Mao et al.—New Silurian Petalocrinidae
Diagnosis.—Horseshoe-shaped articular facets; fused brachials planate; fan bilaterally symmetrical in shape and arrangement of grooves; fused brachials expand in width rapidly for short dis- tance, then are parallel-sided for most of the length for long distance; distal arms with a convex margin; as many as 6–8 distal ambulacra, adjacent arms not in lateral contact; ambula- cral grooves rectilinear and only on oral surface; arm angles are 30°–60°.
Occurrence.—Leijiatun Formation, Aeronian (Llandovery, Silurian), Baisha section, Shiqian County, Guizhou, China.
Description.—Narrow, planate, and slender fused arms. Ambulacral grooves divide almost immediately on the proximal arm, yielding two similar halves. Fused brachials expand in width rapidly for short distance, then are parallel-sided for most of the length for long distance, distal arms with a convex margin. NIGP 163723–163725 are relatively larger than NIGP 163726–163728. Arm height, of NIGP 163723–163725 is respectively 30.4mm, 32.7mm, and 26.9mm; and that of NIGP 163726–163728 is 12.0mm, 11.0mm, and 21.8mm, respec- tively. The maximum width of NIGP 163723–163725 respec- tively is 7.0mm, 7.5mm, and 5.8mm; and that of NIGP 163726–163728 is 4.9mm, 5.9mm, and 9.0mm, respectively. Thickest parts of the six specimens are from 2.6–4.3mm. The arm plates of the six specimens have perfect bilateral
symmetry. The ambulacral grooves on each arm branch exotomously no more than three times, with no branching in the middle part of arm fan yielding 6–8 ambulacral grooves on the distal margin. NIGP 163723–163725, 163727, and 163728 branch three times with eight ambulacra, respectively, and NIGP 163726 branches two times with six finals. Ambulacral grooves rectilinear and are only on oral surface. Width of the ambulacral grooves ~1mm; clear notches for ambulacral cover plates (NIGP 163723, NIGP 163724). Width of the ridge equal to that of ambulacral grooves, flat ridge top. Well-preserved horseshoe-shaped arm facet in NIGP
163723, 163725–163727 with the width between 1.4mm and 3.0mm and a depth of facet 1.0–2.6mm.
Etymology.—The species name ‘stenopetalus’ recognizes the species of Petalocrinus with the narrowest arms.
Materials.—Paratypes NIGP 163724 and NIGP 163725; other materials: NIGP 163726–163728.
Remarks.—Six specimens of Petalocrinus stenopetalus n. sp. include: (1) adult specimens, NIGP 163723–163725, 163727, and 163728; and (2) a juvenile specimen, NIGP 163726, which is relatively small and has fewer branches and finals. This new species is significant for understanding evolu-
tionary trends of Petalocrinus. The arms of P. stenopetalus n. sp. are narrow and elongate with a minimum width and the fewest number of ambulacral grooves. Other species of Petalocrinus typically have subtriangular plates that commonly have more than ten grooves and the arm ratio of height/ maximum width not exceeding two. In contrast, P. stenopetalus n. sp. has eight or fewer distal ambulacra and very elongate arms.
481 The arm plates of the six specimens are perfectly bilateral
symmetrical. The ambulacral grooves on each arm branch exotomously as many as three times, yielding eight ambulacra on the distal margin in adult specimens. After the third ambulacral branch, the width of arms does not increase. NIGP 163726 is a juvenile specimen (Fig. 3.4) with six ambulacral grooves, whereas adults have eight ambulacral grooves. Thus, a general indication of the growth of arms can be deduced for P. stenopetalus n. sp. At the beginning, they grew both in height and width, but after the third branch, they only increased in height.
This new species clearly differs from other genera in arm
form and arrangement of ambulacral grooves. Fused brachials are planate with a very small angle, and ambulacral grooves are rectilinear and only on the oral surface.
Genus Sinopetalocrinus Mu and Lin, 1987 Figures 4, 5.1–5.5, 6
Type species.—Sinopetalocrinus involutus Mu and Lin, 1987.
Sinopetalocrinus involutus Mu and Lin, 1987 Figure 4
1987
Sinopetalocrinus involutus Mu and Lin, p. 12, pl. 5, figs. 5–8.
Holotype.—Holotype NIGP 73967.
Diagnosis.—As many as 32–40 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 >50%.
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 73967, NIGP 73968, NIGP 73969 and NIGP 73970 are type specimens of Sinopetalocrinus involutus from Mu and Lin (1987). NIGP 163729 is a new specimen from the Fengxiang section.
Description.—Ventral side slightly convex. Ambulacra cover- age on aboral surface is >50%. Area of the aboral surface with ambulacra is 23.6mm in height, 26.8mm in width. Thus, the width is larger than the height. Aboral area lacking ambulacra and aboral area with ambulacra are 5.0mm and 7.9mm in thickness, respectively. The arm angle is 101°. Ambulacral grooves as many as 33, both on oral surface and aboral surface. Grooves ~1mm wide. The ridge width is equal to the ambula- cral groove width. Flat ridge top. Unclear notch for ambulacral cover plates on NIGP 163729. Well-preserved, horseshoe- shaped facet 4.4mm in width, 3.0mm in depth.
Materials.—Paratypes NIGP 73968, NIGP 73969 and NIGP 73970; other material: NIGP 163729.
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