J. Paleont., 79(2), 2005, pp. 395–399 Copyright q 2005, The Paleontological Society 0022-3360/05/0079-395$03.00
IMPLICATIONS OF A NEW EARLY ORDOVICIAN ASTEROID (ECHINODERMATA) FOR THE PHYLOGENY OF ASTEROZOANS DANIEL B. BLAKE
AND
THOMAS E. GUENSBURG
Department of Geology, University of Illinois, Urbana 61801, ,
[email protected]., and Physical Sciences Division, Rock Valley College, Rockford, Illinois 61111, ,
[email protected]. ABSTRACT—Eriaster ibexensis n. gen. and sp., from the Ibexian (Lower Ordovician) of Utah, is the oldest-known body-fossil taxon of the class Asteroidea. Although important features are not preserved, the external form of E. ibexensis is suggestive of certain living asteroids and unlike approximately coeval somasteroids. The similar ages of Eriaster and the oldest-known somasteroid challenge the candidacy of the latter as basal to asteroids. Trace fossils assigned to Asteriacites have been recovered from strata as ancient as Lower Cambrian. Asteriacites from younger strata are considered to represent resting traces of asterozoans. Mode of formation of early representatives is problematic; however, their simple existence raises the possibility of extended pre-Ibexian asterozoan evolution, which could explain the morphological disparity found among the earliest-known body-fossil asterozoans.
INTRODUCTION
are rare and therefore even incomplete individuals can contribute to understanding. The importance of Eriaster ibexensis n. gen. and sp. derives from its ancient age and its similarity to some living species.
F
OSSIL ASTEROIDS
Actinals, if present, extend from the marginals to the adambulacrals, but they are not recognized in Eriaster. Accessory spines, spinelets, granules, and pedicellariae are variously developed in asteroids. SKELETAL AND ONTOGENETIC INTERPRETATION
STRATIGRAPHIC POSITIONS
The single-known specimen of Eriaster ibexensis n. gen. and sp. was collected from the Fillmore Formation of the Ibex area of western Utah. The horizon is in the basal, G-2 trilobite zone, and the fossil was collected from near the top of the 1d time zone of the Ibexian, which is equivalent to the very top of the Tremadoc of British terminology. The fossil is slightly younger than the oldest-known somasteroid, Ophioxenikos Blake and Guensburg, 1993, which is also 1d in age. Eriaster n. gen. is slightly older than the previously oldest-known asteroid, Petraster ramseyensis (Hicks, 1873), which is lowermost Arenig, 2a, in age, and equivalent to middle Ibexian in North American terminology. The somasteroids Villebrunaster Spencer, 1951 and Chinianaster Thoral, 1935, from the 2b interval of the Tremadoc and long important in inferences on asterozoan phylogeny (e.g., Spencer, 1951; Fell, 1963; Spencer and Wright, 1966), are a bit younger than Eriaster ibexensis. Recent stratigraphic discussions are found in Vizcaı¨no and Lefebvre (1999) and Webby et al. (2003). TERMINOLOGY
Although the asteroid skeleton consists of thousands of discrete skeletal elements or ossicles, only a relatively small number of separate ossicular types or series are present. Ossicular systems are labeled in Figure 1, and other examples are provided in Blake and Elliott (2003). The ambulacra are constructed of a series of ambulacral and adambulacral ossicles that lead proximally to a differentiated jaw frame. In the present specimen, only a small portion of a single ambulacral is exposed, and the jaw frame is not exposed. The unpaired terminal, found at the tips of the arms of complete individuals, is missing. A more or less distinctive single (in Eriaster n. gen.) or double marginal series marks the edge of the body, separating abactinal ossicles of the dorsal surface from the ossicles of the ventral surface. An unpaired axillary is aligned with the marginal series in the ventral disk interbrachial position of many Paleozoic asteroids. A differentiated primary circlet of abactinals is typical near the dorsal center of the disk. An unpaired carinal series forms the dorsal midline of the arm of some taxa, and the carinal series can be paralleled by an adjacent first lateral and more abradially placed second lateral series.
Identification of number of marginal rows (zero, one, or two) is important to the interpretation of asteroid systematics and phylogeny. Blake (1978) argued that marginal series arise on the proximal side of the terminal ossicle, and only ossicles that arise in this position are available to be considered as possible marginals. Although a terminal is not clearly preserved in Eriaster n. gen., arm shape and taper indicate that the arm is essentially complete; one marginal series, ambital in position, and the first lateral series are considered to have reached the terminal. The second lateral series (adjacent to the marginals) terminated proximal to the arm tip in Eriaster and therefore it cannot be marginal in origin. Because the first lateral series almost certainly reached the tip, its candidacy as the second marginal series must be considered. Marginals in Paleozoic taxa need not be developed as is typical of crown-group asteroids. A double series of marginals is typical of the crown group; however, a number of authors have pointed out that only a single series is present on the arms of some Paleozoic species. Kesling (1967, 1969) described species with a single arm series and he further argued that certain disk ossicles represent a very incomplete superomarginal series. Based largely on comparison with occurrences in modern taxa, Blake and Elliott (2003) disagreed with Kesling’s assessment. Ossicular arrangement in Eriaster is similar to those of Kesling’s examples, and argumentation and illustrations from Blake and Elliott (2003) are useful to interpretation of the new genus. Inferomarginals and superomarginals abut one another in most species, but so-called ‘‘intermarginals’’ occur between marginal series in some taxa. Arguably, in Eriaster, the ‘‘second laterals’’ might be intermarginals and the first laterals superomarginals. However, position of the lateral carinal series next to the carinals on the dorsal surface of the disk and termination of this series at the primary circlet indicate the nonmarginal origin of the lateral series. That is, the first lateral series cannot be superomarginal because the primary circlet ossicles are not (Blake and Elliott, 2003), which would have to be true if the laterals were to be superomarginals. Therefore, only a single marginal series was present in Eriaster. Ontogenetic stage is important because most species add many ossicles during ontogeny, and adults can differ significantly from
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the immature. Small adult sizes are known among living species, and therefore the arm radius of 5.5 mm is not a conclusive indicator. Proportional terminal size also is indicative. The terminal arises early in ontogeny, when it is proportionately large; as the skeleton grows, its relative size decreases. Taper of the arm of Eriaster suggests a small terminal size, and therefore an individual approaching full size. Important to the evaluation here is that the specimen is an asteroid regardless of ontogenetic stage. DISCUSSION
The single known specimen of Eriaster ibexensis n. gen. and sp. reveals little of the nature of the ambulacra, which is basic to phylogenetic inference (e.g., Blake, 2000). The narrow, vaulted arm (suggesting a true asteroid ambulacral furrow), the shape of the adambulacrals, and the position of the ambulacral series away from the ventral surface of the body indicate the asteroid affinities of E. ibexensis. Overall form, the simple primary circlet, and the regular series of dorsal arm and marginal ossicles in Eriaster ibexensis provide no simple differences from certain modern asteroids, although the specimen doesn’t ‘‘fit’’ any extant family. For example, only the Zoroasteridae Sladen, 1889 among crown-group asteroids has a single marginal row, and it has multiple series of actinals and few lateral abactinals (Blake and Elliott, 2003, fig. 1) whereas multiple lateral series but no arm actinals (the ventral disk is not preserved) occur in Eriaster n. gen. A second early true asteroid is Petraster ramseyensis from rocks interpreted as Arenig in age by Pringle (1911). This species was described as Palasterina ramseyensis Hicks, 1873. It was later discussed by Schuchert (1915) and more extensively by Spencer (1918). Material of Petraster ramseyensis was not examined, but earlier authors described two series of marginals, a carinal series on the dorsal surface of the arms, adambulacrals more numerous than marginals, and actinals of irregular arrangement. Eriaster and P. ramseyensis are similar in form but quite different in detail in that Eriaster has only a single series of rather small marginals, lacks actinals, and the marginals and adambulacrals nearly correspond in number. Together, the two ancient fossils demonstrate that the class Asteroidea was established by early in the Ordovician. Approximately coeval somasteroids are different from both the early and the modern asteroids (e.g., Mooi and David, 2000). Somasteroids typically are foliate with ventral series of so-called virgals extending from ambulacrals to marginals. Ophiuroids lack the well-defined abactinal and marginal series. Any younger taxa that might be thought intermediate in some ways between asteroids and somasteroids [e.g., Platanaster Spencer, 1919, see Blake (1994) or Dean (1999)] would have been survivors of already past events. Inferences on morphology and timing of phylogenetic steps in asterozoan evolution must be compatible with the ancient occurrence of Eriaster and Petraster ramseyensis. Regardless of ontogenetic state, form, skeletal configuration, and size suggest Eriaster was an epifaunal small-particle feeder. Asteriacites von Schlotheim, 1820, a trace fossil attributed to stelleroids, has been recorded at least tentatively from rocks as early as the Cambrian (e.g., Alpert, 1976, review of Mikula´s, 1992). Although we know of no way to determine the genesis of
these early Asteriacites occurrences, their very existence must raise the possibility of a long period of asterozoan diversification prior to the Ibexian, and ample time for the establishment of morphologically distinct lineages. Many known early asterozoans are skeletally delicate and therefore not readily preserved; precursors could also have been delicate and unlikely to be preserved. SYSTEMATIC PALEONTOLOGY
Class ASTEROIDEA de Blainville, 1830 Order and family UNKNOWN Discussion.Suprageneric concepts for Paleozoic asteroids are in need of comprehensive reevaluation and Eriaster n. gen. therefore is left in open taxonomy. Eriaster exhibits a skeleton of comparatively large, flat ossicles arranged in closely aligned series. Among Paleozoic taxa sensu Spencer and Wright (1966) with broadly similar arrangements, the Palasterinidae and Promopalaeasteraceae lack the distinct marginals, carinals are absent from the Palaeasteridae, the Neopalaeasteridae lack robust abactinals, and the axillary lies within the inferomarginal sequence, away from the disk edge, in the Mesopalaeasteridae and Xenasteridae. Eriaster perhaps is superficially more like members of the Hudsonasteridae, but its single series of comparatively small marginals is unlike the double series of robust and bulbous marginals of the hudsonasterids. Genus ERIASTER new genus Type species.Eriaster ibexensis new species; the genus is monospecific. Diagnosis.As for the species. Description.As for the species. Etymology.Eri, Greek, early, at dawn; the dawn star. ERIASTER
new species Figure 1
IBEXENSIS
Diagnosis.Disk small, interbrachial arcs angular, dorsal surface arched. Body wall ossicles robust, developed in few series. Ossicles flattened or platelike, polygonal, bearing small pustules. Primary circlet well defined, other disk ossicles quite large, few in number. Arm ossicles consisting of carinal series bordered on each side by two lateral, one marginal series. Marginal series forms ambitus. Adambulacrals inset from arm edge; ossicles approximately equidimensional, robust, adradial edges rounded. Ambulacral series placed above ventral plane of body. Axillary well defined, distal edge exposed at disk edge; virgals lacking. The diagnosis of Eriaster n. gen. is a summary of distinctive features because character polarities cannot now be determined. Description.Five-armed asteroid; body arched, arms elongate, narrow, triangular in cross section; interbrachial arcs narrowly rounded; R approx 5.5 mm, r (now partially collapsed and exaggerated) 1.7–2.0 mm. Body ossicles large, comparatively few in number, shieldlike. Disk ossicles polygonal, approximately equidimensional. Primary circlet incompletely preserved but appearing to consist of 10 ossicles. First carinals somewhat elongate, interbrachials more nearly equidimensional, maximum ossicular dimensions less than 1.0 →
FIGURE 1—1–7, Eriaster ibexensis n. gen. and sp. Holotype, FMNH PE 52741. Fillmore Limestone, Ibex, Utah, Lower Ordovician. Scales 1 mm. 1, Dorsal view of complete specimen; 2, dorsal view of disk; best-preserved ossicles are of the primary circlet; 3, dorsal view of disk and bestpreserved arm; distal carinals remain but proximal carinals have been lost; 4, lateral view of left side of best-preserved arm (to lower right of 1); 5, lateral view of right side of best-preserved arm; carinals, first laterals, marginals, and adambulacrals dominate; compare with 7; 6, poorly preserved ventral surface of arm and portion of disk; 7, inclined view of best-preserved arm showing ossicular arrangement in interbrachial area, compare with 5. Key: 1l, first laterals; 2l, second laterals; a, axillary; ad, adambulacrals; c, carinals; m, marginals; p, primary circlet ossicles.
PALEONTOLOGICAL NOTES
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mm. Madreporite, if present, not identified. Five(?) or more polygonal ossicles present within the primary circlet, now partially displaced, in no discernable pattern, not of uniform size. Ossicular size decreasing toward tip. Arm ossicles longitudinally elongate but becoming more nearly equidimensional nearer tip. Arm ossicles closely fitted, arranged in seven series consisting of carinal series bordered by two lateral, one marginal series on each side of the arm. Carinal series consisting of about nine bilaterally symmetric, longitudinally elongate shieldlike ossicles; each carinal overlapping next distal carinal. First lateral series ossicles equal in number to carinal series, ossicles of the two series nearly aligned or slightly offset. First lateral ossicle comparatively small, lying within juncture of first carinal and interbrachial; second lateral broader than adjacent carinal. Lateral ossicular size gradually decreasing toward arm tip. Second and subsequent first-lateral series ossicles indented medially or at ossicular junctions, suggesting presence of respiratory papulae (indentions indicated by clear calcite). Carinal series, lateral carinal series probably reaching arm tip. Arm ossicles of second lateral series small, becoming fused taphonomically with adjacent ossicles, extending about three-fourths distance to tip (reaching about fifth or sixth marginal) where presence of ossicular material is obscured by notching for respiratory papulae. Second laterals alternating with first laterals. Second lateral series extend from arm to arm across disk interbrachia. At disk, first ossicle of second lateral series interbrachial in position, unpaired, probably abuts interbrachial above, axillary below. At disk, one or more small ossicles can be present between unpaired interbrachial of second lateral series and first ossicles of first lateral series. Abactinal outer surfaces pustulose. Pustules robust, separated rather than abutted over ossicular surface except perhaps closely spaced on distal arm ossicles. Pustule sizes somewhat varied but no clear indication of size classes nor presence of enlarged central pustules. Presence of single large medial pit on some pustules suggests musculature and presence of robust spinelets (none preserved). Marginal series forming rounded ambitus. Ossicles small, cylindrical, longitudinally elongate, approximately corresponding in number to adambulacrals. Eight marginals preserved between axillary and terminal position, with space for about two added ossicles. Axillary 0.8 mm long, 0.6 mm wide, larger than adjacent marginals, ambital edge convexly curved. Adambulacrals robust, inset from arm margin. More than 10 adambulacrals present between mouth frame area and arm tip. Adambulacral adradial margins rounded, adambulacral ventral outline approximately square. Surface irregularity suggests presence of pustules, although details largely lost. Ambulacral ossicles nearly unexposed, recessed above ventral body plane. Etymology.The fossil was collected from rocks of the Ibexian Stage. Type.Holotype, Field Museum of Natural History FMNH PE 52741. A single specimen consisting of the disk, a nearly complete arm, and two somewhat flattened and incomplete arms. Only the nearly complete arm retains the apparent vaulting of the living individual. The tip of the nearly complete arm is leached. The ventral disk region is collapsed and disrupted. Occurrence.Fillmore Limestone, Ibex area, western Utah, Section C of Hintze (1973) at 795 ft. This horizon is in the basal trilobite zone G-2. See section above on Stratigraphic Positions. Discussion.Eriaster exhibits a skeletal arrangement that is common among asteroids [Schuchert (1915); Spencer (e.g., 1916, 1918, 1919)]. Taxa with two marginal series as well as those with many small abactinals are readily differentiated from Eriaster. Neopalaeaster hesslandi Rasmussen, 1952 is superficially similar,
although size and preservation of the known specimen make comparisons difficult. The robust, square outline of N. hesslandi marginals differ from the delicate, elongate marginals of Eriaster, and the axillary is comparatively large. Eriaster is compared with Petraster ramseyensis and the Zoroasteridae, under the Discussion. ACKNOWLEDGMENTS
The writers gratefully acknowledge review by J. Sprinkle of an earlier version of the manuscript and later reviews by R. Mooi and C. Sumrall. P. Jell served as associate editor. REFERENCES
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