Urasterella, McCoy, 1854

Blake, Daniel B. & Rozhnov, Sergei, 2007, Aspects of life mode among Ordovician asteroids: Implications of new specimens from Baltica, Acta Palaeontologica Polonica 52 (3), pp. 519-533 : 527-529

Urasterella ? sp.

Figs. 4 , 5C .

Material.—PIN 4125/770. A single specimen exposed in ventral view, consisting of one complete arm and the disk with the remaining four arms more or less incomplete. The specimen is collapsed and partially dissociated. Disk center to arm tip = 23 mm, collapsed disk radius about 5 mm. Specimens collected at the same quarry and horizon as Cnemidactis sp.

Description.—Five−armed asteroid; arms probably long, slen− der; disk small.

Abactinals and marginals.—Few lateral and dorsal ossicles are exposed; these broadly paxilliform, bases robust, columns distinct, columns expanding toward their tip, tips with deep apparent accessory pits ( Fig. 4E–G ); no accessories exposed. Paxilliform ossicles closest to adambulacrals with bases enlarged relative to those of adjacent abactials, therefore those adjacent to adambulacrals probably are marginals.

Adambulacrals.—Adambulacrals ( Fig. 4A–D ) coin−like, large, approximately eight ossicles in 5 mm interval, width approximately 2 mm. Adambulacrals wide, overall profile convex, abactinal margin arched distally. Medial proximal−face articular ridge U−shaped, separating interadambulacral articular surfaces below from adambulacral−ambulacral surfaces above. Distal face concave with prominent articular facets near abactinal abradial and adradial corners. Outer face narrow, bearing an irregular series of ten or more ring−like pustules of uniform morphology, these varying in size. Many spines preserved near adambulacrals, these robust, pointed, cylindrical to slightly flattened, of varying length but with no clear indication of any differentiation (e.g., bimodality).

Ambulacrals, other ossicular types.—No other ossicular type can be identified with certainty.

Remarks.—The fossil is incompletely preserved and exposed, but overall form and the form of the marginals and adambulacrals are in accord with those of Urasterella ; both adambulacrals and marginals appear broadly similar to those of U. grandis and U. pulchella ( Billings, 1857) . The rather delicate adambulacrals of Urasterella ? sp. are unlike those of U. ruthveni , U. thraivensis Spencer, 1918 , Cnemidactis , Salteraster , and Stiberaster . Ulrichaster shares abactinals of broadly similar size and form, and adambulacrals of a small syntype (USNM 60612B) suggest a similar form, but ventral characters of a larger specimen are unavailable for this genus, and the dorsal surface is unavailable in the fossil.

Figures

Fig. 4. Urasterellid asteroid Urasterella? sp., PIN 4125/770, Middle Ordovician, Volkov, Russia. A. Entire specimen, ventral view; numbers identify ossicles of Fig. 5C. B. Disk region oriented as A, many displaced adambulacrals, spines above and to the left of the scale bar. C. Lower left arm of A, disk, interbrachial angle to left. D. Lower right arm of A, disk to left. E. Two abactinals, lateral view; crowns showing accessory depressions. F. Two abactinals to left, abactinal or possible inferomarginal to right. G. Two abactinals, above, adambulacral below; ossicles toward upper left of same approximate orientation as in D. Scale bars: A–D, 3 mm; E–G, 1 mm.

Fig. 5. Ambulacral series details in Ordovician and extant asteroids, see text for further discussion. A. Family Echinasteridae, Echinaster sp., extant, Florida (USA), scale bars A1, A2, 1 mm; A3–A5, 0.5 mm. A1, furrow view of ambulacrals and adambulacrals, proximal right; adambulacrals are angled (arrow) in the distal direction; cross−furrow tissue grooves (at top) overlie vertical articular plates and grooves, which in turn overlie flattened, ovate, lower cross−fur− row tissue grooves; A2, inclined dorsal view of ambulacrals and adambulacrals, proximal left, longitudinal tissue groove (upper arrow) and angled adambulacrals (lower arrow); A3, proximal view of a right adambulacral, furrow right (see A1); view direction approximately corresponding to C1, except the latter is rotated to the horizontal; black arrow identifies corresponding horizontal U−shaped contact bars in A3, A5, C1, C3, C4; A4, ventral−distal view, white arrow identifies corresponding interadambulacral contact facets above muscle depression in A4, C2, C5; A5, dorsal−proximal view of adambulacral. B. Family Goniasteridae, Peltaster placenta Verrill, 1899, Atlantic Ocean, extant, scale bars, 1 mm. B1, inclined ventral−distal furrow view of adambulacrals and adjacent actinal ossicles showing closely abutted adambulacrals with broad, flat interadambulacral contact surfaces in an extant asteroid; B2, inclined proximal−dorsal view of ambulacral series with vaulted ambulacrals, compare A1, A4, C1; B3, proximal view, adambulacral to left, compare to B4, with tightly abutted actinal ossicles to right; B4, distal view of adambulacral, furrow left; large, flat abutment surface encloses tissue depression. C. Family Urasterellidae, Urasterella? sp., Ordovician, Russia, PIN 4125/770, scale bars, 1 mm; C1, proximal view, ventral right, of adambulacral series with interadambulacral bar (arrow), approximately corresponding to that of Echinaster sp., A; C, C, distal views corresponding to A; C, C, proximal views Ą 3 2 5 4 3 4