Terminology

Terminological usage for asterozoans begins with Spencer and Wright (Reference Spencer, Wright and Moore1966). The primary skeleton forms the body wall. The accessory skeleton includes the generally abundant spines, granules, and pedicellariae seated on all primary ossicular types except arm axials. Axial or ambulacral ossicles form a double series along the axis of the arm. Axials are vaulted to form the ambulacral furrow, and the mouth-angle ossicles (MAOs) are the proximalmost ossicles of the axial series. Axial ossicles articulate with and in more-derived asteroids are immediately dorsal to the adaxial or adambulacral ossicles. The remainder of the skeleton is extraxial. Ambital framework sensu Blake and Guensburg (Reference Blake and Guensburg2015) marginal ossicles form either a single inferomarginal series or a double inferomarginal plus superomarginal series, one or both positioned at the abradial ambital edge. Intermarginal series occur between the double marginal series of some asteroids. Dorsal ossicles, the adradialia, were defined by Spencer and Wright (Reference Spencer, Wright and Moore1966, p. 28) as ossicles of series between the carinals and marginals. Dorsal disk ossicles are arranged in one or multiple more or less regular cycles around the disk axis. Dorsal midarm ossicles can be enlarged and/or otherwise differentiated to form a carinal series. A madreporite links the water vascular system to the environment, although it can be difficult to locate among Paleozoic asteroids. No, few, or many actinal ossicles occur between the inferomarginals and adaxials. Useful descriptive terminology for paxillae was introduced by Ludwig (Reference Ludwig and Bronn1899, p. 510), followed by Spencer (Reference Spencer1918, fig. 82), and adopted here: individual paxillae consist of an expanded paxilla base that gives rise to a cone-like to columnar paxilla shaft typically bearing a paxilla crown of accessories; the paxilla shaft commonly expands at the base of the crown.

Diagnosis

Hadrosidans with disk proportionately small, dorsum rounded, interbrachial angles more or less narrowly rounded; outline of smaller individuals more or less stellate whereas in larger individuals arms proportionately elongate. Dorsal disk expressions varied; ossicles few to many, arranged in irregular to well-defined concentric cycles. Dorsal disk ossicles and arm carinals of most genera more or less similar, paxillar, or plate-like, or thickened and irregular. Adradialia proportionately small, uniform, crescentic, opening and overlapping toward dorsal arm midline. Adradialia aligned in intersecting series, series arced from arm midline. Madreporite dorsal where recognized, can be moderately enlarged. Marginal ossicles in single, inferomarginal series; inferomarginals morphologically more or less clearly differentiated from adradialia, can be similar in size. Inferomarginals typically smaller than adjacent adaxials, ossicles of the two series together more or less resembling a double marginal series. Axillary where recognized inconspicuous, not distinctly differentiated from marginals. Actinal ossicles absent from arms, few if any on disk.

Axials foreshortened, podial basins in two rows throughout arm length. In many urasterellids, proximal axials and MAOs more or less distinctly deflected dorsally toward dorsal disk surface to form actinostome; one or more pairs of proximal axials can abut or nearly abut across the interbrachium to form an adoral carina. MAOs and any accessories comparatively small. Adaxials proportionately large, dominating ventral arm surface and forming part of arm margin; adaxials plate-like, robust, upright to form coin-like columns along arm axis; adaxial transverse accessory bases more or less well developed. Accessories varied, including spinelets and granules; significantly enlarged spines not recognized (edited from Blake, Reference Blake2018.)

Remarks

Fossils later assigned to the Urasterellidae were described and named beginning in the middle of the nineteenth century, mostly on the basis of limited material and, by later standards, with cursory descriptions. Schuchert (Reference Schuchert1915) provided extended review and discussion of urasterellid concepts; these were further reviewed and expanded by Spencer (Reference Spencer1918). Spencer and Wright (Reference Spencer, Wright and Moore1966) provided familial and generic summary, Shackleton (Reference Shackleton2005) reviewed status of Ordovician exemplars, and new urasterellid taxa with some reevaluation were included in Blake, Guensburg, and Lefebvre (Reference Blake, Guensburg and Lefebvre2016) and Blake (Reference Blake2017, Reference Blake2018).

The familial diagnosis of Schuchert (Reference Schuchert1915, p. 172) addressed axial positioning at the arm midline and expression of the marginal series. At the generic level, Schuchert (Reference Schuchert1915, p. 173) accepted only the name Urasterella but included 13 species names, some tentative. Certain interpretations of urasterellid morphology of Schuchert have been challenged by subsequent authors as discussed in the following. Spencer (Reference Spencer1918, p. 125) combined three families, including the Urasterellidae, in his ‘Section C’ of the Asterozoa. Two genera, Urasterella and Salteraster, were recognized. Spencer (Reference Spencer1918, p. 125) stressed occurrence in time in suggesting that “They [i.e., urasterellids] must have originated early because they form an important section of the earliest known Asterozoa.” The familial concept here is largely compatible with that of Spencer and Wright (Reference Spencer, Wright and Moore1966), stressing rounded arm shape, a single marginal series, and differentiation of the carinal series, similar paxilliform adradialia arranged in series and aligned with the adaxials, and adaxials disk-shaped and spine-bearing.

Remarks on generic diagnoses

For the most part, taxonomic diagnoses in the literature emphasize differentia. The expanded treatments here seek to ease comparisons. Needed generic revisions call for identification of added specimens from the type localities of type species, entailing work beyond the scope of the present study.

Remarks on the morphology of the Urasterellidae

Carinal series expression is critical to urasterellid generic concepts but less clearly expressed than might be hoped, in part because of irregular ossicular addition with growth, in part because of similar larger disk and carinal series shapes and sizes, and in part because of specimen disruption with preservation.

The ambulacral column of the Urasterellidae differs from such typical crown-group genera as extant Echinaster Müller and Troschel, Reference Müller and Troschel1840 in that, in the latter, podial pores are developed between successive axials, and axials and adaxials are offset rather than paired whereas essentially similar plate-like adaxial configurations, interadaxial articular faceting, and transverse series of accessory spines are found in both (Figs. 4.8, 6.14, 8.5, 8.6, 8.8).

In his evaluation of Salteraster, Spencer (Reference Spencer1918, p. 150) argued that displaced adradialia demonstrate that ossicular bases were quadrate, the ‘V’ shape an expression of the paxilla shaft (e.g., Figs. 1.6, 1.8, 2.8, 6.2, 7.4, 8.14, 8.15, 8.18, 9.8). In a discussion that can be extended to all urasterellids, Blake (Reference Blake2018, p. 36R) argued that many adradialia exhibit the presence of a true reentrant and that this ossicular shape likely reflects the presence of a respiratory papula comparable to those found among contemporary asteroids. Papulae are complexly distributed over the body wall among extant asteroids, and distribution could have been patchy in Salteraster and other urasterellids, perhaps thereby yielding the configuration noted by Spencer (Reference Spencer1918). Differential preservation can further obscure morphology.

Schuchert (Reference Schuchert1915, p. 172) was of the view that a double set of marginal ossicles was developed in the Urasterellidae, whereas Spencer (Reference Spencer1918, p. 128) recognized only inferomarginals (p. 130), a view supported by Blake (Reference Blake2018) and retained here.

Axial ossicles are paired across the arm midline among extant asteroids, but irregularly and partially offset in earlier representatives, especially so more distally on the arms. The interpretation here is that offset is the plesiomorphic condition among asterozoans and that alignment was emergent within Paleozoic Asteroidea, homoplastic among clades, and commonly enhanced with growth (i.e., axials nearer the disk tend to be more clearly paired than the more distal within a single individual) (Blake, Reference Blake2013, Reference Blake2018), indicating ontogenetic change. Taphonomic events can displace ossicles. Urasterellid axials are particularly vulnerable to distortion because ossicular termini at arm midlines are constricted (Fig. 4.5, 4.9) and ossicles therefore are readily subject to some postmortem displacement.

For various taxa, authors working with Paleozoic asteroids have argued the presence of podial pores (potentially leading to internal ampullae) at the juncture of axials and adaxials, although for urasterellids, Hudson (Reference Hudson1916, p. 117) rejected a hypothesis of pore presence. Blake (Reference Blake2018) argued that podial pores were emergent among Paleozoic Asteroidea and that criteria for evaluation of presence are available. Especially among earlier euaxosidans, many gaps or discontinuities were ascribed to diagenetic alteration of complex articular configurations. Some specimens of A. huxleyi both retain life orientation and expose axial–adaxial junctures that show such skeletal discontinuities (Fig. 4), argued as sites for muscles used in arm flexure; the spacing also provides clearance during movement. In addition, skeletal configurations do not indicate the presence of internal water-vascular tissue receptacles whereas a potential external receptacle is suggested by a Salteraster grandis occurrence (Fig. 8.8). Podial pores are not recognized in the Urasterellidae.

Pits suggestive of the pedicellariae housings found among extant asteroids occur on some adaxials of Salteraster grandis. Pedicellariae recognition minimally demands the presence of two opposed spinelets whereas only a single large spine occurs in S. grandis (Fig. 8.10, 8.12); a hypothesis of pedicellariae presence is rejected, the pits likely housing articular tissues.

Remarks on generic subdivision of the Urasterellidae

Historically, interpretation of taxa herein assigned to the Urasterellidae (and some no longer so assigned) form an intertwined complex of observations and viewpoints, some contradictory. Earlier partitioning of the Urasterellidae by Schuchert (Reference Schuchert1915) and Spencer (Reference Spencer1918) was revisited by Spencer (Reference Spencer1950), who recognized three groups of Urasterella in a diagnostic key (p. 406, ‘Note 4’). The first and second were based on the presence of one or two (only Ulrichaster) rows of mid-dorsal arm ossicles. The third category was represented by Palastropecten Stürtz, Reference Stürtz1886, later synonymized with the ophiuroid Furcaster Stürtz, Reference Stürtz1886 (Spencer and Wright, Reference Spencer, Wright and Moore1966, p. 88). Taxa with a single carinal row were divided into those with differentiated adradialia between the carinals and marginals (only Urasterella) and those in which adradialia are similar. The latter category in turn was separated into Salteraster, with highly swollen arms, and newly proposed Phillipsaster Spencer, Reference Spencer1950, with moderately swollen arms (Spencer, Reference Spencer1950). Three species were assigned to Phillipsaster: the type species Salteraster? coronella Salter, Reference Salter1857, Urasterella ruthveni mut. arisaigensis, sensu Ruedemann, Reference Ruedemann1916, p. 46 (the term ‘mutations’ was used by paleontologists “before the geneticists captured that word”; Simpson, Reference Simpson1961, p. 176), and Salteraster selwyni (McCoy, Reference McCoy1874; see also Withers and Keble, Reference Withers and Keble1934 for S. selwyni).

Spencer and Wright (Reference Spencer, Wright and Moore1966) listed eight available generic names for the Urasterellidae, noting that Palaeaster Hall, Reference Hall1852 included species later assigned elsewhere. Among the remaining names, Spencer and Wright (Reference Spencer, Wright and Moore1966) treated Bohemaster Jaekel, Reference Jaekel1923, as unrecognizable, an interpretation retained here. Three Hunsrück Slate genera proposed by Stürtz (Reference Stürtz1886), Roemeraster, Palasteracanthion, and Protasteracanthion, were synonymized with Urasterella (Spencer and Wright, Reference Spencer, Wright and Moore1966). Modern preparation techniques enable much improved evaluation of adequately preserved Hunsrück Slate specimens, but until such work is undertaken, these three names are held in abeyance. The presence of a double row of carinals was retained for the concept of Ulrichaster by Spencer and Wright (Reference Spencer, Wright and Moore1966, p. 73), and a single row of carinals was recognized for Urasterella, Salteraster, and Phillipsaster, the latter two differing on degree of swelling of the arms, whereas Urasterella was perceived as having flattened arms and paxillae with ‘unequal-sized bases.’

Shackleton (Reference Shackleton2005, p. 99) favored the synonymizing of Salteraster, Phillipsaster, and Ulrichaster with Urasterella; this author's taxonomic treatment emphasized better-understood species. The type of Salteraster, S. asperrimus (Salter, Reference Salter1857), was used for the coding of Urasterella. Because diagnoses of Spencer (Reference Spencer1950) and Spencer and Wright (Reference Spencer, Wright and Moore1966) were based on limited criteria, concerns of Shackleton (Reference Shackleton2005) were well founded; nevertheless, the approach herein differs in part from that of Shackleton as well as that of Blake (Reference Blake2018) in recognizing certain names based on limited data pending possible discovery and study of new specimens. Salteraster and Ulrichaster therefore are retained, but given the idiosyncratic nature of interpretation of arm shape (i.e., degree of swelling), Shackleton (Reference Shackleton2005) is followed in synonymizing Phillipsaster with Urasterella. Diagnoses of Urasterella, Salteraster, and Ulrichaster herein begin with emphases of Schuchert (Reference Schuchert1915), Spencer (Reference Spencer1918, Reference Spencer1950), and Spencer and Wright (Reference Spencer, Wright and Moore1966). Meturaster was recognized by Blake et al. (Reference Blake, Guensburg and Lefebvre2016), and the Carboniferous genera Staurasterias and Anthroosasterias were described by Blake (Reference Blake2017). The availability of a new suite of fossils encompassing a broad size range herein enables assignment of Urasterella huxleyi (Billings, Reference Billings1865) to Anthroosasterias.

Given the many complexities of both the fossils and past interpretations, no simple taxonomic partitioning is possible; the key of Table 1 is intended to serve as a starting guide to specimen identification.

Type species

Uraster ruthveni Forbes, Reference Forbes1848.

Other species

Twelve names were included or tentatively included in the species listing for Urasterella (Schuchert, Reference Schuchert1915, p. 175), including the type, U. ruthveni. Among these, U. asperula is included here and U. pulchella Billings, Reference Billings1857 is tentatively so, although the holotype of the latter is poorly preserved and the configuration of the arm midline is equivocal. Urasterella montanus (Shchurovskiy, Reference Shchurovskiy1874; name transcription S. Rozhnov, 2020) is important because of its Carboniferous age, but efforts to locate types were unsuccessful (Blake, Reference Blake2017). Both Schöndorf (Reference Schöndorf1909, p. 324) and Spencer (Reference Spencer1918, p. 147, 148) described the species in some detail, Spencer accepting it as a Urasterella; diagrams of Shchurovskiy (Reference Shchurovskiy1874) appear plausible but not definitive as a urasterellid. Spencer cited the Schöndorf (Reference Schöndorf1909, p. 326) species occurrence from the lowest horizons of the upper Carboniferous. Recovery of added specimens by collectors has been reported at a popular source, and therefore further information might eventually become available. Of other species listed by Schuchert, ‘U.’ asperrimus is the type species of Salteraster; U. constellata Thorent, Reference Thorent1838 was thought closest to Urasterella (Spencer, Reference Spencer1918, p. 147); ‘U.’ girvanensis Schuchert, Reference Schuchert1915 was recognized as the type species of Cnemidactis Spencer, Reference Spencer1918 (p. 156); ‘U.’ grandis was assigned to Salteraster (Spencer, Reference Spencer1918, p. 154); ‘U.’ hirudo was transferred to ophiuroids (Spencer, Reference Spencer1930, p. 434); ‘U.’ huxleyi is assigned herein to Anthroosasterias; ‘U.’ selwyni McCoy, Reference McCoy1874 was tentatively assigned to Salteraster (Spencer, Reference Spencer1918, p. 155); and ‘U.’ ulrichi Schuchert, Reference Schuchert1915 is the type species of Ulrichaster. In work subsequent to Schuchert (Reference Schuchert1915), U. ruthveni leintwardinensis Spencer, Reference Spencer1918 and U. thraivensis Spencer, Reference Spencer1918 were described whereas the new species ‘U.’ gutterfordensis later was transferred to Ulrichaster (Spencer, Reference Spencer1950, p. 406). Other undocumented names are listed (Spencer, Reference Spencer1918, p. 136, 146). Assignment of the Silurian U. creswelli Withers and Keble, Reference Withers and Keble1934 is in need of review whereas the Devonian Hunsrück Slate ‘U.’ verruculosa Lehmann, Reference Lehmann1957 is no longer assigned to Urasterella (Hotchkiss and Glass, Reference Hotchkiss, Glass, Harris, Böttger, Walker and Lesser2010). Ultimately, evaluation of the concept of Urasterella calls for added specimens of the type species. Generic range: Black River (= Sandbian) Ordovician–Early Devonian (Emsian)–late Carboniferous?

Diagnosis

Urasterellid, arms perhaps weakly petaloid. Disk ossicular configuration comparatively simple and regular: centrale enclosed by circlet of five prominent ossicles bearing longitudinal ridge-like paxilla shaft; one or two additional ossicles present in central disk. Madreporite recognized in some specimens; small and near disk center. Carinal series clearly defined, ossicles abutted; carinal series bordered on each side by row of six-sided ossicles, ossicles of three series bearing longitudinal ridge. Two or three added rows of smaller adradialia on arms, with relatively delicate paxillae shafts. Ossicular form somewhat varied among species. Inferomarginals distinct proximally, more obscure distally; paxillae shafts present. Adaxial series dominating ventral arm surfaces; outer face ridged; spine bases developed.

Remarks

Generic diagnosis for Urasterella is difficult because the type suite of the type species, U. ruthveni, consists of only five incomplete, moldic specimens (Spencer, Reference Spencer1918, p. 140). Generic recognition is retained not only because of name validity but also because of widespread usage. ‘Generic characters’ of Urasterella of Schuchert (Reference Schuchert1915, p. 174) were extended but based on ‘U.’ grandis, which was “thought to be in generic harmony with the genotype” (p. 174). Schuchert noted that U. rutheveni “is not well known, but appears to be congeneric with ‘U.’ grandis and U. pulchella” (p. 187). The generic diagnosis of Schuchert therefore is essentially his species diagnosis for ‘U.’ grandis rather than for the type species of the genus.

Treatment of Schuchert (Reference Schuchert1915) was revisited and reinterpreted by Spencer (Reference Spencer1918) and subsequently augmented by Spencer (Reference Spencer1950, Reference Spencer1951) and Spencer and Wright (Reference Spencer, Wright and Moore1966). The “Generic Characters” of Spencer (Reference Spencer1918, p. 136) for Urasterella were limited to “a Urasterellid [sic] with dissimilar rows of adradialia.” Associated discussion is more extended, but neither the reconstruction of a part of the dorsal surface (Spencer, Reference Spencer1918, p. 140, fig. 90) nor the following description clearly identify a differentiated series adjacent to the carinal series. Spencer went on to rely on a new ‘variety,’ U. ruthveni leintwardinensis, diagrammed (fig. 92, p. 143) with series adjacent to the carinals differing somewhat in form and described as having “bases more distinctly stellate, and the paxilla-shaft more thorn-like, than in the case of the corresponding structures of the radialia”; ‘radialia’ herein referring to carinals. The approach is difficult because of its reliance on specimens that were described as a ‘variety’ and because treatment of Spencer (Reference Spencer1918) begins with the new species U. thraivensis rather than with the type U. ruthveni. The brief diagnosis of Spencer and Wright (Reference Spencer, Wright and Moore1966, p. 71) was “aboral side of arms flat, many paxillae with unequal-sized bases.” The generic diagnosis here is summarized from data of Spencer (Reference Spencer1918, p. 127–145) on British representatives and his concurrence on assignment of the Hunsrück slate U. asperula (Fig. 1.1–1.4) to Urasterella.

Type species

Anthroosasterias stibareus Blake, Reference Blake2017, by monotypy.

Other species

The type species A. stibareus, Gilmore City Formation, Early to Middle Mississippian, Iowa; A. huxleyi (Billings, Reference Billings1865), Point Riche, Newfoundland, original citation, Quebec Group, Chazyan (currently, latest Darriwillian).

Diagnosis

Urasterellid with significant morphologic change accompanying increase in size: small individuals similar to smaller individuals of other urasterellids; in larger individuals, disk proportionately thickened, ossicles numerous, bulbous, ossicular arrangement irregular, arms highly elongate. Carinal series ossicles similar to larger disk ossicles but varied in size, locally offset to form an irregular sequence; configuration particularly complex at disk/arm juncture. Adradialia not bearing attenuated paxillae shafts; adradialia in multiple series. Small madreporite recognized among smaller specimens. Inferomarginals enlarged relative to adradialia. Actinostome, adoral carina strongly differentiated in larger individuals; ossicles of axial series foreshortened approaching actinostome.

Holotype

GSC 554.

Description

Extraxial ossicular differentiation among smaller individuals less distinct than among larger; dorsal ossicles more nearly equidimensional in smaller. Among larger individuals, dorsal disk more or less vaulted or domed, ossicles abundant, organization appearing irregular. Disk ossicles more or less clearly differentiated into size groups with enlarged ossicles toward center, these presumably including centrale enclosed by two more or less irregular circlets of enlarged ossicles. Larger ossicles subcircular, thickened. Smaller ossicles abundant, somewhat varied in size, polygonal to subcircular, some exhibiting papular? reentrants. Madreporite not recognized among available larger specimens but recognized among smaller.

Carinal series well defined more proximally, ossicles of series partially offset; distalmost ossicles of carinal series possibly not clearly differentiated from adjacent adradialia. Adradialia adjacent to carinals not differentiated from more lateral ossicles; adradialia typical of urasterellids but more shield-like rather than bearing prominent paxilla-shaft; adradialia arranged in many series in larger individuals, series extends to disk (Figs 2.7, 3.3, 3.8). Adradialia of arm of smaller individuals of form and arrangement similar to those of larger individuals.

Inferomarginals approximately paired with both adradialia and adaxials; inferomarginals and adradialia sizes similar, adaxials much larger; few smaller marginals irregularly intercalated along arm, not aligned with adradialia or adaxials. Inferomarginals together with adaxials forming arm margins. Inferomarginals subrectangular to elliptical, elongate parallel to arm, ossicles more nearly equidimensional distally on arms, not bearing papular reentrants; sequential ossicles abutted to weakly overlapping. Inferomarginals at disk poorly exposed in most specimens, can be exposed in dorsal aspect, these transversely elongate; inferomarginal series of adjacent arms converging toward disk center (Fig. 3.8). Inferomarginals less clearly differentiated from adradialia in smaller specimens; spines or spine-like prominences developed on at least some proximal marginals (Fig. 5.7). Axillary recognized in some smaller specimens, occurrences in larger specimens obscured with development of adoral carina and actinostome. Actinals not recognized.

Axials more or less typical of family (Fig. 4): axials rectangular in outline, moderately foreshortened, proportionately more strongly so approaching disk (Figs. 3.1, 3.2, 4.1, 4.2). Radial channel small, ossicular terminus narrowed at arm midline (thereby allowing for transverse water-vascular channel and space for arm flexure). Axial positioning at adaxial contact as exposed in ventral aspect can be suggestive of a podial pore (Fig. 4.5, 4.9) but podial opening lacking. Axials tall with deep articular tissue basins extending essentially full ossicular breadth; dorsal edge bordered by well-defined rim enclosing tissue basin. Adaxials more or less typical of family; closely fitted, much larger than inferomarginals. Adaxial proximal, distal faces approximately equidimensional, tissue depressions large; well-developed interossicular facets along dorsal edge (Fig. 4.8). Actinostome deep in larger specimens thereby obscuring MAOs; first seven or eight adaxials abutted across interbrachium to form adoral carina. Adoral carinal variation within and among specimens is varied enough to suggest both facultative twisting and therefore changing superficial appearances during normal life activities and burial with death as well as gradual carinal evolution through geologic time. Actinostome differentiation can be expressed in smaller specimens (Fig. 5.6). MAOs obscured by actinostome in larger specimens, comparatively small in smaller specimens; one robust oral spine occurs on each MAO pair in smaller specimens (Fig. 5.7). Spinelets numerous on MAOs, proximal adaxials (Fig. 3.4).

Remarks

Availability of the new fossil suite documenting transition between the comparatively small holotype of ‘Stenaster’ huxleyi Billings, Reference Billings1865 (Fig. 1.5–1.7) and the holotype of Anthroosasterias stibareus (Fig. 1.8–1.11) enables transfer of huxleyi to Anthroosasterias.

All the new A. huxleyi specimens are on slabs, none fully free as to expose all surfaces. Specimens include 34 on separate blocks, mostly a single specimen on each block, the smallest of these of arm radius (R) approximately 8 mm, the largest approximately R = 120 mm; in addition, a bedding plane surface of a larger slab, approximately 330 mm by approximately 315 mm, exposes 13 more or less complete specimens and additional fragments. All the specimens are small, suggestive of a single recruitment class. Small specimens have not been found on a block with a larger specimen. Although collected over a period of years and therefore different specific quarry blast sites, all but one of the specimens are from a single geographic area and stratigraphic interval that extends across the Bobcaygeon–Verulam formational boundary. A single specimen is from a different stratigraphic formation and locality, see Occurrence. Specimens across the size range differ gradationally in dorsal disk configuration and actinostome and adoral carinal development, with differences first clear in quite small specimens. Because small and large differ, it might be argued that the smaller specimens represent a taxon that did not reach a significantly larger size, thereby leaving the larger to represent a postulated second species. However, the presence of many smaller specimens in the full suite that accumulated through a stratigraphic thickness argues it is unreasonable to expect that no smaller specimens of any such posited ‘larger’ species would have been preserved, and therefore although some morphologic differences among specimens within a limited size range can be recognized (Figs. 2.1–2.3, 5.1–5.8), the comparatively large sample enables interpretation of differences throughout the full size range as ontogenetic, with the differences among smaller specimens documenting difficulties encountered in the interpretation of suites of small specimens of both ancient and extant species. Spencer (Reference Spencer1918, p. 139) also interpreted a small, stellate urasterellid as an immature specimen of his then-new Urasterella thraivensis, the interpretation of A. huxleyi here paralleling the earlier viewpoint of Spencer.

Variation among specimens arguably might reflect evolution during the Bobcaygeon–Verulam depositional interval and/or alternatively, biogeographic change; however, specific source horizons within the stratigraphic sequence are unknown. Therefore, any differences are treated as intraspecific and encompassed within taxonomic description.

Type species

Meturaster belli Blake, Guensburg, and Lefebvre, Reference Blake, Guensburg and Lefebvre2016, only species by monotypy.

Other species

Known only from the holotype of the type species, M. belli, lower Lehman Formation, Middle Ordovician (Darriwilian), Snake Range, Nevada.

Diagnosis

Urasterellid with remnants indicating disk small; arms sturdy, elongate, slender, cylindrical, not tapering notably along (available) length. Adradialia beyond midline series typical of family; small paxilla shaft present. Madreporite not recognized. Inferomarginals proportionately small, elongate, crescentic, each paired with and tightly fitted against adjacent adaxial; dorsal abradial corner of adaxial notched to receive inferomarginal; ossicles of the two series together forming evenly rounded arm margin visually suggestive of a closely fitted marginal pair. Dorsal surfaces of axials evenly rounded. Adaxials robust, abradial ossicular margins at arm edge upright and defining arm margin, demonstrating absence of arm actinals. Accessory bases not recognized; no remnant accessories recognized.

Remarks

The only known specimen is incomplete, with the disk almost entirely lost, but arm configuration is urasterellid and unlike other members of the family.

Type species

Pegoasterella pompom n. gen. n. sp., only species.

Diagnosis

As for the type species, by monotypy.

Etymology

Greek, pegos, strong, solid, urasterellid with relatively block-like, robust ossicles that lack paxilla shafts.

Holotype

PRIP UI-X-4935, including a detached arm fragment.

Paratypes

Three small fragments, PRIP 20010–PRIP 20012.

Diagnosis

Urasterellid with comparatively robust extraxial ossicles. Dorsal disk circlet ossicles enlarged, subspherical; carinal series similar to larger disk ossicles but varied in size, locally offset to form irregular sequence; ossicular surfaces bearing radiating series of pustules and/or more or less continuous linear pustular ridges suggestive of ornamental tuft or cluster. Adradialia in five or more series. Madreporite large, irregular in outline, and fitted among adjacent ossicles. Inferomarginals clearly differentiated from more dorsal ossicles; inferomarginals approximately square and aligned with adaxials. Adradialia, inferomarginals, lateral surfaces of adaxials with discrete pustules rather than fused linear ridges; paxillae shafts lacking.

Occurrences

Holotype, Guttenberg Formation, Decorah Subgroup, Galena Group, Late Ordovician, on Route 2, east side of Wagon Wheel Airport, Rockton, Illinois, NW SE SW Sec. 25, 40N, 1E, Rockton Quadrangle. Paratypes are from the Pooleville Member, Bromide Formation, Turian Stage, Late Ordovician, Geological Enterprises Quarry, Criner Hills, Oklahoma (see Sprinkle, Reference Sprinkle1982). In addition to the illustrated specimens, several hundred small fragments are reposited at the Prairie Research Institute.

Description

Overall configuration of all specimen sizes typical of urasterellids; arms straight-sided; arms of smaller individuals proportionately shorter, more abruptly tapering than arms of larger individuals; transverse arm profile more nearly triangular, becoming more clearly cylindrical with growth but perhaps retaining some angularity. Disk arched, constructed predominantly of comparatively few, bulbous, similar ossicles; enlarged disk ossicles; ossicles of carinal series robust, subspherical or hemispherical, bearing enlarged pustules, pustules disjunct or united to form ridges. Proximal carinal ossicles offset, somewhat varied in size; ossicles locally paired, approaching double-series configuration. Adradialia rectangular, shield-like, papular? reentrants distinct, ossicular surfaces arched, ossicles aligned in series. Madreporite approximately elliptical, closely fitted with adradialia. Inferomarginals square to elliptical or rectangular, abutted or weakly overlapping; inferomarginals smaller than adjacent adaxials. Inferomarginals, adaxials approximately corresponding in number, or inferomarginals slightly more numerous. Axillary, actinal data not available. Axials robust; articular faceting well developed. Data on mouth frame lacking. Some debris suggests presence of granules, small spinelets on adradialia, inferomarginals.

Etymology

Named in honor and memory of Ailsa M. Clark, her many contributions to the understanding of modern echinoderms, and the pleasant memory of our conversation about the naming of her new genus and species, Paxillasterina pompom Clark, Reference Clark1983. Descriptively, the species also is named for the distinctive ossicles of the arms and disk.

Remarks

Surfaces of the three fragmentary Bromide specimens are better preserved than corresponding surfaces of the Guttenberg specimen, which nevertheless is selected as holotype because it is more complete. Inferomarginal shape from the two settings differs; these differences are treated as population differences rather than justifying recognition of a separate name. Pegoasterella pompom exhibits adradialia and inferomarginal arrangements of Urasterella, but extraxial ossicles are robust and postulate. Arms of P. pompom less elongate than those of Anthroosasterias huxleyi.

Type species

Palaeaster asperrima Salter, Reference Salter1857.

Other species

Salteraster asperrimus (Salter, Reference Salter1857), Caradoc, Late Ordovician, the part and counterpart separately numbered (Spencer, Reference Spencer1918, p. 150). Salteraster grandis (Meek, Reference Meek1872), Cinncinnatian, Late Ordovician, Ohio. Further names treated by Spencer (Reference Spencer1918) are held in abeyance pending restudy.

Diagnosis

Urasterellid of conventional form. Disk ossicular configuration comparatively simple: centrale enclosed by more clearly defined inner ring of few ossicles, it in turn enclosed by more irregular outer ring. Disk, carinal ossicles at most weakly differentiated in size and form from adradialia; sequential ossicles of carinal series might be slightly offset and at least suggestive of multiple series. Carinals approximately equidimensional, bases faceted, paxillae shafts ‘boss-shaped,’ those of the adjacent ossicles ‘rod-like’ (Spencer, Reference Spencer1918, p. 131). Adradialia similar; arranged in at least four to five rows. Madreporite small, near disk center. Inferomarginals somewhat irregular in outline, approximately elliptical, elongate parallel to arm axis, weakly overlapping, surfaces weakly arched but not conical; stout paxillae shafts might be developed at least proximally. Inferomarginals larger than immediately adjacent adradialia. Axillary status unknown. Axial transverse ridge linear, narrow; podial basins can be enlarged. Accessory spinelets developed on adradialia; conical and flattened short spines associated with adaxials.

Remarks

The name Salteraster has been broadly cited in the literature, yet generic delineation is problematic. The type specimens of the type species S. asperrimus are moldic, the casts suggesting ossicular surfaces are weathered; therefore, original topography is all but impossible to evaluate. The diagnosis of Urasterella of Schuchert (Reference Schuchert1915) was based on grandis; this generic assignment was rejected by Spencer (Reference Spencer1918, p. 154) because “the rows of adradialia are not differentiated” in grandis as in species correctly assigned to Urasterella. Interpretations of ossicular form in the two genera of Spencer (Reference Spencer1918) were illustrated in his figures 92, 94, and 95. The generic diagnosis of Spencer and Wright (Reference Spencer, Wright and Moore1966, p. 71), “Arms strongly swollen, with single row of carinals separated from marginals by many paxillae,” calls for idiosyncratic evaluation of “strongly” and “many.” Arms of all urasterellids appear to have been more or less rounded in life and relatively readily distorted with preservation because of comparatively small ossicular size.

In synonymizing Salteraster with Urasterella, Shackleton (Reference Shackleton2005, p. 99) referred to S. asperrimus as her exemplar for Urasterella; Blake (Reference Blake2018) followed Shackleton (2018) in synonymizing Salteraster with Urasterella whereas herein Salteraster is separated from other urasterellid genera based on axial and adaxial form and, potentially, dorsal disk arrangement.

Blake (Reference Blake2018, p. 10) suggested proportionately small axials relative to adaxials might reflect a neotenous derivation for the order Kermasida; ossicular size at a tiny arm tip of U. grandis (Fig. 8.2) is consistent with this interpretation, although this particular tip was regenerated.

Type species

Staurasterias elegans Blake, Reference Blake2017 by monotypy.

Other species

Known only from the type species S. elegans, original unit designation ‘Keokuk’; Early Mississippian, Carboniferous, Crawfordsville, Indiana.

Diagnosis

Urasterellid of overall robust form, extraxial ossicles strongly differentiated. Disk ossicles ranging in shape from paxilliform to tabulate. Carinal-series ossicles enlarged, clearly differing from adjacent adradialia; ossicles of carinal series aligned, both transversely and longitudinally bilateral, central portion subcircular, with medial spine base; centers of sequential ossicles overlap proximally; proximal and distal carinal flanges small, triangular; ossicular lateral flanges large, rectangular to subtriangular. Madreporite significantly enlarged, perhaps overlapping adjacent ossicles. Adradialia appear to be of characteristic urasterellid form and arrangement, in about five rows. Inferomarginals approximately aligned with both adradialia and adaxial series; inferomarginals L-shaped, abutting adaxial at corner of ‘L,’ corner thickened, bearing pustule. More proximal axials short, wide relative to more medial axials; only medial axials known to bear flange overlapping next-distal axial. Adaxials scoop-shaped, overlapping proximally. Larger spine pustules present on carinals, inferomarginals, adaxials; granule and spinelets present.

Remarks

The single specimen consists of the disk and proximal arm intervals recurved in a bud-like configuration that obscures ventral surfaces. Generic recognition based on expression of madreporite, carinals, inferomarginals, adaxials, and the ventral expression of the axials. The overlapping flange of the axials is recognized only on a part of the available ambulacral furrow but perhaps only because of limitations of preservation.

Type species

Urasterella ulrichi Schuchert, Reference Schuchert1915.

Other species

Limited to the type species U. ulrichi Schuchert, Reference Schuchert1915, the original unit designation Mohawkian, Late Ordovician (Sandbian), Minnesota. Spencer (Reference Spencer1950) assigned Urasterella gutterfordensis, Spencer, Reference Spencer1918 and Urasterella biradialis Withers and Keble, Reference Withers and Keble1934 to Ulrichaster, interpretations here held in abeyance. The dorsal skeleton of Ulrichaster macrodentatus Jell and Theron, Reference Jell and Theron1999 is “an ordered meshwork (with large perforations) of small tuberculate elongate plates in 4 radial columns (2 columns on either side of arm axis smaller than next columns abradially) linked by less regular struts in transverse rows” (p. 156); this description places ‘U.’ macrodentatus apart from the concept of the Urasterellidae as diagnosed here, and assignment of this species to Ulrichaster and the Urasterellidae is not accepted.

Diagnosis

Urasterellid central disk ossicles and carinals relatively large, thickened, substellate; basal facets short, thickened, ossicles not appearing to have borne well-defined paxilla shaft. Carinals aligned in somewhat irregular double row. Adradialia robust, papular reentrants deep, paxillae shafts elongate, aligned in four or more rows. Madreporite not recognized. First inferomarginal pair exposed proximally on a smaller specimen might be enlarged relative to smaller, rectangular subsequent inferomarginals; proximal inferomarginal pair with conical paxilla shaft.

Remarks

Generic recognition and original separation of species was based primarily on presence of a unique double series of more or less plate-like ossicles at the dorsal arm midline; proportional ossicular size and arm shape was noted by Schuchert (Reference Schuchert1915, p. 185). Complex appearances of carinal series among urasterellids casts some question on generic separation of Ulrichaster.