Blake et al.—Cretaceous seep asteroid
evolutionary histories (Hardy et al., 2010) such as those represented here by both seep occurrences and problematic taxa such as the Radiasteridae. As true of other invertebrates, phylogenetic relationships among major subdivisions of the crown-group Asteroidea are undergoing significant revision with evolving molecular results. At writing, Radiaster has not yet been treated in a molecular phylogenetic analysis whereas Gephyreaster clustered with Pseudarchaster Sladen, 1889, the two assigned to the Pseudarchasteridae within an otherwise traditional paxillosidan and notomyotidan (Benthopectinidae) complex (Mah and Foltz, 2011a).Middle Cretaceous Betelgeusia Blake and Reid (1998) was the first fossil radiasterid to be recognized. Indiaster Rao, 1957 was originally assigned to the Goniasteridae but later transferred to theRadiasteridae (Blake and Reboul, 2011), the genus originally described as “probably of Upper Bathonian age” (Middle Jurassic) (Rao, 1957, p. 213). Given molecular analysis together with the closing of the morphologic gap between Radiaster and Gephyreaster as based on species of Betelgeusia, five fossil and extant genera, Betelgeusia, Gephyreaster, Indiaster, Pseudarchaster,and Radiaster, form a monophyletic
complex.Molecular positioning of Radiaster and subsequent status evaluation of the terms ‘Radiasteridae’ and ‘Pseudarchasteridae’ are left for the future.
Genus Betelgeusia Blake and Reid, 1998
Type species.—Betelgeusia reidi Blake and Reid, 1998, by monotypy.
Other species.—B. exposita Blake and Jagt, 2005; B. orientalis Blake and Reboul, 2011.
Emended diagnosis.—Radiasterid with relatively robust, fas- ciolate marginals; fascicular expression, particularly super- omarginal expression, varied among species. Abactinals subgranular to robust paxillae, not tabulate. Actinal ossicles many, small, aligned in strongly imbricate series that extend from inferomarginals to adambulacrals; well-defined grooves separate exposed crowns of adjacent series. Accessories varied: granules, robust short spinelets, or elongate delicate spinelets; pedicellariae not recognized.
Remarks.—A range of marginal, accessory, and abactinal var- iation is developed among the four species of Betelgeusia, although generic subdivision is not deemed justified. Indiaster is incompletely known, but unlike Betelgeusia, the actinal ossicles appear to be relatively robust, rectangular, and at most, only weakly imbricate. The two marginal series of Indiaster appear to be similar, robust, and closely fitted, the two series not distinctive as in Betelgeusia. Enough remains of the Indiaster specimens to suggest that accessory development was subdued. All primary ossicles of extant Radiaster are small and delicate, the marginals blade-like or subpaxilliform, and paxillae are delicate. Spinelets are small. Diagnostic separation of Gephyreaster and Radiaster (=Mimaster) was based on pre- sence of stout tabulate paxillae and a peculiar MAO accessory arrangement in the former (Fisher, 1911, p. 175), criteria that also serve to separate Gephyreaster from
Betelgeusia.In Gephyreaster as in Betelgeusia, marginals and,
199
in particular, inferomarginals are comparatively robust and closely fitted, although smaller accessories are similar between Gephyreaster and some Betelgeusia. The strongly imbricate, paddle-like actinals of Betelgeusia
aligned in well-defined series form a distinctive generic character complex. The almost upright actinal occurrences of some fragments of B. brezinai (Fig. 5.10, 5.11) might have been accentuated taphonomically, but not significantly so from natural positioning assumed during normal life activities.
Betelgeusia brezinai new species Figures 4.1–4.7; 5.1–5.13
2017 asteroid; Kato et al., p. 223, fig. 2I.
Holotype.—Locality AMNH 3529, holotype AMNH-FI 111823: a small individual in overall good condition and exposing both surfaces, although distorted through sediment compaction and breakage, and partially obscured by foreign shell fragments. Remaining arm radii R=18, 12, 12, 4, and 11mm; r of three remaining interbrachia 8–10mm.
Paratypes.—Locality AMNH-FI 3529, paratypes: an illu- strated, nearly complete specimen exposed in ventral aspect, AMNH-FI 111824, an arm tip with both surfaces exposed, paratype AMNH-FI 111825; five illustrated small fragments, AMNH-FI 111825–AMNH-FI 111830; seven partial paratypes, relatively complete to fragmentary, AMNH-FI 111831– AMNH-FI 111837; and 14 small to tiny paratype fragments of larger specimens, AMNH-FI 111838–AMNH-FI 111851. Locality AMNH 3418, a single paratype, AMNH-FI
111852, exposed in ventral aspect, on a small block of fossiliferous carbonate matrix. Two arms are nearly complete, R=19mm; truncated R between about 9 and 12mm, inter- brachial radius 10mm. Inferomarginals of interbrachia are complete; a single superomarginal series is visible, but no abactinals; no accessory ossicles remain. Approximately 14 inferomarginals between the interbrachial plane and arm tip of nearly complete arm R=19mm, r ~ 9mm. About 13 to 15 marginal pairs between interbrachial midline and terminal.
Occurrences.—Locality AMNH-FI 3529: Didymoceras cheyennense Zone, Pierre Shale, upper Campanian, Late Cretaceous; Pennington County, South Dakota. Exact locality information is available to qualified researchers from the American Museum of Natural History. All available specimens but one were collected at locality 3529. The locality is a low-profile mound consisting of tan-
weathered shales at the top with less-weathered black and dark- gray shales at its base. A 1–2m main carbonate micritic mass (‘core’) is exposed with associated concretions and cemented shell hash carbonates on the core flanks; weathered carbonates are scattered downslope. The original Pierre Shale surrounding the seep was eroded during the Paleocene through early Eocene, exposing the carbonates to weathering. The seep was later reburied by Chadron Formation deposition during the middle to late Eocene, composite sediments including volcanic ash from the west and alluvial gravels and clay from the Black Hills. The seep was reexposed as a result of weathering during the
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