Ausich et al.—Disparid and hybocrinid crinoids from the Brechin Lagerstätte


1889 Iocrinus trentonensis; Miller, p. 257. 1915 Iocrinus trentonensis; Bassler, p. 668. 1943 Iocrinus trentonensis; Bassler and Moodey, p. 525. 1999 Iocrinus trentonensis; Brett and Taylor, p. 65, fig. 74. 2007 Iocrinus trentonensis; Brower, p. 1285, fig. 2.10–2.12. 2008 Iocrinus trentonensis; Brower, p. 59, figs. 2−5. 2011 Iocrinus trentonensis; Brower, p. 270, fig. 2.1–2.2. 2013 Iocrinus trentonensis; Webster and Webster, p. 1707.


Type specimens.—Lectotype, MCZ 106417; paralectotype, MCZ 113426 (see Brower, 2008).


Diagnosis.—Relatively small species with aboral cup distal width to height ratio much more than twice that of aboral cup proximal width versus height; basal plate height ~50% of radial plate height; radial plate height and width approximately equal; transverse ribbing between adjacent aboral cup plates; first primibrachial width versus height 1.5 or less; four to six primibrachials; six to nine secundibrachials; six to fifteen terti- brachials; three to five in-line bifurcations per ray; brachials with distal flanges through most of arm height; anal sac robust.


Occurrence.—Iocrinus trentonensis was originally described from the Trenton Limestone of New York, USA. It is now recognized from the Spillway Member, Rust Limestone (Katian) in New York and the Brechin Lagerstätte from the Bobcaygeon-Verulam contact zone at the Carden Quarry (UMMP 74667.1) and from the lower Verulam Formation at the James Dick Quarry (UMMP 74668) (Ordovician, Katian).


Description.—Crown medium-sized, expanding cone-shaped. Aboral cup flat cone-shaped; height to width ratio ~4.0; plates sharply convex; plate sculpture smooth (Fig. 8.2). Basal circlet ~33% of aboral cup height; basal plates five,


hexagonal, 3.0 times higher than wide; basal-basal plate sutures depressed. Radial circlet ~67% of aboral cup height; radial plates five; A, B, D, and E radial plates simple. Simple radial plates ranging from as wide as high to 1.4 times wider than high, very convex with broad ridge connecting adjacent radial plates, deeply concave at radial-radial-basal plate triple junction. C radial plate compound; C inferradial pentagonal, 1.5 times wider than high, same sculpture as simple radial plates; C superradial plate above aboral cup, subpentagonal, 1.7 times wider than high, constricted medially a short distance above proximal suture; anal X supported on left with slightly shorter suture than C-ray arm to right (Figs. 5.2, 8.2). Radial facets peneplenary, occupying 75% of distal width of radial facet; facet topography unknown. First anal plate above aboral cup, supported beneath by


only C superradial, hexagonal, slightly contorted, ~1.2 times higher than wide. Second anal plate quadrangular,


865


nonramulate; first primibrachial quadrangular, ~1.5 times wider than high; fourth primibrachial axillary in all rays. Secundibra- chials and higher quadrangular, as high as wide; fifth to seventh secundibrachial axillary. Column pentalobate, heteromorphic, holomeric; largest


approximately as high as wide. Based on anal sac plates preserved within crown and isolated, intact anal sac preserved adjacent to Iocrinus crowns; anal sac large, cylindrical, comprised of two columns of robust plates as high as wide (one in CD-interray orientation, one in A-ray orientation); two columns connected laterally by concave, very wide plates with plicated, depressed sutures; sutures covered by small plates (Fig. 8.3). Arms branching four or five times isotomously (Fig. 8.1);


columnals four times wider than high; latus convex (Fig. 8.1). Lumen circular, ~15% of columnal width; columnal facets unknown.


Materials.—UMMP 74667.1, 74667.2, 74668.1, and 74668.2.


Measurements.—UMMP 74667.1: CrH, 48.0; ACH, 3.1; distal ACW, 6.8; CoH 42.8*. UMMP 74668.1: ACH, 4.7; distal ACW, 7.4; CoH, 26.0*.UMMP74668.2: CrH, 49.2.; ACH, 5.0; distal ACW, 7.7; CoH 50.0.


Remarks.—As noted above, Iocrinus trentonensis can be readily differentiated from the other two North American spe- cies of Iocrinus. Iocrinus trentonensis is closest to I. subcrassus and might be a paedomorphic derivative of I. subcrassus,with smaller size, fewer brachial plates within brachitaxes, wider radial plates, a wider aboral cup, and flanges throughout the height of the arms. Of the two species, I. subcrassus is the most widespread geographically of the two species, although both occur in Katian strata of Ontario, Canada. Only I. trentonensis is known from the Brechin fauna. Although this taxon is recognized by relatively small


specimens, Iocrinus trentonensis is regarded as an adult based on the morphology of the brachial plates, which are wider than high rather than higher than wide.


Parvclass Cladida Moore and Laudon, 1943 Superorder Porocrinoidea Wright, 2017 Order Hybocrinida Jaekel, 1918 Family Hybocrinidae Zittel, 1879


Genus Hybocrinus Billings, 1857a


Type species.—Hybocrinus conicus Billings, 1857a, by subsequent designation.


Other species.—Hybocrinus bilateralis Guensburg, 1984; H. crinerensis Strimple and Watkins, 1949; H. nitidus Sinclair, 1945; H. perperamnominatus Brower and Veinus, 1974;


Figure 9. Hybocrinidae and Calceocrinidae: (1–3) Hybocrinus tumidus Billings, 1857a, UMMP 74669; (1) A-ray view of partial crown (note robust, uniserial brachials of atomous arms; note radial plate is broken with an arcuate fracture); (2) CD interray view of partial crown (note the prominent ridges along the convex distal surface of the first anal plate and the prominent grooves on the sides of brachials; compare to Figure 5.5); (3) basal view of calyx; (4, 5) slightly askew E-ray view of crown and column of Cremacrinus inaequalis Billings, 1859 (note damage on some calyx and proximal brachials prior to collection), UMMP 74666; (4) entire crown; (5) enlargement of aboral cup (note short length of column tucked beneath the aboral cup); (6) four partially compressed adult individuals of Hybocystites problematicus Wetherby, 1880 (note prominent sculpture on plates, prominent ridges on brachials and along recumbent ambulacra, and proximal extensions of recumbent ambulacra), UMMP 74670.1–74670.4. Scale bars 2.5mm (5); 5.0mm (remaining).


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