Introduction
Our understanding of the morphology and phylogenetic inclusiveness of edrioasterids (Echinodermata) has been greatly enhanced over the past few decades. Bassler (Reference Bassler1935, Reference Bassler1936) presented a classification dividing edrioasteroids into five families: Stromatocystitidae, Hemicystitidae, Agelacrinitidae, Edrioasteridae, and Cyathocystidae, and this classification was followed by Regnéll, (Reference Regnéll1966). Kesling (Reference Kesling1967) added Pyrgocystidae to this scheme to accommodate taxa with a flexible, highly extendable, peduncular stalk. Bell (Reference Bell1976) revised this classification, essentially combining taxa in Bassler’s Hemicystitidae and Agelacrinitidae into Isorophida at the ordinal level, and restricting Edrioasterida (now at the ordinal level) to squat globular forms, while leaving Stromatocystitidae, Cyathocystidae, and Pyrgocystidae unchanged. Holloway and Jell (Reference Holloway and Jell1983) recognized that Pyrgocystidae was polyphyletic and removed long-stalked rhenopyrgids to a separate family.
Later phylogenetic studies (Smith and Jell, Reference Smith and Jell1990; Guensburg and Sprinkle, Reference Guensburg and Sprinkle1994) began to infer a grouping of edroasteroids that included a discoidal form clade (Isorophida plus Cambraster and related taxa), and an Edrioasterida clade that included edrioasterines, cyathocystids, edrioblastoids, and rhenopyrgids. Later analyses by Sumrall and Zamora (Reference Sumrall and Zamora2011) and Sumrall et al. (Reference Sumrall, Heredia, Rodriguez and Mestre2013) inferred patterns fully congruent with these earlier conclusions. At present, the biggest gap in our understanding is the nature and taxonomic distribution of both the plating bordering the peristome and the aboral structures of the theca and its attachment structures.
Here, we describe two taxa that shed light on some of these issues. Pseudedriophus guensburgi n. gen n. sp. is one of the few edrioasterine taxa that is preserved weathered free of matrix, thereby exposing the bottom surface of the theca. It reveals that the aboral surface is strikingly similar in plating to that of rhenopyrgids by bearing an aboral collar, peduncular stalk, and attachment disk. The edrioblastoid Porosublastus inexpectus n. gen. n. sp. is preserved with some of the ambulacral cover plates stripped from the margin of the deltoids, showing that the deltoids are compound plates combining the interradial plate of the mouth frame and numerous ambulacral floor plates bearing sutural pores.
Morphology
The oral surface of edrioasterids is well understood (Bell, Reference Bell1976; Bell and Sprinkle, Reference Bell and Sprinkle1978; Guensburg and Sprinkle, Reference Guensburg and Sprinkle1994; Sumrall and Deline, Reference Sumrall and Deline2009). All edrioasterid clades share five compound interradial oral elements (sensu Sumrall and Waters, Reference Sumrall and Waters2012; Kammer et al., Reference Kammer, Sumrall, Ausich, Deline and Zamora2013) that may or may not include fused proximal floor plates. Furthermore, all have biserial floor plates bearing sutural podial pores, relatively wide ambulacra when compared to isorophids, enlarged cover plates typically with a one-to-one correspondence to the underlying floor plates, and a wide shelfal expression of the floor plates along the edge of the ambulacra.
Whereas the lower theca and attachment structures of cyathocystids, edrioblastoids, and rhenopyrgids is understood (Bell, Reference Bell1982; Smith and Jell, Reference Smith and Jell1990; Guensburg and Sprinkle, Reference Guensburg and Sprinkle1994; Sumrall et al., Reference Sumrall, Heredia, Rodriguez and Mestre2013), those of globular edrioasterines remain poorly documented because the aboral surface of the theca was affixed to a firm or hard substrate, leaving only the oral surface exposed. Much has been determined about isorophid attachment structures because of rare instances of moldic fossil preservation (Sumrall and Zamora, Reference Sumrall and Zamora2011) and specimens that have weathered free from the substrate (Bell, Reference Bell1976; Sumrall, Reference Sumrall1993, Reference Sumrall2001). However, similar preservation modes are not common among edrioasterines, leaving much of the aboral surface undocumented.
In cyathocystids, there typically is an aboral collar around the oral surface demarking the proximal margin of the pedunculate zone of the aboral theca. The pedunculate zone plating is fused into a turreted structure that attaches to the substrate via toe-like extensions (Bell, Reference Bell1982; Bockelie and Paul, Reference Bockelie and Paul1983). There is no hint of the attachment disk because of extensive plate fusion.
In edrioblastoids, the aboral collar is poorly defined, but marks the boundary of the theca and the peduncular stem. The stalk or stem, though poorly known, is variable between taxa. In some it bears a mosaic of irregular plates, as in Cambroblastus (Smith and Jell, Reference Smith and Jell1990; Zhu et al., Reference Zhu, Zamora and Lefebvre2014), or a series of irregular chevrons or meres as in Lampteroblastus and Astrocystites (Webby, Reference Webby1968; Mintz, Reference Mintz1970; Guensburg and Sprinkle, Reference Guensburg and Sprinkle1994). The aboral disk is known only from Cambroblastus, where it is a polyplated attachment surface that grades into the distal stalk (Zhu et al., Reference Zhu, Zamora and Lefebvre2014).
In rhenopyrgids, the oral surface is bordered by an aboral collar that articulates to an elongate and flexible peduncular stalk. In most taxa, the pedunculate zone is formed from a series of four-piece, highly imbricate, plate circlets that alternate at 45 degrees with circlets above and below. This results in the pedunculate zone bearing eight columns of plates. The pedunculate zone abruptly terminates into an irregularly plated attachment structure—the so-called coriaceous sac (see Holloway and Jell, Reference Holloway and Jell1983, and Sumrall et al., Reference Sumrall, Heredia, Rodriguez and Mestre2013 for discussion of these structures).
These same three divisions are present in the globular edrioasterine P. guensburgi. Here, the aboral collar is well defined as a ring of plates below the ambitus. The peduncular stalk is narrow, tiny-plated, and well developed, and the attachment is a flared ring at the distal end of the aboral stalk.
This complex development of the aboral surface is likely not unique to P. guensburgi. The attachment disk has been described for Edrioaster priscus (Bell, Reference Bell1979; Sumrall and Deline, Reference Sumrall and Deline2009), where it sits in an aboral concavity. It seems likely that this depression results from compression of an aboral peduncular stalk that is covered in matrix. Cross-sections of Edrioaster bigsbyi show a zone of small platelets that is consistent with a short peduncular stalk, and poorly preserved aboral surfaces of E. bigsbyi show the presence of the aboral collar (Bell, Reference Bell1976). Furthermore, Guensburg and Sprinkle (Reference Guensburg and Sprinkle1994) described a short stalk and attachment structure for Paredriophus hintzei, but could not fully document these structures because they were not well exposed in the available material.
The peduncular stalk of Pseudedriophus guensburgi is also reminiscent of the pedunculate zone of discocystinid edrioasteroids (Sumrall, Reference Sumrall1996; Sumrall and Bowsher, Reference Sumrall and Bowsher1996; Sumrall and Parsley, Reference Sumrall and Parsley2003). Here the peduncular plates are arranged into organized columns except in the distalmost portion where the number of columns is reduced (Bell, Reference Bell1977; Sumrall and Parsley, Reference Sumrall and Parsley2003). These structures differ from peduncular stalks of rhenopyrgids where circlets of peduncular plates alternate rather than being aligned (Sumrall et al., Reference Sumrall, Heredia, Rodriguez and Mestre2013).
Interestingly, these aboral structures are demonstrably absent from the middle Cambrian taxon Totiglobus nimius. Here, the base of the theca is a circular structure plated on the outside by a flat radial array of small plates, the outer circlet bearing a ring of radiating ribs on the inside, usually two to three ribs per plate (Bell and Sprinkle, Reference Bell and Sprinkle1978). These probably connected ligaments from the oral surface to the base of the theca, allowing the theca to be pumped up higher in the water column for feeding (the globular state), or retracting it closer to the sea floor when currents were too high (the recumbent state) (Bell and Sprinkle, Reference Bell and Sprinkle1978; Sumrall, Reference Sumrall1993).
The other area of interest is the nature of the interradial plates that form a portion of the mouth frame. Similar plates are seen in most non-eleutherozoan echinoderms where they are termed oral plates (Sumrall, Reference Sumrall2010; Sumrall and Waters, Reference Sumrall and Waters2012; Kammer et al., Reference Kammer, Sumrall, Ausich, Deline and Zamora2013). Whether oral plates are homologues with these similar plates in edrioasteroids has not yet been established. However, these plates can be traced throughout the major edrioasteroid lineages. In Kailidiscus and Walcottidiscus, these interradial elements form part of the abradial floor plate system that is the likely homologue of the biserial floor plates in edrioasterids (Zhao et al., Reference Zhao, Sumrall, Parsley and Peng2010; Sumrall and Zamora, Reference Sumrall and Zamora2011). These interradial elements are bordered proximally by the adambulacral floor plate series that form the mouth frame (Zhao et al., Reference Zhao, Sumrall, Parsley and Peng2010). The interradial oral frame elements are also seen in stromatocystitids and Cambraster (Zamora et al., Reference Zamora, Sumrall and Vizcaïno2013). In isorophids, including pyrgocystids, integrated interradial elements are phylogenetically lost, leaving only radially positioned elements around the mouth frame (Sumrall and Zamora, Reference Sumrall and Zamora2011).
The mouth frame of Edriophus levis has been shown to contain both radial and interradial elements (Bell, Reference Bell1976). The radial elements form the physical peristomial border and look in detail like the oral frame plates of isorophid edrioasteroids (Bell, Reference Bell1976; Sumrall, Reference Sumrall1993; Sumrall and Parsley, Reference Sumrall and Parsley2003). These plates alternate with the interradial elements along pore-bearing sutures from which the floor plates extend distally (Bell, Reference Bell1976). Whether this is the common condition in Edrioasteridae is unclear. It is also unclear whether these radial oral frame plates occur in cyathocystids, edrioblastoids, and rhenopyrgids, but an isolated oral plate of Astrocystites illustrated by Kammer et al. (Reference Kammer, Sumrall, Ausich, Deline and Zamora2013) shows facets along the proximal margin of the peristomial opening consistent with the present of oral frame plates.
Clades of edrioasterids further differ in how the floor plates are incorporated into the theca with respect to the integrated radial elements. In edrioasterines, the first one to two floor plates are sutured to keystone-shaped interradial elements and the floor plates immediately become biserial (Bell, Reference Bell1976). In rhenopyrgids and the plesiomorphic edrioblastoid Cambroblastus, the situation is similar (Smith and Jell, Reference Smith and Jell1990; Sumrall et al., Reference Sumrall, Heredia, Rodriguez and Mestre2013; Zhu et al., Reference Zhu, Zamora and Lefebvre2014). In cyathocystids and derived edrioblastoids, however, all of the floor plates are fused with the interradial elements. The position of the floor plates is marked by the retention of sutural podial pores within the food groove and internal sculpting of the floor plate surface presumably with respect to the system of podia. Although these pores are seen in the proximal tip of the interradial elements in all edrioasterid clades, in taxa where floor plates are sutured, these pores only extend to the first floor plate suture.
Systematic paleontology
All specimens described in this study were collected from the middle Ninemile Shale in the Pseudocybele nasuta Zone (old Zone J), upper Ibexian, (upper Floian, uppermost Lower Ordovician) in Whiterock Canyon, Eureka County, Nevada. Specific collecting localities are listed in the Occurrence sections for each species. All specimens are reposited in the Non-vertebrate Paleontology Laboratory, Jackson School of Geosciences, University of Texas at Austin.
Class Edrioasteroidea Billings, Reference Billings1858
Order Edrioasterida Bell, Reference Bell1976
Suborder Edrioasterina, Bather, Reference Bather1898
Family Edrioasteridae Bather, Reference Bather1899
Genus Pseudedriophus new genus
Type species
Pseudedriophus guensburgi new genus new species
Diagnosis
Same as for species.
Etymology
From the Greek pseudes, false, and Edriophus, a common Late Ordovician edrioasteroid in northeastern North America, which it resembles.
Occurrence
Ninemile Shale (upper Floian), central Nevada.
Remarks
This small edrioasterine closely resembles Paredriophus, which is known from the lower and middle Fillmore Formation and the Kanosh Shale in nearby western Utah. However, Paredriophus typically has a larger and more elongate theca with longer ambulacra, adjacent interrays with numerous plates, single ambulacral cover plates, and a short but thick, multiplated, peduncular stalk. Complete specimens of Pseudedriophus are smaller than known specimens of Paredriophus, but one well-preserved ambulacral fragment showing several of the diagnostic features of Pseudedriophus came from a specimen almost as large as a typical Paredriophus specimen.
Pseudedriophus guensburgi new genus new species
Figure 1 Photographs of edrioasteroid and edrioblastoid specimens from the Ninemile Shale; all×4 except where noted. (1–11), Pseudedriophus guensburgi n. gen. n. sp. (1–3), crushed theca of holotype 1777TX7. (1), oral view of globular theca showing straight wide ambulacra; (2), aboral view of theca showing tips of C and D ambulacra with periproct in between (bottom), and circular collar around tiny-plated area surrounding relatively long peduncular stalk (top); (3), oblique aboral view of theca showing flared, plated, attachment disk at distal end of peduncular stalk (top); (4, 5), lateral and oral views of laterally crushed paratype 1781TX9 showing interambulacral plates and larger set of ambulacral cover plates; (6, 7), oral and lateral views of paratype 1777TX8 showing poorly preserved ambulacral plating but large interambulacral plates and proximal peduncular stalk plating in (7) (right and bottom); (8–10), large fragmented paratype 1778TX16; (8), enlargement of ambulacrum in (9), showing externally exposed floor plates and multitiered cover plates that are preserved in closed position (left) and open position (right); compare with Figure 2, ×8; (9), front of crushed fragment showing interambulacral plating in relation to the ambulacrum; (10), opposite side of crushed fragment showing mostly disarticulated thecal and ambulacral elements, including floor and cover plates; (11), isolated floor plate paratype 1779TX2 showing shape and slightly asymmetrical edges of pores, ×13.6; (12–15), Porosublastus inexpectus n. gen. and sp., holotype 1781TX8; (12), oral surface showing tips of large rounded deltoids and short straight ambulacra covered by simple, biserial, cover plates; (13), Slightly oblique front view showing three best-preserved ambulacra; note large deltoids with porous ornament and stripped cover plates on center ambulacrum showing curved deltoid edges bearing pores between fused floor plates; (14), back view of specimen showing rounded vault and some of pelvis plating; (15), oblique oral view showing biserial plating of ambulacral cover plates and porous ornament on large deltoid plates.
Types
Holotype 1777TX7; paratypes 1781TX9, 1777TX8, 1778TX16 (nearly complete or partial thecae), and 1779TX2 (separate floor plate).
Diagnosis
Edrioasterine having straight, relatively short ambulacra with multiple series of cover plates, few tessellate interray plates, a circlet of large collar plates at the thecal base, and a fairly long, thin, tiny-plated stalk for attachment.
Occurrence
Specimens 1777TX7-8 come from the lower slope float 2 m below and 1 m above the basal ledge (“0 bed”), and 1778TX16 comes from the upper slope float about 10 m above the basal ledge at WR-1 (Front Section). Separate plate 1779TX2 comes from a small float slab an unknown distance up the slope at WR-2 (Narrows Section), and specimen 1781TX9 comes from float about 5 m up the slope at WR-2A (Narrows Section NE Side Slope), all in Whiterock Canyon, WR-1 about one-fourth mi. (0.4 km) and WR-2 & 2A about three-fourths mi. (1.2 km) up Whiterock Canyon from the end of the N-side access road, and about 1.5 mi. (2.4 km) and 2 mi. (3.2 km) W of the Antelope Valley and Copenhagen Canyon road behind Martin Ridge (Nevada Rt. 82), all localities about 35 mi. (56 km) SW of Eureka, Eureka County, central Nevada, western USA.
Description
Terminology used in the description follows Bell (Reference Bell1976). Theca typically small, 12 mm wide in holotype, at least 8 mm high without stalk in paratype 1781TX9, with globular upper portion bearing ambulacra and interambulacra, and nearly flat aboral surface with relatively elongate peduncular stalk in center (Fig. 1.1–1.3). Peristome located centrally on oral surface, bordered by six relatively small, interradially positioned, oral plates one per interambulacrum except for two in CD (Fig. 1.1); presence of internal radially positioned oral frame plates cannot be confirmed in present material; ambulacra extending from laterally elongate peristome in 2-1-2 arrangement (Fig. 1.1); peristomial opening covered by series of oral cover plates; primary orals and lateral bifurcation plates undifferentiated in size and shape from other shared cover plates and ambulacral cover plates; oral cover plates arranged into a biseries with some specimens suggesting presence of small secondaries along perradial suture (Fig. 1.1, 1.5).
Ambulacra relatively wide, up to 3.6 mm proximally in small holotype; plated with large floor plates forming thecal wall and bearing food groove, protected by cover plates; floor plates arranged in simple biseries extending perradially along food groove, much wider than high, roughly rectangular and slightly dumbbell-shaped in oral aspect with notches on proximal and distal margins for sutural pores (Figs. 1.8, 1.11, 2), adradial edge chevron-shaped where articulated with two plates from opposite side of ambulacrum (Fig. 1.11); each floor plate bearing wide externally exposed shelf abradially that slopes upward toward ambulacral midline (Fig. 1.5, 1.8); shelf widest in proximal ambulacra and narrows distally; cover plate articulation marked by ridge positioned three-fourths of plate width toward abradial margin that separates shelf from food groove (Fig. 1.8, 1.11); food groove surface marked by complex series of ridges and grooves, generally two ridges most prominent just adradial to cover plate articulation, proximal ridge extending towards ambulacral midline parallel to plate axis; distal ridge less strongly developed but extending further, deep pit located between abradial tips of two ridges narrows and extends adradially becoming shallow, strongly marked groove along proximal edge of distal ridge (Fig. 1.8, 1.11), second groove extends from abradial edge of sutural pore on proximal side of plate towards ambulacral midline parallel to first grove (Fig. 1.8); some plates show small node between adradial ends of grooves (Fig. 1.8).
Figure 2 Drawing of well-preserved ambulacrum on Pseudedriophus guensburgi n. sp. thecal fragment paratype 1778TX16, showing multiple left-side cover plate exteriors lying on floor plates on left side of ambulacrum, in contrast to wide-open, right side with underlying floor plates exposed, and multiple cover plate interiors lying on adjacent thecal plates, ×11. FP=floor plates (dark shading), P=pore for tube foot, PCP=large primary cover plates (light shading), SCP=smaller secondary cover plates (white, 3–4 series), ?=areas not showing secondary cover plate boundaries.
Cover plates arranged in complex, multitiered pattern, partly developed in small holotype and paratypes (Fig. 1.1, 1.4), but best expressed in larger fragmentary paratype 1778TX16 (Figs. 1.8–1.10, 2); primary cover plate series proportionately very large, approximately half width of entire cover plate series (Figs. 1.8, 2); articulate singly upon each floor plate, square to slightly pentagonal in outline with lateral sutures perpendicular to ambulacral axis; secondary series much smaller, approximately one-half to one-third the size of primary series and more irregular in shape, completely border adradial tips of primary series (Figs. 1.8, 2); more adradial plate series increasingly irregular with two to three rows of small platelets; cover plate interior surfaces ruled with sharp, transverse ridges that branch near articulation point forming sharp V-shaped structures on primary plates (Figs. 1.8, 2); each primary plate bearing one pair of such ridges positioned near proximal and distal margins such that ridges from adjacent cover plates lie directly above podial pores; ridges extend onto more adradial plate series where they become less distinct.
Interambulacra plated with relatively few plates; holotype and small paratypes have approximately eight to 11 large plates in non-anal side interambulacra (Fig. 1.1, 1.4, 1.7) with slightly more but smaller plates on anal side to accommodate periproct, large ambulacrum fragment having few, but proportionately larger interambulacral plates (Fig. 1.9, 1.10); distal-most interambulacra extend over ambitus onto aboral surface where one irregular circlet of small plates spans interambulacral areas to aboral collar circlet (Fig. 1.2, 1.3). Aboral collar circular, restricted to aboral surface, approximately 5 mm in diameter in holotype, with approximately 11–12, roughly trapezoidal plates in circlet approximately 1.5 mm wide with some variation in individual plate size (Fig. 1.2, 1.3).
Distal margin of theca beyond aboral collar strongly demarked and dividable into three regions: proximally flush portion, middle peduncular stalk-like portion, and distal attachment surface of stalk (Fig. 1.2, 1.3). Proximal portion circular in outline, extending from distal edge of aboral collar distally between 1.1 and 1.3 mm, plated by irregular series of minute granular platelets; middle portion columnar, extending about 4 mm in length and 1.8 mm wide in holotype (Fig. 1.2), plated with approximately 15–20 columns of small plates with slightly depressed sutures between columns (Fig. 1.2, 1.3), number of plates in columns unclear in present material; distal portion slightly flared for attachment structure (Fig. 1.2, 1.3), plated with numerous plates, slightly larger than peduncular columns, but otherwise of unclear arrangement, depressed center of bottom surface apparently plated over (Fig. 1.3).
Periproct positioned along midline in distal CD interambulacrum slightly below ambitus (Fig. 1.2), relatively large, details of plating not entirely clear, but peripheral plates small and quadrate, central plates irregular and lathe-shaped. Hydropore and gonopore unclear in present material, hint of pore perforating CD oral plate complex in holotype (Fig. 1.1).
Etymology
Named for Thomas E. Guensburg, who has described many Ordovician edrioasteroids.
Remarks
It is thought that all of the type specimens of Pseudedriophus guensburgi from the Ninemile Shale (upper Floian), including the three small thecae and the ambulacral fragment that is nearly twice as large, belong to the same taxon. All of these are smaller than the type specimens of Paredriophus hintzei from the lower Fillmore Formation (upper Tremadocian), and show consistent morphologic differences. The ambulacra are shorter and wider and there are fewer and proportionately larger interambulacral plates in the adoral theca both of which may be consistent with ontogeny; however, the presence of multiple series of cover plates and longer and thinner attachment stalk on the aboral side of the theca are demonstrably different. We think that these two taxa are closely related, and the slightly older P. hintzei may be the sister taxon to the somewhat younger P. guensburgi.
Suborder Edrioblastoidina Fay, Reference Fay1962
Family Astrocystitidae Bassler, Reference Bassler1935
Genus Porosublastus new genus
Type species
Porosublastus inexpectus new genus new species
Diagnosis
Same as for species.
Etymology
From the Greek porosus, full of holes, and blastos, bud-shaped, because of its porous plate surfaces of the deltoid and radial plates and bud-shaped thecal morphology.
Occurrence
Ninemile Shale (upper Floian), central Nevada, western USA.
Remarks
Porosublastus seems most closely related to the other Early Ordovician edrioblastoid Lampteroblastus Guensburg and Sprinkle (Reference Guensburg and Sprinkle1994) from the slightly older middle Fillmore Formation in nearby western Utah. It differs from Lampteroblastus by having a larger vault, fewer and larger plates in the pelvis, much smaller ridges on the pelvis plates, and much larger stereom pores in all the plate surfaces. It appears more derived than the late Cambrian Cambroblastus from Australia and China, which has a very rudimentary stalk and separate ambulacral floor plates that are not fused with the adjacent deltoids, features not shared with any other member of this family (Smith and Jell, Reference Smith and Jell1990; Zhu et al., Reference Zhu, Zamora and Lefebvre2014). It is less advanced than the Upper Ordovician Astrocystites, which has more reduced thecal plating, especially in the pelvis, very long ambulacra made from large parabolic-shaped deltoids, possible slit-like respiratory structures on several plate circlet sutures, and a large meric stalk (Whiteaves, Reference Whiteaves1897; Webby, Reference Webby1968; Mintz, Reference Mintz1970).
Porosublastus inexpectus new genus new species
Holotype
1781TX8 collected by Sprinkle in 2001.
Diagnosis
Astrocystitid bearing large deltoids with pitted ornament, and lower theca having few relatively large plates that lack large stellate ridges.
Occurrence
From about 8 m above the base of the shale slope at locality WR-2A, the Narrows Section NE side slope, about 1.3 km up Whiterock Canyon from the end of the north-side access road, and about 3.2 km west of the Antelope Valley road behind Martins Ridge (Nevada Rt. 82), about 35 mi. (56 km) southwest of Eureka, Eureka County, central Nevada.
Description
Holotype and only known specimen is a somewhat crushed medium-sized theca; theca apparently elongate bud-shaped, 13 mm high, 14 mm wide just above tips of ambulacra as preserved, vault apparently hemispherical (Fig. 1.14), back somewhat eroded, lower pelvis apparently missing; vault dominated by four (of 5) large interradial deltoids (DD) separating three (of 5), medium-length, straight ambulacra (Fig. 1.13); deltoids triangular in outline, with straight distal margin, nearly straight but convex, lateral sutures that form ambulacral food grooves, and rounded adoral margin (forming edge of disrupted peristome), becoming deeply depressed medially with respect to edges of ambulacra; where ambulacral cover plates missing, lateral deltoid margins showing sharp transition into deep food groove with row of pores just below edge (Fig. 1.13), although fused, outlines of individual floor plates seen in food groove where large platform with two transverse ridges extend adradially into food groove to each podial pore; ambulacral cover plates arranged in simple biseries of parallelogram-shaped, thin plates with zig-zag perradial suture (Fig. 1.13, 1.15), expanding adorally until near peristome, peristomial cover plates somewhat disrupted (Figs. 1.12, 1.15), two ambulacra and intervening deltoid (from back of theca) missing, no 2-1-2 pattern of ambulacral branching visible, no periproct, hydropore, or gonopore visible to identify CD side of theca.
Pelvis more disrupted and incomplete than vault, containing at least two circlets of thecal plates; three to four medium-sized “radials” (RR) present, three on front of theca directly below slightly curved ambulacral tips, pentagonal, folded down middle forming pentagonal cross section for top of pelvis, concave outer profile, long straight top suture, shorter side sutures, shorter oblique bottom sutures; parts of three “interradial” (IRR) or basal (BB) plates below RR, plates broken, possibly in circlet, either hexagonal (IRR) or pentagonal (BB), might have low diagonal ridges up to RR.
Ornament on most thecal and cover plates distinctive, consisting of fine to coarse pores covering different parts of plate surface; deltoids having coarsest pores in depressed middle of plate versus elevated edges (Fig. 1.13), “radials” having coarsest pores in depressed areas alongside central adoral ridge (Fig. 1.13, 1.14), ambulacral cover plates having coarsest pores near perradial suture; surface pores may be expression of plate stereom (Fig. 1.13, 1.15). No thecal respiratory structures seen.
No stalk or stem present, or facet indicating that one was present.
Etymology
From the Latin unexpectens, unexpected, because this specimen is the only edrioblastoid found in twenty-three years of collecting in the Ninemile Shale.
Remarks
Porosublastus inexpectus n. sp. is probably most closely related to Lampteroblastus hintzei from the middle Fillmore Formation. It differs by having a larger vault, apparently fewer and larger pelvis plates, almost no ridges on these pelvis plates, and the distinctive porous surface ornament on several plate sets.
The edrioblastoid is preserved with some of the ambulacral cover plates stripped from the margin of the deltoids, showing that the deltoids are compound plates combining the interradial oral plate of the mouth frame and numerous ambulacral floor plates bearing sutural pores (Fig. 3). Other edrioasterids show this condition to varying degrees. In edrioasterines and rhenopyrgids, the oral plates appear to lack fusion of floor plates (Bell, Reference Bell1976; Sumrall et al., Reference Sumrall, Heredia, Rodriguez and Mestre2013). In cyathocystids, the situation is similar to edrioblastoids, with the oral plates being fused to the floor plate system. Interestingly, this is not the case in the early edrioblastoid Cambroblastus, where the floor plates remain separate elements (Zhu et al., Reference Zhu, Zamora and Lefebvre2014). This suggests that the fused condition evolved independently in edrioblastoids and cyathocystids.
Figure 3 Reconstructed side-layout thecal plating of crushed and damaged holotype 1781TX8 of Porosublastus inexpectus n. gen. n. sp., showing preserved parts of four large deltoids and three sets of ambulacral cover plates in vault, along with three “radial” plates below tips of ambulacra and several underlying “interradial” plates in more damaged pelvis. Center ambulacrum highly restored to show floor plates with pores on sloping edges of adjacent deltoids and partially reconstructed cover plate set, ×3.3. Dashed lines are reconstructed parts of damaged plates; ACP=ambulacral cover plates, D=deltoid, IR=interradial, R=radial.
Acknowledgments
We thank T. Guensburg, Rock Valley College, R. Johns, Austin Community College, D. Sprinkle, Austin, TX, C. Schneider, Edmonton Geological Survey, and F. Gahn, BYU Idaho, for assistance at different times in the field. Early funding for this research was provided by NSF Grant BSR-8906568 (Sprinkle; 1989-92), and later funding by the University of Texas Geology Foundation, and NSF Grant EAR-0745918 (Sumrall). D. Meyer, University of Cincinnati, and T. Kammer, West Virginia University, reviewed the manuscript, and made many helpful suggestions for improvements.
