Anticosti Island

The geology of Anticosti Island comprises Upper Ordovician to lower Silurian strata (Chatterton and Ludvigsen, Reference Chatterton and Ludvigsen2004) that dips gently to the southeast (see Fig. 1.4). The units are dominated by shallow marine carbonate ramp deposits of limestones and shales (Long, Reference Long2007). During deposition of these rocks, the southern margin of Anticosti was most likely exposed to westerly directed tropical cyclones off the Iapetus Ocean, which deposited frequent tempestites (Wilde, Reference Wilde, Barnes and Williams1991). The rhenopyrgid edrioasteroids were found in two members of the lower Silurian (Telychian) Jupiter Formation. Almost all specimens of R. viviani n. sp. came from the Cybèle Member at collection site 1, but a single specimen came from the Pavillon Member at collection site 2 (see Fig 1.4). Within the published literature, the only other edrioasteroid known from Anticosti Island is Isorophusella pleiadae (Sinclair and Bolton, Reference Sinclair and Bolton1965) from the Upper Ordovician Vauréal Formation (Sinclair and Bolton, Reference Sinclair and Bolton1965; Bell, Reference Bell1976). Unpublished edrioasteroid material has been collected more recently, but none of this included rhenopyrgid edrioasteroids (W. Ausich personal communication, 2018).

The Cybèle Member comprises 32 m of gray, sparsely fossiliferous, lithographic limestone deposited on a broad, south-facing carbonate ramp. Fossils are relatively scarce (M. Martin, personal observation, 2015), but dominated by trilobites and highly endemic crinoids (Ausich and Copper, Reference Ausich and Copper2010). The specimens all came from a hard, medium-gray, tempestite limestone. Other fossils include current orientated tentaculitids, disarticulated trilobite fragments, and ostracod debris.

The Pavillon Member is composed of 10 m of thin recessive tempestite limestones that are interbedded within gray/green shales. The member is generally highly fossiliferous and rich in brachiopods and trilobites. Other fossils include large numbers of isolated crinoid columnals and several fairly long partially articulated stem fragments, along with disarticulated trilobite remains, bryozoan fragments, and numerous tentaculitids.

The specimens from Anticosti Island are very well preserved in calcite with some evidence of stereom microstructure present (Fig. 2.5, 2.10). Specimens are preserved either lying on the bedding plane and apparently similarly orientated to other tubular fossils (e.g., tentaculites) (Fig. 2.8), or nearly perpendicular to the bedding (such as Figs. 2.1, 4). All are preserved in a hard gray micritic limestone. Most specimens display partial crushing, but retain some three-dimensionality. The oral surface is usually partially collapsed, with disrupted cover plates (Figs. 2.1–2.3, 6, 7, 9), while others show disarticulation of the suboral constriction and pedunculate zone (Fig. 2.5). The specimens that are preserved perpendicular to bedding surface also suffer either minor crushing or have flattened oral areas (Figs 2.1–2.4) and loss of the distal pedunculated zone and coriaceous sac. None appears to be in life position or within a burrow. All specimens from the Cybèle Member are partially disarticulated and missing parts of the distal pyrgate zone and coriaceous sac. This suggests, in both members, short transportation with preservation occurring after rapid burial.

Figure 2.

Rhenopyrgus viviani n. sp. from Jupiter Formation, Telychian of Anticosti Island, Québec, Canada. (1–9) From Cybéle Member, Rock Pool Ledge site on Jupiter River (1–3) (paratype MPEP 1126.1); (1) lateral view of entire specimen (note it is preserved perpendicular to bedding with flattened oral surface but undisturbed pyrgate zone); (2) detail of oral surface; white arrows point to ridges on adambulacral margin of the oral ossicles, which accommodated cover plates; black arrow points to the ridged ambulacral floor that is presumably an extension of the interadial oral ossicle (coated with ammonium chloride); (3) oral surface without ammonium chloride; again, black arrow points to ridged ambulacral floor; (4) oral view of cover plates showing vestige of 2-1-2 ambulacral arrangement (paratype EE 15752); (5) lateral view, note lack of any floor plate sutures along adambulacral margin of oral plates (paratype EE 15753). (6–8) Holotype (EE 16642); (6) note changing morphology of pyrgate zone ossicles; (7) detail of oral surface and sub-oral constriction; black arrow points to anal pyramid composed of rod-like ossicles; white arrow points to triangular cross section of cover plates; (8) holotype slab showing two individuals (holotype on right). (9) Lateral view with a well-preserved suboral constriction; note a change in morphology of pyrgate zone plates (paratype EE 15754). (10) From Pavillon Member at Rivière aux Plats; distal pyrgate zone and bulbous coriaceous sack-like holdfast comprised of small granular plates (paratype EE 15756). Abbreviations: CL = collar plates, CP = cover plates, O = oral plate. All scale bars represent 1 mm.

Ardmillan specimen (Girvan)

The precise locality of the specimen EE 16524 is simply labelled “Ardmillan” and does not have any further specific detail. Although it is different from the well-known Lady Burn Starfish Bed, it is reminiscent of equivalent rock of the Ardmillan Series of the Upper Ordovician (Hirnantian) exposed on the coast to the south of Girvan, Scotland (in the vicinity of a camp site called ‘Ardmillan’). It therefore seems likely to have come from this locality.

The fossil was preserved in an oval concretion comprised of a massive fine, dark gray, muddy sandstone, including numerous larger clasts of detrital calcite. Unusually for Paleozoic echinoderms, EE 16524 is preserved as a three-dimensional calcite cast (secondary calcite overgrowing primary stereom) composed of a long articulated pyrgate zone missing the adoral structures and basal coriaceous sac (Fig. 3.5). Thus, the imperfect preservation of the specimen and the massive nature of the sediment again suggest short transportation and rapid burial in a mass accumulation.

Figure 3.

Rhenopyrgidae; all whitened with ammonium chloride. (1, 2) Rhenopyrgus grayae, Upper Ordovician, Lady Burn Starfish Bed, Girvan, Ayrshire, Scotland (holotype E23470): (1) details of oral surface with prominent collar plates and no suboral constriction apparent; black arrows highlight exposed floor plates in distal ambulacra; white arrow points to a small, exposed part of a plate of the suboral constriction, suggesting that suboral constriction could be contracted behind collar plates; (2) lateral view. (3, 4) Rhenopyrgus indet. 1, Silurian, Newland Formation, Newlands, Ayrshire, Scotland (E 62753): (3) lateral view of pyrgate zone, arrow points to enlarged plates interpreted here as collar plates; (4) oral view, black arrow highlights disarticulated large D-shaped oral ossicle. (5) Rhenopyrgus indet. 2, Ordovician, Drummuck Series, Ardmillan, Girvan District, Ayrshire, Scotland (EE 16254); lateral view of pyrgate zone; note difference in size and morphology of the pyrgate ossicles suggesting it is different than R. grayae, which is found in similar age rocks that are geographically close; also note disarticulated ridged deltoid-shaped plate closely associated with articulated pyrgate plates. (6–8) Rhenopyrgus indet. 3: (6, 7) Foulon Formation (middle Floian), La Croix de Roquebrun, Saint-Nazaire-de-Ladarez, Hérault, France (UCBL-FSL 713312); (6) lateral view of whole specimen; (7) enlargement of the oral surface, showing confused plate articulation of this region; black arrow highlights possible oral ossicle; white arrows highlight collar plate series; (8) late Tremadocian, beneath Saint-Chinian Formation, Saint-Chinian, SW of Donnadieu, Babeau-Bouldoux, Hérault, France (UCBL-FSL 713316), lateral view. (9–11) Rhenopyrgus coronaeformis Rievers, Reference Rievers1961, Lower Devonian, Emsian, Hunsrück Slate, Bavaria, Germany (holotype SNSB-BSPG 1958 XV 50): (9) detail of oral surface and proximal structures; (10) detail of oral surface; note complicated cover plate articulation surfaces; black arrow highlights grooved adambulacral margin of oral plate; (11) detail of coriaceous sac. Abbreviations, O = oral plate. All scale bars represent 1 mm.

Montagne Noire specimens

The Southern Montagne Noire (France) exhibits a rather complete Lower Ordovician succession. Rhenopyrgid material has been collected from the Saint-Chinian, Foulon, and Landeyran formations.

The Saint-Chinian Formation (<500 m thick) consists of monotonous dark-gray clayey siltstones intercalated by fine- to medium-grained sandstones, interpreted as outer-platform deposits (Álvaro et al., Reference Álvaro, González-Gómez and Vizcaïno2003). The specimen studied (UCBL-FSL713316) here was collected from the E. filacovi Zone (late Tremadocian) and is preserved as a mold within a gray calcareous concretion in three dimensions. The sediment is a fine-grained siltstone.

The Foulon Formation (80 m thick) corresponds to an alternation of shales, bearing siliceous fossiliferous concretions, and fine-grained sandstones deposited via storm-generated turbidity currents that transported sandy material to offshore settings (Noffke and Nitsch, Reference Noffke and Nitsch1994). The specimen studied here (UCBL-FSL713312) was collected from the transitional beds at the uppermost part of the formation at the top of the N. arenosus Zone (middle Floian) and is preserved as a crushed mold within a gray shale (i.e., not within a concretion) (see Fig. 3.6, 3.7).

The Landeyran Formation (~300 m thick) is composed of brown, green, and gray homogeneous shales bearing siliceous concretions, interpreted as bioclastic storm deposits in upper offshore environments (Bonin et al., Reference Bonin, Nardin, Vennin, Vizcaïno, Vennin, Aretz, Boulvain and Munnecke2007). The specimens studied (UCBL-FSL 713313–713315) were all collected from the A. incisus Zone and are preserved both as a mold within a gray calcareous concretion in three dimensions (UCBL-FSL 713315) and as a flattened mold within a gray shale (UCBL-FSL 713313, 713314).

Genera included

Rhenopyrgus Dehm, Reference Dehm1961; Heropyrgus Briggs et al., Reference Briggs, Siveter, Siveter, Sutton and Rahman2017.

Diagnosis (Emended)

Pyrgate edrioasteroids with relatively small oral surfaces bearing five, short, straight rapidly tapering ambulacra; interambulacra formed of single tessellate interradial oral plates and biserial and alternating floor plates that may or may not be fused; cover plates, where present, are biserial and tall proximally; sub oral constriction generally short, composed of circlets of poorly organized imbricate, scale-like plates and periproct, sometimes withdrawn behind collar plates; collar plates large, forming a single imbricate circlet at base of suboral constriction; pyrgate zone comprised of highly imbricated scale-like plates often in eight rows arranged in alternating circlets of four; holdfast formed of coriaceous sac comprised of small unorganized platelets.

Occurrence

North Africa (Morocco), Europe (Germany, UK, France, Russia, Sweden, ?Spain), North and South America. Tremadocian, Lower Ordovician to Eifelian, Middle Devonian.

Remarks

The description of R. viviani n. sp. and a detailed reassessment of previously described taxa (see discussion) with well-preserved oral surfaces have highlighted varying degrees of fusion of the interadial oral plate and the ambulacral floor plates (particularly the externally exposed parts). The ambulacral floor plates of the type species, Rhenopyrgus coronaeformis Rievers, Reference Rievers1961, have totally fused to each other and the interradial oral plate (Fig. 3.9, 3.10), while plate sutures are visible in Rhenopyrgus grayae Bather (Fig. 3.1). This highlights greater structural diversity than was previously thought, which, as well as broadening the familial diagnosis, also enables a re-evaluation of the relationships with other edrioasterids that have a similar oral arrangement (see below).

The discovery of a well-preserved anal pyramid in the holotype of R. viviani n. sp. confirms the suggestion of Sumrall et al. (Reference Sumrall, Heredia, Rodríguez and Mestre2013) and Holloway and Jell (Reference Holloway and Jell1983) that the periproct is located within the suboral constriction, adjacent to an oral plate in the CD interray. This location is similar to that seen in other edrioasterids (Bell, Reference Bell1976) and Heropyrgus (Briggs et al., Reference Briggs, Siveter, Siveter, Sutton and Rahman2017).

Briggs et al. (Reference Briggs, Siveter, Siveter, Sutton and Rahman2017) recently erected Heropyrgus within the Rhenopyrgidae because it shared many key characteristics of skeletal organization and differed only by lacking cover plates and that the floor plates gaped and did not completely floor the ambulacra, although these variations were not included in an emended diagnosis. However, the lack of cover plates and the unusual arrangement of the ambulacral floor are very curious, particularly because the ridged adambulacra margin of the oral and floor plates look very similar to those of other rhenopyrgid genera that clearly bear cover plates (particularly R. viviani n. sp.; Fig. 2.2). As such, taphonomic loss and/or disruption should be considered a possibility for cover plates lacking in specimens assigned to Heropyrgus and potentially the ambulacral gape. That said, in lieu of further evidence, this genus must currently be maintained and its variations included in the family diagnosis.

Following Holloway and Jell (Reference Holloway and Jell1983), Sumrall et al. (Reference Sumrall, Heredia, Rodríguez and Mestre2013) placed rhenopyrgid species into their own family; however, they questioned the validity of classifying specimens based solely on characters of the pyrgate and pedunculate zones or where they lacked well-preserved oral areas. As such, Sumrall et al. (Reference Sumrall, Heredia, Rodríguez and Mestre2013) removed all such examples from the genus Rhenopyrgus, but retained them within the Rhenopyrgidae Holloway and Jell (Reference Holloway and Jell1983) on account of the pyrgate zone being formed of eight rows of plates. We follow this approach here.

Sumrall et al. (Reference Sumrall, Heredia, Rodríguez and Mestre2013) also suggested the removal of generic assignation of rhenopyrgid taxa that lacked a suboral constriction, thereby suggesting Rhenopyrgus grayae (Bather, Reference Bather1915) and possibly R. flos Klug et al., Reference Klug, Kröger, Korn, Rücklin, Schemm-Gregory, de Baets and Mapes2008 should be placed in a separate genus. However, we believe that the lack of a sub-oral constriction is taphonomic in rhenopyrgids (see discussion section). As such, we retain both these species within Rhenopyrgus, which also include taxa with clearly preserved suboral constrictions.

The higher taxonomic relationship of rhenopyrgids was reviewed by Sumrall et al. (Reference Sumrall, Heredia, Rodríguez and Mestre2013) who, in agreement with previous workers, suggested that rhenopyrgids are edrioasterids united with edrioblastoids and cyathocystoids by the reduced number of interambulacral plates, the pseudo-fivefold symmetry of the ambulacral system, and the turreted thecal shape. These groups differ from one another primarily in the form of the pedunculate zone—organized flexible stalk in rhenopyrgids; fused cup in cyathocystids; and an elongate, rigid, multiplated stem in edrioblastoids. However, Sumrall et al. (Reference Sumrall, Heredia, Rodríguez and Mestre2013) left the specific relationships between these families open. Sprinkle and Sumrall (Reference Sprinkle and Sumrall2015) drew closer affinities between cyathocystids and derived edrioasterids because both shared a similar oral surface construction, formed by fusion of the floor plates to each other and the interadial oral plate. However, these comparisons were made on the assumption that all rhenopyrgid taxa did not possess this feature, which we here demonstrate is not the case. We cannot really add anything further to refine these relationships other than highlight that all three turreted edrioasterid families display similar ambulacral structures in various constituent species. As indicated by Sprinkle and Sumrall (Reference Sprinkle and Sumrall2015) for cyathocystids and derived edrioblastoids, this suggests some form of homoplasy/convergence, which was particularly suitable for turreted edrioasterid ecology.

Type species

Pyrgocystis (Rhenopyrgus) coronaeformis Rievers, Reference Rievers1961; Hunsrück Slate, Lower Devonian, (Emsian) of Germany.

Other species

Rhenopyrgus coronaeformis Rievers, Reference Rievers1961; R. flos Klug et al., Reference Klug, Kröger, Korn, Rücklin, Schemm-Gregory, de Baets and Mapes2008; R. viviani n. sp.; R. sp. indet.1, formerly Pyrgocystis procera (Aurivillius) Bather, Reference Bather1915; R. sp. indet. 2, R. sp. indet. 3, R. sp. indet. 4.; R. grayae (Bather, Reference Bather1915); R. whitei Holloway and Jell, Reference Holloway and Jell1983; and R. piojoensis Sumrall et al., Reference Sumrall, Heredia, Rodríguez and Mestre2013.

Emended diagnosis

Rhenopyrgids with fused or unfused ambulacral floor and interradial oral plates; suboral constriction, formed of a zone of highly imbricate elongate plates formed between the oral surface and suboral collar, may or may not be exposed. Cover plates present. Pyrgate zone formed of eight rows of imbricated plates arranged in two circlets of four.

Occurrence

Lower Ordovician (Tremadocian) to Lower Devonian (Emsian); from France (Lower Ordovician, upper Tremadocian and Floian), UK (Katian, Upper Ordovician, and middle Llandovery, lower Silurian), Canada (Rhuddanian, lower Silurian), Argentina (lower Ludlow, upper Silurian), Australia (Lower Devonian), Morocco (Emsian, Lower Devonian), and Germany (Emsian, Lower Devonian).

Remarks

There is significant variation in the fusion of the interradial oral plates with the ambulacral floor plates in different species throughout the range of Rhenopyrgus. Some (e.g., Rhenopyrgus coronaeformis, R. flos, and R. viviani n. sp.) display complete fusion, giving the appearance of only a single D-shaped interradial plate with a broad curved ambital margin. It is tempting to use this character to subdivide the genus further, however there are few other characters that either unify or differentiate these taxa from those with unfused (or less completely fused) oral surfaces. As such, until detailed cladistic analyses are employed, we withhold from formerly erecting a new genus for these taxa with unfused floor plates (because the type species R. coronaeformis clearly displays fused interaidal and ambulacral floor plate elements; see Fig. 3.9, 3.10).

The fusion of the interradial and floor plate systems, however, means that isolated D-shaped oral plates, with either smooth or notched margins, that are found in close association with articulated sections of pyrgate zone comprised of eight rows of plates, which may or may not also lack clearly defined collar and suboral constriction plates (because these plates are determined to be moveable and therefore could have contracted within the pyrgate zone and be obscured, e.g., Fig. 2.3, 2.4), can now be used to imply classification to Rhenopyrgus. Owing to the presence of such associations, Rhenopyrgus sp. indet. 1, formerly Pyrgocystis procera (Aurivillius) Bather, Reference Bather1915, from the lower Silurian Newlands Beds of Newlands, Girvan, Ayrshire, Scotland (Fig. 3.3, 3.4); Rhenopyrgus sp. indet. 2 from the Ordovician of Ardmillan, Girvan, Ayshire, Scotland (see below); Rhenopyrgus sp. indet. 3 from the Montagne Noire, France (see below); R. flos (Klug et al., Reference Klug, Kröger, Korn, Rücklin, Schemm-Gregory, de Baets and Mapes2008) from the Devonian of Morocco; and Rhenopyrgus sp. indet. 4 (as illustrated by Grigo [Reference Grigo2000]) can all be assigned to this genus.

Holotype

EE 16642, paratypes EE 15752–15756 and MPEP 1126.1. Cybèle Member, Jupiter Formation, Telychian, upper Llandovery, lower Silurian from Rock Pool Ledge, Jupiter River, Anticosti Island, Québec, Canada.

Diagnosis

Rhenopyrgus with interambulacra composed of a single broad, relatively low D-shaped compound interadial plate with gently curving adambulacral margin. Distal cover plates distinctly square. Adoral constriction composed of 3–5 circlets. Plates of the pedunculate zone vary in morphology of exposed aboral margin, being curved adorally, but becoming distinctly rhombic (angular) aborally.

Occurrence

Cybèle Member to Pavillon Member, Jupiter Formation, Telychian, upper Llandovery, lower Silurian, Anticosti Island, Québec, Canada (see above for details).

Description

Oral surface domed with straight ambulacra, fairly evenly spaced, but with a vestige of 2-1-2 ambulacral arrangement (Sprinkle, Reference Sprinkle1973) apparent (Fig. 2.2–2.4). External oral surface composed of biserial, alternating, tessellated cover plates with zig-zag perradial suture and tessellated, D-shaped, compound interradial plates (formed of fused interradial oral plate and ambulacral floor plates) lacking in obvious sutures (in un-weathered specimens) and with curved adambulacral margins. Cover plates number 12–16 per interradial oral plate, 6–8 in each ambulacrum (Fig. 2.2–2.7). Proximal cover plates long thin rectangles with triangular or T-shaped cross sections (Fig. 2.7, 2.9), but grade distally into shorter, wider, more equi-dimensional (square) plates (Fig. 2.4, 2.5, 2.9). All cover plates articulate in grooves formed by ridges along the margin of the compound interradial plates (Figs. 2.2, 2.3, 4.2). Below cover plate articulation areas, the ridges anastomose or become off-set, forming a distinct set of ridges. Ambulacral floor not adequately seen in any specimen, but proximally appears to be formed by the shallow, ridged slope of the adambulacral margin of the compound interradial oral plate. Distally, floor plates appear to have a similar morphology (Figs. 2.2, 2.3, 4.2). Aboral margin of interradial oral plates straight and abut along oral surface ambitus with imbricate plates of suboral constriction zone.

Suboral constriction composed of at least 3–5 circlets of less-regularly aligned, highly imbricating, wedge-shaped plates that increase in size distally, but which are comparatively small compared to those of collar and pyrgate zone (Fig. 2.5–2.7, 2.9). Distally, these circlets are bordered by a circlet of larger, rectangular imbricated plates that form the collar (Fig. 2.5–2.7, 2.9). Periproct (Figs. 2.7, 4.1) located in CD interray adjacent to oral plate margin, composed of a single circlet of short imbricated rectangular plates. Gonopore and hydropore unknown.

Pyrgate zone highly organized into alternating circlets of four, highly proximally imbricating plates. Plates form alternate circlets aligned into straight longitudinal columns of at least 52 (in the only known specimen with a complete pedunculate zone) such that eight peduncular ridges are present. Four plates from each circlet meet beneath the more distal circlet to form a ring with a fairly large lumen. Plates are squamose proximally with broadly rounded outline becoming distinctly more angular distally in the holotype and smaller specimens (Fig. 2.6, 2.9), but variation less apparent in large individuals (e.g., in EE 15756). Coriaceous sac bulbous at base, approximately a quarter of length of pedunculate zone and slightly wider. It is formed of numerous irregularly placed platelets (Fig. 2.10).

Etymology

Named for the initial discoverer Mr. Travis Vivian.

Remarks

This taxon is differentiated from most other species of Rhenopygus by the fusion of interradial oral plates and the external portions of the floor plates, thereby giving the impression of cover plates being supported by a single interradial oral ossicle. Of the other Rhenopygus species that also display this fusion, the oral surface of R. viviani n. sp. is not so highly domed, the oral plates are lower, wider, and more rounded adorally, with narrower distal ambulacra. The pedunculate zone is also much more highly variable along the length than in other taxa.

The low ridges or walls between floor plates and along the ambulacral margin of the oral plates (producing a series of grooves) are remarkably similar to the higher ridges seen in Heropyrgus, where they also accommodated the tube feet (Briggs et al., Reference Briggs, Siveter, Siveter, Sutton and Rahman2017). Therefore, it seems reasonable to assume these ridges served a similar function in R. viviani n. sp. and/or are muscle-/ligament-attachment scars for closing the cover plates.

The less-formal arrangement of the plates of the suboral constriction, their high degree of imbrication coupled with a rather inconsistent exposure of plates, and number of circlets present in various individuals from the same locality suggest they were capable of vertical extension and contraction. This agrees with observations of R. piojoensis (Sumrall et al., Reference Sumrall, Heredia, Rodríguez and Mestre2013).

Despite the oral surface being crushed in specimen EE 15756 from the Pavillon Member, it differs from specimens of the Cybèle Member only in there being a lesser amount of change in the morphology of the pedunculate zone plates, although there is still some discernible change. Subtle changes in pedunculate plate morphology were also noted between larger and smaller individuals by Sumrall et al. (Reference Sumrall, Heredia, Rodríguez and Mestre2013) in a population of 84 R. piojoensis, but this variation was not as obvious as in R. viviani n. sp.

Holotype

E 23470. Upper Ordovician, Katian, Ardmillan Series, Drummuck Group, South Threave Formation, Farden Member, Lady Burn Starfish Bed, part of the Craighead inlier.

Diagnosis (Emended)

Rhenopyrgus with highly domed oral surface. Oral plates narrow and high with acutely curved adambulacral margin. Ambulacral zone prominent with long proximal cover plates and wide distal ambulacral zone.

Occurrence

Known only from the Upper Ordovician, Katian, Ardmillan Series, Drummuck Group, South Threave Formation, Farden Member, Lady Burn Starfish Bed, Lady Burn, near Girvan, Scotland, UK.

Description

Rhenopyrgus with well-preserved oral surface comprised of long proximal and distal cover plates arranged in fairly symmetrical pattern, but with a vestige of 2-1-2 pattern discernible (Sprinkle, Reference Sprinkle1973), oral plates fairly rectangular with a narrow ambital margin and acutely curved adoral margin. Three discernible floor plates form distal part of ambulacra and together give the interradial oral area a deltoid shape. Only part of a single plate of the sub-oral constriction is visible between proximal margins of collar plates. Collar plates large and rectangular with curved proximal margins. Two ridges on distal external surface of collar plates run down behind proximal plates of pyrgate zone. Pyrgate zone distinctly tapering and comprised of squamate imbricate plates with curved proximal margins arranged in circlets of four, forming eight rows; 58 circlets in only complete specimen (holotype). Coriaceous sac comprised of a granular membrane with bulbous base.

Remarks

This taxon was erected by Bather (Reference Bather1915) with the diagnosis focused primarily on the morphology of the pyrgate zone. Sumrall et al. (Reference Sumrall, Heredia, Rodríguez and Mestre2013) questioned the validity of erecting taxa based solely on pedunculate zone morphology, and thus we here emend the diagnosis to include characters of the oral surface.

Although Sumrall et al. (Reference Sumrall, Heredia, Rodríguez and Mestre2013) regarded this taxon as a member of the Rhenopyrgidae, the apparent lack of a suboral constriction led them to suggest that it should be placed into a new, undescribed genus. However, we believe that the apparent lack of a suboral constriction is a feature of taphonomy and that the sub-oral constriction has been withdrawn behind the collar plates. Because this specimen is preserved as a mold, the plates of the sub oral constriction are no longer clearly visible. However, a clue to its presence can be seen in Figure 3.1 (highlighted by a white arrow), where the distal portion of a plate is just visible between two collar plates, exactly where one would expect to see such a plate if the sub-oral constriction zone were withdrawn. As such, we feel that there is no need to erect a new genus specifically for this and other rhenopyrgid taxa apparently also lacking a sub-oral constriction.

Rhenopyrgus grayae displays suture lines between the plates of the oral surface, particularly between the interradial oral plates and the external portions of floor plates, distinguishing it from R. viviani n. sp., R. coronaeformis, and R. flos. Of the Rhenopyrgus taxa that also display suture lines between the oral and floor plates, R. grayae can be distinguished by its high, narrow, oral plates with a relatively gentle apical curve and its broad distal ambulacra and cover plates. Both R. whitei and R. piojoensis have more acute apical angles, with the former having distal ambulacra that narrow more abruptly, whilst the latter has narrower distal ambulacral and cover plates.

Occurrence

Known only from the lower Silurian, middle Llandovery, Saugh Hill Group, Newlands Subgroup, Newlands Farm Formation, Newlands Beds. Newlands Farm, Argyle, Scotland, UK, but may extend into the Ardmillan group of the Upper Ordovician (see remarks).

Description

Only known from one relatively complete specimen, E 62753 (Fig. 3.3, 3.4), others comprise only sections of pyrgate zone. This specimen has standard Rhenopyrgus construction, with turreted theca divided proximally to distally into an oral surface, collar, pedunculate zone, and coriaceous sac. Suboral constriction not apparent, but this is assumed to be a feature of taphonomy and that it is obscured by collar plates.

Oral surface only known from one crushed specimen (E 62753) displaying a single large, D-shaped, compound, interradial plate pressed down into top of pyrgate zone (Fig. 3.4). Cover plating and ambulacra unknown. Suboral constriction plating unknown. Collar plates indistinct, but appear wider than other pedunculate plates and form a circlet at the top of the peduncle. Pyrgate zone, slightly curved and composed of ~48 circlets in longest specimen (E 62753). Circlets comprised of fairly narrow squamate plates, with curved proximal margin, although many are broken giving a more angular look (Fig. 3.3). Circlets are arranged at highly inclined angle. Coriaceous sac comprised of numerous small platelets.

Remarks

The material from the Newlands beds was originally assigned to Pyrgocystis procera (Aurivillius) by Bather (Reference Bather1915) owing to similarities in the morphology of the pedunculate zone. However, Bather (Reference Bather1915) expressed concern about the validity of erecting a species solely on the morphology of the pedunculate zone. Sumrall et al. (Reference Sumrall, Heredia, Rodríguez and Mestre2013) suspected that this species was actually a rhenopyrgid on the basis of the pyrgate zone plating and coriaceous sac, but did not formally transfer this species to the genus, owing to uncertainty surrounding the nature of the oral surface and suboral contraction.

On reanalysis of the material, we interpret a large D-shaped plate within the top of the pyrgate zone in specimen E 62753 as a compound interradial plate (Fig. 3.3) seen in other species of Rhenopyrgus with fused floor and interradial oral plates. The position and morphology of this D-shaped plate are similar to the disarticulated oral surfaces of R. flos (see Klug et al., Reference Klug, Kröger, Korn, Rücklin, Schemm-Gregory, de Baets and Mapes2008, pl. 16.4, text-fig. 25), although they lack the prominent ridges on the margins. Specimen E 62753 also displays many other features compatible with Rhenopyrgus, including an enlarged circlet of proximal pedunculate plates, which we interpret as collar plates, while the lack of a discernible suboral contraction may be a feature of taphonomy (i.e., it was withdrawn behind the collar plates—see comments in Functional Morphology section). Thus, we tentatively assign these specimens to the genus Rhenopyrgus. However, we agree with Sumrall et al. (Reference Sumrall, Heredia, Rodríguez and Mestre2013) and with Bather (Reference Bather1915, in part) that species should not be solely based on morphology of the pedunculate zone. Because the type material of Aurivillius (Reference Aurivillius1892) consists only of pedunculate plates, we consider Pyrgocystis procera a nomen dubium, and owing to the imperfect preservation precluding adequate comparison with other Rhenopyrgus species, we assign this material to Rhenopyrgus sp. indet. until better material is found.

Although the incomplete preservation of this occurrence precludes detailed comparison of all features with other named Rhenopyrgus species, useful comparisons can still be made. The pyrgate zone most closely resembles in both size and morphology those of Rhenopyrgus indet. 2, from the nearby Girvan site of “Ardmillan,” but which is probably from slightly older strata (see below). As such, it is described separately, but may in fact belong to the same, albeit indeterminable, taxon. The apparent fused nature of the interradial oral plating discounts affinities to R. grayae (Bather), which is also found from slightly older rocks nearby, or to Heropygus Briggs et al. (Reference Briggs, Siveter, Siveter, Sutton and Rahman2017). Otherwise, it is stratigraphically and morphologically most similar to R. viviani n. sp., but has slightly higher and narrower oral plates.

Occurrence

Part of the Craighead inlier, Katian, Upper Ordovician. “Ardmillan” [?Girvan], Scotland, UK, Ardmillan Series.

Description

Single specimen, displaying long portion (31 mm) of pyrgate zone comprised of at least 38 alternating circlets in eight columns (Fig. 3.5). Pyrgate zone plates large, steeply inclined, widely spaced and highly imbricating. Shape of plate margins unknown because all are damaged. Oral surface, suboral contraction, collar plates, and corriaceous sac are either not preserved or not exposed. Oral plating known only from a single disarticulated D-shaped ossicle with a ridged margin.

Remarks

Only a single, poorly located specimen is known, however a curious disarticulated D-shaped ossicle, found in close association to the articulated portion pyrgate zone (Fig. 3.5), closely resembles a fused interradial plate of Rhenopyrgus. On this rather tentative basis, we assign this specimen to Rhenopyrgus sp. indet. Although the shape of the interradial plate and the size and morphology of the pedunculate zone distinguish it from the stratigraphically similar R. grayae, it instead resembles that of R. sp. indet. 1, formerly described as Pyrgocystis procera Bather, Reference Bather1915 (see above). Owing to the incompleteness of the material from both localities and the apparent stratigraphical differences, we describe it separately and await the discovery of better preserved material for further comparison.

Occurrence

(UCBL-FSL 713316) SW of Donnadieu, Babeau-Bouldoux, Hérault, France, E. filacovi Zone, Saint-Chinian Formation, late Tremadocian, Lower Ordovician; (UCBL-FSL 713312) la Croix de Roquebrun, Saint-Nazaire-de-Ladarez, Hérault, France, N. arenosus Zone, Foulon Formation, middle Floian, Lower Ordovician; (UCBL-FSL 713313–713315) Pont supérieur, Saint-Nazaire-de-Ladarez, Hérault, France, A. incisus Zone, Landeyran Formation, late Floian, Lower Ordovician;

Description

Known from several specimens with well-preserved portions of pyrgate zone and coriaceous sac, but disrupted or obscured portions of oral surface and sub-oral constriction. Oral surface in UCBL-FSL 713312 is disrupted and it is difficult to discern any structures with confidence, precise arrangement or morphology of cover plates difficult to determine; however, suspected interradial oral plates appear D-shaped (see black arrow on Fig. 3.7). Suboral constriction in UCBL-FSL 713312 and UCBL-FSL 713316 obscured by a prominent circlet of rectangular collar plates, with curved proximal edges (white arrows in Fig. 3.7, 3.8). Pyrgate zone long, either distinctly tapering or parallel sided; formed of small, low, squamate, imbricated plates arranged in eight rows of four alternating circlets, all with uniform curved proximal margins (64 circlets in only complete specimen); broader proximally, transitioning abruptly into coriaceous sac composed of small, irregularly placed, granular platelets distally.

Remarks

The specimens from Montagne Noire are the same as those referred to by Vizcaïno and Lefebvre, Reference Vizcaïno and Lefebvre1999 and Vizcaïno et al., Reference Vizcaïno, Álvaro and Lefebvre2001 as ?Pyrgocystis Bather (Reference Bather1915), but which are actually rhenopyrgids (Sumrall et al., Reference Sumrall, Heredia, Rodríguez and Mestre2013). The D-shape of the suspected interradial oral plates in UCBL-FSL 713312 and similarities in the pygate zone mean we tentatively suggest these specimens belong to the genus Rhenopyrgus. They are here described together because there is little to justify their division into different taxa apart from minor variations in stratigraphy. However, pyrgate zone morphology is not a satisfactory taxonomic character, and thus this grouping is provisional.

Despite UCBL-FSL 713312 being relatively complete, the oral surface is highly disrupted making further detailed taxonomically relevant observations impossible. While several specimens display a distinctly tapering pyrgate zone, such morphology is observed in several other rhenopyrgid taxa, particularly R. grayae, and cannot be used to differentiate it as a new species. Thus, because key taxonomic characters to differentiate this taxon from other rhenopyrgid species are not satisfactorily exposed, it seems prudent to wait for better preserved material to become available before erecting a new species. They are recorded here for completeness and are perhaps notable as being the oldest Rhenopyrgus/rhenopyrgid yet identified.