997, p. 29, pl. 23, figs. 21–23) occur 30 m above the level of K. parvicardinis, in strata of Rhuddanian age.

Type species

Biparetis paucirugosus Amsden, Reference Amsden1974; basal Leeman Formation (Hirnantian), Missouri, USA.

Remarks

Cocks and Rong (Reference Cocks, Rong and Kaesler2000) assigned Biparetis to Family Strophomenidae (Subfamily Furcitellinae). However, the sharp dorsal geniculation, concentric rugae, and convergent lobes of the cardinal process at their crest of Biparetis suggest a closer affinity to Family Rafinesquinidae (Subfamily Leptaeninae), as originally proposed by Amsden (Reference Amsden1974), which was adopted by Dewing (Reference Dewing1999), and is followed in this study. At present, Biparetis is a monospecific taxon.

Holotype

OU 6707, basal Leeman Formation (Hirnantian), Girardeau County, Missouri, USA.

Remarks

Only two specimens are available for study, one conjoined mature shell with a damaged posterior, and the other immature. These are assigned to Biparetis paucirugosus based mainly on the strong dorsal geniculation of the shell with weak, concentric rugae (Fig. 10.6, 10.7). The two lobes of the cardinal process have a tendency to converge at their apices in the type material (e.g., Amsden, Reference Amsden1974, pl. 21, figs. 1e, m, n), as is visible also in the immature shell from locality S-2 (Fig. 10.4). Amsden (Reference Amsden1974, pl. 21, figs. 1i–k) showed a relatively narrow shell with a prominent trail, giving the shell an apparent equidimensional (as long as wide) appearance; but the majority of the type specimens have a transversely extended shell outline typical of leptaenids. The Mackenzie Mountains form is mostly similar to paratypes with a transverse outline and faint rugae (e.g., Amsden, pl. 21, fig. 1o, r). The prominent pair of trans-muscle septa in the dorsal valve, as is diagnostic of the genus, can be partly observed when viewed from the damaged posterior inward. Some specimens reported as Lepaena rugosa Dalman, Reference Dalman1828 (see Bergström, Reference Bergström1968) from Hirnantian strata also have weaker concentric rugae similar to Biparetis paucirugosus, such as those from the Porkuni (Hirnantian) strata of Estonia (Hints and Harper, Reference Hints and Harper2015) and the Kuanyinqiao (Hirnantian) and Weiba (upper Hirnantian) formations of South China (Huang et al., Reference Huang, Rong, Harper and Zhou2020a, Reference Huang, Zhou, Harper, Zhan, Zhang, Chen and Rongb; Rong and Huang, Reference Rong and Huang2023); but these leptaenids do not seem to have the prominent pair of trans-muscle septa in the dorsal valve.

The strongly concave dorsal valve of B. paucirugosus seems to have been susceptible to epibiont encrustation, as seen in specimens from Missouri (Amsden, Reference Amsden1974, pl. 21, fig. 1f) and the Mackenzie Mountains (Fig. 10.5, 10.10, 10.11).

Outside of the Edgewood type area in the USA, Biparetis have been found so far only as a rare taxon in the Mackenzie Mountains and on Anticosti Island, eastern Canada. Dewing (Reference Dewing1999) reported only two specimens of B. paucirugosus from the basal Fox Point Member of the Becscie Formation, a level regarded by some as uppermost Hirnantian based on chemostratigraphic and biostratigraphic data (Mauviel et al., Reference Mauviel, Sinnesael and Desrochers2020; Zimmt and Jin, Reference Zimmt and Jin2023).

Type species

Atrypa hemiplicata Hall, Reference Hall1847, “Trenton Limestone” (lower Katian), northwestern New York State.

Parastrophina cf. P. minor (Roy, Reference Roy1941)
Figure 11.1–11.3

Reference Roy1941

Parastrophinella hemiplicata minor Roy, p. 94, fig. 57.

Reference Bolton2000

Parastrophina minor; Bolton, pl. 20, figs. 3–6, 8, 18.

Reference Sproat, Jin, Zhan and Rudkin2015

Parastrophina minor; Sproat et al., p. 172, fig. 14A–T.

Figure 11. (1–3) Parastrophina cf. P. minor (Roy, Reference Roy1941), GSC 131876, incomplete dorsal valve from locality S-2, Hirnantian strata of the lower Whittaker Formation, central Mackenzie Mountains; exterior, interior, and tilted anterior view showing alate plates (arrows in 3) and long cruralium raised anteriorly above valve floor by median septum. (4–14) Brevilamnulella laevis (Sapelnikov and Rukavishnikova, Reference Sapelnikov and Rukavishnikova1975); (4–7) UA 107307, ventral valve from section AV4B, 111.3–111.4 m above base of section, lower Whittaker Formation, Hirnantian, Avalanche Lake, southern Mackenzie Mountains; exterior, interior, tilted anterior, and tilted lateral views, showing high median septum supporting broad V-shaped spondylium; (8–11) UA10736, dorsal valve from section AV1, 77.5 m above base of section, lower Whittaker Formation, Hirnantian, Avalanche Lake, southern Mackenzie Mountains; exterior, interior, tilted anterior, tilted lateral views showing short and low inner hinge plates (arrows in 10, 11); (12–14) GSC 131877, exterior, interior, and tilted apical views of dorsal valve with minor damage, also from section AV1, 77.5 m (arrows indicate inner hinge plates).

Types

Holotype, USNM 28156, by original designation of Roy (Reference Roy1941), Amadjuack Formation (middle Katian, Edenian), Silliman's Fossils Mount, Baffin Island, Canada.

Remarks

The only specimen available for study, an incomplete dorsal valve, is assigned to Parastrophina on account of its well-developed cruralium, which consists of a pair of crural plates merging onto the low median septum, which protrudes above the cruralium floor as a blade-like medial ridge, especially in the anterior part of the structure (Fig. 11.2, 11.3); this morphological feature is typical of the type and other species and Parastrophina, as demonstrated by Sproat et al. (Reference Sproat, Jin, Zhan and Rudkin2015, p. 168, fig. 8). The alate plates are small but well defined (Fig. 11.3). The S-2 specimen is compared to P. minor in its relatively small but moderately convex dorsal valve with strong concentric growth lamellae, but the S-2 specimen differs from other species of Parastrophina in having a strongly pointed umbo and beak. Several species from Dulankara and Anderken strata (upper Sandbian–lower Katian) of Kazakhstan, such as Parastrophina plena Sapelnikov and Rukavishnikova, Reference Sapelnikov and Rukavishnikova1975, P. portentosa (Nikitin and Popov in Nikitin et al., Reference Nikitin, Popov and Holmer1996), and P. iliana Popov, Cocks, and Nikitin, Reference Popov, Cocks and Nikitin2002, have a posteriorly tapering dorsal valve, but these older forms have a much better-developed dorsal median septum that begins at the valve apex (see Popov et al., Reference Popov, Cocks and Nikitin2002; Jin and Popov, Reference Jin and Popov2008). In the S-2 specimen, the dorsal median septum is so low posteriorly as to make the cruralium nearly sessile.

Type species

Clorinda? thebesensis Savage, Reference Savage1913, Leemon Formation (Hirnantian), Illinois, USA.

Types

The specimens figured by Sapelnikov and Rukavishnikova (Reference Sapelnikov and Rukavishnikova1975) are from the Holorhynchus giganteus Zone, Tolen Beds (highest Katian), eastern Kazakhstan.

Remarks

Sapelnikov (Reference Sapelnikov1985) assigned this species to Brevilamnulella because he treated Antigaleatella as a junior synonym of Brevilamnulella. It is more difficult, however, to distinguish Brevilamnulella from early forms of Clorinda, such as Clorinda undata (J. de C. Sowerby, Reference Sowerby and Murchison1839) of earliest Silurian (early Llandovery) age by external morphology alone because both genera have a small, moderate to strongly biconvex shell bearing a ventral sulcus and dorsal fold, as observed by Amsden (Reference Amsden1974), Sapelnikov and Rukavishnikova (Reference Sapelnikov and Rukavishnikova1975), and Sapelnikov (Reference Sapelnikov1985). Such a ventral sulcus and dorsal fold are well developed in relatively large specimens of B. laevis from the Avalanche Lake area (Fig. 11.4–11.7, 11.12–11.14). This problem was also encountered by Temple (Reference Temple1968, Reference Temple1970, Reference Temple1987) in his study of the Hirnantian–Rhuddanian pentamerides from Wales. Internally, Brevilamnulella can be distinguished from Clorinda in having smaller hinge plates, confined largely to the apical area (e.g., Fig. 11.14) and, in transverse cross section, the inner hinge plates range from subparallel to basio-medially inclined to each other in Brevilamnulella, whereas they are basio-laterally divergent from each other in Clorinda (e.g., see Jin et al., Reference Jin, Caldwell and Norford1993). Along their junctions with the valve floor, however, the inner hinge plates become wider apart from each other anteriorly (Fig. 11.10, 11.14). In addition, the crus forms a prominent flange at its junctions with the inner and outer hinge plates in Clorinda, but merges smoothly with the hinge plates in Brevilamnulella.

In the Mackenzie Mountains, Brevilamnulella has been found only from the Avalanche Lake area, not in the S-2 material. These are assigned to B. laevis, originally reported from eastern Kazakhstan (Sapelnikov and Rukavishnikova, Reference Sapelnikov and Rukavishnikova1975; Sapelnikov, Reference Sapelnikov1985) based on their predominantly smooth, subequally biconvex shell with a low and pointed ventral umbo, and a faint plica in the anteriorly developed ventral sulcus in some relatively large specimens (Fig. 11.4). Nearly all the shells from the Avalanche Lake collection are incomplete due to various degrees of damage, and a relatively large dorsal valve with minor damage in the collection is estimated to be 10 mm in length or width (Fig. 11.12–11.14). The type species, B. thebesensis, attains a maximum width of 15 mm (Amsden, Reference Amsden1974), although the Avalanche Lake shells fall within the average shell width (~10 mm) of the type species. The two species differ in that the Avalanche Lake forms have a prominently pointed posterior in both the ventral and dorsal valves.

Farther north along the western margin of Laurentia, a latest Katian species of Brevilamnulella, B. minuta Jin and Blodgett, Reference Jin and Blodgett2020, occurs in east-central Alaska, which belonged to the stable margin of Laurentia. This relatively old species differs from the Mackenzie Mountains form in having a very small shell (< 6 mm in length or width) and a posteriorly tapering shell.

Compared to the miniscule internal structures (hinge plates) of the dorsal valve, the internal structures of the ventral valves are rather prominent relative to shell size, especially in its broadly V-shaped spondylium supported by a high, blade-like median septum that extends for about one-third of the valve length.

Type species

Zygospira putilla Hall and Clarke, Reference Hall and Clarke1894. Edgewood Group (probably Bryant Knob Formation), Hirnantian–Rhuddanian boundary interval, Missouri.

Eospirigerina putilla (Hall and Clarke, Reference Hall and Clarke1894)
Figure 12

Reference Hall and Clarke1894

Zygospira putilla Hall and Clarke, p. 356, pl. 54, figs. 35–37.

Reference Savage1913

Atrypa praemarginalis Savage, p. 129, pl. 6, figs. 14–16. [page range and plate number shown according to reprint dated 1917]

Reference Boucot and Johnson1967

Spirigerina (Eospirigerina) praemarginalis; Boucot and Johnson, p. 91, pl. 1, figs. 1–16.

Reference Amsden1974

Eospirigerina putilla; Amsden, p. 72, pl. 17, figs. 7a–e, pl. 18, figs. 1–19, pl. 19, figs. 1–8.

Reference Amsden1974

Atrypa praemarginalis; Amsden, p. 128, pl. 18, figs. 9a–d.

Reference Lenz1977

Eospirigerina cf. putilla; Lenz, p. 1542, pl. 10, figs. 13, 17–28.

Reference Kulkov, Vladimirskaya and Rybkina1985

Eospirigerina praemarginalis; Kulkov et al., p. 149, pl. 20, figs. 3a–d.

Reference Jin and Chatterton1997

Eospirigerina putilla; Jin and Chatterton, p. 41, pl. 36, figs. 7–23.

Reference Wang and Huang2022

Eospirigerina putilla; Wang and Huang, p. 9, figs. 9A–H.

Figure 12. Eospirigerina putilla (Hall and Clarke, Reference Hall and Clarke1894), six specimens from locality S-2, Hirnantian strata of the lower Whittaker Formation, central Mackenzie Mountains. (1–5) GSC 131878, dorsal, lateral, posterior, apical, and additional, enlarged lateral views; note well-preserved deltidial plates (4), and spiralium visible through damaged part ventral valve (5). (6–9) GSC 131879, dorsal, ventral, posterior, and anterior views; note well-developed fascicostellae in medial part of both valves. (10, 11) GSC 131880, dorsal and ventral views of small shell. (12, 13) GSC 131881, interior and detail view of apical part of dorsal valve showing crura and spiralial lamella. (14, 15) GSC 131882, dorsal and anterior views of distorted shell, with strong growth lamellae. (16–20) GSC 131883, dorsal, ventral, lateral, posterior, and anterior views of immature shell.

Types

Neotype of Zygospira putilla, UI-RX519, selected by Amsden (Reference Amsden1974, p. 73, pl. 18, figs. 8b–f); lectotype of Atrypa praemarginalis (regarded as a subjective junior synonym of Z. putilla by Amsden, Reference Amsden1974), UI X-4757, selected by Amsden (Reference Amsden1974, p. 73, pl. 18, figs. 9a–e (misstated as “figs. 9a–d … IU X-4757” on p. 73). All from the same locality and horizon in Pike County, Missouri, Bryant Knob Formation, Edgewood Group, Hirnantian.

Remarks

The Mackenzie Mountains specimens are assigned to Eospirigerina putilla based on their similarity to both the immature and adult shells figured by Amsden (Reference Amsden1974) from the Edgewood type area. Specimens smaller than 6 mm in length are generally elongate, teardrop-shaped, ventribiconvex, with a dorsal median furrow, ventral carina, and an erect ventral beak; deltidial plates appear to be lacking in the small shells from Avalanche Lake (Jin and Chatterton, Reference Jin and Chatterton1997, pl. 36, figs. 15–19), but are clearly present in well-preserved S-2 specimens (Fig. 12.10, 12.11, 12.15–12.19). With ontogeny, larger shells become less elongate or equidimensional, equibiconvex, with some relatively large shells even becoming dorsibiconvex, such as the shell illustrated by Jin and Chatterton (pl. 36, figs. 7, 8), which is similar to the strongly dorsibiconvex shell of E. putilla illustrated by Amsden (Reference Amsden1974, pl. 19, fig. 2a) from the Leemon Formation of Illinois, and by Rong and Huang (Reference Rong and Huang2023, figs. 4.20, 4.24–4.27) from Hirnantian strata of Yunnan, South China. Most of the Mackenzie Mountains specimens have well-preserved, medially conjoined deltidial plates and an apical foramen (Fig. 12.1, 12.3, 12.6, 12.8, 12.10, 12.13). A similar ontogenetic morphological transformation from elongate younger forms to nearly equidimensional adult ones, with concomitant reduction in size and height of the ventral beak relative to shell size, also has been observed in a large population of silicified specimens of E. putilla from the Hirnantian Wulipo Formation in Yunnan, southwestern China (Wang and Huang, Reference Wang and Huang2022; Rong and Huang, Reference Rong and Huang2023).

The ribs in specimens from both the Avalanche Lake and S-2 localities are strong but uneven due to asymmetrical bifurcations, which are somewhat more frequent in large specimens from S-2 than those from Avalanche Lake. A single strong costa in the dorsal apical area increases anteriorly through bifurcation, reaching up to eight (from left to right furrow that delimits the fold) near the anterior margin of some relatively large shells (e.g., Fig. 12.1). Correspondingly, in the ventral valve, two prominent costae form a carina in the ventral umbonal area, increasingly anteriorly to become a bundle of up to four fascicostellae, which mark the rounded margin on each side of the sulcus (Fig. 12.7, 12.11, 12.16). The tendency to develop fascicostellae also was observed by Amsden (Reference Amsden1974) in E. putilla from the Edgewood type area. Also, as in Edgewood material, a delicate medial capilla in the ventral umbonal may develop anteriorly into a normal medial costa (compare Fig. 12.7 and 12.11 with Amsden, Reference Amsden1974, pl. 18, fig. 8f), but the corresponding medial capilla in the dorsal valve, if present, remains delicate with shell growth (compare Fig. 12.1, 12.10 with Amsden, Reference Amsden1974, pl. 18, fig. 8b). The overall number of costae on each valve may increase to as many as 34 mainly through bifurcation (uncommonly by interaction) at anterior margin of mature shells. In well-preserved shells, very fine growth lines (~15 per mm) can be observed, superimposed by coarser, irregularly spaced (1–2 per mm) growth lamellae (Fig. 12.1, 12.7, 12.10, 12.11, 12.13, 12.14).

Regarding E. putilla based on the Edgewood types, the internal structures of its dorsal valve deserve more attention because some immature specimens shown by Amsden (Reference Amsden1974) have a horizontal plate (“cardinal plate”) arching between the pair of hinge plates. In this respect, these young E. putilla shells resemble adult shells of Alispira gracilis Nikiforova (in Nikiforova and Andreeva, Reference Nikiforova and Andreeva1961) from the lower–middle Llandovery of Siberia. Such a ventrally convex “cardinal plate” is present also in A. gracilis from coeval strata of western Canada (Jin and Norford, Reference Jin and Norford1992). Moreover, young forms of E. putilla are similar to A. gracilis in their elongate shell with a pair of strong costae defining the ventral sulcus. This led Kulkov and Rybkina (Reference Kulkov and Rybkina1982) to regard E. putilla as a species of Alispira. For this study, no such a “cardinal plate” has been observed in either immature or adult specimens of E. putilla, but this may have been due to the poor preservation of the silicified material.

Specimens of Eospirigerina putilla from the Mackenzie Mountains have a certain degree of similarity to E. gaspeensis (Cooper, Reference Cooper1930) from Rhuddanian strata of the Oslo region (Baarli, Reference Baarli2021, p. fig. 12–19) in their slightly elongate outline, pointedly tapering ventral posterior, and a tendency to develop fascicostellae on both sides of the sulcus, but differ in having a much stronger, wider dorsal fold that bears more numerous costae (up to eight fascicostellae) anteriorly, resulting from bifurcations (Fig. 12.1, 12.6, 12.10).