Rare stromatoporoids from the Lower Devonian (Emsian) of Nevada, and their biostratigraphic and paleobiogeographic significance

Abstract Two previously known species of stromatoporoids are reported from Nevada: Stromatoporella perannulata, from the Bartine Member of the McColley Canyon Formation (middle Emsian); and Stictostroma moosense, from the Oxyoke Canyon Sandstone (uppermost Emsian). A new species, Syringodictyon nevadense n. sp., is described from the Coils Creek Member of the McColley Canyon Formation (upper Emsian). Stromatoporella perannulata also occurs in Emsian-age strata on Ellesmere Island, arctic Canada, and in strata of Emsian and/or Eifelian age in Ontario, New York, Ohio, and Kentucky. Outside Nevada, Syringodictyon is confined to the Emsian of Ellesmere Island, and Emsian/Eifelian of Ontario and New York. Stictostroma moosense is known from only the Emsian/Eifelian of Ontario. It is concluded that the three Nevada specimens are Emsian in age. This implies that in other localities where two of the species, S. perannulata and S. moosense, occur in strata where the placement of the Emsian-Eifelian boundary is in question, it might be higher than previously thought. The presence of the two species, plus Syringodictyon, in both the Old World Realm (Nevada, arctic Canada) and the Eastern Americas Realm (Ontario, New York, Ohio, Kentucky), infers a selective marine connection between the two realms that was not available to other taxa. The breach in the inter-realm barrier most likely occurred as a shallow seaway across the Canadian Shield. UUID: http://zoobank.org/e73023c6-88ca-4b4d-9b01-f74a75b99c43


Introduction
Stromatoporoids are very rare components of the fossil fauna of the middle-upper Emsian (upper Lower Devonian) of Nevada. Three specimens were collected from central Nevada ( Fig. 1; Appendix). Three specimens are indeed a small sample size, but considering stromatoporoids' very small populations in the host strata, any amount is significant. All three specimens have both biostratigraphic and paleobiogeographic value.
Although rugose corals are abundant in strata of the study area (Pedder and Murphy, 2004), there are very few stromatoporoids. This situation is most likely due to the fact that stromatoporoids lived in shallower water than most corals (e.g., Walker and Alberstadt, 1975), and/or because corals were better able to resist the effects of fine-grained carbonate or siliciclastic sediments suspended in the water. Two of the stromatoporoid specimens are considered laminar in shape: (1) Stromatoporella perannulata Galloway and St. Jean, 1957, does not include any sediments nor other organisms; and (2) Syringodictyon nevadense n. sp. grew on an undulating surface, so its shape was probably inherited from that surface. The latter's skeleton includes a sedimentary inclusion, suggesting the bedding surface upon which it grew was not highly consolidated. The specimen of Stictostroma moosense (Parks, 1904) encrusted the upper surface of a favositid tabulate coral, inheriting its shape from the coral. Both of these skeletal morphologies are believed to be adaptations to soft substrate (Webby and Kershaw, 2015, p. 432-434, 464-468). Soft sediments accumulate below wavebase, often in deeper water in front of reefs (e.g., Dolphin and Klovan, 1970), or in lagoons in back of reefs or banks (e.g., Laporte, 1967).

Stratigraphy
Lithostratigraphy.-Lithostratigraphic nomenclature adopted in this paper is based on Murphy and Gronberg (1970) for the Roberts Mountains, and Kendall et al. (1983) 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
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, 2004;Morrow, 2007;Pedder, 2010). 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, 1957, 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 .
In an article that is relevant to the present study, Prosh and Stearn (1993) found that four species of stromatoporoids (Stromatoporella perannulata Galloway and St. Jean, 1957;Stictostroma mamilliferum Galloway and St. Jean, 1957; Habrostroma proxilaminatum [Fagerstrom, 1961]; Parallelopora campbelli Galloway and St. Jean, 1957) 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 (1990) detailed conodont biostratigraphic work to support the Emsian age of the Blue Fiord Formation. Prosh andStearn (1993, 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 (1993) concluded that the presence of Emsian-age stromatoporoids in the Detroit River Group of Ontario, as documented by Fagerstrom (1982), 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 (1995) disagreed with Prosh and Stearn (1993), 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  Journal of Paleontology 96(5):979-990 980 …" 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 (1995) discussion, Prosh (1995) 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 andStearn, 1993, 1996). It also occurs in strata of less-certain age in: (1) southwestern Ontario glacial drift (Nicholson, 1873;Parks, 1936); (2) southwestern Ontario, Detroit River Group, including the Formosa Reef Limestone (Fagerstrom, 1982); (3) Niagara Peninsula, Ontario, Onondaga Limestone (St. Jean, 1986); (4) Ohio, Columbus Limestone (Galloway and St. Jean, 1957;Fagerstrom, 1982); and (5) Kentucky, Jeffersonville Limestone (Galloway and St. Jean, 1957).
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, 1982). According to Fagerstrom (1982), Sanford et al. (1968) correlated the Kwataboahegan Formation with the Amherstburg Limestone, lower Detroit River Group, of southwestern Ontario. Klapper and Oliver (1995) placed the Amherstburg Limestone spanning, but mostly above, the Emsian-Eifelian boundary. Telford (1989, 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 (1975, pl. 3) illustrated the Schoharie and Bois Blanc formations as upper Emsian, and the Onondaga Formation as lower-middle Eifelian. Although Norris (1993, 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., 1982)." Recent work by Pinet et al. (2013) and Lavoie et al. (2015) 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, 1873), is known from the Onondaga Limestone of the Niagara Peninsula, Ontario (St. Jean, 1986), and western New York (St. Jean, personal communication, 2010). St. Jean (1986) recognized that a specimen identified as Labechia sp. by Stearn (1983) 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 (1993) and Klapper and Oliver (1995), remains unresolved. My findings, however, support at least a partial Emsian age for a majority of the above-mentioned lithostratigraphic units.

Methods and materials
Measurement.-Procedures used here to measure and count skeletal morphologies are the same as those outlined by Stock (1979, p. 307;1982, 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.
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, 2011); 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 The genus identification of Stictostroma moosense (Parks) has changed a few times in the past. Parks' (1904) assignment to Actinostroma makes no sense because that genus has long pillars and colliculi, rather than short pillars and sheetlike laminae. Parks' (1936) 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 (1957) inclusion of the species in Stromatoporella because no ring-pillars are present.
Pseudostromatoporella moosensis (Parks) of Kazmierczak (1971) 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).
Etymology.-After the state of Nevada, in which it occurs.
Remarks.-There are only two known species of Syringodictyon, S. tuberculatum St. Jean, 1986, 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 There are so few pillars in S. tuberculatum that St. Jean (1986) did not report either the count of pillars per mm, or pillar distance. St. Jean (1986) thought that a specimen of Labechia sp. from the lower Emsian of Ellesmere Island in the Canadian arctic that was described by Stearn (1983) might very well be Syringodictyon. Unfortunately Stearn's specimen is poorly preserved, and he illustrated only a longitudinal section. What Table 4. Comparison of skeletal dimensions (in mm) of specimen USNM 706516 of Stictostroma moosense (Parks, 1904) with ROM 2026D of S. moosense. The mean score is followed by the range in parentheses.   Stearn (1983) reported "pillars" [mamelon columns] 0.3 mm in width in longitudinal section, and 0.3-0.5 mm in diameter in tangential section.

Paleobiogeography
Devonian realms. -Stock and Burry-Stock (2020, 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., 2015). 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 (1977, 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, 1990;Stock in Stock et al., 2015). 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., 1985).
The anomalous nature of the Great Basin Province was taken further by Pedder and Murphy (2004), 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 (2010) 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. (1985) illustrated sea levels in the Emsian as lower than in any other Devonian age. This lowstand is associated with Sloss' (1963) 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., 2009, 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 (2020, 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 Table 6. Comparison in ranges of skeletal dimensions (in mm) of specimen USNM 706517 of Syringodictyon nevadense n. sp. with those of St. Jean (1986) for Syringodictyon tuberculatum. Measurements from Table 5  Journal of Paleontology 96(5):979-990 986 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 (1971, 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 (1978, 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, 2000, 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 (1970), 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., 1989;Johnson et al., 1991)  Other authors have turned to the Canadian Shield for evidence of trans-barrier pathways. Using well logs, Thorpe (1989) 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., 1998), the Ontario-Quebec boundary area (McCracken et al., 2000), and southern Baffin Island, Nunavut (Zhang and Pell, 2014;Fig. 4).

Discussion
It has become clear that stromatoporoids were able to overcome the barrier formed by the Canadian Shield and Transcontinental Arch during the Lochkovian (Stock and Burry-Stock, 2020), and now during the Emsian as well. It is doubtful that stromatoporoids were able to circumnavigate what is now the southwestern end of the Transcontinental Arch, even as planktonic larvae. Adult planktonic and nektonic organisms did not traverse that, or any other, route-if they could, there would be no EAR and OWR. Likewise, neither other benthic organisms nor their larvae did so. Clearly no corridor or sweepstakes route existed; if so, stromatoporoids would have been joined by other organisms, larval or adult, as they breached the barrier. This leaves only one likely option-the stromatoporoids crossed the inter-realm barrier using a highly selective version of a filter, during which several generations existed. Their larvae could travel by currents at least part of the way, as no doubt did the larvae of other of organisms; however, only stromatoporoid adults could inhabit the very shallow conditions present in such a passage (Fig. 4). (1) Map with location of OWR (Old World Realm), EAR (Eastern Americas Realm), Canadian Shield, and Transcontinental Arch (based on Oliver, 1977). 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.

Conclusions
Biostratigraphy.-One of the species described in this paper, Stromatoporella perannulata Galloway and St. Jean, 1957, which occurs in Nevada in the gronbergi Zone (middle Emsian), is conspecific with stromatoporoids in the Canadian arctic, Ontario, New York, Ohio, and Kentucky. Stictostroma moosense (Parks, 1904), which occurs in Nevada is in the serotinus Zone (uppermost Emsian), is conspecific with stromatoporoids in Ontario. The third species, Syringodictyon nevadense n. sp., which occurs in Nevada is in the serotinus Zone (upper Emsian), is only the second species in that genus to be named-the type species occurs in Ontario and New York, and an unnamed species is found in the Emsian of the Canadian arctic. In the case of arctic Canada, strata there containing S. perannulata are known to be Emsian in age, but there is some uncertainty regarding the placement of the Emsian-Eifelian boundary for the age of the stromatoporoid-bearing strata in the other areas mentioned above. My findings lend some credence to the assertion of Prosh and Stearn (1993) that many of the strata in question are Emsian in age.
Paleobiogeography.-Stromatoporella perannulata and Stictostroma moosense occurred at about the same time in both the Eastern Americas Realm and Old World Realm, as did two species of the genus Syringodictyon. The coeval occurrences of these taxa indicate a limited connection between two paleobiogeographic realms.
Evidence from xenoliths in kimberlite pipes in the Canadian Shield, and strata beneath Hudson Bay, indicates deposition of normal-marine sediments well onto the modern margins of the Canadian Shield. There was a shallow-water passage across the Canadian Shield in what was previously seen as an impervious cross-continental barrier, which represented a filter-a seaway containing a few habitats, restricting the kinds of organisms to those capable of inhabiting those depositional settings.
Conclusions drawn here are very similar to those of Stock and Burry-Stock (2020, p. 24, fig. 11), who proposed an Early Devonian (Lochkovian) seaway across the Canadian Shield that allowed the stromatoporoid Habrostroma centrotum (Girty, 1895) to simultaneously inhabit the Eastern Americas Realm (Virginia, New York, Maine) and the Old World Realm (Bathurst and Ellesmere islands, Canadian arctic) in what is now North America.