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 : 526-527

Urasterella McCoy, 1854

= Roemeraster Stürtz, 1886

= Phillipsaster Spencer, 1950

Type species: Uraster ruthveni Forbes, 1848 , Upper Silurian , England .

Diagnosis.—Urasterellid with generally small, paxilliform abactinals; carinal series, primary circlet differentiated but generally weakly so (where dorsal surface is known). Marginals in a single series, similar but not identical in number to adjacent abactinals and adambulacrals; marginals paxilliform, ranging among species from little differentiated from abactinals to robust; lateral edges of marginals not closely fitted between adambulacrals; accessories present. Axillary small (where recognized), extended to MAO. Adambulacrals thin to quite robust; upright, overlapping distally; adambulacral spines robust. Disk appearing to be able to contract to allow proximal adambulacrals to form facultative adoral carina; mouth angle ossicles little enlarged; torus small.

Remarks. —Publication data for Urasterella are variously given in the literature; data here taken from a copy in the library at the University of Illinois, Urbana−Champaign. McCoy (1854: 59) noted that the name Urasterella existed earlier but in manuscript form.

Because of a meager fossil record and the poor preservation of the type material of most recognized species, generic concepts within the Urasterellidae , including that of Urasterella , are difficult. Useful sources for Urasterella include Schuchert (1914, 1915), Spencer (1918, 1950), Spencer and Wright (1966), and Shackleton (2005). Among proposed Urasterella −like genera, Roemeraster Stürtz, 1886 , is Devonian, the remainder are Ordovician. Schuchert (1914, 1915) synonymized Roemeraster and Salteraster with Urasterella ; Spencer and Wright (1966) retained Salteraster as well as Phillipsaster Spencer, 1950 , and Ulrichaster . Separation of genera made use of presence of a well−defined carinal series in Salteraster and a double series in Ulrichaster as well as size and form of the dorsal surface of the arm. Shackleton (2005) was unable to obtain data on the midarm of Urasterella ruthveni Forbes, 1848 , the type species, and she synonymized Salteraster , Phillipsaster , and Ulrichaster with Urasterella in part because of lack of information on the dorsal surface; other presumed generic differences were attributed to taphonomic effects.

A cast of a type of U. ruthveni, SM A−5497a, the type species, was available. The specimen shows the ventral surface with abradial edges of the abactinal series. This specimen, and others representing other species, suggest the presence of facultative adoral carina, this based on subtle differences in ossicular positioning around the mouth frame. Spencer (1918) concluded that superomarginals are not present in Urasterella , although these were recognized by Schuchert (1915); superomarginals were not identified in any of the specimens available to DBB.

Although Salteraster was separated largely based on presence of a carinal series, Shackleton (2005) found presence of such a series to be equivocal in the type species, S. asperrimus ( Salter, 1857) . Casts of the type were available to DBB, who believes the carinal series can be recognized. Abactinal ossicles of specimens of Urasterella available for the current study each overlap the next abactinal toward the arm midline. The arrangement would appear to all but dictate some form of differentiation of arm midline ossicles, the midline functionally analogous to a ridgepole in a gable roof. Spencer and Wright (1966) characterize Urasterella as having flat arms, which appears to be true of specimens of U. ruthveni illustrated by Spencer (1918: pl. 9). Flat arms would seem to lessen the need for a ridgepole; nevertheless, a carinal series appears to be present. Illustration of the type species of Salteraster appears to have an adoral carina ( Spencer and Wright 1966: fig. 64.4b) although this specimen is sharply distorted in a manner that might have squeezed the ossicles together. To the extent discerned on the poorly preserved specimens, morphology of ossicular systems is quite similar but both genera are tentatively retained here based on Spencer and Wright’s conclusion that the arms are comparatively flat, and presumably few rows of abactinals between the carinals and the marginals in the type species of U. ruthveni .

The types of Ulrichaster ulrichi ( Schuchert, 1915) were available, and although the ventral surface is unavailable except in a small specimen assigned to the species by Schuchert (1915), carinals are slightly enlarged, radially symmetrical, and developed in an offset series that appears to be original rather than taphonomic; a double series would appear to be a rather minor variant of the ridgepole specialization, but it is distinctive, and therefore this genus is also retained here.

Spencer (1918) separated Cnemidactis based largely on the nature of accessories, form of the marginals and adambulacrals, the relationships between these two ossicles, and presence of a large torus. The cladistic analysis of Shackleton (2005) placed Cnemidactis adjacent to Urasterella , and she assigned the genus to the Urasterellinae. Shackleton (2005) recognized Stiberaster as the sister group to her Palasteriscinae plus Urasterellinae, and it is here assigned to the Urasterellidae .