科研之友
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摘要
A new stalk articulation named pseudo-synarthry is here described from the mesistele of Vityazicrinus petrachenkoi, a rare deep-sea crinoid from the Central Pacific Ocean. Pseudo-synarthries have an articulation facet displaying a general structure closely resembling the morphology of the true synarthry, i.e., with a strong bilateral symmetry and deep ligament depressions. Pseudo-synarthries differ from synarthries in lacking a hinge-like fulcral ridge and developing two additional depressions bordered by prominent crescent ridges at opposite sides of the bilateral symmetry axis. Pseudo-synarthries develop in the mesistele below a proxistele articulated by symplexies of 6-8 crenular units. Distally, subsidiary radial ridges appear on the outer edge of ligamentary depressions and tend to multiply, transforming pseudo-synarthries into multiradiate syzygies, as previously observed in the genus Guillecrinus. A chaotic polymeric pattern locally appears in the proxistele and mesistele, which tends to develop into a holomeric pattern during growth, devoid of any visible trace of ossicle fusion. Both early columnal ontogeny in comatulid larvae and the transition from a polymeric to a holomeric pattern in the guillecrinid stalk lead to the rejection of the hypothesis that the monomeric columnal results from the fusion of pre-existing ossicles. Among Guillecrinidae, pseudo-synarthries constitute derived characters that are unique to the Vityazicrinus mesistele. The development of radial subsidiary ridges in the distal stalk corresponds to a synapomorphy shared by Vityazicrinus and Guillecrinus, the two only genera currently attributed to this family. Pseudo-synarthries in Vityazicrinus appear as advanced stalk articulations adapted to the deep-sea environment, which are unknown from the fossil record.
- 出版日期2014-9
