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Journal of Paleontology 91(3):477–492
and NIGP 163737 that are 31.4mm and 33.8mm in height, respectively; 6.7mm and 5.4mm in maximum diameter, respectively. A smaller specimen, NIGP 163738, is 20.0mm in height and 4.7mm in maximum diameter. The ambulacral grooves branch no more than three times
with 6–8 distal ambulacral grooves endotomously branched but not spiraled, no branching in the middle of the arm. NIGP 163733 and 162735–162738 have eight distal grooves, and NIGP 163734 has seven distal grooves with a width of ~1mm, which is equal to that of a ridge between adjacent ambulacral grooves. The dorsal central ridges between ambulacra are wider proximally. The notches for ambulacral cover plates alternating on either side, and although poorly preserved are present on NIGP 163733 and NIGP 163734. Well-preserved, horseshoe- shaped proximal facets from six specimens generally have a width between 2.5mm and 3.3mm and a depth from 1.4mm to 2.0mm.
Etymology.—The species name “circularis” is in reference to the endotomously branched but not spiraling distal ambulacra on the cylindrical arms.
Materials.—Paraypes NIGP 163734 and NIGP 163735; other materials: NIGP 163736–163738.
Remarks.—This species differs markedly from other species of the genus in having endotomously branched ambulacral grooves on the cylindrical fused arm plate with no spiraling. Therefore, this morphology is a significant evolutionary link between Petalocrinus and Spirocrinus in aspects of length, branching mode, and number of distal ambulacral grooves, somewhat similar to those of Petalocrinus stenopetalus n. sp. Thus as discussed below, Spirocrinus circularis n. sp. is considered a descendant of Petalocrinus stenopetalus n. sp. or a similar form.
Spirocrinus dextrosus new species Figure 8
Holotype.—Holotype NIGP 163739.
Occurrence.—Shihniulan Formation. Aeronian (Llandovery, Silurian), Shuibatang section, Tongzi County, Guizhou, China.
Diagnosis.—As many as 7–10 distal ambulacra, adjacent arms not in lateral contact, ambulacral grooves spiraled one cycle around fused arm with right-handed spiral.
Description.—Fused brachials cylindrical and tapered at both ends; adjacent arms not in lateral contact; ambulacra with right- handed spiral. The six specimens vary greatly in sizes. Three type specimens (NIGP 163739–163741) are 21.8mm, 25.0mm, and 23.4mm in height; 4.2mm, 5.2mm, and 5.2mm in maxi- mum diameter, respectively. Larger NIGP 163742–163044 are 28.3mm, 30.1mm, and 32.9mm in height; 6.0mm, 6.0mm, and 5.6mm in maximum diameter, respectively. The ambulacral grooves branch no more than three times
with 7–10 distal ambulacral grooves, no branching in the middle of the arm, yielding ten distal ambulacra. All specimens with the same branching pattern. NIGP 163739–163742 and 163744
have eight distal ambulacral grooves and NIGP 163743 has nine. The width of the groove is ~ 1mmand is spiraled one cycle around the fused arm with a right-handed spiral. The groove
width equals that of the ridge width, which is generally flattened. The notches for ambulacral cover plates are not very clear. The proximal horseshoe-shaped arm facet is well preserved in five specimens (NIGP 163739–163743), although part of the facet has been broken away in NIGP 163744. Facets are 1.5–3.4mm in width and 1.4–2.4mm in depth.
Etymology.—The species name ‘dextrosus’ recognizes the right-spiraled ambulacra of Spirocrinus described from China.
Materials.—Paratypes NIGP 163740 and NIGP 163741; other materials: NIGP 163742–163744.
Remarks.—The notable characteristic of the species is the right- spiraled direction of the ambulacral grooves. Other reported species have an ambulacrum with a left-handed ambulacral spiral or an ambulacrum without spiraling. In addition to the direction of the spiral, there is little difference between this species and other left-handed species. Thus, Spirocrinus dextrosus n. sp. also originated from Spirocrinus circularis n. sp., but has the opposite spiraled direction.
Evolution of the Petalocrinidae
With only second-primibrachial characters known for all taxa, an insufficient number of characters exists to complete a comprehensive phylogenetic analysis of petalocrinid species. A small phylogenetic analysis is presented (Fig. 9) only to explore the potential morphological pathways that led to Silurian genera. Whereas this analysis elucidates morphological trends, it cannot be used to develop a phylogenetic classification of petalocrinid species and genera. Li (1993) described the Middle Ordovician petalocrinid and hypothesized potential evolutionary
Eopetalocrinus
trends among Eopetalocrinus, Petalocrinus,and Sinopetalocrinus. Petalocrinidae evolutionary history was also discussed brieflyin Mao et al. (2015).The discovery of the new species described herein allows a more detailed understanding of Petalocrinidae evolution. Following the Late Ordovician mass extinction, crinoid
biodiversity recovered to pre-extinction levels by the Aeronian (middle Llandovery) (Peters and Ausich, 2008). This was accompanied by a rise in sea level with the waning of the Late Ordovician–early Silurian glaciations. As suggested by Fearnhead andHarper (2007), this rise in sea level reestablished epicontinental seas and promoted faunal migrations. A global distribution of Petalocrinus occurred earlier than the typical cosmopolitan Wenlockian faunas (Witzke et al., 1979). During the Llandovery, Petalocrinus first occurred on the Laurentia paleocontinent (Mulloch Hill Formation, Scotland, Rhuddanian; Fearnhead and Harper, 2007). By the Wenlock, Petalocirnus was present on the South China Block, Laurentia, Baltica, and Avalonia paleoconti- nents (Mao et al., 2015). The highest genus-level biodiversity occurred during the
Llandovery in the Upper Yangtze region of China, with the co-occurrence of Petalocrinus, Sinopetalocrinus,and Spirocrinus,
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