Abstract

A new genus and species of Asteroidea (Echinodermata), Embolaster graffhami, Embolasteridae, n. fam., is described from the Upper Ordovician Bromide Formation of Oklahoma. Embolaster is important in part because of the presence of two similar ossicles on each interbrachial plane in the axillary position; only a single axillary is recognized in most Paleozoic asteroids. The axillary traditionally has been considered to be a part of the marginal series, although marginals are almost always paired. The two ossicles are interpreted as axillaries and the homologues of an ancestral proximal marginal pair. The odontophore, which lies distal to the mouth angle pair of most asteroids, tentatively is the homologue of the proximal axillary.

Embolaster also is important because of the presence of an ossicular series between the ambulacrals and the putative adambulacrals. An extra ossicular series in this position is known from taxa traditionally assigned to the ophiuroids; however, other characters of Embolaster indicate asteroid affinities for this genus. The two ossicles lateral to and aligned with the ambulacrals in Embolaster are interpreted as homologous with the similarly arranged radial virgal series of somasteroids, although specific ossicular homologies are problematic.

A part of the mouth frame structure of Embolaster is exposed, although it is tiny and therefore difficult to interpret. The mouth frame arrangement is reminiscent of those of somasteroids, with small, narrow circumorals and apparent buccal podial basins directed toward the mouth opening. Many of the striking characters of Embolaster probably are plesiomorphic; apomorphies are difficult to ascertain.

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