Lithostratigraphy

Lithostratigraphic nomenclature adopted in this paper is based on Murphy and Gronberg (Reference Murphy and Gronberg1970) for the Roberts Mountains, and Kendall et al. (Reference Kendall, Johnson, Brown and Klapper1983) for the Sulphur Spring Range; the reader is referred to these papers for more detailed stratigraphic information. Lithostratigraphic units from which the stromatoporoids were collected are illustrated in Figure 2. The McColley Canyon Formation is found in both the Roberts Mountains and the southern Sulphur Spring Range. In the latter, there are four members, in ascending order: Kobeh Limestone; Bartine Member; Beacon Peak Member; and Coils Creek Member. The Oxyoke Canyon Sandstone and Sadler Ranch Formation occur above the Coils Creek Member. In the Roberts Mountains, there are only three members in the McColley Canyon Formation—the Beacon Peak Member is absent. Above the Coils Creek Member, there is an unconformity, representing the time when the Oxyoke Canyon Sandstone and Sadler Ranch Formation would have been deposited (Vodrážková et al., Reference Vodrážková, Klapper and Murphy2011).

Figure 2.

Chronostratigraphy and lithostratigraphy of the study area (after Morrow and Sandberg, Reference Morrow and Sandberg2008). Stromatoporoid occurrences: (1) Stromatoporella perannulata; (2) Stictostroma moosense; (3) Syringodictyon nevadense n. sp.

Chronostratigraphy

The age of the various strata found in association with the Lower–Middle Devonian (Emsian–Eifelian) is somewhat unsettled (e.g., Pedder and Murphy, Reference Pedder and Murphy2004; Morrow, Reference Morrow2007; Pedder, Reference Pedder2010). The age of these strata, as depicted in Figure 2, is influenced to some extent by the absence of the patulus Zone, which is not found outside the Czech Republic (C.A. Sandberg, personal communication, 2021). At the Romano Ranch localities (Appendix), the partitus Zone (Eifelian) sits conformably on the serotinus Zone (Emsian).

The McColley Canyon Formation in the Sulphur Spring Range and Roberts Mountains is Pragian to Emsian in age (Fig. 2). Stromatoporella perannulata Galloway and St. Jean, Reference Galloway and St. Jean1957, is from the lower Bartine Member. In the southern Sulphur Spring Range, the Coils Creek Member of the McColley Canyon Formation is Emsian in age. Syringodictyon nevadense n. sp. is from the Coils Creek Member. The Oxyoke Canyon Sandstone in the Sulphur Spring Range is of Emsian age. Stictostroma moosense (Parks) is from the Oxyoke Canyon Sandstone.

Stromatoporoid biostratigraphy

Sessile benthic organisms such as stromatoporoids rarely are considered potential candidates in biostratigraphic correlation; however, as with most benthic marine organisms, they most likely had planktonic larvae. Some stromatoporoid species occur over large areas, and their potential utility in correlation has been noted by Webby et al. (Reference Webby, Stearn and Nestor2015).

In an article that is relevant to the present study, Prosh and Stearn (Reference Prosh and Stearn1993) found that four species of stromatoporoids (Stromatoporella perannulata Galloway and St. Jean, Reference Galloway and St. Jean1957; Stictostroma mamilliferum Galloway and St. Jean, Reference Galloway and St. Jean1957; Habrostroma proxilaminatum [Fagerstrom, Reference Fagerstrom1961]; Parallelopora campbelli Galloway and St. Jean, Reference Galloway and St. Jean1957) from the Emsian-age Blue Fiord Formation of Ellesmere Island, arctic Canada, also occur in the Detroit River Group of southwestern Ontario, Canada. They cited Uyeno's (Reference Uyeno1990) detailed conodont biostratigraphic work to support the Emsian age of the Blue Fiord Formation. Prosh and Stearn (Reference Prosh and Stearn1993, p. 2465) stated, “The Detroit River Group of southwestern Ontario contains a diverse but endemic fauna that has made dating difficult and imprecise. Studies of corals, brachiopods, goniatites, and, more recently, conodonts have generally yielded largely or wholly Middle Devonian ages for the Detroit River Group or its immediate correlatives in New York, Ohio, and Michigan.” Prosh and Stearn (Reference Prosh and Stearn1993) concluded that the presence of Emsian-age stromatoporoids in the Detroit River Group of Ontario, as documented by Fagerstrom (Reference Fagerstrom1982), along with then-existing conodont data, indicated an Emsian age for the Detroit River Group, not a Middle Devonian (Eifelian) age. They also inferred that this age revision might also apply to the Onondaga Limestone of New York.

Klapper and Oliver (Reference Klapper and Oliver1995) disagreed with Prosh and Stearn (Reference Prosh and Stearn1993), in that they maintained a middle Devonian (Eifelian) age for the Detroit River Group of central-southwestern Ontario, and the correlative Onondaga Limestone of New York and the Niagara Peninsula of Ontario. They stated (p. 1070), “The conodont and goniatite evidence is much stronger than indicated by [Prosh and Stearn] and too strong to be set aside in response to their new data.” They also concluded (p. 1072), “The ranges of the stromatoporoid species common to the Blue Fiord and Detroit River have not been adequately tested in zonal biostratigraphy …” C.A. Sandberg, accompanied by Klapper, collected and identified conodonts of the lower Eifelian partitus Zone from the Detroit River Group of southwestern Ontario (C.A. Sandberg, personal communication, 2018).

In his reply to Klapper and Oliver's (Reference Klapper and Oliver1995) discussion, Prosh (Reference Prosh1995) noted that the Detroit River and Onondaga faunas are very endemic. He made no significant changes to his original conclusions, saying that (p. 1076) “The Detroit River Group of Ontario is mostly Lower Devonian. The Edgecliff and Nedrow members of the Onondaga Limestone of New York are also Lower Devonian.”

Of the three stromatoporoid species described here from Nevada, Stromatoporella perannulata is the most ubiquitous, occurring in the middle Emsian of Nevada, and Emsian of the Blue Fiord Formation of Ellesmere Island (Prosh and Stearn, Reference Prosh and Stearn1993, Reference Prosh and Stearn1996). It also occurs in strata of less-certain age in: (1) southwestern Ontario glacial drift (Nicholson, Reference Nicholson1873; Parks, Reference Parks1936); (2) southwestern Ontario, Detroit River Group, including the Formosa Reef Limestone (Fagerstrom, Reference Fagerstrom1982); (3) Niagara Peninsula, Ontario, Onondaga Limestone (St. Jean, Reference St. Jean1986); (4) Ohio, Columbus Limestone (Galloway and St. Jean, Reference Galloway and St. Jean1957; Fagerstrom, Reference Fagerstrom1982); and (5) Kentucky, Jeffersonville Limestone (Galloway and St. Jean, Reference Galloway and St. Jean1957).

In the past, Stictostroma moosense has been known exclusively from the Kwataboahegan Formation of eastern Ontario, near the Kwataboahegan River, which flows into James Bay (Fagerstrom, Reference Fagerstrom1982). According to Fagerstrom (Reference Fagerstrom1982), Sanford et al. (Reference Sanford, Norris and Bostock1968) correlated the Kwataboahegan Formation with the Amherstburg Limestone, lower Detroit River Group, of southwestern Ontario. Klapper and Oliver (Reference Klapper and Oliver1995) placed the Amherstburg Limestone spanning, but mostly above, the Emsian-Eifelian boundary. Telford (Reference Telford1989, p. 127) stated, “Strata of the Kwataboahegan Formation are the most fossiliferous of all Devonian units in the Moose River Basin. The fauna is dominated by corals, stromatoporoids and brachiopods …” He (p. 127–128) said that the corals and brachiopods in the Kwataboahegan Formation, “… have many elements in common with the Schoharie-Bois Blanc-Onondaga faunas of the Appalachian Basin and those of the Michigan Basin Detroit River Group.” He also noted that conodonts from the lower Kwatboahegan Formation are probably Emsian in age. For New York, Rickard (Reference Rickard1975, pl. 3) illustrated the Schoharie and Bois Blanc formations as upper Emsian, and the Onondaga Formation as lower–middle Eifelian. Although Norris (Reference Norris1993, fig. 8.4) depicted the Kwataboahegan Formation as lower Eifelian, he stated (p. 678), “The conodonts from the lower third of the [Kwataboahegan] formation are similar to those from the Edgecliff Member of the Onondaga Limestone of New York dated as late Emsian (Uyeno et al., Reference Uyeno, Telford and Sanford1982).” Recent work by Pinet et al. (Reference Pinet, Lavoie, Dietrich, Hu and Keating2013) and Lavoie et al. (Reference Lavoie, Dietrich and Chen2015) placed the Emsian-Eifelian boundary within the Kwataboahegan Formation. In Nevada, S. moosense is found in the uppermost Emsian.

Because Syringodictyon nevadense n. sp. is known from only the upper Emsian of Nevada, species-level correlation is impossible; however, Syringodictyon has limited geographic and stratigraphic ranges. The type species, S. tuberculatum (Nicholson, Reference Nicholson1873), is known from the Onondaga Limestone of the Niagara Peninsula, Ontario (St. Jean, Reference St. Jean1986), and western New York (St. Jean, personal communication, 2010). St. Jean (Reference St. Jean1986) recognized that a specimen identified as Labechia sp. by Stearn (Reference Stearn1983) from the Emsian-age Blue Fiord Formation of Ellesmere Island is most likely Syringodictyon.

In summary, the controversy regarding the age of the Detroit River Group of southwestern Ontario, and correlative units (Onondaga Limestone of Ontario and New York; Columbus Limestone of Ohio; and Jeffersonville Limestone of Indiana and Kentucky), as described by Prosh and Stearn (Reference Prosh and Stearn1993) and Klapper and Oliver (Reference Klapper and Oliver1995), remains unresolved. My findings, however, support at least a partial Emsian age for a majority of the above-mentioned lithostratigraphic units.

Measurement

Procedures used here to measure and count skeletal morphologies are the same as those outlined by Stock (Reference Stock1979, p. 307; Reference Stock1982, p. 657). Eight to 11 skeletal dimensions were obtained from the three Nevada specimens.

Identification

Today, identification of stromatoporoids at the genus level is based on qualitative characteristics, such as shape of pillars, nature of laminae (i.e., sheets or colliculi), and nature of the skeletal microstructure. Identification at the species level, on the other hand, is based on quantitative characteristics, such as the number of laminae in a 1 mm distance, thickness of laminae, and diameter of pillars.

Repositories and institutional abbreviations

ROM, Royal Ontario Museum, Toronto, Ontario, Canada; USNM, U.S. National Museum, Smithsonian Institution, Washington D.C., U.S.A.

Type species

Stromatopora granulata Nicholson, Reference Nicholson1873, p. 94, pl. 4, figs. 3, 3a.

Diagnosis

Laminae continuous, thinner than galleries high. Pillars not superposed, of two types: (1) spool-shaped post-pillars; and (2) ring-pillars formed by upturns of laminae. Microstructure of laminae ordinicellular, of pillars cellular to fibrous.

Remarks

Stromatoporella is unique among the stromatoporoids in possessing common to abundant ring-pillars. Where ring-pillars are superposed, they are called tube-pillars, as in Tubuliporella Khalfina, Reference Khalfina1968.

Stromatoporella perannulata Galloway and St. Jean, Reference Galloway and St. Jean1957
Figure 3.1, 3.2

non Reference Nicholson and Murie1878

Clathrodictyon cellulosum Nicholson and Murie, p. 221, pl. 2, figs. 6–10.

Reference Parks1936

Stromatoporella cellulosa; Parks, p. 108, pl. 4, figs. 6, 7.

Reference Lecompte1951

Stictostroma eriense Parks; Lecompte, p. 137, pl. 20, figs. 2, 2a, b.

Reference Galloway and St. Jean1957

Stromatoporella perannulata Galloway and St. Jean, p. 142, pl. 9, figs. 3a, b.

Reference Galloway and St. Jean1957

Stromatoporella eriensis; Galloway and St. Jean, p. 145, pl. 10, figs. 2a, b.

Reference Flügel and Flügel-Kahler1968

Stromatoporella perannulata; Flügel and Flügel-Kahler, p. 310.

Reference Fagerstrom1982

Stromatoporella perannulata (?); Fagerstrom, p. 38, pl. 7, figs. 2–4.

Reference Fagerstrom1982

Stromatoporella eriense (?); Fagerstrom, p. 39, pl. 7, fig. 5.

Reference Bogoyavlenskaya and Khromych1985

Stromatoporella eriense; Bogoyavlenskaya and Khromych, p. 20.

Reference Bogoyavlenskaya and Khromych1985

Stromatoporella perannulata; Bogoyavlenskaya and Khromych, p. 43.

Reference St. Jean1986

Stromatoporella perannulata; St. Jean, p. 1043, fig. 5.4–5.6.

Reference Prosh and Stearn1993

Stromatoporella perannulata; Prosh and Stearn, figs. 3a, b.

Reference Prosh and Stearn1996

Stromatoporella perannulata; Prosh and Stearn, p. 26, pl. 10, figs. 1, 2.

Reference Stearn1998

Stromatoporella perannulata; Stearn, p. 19, fig. 4.

Figure 3.

(1, 2) Stromatoporella perannulata Galloway and St. Jean, Reference Galloway and St. Jean1957, USNM 706515 (×10); arrows indicate examples of ring-pillars; (1) longitudinal section; (2) tangential section. (3, 4) Stictostroma moosense (Parks, Reference Parks1904), USNM 706516 (×10); (3) longitudinal section; (4) tangential section. (5–8) Syringodictyon nevadense n. sp., holotype, USNM 706517; black dots were applied to thin-sections by Pedder; (5, 6) (×10); (5) longitudinal section; (6) tangential section; (7, 8) (×40); (7) longitudinal section; (8) tangential section.

Figure 4.

Emsian paleogeography and paleobiogeography of North America. (1) Map with location of OWR (Old World Realm), EAR (Eastern Americas Realm), Canadian Shield, and Transcontinental Arch (based on Oliver, Reference Oliver1977). Open circles represent locations of the nine areas from which specimens described in this paper were collected; circles containing dots represent the three kimberlite pipes containing Paleozoic xenoliths. (2) Map showing the proposed location of a shallow seaway across the Canadian Shield.

Holotype

Specimen ROM 1340 Cn, from Onondaga Formation drift, near Simcoe, Ontario (Parks, Reference Parks1936, pl. IV, figs 6–8).

Occurrence

Dry Creek, Roberts Mountains (Appendix), 57–60 m above the base of the Bartine Member of the McColley Canyon Formation (gronbergi Zone).

Description

Specimen laminar fragment 90 mm wide, 54 mm thick. Skeletal dimensions given in Table 1.

Table 1.

Skeletal dimensions (in mm) of specimen USNM 706515 of Stromatoporella perannulata Galloway and St. Jean, Reference Galloway and St. Jean1957. r = range; mdn = median (for counts); x = mean (for measurements); s = standard deviation; N = sample size.

Remarks

Of the species of Stromatoporella described in the literature, the Nevada specimen most closely resembles S. perannulata Galloway and St. Jean of Prosh and Stearn (Reference Prosh and Stearn1996) (Table 2) from the lower and middle Emsian of Ellesmere Island. The latter differs from the Nevada specimen only in having more closely spaced post-pillars, as seen in tangential section.

Table 2.

Comparison of skeletal dimensions (in mm) of specimen USNM 706515 of Stromatoporella perannulata with those of Prosh and Stearn (Reference Prosh and Stearn1996). Measurements from Table 1 are reduced to two decimal places to facilitate comparison with Prosh and Stearn. They reported counts per 2 mm—these have been divided by two to obtain the values below. They did not give means for any of these skeletal characteristics.

Type species

Stromatopora mammillata Nicholson, Reference Nicholson1873, p. 94, pl. 4, fig. 4.

Diagnosis

Laminae continuous, thinner than galleries high. Spool-shaped pillars not superposed. Microstructure of laminae ordinicellular, of pillars cellular to fibrous.

Remarks

Stictostroma lacks the ring-pillars of Stromatoporella Nicholson, Reference Nicholson1886; otherwise these two genera are identical.

Holotype

Specimen ROM 2619 D, from the Middle Devonian, Kwataboahegan River, James Bay, Ontario (Parks, Reference Parks1904, pl. III, figs. 3, 4; Parks, Reference Parks1936, pl. 8, figs. 5–7).

Occurrence

The Oxyoke Canyon Sandstone (serotinus Zone) at Romano Ranch I, southern Sulphur Spring Range (Appendix).

Description

Specimen encrusted favositid tabulate coral. Fragment 45 mm wide, 8 mm thick. Skeletal dimensions given in Table 3.

Table 3.

Skeletal dimensions (in mm) of specimen USNM 706516 of Stictostroma moosense (Parks, Reference Parks1904). r = range; mdn = median (for counts); x = mean (for measurements); s = standard deviation; N = sample size.

Remarks

The microstructure of the laminae and pillars of the specimen is compact, a condition not typical of Stictostroma, which has ordinicellular laminae and pillars that are cellular to fibrous (Stearn, Reference Stearn2011); however, the microstructure of genera such as Stictostroma can be diagenetically altered to compact. Photomicrographs of the specimen were examined by C.W. Stearn, and he strongly agreed that it is a specimen of Stictostroma (C.W. Stearn, personal communication, Reference Stearn2011).

The genus identification of Stictostroma moosense (Parks) has changed a few times in the past. Parks’ (Reference Parks1904) assignment to Actinostroma makes no sense because that genus has long pillars and colliculi, rather than short pillars and sheetlike laminae. Parks’ (Reference Parks1936) reassignment to Clathrodictyon makes some sense, if one takes the compact microstructure at face value. There is less sense in Galloway and St. Jean's (Reference Galloway and St. Jean1957) inclusion of the species in Stromatoporella because no ring-pillars are present.

Pseudostromatoporella moosensis (Parks) of Kazmierczak (Reference Kazmierczak1971) from the Givetian of Poland is not included in synonymy for at least two reasons. Whereas pillars in the Polish specimens that branch upward at the top are capped by straight laminae, those in the Nevada and Ontario specimens are associated with downward deflections in the laminae. The Polish specimens display large gaps in the laminae that are not present in Stictostroma moosense (Parks).

Although I was not able to borrow the holotype specimen from the Royal Ontario Museum, I did borrow a longitudinal section of an apparent topotype specimen (ROM 2026D). Measurements from the latter compare well with those of the Nevada specimen (Table 4) in longitudinal section, with the Nevada specimen having somewhat: (1) more widely spaced laminae; (2) higher galleries; and (3) wider astrorhizal canals. A few other species assigned to other genera by their authors also resemble the Nevada specimen of S. moosense: (1) Clathrodictyon amygdaloides var. subvesiculosum Lecompte, Reference Lecompte1951, from the Couvinian (Eifelian) of Belgium; (2) Clathrodictyon abnorme Yang and Dong, Reference Yang and Dong1979, from the Eifelian of Guangxi, China; (3) Simplexodictyon artyschense (Yavorsky in Thanh et al., Reference Thanh, Nguyen, Khromykh, Nguyen, Nguyen and Phoung1988) from the Pragian of North Vietnam; and (4) Schistodictyon amygdaloides subvesiculosum (Lecompte in Avlar and May, Reference Avlar and May1997) from the lower Eifelian of Germany. These species must be investigated further before synonymy with S. moosense can be established.

Table 4.

Comparison of skeletal dimensions (in mm) of specimen USNM 706516 of Stictostroma moosense (Parks, Reference Parks1904) with ROM 2026D of S. moosense. The mean score is followed by the range in parentheses.

Type species

Stromatopora tuberculatum Nicholson, Reference Nicholson1873, p. 92, pl. 4, figs. 2, 2a.

Emended diagnosis

Laminae continuous, thinner than galleries high. Post-pillars not superposed, spool-shaped. Laminae turn up into many small mamelons, superposed into mamelon columns. Skeletal macrostructures form amalgamated structure in mamelon columns. At least some columns contain axial canal. Microstructure of laminae tubular and ordinicellular, of pillars transversely fibrous.

Remarks

In his original generic diagnosis, St. Jean (Reference St. Jean1986) stated that pillars are rare—this appears to be the case in only the type species because pillars are common in the Nevada species.

Holotype

Specimen USNM 706517; from the upper Coils Creek Member of the McColley Canyon Formation (serotinus Zone) at Romano Ranch IV, southern Sulphur Springs Range, Nevada (Appendix). This specimen consists of only three thin-sections.

Diagnosis

Mamelon centers 1.43 mm apart. Laminae 5.6 per mm, pillars 4.0 per mm. Pillar width in longitudinal section and diameter in tangential section both 0.05 mm. Laminar thickness 0.04 mm, gallery height 0.17 mm. Pillar distance 0.18 mm.

Occurrence

The holotype is the only known specimen.

Description

Specimen laminar fragment 50 mm wide, 9 mm thick. Skeletal dimensions given in Table 5.

Table 5.

Skeletal dimensions (in mm) of specimen USNM 706517 of Syringodictyon nevadense n. sp. r = range; mdn = median (for counts); x = mean (for measurements); s = standard deviation; N = sample size.

Etymology

After the state of Nevada, in which it occurs.

Remarks

There are only two known species of Syringodictyon, S. tuberculatum St. Jean, Reference St. Jean1986, and S. nevadense n. sp. Examination of Table 6 reveals several strong differences between S. nevadense n. sp. and S. tuberculatum. The laminae in S. nevadense n. sp. are more closely spaced (5–7 per mm) than are those in S. tuberculatum (0.8–3.7 per mm), with no overlap in ranges. This characteristic is reflected by differences in gallery height: 0.06–0.28 mm in S. nevadense n. sp.; and 0.24–0.32 mm in S. tuberculatum. The laminae in S. tuberculatum are much thicker (0.04–0.20 mm) than are those of S. nevadense n. sp. (0.03–0.05 mm). Pillar diameters in tangential section are also much greater in S. tuberculatum (0.07–0.43 mm) than those of S. nevadense n. sp. (0.03–0.08 mm). The mamelon centers in S. nevadense n. sp. are much farther apart (0.88–2.38 mm) relative to those of S. tuberculatum (0.63–0.95 mm). There are so few pillars in S. tuberculatum that St. Jean (Reference St. Jean1986) did not report either the count of pillars per mm, or pillar distance.

Table 6.

Comparison in ranges of skeletal dimensions (in mm) of specimen USNM 706517 of Syringodictyon nevadense n. sp. with those of St. Jean (Reference St. Jean1986) for Syringodictyon tuberculatum. Measurements from Table 5 are reduced to two decimal places to facilitate comparison with St. Jean. He reported counts per 4 mm—these have been divided by four to obtain the values below.

St. Jean (Reference St. Jean1986) thought that a specimen of Labechia sp. from the lower Emsian of Ellesmere Island in the Canadian arctic that was described by Stearn (Reference Stearn1983) might very well be Syringodictyon. Unfortunately Stearn's specimen is poorly preserved, and he illustrated only a longitudinal section. What Stearn thought were pillars of Labechia, St. Jean saw as mamelon columns of Syringodictyon. Stearn (Reference Stearn1983) reported “pillars” [mamelon columns] 0.3 mm in width in longitudinal section, and 0.3–0.5 mm in diameter in tangential section.

Devonian realms

Stock and Burry-Stock (Reference Stock and Burry-Stock2020, p. 20–24) summarized in detail the faunal realms of the Devonian Period, and how the geographic distribution of stromatoporoids was affected by these paleogeographic constraints. Traditionally, the Devonian world has been divided into three realms: (1) Eastern Americas Realm (EAR), southeastern North America; (2) Old World Realm (OWR), western and northern North America, northwestern Africa, Europe, Asia, and Australia; and (3) Malvinokaffric realm, southern high latitudes (Stock et al., Reference Stock, Nestor and Webby2015). Stromatoporoids are absent from the Malvinokaffric Realm, most likely due to its cold climate; however, the EAR and OWR were located in tropical to subtropical regions, which were ideal for stromatoporoids.

Oliver (Reference Oliver1977, table 1) determined that for a geological region to be considered a realm, at least 33% of the genera of a particular taxon (e.g., rugose corals, brachiopods, conodonts) must be endemic to that realm. For example, he found that in the EAR of the late Emsian, 92% of the rugose coral genera were endemic, and for the Eifelian, 64% were endemic.

It has been generally accepted that the land barrier separating the EAR from the OWR consisted of the Canadian Shield and the Transcontinental Arch (e.g., Witzke, Reference Witzke1990; Stock in Stock et al., Reference Stock, Nestor and Webby2015). This barrier existed from the Early Devonian (Lochkovian) through the Middle Devonian (early Givetian), until the barrier became submerged by what is known as the Taghanic Onlap (e.g., Johnson et al., Reference Johnson, Klapper and Sandberg1985).

There was one exception to Oliver's (Reference Oliver1977) definition of the OWR, which he illustrated several times (e.g., Oliver, Reference Oliver1990, fig. 1; Reference Oliver1993, fig. 1). He determined that the Great Basin Province, which included Nevada, switched from the OWR to the EAR during the Pragian–early Emsian. In support of this, he cited |Boucot et al. (Reference Boucot, Johnson and Talent1968, Reference Boucot, Johnson and Talent1969) and Boucot (Reference Boucot1975) as, “… [having] convincingly shown that during the Siegenian [Pragian] and early Emsian time, Great Basin (Nevada) brachiopods were dominantly ENA [EAR] (“Appalachian”) types” (Oliver, Reference Oliver1977, p. 105). He also gave an example of the rugose coral Breviphrentis, saying, “This genus ranges from the Gedinnian [Lochkovian] to Eifelian in ENA [EAR]. It apparently moved into the Great Basin area during the Siegenian [Pragian], where it persisted to the late Emsian” (Oliver, Reference Oliver1977, p. 105).

The anomalous nature of the Great Basin Province was taken further by Pedder and Murphy (Reference Pedder and Murphy2004), who emphasized its differences from the EAR and OWR. They stated (p. 845), “… the Great Basin Province began with sparse low-diversity faunas in early Pragian time (nevadensis conodont zone …), and culminated in distinctiveness in middle Emsian time (gronbergi Zone). It ended with the Breviphyllidae and Papiliophyllidae in the late Emsian serotinus Zone, and the arrival of OWR Cystiphyllidae, Ptenophyllidae, and Stringophyllidae in the early Eifelian costatus Zone.” They (p. 847) concluded, “… Rugosa of the Great Basin Province do not fit well into either the OWR or EAR.”

Pedder (Reference Pedder2010) defined the location of the Emsian-Eifelian boundary in association with the Chotec Event, representing eustatic deepening of the ocean. He stated (p. 3) that, “Never again was rugose coral provincialism to be as high as it had been in Emsian time.” The aforementioned fall and subsequent rise in sea level had an effect on stromatoporoid paleobiogeography, described next.

Stromatoporoid paleobiogeography

All three of the genera and two of the species described in this paper occurred in both the EAR and OWR during the Emsian, a time when the EAR had left Nevada, and the OWR returned. Stromatoporella perannulata has been reported in the EAR of Ontario, Ohio, and Kentucky, and in the OWR of Ellesmere Island, arctic Canada. Stictostroma moosense is known from the EAR of Ontario. Syringodictyon nevadense n. sp. is found only in Nevada, and the previously known occurrence of the genus is in Ontario and New York (EAR), with a possible occurrence on Ellesmere Island (OWR).

Johnson et al. (Reference Johnson, Klapper and Sandberg1985) illustrated sea levels in the Emsian as lower than in any other Devonian age. This lowstand is associated with Sloss’ (Reference Sloss1963) transition from the Tippecanoe Sequence to the Kaskaskia Sequence. This sea-level fall may have been a consequence of, “paleoclimatically driven glacio-eustasy and thermo-eustasy” (Elrick et al., Reference Elrick, Beryková, Klapper, Sharp, Joachimski and Fryda2009, p. 179).

Breaching the barrier

We must ask the question, “How did stromatoporoids manage to breach barriers that were closed to other taxa?” Stock and Burry-Stock (Reference Stock and Burry-Stock2020, p. 20–22) summarized the ecological characteristics of stromatoporoid-hosting depositional environments: (1) warm water; (2) normal salinity; (3) shallow subtidal depth; (4) medium to high turbulence; (5) firm substrate; and (6) low turbidity. These characteristics do not seem like the type of criteria necessary either for biostratigraphic correlation or inter-realm transport, especially the exclusion of deep water, and the nature of the substrate. Stromatoporoids were sessile benthic, whereas typical Paleozoic index fossils were nektonic (conodonts; cephalopods) or planktonic (graptolites). Sessile benthic organisms used frequently in biostratigraphy, such as brachiopods, lived in many different depositional environments over a range of depths. An equalizing factor between benthic and pelagic organisms is that many have planktonic larvae. Thorson (Reference Thorson1971, p. 79, 81) stated that ~74% of marine benthic animals today produce planktonic larvae, but these larvae are confined to “relatively shallow waters of the continental shelf.” Bergquist (Reference Bergquist1978, p. 106) noted that many living sponges produce free-swimming larvae that remain in the plankton for 3–48 hours.

There are three types of pathways by which a taxon can cross a barrier: (1) corridor, which is an easily traversed pathway for the majority of organisms at both ends, containing a wide variety of habitats; (2) filter, which contains fewer habitats, restricting those that can inhabit the limited number of habitats; and (3) sweepstakes route, in which two isolated, but similar, habitats are connected for a short period of time. The latter route does not support the entire life history of an organism (e.g., only planktonic larvae can cross the barrier) (Cox and Moore, Reference Cox and Moore2000, p. 37).

Some authors have envisioned straits across the Transcontinental Arch as likely pathways for inter-realm mixing. One example of such a route was proposed by Sandberg (Reference Sandberg1970), who envisioned a northwest-southeast strait between the Williston Basin (OWR) and the North Kansas Basin (EAR) that existed from the Ordovician through the Mississippian. This strait appeared in subsequent publications (e.g., Sandberg et al., Reference Sandberg, Poole and Johnson1989; Johnson et al., Reference Johnson, Sandberg and Poole1991) without explanation. More recently, McCormick et al. (Reference McCormick, Chamberlain and Paterson2018) described northwest-southeast oriented faults in South Dakota and Minnesota, which occurred at the same time as the Midcontinent Rift in the Mesoproterozoic.

Other authors have turned to the Canadian Shield for evidence of trans-barrier pathways. Using well logs, Thorpe (Reference Thorpe1989) described the occurrence of the Emsian-age Stooping River Formation in the Hudson Bay Basin, surrounded by Precambrian rocks of the Canadian Shield. He saw this basin as isolated from basins to the west during the Devonian, and noted that it received sediments intermittently at that time (i.e., there are several unconformities in the Devonian sequence). Even more exciting is the discovery of Devonian-age xenoliths composed of normal-marine sedimentary rocks in Jurassic kimberlites in the Canadian Shield of Northwest Territories (Cookenboo et al., Reference Cookenboo, Orchard and Daoud1998), the Ontario-Quebec boundary area (McCracken et al., Reference McCracken, Armstrong and Bolton2000), and southern Baffin Island, Nunavut (Zhang and Pell, Reference Zhang and Pell2014; Fig. 4).