670


Journal of Paleontology 91(4):662–671


Spiracles also have a complex cover-plate system that could be used to regulate the diameter of the exit pore and, therefore, hydrospire excurrent velocity so that the blastoid could dynamically adjust to changing ambient conditions. In the future, we can use more refined CFD simulations to gauge how sensitive the hydrospires are to current fluctuations and how much control blastoids had to adjust to those changes. The combination of accurate 3D models and CFD simulation soft- ware gives us the toolkit to address these and other equally compelling questions in blastoid research.


Summary


Virtual reconstructions of blastoid hydrospires from acetate peels and CFD studies of the models confirm the basic hypothesis of water flow through blastoid hydrospires from Schmidtling and Marshall (2010). Water entered hydrospires through the hydrospire pores and canals, traveled through the hydrospire fold with significantly reduced velocity allowing gas exchange, and then exited the system through the hydrospire canal and spiracle. Our reconstructions orient blastoid hydro- spires horizontally as part of a living blastoid in feeding position in currents>0.5 cm/s rather than vertically as in previous studies. Therefore, the concept of an “average” velocity of water entering the hydrospire pore is problematic because the system is much more dynamic than previously modeled. From our CFD simulations, we hypothesize that water flow


through blastoids was cilia-driven active flow rather than passive flow. For Monoschizoblasus rofei, an exit velocity from the spiracle of approximately one-half external current velocity produced an internal water flow through the hydrospires that optimized respiration. Our results indicate that the respiratory systems in blastoids


Life, through the University of Tennessee, and funds from the Appalachian State University Foundation. Waters received a Temminck Fellowship at the Naturalis Museum to digitize the acetate peels of blastoids.


References


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are significantlymore complex than previously thought.We have modeled a single condition in spiraculate blastoids: individual hydrospires emptying into individual spiracles with the exception of the anal interray hydrospires.Many other configurations exist in spiraculate blastoids, and each will require similar analysis to understand the flow parameters needed to produce optimal flow through the system for respiration. Fissiculate blastoids have hydrospire systems that are fundamentally different from those of spiraculate blastoids and thatwill require extensive study to fully understand. The possibility that fissiculate blastoids employed cilia-driven active movement of water through the hydrospires seems likely. The results lend support to the concept that hydrospires should play an increased role in providing characters for phylogenetic analysis of blastoid evolution.


Acknowledgments


We thank the reviewers for their thoughtful comments, which significantly improved the manuscript. We thank I.A. Rahman for introducing us to virtual paleontology and allowing us to participate in his projects to image echinoderms at the Swiss Light Source. B.K. Nguyen expresses her gratitude to I.A. Rahman for his encouragement and advice on the process of segmentation and visualization using SPIERS. Funding for this research came from ROA grants to Appalachian State University from NSF DEB 1036260, Assembling the Echinoderm Tree of


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