Urasterellidae Schuchert, 1914

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 : 523-524

Family Urasterellidae Schuchert, 1914

= Cnemidactinidae Spencer, 1918: 155

Description.—Five−armed asteroids; disk small, formed by juncture of elongate, parallel−sided arms; interbrachial arcs angular, arms cylindrical, dorsal midline can be angular, marginals obscured by adambulacrals in ventral view. Madreporite where recognized small, dorsal.

Abactinal ossicles small, usually paxilliform; arm ossicles not radially symmetrical but rather with column near adradial edge; ossicle overlapping next (more adradial) abactinal. Where paxilliform, column expanding gradually to terminus; base more or less shield−like, flanged for papulary? reentrants, terminus bearing spine or spinelet tuft. Abactinals can lack a column, rather bear many pustules. Arm abactinals arranged in both longitudinal and transverse intersecting rows. Carinal series at least usually present; carinal ossicular form similar to that of lateral abactinals but ossicles radially symmetrical except for lateral flanges; ossicles can be enlarged, stout, elongate, forming spine−like row; carinal series can be partially offset, forming a double series. Differentiated series lateral to carinal series present in some species. Primary circlet present, ossicles weakly enlarged, radially symmetrical, similar to carinals; more than 10 ossicles usually present in primary circlet; abactinals within circlet can be numerous, can be regularly arranged (i.e., in a ring); centrale recognized in some species. Additional disk abactinals can be present beyond circlet, these generally similar to arm abactinals.

Madreporite where recognized generally small, abutting primary circlet.

Marginals in a single series, weakly to moderately enlarged relative to abactinals, ranging from similar to abactinals to robust, rectangular, subpaxilliform; accessories similar to those of abactinals.

Single axillary at least usually if not always present; size small to large, dorsal−ventrally elongate, shield−like; typically abutting MAO frame proximally, marginal series laterally, abactinals dorsally. Actinals absent.

Adambulacrals robust to short and plate−like; wide; arced, weakly to strongly overlapping in the distal direction. Adambulacrals bearing large ventral depressions for longitudinal articular tissues, medial transverse articular facets, and dorsal depressions for articular tissues (or water vascular system storage). Outer face bearing transverse series of spine or spinelet bases, bases can be differentiated by size, form.

Ambulacrals more or less slab−like: broad, high, fairly short; paired or nearly so across furrow; dorsal surface rather flat to ridged; cross−furrow articular structures and longitudinal articular structures gracile to moderately robust; ambulacrals strongly overlapping adambulacrals; muscular facets or water vascular pocket present along ventral abradial edge. Furrow for radial canal quite narrow, transverse ridge well developed, podial pores absent, ampullar basin narrow, apparently equally shared between subsequent ossicles. Skeletal gaps absent.

Near−oral adambulacrals not abruptly differentiated in size relative to more distal ossicles; proximal adambulacrals appearing capable of forming facultative adoral carina in at least some taxa. Mouth angle ossicular pair little enlarged relative to adjacent adambulacrals; MAO pair robust, broad, keel−like, each ossicle triangular in outline, outer surfaces rounded; accessory bases, accessories similar to those of adambulacrals but probably more robust. Internal MAO anatomy poorly known but broadly similar to that of other asteroids: oral tip short, blunt; circumoral and first adambulacral articular surfaces well developed. Circumorals poorly known, probably generally little differentiated from proximal ambulacrals but can be narrow or elongate and possibly consisting of fused second and third (i.e., after MAO) ambulacrals; circumoral cross−furrow tissue depressions well−developed. Odontophore data unavailable.

Content.— Urasterella McCoy, 1854 ; Salteraster Stürtz, 1893 ; Cnemidactis Spencer, 1918 ; Ulrichaster Spencer, 1950 ; Stiberaster Blake and Guensburg, 1993 .

Discussion.—An apomorphy−based diagnosis of the Urasterellidae is not attempted here because of the still−tentative nature of the systematics of Paleozoic asteroids. Spencer and Wright (1966) recognized many Paleozoic families differentiated on comparatively few characters. Shackleton (2005) treated the Urasterellidae and the Palasteriscidae as subfamilies of the Palasteriscidae Gregory, 1900 . This author’s diagnosis ( Shackleton 2005: 97) of the Urasterellinae included positioning of the inferomarginals and cross−sectional shape of the ambulacrals as well as character loss based on cladistic analysis. More extended description is provided here because the still incompletely understood early asterozoan fauna suggests continuing revisionary work can be expected.