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Journal of Paleontology 92(2):196–206
(Larson et al., 2014) in response to methane seepage through seafloor faults (Metz, 2010).Larson et al. (2014) hypothesized that they developed along fault lines due to tectonic activity, allowing for dewatering of the thick Cretaceous shales prior to and coin- cident with the Black Hills uplift. At any one locality, the seeps typically occur in groups of three or more, representing either closely connected conduits ormovement of a single seep through time. Howe (1987) reported that most fossil seep core dimensions near Pueblo, Colorado, range from 1 to 8m in width and up to 30m in height, whereas many seep deposits around the Black Hills are larger (Larson et al., 2014). The lithology of ancient seeps has been described by a
number of authors (Kauffmann et al., 1996; Metz, 2008; Hendricks et al., 2011). Using the seep structure model of Handle (2014), Meehan and Landman (2016) related faunal expressions to seep architecture, with variation among seep occurrences resulting from variation among causal factors of original depth, lateral extent of seepage, methane flux, oxygen levels, water temperature (Levin, 2005), seep longevity (Larson et al., 2014), and perhaps as yet unrecognized factors; additional work is needed. Regardless of the lithology and architecture, most of the
carbonate core intervals are micritic and were formed by bacterial-mediated anaerobic oxidation of methane (AOM) coupled with sulfate reduction, the overall chemical reaction being: CH4+SO4
Carbonate precipitation commonly occurred from the seafloor surface to several meters within the substrate (Hendricks et al., 2011; Cochran et al., 2015). Inoceramid bivalve accumulations together with fine clastics provided a varied substrate that supported a diverse primary autotrophic fauna of tube worms and bivalves nourished by methane and/or sulfide chemo- symbionts. The autotrophs were prey for predatory gastropods, crabs (MacAvoy et al., 2002), and cephalopods (Landman et al., 2012). Cemented ‘shell hash’ and seep-associated concretions can be present at the periphery of the main micritic mass (‘core’; Hunter et al., 2016, fig. 3). All the seeps from Colorado to the Black Hills occur in the
−2→HCO3−+HS−+H2O (Levin, 2005).
Pierre Shale, a thick, homogenous deposit of black shale more than 1,000mthick deposited from the early Campanian through early Maastrichtian in a seaway generally 300m deep or less (Gill and Cobban, 1966). Pierre Shale ammonite paleontology and biostratigraphy were summarized by Larson et al. (1997), the pertinent Campanian zonation summarized in Figure 2. The fossil methane seeps encircling the Black Hills are from the Didymoceras nebrascense Zone (base of upper Campanian) through the Baculites grandis Zone (lower Maastrichtian) (Larson et al., 2014), the asteroids from two sites in the Didymoceras cheyennense Zone (upper Campanian), AMNH loc. 3418 and AMNH loc. 3529; a partial arm from AMNH loc. 3495 is lost. Additional asteroid specimens were found at an undocumented seep site in the 1990s, their present location unknown. All asteroid specimens were found on the flanks of the main core micritic masses (Fig. 3). Echinoderm paleoecology of Pierre Shale cold seep envi-
ronments has been studied on the basis of skeletal elemental chemical analyses and stable carbon isotopes (Kato et al., 2017). A crinoid species recovered from seep settings was found to be both morphologically and geochemically differentiated as to
argue that this species is endemic and specialized to the seep environments, whereas Western Interior Seaway echinoids also based on morphologic and geochemical data were judged to have been a part of the background fauna and not restricted to seep settings. Geochemical data are not available for B. brezinai but its morphology does not differ in any obvious manner from morphologies of species of Betelgeusia that are not associated with seep settings, and therefore the new asteroid, like the echinoids, likely is a part of the background fauna.
Terminology.—Terminological usage for the asteroids follows Spencer and Wright (1966) and Blake (2013). The primary skeleton forms the body wall. The accessory skeleton includes the generally abundant spines, granules, and pedicellariae seated on all primary ossicles except ambulacrals. Ambulacral ossicles form a double series along the axis of the arm and serve as the foundation for the water vascular system. Ambulacrals are vaulted to form the ambulacral furrow, and the mouth angle ossicles (MAO) are the proximal-most ossicles of the ambulacral series. Ambulacral ossicles articulate with and are immediately dorsal to the adambulacral ossicles. Marginal ossicles form either a single inferomarginal series or a double inferomarginal plus superomarginal series, one or both posi- tioned near the margin of the asteroid. A primary circlet of abactinals can be differentiated on the dorsal disk, and dorsal midarm ossicles can be enlarged and/or otherwise differentiated to form a carinal series. The madreporite is the outlet of the water vascular system, on the dorsal disk surface of radiasterids. No, few, or many actinals occur between the inferomarginals and adambulacrals.
Repository and institutional abbreviation.—All specimens are housed in the collections at American Museum of Natural His- tory (AMNH), Division of Paleontology.
Systematic paleontology
Order Paxillosida Perrier, 1884 Family Radiasteridae Fisher, 1916
Known genera.—Betelgeusia Blake and Reid, 1998 (middle Cretaceous); Gephyreaster Fisher, 1910 (extant); Indiaster Rao, 1957 (Middle Jurassic); Radiaster Perrier, 1881 (extant).
Remarks.—Although only two extant genera are assigned to the Radiasteridae, taxonomic interpretations of the family have differed (Fisher 1911, 1919; Clark, 1946; Blake, 1987; Clark and Downey, 1992). Two extant genera, Radiaster Perrier, 1881 (=Mimaster), represented by four species, and monospecific Gephyreaster Fisher, 1910 (not Gephyriaster Spencer and Wright, 1966) have been assigned to the family. Because of ambiguities in the literature as well as the relatively robust construction of Gephyreaster as compared to Radiaster, Blake (1987) and Blake and Reid (1998) limited treatment of extant Radiasteridae to Radiaster itself. Molecular methodologies are of increasing importance in
the study of evolution and biogeography, these approaches potentially particularly useful for complex settings and
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