Mao et al.—New Silurian Petalocrinidae
Remarks.—The arms first fused laterally as in Petalocrinus,and then the distal part rolled dorsally, forming an arm-roll. Only a small area in the proximal part of the dorsal surface is not covered by ambulacra. The characters of the ventral furrow ridges and the articular facet are similar to those of Petalocrinus (Mu and Lin, 1987). The aboral area lacking ambulacra of this species is less
than half of the aboral side of the arm, which is larger than Sinopetalocrinus robustus but smaller than Sinopetalocrinus shiqianensis.
Sinopetalocrinus shiqianensis Mu and Lin, 1987 Figure 5.1–5.5
1987
Sinopetalocrinus shiqianensis Mu and Lin, p. 12, pl. 5, figs. 1–4.
Holotype.—Holotype NIGP 73963.
Diagnosis.—As many as 32–35 distal ambulacra; distal part of arms with a rolled margin; adjacent arms not in lateral contact; ambulacral grooves on oral and aboral surface; ambulacra coverage on aboral surface is approximately one-third.
Occurrence.—Leijiatun Formation, Aeronian (Llandovery, Silurian), Sigou section, Shiqian County, Guizhou Province, China; Leijiatun Formation, Aeronian (Llandovery, Silurian), Fengxiang section, Shiqian County, Guizhou Province, China. NIGP 73963, NIGP 73964, NIGP 73965 and NIGP 73966 are type specimens of Sinopetalocrinus shiqianensis from Mu and Lin (1987). NIGP 163730 is a new specimen from the Fengxiang section. Aboral side of arm slightly convex.
Description.—Ambulacra coverage on aboral surface is approximately
one-third.Area of aboral surfacewith ambulacra is 19.2mm in height, 22.4mm in width. Arm thickness is 6.0mm where aboral side of arm lacks ambulacra and 7.0mm where the aboral surface has ambulacral. The armangle is 105°.Ambulacral grooves ~1mm wide, which is narrower than the ridge. Flat ridge top. The distal grooves as many as 32. Unclear notch for ambu- lacral cover plates on NIGP 163730.Well-preserved, horseshoe- shaped facet 2.5mm in width, and 1.6mm in depth.
Materials.—Paratypes NIGP 73964–73966; other material: NIGP 163730.
Remarks.—The aboral area lacking ambulacra in this species is larger than in Sinopetalocrinus involutus and Sinopetalocrinus robustus.
Sinopetalocrinus robustus Mu and Lin, 1987 Figure 6
1987
Sinopetalocrinus robustus Mu and Lin, p. 12, pl. 6, figs. 1–3.
Holotype.—Holotype NIGP 73971.
Diagnosis.—As many as 33 distal ambulacra; distal part of arms with a rolled margin; adjacent arms not in lateral contact; ambulacral grooves on oral and aboral surface; percentage of ambulacra coverage on aboral surface is very high, only a very small area lack of ambulacra.
Occurrence.—Leijiatun Formation, Aeronian (Llandovery, Silurian), Sigou section, Shiqian County, Guizhou Province, China; Leijiatun Formation, Aeronian (Llandovery, Silurian), Fengxiang section, Shiqian County, Guizhou Province, China. NIGP 73971, NIGP 73972 and NIGP 73973 are type specimens of Sinopetalocrinus shiqianensis from Mu and Lin (1987). NIGP 163731 is a new specimen from the Fengxiang section.
Description.—Aboral side of arm slightly convex. Percentage of ambulacra coverage on aboral surface is very high, only a very small area lack of ambulacra. Aboral arm surface with ambulacra is relatively small with a height of 14.3mm and width of 12.1mm. Thickness of aboral surface of arm that lacks ambulacra 2.8mm. Distal area of aboral surface with ambulacra is 4.7mm in thickness. The arm angle is 118°. Ambulacral groove ~1mm wide and ridge <1mm. As many as 33 ambula- cral grooves on the distal arm. Unclear notch for ambulacral cover plates. Well-preserved, horseshoe-shaped facet 3.0mm in width and 2.0mm in height.
Materials.—Paratypes NIGP 73972 and NIGP 73973; other material: NIGP 163731.
Remarks.—This species is characterized by small and thin arm plates; a very small area of the aboral surface that lacks ambu- lacra; and many ambulacral grooves separated by narrow ridges.
Genus Spirocrinus Mu and Wu, 1974 Type species.—Spirocrinus typicus Mu and Wu, 1974.
Other species.—Spirocrinus longus Mu and Wu, 1974. Spirocrinus longus Mu and Wu, 1974 Figure 5.6
1974 1987
Spirocrinus longus Mu andWu, p. 208, pl. 97, figs. 6–8. Spirocrinus longus;Mu and Lin, p. 13, pl. 6, figs. 8–15.
Materials.—NIGP 163732.
Occurrence.—Shihniulan Formation, Aeronian (Llandovery, Silurian), Shuibatang section, Guizhou Province, China.
Remarks.—Arm cylindrical in shape and relatively high, but ambulacra only spiral around half of arm circumference. Arm 31.5mm in height and 4.8mm in width. Fused brachials
Figure 3. Petalocrinus stenopetalus n. sp.: (1) holotype, NIGP 163723; (2) paratype, NIGP 163724; (3) paratype, NIGP 163725; (4) NIGP 163726, juvenile specimen; (5) NIGP 163727; (6) NIGP 163728. a, oral side of arm plate; b, aboral side of arm plate; c, lateral view of arm plate; d, front view of distal furrows; e, facet view of arm plate. Scale bars = 1 cm.
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