Non-technical Summary
Living disc-like hydrozoans such as the by-the-wind sailor and the blue button are often stranded in large numbers on beaches. These animals are delicate and rare as fossils as they require exceptional conditions for preservation. Here we describe a new example from the Bertie Group of upper New York State and Ontario (~420 Ma), which is famous for the remarkably preserved sea scorpions (eurypterids) that occur there. The new hydrozoan, which gets up to 17 cm across and is the second-oldest example known, adds an important new member to this assemblage. It floated on the surface of the ocean and was occasionally incorporated into layers rich in eurypterids and other fossils.
Introduction
The late Silurian (Pridoli) Bertie Group of Ontario, Canada, and upper New York State (Ciurca and Hamell, Reference Ciurca and Hamell1994) has long been known for its well-preserved and diverse fauna of eurypterids (Nudds and Selden, Reference Nudds and Selden2008; Briggs and Roach, Reference Briggs and Roach2020). The Bertie Group Lagerstätten also yield examples of other taxa (Nudds and Selden, Reference Nudds and Selden2008), including bivalve mollusks, gastropods and nautiloids, brachiopods, xiphosurans, scorpions, phyllocarids, an acanthodian (Burrow and Rudkin, Reference Burrow and Rudkin2014), and the plant Cooksonia (Edwards et al., Reference Edwards, Banks, Ciurca and Laub2004). Among additional rare fossils discovered in the Bertie Group by the late Samuel J. Ciurca, Jr. (Briggs and Roach, Reference Briggs and Roach2020), are six (possibly seven) specimens of a new disc-shaped taxon described here.
Disc-shaped fossils are often difficult to assign to a taxonomic group (Stanley, Reference Stanley, Hoffman and Nitecki1986; Lieberman et al., Reference Lieberman, Kurkewicz, Shinogle, Kimmig and MacGabhann2017; Landing et al., Reference Landing, Antcliffe, Geyer, Kouchinsky, Bowser and Andreas2018; MacGabhann et al., Reference MacGabhann, Schiffbauer, Hagadorn, Van Roy, Lynch, Morrison and Murray2019) as they normally represent soft-bodied organisms that have undergone modification during decay and fossilization. Most such fossils are assigned to one of three groups: cnidarian medusae (Young and Hagadorn, Reference Young and Hagadorn2010, Reference Young and Hagadorn2020), eldonids (MacGabhann et al., Reference MacGabhann, Schiffbauer, Hagadorn, Van Roy, Lynch, Morrison and Murray2019), or capitate hydrozoans (Fryer and Stanley, Reference Fryer and Stanley2004). Cnidarian medusae are usually preserved as molds and casts showing a distinct axial area and surrounding bell (Young and Hagadorn, Reference Young and Hagadorn2010, Reference Young and Hagadorn2020). They rarely occur in isolation and are not represented by a thick carbonaceous disc (Young and Hagadorn, Reference Young and Hagadorn2010). Eldonids are an extinct group ranging from the lower Cambrian to the Upper Devonian. They are characterized by a coiled sac at their center that is interpreted as a coelom surrounding the gut (MacGabhann et al., Reference MacGabhann, Schiffbauer, Hagadorn, Van Roy, Lynch, Morrison and Murray2019). The disc comprises internal lobes and radial fibers that bifurcate in a regular pattern (Lieberman et al., Reference Lieberman, Kurkewicz, Shinogle, Kimmig and MacGabhann2017; MacGabhann et al., Reference MacGabhann, Schiffbauer, Hagadorn, Van Roy, Lynch, Morrison and Murray2019). The nature of their preservation indicates that the disc was composed of decay-resistant carbonaceous material (MacGabhann et al., Reference MacGabhann, Schiffbauer, Hagadorn, Van Roy, Lynch, Morrison and Murray2019).
A diversity of rare disc-shaped fossils has been assigned to porpitoid hydrozoans. These fossils were placed historically within the suborder Chondrophorae of the order Siphonophorae (Caster, Reference Caster1942) and referred to as chondrophorines. Garstang (Reference Garstang1946) attempted to resolve the nature and interrelationships of the Siphonophorae, arguing that there are distinct differences between them and Chondrophorae, which led to the elevation of Chondrophorae to a separate order (Totton, Reference Totton1954). More recently, Chondrophorae has been synonymized with order Anthoathecata, and porpitoids have been assigned to suborder Capitata (Schuchert, Reference Schuchert2022). The extant families of Capitata are Velellidae, represented by the sail-bearing genus Velella (by-the-wind sailor), and Porpitidae such as Porpita (blue button). The main features of the Porpitidae include a chitinous float or pneumatophore surrounded by soft tissue that overlies the mouth and downward-directed tentacles (Mackie, Reference Mackie1959; Yochelson et al., Reference Yochelson, Stürmer and Stanley1983; Fryer and Stanley, Reference Fryer and Stanley2004).
Depositional environment
The Bertie Group was deposited along the north rim of the Appalachian Foreland Basin of Laurentia and on the paleosouthern side of the subsiding Algonquin Arch (Burrow and Rudkin, Reference Burrow and Rudkin2014). It crops out over 300 km from southern Ontario to the Herkimer County region of New York State (Edwards et al., Reference Edwards, Banks, Ciurca and Laub2004). During the late Silurian, massive evaporite deposits formed in the center of the basin due to the low paleolatitude, warm arid climate, and reduced connection to the open ocean (Vrazo et al., Reference Vrazo, Brett and Ciurca2016, Reference Vrazo, Brett and Ciurca2017). The Bertie Group was deposited in a near-shore ramp environment separate from these massive evaporite deposits and consists of calcareous shales and chemically precipitated dolomites (waterlimes) that originated in the subtidal and intertidal zone of a shallow lagoonal setting (Burrow and Rudkin, Reference Burrow and Rudkin2014; Vrazo et al., Reference Vrazo, Brett and Ciurca2017).
The disc-shaped fossils described here occur in fine-grained, wavy-bedded, light gray dolostone. Associated thin evaporite layers (gypsum beds) and salt hoppers suggest that the marine environment was frequently hypersaline (Nudds and Selden, Reference Nudds and Selden2008). However, although the salinity was elevated at times, Vrazo et al. (Reference Vrazo, Brett and Ciurca2016) concluded that the eurypterids and associated animals inhabited a shallow subtidal normal marine environment following a transgression. A subsequent regression resulted in hypersaline and increasingly dysoxic conditions leading to desiccation cracks and supersaturated porewater promoting the nucleation of salt hoppers around organic material in the sediment. Thus the Bertie assemblage of eurypterids and associated fossils, including the new taxon described here, likely inhabited a near normal marine setting.
Taphonomy
The specimens are dark compression fossils preserved parallel to the bedding surface (Figs. 1.1, 1.2, 2.1, 2.3). Irregularly spaced concentric ridges around the periphery of the disc are a result of flattened convexity. Where the specimen is flattened at a slight inclination to bedding, these ridges are concentrated on one side (the right side of YPM IP 250362, for example, Fig. 1.1, 1.2), but the preserved specimens depart very little from bilateral symmetry and there is no evidence that outlines are significantly distorted. Energy-dispersive X-ray spectrometer (EDS) analysis with a scanning electron microscope showed that the dark color of the specimens is due to elevated levels of carbon (Fig. 3.1), which is also indicated by a striking contrast between specimen and matrix in back-scattered electron images. There is no evidence that the discs are biomineralized, and they are assumed to have been composed predominantly of chitin as are the floats of extant porpitoids (Bondyale-Juez et al., Reference Bondyale-Juez, Romero-Kutzner, Purcell, Martínez, Packard and Gómez2022). Where the dolomitic matrix is present on and around the margin of the specimens, it contains much less carbon and higher concentrations (wt%) of Ca, Mg, and O (Fig. 3.2). The presence of Si, Al, and low percentages of other minerals may indicate traces of clays and authigenic minerals that formed during diagenesis (Fig. 3). Analyses of eurypterid cuticle, presumably also originally chitinous, from the same locality yielded long-chain aliphatic components similar to type II kerogen as a result of diagenetic in situ polymerization (Gupta et al., Reference Gupta, Tetlie, Briggs and Pancost2007). The discs are darker in color than the eurypterids and have undoubtedly undergone a similar transformation.
Figure 1. Bertratis ciurcae n. gen. n. sp. YPM IP 250362, holotype. (1, 2) Image (1) and interpretive drawing (2) showing lineations (the oblique straight white line on the left side is a scratch that was present when the YPM received the specimen). (3) Enlargement of area arrowed “y” in (2) showing layers of radial structures. (4) Enlargement of area arrowed “b” in (2) showing beads on radial structures.
Figure 2. Bertratis ciurcae n. gen. n. sp. (1, 2) YPM IP 209925, image (1) and interpretive drawing (2) showing lineations (arrow indicates thin surface cuticle analyzed in Fig. 3.1). (3, 4) YPM IP 546800, image (3) and interpretive drawing (4) showing lineations (g? = possible gonozooids; white areas are paint splashes).
Figure 3. Bertratis ciurcae n. gen. n. sp., YPM IP 209925, EDS analyses. (1) Thin surface cuticle arrowed in Figure 2.2. (2) Matrix at edge of specimen.
Rapid burial of the Bertie Group eurypterids is attributed to sediment disruption during storms and/or wave action (Vrazo et al., Reference Vrazo, Brett and Ciurca2017), agents that could have transported the disc-shaped organisms inshore and buried them. Only the decay-resistant internal float (pneumatophore) is well preserved; other morphological features would have degraded faster (Yochelson et al., Reference Yochelson, Stürmer and Stanley1983). Way up information is available for YPM IP 546800 only. It is preserved on the base of a slab providing a concave ventral view; thus, it was deposited convex upward in presumed life attitude. The lack of evidence of mass strandings, which frequently affect extant porpitids, suggests that the disc-shaped organisms were rare in the depositional environment. Their taphonomic pathway may have been promoted by sea-level transgression events as inferred for the eurypterids (Vrazo et al., Reference Vrazo, Brett and Ciurca2016, Reference Vrazo, Brett and Ciurca2017).
Methods
Specimens were photographed with a Canon EOS 60D and analyzed using a Hitachi SU7000 scanning electron microscope (SEM). Elemental analyses were carried out with an energy-dispersive X-ray spectrometer made by Oxford Instruments (Utilm Max-100 EDS system). The SEM was operated at acceleration voltage 15KV at variable pressure mode at 50 Pascals.
Repository and institutional abbreviation
All specimens examined in this study are deposited in the Invertebrate Paleontology Division of the Yale Peabody Museum (YPM IP).
Systematic paleontology
Phylum Cnidaria Verrill, Reference Verrill1865
Class Hydrozoa Owen, Reference Owen1843
Subclass Hydroidolina Collins, Reference Collins2000
Order Anthoathecata Cornelius, Reference Cornelius1992
Suborder Capitata Kuhn, Reference Kuhn1913
Superfamily Porpitoidea Goldfuss, Reference Goldfuss1818
Remarks
Molecular data do not support a monophyletic Anthoathecata (Collins et al., Reference Collins, Schuchert, Marques, Jankowski, Medina and Schierwater2006; Daley et al., Reference Daly, Brugler, Cartwright, Collins and Dawson2007). Capitata is a paraphyletic assemblage of Aplanulata, but there is limited availability of molecular data. The family Porpitidae, which Collins et al. (Reference Collins, Winkelmann, Hadrys and Schierwater2005) recovered as monophyletic, comprises two living genera, Velella and Porpita. Living porpitids are distinguished by a colonial pelagic stage, but it may be a modified individual polyp rather than a colony (Daly et al., Reference Daly, Brugler, Cartwright, Collins and Dawson2007). The float has the greatest potential for fossilization but provides limited morphological characters (Fryer and Stanley, Reference Fryer and Stanley2004).
Genus Bertratis new genus
Type species
Bertratis ciurcae new genus, new species.
Diagnosis
As for type species, by monotypy.
Occurrence
Silurian (Pridoli) Bertie Group. Member A, Williamsville Formation, Ridgemount Quarry South about 8 km west of Fort Erie, Ontario, with the exception of YPM IP 255247, which is from the Phelps Member, Fiddlers Green Formation, Lang (Langheinrich) Quarry, 0.5 km south of Brewer Road, ~1 km southeast of Elizabethtown, Herkimer County, New York State. The locality of YPM IP 250663 is unknown.
Etymology
Bertie, referring to the stratigraphic source of the specimens, and ratis (Latin, feminine) meaning float or raft.
Remarks
Previous reliable records of Paleozoic porpitoids (Table 1) are late Silurian (Ludlovian Series) to Early Devonian in age (Landing et al., Reference Landing, Antcliffe, Geyer, Kouchinsky, Bowser and Andreas2018; Giribet and Edgecombe, Reference Giribet and Edgecombe2020) represented by the Velella-like Plectodiscus discoideus (Rauff, Reference Rauff1939) and the Porpita-like Pseudodiscophyllum Fryer and Stanley, Reference Fryer and Stanley2004. All fossil Velella-like forms were synonymized with Plectodiscus Ruedemann (Reference Ruedemann1916) by Yochelson et al. (Reference Yochelson, Stürmer and Stanley1983) in their redescription of pyritized material from the Devonian Hunsrück Slate, which yields the most complete examples, preserving even the tentacles. The float of Plectodiscus is characterized by pronounced overlapping concentric chambers interpreted as pneumatocysts (Yochelson et al., Reference Yochelson, Stürmer and Stanley1983). The float of the Porpita-like Pseudodiscophyllum, by contrast, is dominated by a regular series of pronounced radial ribs, which are beaded near the center of the disc and bifurcate near the margin at the outer of just two circular ribs (Fryer and Stanley, Reference Fryer and Stanley2004). The float of the new Bertie Group taxon, like Pseudodiscophyllum, lacks well-defined concentric chambers, but it consists of layers of closely spaced fine radial structures that show traces of beading in one place on YPM IP 250362. Bertratis differs from Pseudodiscophyllum in lacking a circular outline and regularly arranged ribs, necessitating the erection of a new genus and species to accommodate it.
Table 1. Paleozoic genera of probable porpitoids best represented in the fossil record.
Holotype
YPM IP 250362.
Diagnosis
Subcircular float, bilaterally symmetrical, narrowing slightly to presumed anterior margin, layers of closely spaced radiating structures, faint concentric features, no obvious sail or similar.
Description
The float (pneumatophore) is subcircular in outline and bilaterally symmetrical (Figs. 1.1, 1.2, 2), allowing the anterior–posterior axis to be identified. For the purposes of description, we designate the more convex margin as anterior. The maximum width lies just over half the distance from front to rear. The posterior margin is slightly convex to nearly straight, showing a shallow indentation in some specimens (Fig. 1.1, 1.2). The four specimens sufficiently well preserved to be measured range in dimensions from 42 × 51 mm (Fig. 2.3, 2.4) to 140 × 172 mm (Fig. 1.1, 1.2). There is no evidence of allometry (Fig. 4). The float is flattened, and its thickness cannot be determined. Specimens show some relief in the central region, which was gently convex whereas the margins were more inclined (Figs. 1.1, 2.1, 2.3).
Figure 4. Bertratis ciurcae n. gen. n. sp., plot of dimensions of four measurable specimens (YPM IP 209925, 250362, 255247, and 546800) revealing strong correlation and absence of allometry.
The float was made up of several thin layers although these rarely separate cleanly where the part and counterpart separate. Three morphological features are evident on the better-preserved specimens. Most obvious are fine radial structures that are present on some part of most specimens (Figs. 1.1, 1.2, 2). They vary in their degree of resolution and spacing but are less than 1 mm wide. They are most pronounced toward the periphery, where they form several layers (Fig. 1.3), and they do not appear to extend to the center (Figs. 1.1, 1.2, 2). A smooth featureless layer underlies the radial lines in the central area of YPM IP 250362, and they are overlain in places by a rougher layer, which may represent adhering sediment. YPM IP 209925 (Fig. 2.1, 2.2) preserves patches of thin cuticle above the radial structures, which may represent the outer cuticle of the float. Concentric lines are more rarely evident than radial ones, but where they are present they appear to be at a similar level (Fig. 1.1, 1.2). They too are generally spaced less than 1 mm apart. In some cases, elongate bead-like structures up to 1 mm long are aligned with the radial lines (left anterior area of YPM IP 250362: Fig. 1.1, 1.2, 1.4). YPM IP 546800 is unusual in preserving a cluster of bulbous structures (center left of the specimen, which was buried convex upward, as viewed from the concave ventral side; Fig. 2.3, 2.4), some up to 1 mm wide. These are reminiscent, in their position and variable orientation, of gonozooids. However, these bulbous structures are preserved at the level of the pneumatophore, so could represent gonozooids only if there were some displacement during flattening of the specimen. There is no clear evidence of a sail.
Etymology
After the discoverer Samuel J. Ciurca, Jr.
Materials
YPM IP 250362 holotype, YPM IP 209925, 255247, 255284, 255285, and 546800 and possibly 250663. All specimens were collected by Samuel Ciurca except for YPM IP 255247 and 250663, which he purchased on eBay.
Remarks
We assign Bertratis ciurcae to the Porpitoidea but do not place it in a family. The absence of information on the life cycle and lack of preserved morphology other than the float prevent a confident placement in the living family Porpitidae.
Discussion and conclusions
Waggoner and Collins (Reference Waggoner and Collins2004) argued that the occurrence of Paleozoic, as well as Neoproterozoic, porpitids (chondrophorines) is improbable because their use in calibrating molecular clock estimates placed the origin of Cnidaria at more than 1,500 Ma. Phylogenomics of Cnidaria have become more refined (e.g., Park et al., Reference Park, Hwang, Lee, Song, Seo and Won2012; Kayal et al., Reference Kayal, Bentlage, Sabrina Pankey, Ohdera, Medina, Plachetzki, Collins and Ryan2018) and Landing et al. (Reference Landing, Antcliffe, Geyer, Kouchinsky, Bowser and Andreas2018), in their review of the early evolution of colonial animals, concluded that the oldest undoubted examples of porpitoids are Silurian. Bertratis adds a third genus to the list of Paleozoic porpitoids (Table 1). The other Silurian example (Fryer and Stanley, Reference Fryer and Stanley2004) is older.
Bertratis reaches a maximum dimension of just over 17 cm, making it the largest Paleozoic capitate float known and suggests that these hydrozoans were important consumers of zooplankton. Schuchert (Reference Schuchert2010, p. 480) noted that living Porpita reaches diameters of 5 cm whereas Velella reaches sizes of up to 10 × 5 cm. Thus the largest specimen of Bertratis is notably larger than living porpitids, but its maximum dimension is less than twice that of the largest Velella. The occurrence of Bertratis in the Bertie Group Lagerstätten reflects the need for exceptional conditions to facilitate preservation. The Bertie Group represents nearshore marine settings with a restricted fossil assemblage dominated by eurypterids, which may have congregated to molt. Open marine taxa are uncommon but include nautiloids. The discovery of Bertratis ciurcae adds a rare taxon to the Bertie Group biota. Its thick organic float indicates a preservation potential at least as high as that of eurypterid cuticle. The range of size and low number of specimens show that it was not aggregated by wind or currents in this case. It was rare in this depositional setting and may have lived on the surface of the open ocean like modern porpitids.
Acknowledgments
We are grateful to S. Butts and J. Utrup (Yale Peabody Museum) for collections support. J. Utrup identified a number of specimens while unpacking material from the Ciurca bequest. She took the photographs, and Z. Jiang assisted with scanning electron microscopy and EDS analysis. B. Roach prepared a specimen. The final version benefited from insightful comments from G. Young and an anonymous reviewer.
Declaration of competing interests
The authors declare that they have no competing interests.
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