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Journal of Paleontology 92(5):850–871
H. pristinus Billings, 1858; H. punctatocristatus Brower and Veinus, 1974; H. punctatus (Miller and Gurley, 1895); and H. tumidus Billings, 1857a.
Occurrence.—Ordovician (Sandbian), Illinois, Iowa, Kentucky, Oklahoma, Minnesota, Tennessee, Virginia, and Wisconsin, USA; (Katian), Ontario, Canada.
Remarks.—Billings (1857a) assigned two new species (Hybocrinus conicus and H. tumidus) from the Upper Ordovician to his new genus Hybocrinus; in 1858, he described a third species, H. pristinus (see Billings, 1958). These are the three species of Hybocrinus known from Ontario, Canada. As noted below, only H. tumidus is recognized from the Brechin fauna, and H. pristinus is considered a junior synonym of H. tumidus. Species characters in Hybocrinus are: overall calyx
symmetry about the oral-aboral axis, calyx outline in oral view, calyx plate sculpture, summit attitude, basal plate size, anal X shape, nature of the distal margin of the first anal plate, presence or absence of a stout A-ray arm, and position of column facet.
Hybocrinus tumidus Billings, 1857 Figures 5.5, 9.1–9.3
1857a Hybocrinus tumidus Billings, p. 275. 1858 Hybocrinus pristinus Billings, 1858, p. 25, figs. 4, 5. 1859 Hybocrinus pristinus; Billings, p. 23, pl. 1, fig. 2a. 1859 Hybocrinus tumidus; Billings, p. 28, pl. 2, fig. 1a–e. 1867 Hybocrinus pristinus; Grewingk, p. 103. 1867 Hybocrinus tumidus; Grewingk, p. 103. 1868 Hybocrinus pristinus; Shumard, p. 378. 1868 Hybocrinus tumidus; Shumard, p. 378. 1868 Hybocrinus pristinus; Bigsby, p. 20. 1868 Hybocrinus tumidus; Bigsby, p. 20. 1880 Hybocrinus tumidus; Wetherby, p. 152, pl. 5, fig. 2a–c.
1889 Hybocrinus pristinus; Miller, p. 255. 1889 Hybocrinus tumidus; Miller, p. 255. 1910 Hybocrinus pristinus; Grabau and Shimer, p. 501.
1910 Hybocrinus tumidus; Grabau and Shimer, p. 501, fig. 1812.
1915 Hybocrinus pristinus; Bassler, p. 654. 1915 Hybocrinus tumidus; Bassler, p. 654. 1943 Hybocrinus pristinus; Bassler and Moodey, p. 515. 1943 Hybocrinus tumidus; Bassler and Moodey, p. 515.
1944 Hybocrinus tumidus; Moore and Laudon, p. 151, pl. 53, fig. 3.
1946 Hybocrinus tumidus; Wilson, p. 31. 1973 Hybocrinus tumidus; Webster, p. 151. 1975 Hybocrinus tumidus; Strimple, p. 51, fig. 1. 1981 Hybocrinus tumidus; Parsley, p. 3, pl. 1, figs. 6, 9. 1986 Hybocrinus tumidus; Webster, p. 174. 1988 Hybocrinus tumidus; Webster, p. 96. 2013 Hybocrinus tumidus; Webster and Webster, p. 1666. 2013 Hybocrinus pristinus; Webster and Webster, p. 1665.
Type specimens.—Wilson (1946) recognized cotypes (GSC 1419b and 1419c) from Ottawa, Canada in the Trenton Limestone.
Diagnosis.—Calyx asymmetrical about oral-aboral axis; calyx with subpentagonal outline in oral view; calyx plate sculpture coarsely granulose; summit surface tilted; basal plates unequal in size; anal X nearly equidimensional or wider than high; distal margin of first anal plate with deep grooves; A-ray arm stout; position of column facet asymmetrical.
Occurrence.—Hybocrinus tumidus was originally described from the Cobourg beds (now Lindsay Formation) at Ottawa, Canada, and H. pristinus was originally described from the Aylmer Formation (Sandbian) of Quebec, Canada. In addition, this taxon was reported from the Hull (now Bobcaygeon For- mation) and Sherman Falls members, Ottawa (now Verulam) Formation, Ontario (Katian), Canada; and the Curdsville Member, Hermitage Formation, Kentucky (Katian), USA. Since then, it has been recognized from the Curdsville Limestone Member of the Lexington Limestone in Mercer County, Kentucky, USA. In the Brechin Lagerstätte H. tumidus is known from the Bobcaygeon-Verulam contact zone at the Carden Quarry (UMMP 74669) and LaFarge Quarry and from the upper Bobcaygeon at the Kirkfield Quarry (Ordovician, Katian).
Description.—Crown medium-sized. Aboral cup medium bowl- to medium globe-shaped; maximum width to maximum height ratio ~1.0; plates gently to markedly convex; sculpture with irregular-shaped, small nodes arranged in rows parallel to plate boundaries, combined with fine pitting (Fig. 9.3); in narrow margin along plate boundaries, fine ridges perpendicular to plate boundaries that abut similar ridges on opposing plates. Basal circlet ~25% of aboral cup height, varying in
1882 Hybocrinus tumidus; Carpenter, p. 298, fig. A, pl. 11, figs. 3–5.
different rays. Basal plates five, hexagonal, very differently sized (Fig. 9.3), typically as high as wide. Radial circlet ~75% of aboral cup height; radial plates five, A, B, D, and E radial plates simple; C radial plate compound. Radial plates of very different sizes and shapes, symmetrical or asymmetrical about the oral- aboral axis. C inferradial hexagonal, slightly wider than high; superradial above to right; first anal plate above to left. Radial facets angustary, horseshoe-shaped, declivate; A, B, and E radial facets symmetrically positioned on radial plate; C and D radial facets asymmetrically positioned on radial plate. Details of radial facet topography unknown. Radianal (Fig. 9.2) supported equally beneath by C and D
inferradials, supporting above C radial plate to right and anal X to left. Anal X partially in aboral cup; distal margin arcuate, strongly crenulate (Figs. 5.5, 9.2). Oral surface plating unknown; surface tilted relative to oral-aboral axis. Arms atomous; primibrachials rectilinear uniserial
(Fig. 9.1), as high as wide, distinctly convex, deep; lateral sides of brachials strongly crenulate (Fig. 9.2). Proximal columnals very thin, homeomorphic; wavy
sutures between columnals; other aspects of column not known. Lumen strongly pentalobate, ~50% of columnal width; columnal facets unknown.
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