Naraoiids are nektaspidid arthropods that display two exoskeletal shields, typically lack thoracic tergites, and have an exceptional Cambrian diversity. Despite their predominantly Cambrian fossil record, Naraoia species have been documented in deposits as young as the late Silurian (Pridoli). At present, only one specimen of the Silurian taxon—Naraoia bertiensis—has been documented. Here we report the second known Silurian Naraoia specimen assigned to N. cf. bertiensis from the Phelps Member of the Fiddlers Green Formation, extending the fossil record from the Williamsville Member. We use paleo-elevation models to explore naraoiid paleobiogeography, illustrating a striking decrease in naraoiid distribution from the Cambrian into the Ordovician and Silurian. These models allow us to explore the proposal that geologically younger naraoiids migrated to deep water, off shelf environments, inhabiting these refugia-like conditions after the Cambrian.

Naraoia, Naraoiidae, paleo-elevation modeling, Phelps Member, Pridoli, Silurian

Naraoiidae is a family of nektaspidid arthropods known primarily from the Cambrian fossil record (Whittington 1977; Peng et al. 2012; Mayers et al. 2019) that consists of four genera—Naraoia, Liwia, Soomaspis, and Tariccoia—and one sub-genus Naraoia (Misszhouia) (Dzik and Lendzion 1988; Fortey and Theron 1994; Mayers et al. 2019; Lerosey-Aubril et al. 2020; Bond and Edgecombe 2021; Pérez-Peris et al. 2021). Naraoiids have two exoskeletal shields (Caron et al. 2004; Gozalo et al. 2018; Zhai et al. 2019; Bond and Edgecombe 2021), lack a calcified exoskeleton (Budd 1999), and show few to no thoracic tergites (Dzik and Lendzion 1988; Fortey and Theron 1994; Pérez-Peris et al. 2021). While most documented naraoiids are Cambrian-aged, the naraoiid fossil record extends to the late Silurian (Pridoli) (Caron et al. 2004; Zhai et al. 2019; Bond and Edgecombe 2021). Across their evolution, naraoiids primarily inhabited outer shelf paleoenvironments on clastic shelves or carbonate platforms (Bond and Edgecombe 2021).

The apparent rarity of Ordovician and Silurian naraoiids is not an evolutionary pattern, but evidence of taphonomic biases and a lack of appropriate fossil sites (Caron et al. 2004; Muscente et al. 2017; Pérez-Peris et al. 2021). As such, documentation of non-Cambrian species is imperative for understanding naraoiid evolution. Here we report the second specimen of the youngest naraoiid, Naraoia cf. bertiensis Caron et al., 2004, extending the distribution from Ontario, Canada to New York State, adding more anatomical information for this rare species. We also consider naraoiid paleobiogeography using paleo-elevational modeling, exploring possible water depths inhabited by these taxa between the Cambrian and end-Silurian.

The two known specimens of Silurian Naraoia come from the Pridolian Bertie Group. The group was deposited near the geographic centre of the northeast/southwest trending Appalachian foreland basin (Brett et al. 1990). Paleogeographic reconstructions place this part of Laurentia ~ 30°S of the paleo-equator, within high-pressure latitudes where descending dry air produces high evaporation rates (Cocks and Torsvik 2011). Circulation between the foreland basin and both the Iapetus Ocean and the epeiric sea covering the majority of Laurentia was restricted by the Taconic Highlands and the Algonquin Arch, respectively (Brett et al. 1998).

The Bertie Group includes several units of fossiliferous, massive dolostone and argillaceous dolostone with minor limestone, argillaceous limestone, and thin evaporites (mainly gypsum) gently dipping to the south/southwest (Vrazo et al. 2016). In Canada, the Bertie is a formation with four members (Falkirk, Scajaquada, Williamsville, Akron) (Haynes and Parkins 1992), while in New York State, it is a group comprised of five formations (Fort Hill, Oatka, Fiddlers Green, Scajaquada/Forge Hollow, Williamsville) (Ciurca Jr 1978). We use the terminology employed in New York as the examined specimen comes from Passage Gulf, Herkimer County, New York.

The Bertie Group overlies the Salina Group—a geological unit comprised of three formations containing dolostone and shale with abundant and economically important interbedded gypsum, anhydrite, and halite (Ciurca Jr 1978; Fig. 1). A restricted fauna in the Salina Group suggests that evaporitic conditions led to high salinity during deposition (Kluessendorf 1994). The Bertie Group sedimentary succession has a maximum thickness of 18 m, and crops out from near Herkimer, west through Buffalo to Dunnville, Ontario, Canada, over a distance of ~410 km.

Figure 1. 

Stratigraphic collum of the passage Gulf locality showing location of new specimen. Modified from Edwards et al. (2004) and Lamsdell and Briggs (2017).

The Bertie Group formed on the proximal margin of a carbonate ramp originating on the paleo-southeastern side of the Algonquin Arch and gently dipping into the Appalachian Basin (Ciurca Jr 1973; Hamell 1982). Low slopes likely intensified the effects of sea level fluctuations, and three shallowing-upward cycles are recorded (Brett et al. 2000). Marginal marine environments in the Bertie Group ranged from shallow subtidal to supratidal (Ciurca Jr 1973, 1978), and the shallowest environments are interpreted as sabkhas based on the presence of microbialites (thrombolites/cryptalgal laminations), desiccation cracks, and evaporites (gypsum, anhydrite and halite) (Ciurca Jr and Hamell 1994; Brett et al. 2000; Vrazo et al. 2016). Furthermore, the Ellicott Creek Breccia is composed of ripped up thrombolites, suggesting shallow depths (Brett et al. 2000) and may represent a tsunamite deposit (Ciurca Jr and Hamell 1994). The deepest environments in the Bertie Group, found above flooding surfaces, are shallow subtidal, represented by fine-grained dolostone with conchoidal fractures bearing a normal marine fauna (Brett et al. 2000).

Four units within the Bertie Group, the Fort Hill Formation, the Morganville and Phelps members of the Fiddlers Green Formation, and the Moran Corner Member of the Akron Formation, are considered ‘waterlimes’. The term ‘waterlime’ describes the extremely fine-grained Silurian dolostone units in Ohio, Michigan, western New York, and southern Ontario used to produce hydraulic cement (Newberry 1870). ‘Waterlime’ is therefore not a formal geological term, despite extensive use in the literature. The Bertie ‘waterlime’ lithology is extremely fine-grained, clayey dolostone with conchoidal fracture that yields exceptionally well-preserved fossils, including soft-bodied organisms (Brett et al. 2000). Andrews et al. (1974) interpreted the Phelps Member as a primary dolomite. However, direct precipitation from seawater requires a salinity of or above ~70 ppt (Scruton 1953), which is unlikely based on the diverse fauna (Burrow and Rudkin 2014). Further, diagenetic alteration to molds or coarse calcite filled voids of faunal elements with biomineralized skeletons (e.g., gastropods, bivalves and brachiopods) argues for sub-burial dolomitization (Vrazo et al. 2016).

Interpretations of salinity levels in the Appalachian foreland basin during deposition of the Bertie Group range from relatively fresh to hypersaline. Brackish conditions are suggested by land plants like Cooksonia and faunal elements capable of tolerating low salinities including inarticulate linguliform brachiopods and gastropods, limited evidence for taxa that require marine conditions (e.g., echinoderms) and limited bioturbation (e.g. Kjellesvig-Waering 1950; Plotnick 1999; Vannier et al. 2001; Edwards et al. 2004 and Vrazo et al. 2014). Hypersalinity during deposition has also been argued based on proposed euryhaline tolerances of eurypterids, the presence of salt hoppers, halite molds and casts, and desiccation cracks that frequently occur on bedding planes with eurypterid fossils (Hamell 1982; Selden 1984; Tollerton Jr and Muskatt 1984; Hamell and Ciurca Jr 1986; Ciurca Jr and Hamell 1994; Braddy 2001).

Recognizing the post-depositional timing of salt hopper formation is important for paleoenvironmental reconstructions. Salt hoppers grow quickly from a supersaturated fluid and are the most common evaporitic feature in the Bertie Group. These three-dimensional, pyramid-shaped structures can form under high salinity conditions a) at the air-water interface, b) within the water column, c) on the substrate surface, or d) within subsurface sediment. When salt hoppers form below the sediment-water interface they displace the surrounding sediment and crosscut sedimentary and biogenic laminations (Vrazo et al. 2016). In the Bertie Group, salt hoppers displace sedimentary and biogenic (thrombolitic) laminations and are regularly found within the outer margins of body fossils. The organic remains therefore acted as nucleation points for salt hopper growth post-burial (Vrazo et al. 2016).

Two formations, the Williamsville and Fiddlers Green, contain Konservat-Lagerstätten that consist of extremely fine-grained, nearly lithographic dolostone with conchoidal fracture (Vrazo et al. 2017). Naraoia bertiensis was originally described from the Williamsville Member at Ridgemount, Ontario, Canada about 13.5 km northwest of Buffalo, NY (Caron et al. 2004). The second known Naraoia specimen, considered here, is from the Phelps Member of the Fiddlers Green Formation at Passage Gulf, ~352 km east of Buffalo. At Passage Gulf, the Fiddlers Green and Forge Hollow formations are the only units exposed (Fig. 1). The Fiddlers Green Formation is subdivided into the Morgansville, Victor, Phelps, and Ellicott Creek Breccia members. The Phelps Member is ~1 m of fine-grained dolostone exhibiting conchoidal fracture (Brett et al. 2000). In its upper part, the Phelps Member contains ripples, desiccation cracks and wrinkled laminae that are likely algal in origin, suggesting upward shallowing into an intertidal to supratidal environment (Ciurca Jr and Honan 1965; Brett et al. 2000). The Fiddlers Green Formation at Passage Gulf preserves a high diversity fauna including organisms that can tolerate high salinities (gastropods, ostracods, phyllocarids), normal marine indicators (nautiloid cephalopods, synziphosurines), and elements derived from land (scorpions, early land plants Cooksonia and Hostinella), suggesting brackish water conditions (Hamell 1982; Tollerton Jr and Muskatt 1984; Haynes and Parkins 1992; Caron et al. 2004; Lamsdell and Briggs 2017; Bicknell et al. 2019; Bicknell and Pates 2020; Ruebenstahl et al. 2021).

The exceptional preservation of soft-bodied material (e.g. land plants) and non-mineralized, chitinous exoskeletons (eurypterids, scorpions, synziphosurines, chasmataspidids) requires inhibition of bioturbation, scavenging, and bacterial decay. Evaporitic structures have been used to suggest that eurypterids were adapted to living in hypersaline environments (Kluessendorf and Mikulic 1991; Ciurca Jr and Hamell 1994; Edwards et al. 2004). However, more recent work suggests that eurypterid accumulations are most common following small-scale transgressions that freshened shallow subtidal marine water to near-normal conditions (Vrazo et al. 2017). Eurypterids likely gathered in the newly opened habitat to moult and reproduce (Braddy 2001). Exuviae were then buried either by storm sedimentation or due to baffling by microbialite structures. Storm sedimentation is more likely as is indicated by the presence of windrows (Ciurca Jr 1978). Following burial, sea level highstand led to high temperature, higher salinity and dysoxia in the water column, inhibiting decay of eurypterid cuticle (Vrazo et al. 2017).

The examined specimen is housed within the New York State Museum, Albany, New York Paleontology Collection (prefix NYSM) and has the specimen number NYSM 19522. The specimen was submerged in diluted 90% ethanol and imaged with an Olympus E-M1MarkIII using an Olympus 60 mm macro lens under LED lighting. Measurements of the specimens were gathered from images using ImageJ (Schneider et al. 2012). When describing the specimen, we followed the systematic and morphological terminology of Caron et al. (2004), Mayers et al. (2019), and Bond and Edgecombe (2021).

We generated naraoiid paleogeographic distributions from the Cambrian to the Silurian. We acquired occurrence records of naraoiid observations from the Paleobiology Database (PBDB; Downloaded September 2024, Suppl. material 1) and retained occurrences of taxa possessing at least a genus epithet (in the PBDB ‘accepted name’ category) and could be verified in the literature. Further occurrence filtering consisted of omitting specimens for which latitude and longitude could not be computed. Time intervals were binned to the Cambrian (538.8–485.4 Mya; 142 occurrences), the Ordovician (485.4–443.8 Mya; four occurrences), and Silurian (443.8–419.2 Mya; two occurrences) (Suppl. materials 1, 2). We derived paleo-elevational maps using the Paleo-Digital Elevation Model (PaleoDEM) dataset from Scotese and Wright (2018) and selecting the 1°×1° PALEOMAP dataset. We derived PALEOMAP paleocoastlines from the online GPlates portal (Müller et al. 2018) (https://gwsdoc.gplates.org/) using the R package sf (Pebesma 2018). Since PALEOMAP PaleoDEMs consist of three-million-year time slices that approximate geological stages, we paleo-rotated naraoiid occurrences to the median time slice of each age (Cambrian: 515 Mya; Ordovician: 465 Mya; Silurian: 430 Mya; Suppl. material 2). We selected these rasters to encapsulate the elevational gradient of each period using the R package raster (Hijmans 2023). We plotted the paleorotated occurrences of naraoiids using PALEOMAP rasters in QGIS v3.36.1 (Baghdadi et al. 2018).

We explored changes in water depth occupied by naraoiids using the PALEOMAP PaleoDEMs. We used the oldest and youngest stratigraphic time intervals for each naraoiid occurrence (min_ma and max_ma in the PBDB; Suppl. material 3) to generate minimum and maximum ages for each paleo-rotated specimen coordinate. We used these coordinates to extract bathymetrical readings for specimen observations to consider water depths ranges for naraoiids. Four data points indicated positive bathymetric readings suggesting a position above sea level. As naraoiids are marine organisms (Whittington 1977), these values are erroneous. We therefore exclude these specimens in a detailed examination of water depth. We plotted these remaining data in an R environment (R Development Core Team 2024) using ggplot2 (Wickham 2011), deeptime (Gearty 2024), and palaeoverse packages (Jones et al. 2023) (Suppl. materials 3, 4). All data and code are available from the following link: https://doi.org/10.17605/OSF.IO/TNYEQ.

Naraoiid geographic and environmental distributions shift between the Cambrian and Silurian (Fig. 3) (Pérez-Peris et al. 2021). Cambrian naraoiids are predominantly located in low-latitude regions—Laurentia, Baltica, South China, and Gondwana (Fig. 3A; Whittington 1977; Zhang et al. 2007; Mayers et al. 2019; Lerosey-Aubril et al. 2020), generally inhabiting the open marine environment. Ordovician naraoiids are only observed in Gondwana (Fig. 3B) in open marine (Budil et al. 2003; Pérez-Peris et al. 2021), restricted (Hamman et al. 1990), and brackish-to-marine settings (Fortey and Theron 1994). The youngest forms are limited to Laurentia (Fig. 3C; Caron et al. 2004).

Figure 3. 

Naraoiid localities from the Cambrian to Silurian. A. Observations during the Cambrian; B. Observations during the Ordovician; C. Observations during the Silurian, including the new specimen. Paleo-elevational maps were acquired using Scotese and Wright (2018) and data from the PBDB.

Predicted paleo-elevation for areas occupied by naraoiids shift from the Cambrian to the end-Silurian (Fig. 4). The PBDB data indicate that Cambrian forms range between 160–800 m below sea level. Ordovician forms range about 200 m below sea level and Silurian forms range between 200 and 520 m below sea level.

Figure 4. 

Depiction of water depth inhabited by naraoiids through time. An increase in water depth from Ordovician to Silurian indicates habitation of deeper water conditions in younger species.

Two argillaceous, very fine-grained dolomite units within the Bertie Group (e.g., Phelps Member and Williamsville Formation) famously preserve eurypterids in exceptional abundance and detail (see Clarke and Ruedemann 1912, Kjellesvig-Waering 1964; Ciurca Jr 1978; Brett et al. 2000; Tetlie et al. 2008; Ruebenstahl et al. 2021; Larson and Briggs 2023). Because the preservation of non-biomineralized arthropods within these units is common, the rarity of Naraoia in the Bertie Group therefore requires some explanation. As the Phelps Member preserves evidence of marine windrows (Ciurca Jr 1978), the N. c.f. bertiensis specimen considered here may be allochthonous. Re-examination of bedding planes within the Bertie Group that preserve Naraoia may uncover other specimens.

Naraoiid distribution between the Cambrian and Silurian shifts from cosmopolitan to highly restricted (see Pérez-Peris et al. 2021 and Results) (Fig. 3). Such a change likely reflects a taphonomic and/or sampling bias, rather than a real biogeographic pattern. Muscente et al. (2017) comprehensively evaluated exceptionally preserved fossil assemblages. Their results show a decrease in the number of Lagerstätten representing open marine environments across the Cambrian-Ordovician interval. Consequently, non-biomineralized open marine taxa are rarely recorded in the Ordovician and Silurian fossil record, leading to the absence of naraoiids in the Ordovician of Laurentia, Baltica, South China, and other low to mid-latitude areas. Since Naraoia is present both in the Cambrian and Silurian strata of Laurentia, it likely was present in the same area during the Ordovician period and is absent in the fossil record either due to the lack of suitable deposits with exceptional preservation or the limited sampling of deeper outer shelf areas in Laurentia. It is unparsimonious to propose that Naraoiidae went locally extinct in Laurentia and then had a Silurian migration. More plausible is that these forms inhabited outer shelf refugia (see Bond and Edgecombe 2021) and such Ordovician-aged deposits in Laurentia have not been explored in detail (Muscente et al. 2017; Pérez-Peris et al. 2021). Moreover, the rare findings of Naraoia hammani in deep water settings of the Prague Basin (Budil et al. 2003) suggest that naraoiids were probably much more widespread in the open marine environments during the Ordovician period. Identifying and examining such deposits in Laurentia (and elsewhere) may therefore uncover more Ordovician naraoiids (see discussion in Bond and Edgecombe 2021 and Pérez-Peris et al. 2021).

Potential biases in the bathymetric data are associated with the paleo-elevational model used to rotate occurrences to the geologic past, the age range of specimens, and the paleo-elevational DEMs resolution. Digitisation and a lack of well sampled representation of specimens in the PBDB can strongly impact the reconstructed sea level for data points, as sparse or low-resolution locality records fail to generate a consensus of bathymetric trends of marine invertebrates over time. Furthermore, depending on the paleo-elevational model used (compare Scotese and Wright 2018 with Seton et al. 2012 or Matthews et al. 2016), benthic shelf-residing taxa may mistakenly be rotated on land. This substantially impacts depth predictions for species. Additionally, imprecise early and late time intervals for observations can result in large differences in minimum and maximum bathymetric ranges. Finally, the coarseness in spatial grain of our DEM’s may reflect loss of information regarding naraoiid bathymetric preferences, as the transition from coastal to deep-water conditions were represented by only a few pixels. Indeed, we removed four naraoiid occurrences from the water depth examination due to positive elevation values. These reflect numerical errors associated with data resolution. Potential solutions to such issues include re-examination of data points, both in geographic and age resolution, inputting more data from the primary literature into the PBDB, and development of more precise plate models with expressed aim of correctly reconstructing environments and water depth. Although deriving bathymetric patterns of fossil invertebrates from original papers would be more precise on the regional-scale, due to more extensive sampling, utilisation of DEM’s is more optimal for global distributions due to the even sampling of pixels (Goodman et al. in press).

This research was funded by an MAT Program Postdoctoral Fellowship (to R.D.C.B) and the Institute of Geology of the Czech Academy of Sciences (institutional support RVO 67985831 to L.L.). We also thank members of the Paleobiology Database community that entered data used in this study. This is Paleobiology Database publication no. 522. We thank Jean-Bernard Caron for images of the Naraoia bertiensis holotype. Finally, we thank David Rudkin, Julien Kimmig, and Petr Budil for comments on the manuscript that improved the scope of the work.