1257

J. Paleont., 81(6), 2007, pp. 1257–1265Copyright ! 2007, The Paleontological Society0022-3360/07/0081-1257$03.00

A NEW, PHYLOGENETICALLY SIGNIFICANT EARLY ORDOVICIANASTEROID (ECHINODERMATA)

DANIEL B. BLAKE,1 THOMAS E. GUENSBURG,2 JAMES SPRINKLE,3 AND COLIN SUMRALL4

1Department of Geology, University of Illinois, Urbana 61801, !dblake@uiuc.edu", 2Physical Sciences Division, Rock Valley College, Rockford, Illinois61114, !t.guensburg@rvc.cc.il.us", 3Department of Geological Sciences, University of Texas, Austin 78712, !echino@mail.utexas.edu", and

4Department of Earth and Planetary Sciences, University of Tennessee, Knoxville 37996, !csumrall@utk.edu"

ABSTRACT—A new monospecific family of asteroids (Echinodermata) is based on Eukrinaster ibexensis n. gen. and sp. from the LowerOrdovician of Utah and Nevada. Eukrinaster, Arenig in age, is one of the earliest of known asterozoans. The new, relatively well-preservedfossils yield important information on character state distribution that will be useful for the interpretation of phylogenetic relationships amongthe three asterozoan classes, the Somasteroidea, Ophiuroidea, and Asteroidea. In addition, overall form is suggestive of certain living asteroids:to the extent that form equates with function, similarities suggest ecologic parallels in these only distantly related asteroids inhabiting ecologicallydistinct worlds.

INTRODUCTION

MOST ASTEROZOANS have fragile skeletons, and as a result,the fossil record is scanty and fossils tend to be poorly

preserved and incomplete. Because of these constraints, the originand early history of subphylum diversification is not well under-stood. The recently discovered specimens of Eukrinaster ibexensisn. gen. and sp. are an important addition to the limited record,and the purpose of the present paper is to describe and illustratethis new taxon and to evaluate its morphology.

Earlier researchers offer diverse perspectives on asterozoanphylogeny (e.g., Spencer, 1914–1940, 1951; Schuchert, 1915;Fell, 1963; Spencer and Wright, 1966; and Shackleton, 2005) pre-sented the first comprehensive cladistic treatment of early aster-ozoans. One of us (DBB) is developing a cladistic hypothesis ofasterozoan phylogeny, preliminary portions of which were pub-lished in Blake (1998) and Blake and Hagdorn (2003). Publicationof more complete treatment must await documentation of newtaxa, including Eukrinaster n. gen.; however, Eukrinaster ibex-ensis n. gen. and sp. is considered below in light of earlier dis-cussions on asterozoan morphology and phylogeny.

STRATIGRAPHIC POSITION

Four of the five asteroids studied here were collected from tri-lobite zones I and J (Presbynileus ibexensis to Pseudocybele na-suta zones) of the Fillmore and Wah Wah Formations in the east-ern Ibex area of western Utah; the fifth is from Zone J of theNinemile Shale in Whiterock Canyon, central Nevada (see Sprin-kle and Guensburg, 1995; Ross et al., 1997). These units correlateto the Whitlandian Stage, early Middle Arenig in British termi-nology, the Latorp or earlier of the Baltic Region, Bed 11 of theCowhead Formation in Newfoundland, the middle Horn ValleySiltstone in Australia, the La Maurerie of southern France, andthe lower to middle San Juan Formation of Argentina (Nielson,1992).

Eriaster Blake and Guensburg, 2005, was not collected by theauthors; it was reported to be from the G2 trilobite horizon, al-though its morphology appears relatively derived for such an earlyoccurrence. These authors reported the specimen as Late Trema-doc in age, although Nielsen (1992) placed this interval in theEarly Arenig. Either way, the fossil is slightly older than the spec-imens of Eukrinaster. No unequivocal Tremadoc asterozoans arepresently known from Laurentia.

Changes around the transition from the Tremadoc to the Arenigrepresent a significant echinoderm transition to a more derivedfaunal assemblage, in Laurentia characterized by diversificationof standardized disparid, cladid, and camerate crinoids. The as-terozoan record is too sparse and earliest asterozoans too diverseto permit easy interpretation of phylogenetic events based solelyon stratigraphic position; nevertheless, events are suggestive of

those in crinoids with the oldest taxa, somasteroids, appearingnear the base of the Arenig.

TERMINOLOGY

Spencer and Wright (1966) recognized the Somasteroidea, As-teroidea, and Ophiuroidea as subclasses of the class StelleroideaLamarck, 1816. The Stelleroidea was the only class these authorsassigned to the subphylum Asterozoa Zittel, 1895. General usageseems to prefer recognition of asteroids and ophiuroids at the classlevel, and this convention is followed here. ‘‘Asterozoa’’ has auseful equivalence with Echinozoa, Crinozoa, and Blastozoa, andit is employed. The term ‘‘Stelleroidea’’ is not used.

Terminological usage follows Spencer and Wright (1966) andBlake and Hagdorn (2003). In summary, asteroid ambulacra areconstructed of a series of ambulacral and adambulacral ossiclesthat arise at the proximal side of the unpaired terminal at the distaltip of the arm and extend proximally to the mouth frame. Dom-inant in the mouth frame are the paired mouth angle ossicles(MAO). Phylogenetic origin of the MAO has engendered somediscussion but consensus now is that they are ambulacral in origin(reviewed in Gordon, 1929; Smith and Jell, 1990; Mooi and Da-vid, 2000; Shackleton, 2005). Smith and Jell (1990) recommendedreplacement of ‘‘mouth angle ossicle’’ with ‘‘1st Amb’’ whereasboth Mooi and David (2000) and Shackleton (2005) retained theolder terminology. MAO is a descriptive term whereas ‘‘1stAmb’’ carries a genetic component; MAO is preferred here. Fol-lowing Shackleton (2005), the term circumoral is used for theambulacral ossicle immediately distal to the MAO. The torus isan unpaired ossicle found in some asterozoans at the proximaledge of the mouth angle pair; it is directed toward the mouthregion. The edge of the body is framed by a more or less dis-tinctive single, or in the present species, double, marginal series.The marginal framework separates abactinal ossicles of the dorsalsurface from the actinal ossicles found between the marginals andthe adambulacrals. Marginals also arise at the proximal side ofthe terminal. An unpaired axillary is usually aligned with the mar-ginal series in the ventral disk at the interbrachial position ofmany Paleozoic asteroids, either exposed at the disk margin orforming a part of the ventral surface. The axillary generally abutsthe mouth angle ossicles. A differentiated primary circlet of abac-tinals is typical near the dorsal center of the disk, and an unpairedcarinal series lies at the dorsal midline of the arm of some taxa,although neither group is recognized in the new species. Acces-sory spines, spinelets, granules, and pedicellariae variously investthe larger ossicles of the primary skeleton. In describing asteroidossicles, ‘‘length’’ is that dimension essentially parallel to themouth-arm tip axis, ‘‘width’’ is transverse to this axis in the hor-izontal plane, and ‘‘height’’ is the third, vertical dimension. Usage

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FIGURE 1—Eukrinaster ibexensis n. gen. and sp. Scale bars, 5 mm. Overall views of the four relatively complete specimens, Upper Fillmore Formation,Ibex area, west-central Utah, Lower Ordovician. 1, paratype, TMM 1982TX1, dorsal view, form is suggestive of some crown-group taxa; 2, paratype, TMM1810TX8, dorsal view, the disk is largely lost; 3, paratype, TMM 1965TX11, dorsal view; the rounded ambulacrals dominate but abactinal and marginal seriesare distinct; 4, holotype, TMM 1965TX10; ventral view, actinal series dominate, ambulacral series and the mouth frame are incompletely exposed; 5, slabwith paratype TMM 1965TX11 (left) and holotype TMM 1965TX10; line of slab fracture is marked by arrows and passes through a large clast; other clastsare to the left and below the paratype; brachiopods and trilobite fragments are visible. Key: a, ambulacrals; ab, abactinals; c, actinals; m, marginals.

1259BLAKE ET AL.—EARLY ASTEROID

can be counterintuitive, e.g., the rodlike adambulacrals of Eukri-naster n. gen. are wide but short (and therefore not, for example,‘‘elongate’’).

HOW MANY TAXA?Assignment of disparate fragments of complex, articulated as-

teroids to species can be problematic. Adding to the difficulty,many fossil asteroids, including all of the present specimens, ex-pose either the dorsal or ventral surface, but not both. Yet as-signment of specimens to species is crucial to subsequent inter-pretation.

Body outline differs rather subtly among the four relativelycomplete available specimens (Fig. 1), but shape can vary signif-icantly within extant populations of single asteroid species (e.g.,Pearse, 1965). Further, taphonomic events can significantly alterasteroid shape, especially in species such as E. ibexensis n. gen.and sp. that are constructed of comparatively small ossicles.Shape variation among the present fossils does not provide evi-dence for presence of more than a single species.

The abactinal, ambulacral, and marginal ossicles are similar onall three available dorsally exposed specimens (Figs. 1, 2). Themouth frame of Texas Memorial Museum (TMM), University ofTexas at Austin TMM 1982TX1 (Fig. 1.1, partially revealedthrough the draped abactinals) seems more delicate than that ofthe better-exposed TMM 1965TX11 (Fig. 1.3). Nevertheless, thethree appear to belong to a single species. The two ventrally ex-posed specimens, TMM 1965TX10 (Fig. 1.4) and TMM1780TX11 (Fig. 3.2), share actinal, ambulacral, and adambulacralform, as well as overlap of the actinals onto the adambulacrals.They represent a single taxon. (Because of folding of the speci-men, only the ventral surface of TMM 1780TX11, Fig. 3.2, isexposed.)

Difficulties arise in comparing dorsal and ventral surfaces.Specimens TMM 1965TX11 and TMM 1965TX10 were discov-ered on a single, now-broken block (Fig. 1.5). Both rest on thesame hardground bedding surface with the centers of the mouthframes about 8 cm apart. Specimen size, overall form, presenceof rather small ossicles, and preservational style are all similar.Marginals are difficult to identify on TMM 1965TX10 becauseof their similarity to and interdigitation with actinals, but there isno evidence for more than a single taxon.

Specimen TMM 1965TX11 exposes the dorsal surface of theambulacrals and inferred adambulacrals whereas TMM1965TX10 exposes only the ventral surfaces of the same twoossicular types. Adambulacrals appear conventional in both spec-imens, exposing only the ventral surface, although in both theactinal series extends over and partially obscures the adambula-crals (Figs. 2.6, 3.7, 3.8). Dorsally, in both TMM 1810TX8 andTMM 1965TX11, adambulacrals can be identified by their posi-tion lateral to the ambulacrals. Proximal adambulacrals are verywide, transverse rods (Figs. 3.10, 4.2, 4.7), the series narrowingdistally as the gap between the ambulacrals and marginals is nar-rowed (Figs. 2.2, 3, 4). In one area (Fig. 4.7), the abradial tips ofadambulacrals can be seen to be braced against the actinals. Thereis no indication of a virgal series, and rounding of the abradialtips suggests no such series has been lost taphonomically; further,there is no obvious reason why only virgal series might be whollylost during preservation. The inference of virgal absence, how-ever, ultimately relies in no small part on negative evidence. Verywide adambulacrals have been described from e.g., PlatanasterSpencer, 1919, and Palasteriscus Sturtz, 1886, therefore wideadambulacrals in Eukrinaster n. gen. are neither new nor prob-lematic; what is problematic is that this shape cannot be recog-nized in either specimen exposing only ventral surfaces. Althoughpartially obscured by actinals, the adambulacrals in these speci-mens do not appear wide, and no wide ossicles deflect the ventralsurface (Fig. 3.1, 3.9) as they do the dorsal surface (Fig. 2.2).The disparity in appearance between dorsal and ventral surfaces

of the adambulacrals is attributed to a T-shaped adradial trans-verse section (Fig. 4.5) yielding distinctive appearances whenviewed from above or below.

A deep podial basin is readily recognized on the ventral sur-face. Deep dorsal weathering appears to expose the basin (Fig.4.4) although the central portions of the weathered surfaces donot show the obvious skeletal discontinuity that would indicatethe open basin. Nevertheless, dorsal and ventral appearances donot indicate different species.

Because of the many similarities, the five specimens are as-signed to a single species.

MORPHOLOGY AND CLASS AFFINITIES OFEKRINASTER IBEXENSIS N. GEN. AND SP.

Eukrinaster ibexensis is among the most ancient of known as-teroids, and although similar to other early asterozoans in manyfeatures, it is also highly suggestive in overall form and ossiculararrangement of certain surviving taxa, e.g., members of the familyGoniasteridae Forbes, 1841. It thus demonstrates the early estab-lishment of basic asteroid body arrangement and presumably cor-related aspects of function.

Fossil compaction can obscure original body outline; nonethe-less, many early asteroids have elongate arms and acute interbra-chial angles (e.g., Urasterellidae Schuchert 1914) whereas disksare enlarged in somasteroids and some ophiuroids (see Spencerand Wright, 1966). Because ossicles are small relative to bodysize in asteroids of all ages, disks are enlarged by ossicular ad-dition rather than by ossicular enlargement. Actinals are numerousin Eukrinaster n. gen. as they are in many crown-group asteroids;however, actinal arrangement is irregular in this Ordovician fossilwhereas it is typically regular in crown-group asteroids. This dif-ference provides an argument against close phylogenetic linkagebased on ossicular-type presence (Blake and Hagdorn, 2003;Blake and Hotchkiss, 2004).

The many small, undifferentiated (e.g., no carinal series norprimary circlet) abactinal ossicles are suggestive of somasteroidsor early ophiuroids. The apparently tough dermal?-granular layerseems unusual among early asterozoans, although presence ofonly scattered granules in different fossils raises the possibility oftaphonomic loss. The apparent extension of the dermal layer ontothe adambulacrals and mouth-angle ossicles in Eukrinaster is par-ticularly unusual, even within the crown group, although it isknown, e.g., in the Mithrodiidae Viguier, 1878, where it appearsto provide a tough protective covering.

SYSTEMATIC PALEONTOLOGY

Class ASTEROIDEA de Blainville, 1830Order PLATYASTERIDA?

Discussion.!Shackleton (2005) did not recognize ordinal-levelunits, but a tentative assignment is included here, seeking to fitthe new taxon into the perspectives of earlier workers.

Following Spencer and Wright (1966), paxilliform abactinalsare found in the orders Platyasterida and the Paxillosida Perrier,1884. Within the Paxillosida, presence of two series of marginalsseparates Eukrinaster n. gen. from the suborder HemizoninaSpencer, 1951; hemizoninans either have only one marginal rowor superomarginals have been recognized only on the disk (butsee Blake and Elliott, 2003). Spencer and Wright (1966) assignedonly crown-group families to their then-new Diplozonina, a sub-order of the Paxillosida. In this suborder, as in all crown-groupasteroids and unlike stem-group taxa, ambulacrals are offset onadambulacrals and dorsal podial pores are present; mouth frame,ambulacral, and actinal construction of Eukrinaster n. gen. are allvery different from corresponding expressions in the crowngroup.

The Platyasterida sensu Spencer and Wright (1966) was limitedto Platanaster and Palasteriscus; these authors stressed transverse

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FIGURE 2—Eukrinaster ibexensis n. gen. and sp., details. Scale bars, 2 mm. 1, paratype, TMM 1982TX1; lower right interbrachium of Figure 1.1, abactinalswith accessory granules, superomarginals below; 2, paratype, TMM 1810TX8, inclined lateral view of midarm region of left arm of Figure 1.2; double seriesof ambulacrals, dermal?-granular layer retained laterally covering adambulacrals, marginals offset; 3–5, 7, 8, paratype, TMM 1965TX11, 3, 4, distal intervalof upper left arm of Figure 1.3, 3, dorsal view, robust adambulacrals abut ambulacrals and marginals; the dermal?-granular layer was partially torn loosebefore final burial. Contacts between ambulacrals and adambulacrals are upright, flat; marginals offset; 4, inclined distal view of arm, tip missing, marginalsoffset and blocklike; distal ambulacrals differ from near-oral ambulacrals, see 7, 8 (arrow); Figure 4.2; 5, Inclined view of arm; shape of ambulacrals, marginals

1261BLAKE ET AL.—EARLY ASTEROID

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overlap in the interbrachia, portions of the dermal?-granular layer remain; 7, 8, proximalmost ambulacrals and circumoral, ambulacral configuration is suggestiveof some early ophiuroids with recurved edge (8, arrow), see Figure 4.6; abactinals collapsed into disk show no indication of differentiation of primary circlet;6, paratype, TMM 1965TX10, arrangement of ventral ossicles with three broad, grooved ventral spines and a possible axillary. Key: d, adambulacrals; dg,dermal?-granular layer; p, adambulacral spine; X?, possible axillary; other abbreviations as in Figure 1.

alignment of ossicles and the presence of channeling, or respira-tory fascioles. Eukrinaster shares neither feature, but small, gran-ular to paxilliform abactinals, extreme width of adambulacrals,and ambulacral form are shared with one or both of these genera,allowing tentative inclusion.

Family EUKRINASTERIDAE, new familyType genus.!Eukrinaster n. gen.; the family is monogeneric.Diagnosis.!As for the type species.Description.!As for the type species.Etymology.!Based on the type genus.Occurrence.!As for the type species.Discussion.!Shackleton (2005) recognized Petraster kinahani

(Baily, 1878) at the generic level as a basal plesion within theAsteroidea. Eukrinaster differs from P. kinahani in body outlineand corresponding number of disk ossicles, absence of a distinctaxillary, and continuity of the superomarginal series. Apomor-phies above P. kinahani (Shackleton, 2005) included loss of ven-tral and interradial plating, both developed in Eukrinaster. In ad-dition, Shackleton (2005) noted presence of spines associated withthe adambulacrals that are unlike those of Eukrinaster.

With the Palasteriscidae Gregory, 1900, the Eukrinasteridaeshares wide adambulacrals, abactinal form, and aspects of am-bulacral shape. Two marginal series, found in Eukrinaster but notthe Palasteriscidae, was stressed as significant by both Kesling(1969) and Spencer and Wright (1966), although the latter authorsincluded Palasteriscus in the family, which entirely lacks mar-ginals. Positioning of the ambulacrals differs between the twofamilies, as does presence of a robust dermal?-granular layer.Presence of only a single adambulacral spine in the Eukrinaster-idae is distinct but likely strongly correlated with the dermal?-granular layer. DBB recognizes presence of at least a few actinalsin genera assigned to the Palasteriscidae, and these are numerousin Eukrinaster.

Genus EUKRINASTER new genusType species.!Eukrinaster ibexensis n. sp.; the genus is mono-

specific.Diagnosis.!As for the family.Description.!As for the species.Etymology.!From eukrines, Gr., clear, distinct, well separated; in reference

to the distinctive nature of the fossil.Occurrence.!As for the type species.

EUKRINASTER IBEXENSIS new speciesFigures 1–3, 4.1–4.5, 4.7–4.9, 5.3

Diagnosis.!Dense dermal?-granular layer investing all pri-mary ossicles except ambulacrals, including portions of adam-bulacrals and mouth-angle ossicles. One large, paddle-shapedspine on each adambulacral; dermal?-granular layer suggests nofurther adambulacral spines present. No MAO spines recognized.Ambulacral ossicles buttressed by upright ridge on nose of adam-bulacrals; ambulacral-adambulacral contact ridge robust. Adam-bulacrals extend adradially into interior. Proximal adambulacralsvery wide, braced internally by actinals; adambulacral series nar-rowed distally. Marginals in double series, little enlarged relativeto adjacent ossicles. Arm marginals approximately equidimen-sional and offset, their lateral margins nearly abutted; interbra-chial marginals similar but slightly smaller, overlapping.

Probable non-apomorphic characters aiding in recognition in-clude overall body form with presence of an abactinal skeleton

of uniform, densely arranged paxilliform granules; carinal abac-tinal series and primary circlet apparently lacking. Actinals nu-merous, irregular in arrangement. Axillary, if present, only weak-ly differentiated. Dorsal surface of ambulacrals rounded. Torus,buccal slit both present.

Description.!Five-armed asteroid, outline defined by marginals. Arms at-tenuated, triangular; interbrachia rounded. Skeletal arrangement suggests dor-sal surface of low convexity in life.

Abactinal surface formed by uniform, small (0.25–0.5 mm), subsphericalto weakly paxilliform, closely fitted ossicles aligned in series (Figs. 1.1–1.3,2.1–2.5, 4.2–4.4, 4.9). Dorsal surface of abactinals domal, smooth; columnbelow dome weakly constricted, irregular. Locally, abactinal size appears tovary weakly; carinal series, primary circlet not recognized (collapse of diskand taphonomic abrasion over ambulacral series might obscure differentiationif it were present but only weakly developed). Primary abactinals covered bydense probably originally tough tissue layer incorporating fine granules; gran-ules now collapsed about the primaries. (In one area, Fig. 2.3, 2.4, the der-mal?-granular layer has retained its integrity, and is folded back.) Enlargedspines, spinelets not developed. Madreporite not recognized.

Marginals in two series, not significantly enlarged relative to adjacent abac-tinals, actinals; marginal series offset (i.e., alternating) along arm (Fig. 2.2–2.4). Arm superomarginal length 0.75–1.0 mm, height similar, width slightlyless. Superomarginal dorsal outline pentagonal to trapezoidal, surface domed,arrangement upright, lateral surfaces abutted. Marginals near interbrachialmidlines slightly smaller, dorsal edge of each overlapping next proximal mar-ginal. Inferomarginals similar in form to superomarginals, up to about 1 mmin breadth, rectangular, outer face domed. Inferomarginal series locally onarms partially penetrated by actinals (Fig. 3.1, 3.9). Marginals, actinals nowlargely stripped of dermal?-granular layer but this layer preserved locallybetween marginals (Figs. 2.3, 2.4, 4.9) and between MAO pair (Fig. 3.2–3.5).

Axillary (Figs. 2.6, 3.1) not clearly differentiated; if present, similar in size,form to marginals.

Actinals (Figs. 1.4, 3.1, 3.9) polygonal, 0.5–1 mm in longest dimension,outlines rectangular to hexagonal. Actinals tightly fitted, not arranged in well-defined series. Outer surfaces weakly domed.

Adambulacral ossicles in life partially obscured by actinals, granular ac-cessories (Fig. 2.6, 3.6, 3.8). Adambulacral ossicles paired with ambulacrals.Adambulacrals upright, rectangular in ventral outline; adradial end of adam-bulacral bears prominent nose that abuts ambulacral (Fig. 3.7); adambulacralnose extends upward into arm interior contributing to ambulacral furrow (Fig.5). Adambulacral abuts ambulacral at upright abradial edge of ambulacral(Fig. 5). Nose placed approximately medially on adradial (furrow) surface ofadambulacral, flanked on both sides by smooth, curved surfaces that faceadradially. Proximal adambulacrals very wide, extending well across disk inarm interior (Figs. 2.2, 2.3, 3.10, 4.7). Distal intervals of arm narrowed, withwidth of adambulacrals correspondingly reduced (Fig. 2.3). Few adambulacralspines (Figs. 2.6, 3.8) preserved, these appear limited to single flattened, pad-dle-shaped spine at adradial end of outer face of each adambulacral; spinetips rounded, surfaces of some with longitudinal grooves. Potential spine basesnot recognized on comparatively few well-exposed noses.

Ambulacral ossicles rectangular, slightly wider than long, paired across armaxis in some areas, offset elsewhere (Fig. 4.8, 4.9). Cross-furrow and longi-tudinal articular structures large but surface relief subdued. Radial water canalbroad, surface concave. Transverse ridge sharp, prominent, J-shaped, base ofJ curved proximally to form wall of radial canal; gap for transverse watercanal distal to transverse portion of ridge well developed (Fig. 3.6). Podialbasin broad, approximately equally shared between proximal and distal am-bulacrals; basin flange of distal ambulacral overlapping basin of adjacent moreproximal ambulacral. Abradial ends of transverse ridge forming a Y-shapedarticular facet and podial basin rim (Figs. 2.6, 3.8). Dorsal surface of ambu-lacrals rounded, semicircular in outline; proximal and distal extremities ofnear-oral ambulacrals recurved to form rim suggestive of those of certainophiuroids (Figs. 2.7, 2.8; 4.2–4.9).

Mouth angle ossicles (Fig. 3.2–3.5, 3.9) not significantly enlarged relativeto adjacent adambulacrals; MAO appearing quite flat (narrow) with ventraledges narrow, ridgelike; spine bases not recognized, dense dermal?-granularlayer preserved between dilated adoral edges of ossicles. Circumoral robust(Figs. 2.7, 2.8; 4.2, 4.4). Adambulacrals adjacent to MAO curved, similar inform to adjacent MAO, form gradually changing distally. In one interbrach-ium, first adambulacrals displaced slightly behind and partially enclosing

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FIGURE 3—Eukrinaster ibexensis n. gen. and sp., details. Scale bars for figures 1, 2, 9, 10: 2 mm; for 3–8: 1 mm. 1, 8, 9, paratype, TMM 1965TX10, 1,ventral view of upper right interbrachium of Figure 1.4; actinal arrangement, possible axillary; 8, ambulacrals flair abradially forming Y-shaped juncture withadambulacrals, the latter partially obscured by actinals; three grooved furrow spines of Figure 2.6; 9, right arm of Figure 1.4, ventral surface, mouth area withan adambulacral behind MAO; marginals, actinals similar, difficult to differentiate in ventral view; 2–7, paratype, UT 1780TX11, 2, entire fragment; torus(top arrow) visible in 5; adambulacrals (middle arrow) of 7; ambulacrals (bottom arrow) of 6; 3–5, three views of MAO pair, the arrows indicating the samepoint, the MAO pair pulled apart with dermal?-granular material between the ossicles, the torus at the blunt proximal tip; 6, ambulacrals, furrow up; basinsare partially closed abradially (arrow) and the more distal overlaps the more proximal (proximal right); ambulacrals abut the nose of the adambulacrals, whichare partially obscured by actinals; 7, prolonged adradial nose of adambulacral, which buttresses the ambulacrals; arrow points to adradial face of nose of topof three adambulacrals; sides of the nose are curved, perhaps closing ampullar basin; 10, paratype, TMM 1965TX11, dorsal view of upper left arm, Figure1.3, mouth frame to left, portion of ambulacral series missing, black ‘‘d’’ shows adambulacral series narrowing distally, white ‘‘d’’ is adambulacrals of Figure4.7. Key: o, mouth angle ossicle, other abbreviations as in Figure 2.

1263BLAKE ET AL.—EARLY ASTEROID

FIGURE 4—1–5, 7–9, Eukrinaster ibexensis, details. Scale bars for 1–5, 7–9: 1 mm; for 6: 5 mm; 1, paratype TMM 1982TX1, mouth frame splayed buccalslits; arrangement of abactinals does not suggest differentiation of carinals, primary circlet; 2–5, 7–9, paratype TMM 1965TX11, 2, dorsal view of mouthregion showing more ophiuroid-like shape of near-oral ambulacrals, see 4, 6; adambulacrals transversely elongate; 3, tip of rotated lower right arm of Figure1.3, abactinals are aligned, ambulacrals (above) and marginals; 4, proximal interval of lower right arm of Figure 1.3, constriction of ambulacrals to left ofarrow suggests breaching of dorsal surface to expose podial basin; enlarged proximal ambulacrals suggest those of ophiuroids, see 6; 5, adradial (furrow)view of six adambulacrals (between arrows) with podial basin reentrants; opposite series of ambulacrals is below; 7, dorsal view of five transversely elongateadambulacrals; arrow points to abradial terminus, rests on interior surfaces of actinals; 8, 9, dorsal views of ambulacral series showing irregularity of offsetof ossicles across the arm axis; 6, Aspidosoma goldfussi Schondorf, 1910, an ophiuroid, mouth frame to left, ambulacral series in ophiuroids are spool-likewith flared termini and similar to proximal ambulacrals of Eukrinaster n. gen. Key: s, buccal slits, other abbreviation as in Figure 1.

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FIGURE 5—1, generalized orientation arm cross-section. The left side isshown with one marginal, the right with both supero- and inferomarginals;accessories include granules, spinelets, a slender spine on the superomarginal,and a robust spine on the middorsal carinal abactinal. Ossicular types includedin 2, 3 are blackened, other types textured; 2, reconstruction of one side ofan arm of Promopalaeaster Schuchert, 1914; and 3, of Eukrinaster ibexensisn. gen. and sp. Promopalaeaster, although Late Ordovician, exhibits the es-sential ambulacral-adambulacral arrangement found in the crown group. InEukrinaster n. gen. a comparatively narrow ambulacral abuts the transverselyelongate adambulacral whereas the transversely elongate ambulacral of Prom-opalaeaster rests on a comparatively narrow adambulacral. An upwardly di-rected adradial nose on the adambulacral of Eukrinaster, not found in Prom-opalaeaster, contributes to vaulting of the ambulacral furrow. In both genera,the ambulacral is recessed, i.e., abutting the adambulacral above the ventralplane of the adambulacral. Extension of the actinals over the adambulacrals;the robust dermal?-granular layer, and the single paddle-shaped furrow spineare notable in Eukrinaster. Key: a, ambulacrals; ab, abactinals; c, actinals; d,adambulacrals; m, marginals.

mouth-angle ossicles (Figs. 1.4, 3.9). Torus (Fig. 3.2–3.5) small, largely ob-scured by skeletal-dermal layer. Dorsal interior of disk not clearly exposedbut symmetrical collapse of abactinals indicates presence of buccal slit ex-tending beyond circumorals (Fig. 4.1).

Etymology.!Most available specimens were collected in the Ibex area ofwest-central Utah.

Material and occurrence.!The five types are deposited in the Texas Me-morial Museum (TMM), University of Texas at Austin. Holotype TMM1965TX10 is a relatively complete ventral surface on a slab found by Sumrallthat also contains paratype TMM 1965TX11, which is a dorsal surface miss-ing one arm and the tips of two others. The slab was collected from the southflank of Square Top about 15 m (50 ft) above the base of the Wah WahFormation (Zone J), at the top of Hintze’s (1973) Square Top Measured Sec-tion (his ST, our SQ). Paratype TMM 1810TX8 is a dorsal surface includingthe disk and three nearly complete arms that was collected by Guensburgabout 9 m (30 ft) below the top of the Fillmore Formation (Zone I), near theridgeline knob marked 5,511 (ft), just south of Square Top (SQ[S]). ParatypeTMM 1982TX1 is a nearly complete dorsal surface missing the tip of onearm that was collected by Sprinkle about 15 m (50 ft) below the top of theFillmore Formation (Zone I), on the descending ridge just east of Square Top(SQ[Back]). The four specimens were collected from a relatively small areaof the eastern Ibex region, about 80 km (50 mi.) southwest of Delta, MillardCounty, western Utah. Paratype TMM 1780TX11 is a small arm fragmentextending from the mouth angle ossicles well out onto the ventral arm, whichis folded back over itself hiding the dorsal surface. The specimen was col-lected by Sprinkle from a steeply dipping side slope several meters below theapparent ‘‘basal ledge’’ in what is probably the middle of the Ninemile Shale(Zone J), Narrows Section (WR-2), 0.8 km (0.5 mi) up Whiterock Canyonfrom the end of the dirt track, Monitor Range, about 48 km (30 mi) southwestof Eureka, Eureka County, central Nevada.

Discussion.!Detail is well preserved where protected by sed-iment at the outcrop, but exposed external surfaces, especiallydorsal surfaces of ambulacrals and the marginals, are leached andoriginal ossicular form is difficult to decipher for parts of theossicles. No madreporite was recognized, perhaps because of al-teration, because some areas of the dorsal disk suggest possiblepresence. An axillary, if present, is little differentiated from ad-jacent marginals.

The combination of characters of Eukrinaster n. gen. is dis-tinctive, see discussions above. Sediment compaction tends to flat-ten asteroids, thereby enlarging apparent disk size, but large ac-tinal surfaces constrained in part by a fairly robust marginalframe, together with absence of obvious significant distortion, in-dicate a large disk in living Eukrinaster.

ACKNOWLEDGMENTS

We thank T. Abbott, Delta, Utah, and M. Donivan, Salt Lake City, Utah,for access to localities at Ibex and help collecting specimens. (Reviewer 1)and (Reviewer 2) reviewed the manuscript and provided helpful comments.Specimens were collected using funds from NSF grants BSR-8906568 (Sprin-kle) and EAR-9304253 (Sprinkle, Guensburg, Sumrall), CRDF CooperativeResearch Grant RG1-242 (Sprinkle, Guensburg), a PRF-ACS grant to M. Wil-son, College of Wooster (Guensburg), and funds from the Geology Founda-tion, University of Texas at Austin (Sprinkle, Sumrall) and Rock Valley Col-lege (Guensburg). P. Jell and R. Mooi provided helpful reviews and B. Prattserved as Associate Editor.

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ACCEPTED 11 JULY 2006