490
Journal of Paleontology 91(3):477–492
noted above, the analysis depicted in Figure 9 does not include enough taxa to allow confident phylogenetic conclusions. Further, from a Linnaean, rank-based perspective, P. stenopetalus n. sp. has all of the diagnostic characters of Petalocrinus and lacks diagnostic characters of Spirocrinus. Thus, for the present, P. stenopetalus n. sp. is retained in Spirocrinus. Sinopetalocrinus species form a monophyletic group derived
from Petalocrinus inferior, a sister group of P. stenopetalus n. sp. and the Spirocrinus species used in this analysis (Fig. 9). Based on current analysis, Petalocrinus should be regarded as a paraphyletic genus until a more comprehensive species-level phylogeny can be attempted. As noted in Mao et al. (2015), this arm morphology may
have been an adaptation to the multidirectional flow conditions present in a reefal setting. Also, on Spirocrinus arms, the ambulacral groove was much wider than on species of either Petalocrinus or Sinopetalocrinus, which is an indication that Spirocrinus individuals fed on a wider range of food sizes (Ausich, 1980). Alternatively, a more typical Petalocrinus species (e.g., P. inferior) evolved into Sinopetalocrinus by the expansion of the ambulacra from only the aboral surface to both the oral and aboral surfaces (Figs. 4–6, 9). The increased length of the ambulacra on an arm allowed particle capture on both the oral and aboral sides of the arm (Mao et al., 2015). Again, this configuration may be better adapted to the multidirectional flow conditions present in a reefal setting.
Figure 9. Phylogenic tree representing the relationship among genera of petalocrinids from China. The cladogram corresponds to the 50% majority-rule tree of 10 trees, with majority-rule values above and bootstrap/jackknife support below at bifurcations. Tree length = 18, CI = 0.889, RI = 0.895, and RC = 0.795.
the latter two endemic to the South China Block. After the Llandovery the Petalocrinidae had a much lower morphologic dis- parity globally with the final two species of Petalocrinus occurring in theWenlock of Laurentia and Baltica. Finally, the EarlyDevonian petalocrinid, Vadarocrinus of the Perunica paleocontinent, occurred in the present-day Czech Republic and is the youngest known occurrence of the Petalocrinidae. It is still unknown why there are two long-term durations, Darriwilian–Hirnantian and Ludlow–earliest Devonian, that lack representatives of this family. Petalocrinus evolved from the Ordovician crinoid Eopetalo-
crinus by a change from having the arms (second primibrachials) in lateral contact, nearly forming a solid plate to second primibrachials with the lateral edges concave, yielding gaps between adjacent arms. Two distinct lineages evolved from the basic Petalocrinus morphology (Fig. 9). Petalocrinus stenopetalus n. sp. retained planar second primibrachials, but it evolved narrow and elongate second primibrachials compared to other species of Petalocrinus (Fig. 3). This change in shape produced a much less dense filtration fan, but the length of the total ambulacra remained approximately the same. From P. stenopetalus n. sp., Spirocrinus evolved a circular cross-sectional shape for the second primibrachials, and the ambulacra extended around the circumference of the arm plate (Figs. 7–9).LikeP. stenopetalus n. sp., the Spirocrinus second primibrachials aremuch higher than those of a typical Petalocrinus, and the length of the total ambulacra remained approximately the same. From a strict phylogenetic perspective (Fig. 9), P. stenopetalus n. sp. could be placed in Spirocrinus.However, as
Conclusions
Members of Silurian Petalocrinidae occur in carbonate facies of the Upper Yangtze Epicontinental Sea and are especially common from the coeval Shihniulan and Leijiatun formations. Three new taxa are described that further our understanding of the phylogenetic relationships within the Petalocrinidae. Petalocrinus stenopetalus n. sp. and Spirocrinus circularis n. sp. are key transitional taxa that allow us to establish a definite evolutionary trend from Petalocrinus to Spirocrinus. Genus and species divergences happened within a very short time during the Llandovery of China. Thereafter, when Petalocrinus became cosmopolitan, the pace of speciation was considerably slower, and no new genera arose during the Silurian.
Acknowledgments
This research was supported by National Natural Science Foundation of China (granted No. β2022, XDB10010503) and the National Science Foundation (WIA, DEB 1036416). We offer our sincere gratitude to J. Jin and S. Zamora for editing, and two reviewers for the very useful comments on the earlier version of the manuscript.
References
Ausich, W.I., 1980, A model for niche differentiation in Lower Mississippian crinoid communities: Journal of Paleontology, v. 54, p. 273–288.
Ausich, W.I., 1984a, Calceocrinids from the early Silurian (Llandoverian) Brassfield Formation of southwestern Ohio: Journal of Paleontology, v. 58, p. 1167–1185.
Ausich, W.I., 1984b, The genus Clidochirus from the early Silurian of Ohio (Crinoidea, Llandoverian): Journal of Paleontology, v. 58, p. 1341–1346.
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