Ausich et al.—Disparid and hybocrinid crinoids from the Brechin Lagerstätte
Measurements.—UMMP 74655.1: CrH, 28.0*; ACH, 2.8; CoH, 20.0*. UMMP 74656: CrH, 21.5; ACH, 2.6; maximum ACW, 3.5; CoH, 10.2* (although nearly complete). UMMP 74657: CrH, 11.6*;ACH, 2.2; maximumACW, 4.1; CoH, 6.0*.
Remarks.—On several specimens, a portion of the calcite skeleton is preserved with a very dark coloration. This most typically occurs on aboral cup plates, but this coloration can extend onto the proximal column and proximal brachials (e.g., UMMP 74657). Whether this represents secondary mineralization or an expression of preserved organic molecules (O’Malley et al., 2013) is not known.
Family Cincinnaticrinidae Warn and Strimple, 1977 Genus Isotomocrinus Ulrich, 1925
Type species.—Isotomocrinus typus Ulrich, 1925, by mono- typy. However, I. typus is has been designated a subjective junior synonym of Heterocrinus tenuis Billings, 1857b (see Warn and Strimple, 1977).
Other species.—Isotomocrinus apheles Ausich, Bolton, and Cummings, 1998 and I. minutus Kolata, 1975.
Occurrence.—Ordovician (Darriwilian), Newfoundland, Canada; (Sandbian), Illinois, Iowa, Minnesota, Tennessee, and Wisconsin, USA; and (Katian), New York, USA, and Ontario, Canada.
Remarks.—As with Daedalocrinus, Ulrich (1925) proposed a new disparid species as the type species of a new genus; however, that species was subsequently regarded as a junior synonym. Ulrich (1925) designated Isotomocrinus typus as the type species of Isotomocrinus. Although not recognized as such by Webster and Webster (2013), Warn and Strimple (1977) and Ausich et al. (1998) regarded I. typus as a junior subjective synonym of I. tenuis, and that opinion is followed here. Thus, three species of Isotomocrinus are considered valid:
I. apheles, I. minutus, and I. tenuis. These three species are distinguished on the basis of shape of the aboral cup, number of primibrachials, number of secundibrachials, and arm branching.
Isotomocrinus tenuis (Billings, 1857b) Figure 4.4, 4.5
1857b Heterocrinus tenuis Billings, p. 273. 1868 Heterocrinus tenuis; Shumard, p. 377. 1868 Heterocrinus tenuis; Bigsby, p. 20. 1886 Stenocrinus tenuis; Wachsmuth and Springer, p. 132 (p. 208).
1889 Heterocrinus tenuis; Miller, p. 252. 1910 Heterocrinus tenuis; Grabau and Shimer, p. 502. 1915 Heterocrinus tenuis; Bassler, p. 612.
1925 Heterocrinus juvenis Fritz, p. 10, pl. 1, figs. 7, 11, 12, figs. 2, 3 (non Hall, 1866).
1925 Isotomocrinus typus Ulrich, p. 86, fig. 5. 1938 Isotomocrinus typus; Bassler, p. 119. 1943 Isotomocrinus typus; Bassler and Moodey, p. 525.
857
1944 Isotomocrinus typus; Moore and Laudon, p. 149, pl. 52, fig. 11.
1971 Ectenocrinus n. sp.; Steele and Sinclair, p. 3, pl. 16, figs. 10, 11.
1973 Isotomocrinus typus; Webster, p. 158. 1975 Isotomocrinus tenuis;Kolata, p. 26, fig. 8, pl. 4, figs. 9–11.
1977 Isotomocrinus tenuis; Warn and Strimple, p. 62, pl. 8, fig. 15.
1978 Isotomocrinus tenuis; Brower and Veinus, p. 456, pl. 16, fig. 4.
1986 Isotomocrinus tenuis; Webster, p. 180. 1999 Isotomocrinus typus; Brett and Taylor, p. 69, fig. 82. 2005 Isotomocrinus tenuis; Sloan, p. 153, fig. 4–60.5. 2013 Isotomocrinus tenuis; Webster and Webster, p. 1710. 2013 Isotomocrinus typus; Webster and Webster, p. 1711.
Holotype.—GSC 1438.
Diagnosis.—Steep-sided medium cone-shaped aboral cup; 4 or 5 primibrachials,; 4–9 secundibrachials; poor isotomous arm branching.
Occurrence.—The original description of Isotomocrinus tenuis was based on material from the ‘Trenton Limestone’ at Ottawa andMontreal,Canada,which are nowreferred to as theHull beds. In the Brechin Lagerstätte, this taxon is known from the Bobcaygeon-Verulamcontact zone at theCardenQuarry (UMMP 74659) and LaFarge Quarry (UMMP 74658, UMMP 74660) and from the upper Bobcaygeon Formation at the Kirkfield Quarry (Ordovician, Katian). In addition, this taxon has been described from the Buckhorn Member of the Ion Formation in Illinois (Sandbian), the Platteville Group of Illinois (Sandbian), and the Trenton Limestone of New York (Katian), USA.
Description.—Crown medium-sized, constricted at level of proximal primibrachials then an expanding cone thereafter. Aboral cup medium cone-shaped, as high as wide; plates gently convex; plate sculpture smooth (Fig. 4.5). Basal circlet ~50% of aboral cup height; basal plates five,
hexagonal, 1.6 times higher than wide. Radial circlet ~50% of aboral cup height; radial plates five; B and D radial plates simple; A, C, and E radial plates compound; simple radial plates pentagonal, 1.5 times higher than wide; inferradial plates tetragonal, as high as wide; superradial plates pentagonal, slightly higher than wide, extending to approximate level of adjacent radial plates; proximal anal plate on upper left shoulder; C-ray arm on upper right shoulder. Radial facets plenary; details of radial facet topography unknown. Distal corner of first anal plate supported beneath by C superradial and D radial, pentagonal with uneven sides,
approximately as wide as high. More distal anal plates above first anal plate similar to brachials, approximately as high as wide.
absent; first primibrachial trapezoidal, proximal width 1.5 times greater than distal width, height approximately same as proximal width; subsequent primibrachial width 1.25 times wider than high. Axillaries larger than nonaxillary plates. Primaxils from fourth to fifth primibrachial; secundaxil from
Arms branching three times with poor isotomy; ramules
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