668


Journal of Paleontology 91(4):662–671


Figure 4. Models of Monoschizoblastus rofei:(1) reconstruction of Monoschizoblastus rofei in feeding position with stem bent into the current with a brachiolar filtration fan; (2) semitransparent oral view of Monoschizoblastus rofei showing the hydrospires leading into spiracles in relation to brachioles; (3) lateral cut view showing the hydrospires, hydrospire pores, and pore canals.


and allowed the hydrospire pores and spiracles to act as open- ings under ambient environmental conditions. The simulation produced unexpected results with respect to fluid flow through the hydrospire. Because the hydrospire pores are very small


openings, the majority of the water flowed around the theca and filtration fan as expected. Water entered the hydrospire pores at varying velocities because water flow is predominantly parallel to pore opening rather than largely perpendicular. Water predominantly enters the hydrospire through the aboral- most pores, travels through the hydrospire fold at a reduced velocity, and then exists the hydrospire through the adoral-most hydrospire pores (Fig. 5.1). Very little water exits the spiracle. Water actually enters the hydrospire through the spiracle as a result of the zone of turbulent eddying that formed downstream of the blastoid oral surface.


A new model for cilia-driven active flow in blastoid hydrospires


The results from the CFD simulations of Monoschizoblastus


rofei in feeding orientation suggest that a reevaluation of active flow in blastozoan respiratory structures is necessary. Although studies as far back as Paul (1978) have suggested the possibility of cilia-driven active flow in blastozoans, the hypothesis has been difficult to evaluate because the cilia are rarely preserved in the fossil record.We concur with the hypothesis of Paul (1978) and Huynh et al. (2015) that, if present in the hydrospire, cilia would be located in the hydrospire tube. We designed CFD simulations to test for active flow in hydrospires of Monoschizoblastus rofei using the CFD module


of Solidworks with conditions as described previously with the following exceptions: (1) the hydrospire was oriented horizon- tally; (2) external water velocity was defined at 0.5 cm/s, 2 cm/s, 5 cm/s, and 10 cm/s; (3) hydrospire pores were allowed to operate as openings under ambient environmental conditions; and (4) we defined the velocity of water exiting the spiracle with the rationale that cilia-driven flow in the hydrospire tube would pump water out of the spiracle at a given velocity. Drawing on results of our previous CFD simulations, we used a value of 0.9 cm/s as the initial excurrent velocity of water exiting the spiracle. In CFD simulations with an external current velocity of 0.5 cm/s (Fig. 5.2), and 2.0 cm/s (Fig. 5.3) with spiracular exit velocity of 0.9 cm/sec, the fundamental water flow through the hydrospire is analogous to that modeled in passive flow (com- pare to Fig. 3.4) even though the orientation of the hydrospire has rotated 90º and the incurrent pore velocity has changed. When water flow was simulated with an external current


velocity of 5 cm/s and spiracular exit velocity of 0.9 cm/s, the pattern of water flow through the hydrospire was abnormal and similar to that seen in Figure 5.1. Even with the assumption of active cilia-driven flow, the exit velocity of water leaving the spiracle was too low (relative to the current velocity) to maintain water flow through the hydrospire optimized for gas exchange.


The hydrospire suffered significant respiratory leakage (adoral flow in the hydrospire fold) and reduced flow of water exiting the spiracle (Fig. 5.4). This result suggested that an optimized ratio between


external current velocity and the exit velocity ofwater leaving the spiracle exists for proper respiratory function in the hydrospire. We reran the CFD simulation with an external current velocity of 5 cm/s and spiracular exit velocity of 2.5 cm/s (Fig. 5.5).


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