Non-technical Summary
Movement of ophiuroids (brittle stars) on the substrate is recognized through arm and disk imprints and burrows. In the rock record, these features are recognized by specific physical attributes of trace fossils. To date, arm imprints were reported only from the Carboniferous through Cretaceous with possible occurrences in the Upper Ordovician and Upper Devonian. Burrows with facetted margins similar in shape to disks have been reported only from the Carboniferous. The present study describes trace fossils of both forms in Lower Devonian strata from the northern Appalachians in western Newfoundland, Canada. The trace fossils occur in what was a physically stressed, tide-influenced, and likely marginal marine coastal setting subject to constant movement of sand and mud across the seafloor. Arm imprints, formed through compaction of a muddy substrate beneath the load of an ophiuroid arm, capture imprints of tube feet (that are part of the water vascular system of an ophiuroid) and parts of the underlying skeletal structure. The degree of preserved detail is a function of the depth of the impression and the rate of horizontal movement of the ophiuroid arm; most of the imprints in the Lower Devonian material are rich in detail demonstrating little horizontal movement. A few curved imprints fit with quantified flexibility of modern ophiuroids. Partially facetted burrows are distributed among the arm imprints and reflect feeding, hiding, or nestling activity by ophiuroids wherein only the disk has been buried. The study reinforces the ecological breadth of ophiuroids, in this case occurring within a physically stressed marginal marine setting.
Introduction
A fossil record of surface imprints and burrows is associated with surface to near-surface activity of asterozoans (asteroids, ophiuroids, somasteroids). Classic sea-star-shaped imprints (e.g., Asteriacites von Schlotheim, Reference von Schlotheim1820; see Seilacher, Reference Seilacher1953 and Knaust and Neumann, Reference Knaust and Neumann2016) have a post-Cambrian ichnological record that matches the related skeletal-fossil record (Spencer and Wright, Reference Spencer, Wright and Moore1966; Häntzshel, Reference Häntzshel and Teichert1975). These ichnofossils have been grouped within the ichnofamily Asteriacitidae (Knaust and Neumann, Reference Knaust and Neumann2016). A second kind of imprint includes strap-like geometries that are narrow, locally sinuous to arcuate to hook-shaped and are represented by three ichnogenera: Biformites Linck, Reference Linck1949; Arcichnus Sutcliffe, Reference Sutcliffe1997; and Harpichnus Vallon et al., Reference Vallon, Schweigert, Bromley, Röper and Ebert2015. These ichnogenera have been interpreted to reflect impressions created through movement of ophiuroid arms (Schlirf, Reference Schlirf2012) although Morton et al. (Reference Morton, Myers, Gingras and Zonneveld2023) have recently questioned the ophiuroid affinity of Biformites. Compared with the post-Cambrian ophiuroid skeletal record (Spencer and Wright, Reference Spencer, Wright and Moore1966), the ichnofossil record for this group, placed within the ichnofamily Biformitidae (Knaust and Neumann, Reference Knaust and Neumann2016), appears concentrated within Upper Mississippian through Lower Cretaceous rocks (Linck, Reference Linck1949; Boyer, Reference Boyer1979; Pickerill and Forbes, Reference Pickerill and Forbes1979; Yang and Song, Reference Yang and Song1985 cited by Buckman et al., Reference Buckman, Doughty, Benton and Jeram1998; Yang et al., Reference Yang, Zhang and Yang2004; Schlirf, Reference Schlirf2012; Knaust et al., Reference Knaust, Warchol and Kane2014; Knaust and Neumann, Reference Knaust and Neumann2016; Singh et al., Reference Singh, Bhargava, Mikuláš, Prasad, Singla and Kaur2017; Feng et al., Reference Feng, Chen, Benton, Wu, Bottjer and Thompson2019). However, similar forms have been reported from Upper Ordovician and Upper Devonian strata (Pickerill and Forbes, Reference Pickerill and Forbes1979; McDowell et al., Reference McDowell, Avary, Matchen and Britton2007).
Ophiuroid burrows form a second product of surface to near-surface activity associated with dwelling, resting, hiding, or searching for food (Fell, Reference Fell and Boolootian1966b; Frey et al., Reference Frey, Howard and Hong1987; Broach et al., Reference Broach, Miller and Bowser2016; Buatois et al., Reference Buatois, Wisshak, Wilson and Mángano2017). Ophiuroid burrows have not been described ichnotaxonomically apart from the pentamerally facetted Pentichnus pratti Maerz, Kaelser, and Hakes, Reference Maerz, Kaesler and Hakes1976 reported only from the Carboniferous (Maerz et al., Reference Maerz, Kaesler and Hakes1976; Seilacher, Reference Seilacher1983, Reference Seilacher and Said1990; Rindsberg, Reference Rindsberg1994; Mángano et al., Reference Mángano, Buatois, West and Maples2002).
As surface movement and burrowing are recognized features of modern ophiuroids (Broach et al., Reference Broach, Miller and Bowser2016), gaps in the post-Cambrian ichnofossil record may be examples of spatiotemporal sampling bias (Butler et al., Reference Butler, Brusatte, Andres and Benson2012). This is supported here by description of Biformites insolitus Linck, Reference Linck1949 and partially facetted burrows that occur together within a Lower Devonian (Lochkovian) succession of the Clam Bank Formation, western Newfoundland (Fig. 1). The host siliciclastic lithofacies represents a peritidal paleoenvironment (Burden et al., Reference Burden, Quinn, Nowlan and Bailey-Nill2002; Dix et al., Reference Dix, Pignotta and White2023). Differential depths of imprints of B. insolitus create a range of fine-scale ornamentation enabling a comparison with skeletal and tube-feet structure of Paleozoic ophiuroids (Glass and Blake, Reference Glass and Blake2004; Glass, Reference Glass2006; Hotchkiss and Glass, Reference Hotchkiss and Glass2012; Clark et al., Reference Clark, Bhullar, Darroch and Briggs2017, Reference Clark, Hutchinson and Briggs2020; Carter et al., Reference Carter, Sutton, Briggs, Rahman, Siveter and Siveter2021). Incompletely facetted near-surface burrows, some of which exhibit possible higher order (6- to 8-fold) symmetry, are distributed among B. insolitus. A comparison with the holotype of Pentichnus pratti (Maerz et al., Reference Maerz, Kaesler and Hakes1976) and other examples of this ichnogenus (Rindsberg, Reference Rindsberg1994; Mángano et al., Reference Mángano, Buatois, West and Maples2002; Buatois et al., Reference Buatois, Wisshak, Wilson and Mángano2017) is used to address whether non-pentameral symmetry might broaden the ichnotaxobase of facetted burrows. Field and aquaria examples of ophiuroid burrow cross sections (e.g., Howard, Reference Howard1968; Howard and Frey, Reference Howard and Frey1975; Christensen and Colacino, Reference Christensen and Colocino2000; Gingras et al., Reference Gingras, Dashtgard, MacEachern and Pemberton2008) do not obviously support production of a facetted burrow margin, but I present examples of modern surface-restricted burrowing that underscores a likely broader spectrum of near-surface burrow habits.
Study locality and selected bedrock geology of the Port au Port Peninsula, western Newfoundland. A general lithostratigraphic section of the upper Silurian (?)–Lower Devonian Clam Bank Formation references the section northeast of Salmon Cove. Lithostratigraphic marker beds or contacts, A–D, represent prominent changes in base level. The parts of the section without a black outline refer to covered intervals with interpreted lithology. Locations of talus blocks bearing Biformites insolitus and facetted burrows are indicated with the following reference databases: CB22–48.63178°N, 59.07085°W; CB23–48.63329°N, 59.06676°W. Label abbreviations: C—MOrd, Cambrian to Middle Ordovician; UOrd, Upper Ordovician; RHT, Round Head Thrust; TCT, Tea Cove Thrust; particle grain sizes: M, mud; S, sand (f, fine; m, medium; c, coarse); Cg, gravel; and rock types: mudst, mudstone; sltst, siltstone; ss, sandstone.
A Lower Devonian association of Biformites insolitus and burrows partially fills a gap in the current range of the related ichnological fossil records. The occurrence reinforces well-documented behavior of ophiuroids to exploit and colonize stressed (physical, salinity) paleoenvironments that are just part of an ecological breadth known from modern and fossil records (Binyon, Reference Binyon and Boolootizn1966; Turner and Meyer, Reference Turner and Meyer1980; Pagett, Reference Pagett, Jones and Wolff1981; Mángano et al., Reference Mángano, Buatois, West and Maples1999, Reference Mángano, Buatois, West and Maples2002; Mángano and Buatois, Reference Mángano, Buatois and McIlroy2004; Martínez, Reference Martínez2008; Boos et al., Reference Boos, Gutow, Mundry and Franke2010; Salamon et al., Reference Salamon, Niedzwiedzki, Lach, Brachaniec and Gorzelak2012; Kuk-Dzul et al., Reference Kuk-Dzul, Solís-Marín, Herrera-Dorantes and Ardisson2019).
Geological setting
The Clam Bank Formation forms an ~600 m thick succession of steeply overturned strata exposed along the northwest coast of the Port au Port Peninsula, western Newfoundland (Fig. 1). The unit represents the most northern Devonian foreland-basin succession within the Appalachian orogen (Ziegler, Reference Ziegler, McMillan, Embry and Glass1988; Bourque et al., Reference Bourque, Malo and Kirkwood2000; Ettensohn, Reference Ettensohn and Miall2008; White et al., Reference White, Waldron and Harris2020).
Exposed strata represent the lower half of an ~1.3 km thick stratigraphic succession of which the upper half lies offshore dipping gently seaward beneath Lower Devonian (Emsian) strata of the Red Island Road Formation that underlies Red Island (Fig. 1; Quinn et al., Reference Quinn, Bashforth, Burden, Gillespie, Springer and Williams2004; White et al., Reference White, Waldron and Harris2020). The lower boundary of the Clam Bank Formation lies inland and covered, where it structurally abuts the Lourdes thrust sheet (Lacombe et al., Reference Lacombe, Waldron and Williams2020) to the south. The Lourdes thrust sheet is an amalgam of allochthonous Cambrian to Middle Ordovician slope strata arising from west-vergent thrusting during the Acadian orogeny in the Late Devonian (Lacombe et al., Reference Lacombe, Waldron and Williams2020). The Clam Bank Formation is downfaulted against platform carbonates of the Upper Ordovician Long Point Group to the east (Fig. 1; Quinn et al., Reference Quinn, Harper, Williams and Clarkson1999; Dix and Burden, Reference Dix and Burden2018).
A longstanding late Silurian (Přídolían) age associated with the Clam Bank Formation (Boucot, Reference Boucot and Kay1969; Berry and Boucot, Reference Berry and Boucot1970; O’Brien, Reference O’Brien1973; Wright et al., Reference Wright, Quinn, Briggs and Williams1995) is now restricted to the lower tens of meters of the formation (Fig. 1; Burden et al., Reference Burden, Quinn, Nowlan and Bailey-Nill2002; Dix et al., Reference Dix, Pignotta and White2023). A palynomorph assemblage recovered from calcareous siliciclastics comprising part of the lower half of the formation northeast of Salmon Cove (Fig. 1) contains miospores that first appear in the Lochkovian of the Early Devonian (Burden et al. Reference Burden, Quinn, Nowlan and Bailey-Nill2002; Edwards et al., Reference Edwards, Morris, Richardson and Kenrick2014). It is this interval that hosts the ichnofossils reported here. The upper half (~400 m) of the formation remains undated (Fig. 1) but is of Early Devonian age given that a coeval offshore succession extends beneath Emsian strata that underlie Red Island to the southwest (Fig. 1; Quinn et al., Reference Quinn, Bashforth, Burden, Gillespie, Springer and Williams2004; White et al., Reference White, Waldron and Harris2020).
The Clam Bank succession
Regional structure and coastal erosion have created two coastal sections, both young toward Red Head (Fig. 1). Section S1 extends southwest from Clam Bank Cove, and section S2 extends northeast of Salmon Cove, but only the latter provides a continuous section (Dix et al., Reference Dix, Pignotta and White2023). There are few bedding-plane exposures due to overturned bedding striking oblique to the shoreline, but there is an abundance of small to large talus slabs distributed along the upper beach that provide access to bedding surfaces.
The Clam Bank Formation displays an upsection change in facies associations reflecting a change from coastal zone paleoenvironments to an alluvial–plain succession with synsedimentary calcrete (Fig. 1; Wright et al., Reference Wright, Quinn, Briggs and Williams1995; Burden et al., Reference Burden, Quinn, Nowlan and Bailey-Nill2002; Dix et al., Reference Dix, Pignotta and White2023). Four lithostratigraphic transitions (A–D, Fig. 1) demarcate prominent changes in base level (Dix et al., Reference Dix, Pignotta and White2023): A, base-level rise with shoreface transgression across coastal-plain facies; B, base-level fall producing a mid-formation disconformity overlain by aeolian facies; C, base-level rise reflected by a possible marine ravinement with limestone breccia; and D, base-level fall producing a gradational change to stacked alluvial channels host to synsedimentary calcrete.
In contrast to very fossiliferous limestone in section S1 near Red Point, skeletal remains in section S2 are uncommon and form thin lenses restricted to intervals of calcareous siliciclastic and lime mudstone between markers A and B in the lower part of the formation (Fig. 1). Trace fossils are locally abundant in these intervals and within the upper (siliciclastic) half of the formation but have remained unstudied apart from reports of Diplichnites isp. in strata positioned a few meters below horizon D on either side of Red Head (Fig. 1; Wright et al., Reference Wright, Quinn, Briggs and Williams1995; Williams et al., Reference Williams, Nowlan and Boyce2001).
The ichnofossil Biformites insolitus Linck, Reference Linck1949 and facetted burrows described here were found on two talus slabs separated by a distance of ~200 m within the upper intertidal zone between markers A and B (Fig. 1). The composition and depositional fabric of these slabs are similar as that of calcareous, mostly siliciclastic, successions (1–30 m in thickness) of steeply overturned bedrock exposed in places within the intertidal zone between these two markers. The outcrops extend through beachface to a continuous rock escarpment forming the supratidal limit of the narrow high-energy beach.
The calcareous bedrock consists of thinly interstratified, very fine- to medium-grained, calcareous sandstone and calcareous siltstone (Fig. 2.1), minor lime mudstone, and rare lenses to laminae of skeletal packstone to grainstone (Figs. 1, 2.2, 2.3). These bedrock intervals are intercalated with non-calcareous successions (Fig. 1) of unfossiliferous fine-grained siliciclastics (arenite, siltstone) in which there is evidence for local paleosol development, dewatering structures, and mudcracks.
Lithostratigraphic elements of calcareous siltstone and fine-grained calcareous arenite host to trace fossils. (1) Steeply overturned bedding (stratigraphic top to the left) exposing typical finely interstratified calcareous lithofacies; chisel length is 15 cm. (2) Irregularly interstratified siltstone (dark) and discontinuous beds, laminae, and lenses of fine-grained arenite (light). Highlighted are: erosional (stranded?) current-ripple lamination (black arrow), wave-rippled laminae (white arrow), and a synsedimentary fissure (f). (3) Rhythmically interstratified siltstone (dark) and fine-grained arenite (light) overlying (black arrow) amalgamated lenses of fine-grained arenite that disconformably overlie a vertically burrowed siltstone (white arrow) with burrows filled by the overlying arenite. Scale bars = 2.5 cm.
In detail, the depositional fabric of the calcareous successions includes an irregular to rhythmic interstratification of arenite and siltstone consisting of erosionally bound laminae to thin beds (often discontinuous over several meters) and lenses (Fig. 2.2). Local medium-thick amalgamated sandstone beds are present (Fig. 2.3). Sedimentary bedforms include ripple-laminated lenticular bedding, isolated erosional ripples, current or combined flow waveforms (Fig. 2.2), shallow (<30 cm) channelization filled with thinly stratified calcareous siliciclastics, erosional surfaces (often capping bedforms), and local desiccation features. Locally abundant synsedimentary deformation features include ball-and-pillow structures, sedimentary boudinage, and fissures (Fig. 2.2). Rare body fossils of small-spired gastropods, ostracodes, and thin-shelled tiny (millimeter scale) brachiopods are associated with grainstone and packstone deposits. Crinoid ossicles are very rare and, when present, are tiny (~75 microns) and form thin sediment clusters. The most common trace fossil type visible in the calcareous bedrock is a small-diameter vertical burrow filled with fine-grained sandstone (Fig. 2.3). However, talus slabs reveal bedding planes with a more diverse, yet to be documented, infaunal trace-fossil suite.
The association of interstratified silt- and sand-sized sediment, paleoerosional and paleoexposure surfaces, and local channelization suggest that the calcareous successions characterize deposition within coastal deltaic and estuarine tidal paleoenvironments (Dalrymple, Reference Dalrymple, James and Dalrymple2010; Dix et al., Reference Dix, Pignotta and White2023). The limited skeletal fossil content suggests a euryhaline peritidal setting, one also strongly influenced by movement of siliciclastic sediment. Synsedimentary deformation allied with these calcareous successions may indicate orders of base-level variation linked to periods of seismicity.
Materials and methods
This study incorporates two fossil datasets arising from coastal impact of extratropical (Category 4) Hurricane Fiona in eastern Canada in September 2022 (Pasch et al., Reference Pasch, Reinhart and Alaka2023). During fieldwork in July 2022, I discovered a large talus block in the upper intertidal zone with abundant ichnofossil traces described and illustrated here as dataset CB22. The block size and weight precluded removal, and splitting the rock was not possible with tools at hand. On returning to the field in July 2023, I discovered that the talus block had been removed through coastal erosion related to the hurricane, part of a considerable volume of material transported in and out of the depositional zone. A new talus block with similar ichnofossil traces was discovered, again in the upper intertidal zone, and four bedding-plane slabs (NFM 3989–3892) represent dataset CB23.
Field evaluation
Ichnofossils of datasets CB22 and CB23 occur across bedding soles of large (0.6 × 0.6 m2 and 0.8 × 0.8 m2, respectively) talus slabs of finely interlaminated calcareous siltstone and very fine-grained calcareous quartz arenite. In both cases, outcrop sources were not identified, but nearby sources are inferred because the blocks display a similar lithofabric as the calcareous siliciclastic succession comprising bedrock exposures within tens of meters to the southwest and northeast of each block along the beachface and similar bedrock that underlies the escarpment topographically above and behind the beachface. For dataset CB23, the block was split near its edges to recover fragments of fossil-bearing surfaces. Related to both datasets, ichnofossils were examined and described in the field with the aid of a hand lens, with capture of high-resolution digital (4,608 × 3,456 pixels) images using a Nikon Coolpix AW100 in macro-mode with a minimum lens working distance of ~2 cm.
Laboratory analysis
Additional observations of material from dataset CB23 were made with a Motic binocular microscope. Images of both datasets were imported into Adobe Photoshop Elements with grayscale transformation and modification of lighting and sharpening.
Repositories and institutional abbreviations
Material (NFM F-3989 to –3992) composing dataset CB23 is reposited in the Natural History Invertebrate Paleontology Collections at The Rooms Corporation, St. John’s, Newfoundland, Canada. The photo collection representing dataset CB22 is presented in this study.
Systematic paleontology
Ichnofamily Biformitidae Knaust and Neumann, Reference Knaust and Neumann2016
Ichnogenus Biformites Linck, Reference Linck1949
Type species
Biformites insolitus Linck Reference Linck1949 from the Upper Triassic (Keuper) Stuttgart Formation or the Schilfsandstein (old name), southwest Germany.
Biformites insolitus Linck, Reference Linck1949
Incident-light photographs of rock slabs with Biformites insolitus, database CB23. (1) Distribution and variety of traces on slab NFM 3990: strap-like ridges (black arrows) with varying finer ornamentation of conical protuberances; a narrow ridge (left black arrow) displays a medial depression filled with siltstone; two narrow curved ridges (white arrows) taper away from a rounded burrow (a). The white box identifies the view of (3). Scale bar = 1 cm. (2) Strap-like ridge (lower arrow) with medial depression (dark) filled with siltstone and an axial trend of paired conical protuberances (upper arrow) extending from beneath apparent branching of three strap-like unadorned ridges. Slab NFM 3989, scale bar = 1 cm. (3) Close-up view of area on slab NFM 3990 defined in (1) showing gradation of ornamentation along a strap-like ridge (lower arrow) with appearance (to the left) of paired conical protuberances. A second horizontally oriented axial trend (upper arrow) is defined by a row of paired conical protuberances. Scale bar = 5 mm. (4) Axial trend of ridge divided into blocky protuberances (black arrow) with developed conical protuberances appearing along the axial trend (white arrow). Slab NFM 3992, scale bar = 2.5 mm.
(1) Bedding sole of ichnofossil-rich talus block, database CB22. Convex hypichnia of paired protuberances define the axis of short strap-like imprints (Biformites insolitus), overprints of branching (a), and examples of ridges with vaguely defined protuberances (b). Vertical, incompletely facetted burrows (unlettered arrows) with one example (c) cutting the edge of a burrow, Palaeophycus (p). (2) Variation in abundance of paired conical, blocky to irregular protuberances of B. insolitus with evidence of medial grooves (a, b) along strap-like imprints. There are examples of apparent branching (a) and strong curvature of strap-like imprints (c). (3) Prominent ridge-like convex hypichnia (a, c) are either unadorned of additional ornamentation or display parts with vaguely defined protuberances (a). Hypichnia of lower relief are always associated with paired rounded conical to blocky and irregular protuberances (b). Apparent branching (b, c) reveals the role of hypichnial overprinting (b) best defined in the latter example by contrast in hypichnial relief. Scale bars = 10 mm.
(1) Details of paired rounded conical to blocky protuberances of Biformites insolitus, database CB22. (2) Protuberances showing isolated conical protuberance (a) versus transverse bar geometry (b). A medial groove (c) is locally developed according to the depth of imprint. (3) Paired to irregularly developed protuberances along parallel strap-like imprints. (4) Apparent branching and strong curvature of B. insolitus around a vertical incompletely facetted burrow (d). Examples of isolated (a) and transverse bar (b) protuberances along with a continuous ridge (c) arise through different depths of imprints. Scale bars = 5 mm.
Reference Linck1949 Biformites insolitus Linck, p. 44, fig. 1, pl. 4, figs. 1, 2
Holotype
Not defined; lectotype and paratypes are exposed on the same rock slab (SMNS 22010) illustrated by Schlirf (Reference Schlirf2012, figs. 8, l0B, E) from the upper Schilfsandstein, Stuttgart Formation, middle Keuper, Maulbronn, Baden-Warttemberg, southern Germany.
Diagnosis
Narrow, vermiform, slightly tapering, horizontal, straight to curved convex hypichnial ridges with delicate to robust, ideally bisymmetrically arranged, perpendicular elongate, or polygonal to round, positive or negative protuberances. Emended after Schlirf (Reference Schlirf2012).
Description
Bedding sole fragments composing dataset CB23 (NFM 3989–3992) illustrate segments of straight to gently arcuate strap-like convex hypichnia varying in width from 2 to 3 mm extending over lengths of 10 to 20 mm (Fig. 3); the widest segment is 5 mm; the longest is 45 mm. The segments are composed of very fine-grained arenite with ornamentation projecting through host siltstone. An axial (strap-like) expression of segments arises through two styles of ornamentation. Some segments display only a bilobate ridge with a developed to obscure medial depression filled with siltstone (Fig. 3.2). Some ridges lack any fine ornamentation or exhibit poorly resolved rounded protuberances distributed locally (Fig. 3.3). In some cases, an axial expression of a ridge is inferred from distribution and arrangement of conical, irregular, and blocky protuberances (Fig. 3.1 [a]). Most traces, however, illustrate a spectrum of variation from ridge with paired protuberances to an axial trend of paired protuberances (Fig. 3.2, 3.3). An irregular medium depression (Fig. 3.3) often bisects a ridge and conical protuberances. There are some ridges without ornamentation (Fig. 3.2). In this example (Fig. 3.2), three ridges appear to display branching, with a fourth segment emerging from beneath consisting of an axial expression of paired protuberances. Ridges without ornamentation typically express the greatest relief. In some areas, segments display parallel alignment (Fig. 3.1).
Images composing dataset CB22 illustrate a denser distribution of traces. Strap-like hypichnial segments of a fine-grained sandstone in siltstone form straight, sinuous, and curved geometries (Figs. 4, 5). Segments are mostly 2–3 mm wide and 4 to 7 cm long (Fig. 4.1), with the longest being ~12 cm (Fig. 4.3). Endmember ornamentation of hypichnia is characterized by ridge continuity superimposed by finer ornamentation. The more elevated hypichnial traces form the most continuous ridges with or without a medial groove (Fig. 4.2 [b], 4.3 [a]) filled with siltstone. As in database CB22, some ridges exhibit either no ornamentation or poorly resolved rounded protuberances distributed locally (4.2 [c]). Likewise, an axial expression of a ridge is sometimes inferred from distribution and arrangement of conical, irregular to blocky protuberances (Figs. 4.2 [b], 5). Apparent branching along some ridges (Fig. 4.2, 4.3) represents impression overprints of differing axial orientation (Figs. 4.1 [a], 4.3 [b], 5.4 [b]). In database CB22, B. insolitus appears randomly oriented across the bedding sole (Fig. 4.1) except for an area in which traces occur in parallel (Fig. 5.3). Two hypichnial ridges with moderate development of finer ornamentation exhibit strong curvatures (Figs. 4.2, 5.4) with in-plane bedding angles (Tomholt et al., Reference Tomholt, Friesen, Berdichevsky, Fernandes, Pierre, Wood and Weaver2020) of 8–10°C.
The abundance and character of finer ornamentation in both datasets vary according to the depth of the imprint as reflected in the differential elevation of the strap-like hypichnia. At a minimal depth of penetration, only conical (with rounded to flattened top) protuberances appear oriented in parallel and opposite, separated by a medial groove (Figs. 3.1, 3.2, 5.1), and seemingly isolated but paired protuberances occur scattered across the bedding sole (Figs. 3.1, 5.1). With an increased elevation of the hypichnion, there is improved definition of the medial groove. In dataset CB22, there is a gradation of protuberance expression from paired protuberances to blocky to irregular masses that are oriented transverse to the axis of the imprint (Fig. 5.2–5.4). Continued increase in the hypichnion relief results in development of a ridge, in places flat-topped, with protuberance and medial groove distribution poorly resolved or lacking (Fig. 5.2–5.4).
Remarks
The diagnosis is emended from Schlirf (Reference Schlirf2012) on the basis of this present study. Knaust and Neumann (Reference Knaust and Neumann2016) regrouped previously assigned sinuous convex hypichnia with little to no ornamentation—that is, Zhadaichnus zhadaensis Yang and Song, Reference Yang and Song1985 (see Yang et al., Reference Yang, Zhang and Yang2004) and Ophioichnus aysenensis (Bell, Reference Bell2004)—into Biformites, with B. zhadaensis (Yang and Song) as the senior synonym. These differ from the horseshoe-shaped trackway imprints of Archichnus isp. (Sutcliffe, Reference Sutcliffe1997). Pickerill and Forbes (Reference Pickerill and Forbes1979) conferred non-branching burrows in Upper Ordovician strata (Quebec, Canada) to Biformites on the basis of the presence of an interlocking mosaic of offset transverse protuberances. An association with burrowing had been suggested in other early work (Linck, Reference Linck1949; Schindewolf and Seilacher, Reference Schindewolf and Seilacher1955; Boyer, Reference Boyer1979), but re-examination of type material (Schlirf, Reference Schlirf2012) confirmed the ichnotaxon as a surface imprint. This is borne out by the present study, with various expressions of ornamentation in datasets CB22 and CB23 supporting passive fill of a surface depression. The Clam Bank imprints differ from those referred to Pteridichnites biseriatus Clarke and Swartz, Reference Clarke and Swartz1913, in which the hyporelief geometry consists of more irregular lobate to narrow lenticular shapes oriented transverse to the strap-like axis along with examples of offset protuberances (McDowell et al., Reference McDowell, Avary, Matchen and Britton2007; see their figs. 4c, 4d, 5c).
Both strongly ornamented and relatively unadorned ridges (with medial grooves) were regarded to characterize Biformites insolitus by Linck (Reference Linck1949; his fig. 1 and plate 4, figs. 1, 2), but only the ornamented ridges have been referred to in subsequent literature (Häntzshel, Reference Häntzshel and Teichert1975; Morton et al., Reference Morton, Myers, Gingras and Zonneveld2023). The Clam Bank material shows that the degree of ornamentation and presence of a medial groove change along the same imprint (Figs. 3–5). Apparent branching arises from arm overprints of differing axial orientations (Figs. 3.2, 3.3, 4.4), with evidence for imprint truncation lost with deeper imprints (Fig. 3.2, 3.3). These features reinforce the association with arm-impression overlap (Bell, Reference Bell2004; Schlirf, Reference Schlirf2012; Knaust and Neumann, Reference Knaust and Neumann2016; Feng et al., Reference Feng, Chen, Benton, Wu, Bottjer and Thompson2019).
Unnamed facetted burrows
Incident-light photographs (left) and interpretative drawings (right) of individual facetted vertical burrows. (1–4) Database CB22. (5) Slab NFM 3989 in database CB23. Drawings illustrate facet margins (solid lines), contiguous burrow outline (dashed line), overlying host siltstone (gray), and in (1), an apparent medial ridge (thick dashed line). For comparison, a transversely rounded burrow with related ridges is shown in Figure 3.1. (6) Box-and-whisker plot with a common mean (X) and median (horizontal line) value (~130°) for interfacial (interior) angles of facetted margins. Scale bars = 5 mm.
Description
Both Clam Bank datasets include small (5–7 mm diameter) vertical, shallow (<1 cm), and partially facetted burrows. These form shallow convex hypichnia composed of very fine-grained arenite in siltstone (Fig. 6), part of the host interlaminated siltstone and sandstone making up the block. Burrow depths appear restricted to the thickness of the siltstone, <1 cm. In transverse section, hypichnia are rounded to ovoid; in longitudinal section, they are rounded plugs as suggested by differentially weathered host siltstone across rounded hypichnial surfaces (Fig. 6.1, 6.2). Parts of burrow margins are facetted or form sharp facial perimeters (Fig. 6); incomplete preservation of faceting precludes definition of symmetry, but two examples suggest a six- to eightfold order (Fig. 6.2, 6.3). The mean and median values of interfacial angles are similar, ~130° (Fig. 6.6). In database CB22, one hypichnion exhibits a subordinate narrow ridge (Fig. 6.1) that would have been a groove along the bottom of the burrow. Two ridges of similar size but associated with a non-facetted burrow appear in dataset CB23 (Fig. 3.1 [a]). Burrows occur distributed among but do not truncate B. insolitus (Fig. 4.1) whereas a burrow cross-cuts Palaeophycus isp. (Fig. 4.1 [c]).
Remarks
Facetted forms of plug-shaped shallow vertical burrows (Knaust, Reference Knaust, Knaust and Bromley2012, fig. 2C) are referred to Pentichnus pratti Maerz, Kaelser, and Hakes, Reference Maerz, Kaesler and Hakes1976, with a distinctive transverse pentagonal symmetry (Fig. 7.1), and interpreted to document an asterozoan trace maker, likely an ophiuroid (Maerz et al., Reference Maerz, Kaesler and Hakes1976; Mángano et al., Reference Mángano, Buatois, West and Maples2002; Buatois et al., Reference Buatois, Wisshak, Wilson and Mángano2017). Rindsberg (Reference Rindsberg1994) interpreted very shallow forms of Pentichnus isp. in thinly laminated sediment (Fig. 7.2) as resting traces of stalkless (cladid) crinoids. The only other reports of Pentichnus pratti come from the Pennsylvanian Kanwaka Shale, Kansas, USA (Fig. 7.3, 7.4; Mángano et al., Reference Mángano, Buatois, West and Maples2002, Buatois et al., Reference Buatois, Wisshak, Wilson and Mángano2017). Mángano et al (Reference Mángano, Buatois, West and Maples2002) reassigned Asteriacites gugelhupf Seilacher, Reference Seilacher1983 to the ichnogenus Pentichnus due to a similar cross-sectional symmetry (Seilacher, Reference Seilacher and Said1990, his plate 32, fig. 3). P. gugelhupf represents a significantly deeper (10–15 cm) burrow (Seilacher, Reference Seilacher1983, Reference Seilacher and Said1990) than P. pratti.
Images (transverse views) of the ichnogenus Pentichnus pratti. (1) Rounded pentagonal shape of the holotype (KUMIP 110618) elevated above background matrix (see Maerz et al., Reference Maerz, Kaesler and Hakes1976, their plate 2, fig. 4b), from the upper Pennsylvanian (Virgilian) Rock Bluff Limestone (equivalent to the Leavenworth Limestone Member of the Oread Limestone), Kansas, USA. (2) GSA-T138 from the upper Mississippian (Chesterian) Hartselle Formation, Alabama (see Rindsberg, Reference Rindsberg1994, his plate 1C, old catalogue number GSA 1052-247). (3) Specimen surrounded by matrix from the upper Pennsylanian (Virgilian) Stull Shale Member, upper Kanwaka Shale, Waverly trace fossil site, Kansas, USA (see Buatois et al., Reference Buatois, Wisshak, Wilson and Mángano2017, their fig. 31B). (4) KUMIP 288535 surrounded by matrix of the upper Pennsylanian (Virgilian) Stull Shale Member of the upper Kanwaka Shale, Waverly trace fossil site, Kansas, USA (see Mángano et al., Reference Mángano, Buatois, West and Maples2002, their fig. 44A). Images are provided by the Kansas University Biodiversity Institute and Natural History Museum, the Geological Survey of Alabama, and L. Buatois. Scale bars = 10 mm.
The Clam Bank material is up to two times smaller in diameter than the holotype of Pentichnus pratti and other published examples of this ichnogenus (compare Figs. 6 and 7). Nonetheless, partial development or preservation of facetted margins suggests a common association. A relation to stalkless crinoids is unlikely given that they first appeared by the Late Devonian (Baumiller and Messing, Reference Baumiller and Messing2007).
Discussion
The range of ornamentation associated with the Clam Bank material allows relating Biformites insolitus Linck, Reference Linck1949 to ophiuroid arm movement. The spatially associated burrows further underscore the range of activity on and within the near surface. Discovery of these two ichnofossils in Lower Devonian strata reduces the current disparity between the Paleozoic ichnological record of ophiuroids as trace makers and the post-Cambrian skeletal record (see Introduction). Spatiotemporal sampling bias (Butler et al., Reference Butler, Brusatte, Andres and Benson2012) may well explain the apparent disparity between the ichnofossil and skeletal fossil records but, as discussed in the following, the Clam Bank depositional setting offers clues as to how sedimentological and paleoecological factors may have influenced ophiuroid activity along with the preservation potential of the traces.
Ophiuroid tube feet, ambulacral structures, and arm movement
Surface imprints on the seafloor, whether related to resting (cubichnia) or locomotion (repichnia) activities, are the result of a physical load directed down onto or moved along the substrate. Mechanical compaction of mud (silt and/or clay) results in a greater volume reduction of the host sediment than in sand (Weller, Reference Weller1959). Differential compaction of silt, followed by sand filling the resulting depression, allowed for preservation of the observed differential three-dimensional perspectives of axial trends of ornamented (conical to rectangular protuberances, medial depressions) and unornamented ridges composing the strap-like ichnofossils in the Clam Bank Formation.
Examples of isolated and paired conical protuberances (Figs. 3–5) are interpreted to indicate slight contact of an ophiuroid underarm with the substrate, with only tube feet penetrating the surface sediment. These rounded protuberances are similar in geometry to illustrations of modern tube feet (Buchanan and Woodley, Reference Buchanan and Woodley1963; Nichols, Reference Nichols and Boolootian1966); are of similar diameter (100-400 microns) as examples of the largest pyritized ophiuroid tube feet from the Upper Ordovician, mid-Silurian, and Lower Devonian (Glass and Blake, Reference Glass and Blake2004; Glass, Reference Glass2006; Clark et al., Reference Clark, Bhullar, Darroch and Briggs2017; Carter et al., Reference Carter, Sutton, Briggs, Rahman, Siveter and Siveter2021); and are similar to tube-feet imprints interpreted by Mángano et al. (Reference Mángano, Buatois, West and Maples1999). No other skeletal structure extends at right angle beneath an ophiuroid arm; spines, if present, are distributed along the top of the arm or extend downward and away from the sides of arms (Spencer and Wright, Reference Spencer, Wright and Moore1966). In the case of the latter, there is an ordered sequence of conical protuberances parallel to the margin of hypichnial ridges (Bell, Reference Bell2004).
With increased depth of an imprint (corresponding to greater relief of the hypichnion), the axial medial groove that emerges separating protuberances (compare Figs. 3.2, 3.3, and 5.1) is interpreted to represent the ambulacral groove. However, this disappears with deeper impressions that contain transverse block-shaped protuberances (Fig. 5) that match the paired ambulacral plates and/or laterals forming the oral skeletal structure of an ophiuroid arm as described from Paleozoic ophiuroids (Spencer and Wright, Reference Spencer, Wright and Moore1966; Glass and Blake, Reference Glass and Blake2004; Hotchkiss and Glass, Reference Hotchkiss and Glass2012; Carter et al., Reference Carter, Sutton, Briggs, Rahman, Siveter and Siveter2021).
From preservation of the tube feet and ambulacral plate detail comes the corollary interpretation that ridges without ornamentation likely identify a smearing out of detail arising from horizontal arm movement. Arm movement in the locomotion of modern ophiuroids is commonly described qualitatively as “rowing,” “paddling,” “reverse rowing,” and “breast stroke,” and all forms involve some of the arms actively engaged in producing curved or hook-like trackways whereas others are passively dragged or pushed in the direction of motion (Schatz et al., Reference Schatz, Mángano, Aitken and Buatois2013; Broach et al., Reference Broach, Miller and Bowser2016; Wakita et al., Reference Wakita, Kagay and Aonuma2020; Morton et al., Reference Morton, Myers, Gingras and Zonneveld2023). By contrast, the morphology of Biformites insolitus that is usually cited follows the illustration from Linck (Reference Linck1949, p. 40, Abb. 1): short ridges bisected by a medial groove and dissected by cross-ribs (see Häntzshel, Reference Häntzshel and Teichert1975; Morton et al., Reference Morton, Myers, Gingras and Zonneveld2023). However, Linck (Reference Linck1949) described and illustrated (Tafel IV; Fig. 2) unadorned ridges (with a medial line) as being part of the concept of B. insolitus. With a force of horizontal motion exceeding the downward directed force of an arm, hypichnial ridges should display no or little ornamentation (Schlirf, Reference Schlirf2012). The Clam Bank material illustrates variation in the degree of ornamentation along a single axis (Figs. 3, 4) as might be expected in the complex balance between downward and horizontal forces of motion along the length of an arm. This is compatible to Linck’s (Reference Linck1949) concept of this ichnogenus and questions the taxonomic utility of unadorned ridges separately referred to as Biformites zhadaensis (Knaust and Neumann, Reference Knaust and Neumann2016; Feng et al., Reference Feng, Chen, Benton, Wu, Bottjer and Thompson2019).
Thus, B. insolitus is interpreted here to reflect one part of a spectrum of ophiuroid arm movement. The relatively abundant ornamentation interpreted as tube-feet imprints (Figs. 3–5) in the Clam Bank material suggests that vertical downward force was predominant. Such downward motion could have been directed either as a single vertically oriented movement of the arm or through a proximal-to-distal downlapping of the arm onto the substrate. This contrasts with surface scratches interpreted as evidence of lateral motion of tube feet beneath modern asterozoans and recorded from examples of the ichnospecies Asteriacites lumbricalis von Schlotheim, Reference von Schlotheim1820 (Mángano et al., Reference Mángano, Buatois, West and Maples1999; Ishida et al., Reference Ishida, Fujita, Kiyomoto, Röper, Komatsu, Kato, Kamada, Shigeta and Kumagae2017). In the Clam Bank material, a combination of sustained widths of most imprints of B. insolitus (Figs. 3, 4) and relatively short lengths of imprints suggests that downward movement of arms likely occurred with the animal arched above the substrate, resulting in the middle to distal (but not terminal) parts of arms being in contact with the substrate. This positioning and movement of the disk above the sediment–water interface has been illustrated in aquaria and field studies (Reese, Reference Reese and Bootolian1966; see Feder, Reference Feder1981, figs. 1C, D, 2E; Walker et al., Reference Walker, Miller, Bowser, Furbish and Gualda2013, fig. 3). Strongly curved imprints in the Clam Bank material (Figs. 4.2, 5.4) fit with observed arm flexibility during arching above and along the substrate (Woodley, Reference Woodley1975; Feder, Reference Feder1981, fig. 2C, D), and the degree of arm curvature falls within the experimental range of in-plane angles related to arm flexure (Tomholt et al., Reference Tomholt, Friesen, Berdichevsky, Fernandes, Pierre, Wood and Weaver2020).
Burrows and the trace maker
The distribution of burrows among B. insolitus is a spatial combination of traces reflecting surface activities in common with modern ophiuroids (Broach et al., Reference Broach, Miller and Bowser2016). The burrows may identify fodinichnia, or burrowing in search of food, with burrow depths corresponding to depth of a food horizon (Gielazyn et al., Reference Gielazyn, Stancyk and Piegorsch1999; Boos et al., Reference Boos, Gutow, Mundry and Franke2010). Field and aquaria studies examining longitudinal cross sections of ophiuroid burrows illustrate extension of arms vertically within a thoroughly bioturbated core and extending above the sediment–water interface (Howard, Reference Howard1968; Howard and Frey, Reference Howard and Frey1975; Christensen and Colacino, Reference Christensen and Colocino2000; Gingras et al., Reference Gingras, Dashtgard, MacEachern and Pemberton2008). These accounts do not lend much support to preservation of facetted margins in transverse sections. Yet there are examples of ophiuroids in modern tropical reef settings that display only near-surface burial of the disk, with arms freely extending up into the water column (Charpin, Reference Charpin2023). This style of anchoring does not appear addressed in previous work yet might yield the trace of a facetted burrow. In this context, incomplete facetted margins in the Clam Bank material (Fig. 6) may document disruption by proximal arm movement; the small hypichnial ridge beneath one burrow (Fig. 6.1) may be the imprint of an arm beneath the trace maker, and narrow curved ridges extending away from a rounded burrow (Fig. 3.1) may also represent the surface imprint of arms extending from the burrow. Ichnotaxonomic definition of the Clam Bank material is deferred due to an uncertainty of the order of symmetry arising from incompletely developed facetted margins.
Some of the burrow geometry appears to fit an irregular five-sided polygon (Fig. 6.1–6.3) whereas the magnitude of the mean interfacial angle (~130°; Fig. 6.6) accommodates an octagonal shape. This difference might underscore biological factors in cross-sectional burrow geometry. Potential controls include phylogenetic and ontogenetic development of bilateral symmetry, which may lead to apparent or real higher-order symmetry (Ubaghs, Reference Ubaghs and Moore1967), and fissiparity, or asexual reproduction, resulting in a characteristic supernumerary (six to eight) of arms. The latter influence occurs in nearly half of extant ophiuroid families, more than any other group of echinoderms (Clark, Reference Clark and Millott1967). The extant fissiparous genus Ophiactis Lütken, Reference Lütken1856, for example, has a multifaceted-shaped disc that, depending on imprint quality, could identify an eight- to 10-fold symmetry. Ill-defined or disrupted imprints of rounded central discs might further complicate definitions of symmetry. Rounded to ovoid discs of Early Devonian genera occur within the extinct family Ophiurinidae Gregory, Reference Gregory1897 (Silurian to Mississippian) and the genus Ophiurina Sturtz, Reference Sturtz1890 (Spencer and Wright, Reference Spencer, Wright and Moore1966).
At present, two possible solutions emerge for the ichnological affinity of the Clam Bank burrows: (1) they are a smaller variant of Pentichnus isp. and, if correct, expand the stratigraphic range of the ichnogenus within the Paleozoic; or (2) they define a new ichnogenus of higher-order symmetry, which broadens the ichnotaxobase of facetted burrows. More study of ophiuroid burrows through the Phanerozoic is needed to resolve these possibilities.
Paleoecology
Surface and near-surface biogenic structures have very low preservation potential (Seilacher, Reference Seilacher2007). This becomes even further reduced in a tide-influenced coastal setting, as represented by the lower Clam Bank Formation, wherein shifting sands and muds, alternating erosion and deposition, and periods of exposure and desiccation influenced paleoenvironmental conditions. The Lower Devonian ichnofossil record presented here (Figs. 3–6), therefore, is a remarkable snapshot of surface activity in a paleoceanographically dynamic nearshore setting.
The Clam Bank material reinforces the (paleo)ecological breadth of ophiuroids relative to other echinoderms. Some ophiuroids exhibit the capacity to penetrate and adapt to brackish coastal waters (Binyon, Reference Binyon and Boolootizn1966; Turner and Meyer, Reference Turner and Meyer1980; Pagett, Reference Pagett, Jones and Wolff1981; Martínez, Reference Martínez2008; Kuk-Dzul et al., Reference Kuk-Dzul, Solís-Marín, Herrera-Dorantes and Ardisson2019) and euryhaline (brackish–hypersaline) conditions (Kuk-Dzul et al., Reference Kuk-Dzul, Solís-Marín, Herrera-Dorantes and Ardisson2019) and even display intra-species variation in salinity tolerances along a coastline (Pagett, Reference Pagett, Jones and Wolff1981). In the fossil record, ichnofossils making up Biformitidae are reported from subtidal marine facies (Pickerill and Forbes, Reference Pickerill and Forbes1979), semi-restricted low-energy lagoonal deposits (Feng et al., Reference Feng, Chen, Benton, Wu, Bottjer and Thompson2019), and both intertidal (Boyer, Reference Boyer1979) and hypersaline flat facies (Salamon et al., Reference Salamon, Niedzwiedzki, Lach, Brachaniec and Gorzelak2012). This environmental breadth demonstrates that salinity adaptation has been a longstanding characteristic of ophiuroids. The facies association host to ophiuroid traces in the lower Clam Bank Formation documents a physically stressed, peritidal paleoenvironment related to periods of siliciclastic sediment transport, deposition, erosion, episodic exposure, and synsedimentary deformation. The resulting turbidity, substrate instability, and potential salinity variation characteristic of these types of settings (Dalrymple, Reference Dalrymple, James and Dalrymple2010) impose significant environmental pressures such that physio-chemical coupling may explain the near absence of crinoids that are usually taken to define stenohaline conditions (Fell, Reference Fell and Boolootian1966a; Ausich and Simms, Reference Ausich, Simms, Hess, Ausich, Brett and Simms1999) removed from physical sedimentary pressures (Lane and Webster, Reference Lane, Webster, Broadhead and Waters1980). Yet these same factors benefit the negative phototaxis nature of ophiuroids (Fell, Reference Fell and Boolootian1966b) and their opportunistic behavior in exploiting environments with feeding versatility related to suspension and deposit feeding, scavenging, and predation (Warner, Reference Warner, Jangoux and Lawrence1982).
Predation and substrate selectivity also impact the potential development of ophiuroid burrows (Zimmerman et al., Reference Zimmerman, Stancyk and Clements1988; Boos et al., Reference Boos, Gutow, Mundry and Franke2010). These studies have shown fine-grained sand to sandy silt as the preferred target for modern ophiuroid burrowing, a size range reinforced in this present study by the burrowed siltstone of the Clam Bank Formation, and the preferred grain size of substrates hosting the ichnogenus Pentichnus (Maerz et al., Reference Maerz, Kaesler and Hakes1976; Mángano et al. Reference Mángano, Buatois, West and Maples2002; Mángano and Buatois, Reference Mángano, Buatois and McIlroy2004). Littoral settings usually display an overall reduction of fine-grained sediment due to increased wave and current energy with transport of fines either landward or seaward (Hill et al., Reference Hill, Nittrouer, Austin, Field, Kravitz, Syvitski and Wiberg2007). Thus, elevated potential for development of ophiuroid burrows seems best associated with peritidal flats and the middle to outer shelf.
Acknowledgments
This work is an outcome of a study of the Clam Bank Formation funded by a Discovery Grant to G.R. Dix from the Natural Sciences and Engineering Research Council, Canada. Material was collected under paleontological investigation permits P22.03 and P23.02 issued by the provincial government of Newfoundland and Labrador. Thanks to the Kansas University Biodiversity Institute and Natural History Museum, Geological Survey of Alabama, and L. Buatois for providing images that compose Figure 7 and their permissions to publish. Two anonymous reviewers and AE G. Mángano are thanked for their constructive comments.
Competing interests
The author declares none.
