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Buried or brined? Eurypterids and evaporites in the Silurian Appalachian basin
... Such sites occur from the Middle Ordovician (Lamsdell et al. 2015) onward and across multiple paleocontinents, but perhaps the most well known are those found in nearshore settings in the Silurian of Laurentia and Baltica (Clarke and Ruedemann 1912;Kaljo 1970;Kluessendorf 1994). Regular preservation in these marginal, often hypersaline, marine environments has been attributed to various factors, including an ecological preference for restricted settings in which certain groups may have molted and/or mated en masse (Braddy 2001); the prevalence of eurypterid molts, as opposed to more decayprone carcasses ); a paucity of biostratinomic activity concomitant with locally restricted environments (e.g., bioturbation, predation; Vrazo and Braddy 2011); and environmental conditions conducive to preservation (e.g., anoxia, hypersalinity; Kluessendorf 1994; Vrazo et al. 2016). ...
... Although the broad paleoenvironmental and paleoecological drivers for these eurypterid occurrences are generally understood, inference of specific depositional environments and, in turn, eurypterid habitats in these Lagerstätte deposits typically has been hindered by a lack of sedimentary evidence at the bed level, and coarse lumping of contemporaneous fauna and adjacent stratigraphy (Clarke and Ruedemann 1912;Alling and Briggs 1961;Leutze 1961;Selden 1984;Feldman et al. 1993;Plotnick 1999;Vrazo et al. 2014). In considering these past limitations, a growing number of studies suggest that accurate inference of eurypterid habitats and faunal associations within these depositionally ambiguous units can only be made using a highresolution stratigraphic approach (Ciurca 1973;Maples and Schultze 1988;Plotnick 1999;Tetlie and Poschmann 2008;Vrazo et al. 2014Vrazo et al. , 2016. Using this approach, Ciurca (1973Ciurca ( , 1978 and others (Hamell and Ciurca 1986;Ciurca, 1990;Ciurca and Hamell 1994) documented cyclicity in eurypterid-rich dolomitic "waterlimes" in northern Appalachian basin Silurian localities. ...
... More recently, Vrazo et al. (2014) revealed a strong stratigraphic and water-depth control on upper Silurian eurypterid occurrences in the central Appalachian basin. Vrazo et al. (2016) subsequently demonstrated that regional sealevel fluctuations in nearshore settings could result in conditions that were conducive first to eurypterid inhabitation (i.e., near-euhaline conditions) and then to preservation (i.e., hypersalinity and/or anoxia). ...
Recent studies of eurypterid paleoecology suggest that formation of eurypterid Lagerstätten in the mid-Paleozoic of Laurentia was controlled by the presence of an ecological–taphonomic window that recurred predictably in nearshore, marginal environments during transgressions. We tested this hypothesis by performing a high-resolution taxonomic, environmental, and stratigraphic survey and quantitative analysis of all Silurian–Lower Devonian eurypterid-bearing intervals in the Appalachian basin, the most prolific region for eurypterid remains in the world. Canonical correspondence analysis of sedimentological and faunal associations revealed a strong lithologic gradient between groupings of eurypterid genera and associated taxa across the basin, and a significant association of eurypterids with microbialites (thrombolites, stromatolites) and evaporitic structures. Field observations confirmed that, stratigraphically, eurypterids in the basin frequently occur above the microbialite structures and beneath evaporites and other indicators of increased salinity or subaerial exposure. Following interpretation of these features within a sequence stratigraphic framework, we present a preservational model in which (1) eurypterids inhabited nearshore settings following freshening conditions concomitant with minor transgressions, (2) their remains were subsequently buried by storms or microbialite sediment baffling, and (3) subsequent long-term preservation of tissues was facilitated by regression and cyclical shallowing-up successions that promoted hypersalinity and anoxia. In the central and southern region of the basin, where microbial structures and evidence for hypersalinity are less common, a similar pattern of cyclical shallowing-upward deposition within eurypterid-bearing units holds. Thus, eurypterid preservation appears to reflect a combination of ecological preferences and abiotic conditions that promoted inhabitation and eventual preservation within the same setting. This study provides the first quantitative support for a sea level–based control on preservation of eurypterids and adds to the growing body of evidence that suggests that analysis of exceptional preservation in the fossil record benefits from interpretation within a sequence stratigraphic framework.
... Such sites occur from the Middle Ordovician (Lamsdell et al. 2015) onward and across multiple paleocontinents, but perhaps the most well known are those found in nearshore settings in the Silurian of Laurentia and Baltica (Clarke and Ruedemann 1912;Kaljo 1970;Kluessendorf 1994). Regular preservation in these marginal, often hypersaline, marine environments has been attributed to various factors, including an ecological preference for restricted settings in which certain groups may have molted and/or mated en masse (Braddy 2001); the prevalence of eurypterid molts, as opposed to more decayprone carcasses ); a paucity of biostratinomic activity concomitant with locally restricted environments (e.g., bioturbation, predation; Vrazo and Braddy 2011); and environmental conditions conducive to preservation (e.g., anoxia, hypersalinity; Kluessendorf 1994; Vrazo et al. 2016). ...
... Although the broad paleoenvironmental and paleoecological drivers for these eurypterid occurrences are generally understood, inference of specific depositional environments and, in turn, eurypterid habitats in these Lagerstätte deposits typically has been hindered by a lack of sedimentary evidence at the bed level, and coarse lumping of contemporaneous fauna and adjacent stratigraphy (Clarke and Ruedemann 1912;Alling and Briggs 1961;Leutze 1961;Selden 1984;Feldman et al. 1993;Plotnick 1999;Vrazo et al. 2014). In considering these past limitations, a growing number of studies suggest that accurate inference of eurypterid habitats and faunal associations within these depositionally ambiguous units can only be made using a highresolution stratigraphic approach (Ciurca 1973;Maples and Schultze 1988;Plotnick 1999;Tetlie and Poschmann 2008;Vrazo et al. 2014Vrazo et al. , 2016. Using this approach, Ciurca (1973Ciurca ( , 1978 and others (Hamell and Ciurca 1986;Ciurca, 1990;Ciurca and Hamell 1994) documented cyclicity in eurypterid-rich dolomitic "waterlimes" in northern Appalachian basin Silurian localities. ...
... More recently, Vrazo et al. (2014) revealed a strong stratigraphic and water-depth control on upper Silurian eurypterid occurrences in the central Appalachian basin. Vrazo et al. (2016) subsequently demonstrated that regional sealevel fluctuations in nearshore settings could result in conditions that were conducive first to eurypterid inhabitation (i.e., near-euhaline conditions) and then to preservation (i.e., hypersalinity and/or anoxia). ...
Konservat-Lagerstätten are increasingly being shown to be predictable within a sequence stratigraphic framework. In this study, we hypothesize that formation of eurypterid Lagerstätten in the mid-Paleozoic of Laurentia was controlled by an ecological-taphonomic window that recurred predictably in nearshore, marginal environments during transgressions. To test this hypothesis, a taxonomic and environmental survey was performed on all Silurian–Early Devonian-age eurypterid-bearing intervals in the Appalachian basin, the most prolific region for eurypterid remains in the world. Canonical correspondence analysis (CCA) indicates a strong lithological gradient between groupings of eurypterid genera and associated taxa across basin. Results from the CCA and quantitative analyses of sedimentological and faunal associations, in conjunction with field observations, indicate that: a) associated faunal diversity is greatest within eurypterid-bearing units, and b) eurypterids in the basin frequently occur above microbialitic structures (thrombolites, stromatolites) and beneath indicators of increased salinity or sub-aerial exposure (evaporitic minerals, desiccation cracks). Based on these parameters, we present a sequence stratigraphic model in which microbialites represent flooding surfaces and evaporites/sub-aerial exposure features represent the tops of shallowing-upward parasequences. In this scenario, eurypterids primarily occurred within freshening conditions concomitant with minor transgressions; subsequent shallowing-up successions promoted hypersalinity and anoxia, which facilitated soft-tissue preservation. In the central and southern region of the basin, where microbial structures and evidence for hypersalinity are less common, a similar pattern of cyclical shallowing-upward deposition within eurypterid-bearing units holds. Thus, eurypterid preservation appears to reflect a combination of ecological preferences and abiotic conditions that promote inhabitation and eventual preservation within the same setting. Ultimately, this model suggests that formation of eurypterid Lagerstätten in the mid-Paleozoic are controlled primarily by fluctuations in water depth.
... Two intervals of the Paleozoic have a disproportionately large number of non-concretionary marine Lagerstätten, the Cambrian and Silurian (e.g., Allison and Briggs, 1993;Bottjer et al., 2002b;Muscente et al., 2017). With few exceptions (e.g., eurypterids; Clarke and Ruedemann, 1912;Andrews et al., 1974;Kluessendorf, 1994;Ciurca Jr. and Tetlie, 2007;Tetlie et al., 2007;Vrazo et al., 2016), exceptionally preserved fossils from Silurian deposits are not as well-known and are less studied than those of the Cambrian. Strictly from a large-scale perspective, Cambrian and Silurian Lagerstätten share some interesting similarities. ...
... One important distinction between classic siliciclastic-hosted Konservat-lagerstätten and those hosted in non-concretionary carbonates is the inferred influence of microbial entombment in some of the carbonate deposits. Microbial entombment has been documented from the Crato Formation (Varejão et al., 2019), but also can be inferred from photographs of specimens from other deposits published previously (e.g., Bertie Group; Vrazo et al., 2016). Microbial entombment has not been demonstrated to date from classic Burgess Shale-type deposits, or from Hunsrück-type deposits. ...
... Unlike the Big Hill Lagerstätte, the Bertie, as well as the Airport Cove and William Lake biotas, shows evidence of hypersalinity in the form of evaporite moulds. A new model, however, indicates that these units were deposited under normal to near-normal salinity subtidal settings, and that the evaporite moulds reflect early stage diagenetic overprinting (Vrazo et al. 2016). Notably, the eurypterid-bearing intervals of the Bertie Group each appear to have been deposited during a small-scale transgression (Vrazo et al. 2016), and the same may apply to the fossil-bearing intervals of the Big Hill Formation. ...
... A new model, however, indicates that these units were deposited under normal to near-normal salinity subtidal settings, and that the evaporite moulds reflect early stage diagenetic overprinting (Vrazo et al. 2016). Notably, the eurypterid-bearing intervals of the Bertie Group each appear to have been deposited during a small-scale transgression (Vrazo et al. 2016), and the same may apply to the fossil-bearing intervals of the Big Hill Formation. ...
A new exceptionally preserved marginal marine biota is reported from the Late Ordovician Big Hill Formation of Stonington Peninsula in Michigan’s Upper Peninsula. The new Lagerstätte hosts a moderately diverse fauna of medusae, linguloid brachiopods, non-mineralized arthropods and orthocone nautiloids, alongside dasycladalean green algae. The biota is similar to those of Lagerstätten from the Late Ordovician of Canada, revealing an extensive distribution of a distinctive marginal marine palaeocommunity in Laurentia at this time. The Big Hill biota extends the geographical range of exceptionally preserved Late Ordovician faunas in Laurentia and indicates that further examples remain to be discovered.
... 5 There is some debate, however, about the salinity of the water where the fossils are preserved, and conditions may not have been hypersaline. 56 Nonetheless benthic animals are rare in the unit that yields R. superbus and are confined to small gastropods and brachiopods that represent less than 2% of specimens from Bed A in Ridgemount Quarry South in the YPM Ciurca collection, excluding the benthic alga Inocladus lesquereuxi 57 and the plant Cooksonia, 28 which was washed in from the land. Eurypterids are the main faunal element, but they are represented exclusively by exuviae, and there is a consensus that their normal habitat was elsewhere-they congregated and molted in this setting. ...
Pterobranchs, a major group of the phylum Hemichordata, first appear in the fossil record during the Cambrian, and there are more than 600 fossil genera dominated by the mainly planktic graptolites of the Paleozoic, which are widely used as zone fossils for correlating sedimentary rock sequences. Pterobranchs are rare today; they are sessile marine forms represented by Rhabdopleura, which is considered the only living graptolite, and Cephalodiscus. Unlike their sister taxon, the colonial graptolites, cephalodiscids are pseudocolonial. Here, we describe a problematic fossil from the Silurian (Pridoli) Bertie Group of Ontario (420 mya), a sequence of near-shore sediments well known for its remarkably preserved diversity of eurypterids (sea scorpions). The fossil, Rotaciurca superbus, a new genus and species, was familiarly known as Ezekiel’s Wheel, with reference to the unusual circular arrangement of the tubes that compose it. The structure and arrangement of the tubes identify Rotaciurca as a pterobranch, and phylogenetic analysis groups it with the cephalodiscids. We place it in a new family Rotaciurcidae to distinguish it from Cephalodiscidae. A large structure associated with the tubes is interpreted as a float, which would distinguish Rotaciurca as the only known planktic cephalodiscid—thus cephalodiscids, like the graptolites, invaded the water column. This mode of life reflects the rarity of pseudocolonial macroinvertebrates in planktic ocean communities, a role occupied by the tunicates (Chordata) known as salps today. Our estimates of divergence times, the first using relaxed total-evidence clocks, date the origins of both hemichordates and pterobranchs to the earliest Cambrian (Fortunian).
... However, there would normally be more males than females in such a gathering (but see Vrazo and Braddy 2011), and the range of sizes suggests that the majority of individuals within these accumulations had not yet reached sexual maturity. More recently Vrazo, Brett, and Ciurca (2016) have argued that the salt hoppers thought to indicate hypersalinity formed later within the sediment and that the eurypterids occupied more normal marine or brackish conditions. It is possible that the larger E. lacustris were not molting in the same shallow coastal regions as the smaller animals even if they were breeding in these settings. ...
... Actinopterygian fish were the most common freshwater fish in the Carboniferous and Permian (Gray, 1988). Late Devonian-Early Carboniferous eurypterids are mostly restricted to brackish or freshwater environments (Braddy, 2001;Lamsdell and Braddy, 2010;Lamsdell et al., 2019) and were not tolerant of hypersalinity (Vrazo et al., 2016). Modiolus and Naiadites bivalves are typical of brackish to freshwater deposits in the Mississippian (Ballèvre and Lardeux, 2005;Bennison, 1960;Trueman and Weir, 1946), and of freshwater-brackish deposits in the Pennsylvanian (Eagar and Weir, 1971;Rogers, 1965). ...
The Ballagan Formation of northern Britain provides an exceptional record of Early Mississippian ecosystems that developed as tetrapods emerged onto land. In this paper, we study two 500-metre sections of the formation near Berwick-upon-Tweed, which are characterised by abundant ferroan dolostone beds. Five lithofacies are identified: cemented siltstone and sandstone, homogeneous dolomicrite, mixed dolomite and siltstone, mixed calcite and dolomite, and dolomite with evaporite minerals. Cemented sediments have non-planar to planar subhedral dolomite crystals, up to 40 μm in size, whereas other facies predominantly comprise dolomicrite or planar euhedral dolomite rhombs 15 μm in size, with patches of larger rhombs indicating partial recrystallisation. The macro- and microfossil content of the dolostones is dominated by sarcopterygian (rhizodont) and actinopterygian fish, bivalves, Serpula, ostracods and Chondrites trace fossils; with rarer Spirorbis, chondrichthyans (Ageleodus, hybodonts and ?ctenacanths, xenacanths), non-gyracanth acanthodians, gastropods, eurypterids, brachiopods, plant debris, wood, lycopsid roots, charcoal, megaspores, phycosiphoniform burrows, Zoophycos? and Rhizocorallium. The oxygen and carbon isotope composition of dolomites range from –3.6‰ to –1.7‰ (for δ¹⁸O) and –2.6‰ to +1.6‰ (for δ¹³C) respectively indicating dolomite growth in mixed salinity waters. Frequent marine storm-surge events transported marine waters and animals into floodplain lakes, where evaporation, interstitial sulphate-reducing bacteria, iron reduction and methanogenesis allowed dolomite growth in the shallow sub-surface. Secondary pedogenic modification (by roots, brecciation, desiccation, and soil forming processes) is common and represents lake evaporation with, in some cases, saline marsh development. The dolostone facies are part of a complex environmental mosaic of sub-aerial dry floodplain, wet marshy floodplains, rivers, and lakes ranging in salinity from freshwater to hypersaline. Marine influence is strongest at the base of the formation and decreases over time, as the floodplain became drier, and forested areas became more established. Coastal lakes were an important habitat for animals recovering from the end-Devonian Hangenberg Crisis and may have acted as a pathway for euryhaline fishes, molluscs and arthropods to access freshwater environments.
... The Přídolí Bertie Group of Ontario and New York State is dominated by eurypterids, xiphosurans, scorpions and phyllocarids preserved in fine carbonate muds (Clarke & Ruedemann 1912: Copeland & Bolton 1985. The presence of salt hoppers was long regarded as evidence of hypersaline conditions, but it is likely that they were precipitated in the sediment after deposition and the Bertie Group represents a shallow subtidal setting (Vrazo et al. 2016). The deeper marine Herefordshire fauna includes numerous sponges, two trilobite genera, and a greater representation of major arthropod groups, as well as four major (fully stenohaline) echinoderm taxa (Box 1, Table 1). ...
The Herefordshire Lagerstätte ( c . 430 Ma) from the UK is a rare example of soft-tissue preservation in the Silurian. It yields a wide range of invertebrates in unparalleled three-dimensional detail, dominated by arthropods and sponges. The fossils are exceptionally preserved as calcitic void infills in early diagenetic carbonate concretions within a volcaniclastic (bentonite) horizon. The Lagerstätte occurs in an outer shelf/upper slope setting in the Welsh Basin, which was located on Avalonia in the southern subtropics. The specimens are investigated by serial grinding, digital photography and rendering in the round as ‘virtual fossils’ by computer. The fossils contribute much to our understanding of the palaeobiology and early history of the groups represented. They are important in demonstrating unusual character combinations that illuminate relationships; in calibrating molecular clocks; in variously linking with taxa in both earlier and later Paleozoic Lagerstätten; and in providing evidence of the early evolution of crown-group representatives of several groups.
... Eurypterids, including stylonurids, in the Late Devonian were almost entirely constrained to freshwater habitats (Lamsdell and Braddy, 2010). In addition, a previous study of eurypterids preserved in evaporitic settings has demonstrated that the onset of hypersaline conditions (and any subsequent formation of associated intrasedimentary evaporites) occurred after burial of the eurypterid specimens, and thus are not reflective of life habitat (Vrazo et al., 2016). ...
We describe a new stylonurid eurypterid from the evaporitic potassium-salt deposits of the Upper Devonian (Famennian) Soligorsk Formation in the Pripyat Trough of Belarus. All specimens are assigned to Soligorskopterus tchepeliensis new genus new species, which represents the first formally described eurypterid species from Belarus. The occurrence of well-preserved eurypterids in these unusual evaporite deposits is most likely due to transport from freshwater stream habitats into a hypersaline setting following death. Soligorskopterus tchepeliensis n. gen. n. sp. appears to be intermediate between the traditionally considered parastylonurids and stylonurids and thus extends our understanding of stylonurid evolution in the mid-Paleozoic. Soligorskopterus n. gen. extends the occurrence of Famennian eurypterids into eastern Laurussia and the Stylonuridae into the Upper Devonian, and this taxon could be part of a global eurypterid habitat shift that took place in the Late Devonian.
UUID: http://zoobank.org/466b0195-6199-495b-9270-6f032e2d7493
... Animal taxa, when present, typically include worms and non-biomineralized arthropods, such as phyllocaridas and eurypterids. All appear to be associated with normal or near-normal marine salinity (see Vrazo, Brett, & Ciurca, 2016; for a recent reappraisal of the depositional environment of the Bertie Group and similar Upper Silurian Lagerstätten). Notably, many of the Silurian macroalga-bearing deposits were accumulated in inter-reef environments. ...
Non-calcified marine macroalgae ("seaweeds") play a variety of key roles in the modern Earth system, and it is likely that they were also important players in the geological past, particularly during critical transitions such as the Cambrian Explosion (CE) and the Great Ordovician Biodiversification Event (GOBE). To investigate the morphology and ecology of seaweeds spanning the time frame from the CE through the GOBE, a carefully vetted database was constructed that includes taxonomic and morphometric information for non-calcified macroalgae from 69 fossil deposits. Analysis of the database shows a pattern of seaweed history that can be explained in terms of two floras: the Cambrian Flora and the Ordovician Flora. The Cambrian Flora was dominated by rather simple morphogroups, whereas the Ordovician Flora, which replaced the Cambrian Flora in the Ordovician and extended through the Silurian, mainly comprised comparatively complex morphogroups. In addition to morphogroup representation, the two floras show marked differences in taxonomic composition, morphospace occupation, functional-form group representation, and life habit, thereby pointing to significant morphological and ecological changes for seaweeds roughly concomitant with the GOBE and the transition from the Cambrian to Paleozoic Evolutionary Faunas. Macroalgal changes of a similar nature and magnitude, however, are not evident in concert with the CE, as the Cambrian Flora consists largely of forms established during the Ediacaran. The cause of such a lag in macroalgal morphological diversification remains unclear, but an intriguing possibility is that it signals a previously unknown difference between the CE and GOBE with regard to the introduction of novel grazing pressures. The consequences of the establishment of the Ordovician Flora for shallow marine ecosystems and Earth system dynamics remain to be explored in detail but could have been multifaceted and potentially include impacts on the global carbon cycle.
... The Bertie Group is a 15-18 m thick sequence that outcrops in western and central New York State (Fig. 1a), and in southwestern Ontario, Canada, and is composed primarily of dolomitic limestone. The occurrence of evaporites and large halite pseudomorphs has led to suggestions that the eurypterids may have inhabited a hypersaline setting (Ciurca & Hamell, 1994), but a recent study has shown that the growth of halite structures was likely post-depositional and that the eurypterids may have lived in normal marine salinities (Vrazo, Brett & Ciurca, 2016). The Bertie Group is well known for its numerous eurypteridbearing horizons, and the Phelps Member at Millers Mills Quadrangle boasts a particularly diverse biota (Fig. 1b): in addition to five species of eurypterids (Eurypterus remipes, Acutiramus macrophthalmus, Dolichopterus jewetti, Pterygotus cobbi and Buffalopterus pustulosus), the locality yields Proscorpius osborni, one of the oldest scorpions (Dunlop, Tetlie & Prendini, 2008), and the early terrestrial plants Cooksonia and Hostinella (Edwards et al. 2004), along with the synziphosurine Pseudoniscus, phyllocarids, ostracodes, gastropods and cephalopods including Hexameroceras. ...
A single specimen of a new species of the chasmataspidid Diploaspis Størmer, 1972 is described from the upper Silurian (Pridoli) Phelps Member of the Fiddlers Green Formation (Bertie Group) in Herkimer County, New York State, USA. Diploaspis praecursor sp. nov. is distinguished by the shape of the posterolateral margins of the buckler, which are drawn out into angular epimera, and by the lack of elongate tubercles on the postabdomen. This discovery increases the taxonomic diversity of the Bertie Group by extending the geographic extent of Diploaspididae into North America. D. praecursor pre-dates previously known species of Diploaspis by more than 10 million years.
... Also the presence of eurypterid fragments cannot be used as evidence for hypersaline conditions, as the common occurrence of these organisms in hypersaline facies seems to result from a combination of their behavior and taphonomy, but not actual physiological tolerance (e.g. Braddy, 2001;Vrazo et al., 2016). Potential restricting factors include therefore fluctuating salinity, oxygen depletion, brackish influence, or elevated temperature, all of which are typical of shallow lagoonal environments with limited water mixing. ...
The decline of the upper peak of the Homerian Mulde carbon isotope excursion (CIE) is used in low- to mid-paleolatitudes as a marker for the Wenlock/Ludlow boundary, which is otherwise difficult to constrain in carbonate successions. In the Midland Platform (England) the CIE ends just below the boundary or ranges through it, whereas in Baltic sections it has been placed substantially below the inferred base of the Ludlow Stage. Difficulties in correlating the Baltic sections are caused by widespread development of lagoons and sabkhas with specialized conodont and vertebrate faunas. We describe here a lagoonal section from Gothemshammar, eastern Gotland (Sweden), spanning the entire upper peak of the Mulde CIE. Based on integrated conodont, δ13Ccarb and sequence stratigraphy, a hardground present at the lowering limb of the CIE is correlated with a sequence boundary present across the Baltic Basin, in the Midland Platform, and the southern shelf of Laurentia. This sequence boundary corresponds to a global eustatic regression and can serve as a correlative horizon in restricted facies with depauperate or specialized fauna. The Wenlock-Ludlow boundary is placed in the transgressive strata overlying this boundary. Species richness and abundance of thelodonts, anaspids, and osteostracans at Gothemshammar represent one of the first diversity peaks of vertebrates in the Silurian. Associated conodonts are characteristic for late Wenlock marginal-marine environments and distinguished by large, robust elements. We quantitatively assess the conodont assemblages to evaluate to which degree the overrepresentation of large elements in these facies is produced by taphonomic processes. The taphonomic alteration differs between species and facies, but is lowest for the shallow-water specialist Ctenognathodus murchisoni. Regardless, the use of this species as an index taxon is discouraged based on its strong facies affinity. Instead, the integrated approach proposed here indicates that the Wenlock/Ludlow boundary is situated lower in the Baltic sections than previously identified.
Fossil capitate hydrozoans require exceptional conditions for preservation. Here we describe Bertratis ciurcae new genus, new species from the Silurian (Pridoli) of southern Ontario and upper New York State, where it occurs in association with a diverse assemblage of eurypterids. Only the float (pneumatophore) is well preserved, surviving as a thick carbonaceous compression. The new taxon is the largest fossil capitate reported, reaching a width of 17 cm, and the third Porpita-like example known from the Paleozoic. It was a rare pelagic component of the biota of the well-known Bertie Group Lagerstätten.
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The endostoma of eurypterids is a small ventral structure previously known from only 11 specimens and in detail from only a single species, Eurypterus tetraganophthalmus. Four previously undescribed eurypterid specimens from the Samuel J. Ciurca Jr. Collection, Yale Peabody Museum of Natural History, likely belonging to Eurypterus, and a previously documented specimen of the pterygotid Acutiramus cummingsi in the New York State Museum, are considered here to reinterpret the morphology and homologies of the endostoma. We demonstrate that the endostoma is a bilateral, flaplike appendage, with distal margins bearing anteriorly facing setae. An anterior embayment and posterior medial notch are noted. The latter represents the point where the endostoma articulates with an oval sclerite. Here we propose that, contrary to previous interpretations, the endostoma is an appendage, likely of the seventh body segment, and is probably homologous to the xiphosuran chilaria. This has direct implications for the hypotheses of homology for the metastoma, a ventral structure common to eurypterids and chasmataspids.
A 1901 report by the Smithsonian Custodian of Paleozoic Plants noted that the nonbiomineralized taxa Buthotrephis divaricata White, 1901, B. newlini White, 1901, and B. lesquereuxi Grote and Pitt, 1876, from the upper Silurian of the Great Lakes area, shared key characteristics in common with the extant green macroalga Codium . A detailed reexamination of these Codium -like taxa and similar forms from the lower Silurian of Ontario, New York, and Michigan, including newly collected material of Thalassocystis striata Taggart and Parker, 1976, aided by scanning electron microscopy and stable carbon isotope analysis, provides new data in support of an algal affinity. Crucially, as with Codium , the originally cylindrical axes of all of these taxa consist of a complex internal array of tubes divided into distinct medullary and cortical regions, the medullary tubes being arranged in a manner similar to those of living Pseudocodium . In view of these findings, the three study taxa originally assigned to Buthotrephis , together with Chondrites verus Ruedemann, 1925, are transferred to the new algal taxon Inocladus new genus, thereby establishing Inocladus lesquereuxi new combination, Inocladus newlini new comb., Inocladus divaricata new comb., and Inocladus verus new comb. Morphological and paleoecological data point to a phylogenetic affinity for Inocladus n. gen. and Thalassocystis within the Codium -bearing green algal order Bryopsidales, but perhaps nested within an extinct lineage. Collectively, this material fits within a large-scale pattern of major macroalgal morphological diversification initiated in concert with the Great Ordovician Biodiversification Event and apparently driven by a marked escalation in grazing pressure.
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The eurypterids of the Silurian Bertie Group of North America are some of the most striking fossils known, with their dark organic cuticle outlined against a background of light coloured dolomite. Eurypterus is the most abundant genus but the pterygotids with their long grasping claws have fired our imagination as giant extinct predators. Large complete specimens are rarely found—the specimens are dominated by moulted exoskeletons that often disarticulate—but a giant that emerged from the rock more than 50 years ago is being prepared to go on display.
Stromatolites are abundant in shallow marine sediments deposited before the evolution of animals, but in the modern ocean they are restricted to locations where the activity of animals is limited. Overall decline in the abundance of stromatolites has, therefore, been attributed to the evolution of substrate-modifying metazoans, with Phanerozoic stromatolite resurgences attributed to the aftermaths of mass extinctions. Here we use a comprehensive stratigraphic database, the published literature, and a machine reading system to show that the rock record–normalized occurrence of stromatolites in marine environments in North America exhibits three phases: an initial Paleoproterozoic (ca. 2500 Ma) increase, a sustained interval of dominance during the Proterozoic (2500–800 Ma), and a late Neoproterozoic (700–541 Ma) decline to lower mean prevalence during the Phanerozoic (541–0 Ma). Stromatolites continued to exhibit large changes in prevalence after the evolution of metazoans, and they transiently achieved Proterozoic-like prevalence during the Paleozoic. The aftermaths of major mass extinctions are not well correlated with stromatolite resurgence. Instead, stromatolite occurrence is well predicted by the prevalence of dolomite, a shift in carbonate mineralogy that is sensitive to changes in water-column and pore-water chemistry occurring during continent-scale marine transgressive-regressive cycles.
Background
The relationships between early jawed vertebrates have been much debated, with cladistic analyses yielding little consensus on the position (or positions) of acanthodians with respect to other groups. Whereas one recent analysis showed various acanthodians (classically known as ‘spiny sharks’) as stem osteichthyans (bony fishes) and others as stem chondrichthyans, another shows the acanthodians as a paraphyletic group of stem chondrichthyans, and the latest analysis shows acanthodians as the monophyletic sister group of the Chondrichthyes.
Methodology/Principal Findings
A small specimen of the ischnacanthiform acanthodian Nerepisacanthus denisoni is the first vertebrate fossil collected from the Late Silurian Bertie Formation Konservat-Lagerstätte of southern Ontario, Canada, a deposit well-known for its spectacular eurypterid fossils. The fish is the only near complete acanthodian from pre-Devonian strata worldwide, and confirms that Nerepisacanthus has dentigerous jaw bones, body scales with superposed crown growth zones formed of ondontocytic mesodentine, and a patch of chondrichthyan-like scales posterior to the jaw joint.
Conclusions/Significance
The combination of features found in Nerepisacanthus supports the hypothesis that acanthodians could be a group, or even a clade, on the chondrichthyan stem. Cladistic analyses of early jawed vertebrates incorporating Nerepisacanthus, and updated data on other acanthodians based on publications in press, should help clarify their relationships.
Stoermeropterus conicus (Eurypterida: Eurypterina), from the Telychian (Upper Llandovery, Silurian) of Pentland Hills near Edinburgh, Scotland, is described from material originally assigned to three different species (Nanahughmilleria conica, Drepanopterus bembycoides and Drepanopterus lobatus). Two other existing eurypterid species, Hughmilleria lata from the Wenlock of Norway and Drepanopterus nodosus from the Pridoli of North America are recognized as being congeneric with S. conicus, united principally by their possession of moveable mesosomal spines on the mesosoma, genital spatulae and a bulbous telson ‘boss’. Several characters support the assignment Stoermeropterus to Moselopteridae, the most basal eurypterine clade, including its possession of a pediform appendage VI with a modified ‘podomere’ 7a and the possession of a coxal ‘ear’, which may represent the remains of a much-reduced exopod. Stoermeropterus conicus resolves phylogenetically as the most basal known eurypterine, and can aid in reconstructing the eurypterid ground plan. As such an attempt is made to reconstruct the characteristics that are plesiomorphic for Eurypterida through comparison with basal Eurypterina, Stylonurina, chasmataspidids and synziphosurines. Several characteristics previously thought to be autapomorphies of Stylonurina, such as a three-segmented genital operculum, are now shown to actually be plesiomorphic conditions in respect to Eurypterida as a whole, while other apparently derived characteristics such as an epistoma and genital spatulae may be characters that are present in all eurypterids during the juvenile and are respectively either paedomorphically retained into adulthood or become hypertrophied in various species. Following the identification of a metastoma and genital appendage in some chasmataspidid species, the sole currently known eurypterid autapomorphy is identified as the fusion of the opercular plates of somites VIII and IX. Evolutionary relationships among the traditional ‘merostome’ groups are reviewed, primarily in light of segment articulations and the development of the appendage of somite VII. The concept that synziphosurines may represent a paraphyletic stem grade to a group inclusive of xiphosurids, chasmataspidids, eurypterids, and arachnids is proposed.
Studies of the South Platte-Platte River in Colorado and Nebraska substantiate Ore's (1964) conclusion that braided patterns in streams are created mainly by accretion of longitudinal bars and dissection of transverse bars. Distribution of bars in the South Platte-Platte River depends on texture of the bed load. Coarse, poorly sorted sediment favors formation of longitudinal bars, and finer grained, better sorted materials form transverse bars. The relative proportion of transverse to longitudinal bars increases downstream, following the river's tendency to fractionate its load into finer sizes downstream. This is accompanied by an increase in the ratio of planar cross-stratification to horizontal stratification and a decrease in cross-channel topographic relief expressed as a bed-relief index.
Relative abundances of planar cross-stratification and horizontal stratification, as well as bed-relief indices were measured in sandstones and conglomerates of the Lower Silurian Shawangunk Conglomerate, Green Pond Conglomerate, and Tuscarora Sandstone in New Jersey and Pennsylvania. These formations display downslope trends similar to those of the South Platte and Platte Rivers, and, combined with paleocurrent, grain-size distribution, and other data, suggest that the coarse eastern facies (Green Pond, Shawangunk) represent proximal braided stream deposits with longitudinal bars that grade westward and northwestward into finer grained distal braided stream sediments (Tuscarora) characterized by transverse bars.
Stylonurid eurypterids (Arthropoda: Chelicerata) include some of the largest known arthropods – bizarre sweep-feeding hibbertopterids from the Carboniferous to end-Permian. New material of Drepanopterus abonensis, a stylonurid from the Late Devonian (Famennian) of Portishead, south-west England, offers key insights into this genus and its affinities. A redescription utilising the new material enables D. abonensis to be assigned as basal member of the Superfamily Hibbertopteroidea, the large-sweep-feeding forms, possessing a cleft metastoma and blades (modified blunt spines) on their anterior prosomal appendages. D. abonensis also shares characters such as a clavate telson and median ridge on the carapace with the proposed hibbertopteroid sister group the Kokomopteroidea. Hibbertopteroid eurypterids are the most long-ranging stylonurids, surviving the decline and extinction of the other eurypterid families in the Late Devonian, their survival probably because of their sweep-feeding mode of life, which was not in direct competition with their eurypterine relatives and other predators.
The functional morphology and autecology of leperditicopid arthropods (Ordovician-uppermost Devonian) are analyzed in the light of well-preserved specimens from the Devonian of China and detailed comparisons with recent ostracodes. Leperditicopids were large, bivalved arthropods (adults ranging from 5 to about 50 mm in length) typically with an asymmetric carapace (strong ventral overlap), a complex muscular system (powerful adductors, extrinsic muscles, tendinous structures) whose insertions are preserved as scars on the inner surface of the exoskeleton and steinkerns, and an extensive radiating network of integumental sinuses probably involved in gaseous and ionic exchanges (oxygen uptake and transport, osmoregulation). The conspicuous chevron scars adjacent to the adductor scars are interpreted as the anchoring spots of mandibular tendinous structures possibly involved in the opening mechanism of the valves. The ultrastructure of the carapace is comparable to that of thick-shelled recent myodocopid ostracodes. A review of leperditicopid occurences (depositional environment, associated faunas and floras) shows that the group preferentially occupied very shallow marginal habitats (tidal flats, reef-flats, lagoons, embayments, or estuarian complexes) that were subjected to environmental stress (salinity, temperature, moisture). This ecological range implies specific adaptations (osmoregulation, resistance to desiccation) supported by morphological evidence (e.g., circulatory system, carapace closing system, thick shell). Most leperditicopids had epibenthic lifestyles and were probably detritus feeders. They may have been adapted (powerful mandibles) to scrape food on algal/microbial mats. Their typical pattern of occurrence (monospecificity, large numerical abundance) displays some of the characteristics of opportunistic populations (e.g., recent ostracodes, branchiopods) living in variable environments. Morphological similarities with Ostracoda are important (e.g., muscular and tendinous features, circulatory system, valve overlap, carapace ultrastructure) but taxonomic relationships with that group remain inconclusive because of the lack of evidence from soft parts.
It is widely accepted that carbonates, especially those which originate as banks or reefs, and the evaporites with which they are closely associated represent mutually exclusive, successive phases of deposition: (1) a stage of carbonate bank or reef deposition under conditions of normal marine circulation, followed by (2) a time of sulfate deposition under conditions of restricted circulation, (3) an episode of further restriction leading to chloride precipitation, and, in some instances, ending in (4) an ultimate stage of bittern-salt accumulation. A multistage history of carbonate-evaporite deposition requires paleogeographic interpretations that appear bizarre in view of the epeiric setting of most deposits. An alternative is sought in deposition from density-layered waters with an upper layer of nearly seawater salinity and a lower layer or layers saturated with respect to one or more evaporite salts. Brine is supplied to such a system by evaporation on shelf areas surrounding a basin, and saturation is maintained by contact with salts on the basin floor. In this manner layered solutions persist for geologically significant spans of time, in spite of diffusion and mixing, while carbonate deposition continues in the upper dilute waters. Episodic exposure of the lower brine to evaporation is required for evaporite deposition. S ch exposure may be the result of displacement of the upper layer by wind, or may accompany long-period, high-amplitude, internal seiches. Layered systems and the effects of winds and internal seiches are demonstrable in present-day saline lakes; the processes involved are amenable to laboratory investigation.
The stratigraphy of the Cayugan series strongly contrasts classical terminology of the basin margins with subsurface relationships of the basin centers. Both lithofacies and paleontological facies are sharply defined and narrowly restricted so that the few outcrops in the basin margin carbonate facies serve poorly to define a proper framework for Cayugan stratigraphy. Basinal rocks are exposed only in New York state and in the Appalachian fold-belt in Pennsylvania, Maryland, and Virginia where the rocks are largely atypical of the bulk of the Cayugan series. A review of the evolution of Cayugan stratigraphic terminology, and the nature and distribution of the faunas demonstrates the provincial nature of knowledge concerning correlation among the regions of classical study. A different stratigraphic terminology is used in New York, Pennsylvania-Maryland, Ohio, Indiana, Wisconsin, and Michigan-Ontario. In most regions the terminology of the basin margins is inappropriate for the basin centers. Moreover, in the faunas reported, out of a total of 243 species, 199 (82 per cent) are reported from only one locality, and 225 (93 per cent) are reported from one or two localities. To solve some of these problems a regional framework of sedimentation is established so that there can be a standard of reference within which stratigraphic classification and correlation over the entire Cayugan depositional basin can be made. The framework of sedimentation establishes general stratigraphic relations between disconnected outcrops, outcrop and subsurface, and subsurface units within and between basins. It is prescribed from regional lithofacies analyses of the distribution of evaporite rock types, limestone-dolomite relations, terrigenous detrital debris, and thicknesses of rock units. The sedimentary framework for Cayugan rocks involves the development and growth of Niagaran reef platforms around the Michigan and Ohio basins which prevented free interchange of oceanic brine between the open sea and the basin, and which formed the ultimate control on subsequent evaporite deposition. In the eastern part of the Appalachian basin evaporites were deposited in New York and northern Pennsylvania while open marine limestone and shale were being deposited in southern Pennsylvania and Maryland in the Wills Creek-Tonoloway sea. Oceanic brine flow between the southern open ocean and the northern evaporite basin was restricted by the distal end of the Bloomsburg-Vernon delta complex, deposited by westward flowing streams carrying red terrigenous debris onto the eastern shelf of the Appalachian basin.
Conodonts of the genus Ctenognathodus form an important part of the fossil fauna through the Wenlock-Ludlow boundary beds on Saaremaa Island. In the collections from the Anikaitse, Hülge, and Soeginina cliffs of the uppermost Rootsiküla and lowermost Paadla stages three new species of Ctenognathodus are identified, together with two new species of Ozarkodina. One more new species, Ctenognathodus jeppssoni sp. nov., is described from the Sauvere Beds of the Paadla Stage. Conodont evidence suggests that the Soeginina Beds in the stratotype section are the lowest stratigraphic unit of the Paadla Stage and the Ludlow Series, as presumed earlier by cyclostratigraphic investigations. The layers of the Anikaitse cliff with unique conodont fauna are considered as an independent rock-unit - the Anikaitse Beds.
A comprehensive facies model of the Tonoloway Limestone shows an array of depositional environments from carbonate lagoon-sabkha on the NNE to deeper ramp on the SSW. This model is significant to the developmental history of the central Appalachian basin because carbonate-ramp facies have not been previously recognized in the pre-Helderberg part of the Upper Silurian interval. Four regional facies-environments are recognized: Carbonate Lagoon-Sabkha, Sandy Lagoon-Sabkha, Shallow Ramp, and Deep Ramp-Tempestites. In the carbonate lagoon, moderate to low energy, skeletal/ oolitic sand flats passed landward into subtidal mudflats, muddy intertidal flat, and sabkha. Conditions varied from restricted to open-marine, and depths were everywhere above wave base (15 m). The minor Sandy Facies (including the Indian Springs Sandstone Member) represent quartz-rich counterparts to the restricted carbonate lagoon-sabkha. The shallow ramp consisted of high-energy crinoid shoals and deeper, offshoal biostromes, floatstone/wackestone/packstone, and nodular lime sediments. Frame builders included stromatoporoids, bryzoans, and corals. Maximum depth was between normal wave base (15 m) and storm wave base (40 m). Graded carbonate tempestites formed on the deep ramp, where water depths reached storm wave base. Open-marine conditions prevailed on both parts of the ramp. The lagoon/sabkha-shallow ramp-deep ramp profile existed during the middle of Tonoloway time and extended from Pennsylvania through Maryland, Virginia, and West Virginia, with the ramp sloping gently S. The ramp developed from a regional epeiric platform coincident with a northeastward transgression between two regressions, all within the 1-3 Ma total span of Tonoloway time. Both eustatic activity and tectonic mechanisms, possibly related to basement-fault reactivation, acted as controls on ramp development.
In order to establish the ancient setting of the Salina Formation, the authors related environments of deposition of the Tonoloway Limestone to the stratigraphic and geographic distribution of evaporites in the Salina Formation. Final deductions were based on a combination of surface and subsurface stratigraphy, paleontology and carbonate petrology. With paleoenvironments of the Salina and Tonoloway understood, a regional framework was established for this part of the Appalachian basin during the Late Silurian time. From this model, areas of possible oil and gas accumulation were then outlined.
Layered halite rock is presently forming in depressions on supratidal flats off the mouth of the Colorado River, Mexico. The halite crystals display a chevron structure essentially similar to that which occurs in many ancient layered salt deposits. It is apparent in the field that the layers are not the records of annual events but that each one is the product of a long period of 'reworking' of brinepan halite and ground water-brine diagenesis.
Evaporites are soluble in most subsurface situations. Even preserved evaporite beds show varying degrees of dissolution about their edges. The longer an evaporite bed remains in the subsurface the more likely it is to dissolve. Preservation or loss it is not a question of whether an evaporite will dissolve in the subsurface, but when. This is why there are few preserved beds of evaporite salts older than Neoproterozoic. As thick evaporite units dissolve they leave behind characteristic breccias and other textures and replacements that allow the recognition of former extensive salt beds. The breccias can be interstratal beds or trans-stratal chimneys, distinguishing the two forms differentiates between blanket or more focused inflows. Dissolution can be driven by basinal fluids escaping from below or by fluids infiltrating from above. Salt units may not dissolve until they have flowed into diapirs and welds (halokinetic breccias) or have acted as décollement surfaces in fold and thrust belts (rauhwacke). All evaporite salt units will ultimately dissolve, many disappear in the diagenetic realm while others may be preserved until they attain metamorphic grades or interact with a magmatic body.
The monograph offers a comprehensive discussion of the role of evaporites in hydrocarbon generation and trapping. For the first time, diverse knowledge on exploitable salts has been assembled and organized, along with a summary of evaporate karst hazards as well as a summary of exploitative methods and pitfalls in dealing with evaporites in conventional and solution mining. Written by a field specialist in research and exploration, the book presents a comprehensive synthesis of the low temperature realm of evaporite evolution. It is aimed for earth science professionals, sedimentologists, oil and gas explorers, mining geologists, as well as environmental geologists.
Modern oceanographic studies in estuaries and other restricted arms of the sea have shown that a characteristic circulation pattern exists. Surface currents flow from regions of low salinity to regions of higher salinity in response to hydrostatic head and are accompanied at depth by oppositely directed currents flowing from high to low salinity regions because of density distribution. Salts are deposited in restricted estuaries where evaporation exceeds precipitation plus runoff. The necessary restrictions of the estuary or basin are in part dynamic and in part static. Dynamic restriction is caused by the hydrostatic head and by frictional stresses between the oppositely directed surface and bottom currents, and between the bottom current and the channel floor. Static re triction is produced by topographic confinement. When high concentrations are developed a strong horizontal salinity gradient exists which produces lateral segregation of different salts during precipitation. The escaping deep current returns to the sea those salts which have not been precipitated. Fluctuations in equilibrium caused principally by changes in excess of evaporation or in degree of channel closure cause migrations of the horizontal salinity gradient along the longitudinal axis of the basin which produce vertical differentiation of salts. The order in which salts occur in the vertical column can be predicted from oceanographic and chemical data and agrees well with many vertical sequences which have been described.
Large hopper-shaped halite cubes (up to 10cm on a side) grow displacively within Holocene sediments near the southern end of the Dead Sea, under conditions of high temperatures, ionic concentrations and rapid rates of crystallization. Two theories of formation are presented: downward diffusion of ions versus upward diffusion, and 'evaporative pumping'. Both alternatives are considered possible, and the correct choice probably depends upon the location of the sediments at the time of precipitation - whether subaqueous or subaerial - a choice indeterminate at present, because of human disturbance to the sediments prior to collection. Displacively formed halite crystals, similar in morphology to the Dead Sea cubes, are common in evaporite-bearing carbonate rocks of many regions, and are believed to have formed under conditions analgous to those in the present Dead Sea.-from Authors
Eurypterids are generally rare in the fossil record, but occasionally occur in abundance. The genus Eurypterus, in particular, is well known from certain upper Silurian Lagerstätten of the northern Appalachian basin (New York and Ontario), but occurs far less frequently in the central and southern Appalachian basin (Pennsylvania, Maryland, and West Virginia, respectively). The recent discovery of an exceptionally preserved mass assemblage of Eurypterus in the upper Tonoloway Formation (upper Ludlow–Přídolí) of Pennsylvania provides new information on the behavior and life habitat of the genus in this region. Eurypterids at this locality are found in thinly laminated, calcareous shale deposited within the lower intertidal to shallow subtidal zone of a coastal mudflat or sabkha. Rare associated fauna of limited diversity, and evaporitic and desiccation features in associated beds, suggest a stressed environment with variable salinity and possible hypoxic conditions. Most eurypterids are disarticulated and fragmentary, but several fully articulated, exceptionally preserved specimens are present. Exoskeletal features and taphonomic indices values indicate a molt rather than death assemblage, and the presence of arthropod trackways suggests that Eurypterus sp. may have molted en masse in the vicinity of the burial site. Sequence stratigraphic interpretation of the site suggests that preservation of eurypterid remains is the result of occupation of ephemeral environmental (salinity/oxygen) conditions during a transgression. The occurrence of this new Lagerstätte within the upper Silurian succession of the central Appalachians, an interval which had heretofore yielded only rare, fragmentary remains, indicates that eurypterids were more prevalent in this region than previously thought. INTRODUCTION
The Kokomo Limestone Member of the Wabash Formation is of late Silurian age. This member consists of about 30 m of thinly-laminated carbonates in the Logansport area of north-central Indiana. The almost complete lack of fossils throughout the member indicates a hypersaline depositional environment. Previous workers interpreted the Kokomo Limestone as a product of an intertidal and subtidal environment, or reported the presence of syneresis cracks to argue for a restricted lagoon and intertidal environment. In this paper we show that desiccation mudcracks, storm-produced intraclasts, intraforma-tional conglomerate, and bird's-eye structures taken together indicate an intertidal and supratidal environ-ment during deposition of the Kokomo Limestone. Microscopic supporting evidence includes micro-cracks, intraformational micro-breccia/conglomerate, an abundance of dolomite, anhydrite/gypsum pseudomorphs, and fenestrate structures lined with calcite, dolomite and tar. Couplets of dark and light laminae were previously interpreted as annual cycles, but we interpret them as daily tidal cycles, bimonthly spring tide deposits, and occasional storm tide deposits. We also show evidence of unconformities both at the base and at the top of the Kokomo Limestone Member. The Kokomo Limestone Member of the Wabash Formation, Salina Group, is of late Silurian age, latest Ludlovian to early Prido-lian (middle Cayugan) (Shaver et al. 1986). The contact with the underlying Mississinewa Shale is sharp and disconformable (Cummings & Shock 1928), but Pinsak & Shaver (1964) report lacking proofs for either disconformity or conformity. The upper contact with the Kenneth Limestone is sharp but poorly un-derstood. According to Shaver et al. (1986), sand grains and other clastic sediments asso-ciated with this contact are not evidence of intra-Silurian unconformity but rather of pre-Middle Devonian karst. Pinsak and Shaver (1964) correlate the Kokomo Limestone to the A-2 unit of the Salina Formation of Michigan. The Kokomo Limestone is exposed in several quarries in the Logansport area of north-cen-tral Indiana. Thinly-laminated beds of the Ko-komo Limestone are alternating light and dark layers less than 1 mm thick. Outcrop expo-sures also reveal stacking of small scale (2–5 cm) and large scale (30–100 cm) light and dark beds. Even though most of the unit is dolomite, as noted by previous workers (Pinsak & Shav-er 1964; Tollefson 1979; Nellist 1986), the Kokomo Member is still formally called a limestone rather than a dolomite. In this paper we adopt Nellist's (1986) subdivision of the Kokomo Limestone Member into the Morgan Hill Bed (Tollefson's beds A and B) and the Eel Bed (Tollefson's beds C, D, E and F). Tol-lefson (1979) considered the Kokomo Lime-stone depositional environment as a restricted, hypersaline intertidal environment on the iso-lated Wabash platform, which near the end of deposition changed into a more normal marine subtidal environment. Nellist (1986), however, interpreted the Kokomo Limestone deposi-tional environment as a subtidal, restricted la-goon that near the end of deposition became shallower and, in parts, intertidal. Tollefson (1979) used the presence of broken and dis-torted laminae in the Morgan Hill Bed of the Kokomo Limestone as evidence of desiccation and subaerial exposure indicating presence of mudcracks, but Nellist (1986) interpreted the same features as subaquaeous syneresis cracks and considered the lamination couplets to be of seasonal character. In this paper we dem-onstrate the presence of desiccation mud-cracks not only in the outcrop, but also in thin
Synopsis
Eurypterids of the Superfamily Stylonuroidea Diener 1924 sensu Størmer (1974, 373) from the Pentland Hills, Midlothian, are redescribed and the evidence which these forms may give concerning the life environment of the Gutterford Burn Eurypterid Bed (Upper Llandovery), from which most of them have been obtained, is considered. Five species are recognised. Parastylonurus ornatus (Laurie) is redescribed with special reference to the organs of locomotion and reproduction. A new form from the Gutterford Burn is described as Parastylonurus hendersoni sp. nov. Stylonurus macrophthalmus Laurie is designated the type species of the new genus Hardieopterus and Stylonurus knoxae Lamont as the type species of the new genus Lamontopterus . The unique holotype of Lamontopterus knoxae is of morphological interest in showing evidence of the gut.
Stylonuroids from other Scottish Silurian localities are also considered. Two new species, Brachyopterella ritchiei sp. nov. from Seggholm and Hardieopterus (?) lanarki sp. nov. from the Logan Water are described. Stylonurella spinipes (Page) is redescribed and this species is reported for the first time from Seggholm.
The new information provided by this study has raised a number of problems of classification and the criteria upon which eurypterids are classified, and particularly those applied to the stylonuroid eurypterids, are re-assessed and a new classification of the Stylonuroidea proposed. This has required the description of new taxa; a new family, the Parastylonuridae, is proposed to accommodate Parastylonurus, Hardieopterus and probably Lamontopterus which share a unique combination of characters of the prosomal appendages and metastoma. A new genus, Kiaeropterus , is described to accommodate certain species previously assigned to Stylonurella but displaced from that genus as now emended.
A reconstruction of Parastylonurus ornatus in the walking position is attempted and has resulted in the recognition of a new generation of problems related to the functional morphology of these animals. Criteria which may appropriately be used in deciding the manner in which they walked and the posture which they adopted when doing so, are discussed. It is suggested that, in response to certain anatomical and physiological constraints which otherwise would have rendered the animals unstable in the walking position, the post-abdomen and telson have been specialised as hydrodynamic structures for monitoring water movements and vectoring the animal for maximum walking efficiency. The significance of this hypothesis is discussed in relation to the functional morphology and evolution of other members of the Stylonuroidea.
The distribution and dispersal histories of Eurypterida are examined in a phylogenetic and palaeogeographic context. It is suggested that all the eurypterid clades originated and had most of their evolutionary history on the palaeocontinents of Laurentia, Baltica, Avalonia, the Rheno–Hercynian Terrane and some presence in Siberia. The Siluro–Devonian Stylonurina and the eurypterine superfamilies Eurypteroidea and Waeringopteroidea are confined in their entirety to these continents. The Late Palaeozoic records of Adelophthalmoidea and Hibbertopteroidea in China, South America and Africa can be explained by ‘rafting’ on the continental plates that amalgamated into the supercontinent Pangaea. However, two clades have distributions that suggest abilities to cross open stretches of ocean; the Silurian and Devonian pterygotoids and some Devonian adelophthalmoids. However, one Silurian mixopteroid and the basal swimming form Onychopterella outside the ‘core-area’ suggest the dispersal patterns are more complicated. The pterygotoids might be the only clade whose members frequently undertook oceanic excursions. Since almost all eurypterid evolution took place in Laurentia, Baltica and Avalonia (and some in the Early Devonian of the Rheno–Hercynian Terrane) prior to the Carboniferous, it is unlikely that many taxonomically rich pre-Carboniferous eurypterid faunas will be encountered outside of these palaeocontinents.
The eurypterids (Arthropoda: Chelicerata), some of the earliest arthropods to undertake amphibious excursions onto land, are generally rare in the fossil record, but are sometimes found in great abundance, for example in the Late Silurian Bertie Group of New York State. The mass-moult-mate hypothesis has been proposed to explain such occurrences, whereby eurypterids undertook mass migrations into near shore settings and lagoons to moult, mate and spawn, similar to the behaviour of living horseshoe crabs. This hypothesis is tested using measurements from over 600 Eurypterus specimens from three localities in the Bertie Group; Eurypterus remipes, from the Fiddlers Green Formation, and the slightly larger Eurypterus lacustris, from the overlying Williamsville Formation. Disarticulation patterns support previous evidence for moulted assemblages. A significant predominance of female exuviae is noted at each locality, unlike studies on modern Limulus populations. Therefore, a modified mass-mate-spawn-moult hypothesis is proposed here: males returned to deeper waters after mating, whereas females, having mated, remained at the breeding sites to deposit their eggs before moulting. After hatching, eurypterid larvae and juveniles remained in these spawning grounds until they matured and could move to deeper water, in comparison with Limulus. This hypothesis is also discussed in light of recent work on the gender determination of the type “A” and type “B” genital appendages.
The Tuscarora Formation (Lower Silurian) of central Pennsylvania owes its origin to a combination of tectonic, eustatic, and depositional events. Tuscarora deposition began at the beginning of the Silurian period at the time of both glacioeustatic sea-level rise and renewed tectonic elevation of the Taconic source terrain. The basis for these interpretations is the analysis of five major lithofacies in the exposed Tuscarora Formation of central Pennsylvania.-from Author
New paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the Silurian and Devonian. In this paper, the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data. The data from the three fields of paleomagnetism, paleoclimatology, and biogeography are generally in excellent agreement, and an internally consistent paleogeographic evolutionary picture of the interactions between North America, Gondwana, and intervening displaced terranes is emerging. During the interval of the Ordovician, Silurian, and Devonian, North America stayed in equatorial paleoposition, while rotating counterclockwise. The northwest African part of Gondwana was in high southerly latitudes during the Late Ordovician and was fringed by peri-Gondwanide terranes, such as southern Europe (Armorica) and Avalonian basement blocks now found in eastern Newfoundland, Nova Scotia, the Boston Basin, the Appalachian Piedmont, and northern Florida. Subsequently, Gondwana and the peri-Gondwanide terranes displayed rapid drift with respect to the pole. This drift translates into the following pattern of movement for northwest Africa. During the latest Ordovician-Early Silurian, this area moved rapidly northward from polar to subtropical latitudes, followed by equally rapid southward motion from subtropical to intermediate (about 50°S) paleolatitudes during the Late Silurian-Middle Devonian. It is likely that significant east-to-west motion accompanied the latter shift in paleolatitudes, with the Caledonian-Acadian orogeny the result of Silurian to Early Devonian convergence and collision between Gondwana and North America. This collision sandwiched several of the intervening displaced terranes between Gondwana and North America. Subsequent to this collision, Gondwana was separated in the Late Devonian by a medium-width ocean from North America and the Avalonian and southern European blocks which were left behind adjacent to North America. This new ocean closed during the Carboniferous, and the resulting convergence and collision were the cause of the Hercynian-Alleghanian orogenic belt. Problems remaining for future research, besides the further gathering of reliable paleopoles, involve the uncertain pre-Devonian position of the southern British Isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid Late Ordovician-Silurian apparent polar wander for Gondwana.
Eurypterid (Eurypterida, Chelicerata) palaeoecology is reviewed, in light of a statistical analysis of 74 Silurian and Early Devonian eurypterid assemblages. Kjellesvig-Waering's three eurypterid ‘biofacies’ model is rejected as this analysis indicates considerable mixing of eurypterids from each supposed biofacies. A new model is proposed, which regards each eurypterid genus as having a broad palaeoecological range, determined by several environmental and palaeophysiological criteria, including an ‘ontogenetic gradient’, whereby juveniles preferentially adopted marginal and near shore habits (here interpreted as representing a true population distribution and not a taphonomic or collecting bias). A ‘mass–moult–mate’ hypothesis is proposed, which depicts eurypterids as having migrated en masse into nearshore and marginal environments (e.g. lagoons) to moult and mate. This hypothesis is evidenced by: (i) the occurrence of abundant accumulations of eurypterid remains (e.g. the Bertie and Saarema assemblages), which are interpreted as concentrations of exuviae in the absence of any evidence that these remains represent mass mortalities. (ii) Eurypterid respiratory and reproductory palaeobiology (i.e. eurypterids possessed accessory aerial respiratory tissues, enabling amphibious excursions and they were capable of spermatophore exchange and storage, allowing the delay of fertilisation of their eggs). (iii) Occurrences of abundant, variously sized, sub-parallel, eurypterid trackways, indicating migrations en masse, in a similar direction, across marginal environments. Comparisons with modern analogues (semi-terrestrial crabs and xiphosurans) are made, which employ a similar behaviour.